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Experimental study on flow boiling characteristics of pure refrigerant (R134a) and zeotropic mixture (R407C) in a rectangular micro-channel
International Journal of Heat and Mass Transfer 104 (2017) 351–361
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Experimental study on flow boiling characteristics of pure refrigerant (R134a) and zeotropic mixture (R407C) in a rectangular micro-channel Chao Dang, Li Jia ⇑, Mingchen Xu, Qian Huang, Qi Peng Institute of Thermal Engineering, School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing 100044, China Beijing Key Laboratory of Flow and Heat Transfer of Phase Changing in Micro and Small Scale, Beijing 100044, China
a r t i c l e
i n f o
Article history: Received 27 June 2016 Received in revised form 20 August 2016 Accepted 20 August 2016
Keywords: Flow boiling Visualization Zeotropic mixture Ma number New correlation
a b s t r a c t In the present study, the flow boiling characteristics of pure refrigerant R134a and zeotropic mixture R407C are experimentally investigated in a single visualized rectangular micro-channel heated on three sides with the cross-sectional area of 1 mm 1 mm and length of 106 mm. Boiling heat transfer coefficients are obtained at the saturation temperature of 21 °C under the heat flux and mass flux ranging from 30–150 kW/m2 and 35–1400 kg/m2 s, respectively. The boiling curves of the two refrigerants are also discussed. Based on the visualization results, seven flow types are identified and the flow pattern maps are plotted. Through the comparative study, the phenomena of advance into churn-annular flow and coexistent bubbles with the flow patterns from confined bubble to annular are observed for R407C. The boiling heat transfer coefficient of R407C is slightly higher than that for R134a at lower vapor quality while the opposite situation appears with the increasing vapor quality after that. During churn-annular to annular flow stage, the boiling heat transfer coefficient of R407C presents a declining trend which is obviously different from the relatively stable value of R134a. The nucleate boiling heat transfer of R407C is suppressed during bubbly flow but promoted in confined bubble to slug flow stage compared with R134a. The CHF of R407C is higher than that of R134a. A correlation for the flow boiling heat transfer coefficient of mixtures is proposed in consideration of Ma number and predicts satisfactorily the database of R407C and R404A. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Flow boiling in a mini/micro-channel has been investigated extensively during the past several years. Due to the advantages such as high heat transfer coefficient and large specific surface area, the research can be promisingly applied to design compact and lightweight heat exchangers or other cooling devices [1]. Additionally, since the depletion of ozone layer and greenhouse effect, chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) refrigerants have been replaced by some hydrofluorocarbons refrigerants (HFCs) which are chlorine free. Among the newly recommended substances, pure refrigerant (R134a), quasi-azeotropic mixtures (R404A, R410A) and zeotropic mixtures (R407C, R417A) are considered to be the eligible substitutes and some of them are currently in application. Despite the fact that zeotropic mixtures present a relatively lower heat transfer performance ⇑ Corresponding author at: Institute of Thermal Engineering, School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing 100044, China. E-mail address: [email protected] (L. Jia). http://dx.doi.org/10.1016/j.ijheatmasstransfer.2016.08.067 0017-9310/Ó 2016 Elsevier Ltd. All rights reserved.
compared with pure refrigerants [2], it is still necessary to investigate flow boiling characteristics and mechanism for multicomponent mixtures, since expected features such as delay dryout and excellent thermal and physical properties were observed [3]. Few works have been done in regard to bubble growth behavior and special correlations for zeotropic mixtures like R407C. Flow boiling heat transfer of zeotropic mixtures has been studied by some scholars recently. Kundu et al. [4,5] conducted an experimental study on the characteristics of flow boiling heat transfer in a smooth horizontal tube for different fluids including pure refrigerant (R134a), quasi-azeotropic mixture (R410A) and zeotropic mixture (R407C). They reported that the boiling heat transfer coefficients of R134a are always higher than those of R407C for same operating conditions. Also, flow regimes visualizations for R134a and R410A were compared with a widely approved flow pattern map proposed by Wojtan et al. [6] for macro-scale, and an assessment of predictive methods available for local heat transfer coefficients was presented in their literatures. Rollmann and Spindler [7] investigated the heat transfer of R407C and the pressure drop of R407C and R410A during flow boiling in a horizontal microfin tube. Based on the measurements, some
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Nomenclature Ah Bd Bo D cp H h hlv I G g L M Ma MAE Pr Q q Rel T U W x
heated area (m2) bond number, g DqD2/r (–) boiling number, q/Ghlv hydraulic diameter (m) specific heat at constant pressure (J/kg K) height of the micro channel (m) heat transfer coefficient (W/m2 K) latent heat of vaporization (J/kg) current (A) mass flux (kg/m2 s) gravitational acceleration (m/s2) length of the micro-channel (m) mass flow rate (kg/s) Marangoni number mean absolute error Prandtl number heating power (W) heat flux (W/m2) liquid Reynolds number, (1 x)GD/qm temperature (°C) voltage (V) width of the micro-channel (m) vapor quality
correlations for the Nusselt number (Nu(x, Bo, Re, Pr)) and total pressure drop were derived with reasonable mean deviations. Leão et al. [8] pointed out that none of the heat transfer predictive methods evaluated in their study is accurate enough to predict the R407C database through the experimental study on flow boiling heat transfer in microchannels. Aprea et al. [9] presented that the R407C heat transfer coefficients are lower than those of R417A at low vapor quality. While for high vapor quality, the R407C heat transfer coefficients become identical with those of R417A. However, the temperature glide of R407C is about 2 °C higher than that of R417A, which may be a slight design advantage for counter type heat exchangers. Zhang et al. [10] explored flow boiling of R417A in different tubes and demonstrated that the enhancement parameters show different situations in different internally grooved tubes. Several experiments on flow boiling characteristics have been conducted with refrigerant mixtures. Guo et al. [11] found that the mixture of R134a and R245fa with 0.82/0.18 in mass fraction show lower pressure drop, but higher flow boiling heat transfer coefficient than that of pure R245fa. Li et al. [12] presented that the nucleate boiling heat transfer is noticeably suppressed at low vapor quality, while the forced convective heat transfer is significantly suppressed at high vapor quality. It indicated that the heat transfer is greatly influenced by mass diffusion resistance and temperature glide of the mixtures. Besides, some modified correlations were obtained by introducing suppression factors into both existing and proposed correlations considering the effects of mass diffusion of the mixtures. Shah [13] proposed a method for predicting heat transfer during flow boiling of mixtures in plain tubes. He applied the Thome and Shakir [14] correlation to nucleate boiling term and the Bell and Ghaly [15] correlation to the convective boiling term. A visualization experimental study on the flow boiling of a mixture of R134a and R245fa was conducted by Abadi et al. [16] in a vertical circular glass tube with 3 mm inner diameter. Flow regimes such as bubbly flow, slug flow and annular flow were
Greek symbols b percentage of data within 25% e heating efficiency g heat loss ratio k thermal conductivity (W/m K) m kinematic viscosity (m2/s) q density (kg/m3) r surface tension (N/m) Subscripts exp exponential value in inlet l liquid v vapor mix mixture out outlet pre predicted value sat saturation sub subcooled tp two phase w wall 0 reference value
recognized in each case, while throat-annular flow was observed in limited cases presented in their works. Many two phase heat transfer correlations for flow boiling of pure refrigerants or water in small to mini/micro tubes were available in literatures. For the approach of superposition model, Saitoh et al. [17] proposed a modified Chen-type correlation taking into account the effect of tube diameter, which is characterized by the Weber number in gas phase. The measurements were collected for R134a in horizontal tubes of diameters ranging from 0.51 to 11 mm. As for asymptotic model, the correlation presented by Mikielewicz [18] has been developed on a theoretical basis considering the energy dissipation in the flow where boiling occurs. A model due to Müller-Steinhagen and Heck [19] is the most effective selection of the two phase flow multiplier for refrigerants calculations. Another recommended simple approach is fitting statistical correlations using the most important dimensionless groups. A modified Lazarek and Black correlation has been suggested by Kew and Cornwell [20] to allow for the observed increase in heat transfer coefficient with vapor quality in their study. Fang [21] developed a correlation for two-phase flow boiling heat transfer coefficient based on the database consisting of 2286 data points of R134a flow boiling heat transfer compiled from 19 published papers, and the correlation also showed high prediction accuracy for other refrigerants such as R22, R245fa and R410A. Li and Wu [22] proposed a general correlation for flow boiling in mini/ micro-channels considering Boiling number, Bond number and Reynolds number. Large amounts of experimental data points (3744) were collected, which covered a wide range of working fluids, operational conditions and different micro-channel dimensions. Mahmoud and Karayiannis [23] and Xu et al. [24] also conducted studies on evaluation of existing correlations and provided some guidance for the selection of appropriate correlations in specific applications during flow boiling heat transfer. Given the present research situation, only a few works in related to visualization of zeotropic mixtures have been done
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during flow boiling heat transfer [16,25–27]. Particularly, seldom studies were conducted in the micro-channel with refrigerants. Unlike pure refrigerant, zeotropic mixtures experience a temperature glide in phase change and can be affected by concentration of each component [3,28]. Therefore, the flow pattern as well as bubble growth and departure behavior is not completely understood for zeotropic mixtures. Moreover, some predictive methods proposed for flow boiling heat transfer of zeotropic mixtures [7,12,13] were presented by adjusting exponents or coefficients of correlations. But, no new parameters or dimensionless numbers under mechanisms were introduced. Much work should be done for special correlation of zeotropic mixtures. In the present study, flow boiling experiments of zeotropic mixture R407C and pure refrigerant R134a as reference were conducted in a single rectangular micro-channel with crosssectional area of 1 mm 1 mm. The micro-channel was heated on three sides. The top part of the test section was transparent, and flow patterns and bubble behaviors of the two test fluid were observed as the visualization results. Heat transfer coefficients were obtained under the conditions that the ranges of refrigerant mass flux and heat flux were 35–1400 kg/m2 s and 30–150 kW/ m2, respectively. Boiling curves for R407C and R134a were also discussed in this paper. Based on the modified correlation for flow boiling heat transfer of R134a, an empirical correlation for zeotropic mixture R407C was developed by introducing an impact factor in related to Marangoni number and vapor quality. The experimental data of quasi-azeotropic mixtures R404A was collected to verify the application range of the suggested correlation.
measurements. A preheater preceded the test section to control the inlet temperature of the flow by adjusting voltage regulator, and a sight glass ahead of test section was used to monitor the inlet state of refrigerants. The flow rate was controlled with two needle valves on both sides of test section. The outlet flow was cooled down and condensed in a plate heat exchanger (condenser) and then collected into the reservoir 2. In addition, temperature and pressure transducer were installed along the test loop to monitor the measurement values logging by a data acquisition system during experiments. 2.2. Test section The schematic diagram of experimental test section is shown in Fig. 2. The micro-channel was processed on a copper base with cross-sectional area of 1 mm 1 mm and length of 106 mm. A transparent PC plate was fastened between the copper base and
1
2 3
10
4 5
2. Experimental setup 2.1. Test loop
9
The experimental apparatus is schematically shown in Fig. 1. The apparatus was an open loop system for stable and smooth flow as well as convenient adjustment. At the beginning and the end of the test loop, two reservoirs were placed in a water bath and a refrigerator respectively to form a pressure-driven flow. The subcooler after the reservoir 1 was arranged for ensuring liquid refrigerants to be subcooled and enter into the Coriolis-type mass flow meter (SIEMENS MASSFLO MASS 2100 DI 1.5) for accurate
8
1. Stainless cover; 2. Rubber gasket; 3. Transparent PC plate; 4. PTFE gasket; 5. Micro-channel base; 6. PTFE piece; 7. Heater positions; 8. Micro-channel; 9. Thermocouple positions; 10. buffer zone Fig. 2. Schematic diagram of the experimental test section.
T|P
T|P
6
7
T|P DP
~|W
Sight glass
Preheater
needle valve 2
needle valve 1 ~|W
Flowmeter
Condenser T|P
T|P
Test section
T|P
Subcooler
Reservoir 2 Reservoir 1 Refrigerator Waterbath Fig. 1. Schematic diagram of the experimental test apparatus.
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Tin
2.4. Uncertainty analysis
Tout
The uncertainty analysis in the present study was performed base on the following equation proposed by Moffat [29]:
dR ¼
" N X @R
Outlet
Inlet Visualized zone Pin
Pout
Fig. 3. Positions of inlet/outlet and transducers on test section.
stainless cover for visualization of flow patterns during experiments. PTFE and rubber gaskets were used for sealing and weakening rigid contact, respectively. Two PTFE pieces with manufactured buffer zone were added to each end of the micro-channel on the copper base to reduce heat losses from the ends. T-type thermocouples were installed at the bottom of channel wall in order to obtain temperature gradient and channel surface temperature along the flow direction. Two cartridge heaters (MISUMI C-MJCHA8-100-V220-W330) were horizontally inserted at the bottom of copper base with a total power of 660 W which could be adjusted by a voltage regulator. The inlet/outlet and transducer (HSTL-103 PT100 and SIEMENS P200 7MF1565) positions of test section were designed on the stainless cover shown in Fig. 3. The flow patterns were recorded using a high-speed camera (PHOTRON FASTCAM UX50), and the visualized zone was delimited near the outlet region of the micro-channel to match more comprehensive data of vapor quality due to the constant subcooled inlet condition in the present study.
i¼1
@V i
2 #1=2 ð1Þ
dV i
where dR represents the overall uncertainty of calculated parameter R, which is the dependent variable affected by a set of measurements Vi, and dVi is the uncertain of the independent variable Vi . The uncertainties of derived quantities such as mass flux G, heat flux q, surface temperature Tw and vapor quality x can be directly calculated using the Eq. (1) based on the accuracies of measurement devices. However, the uncertainty of saturated temperature Tsat is not only related to the accuracy of local pressure but also uncertainty in the equation of state. Maximum deviations of ±0.08 °C and ±0.09 °C for R134a and R407C respectively were obtained with the combination of NIST REFPROP 9.0 and uncertainty of pressure. Table 3 summarizes the experimental uncertainties associated with measured (accuracies of main devices) and calculated parameters. The maximum uncertainty of heat transfer coefficient was ±7.6% which demonstrated the accuracy of indirect measurements. 3. Data reduction For visualization requirements, the upper surface of test section was not covered by thermal insulation material and directly exposed to the surrounding environment, which caused the heat loss of test section to the ambient [30]. Single-phase heat transfer experiments were carried to validate the test section prior to flow boiling tests to estimate the thermal losses.
2.3. Experimental conditions
3.1. Data reduction
Each group of experiments was conducted by changing mass flow rate and heat flux conditions. Boiling curves were obtained by regulating heat flux at the selected mass velocities. Each of the test data including visualization results was recorded in a stable experimental state. All the refrigerants and experimental conditions used in the present study are listed in Tables 1 and 2, respectively. The thermal and physical properties of the refrigerants were acquired from NIST REFPROP 9.0.
During the single-phase experiments, the heating power is balanced with the sensible heat gained by the working fluid (R134a) and thermal losses through different ways such as convection and thermal radiation:
Q ¼ Q eff þ Q loss
ð2Þ
where Q = U I is the total heating power supplied by the cartridge heaters; Qloss represents the heat loss of the test section; Qeff is the
Table 1 Test refrigerants. Property
R134a
R407C
R404A
Component Molar mass Liquid density (25 °C) Critical temperature Critical pressure hlv (1 atm) ODP/GWP ASHRAE safety
Pure 102.03 g/mol 1206.7 kg/m3 101.1 °C 4066.6 kPa 216.97 kJ/kg 0/1300 A1
R32/R125/R134a, 23/25/52 (weight percent) 86.204 g/mol 1154.5 kg/m3 86.74 °C 4620 kPa 248.28 kJ/kg 0/1700 A1
R32/R143a/R134a, 44/52/4 (weight percent) 97.604 g/mol 1062.7 kg/m3 72.4 °C 3688.7 kPa 199.73 kJ/kg 0/3850 A1
Table 2 Experimental conditions. Test
Refrigerant
Mass flux (kg/m2 s)
Heat flux (kW/m2)
Vapor quality
Tsat (°C)
Tsub (°C)
Single phase Two phase Boiling curve
R134a R134a, R407C, R404A R134a, R407C
300–1800 40–1500 250–480
3–16 30–150 30–330
– 0–1 0.15–1
– 21, 30 21
– 1–2 1–2
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C. Dang et al. / International Journal of Heat and Mass Transfer 104 (2017) 351–361 Table 3 Uncertainty of experimental parameters. Measured
Calculated
Parameter
Uncertainty
Parameter
Uncertainty
H, W, L M T (PT100) T (T-type) p U, I
±0.01 mm ±0.1% ±0.2 °C ±0.4 °C ±0.25% ±0.5%
G q Tw Tsat x h
±1.4% ±1.0% ±1.2% ±0.08–0.09 °C ±0.7% ±1.9–7.6%
power. Actually, the data of lower heating power were not very important since larger values were selected during flow boiling tests. The Nusselt number data in the single-phase experiments were compared to the predicted values obtained from the widely accepted correlations proposed by Dittus and Boelter [33] and Gnielinski [34], as shown in Fig. 5. The comparison results between the experimental Nusselt number data and the predicted values using Gnielinski correlation were within the error range of ±10% and presented satisfied reproducibilities, as shown in Fig. 6. The single-phase tests demonstrated that the test section was suitable for flow boiling heat transfer experiments.
effective heat obtained by the single-phase test refrigerant flowing through the channel:
ð3Þ
where M is the mass flow rate; cp,l is the specific heat at constant pressure; Tl,in and Tl,out are inlet and outlet temperatures of the liquid, respectively. The heat loss ratio for single-phase tests can be described as follows:
g ¼ 1 Q eff =Q
ð4Þ
Thus, the heat loss ratio gained from single-phase experiments was used to calculate the heat flux. The similar method was used by Hetsroni et al. [31] and Bogojevic et al. [32] to determine the thermal losses. As for the flow boiling heat transfer tests, the vapor quality at the exit of the test section was calculated using the following equation due to the inlet flow was subcooled in the present study:
x¼
1 eQ cp;l ðT sat T in Þ h lv M
q ðT w T sat Þ
4.1. Flow patterns Visualization results of R134a and R407C were received during flow boiling tests. In order to obtain more precise matching results of flow pattern and vapor quality, the visualized zone was located near the exit of the micro-channel, and the corresponding vapor quality at the outlet was accurately calculated with the condition of subcooled inlet flow. Thus, the relevant uncertainty of vapor quality with flow pattern could be eliminated as much as possible.
100 90 80
ð5Þ
70
where e = 1 g represents the heating efficiency of test section; Tin is the inlet temperature of the test section; Tsat is the saturated temperature of working refrigerant at the inlet pressure. It is noteworthy that the saturated temperature of zeotropic mixtures is not only related to the pressure, but also the quality. The saturated temperature of mixtures can be obtained through the iterative computation of the two parameters using NIST REFPROP 9.0, which contains the programs of thermodynamic vapor-liquid equilibrium and energy balance for calculating the properties of mixtures. [12,16]. The local heat transfer coefficient of the test section is defined as the follows:
hexp ¼
4. Results and discussion
(%)
Q eff ¼ Mcp;l ðT l;out T l;in Þ
50
30 20
Ah
2
4
6
8
10
12
14
Q (W) Fig. 4. Heating efficiency of test section in single-phase experiments.
ð6Þ
100 90 80 70
ð7Þ
where Ah = (2H + W) L represents the heated area of the microchannel. It is appropriate to assume that the heat flux at each side of rectangular channel is approximately uniform because of high thermal conductivity of copper material and small channel dimensions.
R134a Nudata-Turbulent Dittus-Boelter Gnielinski
60 Nu (-)
eQ
R134a Experimental data Mean value (77.56%)
40
where Tw is the wall temperature of the micro-channel derived from Fourier’s law of heat conduction; q is the heat flux derived from:
q¼
60
50 40 30 20 10
3.2. Single-phase validation The working fluid of R134a was used to validate the test loop and section before two phase experiments. The average heating efficiency during single-phase tests was about 77.56% shown in Fig. 4, and the heating efficiency increases slightly with the heating
0 3000
4000
5000
6000
7000
8000
9000 10000 11000
Re (-) Fig. 5. Comparison of Nusselt number data between experiments and the prediction of correlations.
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100 90 80
+10%
R134a Nudata-03/20/2016
-10%
Nudata-05/18/2016
Nuexp (-)
70 60 50 40 30 20 10 10
20
30
40
50
60
70
80
90
100
Nupre- Gnielinski (-) Fig. 6. Experimental reproducibility.
Figs. 7 and 8 present the typical flow patterns of R134a and R407C at different mass flux with same operating conditions, respectively. Since the confined effect of micro-channel, the characteristics of bubble growth and departure are not exactly the same as the situations in macro-scale [35]. In the present experiments, seven types of flow pattern were identified: bubbly flow, confined bubble flow, slug flow, churn annular flow, annular flow, annular-dry out flow and dry out flow. All of them were observed both in the tests of R134a and R407C. With mass flux decreasing, the flow patterns experienced the transition from bubbly flow to dry out flow with the corresponding increase of vapor quality, shown in (a)–(g) of both Figs. 7 and 8. However, several interesting phenomena observed for R407C were different from that of R134a. As shown the bubbly flow in Figs. 7(a) and 8(a), most of dispersed vapor bubbles of R407C were
suspended away from the channel wall and moving in the flowing liquid. While the bubbles of R134a were near the wall, and some of them even attached to the surface leading to slip flow. The most likely reason is that the less volatile components (higher boiling point) of R407C rapidly fill the vacancy position left by the departure of bubbles generated by more volatile components (lower boiling point). While there is no concentration effect or temperature glide in R134a. For R407C, unconfined bubbles obviously existed in the confined and slug flow through the comparison of Figs. 7(b)–(c) and 8(b)–(c). As for churn annular and annular flow, small bubbles were observed inside the flow of R407C and the liquid film fluctuation of R407C was stronger than that of R134a as shown in Figs. 7(d)–(e) and 8(d)–(e). The similar phenomenon also occurred in the literature reported by Abadi et al. [16]. Comparing Figs. 7(f)–(g) and 8(f)–(g), the images and corresponding conditions indicated that R407C would delay the dry out during flow boiling. These characteristics of R407C mentioned above could differently affect the heat transfer coefficient. In order to classify and analyze the situations of flow pattern transition and make sure the mechanism of the variation of heat transfer coefficient during the flow boiling experiments, the flow pattern maps of R134a and R407C were presented, shown in Figs. 9 and 10. The flow pattern maps were plotted at a saturation temperature of 21 °C in coordinates mass flux versus vapor quality (G vs. x), which were sorted out the observations with specific heat fluxes (q = 35.4, 68.6, 142.6 kW/m2) and varying mass flux to obtain the flow pattern transitions at corresponding vapor quality. The transition to churn-annular flow of R134a (shown in Fig. 9) had similar trends to recent studies on pure refrigerants both in macro and mini channels [36]. While R407C presented a characteristic of advance into churn-annular flow stage coexisted with small bubbles through the comparison of Figs. 9 and 10. As for bubbly, confined or slug flow, these flow patterns only occurred within a relatively certain range of vapor quality. The transitions of flow pattern were more dependent on the variation of vapor quality for each kind of refrigerant.
Fig. 7. Flow patterns of R134a at different mass flux (Tsat = 21 °C, q = 35.4 kW/m2).
C. Dang et al. / International Journal of Heat and Mass Transfer 104 (2017) 351–361
357
Fig. 8. Flow patterns of R407C at different mass flux (Tsat = 21 °C, q = 35.4 kW/m2).
Fig. 9. Flow pattern map for R134a at Tsat = 21 °C in a single rectangular micro-channel (1 mm 1 mm).
4.2. Heat transfer characteristics Local boiling heat transfer coefficient was derived by the Eqs. (5)–(7) based on the experimental data of flow boiling tests in steady state. Though the boiling heat transfer characteristics of R134a have been widely investigated by many scholars, only a few studies about the flow boiling in a single rectangular microchannel were reported recently [37]. As for zeotropic refrigerant R407C, there was no research publication. Therefore, the experiments on pure refrigerant R134a were conducted as a comparative reference for the results of R407C. Fig. 11 shows the comparison of boiling heat transfer coefficient of R134a variation with vapor quality at different heat flux in the saturated temperature of 21 °C. Obviously, the boiling heat transfer coefficient increased with the increase of heat flux. The reason is
that the increasing number of bubbles attributed by higher heat flux results in the augmentation of turbulence intensity [37]. The boiling heat transfer coefficient decreased firstly and then increased within the regions of lower vapor quality, where the flow patterns were mainly concentrated in bubbly, confined bubble and slug flow, as shown in Fig. 9. Since the increase of vapor quality corresponded to the decrease of mass flux, the downtrend of boiling heat transfer coefficient thusly appeared at first due to the intensely weakened single phase convective heat transfer. Then, the nucleate boiling gradually dominated the heat transfer mechanism and the effect of convective heat transfer could be neglected. As the vapor quality continued to increase, the heat transfer coefficient kept a relatively stable and high value. The convective boiling played the dominant role in this situation due to the transition of flow pattern to churn-annular and annular flow. Along with the appearance of partial dry out at higher vapor quality as depicted in Fig. 7(f), the heat transfer coefficient would present a downward trend because of the increase of wall temperature. When the dry-out flow occurred, the boiling heat transfer coefficient decreased dramatically. There are many differences between R407C and R134a in the boiling heat transfer coefficient, as shown in Fig. 12. During the bubbly flow, the effect of single phase convective heat transfer on R407C is notably stronger than R134a, since the heat transfer coefficient decreases obviously at lower vapor quality. On the one hand, the bubbles generated by more volatile component of R407C are suspended away from the channel wall and moving along with the flowing liquid. On the other hand, the less volatile components fill the vacancy positions left by the departure of bubbles, so the convective heat transfer between the liquid and the wall is promoted regardless of the relatively suppressed nucleate boiling at the same time due to the dominating mechanism of single phase convective heat transfer. This phenomenon can be illustrated in Fig. 8(a). With the decrease of mass flux, the relevant rise of vapor quality accompanies with the increase of heat transfer coefficient caused by the enhanced nucleate boiling, which also
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20000 R407C Tsat=21
18000
q=35.4 kW/m2 q=68.6 kW/m2 q=142.6 kW/m2
Tsub=1~2
16000
htp (W/m2 K)
14000 12000 10000 8000 6000 4000 2000 0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
x (-) Fig. 10. Flow pattern map for R407C at Tsat = 21 °C in a single rectangular microchannel (1 mm 1 mm).
Fig. 12. Boiling heat transfer coefficient versus vapor quality for R407C at Tsat = 21 °C with different heat flux.
18000
350
R134a Tsat=21
16000
Tsub=1~2
300
12000
250
10000
200
q (kW/m2)
htp (W/m2 K)
14000
8000 6000 q=35.4 kW/m q=68.6 kW/m2 q=142.6 kW/m2
2000 0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Tsat=21
50 0.9
1.0
0
1.1
x (-) Fig. 11. Boiling heat transfer coefficient versus vapor quality for R134a at Tsat = 21 °C with different heat flux.
proves that the effect of single phase convective heat transfer is negligible after that. During churn-annular and annular flow stage, the heat transfer coefficient of R407C is lower than R134a and continues to decline with a gradually slowdown rate. The situation is obviously different with that of R134a which keep a stable value at this stage. The most likely reason is that the more volatile component decreases with the continuous evaporation, meanwhile, the hysteresis boiling of less volatile components prevent the heat from being transferred quickly. But the slowdown trend of decreasing heat transfer coefficient is presented due to the occurrence of small bubbles and liquid fluctuation caused by the boiling of less volatile components shown in Fig. 8(d)–(e). Additionally, the boiling heat transfer coefficient of R407C is slightly higher than that of R134a at lower vapor quality in bubbly to slug slow stage, during which the disturbance of the boiling fluid is enhanced by the coexisted bubbles as shown in Fig. 8(b)–(d). The heat flux versus the wall superheat (Tw Tsat) for R134a and R407C at different mass flux in a saturated temperature of 21 °C are given in Fig. 13. The effect of mass flux on the boiling curve was mainly presented at higher wall superheat (Tw Tsat > 15 °C) both for R134a and R407C. It took greater heat flux to reach the same wall superheat with the increase of mass flux. The variation of mass flux had no significant effect on the boiling curve at lower wall superheat (Tw Tsat < 15 °C). The reason of this occurrence is
Tsub=1~2
R134a, G=273.2 kg/m2 s R134a, G=472.2 kg/m2 s R407C,G=266.7 kg/m2 s R407C,G=461.8 kg/m2 s
100
2
4000
150
0
10
20
30
40
50
60
70
80
Tw-Tsat ( Fig. 13. Boiling curves of R134a and R407C with various mass flux at Tsat = 21 °C.
that the dry out flow will firstly appear in the case of lower mass flux under the same heat flux. The increasing rate of wall superheat for R407C is a little larger than that of R134a at lower wall superheat (Tw Tsat < 15 °C), while the situation completely reverses at higher wall superheat (Tw Tsat > 15 °C). It shows that R407C presents a higher CHF than R134a. This can be explained by the concentration effect caused by temperature glide in the phase change of R407C due to the different boiling point for each component, which is involved in the Ma number through the impact on physical properties proposed by Fujita and Bai [38]:
Ma ¼
Dr
r
ql m2l gðql qv Þ
Pr
ð8Þ
where r is the surface tension of the fluid; Dr represents the difference of the fluid surface tension between the bubble point and dew point; ql, qv are liquid density and vapor density, respectively; and ml and Pr represent the liquid kinematic viscosity and Prandtl number, respectively. Moreover, Marangoni effect helps the region with a breakup persist to a higher heat flux [3] and plays an important role on the boiling heat transfer of zeotropic mixtures [39].
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4.3. Correlation with Marangoni number
Table 4 The statistical assessment of the six examined micro-scale correlations for R134a.
As shown above, several differences were observed on the heat transfer coefficient between the zeotropic mixture R407C and pure refrigerant R134a, especially in the slug to annular-dry out stage (x 0.2–1). A number of mini/micro-scale heat transfer correlations were widely discussed in literatures for pure fluid such as refrigerants and water, and several reviews have even been published for the evaluation of existing correlations for pure fluid in macro to micro scales [23,24]. While hardly any micro-scale correlation has been proposed so far for zeotropic mixtures with big temperature glide such as R407C. In fact, accurate heat transfer correlation for the slug to annular-dry out stage of R407C can be developed based on the modified correlation of R134a in the micro-channel, which can be selected from the accessible correlations mentioned above. The experimental data of R134a in the present study were globally compared with the recommended correlations for micro-scale [17,18,20–22,40] as shown in Fig. 14. Obviously, Mikielewicz correlation [18] and Kew and Cornwell correlation [20] gave a limited prediction with larger MAE defined by Eq. (9) and most of the data located out of the ±25% error bands. The two correlations excessively relied on the term of (1 x) were not appropriate to predict the heat transfer coefficient for convective boiling heat transfer in micro-channel. Better statistical assessments were presented on other four correlations with relatively higher MAE and b (percentage of data within ±25%) values as shown in Table 4.
b
Saitoh et al. [17] Mikielewicz [18] Kew and Cornwell [20] Fang [21] Li and Wu [22] Mohamed and Karayiannis [40]
18.4 61.7 40.4 21.0 15.7 23.2
87.5 8.3 29.2 62.5 83.3 70.8
higher vapor quality compared with the experimental situation. Therefore, only Li and Wu [22] correlation examined in this paper behaved an acceptable agreement between the experimental and predicted trends within the whole considered range of vapor quality. But some corrections were needed since the predicted values were integrally higher than the experimental values resulted in a slightly lower b value. Thusly, the selected statistical correlation of Li and Wu was modified for R134a in a single micro-channel as follows: 0:36 0:4
htp;R134a ¼ 260Bo0:275 ðBdRel
Þ
0:45 kl q D q0
ð11Þ
16000 14000
y ¼ a þ be
N 1X jhpre hexp j 100 N i¼1 hexp
ð9Þ
The number of data within 25% 100% The number of data
ð10Þ
In order to verify the consistency of experimental trend and performance of the other four correlations, the local assessment was performed as shown in Fig. 15. The correlations of Fang [21] and Mahmoud and Karayiannis [40] presented similar performance that the heat transfer coefficient decreased quickly with relatively higher vapor quality (x > 0.4), both of which were failed to capture the experimental trend. Despite the applicable prediction at lower vapor quality using Saitoh et al. [17] correlation, an opposite trend occurred for predicting the heat transfer coefficient at
20000 R134a
18000
htp,exp (W/m2 K)
MAE
Based on the analysis of experimental results for R134a, the heat transfer coefficient is greatly influenced by the heat flux. Meanwhile the decreasing mass flux corresponding to the increasing Bo number does not lead to the decrease of heat transfer coefficient, and which slightly increases in the first and then keeps a relatively stable value. So the revised exponent of Bo number was recommended to enlarge the effect of Bo number in Eq. (11). Posteriorly, the predicted values of Li and Wu correlation were high at lower heat flux but low at higher heat flux, which is most probably caused by the non-linear relationship between the heat flux and wall superheat. The brief term of (q/q0), where q0 = 68600 W/m2 for R134a in a single micro-channel, was put forward to balance the error between experimental and predicted values. In order to develop a new correlation for zeotropic mixture R407C, the experimental data of heat transfer coefficient for R407C and R404A were compared with the predicted values using the modified correlation for R134a. The database of quasiazeotropic mixture R404A with a smaller temperature glide were collected to assist the development of new correlation and extended its application range. The comparison results present a positive deviation at low vapor quality but negative deviation at higher vapor quality, as shown in Fig. 16. The deviation versus vapor quality for both of R407C and R404A presents the exponential decay trend, which can be fitly described as the following form:
MAE ¼
b¼
Correlation
+25%
xc
12000
-25%
10000 8000 6000 4000 2000 0
0
2000 4000 6000 8000 100001200014000160001800020000 htp,pre (W/m2 K) Saitoh et al. [17] Mikielewicz [18] Kew and Cornwell [20]
Fang [21] Li and Wu [22] Mohamed and Karayiannis [40]
Fig. 14. The comparison with six examined micro-scale correlations for R134a.
ð12Þ
But the attenuation amplitude and minimum value of the deviation were different for R407C and R404A. That is mainly because the different temperature glides result in different concentration effects, which can be appropriately expressed using Ma number proposed by Fujita and Bai [38] shown in Eq. (8). Hence, considering the Marangoni effect, the correction factor for mixture was befittingly fitted as a function of Ma number and vapor quality, as shown in Fig. 17. The correction factor described the extent of enhancement or attenuation for the flow boiling heat transfer coefficient of mixtures compared with the predicted values of pure refrigerant based on the modified correlation shown in Eq. (11). The effect of correction factor for mixture decreases with the decrease of Ma number since the closer to the pure fluid, the smaller the temperature glide. The correction factor for mixture is described as the following equation:
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18000 16000
1.4 Experimental data Saitoh et al. [17] Fang [21] Li and Wu [22] Mohamed and Karayiannis [40]
R134a q=35.4 kW/m2
14000
1.0 hexp,mix/htp,R134a (-)
htp (W/m2 K)
12000
1.2
10000 8000 6000 4000
±25% Error band
0.8 0.6 0.4
Experimental data R407C-Ma=2.79e-4 R404A-Ma=2.78e-5
2000
0.2
0 0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0.0 0.1
x (-)
0.2
0.3
0.4
(a) 20000 18000 16000
htp (W/m2 K)
14000
0.7
0.8
0.9
1.0
Fig. 16. The deviation of the experimental heat transfer coefficient for R407C and R404A with respect to the predicted values based on the modified correlation for R134a.
1.4
12000
1.2
10000 8000
1.0
6000
Fmix (-)
±25% Error band
4000 2000 0 0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0.8 0.6 0.4
x (-)
0.2
(b) 26000 R134a 24000 q=142.6kW/m2 22000 20000 18000 16000 14000 12000 10000 8000 6000 4000 2000 0 0.1 0.2 0.3
0.0 0.1
Experimental data Saitoh et al. [17] Fang [21] Li and Wu [22] Mohamed and Karayiannis [40]
18000 16000
0.4
0.5
0.6
0.7
0.8
0.9
1.0
4 0:36 x 0:2
þ 0:6ðMa 10 Þ
e
ð13Þ
Then, the new correlation for mixture is proposed as follows:
htp;mix ¼ 260F mix Bo
0.4
0.5
0.6
0.7
0.8
0.9
1.0
20000
Fig. 15. The comparison with the recommended micro-scale correlations at various heat flux for R134a at Tsat = 21 °C.
0:275
0.3
±25% Error band
(c)
F mix ¼ 0:25Ma
0.2
Fig. 17. The correction factor for mixture as a function of Ma number and vapor quality.
x (-)
0:123
Prediction curve R407C-Ma=2.79e-4 R404A-Ma=2.78e-5
x (-)
0:36 0:4 ðBdRel Þ
0:45 kl q D q0
ð14Þ
htp,mix,exp (W/m2 K)
htp (W/m2 K)
0.6
x (-)
Experimental data Saitoh et al. [17] Fang [21] Li and Wu [22] Mohamed and Karayiannis [40]
R134a q=68.6kW/m2
0.5
New correlation for mixture MAE= 6.1% R407C R404A
+15% -15%
14000 12000 10000 8000 6000 4000 2000 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000 htp,mix,pre (W/m2 K)
Fig. 18. The global comparison with new correlation for R407C and R404A.
The suggested correlation for mixture is validated for both of R407C and R404A with different database at various test conditions within the range of vapor quality from 0.15 to 1, where the flow
C. Dang et al. / International Journal of Heat and Mass Transfer 104 (2017) 351–361
patterns are mainly concentrated in slug to annular dry out flow and the heat transfer mechanism is dominated by convective boiling. Fig. 18 depicts the global comparison between the suggested correlation and the current exponential data of R407C and R404A. The correlation predicts 95.6% of all data within the ±15% error bands at a MAE value of 6.1%. Therefore, the proposed correlation presents good prediction accuracy for R407C and R404A. Meanwhile, the proposed correlation in the present paper may provide some guidance for the research on zeotropic mixtures and be prospectively verified. 5. Conclusion The flow boiling characteristics of pure refrigerant R134a and zeotropic mixture R407C in a single rectangular micro-channel were experimentally investigated in this study. The flow types were identified and the flow pattern maps were plotted based on the visualization results. The heat transfer coefficient and the boiling curve with the effects of mass flux, heat flux and vapor quality were analyzed and compared for both of the two refrigerants. The predictive method for mixtures was also discussed. The main results can be summarized as follows: (1) Seven flow types are identified. The coexistence of some bubbles and the confined bubble flow to annular flow are observed for R407C, which also presents a characteristic of advance into churn-annular flow stage compared to R134a. (2) In the range of vapor quality corresponding to churnannular to annular flow stage, the heat transfer coefficient of R134a maintains a relatively stable and high value with the increasing vapor quality, but the decreasing tendency is presented for R407C as well as the lower heat transfer coefficient in the similar circumstance. (3) R407C shows a higher CHF than R134a. The effect of mass flux on the boiling curve is mainly presented at higher wall superheat, and the larger the mass flux, the higher the CHF. (4) A new correlation is developed for predicting the flow boiling heat transfer coefficient of zeotropic mixtures in view of Marangoni effect. With a valid range of vapor quality (x = 0.15–1), the correlation predicts 95.6% of all experimental data within the ±15% error bands at a MAE value of 6.1%. Acknowledgement This research was supported by Natural Science Foundation of China (No. 51376019). References [1] T.N. Tran, M.W. Wambsganss, D.M. France, Small circular-and rectangular channel boiling with two refrigerants, Int. J. Multiphase Flow 22 (1996) 485– 498. [2] A. Greco, Convective boiling of pure and mixed refrigerants: an experimental study of the major parameters affecting heat transfer, Int. J. Heat Mass Transfer 51 (2008) 896–909. [3] P.H. Lin, B.R. Fu, C. Pan, Critical heat flux on flow boiling of methanol-water mixtures in a diverging microchannel with artificial cavities, Int. J. Heat Mass Transfer 54 (2011) 3156–3166. [4] A. Kundu, R. Kumar, A. Gupta, Heat transfer characteristics and flow pattern during two-phase flow boiling of R134a and R407C in a horizontal smooth tube, Exp. Therm. Fluid Sci. 57 (2014) 344–352. [5] A. Kundu, R. Kumar, A. Gupta, Comparative experimental study on flow boiling heat transfer characteristics of pure and mixed refrigerants, Int. J. Refrig. 45 (2014) 136–147. [6] L. Wojtan, T. Ursenbacher, J.R. Thome, Investigation of flow boiling in horizontal tubes: Part II—development of a new heat transfer model for stratified-wavy, dryout and mist flow regimes, Int. J. Heat Mass Transfer 48 (2005) 2970–2985. [7] P. Rollmann, K. Spindler, New models for heat transfer and pressure drop during flow boiling of R407C and R410A in a horizontal microfin tube, Int. J. Therm. Sci. 103 (2016) 57–66.
361
[8] H.L.S.L. Leão, F.J.D. Nascimento, G. Ribatski, Flow boiling heat transfer of R407C in a microchannels based heat spreader, Exp. Therm. Fluid Sci. 59 (2014) 140– 151. [9] C. Aprea, A. Greco, A. Rosato, Comparison of R407C and R417A heat transfer coefficients and pressure drops during flow boiling in a horizontal smooth tube, Energy Convers. Manage. 49 (2008) 1629–1636. [10] X. Zhang, J. Zhang, H. Ji, D. Zhao, Heat transfer enhancement and pressure drop performance for R417A flow boiling in internally grooved tubes, Energy 86 (2015) 446–454. [11] C. Guo, J. Wang, X. Du, L. Yang, Experimental flow boiling characteristics of R134a/R245fa mixture inside smooth horizontal tube, Appl. Therm. Eng. 103 (2016) 901–908. [12] M. Li, C. Dang, E. Hihara, Flow boiling heat transfer of HFO1234yf and HFC32 refrigerant mixtures in a smooth horizontal tube: Part II. Prediction method, Int. J. Heat Mass Transfer 64 (2013) 591–608. [13] M.M. Shah, A method for predicting heat transfer during boiling of mixtures in plain tubes, Appl. Therm. Eng. 89 (2015) 812–821. [14] J.R. Thome, S. Shakir, A new correlation for nucleate boiling of aqueous mixtures, AIChE Symp. Ser. 83 (257) (1987) 46–57. [15] K.J. Bell, A.M. Ghaly, An approximate generalized method for multicomponent partial condenser, AIChE Symp. Ser. 69 (1973) 72–79. [16] G.B. Abadi, E. Yun, K.C. Kim, Flow boiling characteristics of R134a and R245fa mixtures in a vertical circular tube, Exp. Therm. Fluid Sci. 72 (2016) 112–124. [17] S. Saitoh, H. Daiguji, E. Hihara, Correlation for boiling heat transfer of R-134a in horizontal tubes including effect of tube diameter, Int. J. Heat Mass Transfer 50 (2007) 5215–5225. [18] D. Mikielewicz, A new method for determination of flow boiling heat transfer coefficient in conventional-diameter channels and minichannels, Heat Transfer Eng. 31 (4) (2010) 276–287. [19] H. Müller-Steinhagen, K. Heck, A simple friction pressure drop correlation for two-phase flow in pipes, Chem. Eng. Process. 20 (6) (1986) 297–308. [20] P.A. Kew, K. Cornwell, Correlations for the prediction of boiling heat transfer in small diameter channels, Appl. Therm. Eng. 17 (8–10) (1997) 705–715. [21] X. Fang, A new correlation of flow boiling heat transfer coefficients based on R134a data, Int. J. Heat Mass Transfer 66 (2013) 279–283. [22] W. Li, Z. Wu, A general correlation for evaporative heat transfer in micro/minichannels, Int. J. Heat Mass Transfer 53 (2010) 1778–1787. [23] M.M. Mahmoud, T.G. Karayiannis, Heat transfer correlation for flow boiling in small to micro tubes, Int. J. Heat Mass Transfer 66 (2013) 553–574. [24] Y. Xu, X. Fang, G. Li, D. Li, Y. Yuan, An experimental study of flow boiling heat transfer of R134a and evaluation of existing correlations, Int. J. Heat Mass Transfer 92 (2016) 1143–1157. [25] T.W. Lim, J.U. Park, Prediction of the transition criterion to annular flow in twophase flow pattern of refrigerants, JSME Int. J. Ser. B 46 (2003) 618–624. [26] R. Balakrishnan, L.M. Dhasan, S. Rajagopal, Flow boiling heat transfer coefficient of R-134a/R-290/R-600a mixture in smooth horizontal tubes using varied heat flux method, Appl. Therm. Eng. 29 (2009) 1778–1785. [27] B. Raja, D.M. Lal, R. Saravanan, Stratified flow boiling heat transfer study of a HFC/HC refrigerant mixture in smooth horizontal tubes, Heat Mass Transfer 46 (2010) 323–331. [28] B.R. Fu, P.H. Lin, M.S. Tsou, C. Pan, Flow pattern maps and transition criteria for flow boiling of binary mixtures in a diverging microchannel, Int. J. Heat Mass Transfer 55 (2012) 1754–1763. [29] R.J. Moffat, Describing the uncertainties in experimental results, Exp. Therm. Fluid Sci. 1 (1988) 3–17. [30] L. Yin, L. Jia, P. Guan, D. Liu, Experimental investigation on bubble confinement and elongation in microchannel flow boiling, Exp. Therm. Fluid Sci. 54 (2014) 290–296. [31] G. Hetsroni, A. Mosyak, E. Pogrebnyak, Z. Segal, Explosive boiling of water in parallel micro-channels, Int. J. Multiphase Flow 31 (2005) 371–392. [32] D. Bogojevic, K. Sefiane, G. Duursma, A.J. Walton, Bubble dynamics and flow boiling instabilities in microchannels, Int. J. Heat Mass Transfer 58 (2013) 663– 675. [33] F. Dittus, L. Boelter, Heat transfer in automobile radiators of the tubular type, Univ. Calif. Publ. Eng. 2 (1930) 443–461. [34] V. Gnielinski, New equations for heat and mass transfer in turbulent pipe and channel flows, Int. Chem. Eng. (1976) 403–406. [35] L. Yin, L. Jia, P. Guan, F. Liu, An experimental investigation on the confined and elongated bubbles in subcooled flow boiling in a single microchannel, J. Therm. Sci. 21 (6) (2012) 549–556. [36] S. Saisorn, J. Kaew, Flow pattern and heat transfer characteristics of R-134a refrigerant during flow boiling in a horizontal circular mini-channel, Int. J. Heat Mass Transfer 53 (2010) 4023–4038. [37] C. Keepaiboon, S. Wongwises, Two-phase flow patterns and heat transfer characteristics of R134a refrigerant during flow boiling in a single rectangular micro-channel, Exp. Therm. Fluid Sci. 66 (2015) 36–45. [38] Y. Fujita, Q. Bai, Critical heat flux of binary mixtures in pool boiling and its correlation in terms of Marangoni number, Int. J. Refrig. 20 (8) (1997) 616– 622. [39] W.R. Mcgillis, V.P. Carey, W.R. Mcgillis, V.P. Carey, On the role of Marangoni effects on the critical heat flux for pool boiling of binary mixtures, J. Heat Transfer-Trans. ASME 118 (1996) 103–109. [40] M.M. Mahmoud, T.G. Karayiannis, A statistical correlation for flow boiling heat transfer in micro tubes, in: Proceedings of the Third European Conference on Microfluidics–Microfluidics, Springer, Heidelberg, 2012, pp. 3–5.
Contents lists available at ScienceDirect
International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt
Experimental study on flow boiling characteristics of pure refrigerant (R134a) and zeotropic mixture (R407C) in a rectangular micro-channel Chao Dang, Li Jia ⇑, Mingchen Xu, Qian Huang, Qi Peng Institute of Thermal Engineering, School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing 100044, China Beijing Key Laboratory of Flow and Heat Transfer of Phase Changing in Micro and Small Scale, Beijing 100044, China
a r t i c l e
i n f o
Article history: Received 27 June 2016 Received in revised form 20 August 2016 Accepted 20 August 2016
Keywords: Flow boiling Visualization Zeotropic mixture Ma number New correlation
a b s t r a c t In the present study, the flow boiling characteristics of pure refrigerant R134a and zeotropic mixture R407C are experimentally investigated in a single visualized rectangular micro-channel heated on three sides with the cross-sectional area of 1 mm 1 mm and length of 106 mm. Boiling heat transfer coefficients are obtained at the saturation temperature of 21 °C under the heat flux and mass flux ranging from 30–150 kW/m2 and 35–1400 kg/m2 s, respectively. The boiling curves of the two refrigerants are also discussed. Based on the visualization results, seven flow types are identified and the flow pattern maps are plotted. Through the comparative study, the phenomena of advance into churn-annular flow and coexistent bubbles with the flow patterns from confined bubble to annular are observed for R407C. The boiling heat transfer coefficient of R407C is slightly higher than that for R134a at lower vapor quality while the opposite situation appears with the increasing vapor quality after that. During churn-annular to annular flow stage, the boiling heat transfer coefficient of R407C presents a declining trend which is obviously different from the relatively stable value of R134a. The nucleate boiling heat transfer of R407C is suppressed during bubbly flow but promoted in confined bubble to slug flow stage compared with R134a. The CHF of R407C is higher than that of R134a. A correlation for the flow boiling heat transfer coefficient of mixtures is proposed in consideration of Ma number and predicts satisfactorily the database of R407C and R404A. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Flow boiling in a mini/micro-channel has been investigated extensively during the past several years. Due to the advantages such as high heat transfer coefficient and large specific surface area, the research can be promisingly applied to design compact and lightweight heat exchangers or other cooling devices [1]. Additionally, since the depletion of ozone layer and greenhouse effect, chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) refrigerants have been replaced by some hydrofluorocarbons refrigerants (HFCs) which are chlorine free. Among the newly recommended substances, pure refrigerant (R134a), quasi-azeotropic mixtures (R404A, R410A) and zeotropic mixtures (R407C, R417A) are considered to be the eligible substitutes and some of them are currently in application. Despite the fact that zeotropic mixtures present a relatively lower heat transfer performance ⇑ Corresponding author at: Institute of Thermal Engineering, School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing 100044, China. E-mail address: [email protected] (L. Jia). http://dx.doi.org/10.1016/j.ijheatmasstransfer.2016.08.067 0017-9310/Ó 2016 Elsevier Ltd. All rights reserved.
compared with pure refrigerants [2], it is still necessary to investigate flow boiling characteristics and mechanism for multicomponent mixtures, since expected features such as delay dryout and excellent thermal and physical properties were observed [3]. Few works have been done in regard to bubble growth behavior and special correlations for zeotropic mixtures like R407C. Flow boiling heat transfer of zeotropic mixtures has been studied by some scholars recently. Kundu et al. [4,5] conducted an experimental study on the characteristics of flow boiling heat transfer in a smooth horizontal tube for different fluids including pure refrigerant (R134a), quasi-azeotropic mixture (R410A) and zeotropic mixture (R407C). They reported that the boiling heat transfer coefficients of R134a are always higher than those of R407C for same operating conditions. Also, flow regimes visualizations for R134a and R410A were compared with a widely approved flow pattern map proposed by Wojtan et al. [6] for macro-scale, and an assessment of predictive methods available for local heat transfer coefficients was presented in their literatures. Rollmann and Spindler [7] investigated the heat transfer of R407C and the pressure drop of R407C and R410A during flow boiling in a horizontal microfin tube. Based on the measurements, some
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Nomenclature Ah Bd Bo D cp H h hlv I G g L M Ma MAE Pr Q q Rel T U W x
heated area (m2) bond number, g DqD2/r (–) boiling number, q/Ghlv hydraulic diameter (m) specific heat at constant pressure (J/kg K) height of the micro channel (m) heat transfer coefficient (W/m2 K) latent heat of vaporization (J/kg) current (A) mass flux (kg/m2 s) gravitational acceleration (m/s2) length of the micro-channel (m) mass flow rate (kg/s) Marangoni number mean absolute error Prandtl number heating power (W) heat flux (W/m2) liquid Reynolds number, (1 x)GD/qm temperature (°C) voltage (V) width of the micro-channel (m) vapor quality
correlations for the Nusselt number (Nu(x, Bo, Re, Pr)) and total pressure drop were derived with reasonable mean deviations. Leão et al. [8] pointed out that none of the heat transfer predictive methods evaluated in their study is accurate enough to predict the R407C database through the experimental study on flow boiling heat transfer in microchannels. Aprea et al. [9] presented that the R407C heat transfer coefficients are lower than those of R417A at low vapor quality. While for high vapor quality, the R407C heat transfer coefficients become identical with those of R417A. However, the temperature glide of R407C is about 2 °C higher than that of R417A, which may be a slight design advantage for counter type heat exchangers. Zhang et al. [10] explored flow boiling of R417A in different tubes and demonstrated that the enhancement parameters show different situations in different internally grooved tubes. Several experiments on flow boiling characteristics have been conducted with refrigerant mixtures. Guo et al. [11] found that the mixture of R134a and R245fa with 0.82/0.18 in mass fraction show lower pressure drop, but higher flow boiling heat transfer coefficient than that of pure R245fa. Li et al. [12] presented that the nucleate boiling heat transfer is noticeably suppressed at low vapor quality, while the forced convective heat transfer is significantly suppressed at high vapor quality. It indicated that the heat transfer is greatly influenced by mass diffusion resistance and temperature glide of the mixtures. Besides, some modified correlations were obtained by introducing suppression factors into both existing and proposed correlations considering the effects of mass diffusion of the mixtures. Shah [13] proposed a method for predicting heat transfer during flow boiling of mixtures in plain tubes. He applied the Thome and Shakir [14] correlation to nucleate boiling term and the Bell and Ghaly [15] correlation to the convective boiling term. A visualization experimental study on the flow boiling of a mixture of R134a and R245fa was conducted by Abadi et al. [16] in a vertical circular glass tube with 3 mm inner diameter. Flow regimes such as bubbly flow, slug flow and annular flow were
Greek symbols b percentage of data within 25% e heating efficiency g heat loss ratio k thermal conductivity (W/m K) m kinematic viscosity (m2/s) q density (kg/m3) r surface tension (N/m) Subscripts exp exponential value in inlet l liquid v vapor mix mixture out outlet pre predicted value sat saturation sub subcooled tp two phase w wall 0 reference value
recognized in each case, while throat-annular flow was observed in limited cases presented in their works. Many two phase heat transfer correlations for flow boiling of pure refrigerants or water in small to mini/micro tubes were available in literatures. For the approach of superposition model, Saitoh et al. [17] proposed a modified Chen-type correlation taking into account the effect of tube diameter, which is characterized by the Weber number in gas phase. The measurements were collected for R134a in horizontal tubes of diameters ranging from 0.51 to 11 mm. As for asymptotic model, the correlation presented by Mikielewicz [18] has been developed on a theoretical basis considering the energy dissipation in the flow where boiling occurs. A model due to Müller-Steinhagen and Heck [19] is the most effective selection of the two phase flow multiplier for refrigerants calculations. Another recommended simple approach is fitting statistical correlations using the most important dimensionless groups. A modified Lazarek and Black correlation has been suggested by Kew and Cornwell [20] to allow for the observed increase in heat transfer coefficient with vapor quality in their study. Fang [21] developed a correlation for two-phase flow boiling heat transfer coefficient based on the database consisting of 2286 data points of R134a flow boiling heat transfer compiled from 19 published papers, and the correlation also showed high prediction accuracy for other refrigerants such as R22, R245fa and R410A. Li and Wu [22] proposed a general correlation for flow boiling in mini/ micro-channels considering Boiling number, Bond number and Reynolds number. Large amounts of experimental data points (3744) were collected, which covered a wide range of working fluids, operational conditions and different micro-channel dimensions. Mahmoud and Karayiannis [23] and Xu et al. [24] also conducted studies on evaluation of existing correlations and provided some guidance for the selection of appropriate correlations in specific applications during flow boiling heat transfer. Given the present research situation, only a few works in related to visualization of zeotropic mixtures have been done
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during flow boiling heat transfer [16,25–27]. Particularly, seldom studies were conducted in the micro-channel with refrigerants. Unlike pure refrigerant, zeotropic mixtures experience a temperature glide in phase change and can be affected by concentration of each component [3,28]. Therefore, the flow pattern as well as bubble growth and departure behavior is not completely understood for zeotropic mixtures. Moreover, some predictive methods proposed for flow boiling heat transfer of zeotropic mixtures [7,12,13] were presented by adjusting exponents or coefficients of correlations. But, no new parameters or dimensionless numbers under mechanisms were introduced. Much work should be done for special correlation of zeotropic mixtures. In the present study, flow boiling experiments of zeotropic mixture R407C and pure refrigerant R134a as reference were conducted in a single rectangular micro-channel with crosssectional area of 1 mm 1 mm. The micro-channel was heated on three sides. The top part of the test section was transparent, and flow patterns and bubble behaviors of the two test fluid were observed as the visualization results. Heat transfer coefficients were obtained under the conditions that the ranges of refrigerant mass flux and heat flux were 35–1400 kg/m2 s and 30–150 kW/ m2, respectively. Boiling curves for R407C and R134a were also discussed in this paper. Based on the modified correlation for flow boiling heat transfer of R134a, an empirical correlation for zeotropic mixture R407C was developed by introducing an impact factor in related to Marangoni number and vapor quality. The experimental data of quasi-azeotropic mixtures R404A was collected to verify the application range of the suggested correlation.
measurements. A preheater preceded the test section to control the inlet temperature of the flow by adjusting voltage regulator, and a sight glass ahead of test section was used to monitor the inlet state of refrigerants. The flow rate was controlled with two needle valves on both sides of test section. The outlet flow was cooled down and condensed in a plate heat exchanger (condenser) and then collected into the reservoir 2. In addition, temperature and pressure transducer were installed along the test loop to monitor the measurement values logging by a data acquisition system during experiments. 2.2. Test section The schematic diagram of experimental test section is shown in Fig. 2. The micro-channel was processed on a copper base with cross-sectional area of 1 mm 1 mm and length of 106 mm. A transparent PC plate was fastened between the copper base and
1
2 3
10
4 5
2. Experimental setup 2.1. Test loop
9
The experimental apparatus is schematically shown in Fig. 1. The apparatus was an open loop system for stable and smooth flow as well as convenient adjustment. At the beginning and the end of the test loop, two reservoirs were placed in a water bath and a refrigerator respectively to form a pressure-driven flow. The subcooler after the reservoir 1 was arranged for ensuring liquid refrigerants to be subcooled and enter into the Coriolis-type mass flow meter (SIEMENS MASSFLO MASS 2100 DI 1.5) for accurate
8
1. Stainless cover; 2. Rubber gasket; 3. Transparent PC plate; 4. PTFE gasket; 5. Micro-channel base; 6. PTFE piece; 7. Heater positions; 8. Micro-channel; 9. Thermocouple positions; 10. buffer zone Fig. 2. Schematic diagram of the experimental test section.
T|P
T|P
6
7
T|P DP
~|W
Sight glass
Preheater
needle valve 2
needle valve 1 ~|W
Flowmeter
Condenser T|P
T|P
Test section
T|P
Subcooler
Reservoir 2 Reservoir 1 Refrigerator Waterbath Fig. 1. Schematic diagram of the experimental test apparatus.
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C. Dang et al. / International Journal of Heat and Mass Transfer 104 (2017) 351–361
Tin
2.4. Uncertainty analysis
Tout
The uncertainty analysis in the present study was performed base on the following equation proposed by Moffat [29]:
dR ¼
" N X @R
Outlet
Inlet Visualized zone Pin
Pout
Fig. 3. Positions of inlet/outlet and transducers on test section.
stainless cover for visualization of flow patterns during experiments. PTFE and rubber gaskets were used for sealing and weakening rigid contact, respectively. Two PTFE pieces with manufactured buffer zone were added to each end of the micro-channel on the copper base to reduce heat losses from the ends. T-type thermocouples were installed at the bottom of channel wall in order to obtain temperature gradient and channel surface temperature along the flow direction. Two cartridge heaters (MISUMI C-MJCHA8-100-V220-W330) were horizontally inserted at the bottom of copper base with a total power of 660 W which could be adjusted by a voltage regulator. The inlet/outlet and transducer (HSTL-103 PT100 and SIEMENS P200 7MF1565) positions of test section were designed on the stainless cover shown in Fig. 3. The flow patterns were recorded using a high-speed camera (PHOTRON FASTCAM UX50), and the visualized zone was delimited near the outlet region of the micro-channel to match more comprehensive data of vapor quality due to the constant subcooled inlet condition in the present study.
i¼1
@V i
2 #1=2 ð1Þ
dV i
where dR represents the overall uncertainty of calculated parameter R, which is the dependent variable affected by a set of measurements Vi, and dVi is the uncertain of the independent variable Vi . The uncertainties of derived quantities such as mass flux G, heat flux q, surface temperature Tw and vapor quality x can be directly calculated using the Eq. (1) based on the accuracies of measurement devices. However, the uncertainty of saturated temperature Tsat is not only related to the accuracy of local pressure but also uncertainty in the equation of state. Maximum deviations of ±0.08 °C and ±0.09 °C for R134a and R407C respectively were obtained with the combination of NIST REFPROP 9.0 and uncertainty of pressure. Table 3 summarizes the experimental uncertainties associated with measured (accuracies of main devices) and calculated parameters. The maximum uncertainty of heat transfer coefficient was ±7.6% which demonstrated the accuracy of indirect measurements. 3. Data reduction For visualization requirements, the upper surface of test section was not covered by thermal insulation material and directly exposed to the surrounding environment, which caused the heat loss of test section to the ambient [30]. Single-phase heat transfer experiments were carried to validate the test section prior to flow boiling tests to estimate the thermal losses.
2.3. Experimental conditions
3.1. Data reduction
Each group of experiments was conducted by changing mass flow rate and heat flux conditions. Boiling curves were obtained by regulating heat flux at the selected mass velocities. Each of the test data including visualization results was recorded in a stable experimental state. All the refrigerants and experimental conditions used in the present study are listed in Tables 1 and 2, respectively. The thermal and physical properties of the refrigerants were acquired from NIST REFPROP 9.0.
During the single-phase experiments, the heating power is balanced with the sensible heat gained by the working fluid (R134a) and thermal losses through different ways such as convection and thermal radiation:
Q ¼ Q eff þ Q loss
ð2Þ
where Q = U I is the total heating power supplied by the cartridge heaters; Qloss represents the heat loss of the test section; Qeff is the
Table 1 Test refrigerants. Property
R134a
R407C
R404A
Component Molar mass Liquid density (25 °C) Critical temperature Critical pressure hlv (1 atm) ODP/GWP ASHRAE safety
Pure 102.03 g/mol 1206.7 kg/m3 101.1 °C 4066.6 kPa 216.97 kJ/kg 0/1300 A1
R32/R125/R134a, 23/25/52 (weight percent) 86.204 g/mol 1154.5 kg/m3 86.74 °C 4620 kPa 248.28 kJ/kg 0/1700 A1
R32/R143a/R134a, 44/52/4 (weight percent) 97.604 g/mol 1062.7 kg/m3 72.4 °C 3688.7 kPa 199.73 kJ/kg 0/3850 A1
Table 2 Experimental conditions. Test
Refrigerant
Mass flux (kg/m2 s)
Heat flux (kW/m2)
Vapor quality
Tsat (°C)
Tsub (°C)
Single phase Two phase Boiling curve
R134a R134a, R407C, R404A R134a, R407C
300–1800 40–1500 250–480
3–16 30–150 30–330
– 0–1 0.15–1
– 21, 30 21
– 1–2 1–2
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C. Dang et al. / International Journal of Heat and Mass Transfer 104 (2017) 351–361 Table 3 Uncertainty of experimental parameters. Measured
Calculated
Parameter
Uncertainty
Parameter
Uncertainty
H, W, L M T (PT100) T (T-type) p U, I
±0.01 mm ±0.1% ±0.2 °C ±0.4 °C ±0.25% ±0.5%
G q Tw Tsat x h
±1.4% ±1.0% ±1.2% ±0.08–0.09 °C ±0.7% ±1.9–7.6%
power. Actually, the data of lower heating power were not very important since larger values were selected during flow boiling tests. The Nusselt number data in the single-phase experiments were compared to the predicted values obtained from the widely accepted correlations proposed by Dittus and Boelter [33] and Gnielinski [34], as shown in Fig. 5. The comparison results between the experimental Nusselt number data and the predicted values using Gnielinski correlation were within the error range of ±10% and presented satisfied reproducibilities, as shown in Fig. 6. The single-phase tests demonstrated that the test section was suitable for flow boiling heat transfer experiments.
effective heat obtained by the single-phase test refrigerant flowing through the channel:
ð3Þ
where M is the mass flow rate; cp,l is the specific heat at constant pressure; Tl,in and Tl,out are inlet and outlet temperatures of the liquid, respectively. The heat loss ratio for single-phase tests can be described as follows:
g ¼ 1 Q eff =Q
ð4Þ
Thus, the heat loss ratio gained from single-phase experiments was used to calculate the heat flux. The similar method was used by Hetsroni et al. [31] and Bogojevic et al. [32] to determine the thermal losses. As for the flow boiling heat transfer tests, the vapor quality at the exit of the test section was calculated using the following equation due to the inlet flow was subcooled in the present study:
x¼
1 eQ cp;l ðT sat T in Þ h lv M
q ðT w T sat Þ
4.1. Flow patterns Visualization results of R134a and R407C were received during flow boiling tests. In order to obtain more precise matching results of flow pattern and vapor quality, the visualized zone was located near the exit of the micro-channel, and the corresponding vapor quality at the outlet was accurately calculated with the condition of subcooled inlet flow. Thus, the relevant uncertainty of vapor quality with flow pattern could be eliminated as much as possible.
100 90 80
ð5Þ
70
where e = 1 g represents the heating efficiency of test section; Tin is the inlet temperature of the test section; Tsat is the saturated temperature of working refrigerant at the inlet pressure. It is noteworthy that the saturated temperature of zeotropic mixtures is not only related to the pressure, but also the quality. The saturated temperature of mixtures can be obtained through the iterative computation of the two parameters using NIST REFPROP 9.0, which contains the programs of thermodynamic vapor-liquid equilibrium and energy balance for calculating the properties of mixtures. [12,16]. The local heat transfer coefficient of the test section is defined as the follows:
hexp ¼
4. Results and discussion
(%)
Q eff ¼ Mcp;l ðT l;out T l;in Þ
50
30 20
Ah
2
4
6
8
10
12
14
Q (W) Fig. 4. Heating efficiency of test section in single-phase experiments.
ð6Þ
100 90 80 70
ð7Þ
where Ah = (2H + W) L represents the heated area of the microchannel. It is appropriate to assume that the heat flux at each side of rectangular channel is approximately uniform because of high thermal conductivity of copper material and small channel dimensions.
R134a Nudata-Turbulent Dittus-Boelter Gnielinski
60 Nu (-)
eQ
R134a Experimental data Mean value (77.56%)
40
where Tw is the wall temperature of the micro-channel derived from Fourier’s law of heat conduction; q is the heat flux derived from:
q¼
60
50 40 30 20 10
3.2. Single-phase validation The working fluid of R134a was used to validate the test loop and section before two phase experiments. The average heating efficiency during single-phase tests was about 77.56% shown in Fig. 4, and the heating efficiency increases slightly with the heating
0 3000
4000
5000
6000
7000
8000
9000 10000 11000
Re (-) Fig. 5. Comparison of Nusselt number data between experiments and the prediction of correlations.
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100 90 80
+10%
R134a Nudata-03/20/2016
-10%
Nudata-05/18/2016
Nuexp (-)
70 60 50 40 30 20 10 10
20
30
40
50
60
70
80
90
100
Nupre- Gnielinski (-) Fig. 6. Experimental reproducibility.
Figs. 7 and 8 present the typical flow patterns of R134a and R407C at different mass flux with same operating conditions, respectively. Since the confined effect of micro-channel, the characteristics of bubble growth and departure are not exactly the same as the situations in macro-scale [35]. In the present experiments, seven types of flow pattern were identified: bubbly flow, confined bubble flow, slug flow, churn annular flow, annular flow, annular-dry out flow and dry out flow. All of them were observed both in the tests of R134a and R407C. With mass flux decreasing, the flow patterns experienced the transition from bubbly flow to dry out flow with the corresponding increase of vapor quality, shown in (a)–(g) of both Figs. 7 and 8. However, several interesting phenomena observed for R407C were different from that of R134a. As shown the bubbly flow in Figs. 7(a) and 8(a), most of dispersed vapor bubbles of R407C were
suspended away from the channel wall and moving in the flowing liquid. While the bubbles of R134a were near the wall, and some of them even attached to the surface leading to slip flow. The most likely reason is that the less volatile components (higher boiling point) of R407C rapidly fill the vacancy position left by the departure of bubbles generated by more volatile components (lower boiling point). While there is no concentration effect or temperature glide in R134a. For R407C, unconfined bubbles obviously existed in the confined and slug flow through the comparison of Figs. 7(b)–(c) and 8(b)–(c). As for churn annular and annular flow, small bubbles were observed inside the flow of R407C and the liquid film fluctuation of R407C was stronger than that of R134a as shown in Figs. 7(d)–(e) and 8(d)–(e). The similar phenomenon also occurred in the literature reported by Abadi et al. [16]. Comparing Figs. 7(f)–(g) and 8(f)–(g), the images and corresponding conditions indicated that R407C would delay the dry out during flow boiling. These characteristics of R407C mentioned above could differently affect the heat transfer coefficient. In order to classify and analyze the situations of flow pattern transition and make sure the mechanism of the variation of heat transfer coefficient during the flow boiling experiments, the flow pattern maps of R134a and R407C were presented, shown in Figs. 9 and 10. The flow pattern maps were plotted at a saturation temperature of 21 °C in coordinates mass flux versus vapor quality (G vs. x), which were sorted out the observations with specific heat fluxes (q = 35.4, 68.6, 142.6 kW/m2) and varying mass flux to obtain the flow pattern transitions at corresponding vapor quality. The transition to churn-annular flow of R134a (shown in Fig. 9) had similar trends to recent studies on pure refrigerants both in macro and mini channels [36]. While R407C presented a characteristic of advance into churn-annular flow stage coexisted with small bubbles through the comparison of Figs. 9 and 10. As for bubbly, confined or slug flow, these flow patterns only occurred within a relatively certain range of vapor quality. The transitions of flow pattern were more dependent on the variation of vapor quality for each kind of refrigerant.
Fig. 7. Flow patterns of R134a at different mass flux (Tsat = 21 °C, q = 35.4 kW/m2).
C. Dang et al. / International Journal of Heat and Mass Transfer 104 (2017) 351–361
357
Fig. 8. Flow patterns of R407C at different mass flux (Tsat = 21 °C, q = 35.4 kW/m2).
Fig. 9. Flow pattern map for R134a at Tsat = 21 °C in a single rectangular micro-channel (1 mm 1 mm).
4.2. Heat transfer characteristics Local boiling heat transfer coefficient was derived by the Eqs. (5)–(7) based on the experimental data of flow boiling tests in steady state. Though the boiling heat transfer characteristics of R134a have been widely investigated by many scholars, only a few studies about the flow boiling in a single rectangular microchannel were reported recently [37]. As for zeotropic refrigerant R407C, there was no research publication. Therefore, the experiments on pure refrigerant R134a were conducted as a comparative reference for the results of R407C. Fig. 11 shows the comparison of boiling heat transfer coefficient of R134a variation with vapor quality at different heat flux in the saturated temperature of 21 °C. Obviously, the boiling heat transfer coefficient increased with the increase of heat flux. The reason is
that the increasing number of bubbles attributed by higher heat flux results in the augmentation of turbulence intensity [37]. The boiling heat transfer coefficient decreased firstly and then increased within the regions of lower vapor quality, where the flow patterns were mainly concentrated in bubbly, confined bubble and slug flow, as shown in Fig. 9. Since the increase of vapor quality corresponded to the decrease of mass flux, the downtrend of boiling heat transfer coefficient thusly appeared at first due to the intensely weakened single phase convective heat transfer. Then, the nucleate boiling gradually dominated the heat transfer mechanism and the effect of convective heat transfer could be neglected. As the vapor quality continued to increase, the heat transfer coefficient kept a relatively stable and high value. The convective boiling played the dominant role in this situation due to the transition of flow pattern to churn-annular and annular flow. Along with the appearance of partial dry out at higher vapor quality as depicted in Fig. 7(f), the heat transfer coefficient would present a downward trend because of the increase of wall temperature. When the dry-out flow occurred, the boiling heat transfer coefficient decreased dramatically. There are many differences between R407C and R134a in the boiling heat transfer coefficient, as shown in Fig. 12. During the bubbly flow, the effect of single phase convective heat transfer on R407C is notably stronger than R134a, since the heat transfer coefficient decreases obviously at lower vapor quality. On the one hand, the bubbles generated by more volatile component of R407C are suspended away from the channel wall and moving along with the flowing liquid. On the other hand, the less volatile components fill the vacancy positions left by the departure of bubbles, so the convective heat transfer between the liquid and the wall is promoted regardless of the relatively suppressed nucleate boiling at the same time due to the dominating mechanism of single phase convective heat transfer. This phenomenon can be illustrated in Fig. 8(a). With the decrease of mass flux, the relevant rise of vapor quality accompanies with the increase of heat transfer coefficient caused by the enhanced nucleate boiling, which also
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20000 R407C Tsat=21
18000
q=35.4 kW/m2 q=68.6 kW/m2 q=142.6 kW/m2
Tsub=1~2
16000
htp (W/m2 K)
14000 12000 10000 8000 6000 4000 2000 0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
x (-) Fig. 10. Flow pattern map for R407C at Tsat = 21 °C in a single rectangular microchannel (1 mm 1 mm).
Fig. 12. Boiling heat transfer coefficient versus vapor quality for R407C at Tsat = 21 °C with different heat flux.
18000
350
R134a Tsat=21
16000
Tsub=1~2
300
12000
250
10000
200
q (kW/m2)
htp (W/m2 K)
14000
8000 6000 q=35.4 kW/m q=68.6 kW/m2 q=142.6 kW/m2
2000 0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Tsat=21
50 0.9
1.0
0
1.1
x (-) Fig. 11. Boiling heat transfer coefficient versus vapor quality for R134a at Tsat = 21 °C with different heat flux.
proves that the effect of single phase convective heat transfer is negligible after that. During churn-annular and annular flow stage, the heat transfer coefficient of R407C is lower than R134a and continues to decline with a gradually slowdown rate. The situation is obviously different with that of R134a which keep a stable value at this stage. The most likely reason is that the more volatile component decreases with the continuous evaporation, meanwhile, the hysteresis boiling of less volatile components prevent the heat from being transferred quickly. But the slowdown trend of decreasing heat transfer coefficient is presented due to the occurrence of small bubbles and liquid fluctuation caused by the boiling of less volatile components shown in Fig. 8(d)–(e). Additionally, the boiling heat transfer coefficient of R407C is slightly higher than that of R134a at lower vapor quality in bubbly to slug slow stage, during which the disturbance of the boiling fluid is enhanced by the coexisted bubbles as shown in Fig. 8(b)–(d). The heat flux versus the wall superheat (Tw Tsat) for R134a and R407C at different mass flux in a saturated temperature of 21 °C are given in Fig. 13. The effect of mass flux on the boiling curve was mainly presented at higher wall superheat (Tw Tsat > 15 °C) both for R134a and R407C. It took greater heat flux to reach the same wall superheat with the increase of mass flux. The variation of mass flux had no significant effect on the boiling curve at lower wall superheat (Tw Tsat < 15 °C). The reason of this occurrence is
Tsub=1~2
R134a, G=273.2 kg/m2 s R134a, G=472.2 kg/m2 s R407C,G=266.7 kg/m2 s R407C,G=461.8 kg/m2 s
100
2
4000
150
0
10
20
30
40
50
60
70
80
Tw-Tsat ( Fig. 13. Boiling curves of R134a and R407C with various mass flux at Tsat = 21 °C.
that the dry out flow will firstly appear in the case of lower mass flux under the same heat flux. The increasing rate of wall superheat for R407C is a little larger than that of R134a at lower wall superheat (Tw Tsat < 15 °C), while the situation completely reverses at higher wall superheat (Tw Tsat > 15 °C). It shows that R407C presents a higher CHF than R134a. This can be explained by the concentration effect caused by temperature glide in the phase change of R407C due to the different boiling point for each component, which is involved in the Ma number through the impact on physical properties proposed by Fujita and Bai [38]:
Ma ¼
Dr
r
ql m2l gðql qv Þ
Pr
ð8Þ
where r is the surface tension of the fluid; Dr represents the difference of the fluid surface tension between the bubble point and dew point; ql, qv are liquid density and vapor density, respectively; and ml and Pr represent the liquid kinematic viscosity and Prandtl number, respectively. Moreover, Marangoni effect helps the region with a breakup persist to a higher heat flux [3] and plays an important role on the boiling heat transfer of zeotropic mixtures [39].
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4.3. Correlation with Marangoni number
Table 4 The statistical assessment of the six examined micro-scale correlations for R134a.
As shown above, several differences were observed on the heat transfer coefficient between the zeotropic mixture R407C and pure refrigerant R134a, especially in the slug to annular-dry out stage (x 0.2–1). A number of mini/micro-scale heat transfer correlations were widely discussed in literatures for pure fluid such as refrigerants and water, and several reviews have even been published for the evaluation of existing correlations for pure fluid in macro to micro scales [23,24]. While hardly any micro-scale correlation has been proposed so far for zeotropic mixtures with big temperature glide such as R407C. In fact, accurate heat transfer correlation for the slug to annular-dry out stage of R407C can be developed based on the modified correlation of R134a in the micro-channel, which can be selected from the accessible correlations mentioned above. The experimental data of R134a in the present study were globally compared with the recommended correlations for micro-scale [17,18,20–22,40] as shown in Fig. 14. Obviously, Mikielewicz correlation [18] and Kew and Cornwell correlation [20] gave a limited prediction with larger MAE defined by Eq. (9) and most of the data located out of the ±25% error bands. The two correlations excessively relied on the term of (1 x) were not appropriate to predict the heat transfer coefficient for convective boiling heat transfer in micro-channel. Better statistical assessments were presented on other four correlations with relatively higher MAE and b (percentage of data within ±25%) values as shown in Table 4.
b
Saitoh et al. [17] Mikielewicz [18] Kew and Cornwell [20] Fang [21] Li and Wu [22] Mohamed and Karayiannis [40]
18.4 61.7 40.4 21.0 15.7 23.2
87.5 8.3 29.2 62.5 83.3 70.8
higher vapor quality compared with the experimental situation. Therefore, only Li and Wu [22] correlation examined in this paper behaved an acceptable agreement between the experimental and predicted trends within the whole considered range of vapor quality. But some corrections were needed since the predicted values were integrally higher than the experimental values resulted in a slightly lower b value. Thusly, the selected statistical correlation of Li and Wu was modified for R134a in a single micro-channel as follows: 0:36 0:4
htp;R134a ¼ 260Bo0:275 ðBdRel
Þ
0:45 kl q D q0
ð11Þ
16000 14000
y ¼ a þ be
N 1X jhpre hexp j 100 N i¼1 hexp
ð9Þ
The number of data within 25% 100% The number of data
ð10Þ
In order to verify the consistency of experimental trend and performance of the other four correlations, the local assessment was performed as shown in Fig. 15. The correlations of Fang [21] and Mahmoud and Karayiannis [40] presented similar performance that the heat transfer coefficient decreased quickly with relatively higher vapor quality (x > 0.4), both of which were failed to capture the experimental trend. Despite the applicable prediction at lower vapor quality using Saitoh et al. [17] correlation, an opposite trend occurred for predicting the heat transfer coefficient at
20000 R134a
18000
htp,exp (W/m2 K)
MAE
Based on the analysis of experimental results for R134a, the heat transfer coefficient is greatly influenced by the heat flux. Meanwhile the decreasing mass flux corresponding to the increasing Bo number does not lead to the decrease of heat transfer coefficient, and which slightly increases in the first and then keeps a relatively stable value. So the revised exponent of Bo number was recommended to enlarge the effect of Bo number in Eq. (11). Posteriorly, the predicted values of Li and Wu correlation were high at lower heat flux but low at higher heat flux, which is most probably caused by the non-linear relationship between the heat flux and wall superheat. The brief term of (q/q0), where q0 = 68600 W/m2 for R134a in a single micro-channel, was put forward to balance the error between experimental and predicted values. In order to develop a new correlation for zeotropic mixture R407C, the experimental data of heat transfer coefficient for R407C and R404A were compared with the predicted values using the modified correlation for R134a. The database of quasiazeotropic mixture R404A with a smaller temperature glide were collected to assist the development of new correlation and extended its application range. The comparison results present a positive deviation at low vapor quality but negative deviation at higher vapor quality, as shown in Fig. 16. The deviation versus vapor quality for both of R407C and R404A presents the exponential decay trend, which can be fitly described as the following form:
MAE ¼
b¼
Correlation
+25%
xc
12000
-25%
10000 8000 6000 4000 2000 0
0
2000 4000 6000 8000 100001200014000160001800020000 htp,pre (W/m2 K) Saitoh et al. [17] Mikielewicz [18] Kew and Cornwell [20]
Fang [21] Li and Wu [22] Mohamed and Karayiannis [40]
Fig. 14. The comparison with six examined micro-scale correlations for R134a.
ð12Þ
But the attenuation amplitude and minimum value of the deviation were different for R407C and R404A. That is mainly because the different temperature glides result in different concentration effects, which can be appropriately expressed using Ma number proposed by Fujita and Bai [38] shown in Eq. (8). Hence, considering the Marangoni effect, the correction factor for mixture was befittingly fitted as a function of Ma number and vapor quality, as shown in Fig. 17. The correction factor described the extent of enhancement or attenuation for the flow boiling heat transfer coefficient of mixtures compared with the predicted values of pure refrigerant based on the modified correlation shown in Eq. (11). The effect of correction factor for mixture decreases with the decrease of Ma number since the closer to the pure fluid, the smaller the temperature glide. The correction factor for mixture is described as the following equation:
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18000 16000
1.4 Experimental data Saitoh et al. [17] Fang [21] Li and Wu [22] Mohamed and Karayiannis [40]
R134a q=35.4 kW/m2
14000
1.0 hexp,mix/htp,R134a (-)
htp (W/m2 K)
12000
1.2
10000 8000 6000 4000
±25% Error band
0.8 0.6 0.4
Experimental data R407C-Ma=2.79e-4 R404A-Ma=2.78e-5
2000
0.2
0 0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0.0 0.1
x (-)
0.2
0.3
0.4
(a) 20000 18000 16000
htp (W/m2 K)
14000
0.7
0.8
0.9
1.0
Fig. 16. The deviation of the experimental heat transfer coefficient for R407C and R404A with respect to the predicted values based on the modified correlation for R134a.
1.4
12000
1.2
10000 8000
1.0
6000
Fmix (-)
±25% Error band
4000 2000 0 0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0.8 0.6 0.4
x (-)
0.2
(b) 26000 R134a 24000 q=142.6kW/m2 22000 20000 18000 16000 14000 12000 10000 8000 6000 4000 2000 0 0.1 0.2 0.3
0.0 0.1
Experimental data Saitoh et al. [17] Fang [21] Li and Wu [22] Mohamed and Karayiannis [40]
18000 16000
0.4
0.5
0.6
0.7
0.8
0.9
1.0
4 0:36 x 0:2
þ 0:6ðMa 10 Þ
e
ð13Þ
Then, the new correlation for mixture is proposed as follows:
htp;mix ¼ 260F mix Bo
0.4
0.5
0.6
0.7
0.8
0.9
1.0
20000
Fig. 15. The comparison with the recommended micro-scale correlations at various heat flux for R134a at Tsat = 21 °C.
0:275
0.3
±25% Error band
(c)
F mix ¼ 0:25Ma
0.2
Fig. 17. The correction factor for mixture as a function of Ma number and vapor quality.
x (-)
0:123
Prediction curve R407C-Ma=2.79e-4 R404A-Ma=2.78e-5
x (-)
0:36 0:4 ðBdRel Þ
0:45 kl q D q0
ð14Þ
htp,mix,exp (W/m2 K)
htp (W/m2 K)
0.6
x (-)
Experimental data Saitoh et al. [17] Fang [21] Li and Wu [22] Mohamed and Karayiannis [40]
R134a q=68.6kW/m2
0.5
New correlation for mixture MAE= 6.1% R407C R404A
+15% -15%
14000 12000 10000 8000 6000 4000 2000 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000 htp,mix,pre (W/m2 K)
Fig. 18. The global comparison with new correlation for R407C and R404A.
The suggested correlation for mixture is validated for both of R407C and R404A with different database at various test conditions within the range of vapor quality from 0.15 to 1, where the flow
C. Dang et al. / International Journal of Heat and Mass Transfer 104 (2017) 351–361
patterns are mainly concentrated in slug to annular dry out flow and the heat transfer mechanism is dominated by convective boiling. Fig. 18 depicts the global comparison between the suggested correlation and the current exponential data of R407C and R404A. The correlation predicts 95.6% of all data within the ±15% error bands at a MAE value of 6.1%. Therefore, the proposed correlation presents good prediction accuracy for R407C and R404A. Meanwhile, the proposed correlation in the present paper may provide some guidance for the research on zeotropic mixtures and be prospectively verified. 5. Conclusion The flow boiling characteristics of pure refrigerant R134a and zeotropic mixture R407C in a single rectangular micro-channel were experimentally investigated in this study. The flow types were identified and the flow pattern maps were plotted based on the visualization results. The heat transfer coefficient and the boiling curve with the effects of mass flux, heat flux and vapor quality were analyzed and compared for both of the two refrigerants. The predictive method for mixtures was also discussed. The main results can be summarized as follows: (1) Seven flow types are identified. The coexistence of some bubbles and the confined bubble flow to annular flow are observed for R407C, which also presents a characteristic of advance into churn-annular flow stage compared to R134a. (2) In the range of vapor quality corresponding to churnannular to annular flow stage, the heat transfer coefficient of R134a maintains a relatively stable and high value with the increasing vapor quality, but the decreasing tendency is presented for R407C as well as the lower heat transfer coefficient in the similar circumstance. (3) R407C shows a higher CHF than R134a. The effect of mass flux on the boiling curve is mainly presented at higher wall superheat, and the larger the mass flux, the higher the CHF. (4) A new correlation is developed for predicting the flow boiling heat transfer coefficient of zeotropic mixtures in view of Marangoni effect. With a valid range of vapor quality (x = 0.15–1), the correlation predicts 95.6% of all experimental data within the ±15% error bands at a MAE value of 6.1%. Acknowledgement This research was supported by Natural Science Foundation of China (No. 51376019). References [1] T.N. Tran, M.W. Wambsganss, D.M. France, Small circular-and rectangular channel boiling with two refrigerants, Int. J. Multiphase Flow 22 (1996) 485– 498. [2] A. Greco, Convective boiling of pure and mixed refrigerants: an experimental study of the major parameters affecting heat transfer, Int. J. Heat Mass Transfer 51 (2008) 896–909. [3] P.H. Lin, B.R. Fu, C. Pan, Critical heat flux on flow boiling of methanol-water mixtures in a diverging microchannel with artificial cavities, Int. J. Heat Mass Transfer 54 (2011) 3156–3166. [4] A. Kundu, R. Kumar, A. Gupta, Heat transfer characteristics and flow pattern during two-phase flow boiling of R134a and R407C in a horizontal smooth tube, Exp. Therm. Fluid Sci. 57 (2014) 344–352. [5] A. Kundu, R. Kumar, A. Gupta, Comparative experimental study on flow boiling heat transfer characteristics of pure and mixed refrigerants, Int. J. Refrig. 45 (2014) 136–147. [6] L. Wojtan, T. Ursenbacher, J.R. Thome, Investigation of flow boiling in horizontal tubes: Part II—development of a new heat transfer model for stratified-wavy, dryout and mist flow regimes, Int. J. Heat Mass Transfer 48 (2005) 2970–2985. [7] P. Rollmann, K. Spindler, New models for heat transfer and pressure drop during flow boiling of R407C and R410A in a horizontal microfin tube, Int. J. Therm. Sci. 103 (2016) 57–66.
361
[8] H.L.S.L. Leão, F.J.D. Nascimento, G. Ribatski, Flow boiling heat transfer of R407C in a microchannels based heat spreader, Exp. Therm. Fluid Sci. 59 (2014) 140– 151. [9] C. Aprea, A. Greco, A. Rosato, Comparison of R407C and R417A heat transfer coefficients and pressure drops during flow boiling in a horizontal smooth tube, Energy Convers. Manage. 49 (2008) 1629–1636. [10] X. Zhang, J. Zhang, H. Ji, D. Zhao, Heat transfer enhancement and pressure drop performance for R417A flow boiling in internally grooved tubes, Energy 86 (2015) 446–454. [11] C. Guo, J. Wang, X. Du, L. Yang, Experimental flow boiling characteristics of R134a/R245fa mixture inside smooth horizontal tube, Appl. Therm. Eng. 103 (2016) 901–908. [12] M. Li, C. Dang, E. Hihara, Flow boiling heat transfer of HFO1234yf and HFC32 refrigerant mixtures in a smooth horizontal tube: Part II. Prediction method, Int. J. Heat Mass Transfer 64 (2013) 591–608. [13] M.M. Shah, A method for predicting heat transfer during boiling of mixtures in plain tubes, Appl. Therm. Eng. 89 (2015) 812–821. [14] J.R. Thome, S. Shakir, A new correlation for nucleate boiling of aqueous mixtures, AIChE Symp. Ser. 83 (257) (1987) 46–57. [15] K.J. Bell, A.M. Ghaly, An approximate generalized method for multicomponent partial condenser, AIChE Symp. Ser. 69 (1973) 72–79. [16] G.B. Abadi, E. Yun, K.C. Kim, Flow boiling characteristics of R134a and R245fa mixtures in a vertical circular tube, Exp. Therm. Fluid Sci. 72 (2016) 112–124. [17] S. Saitoh, H. Daiguji, E. Hihara, Correlation for boiling heat transfer of R-134a in horizontal tubes including effect of tube diameter, Int. J. Heat Mass Transfer 50 (2007) 5215–5225. [18] D. Mikielewicz, A new method for determination of flow boiling heat transfer coefficient in conventional-diameter channels and minichannels, Heat Transfer Eng. 31 (4) (2010) 276–287. [19] H. Müller-Steinhagen, K. Heck, A simple friction pressure drop correlation for two-phase flow in pipes, Chem. Eng. Process. 20 (6) (1986) 297–308. [20] P.A. Kew, K. Cornwell, Correlations for the prediction of boiling heat transfer in small diameter channels, Appl. Therm. Eng. 17 (8–10) (1997) 705–715. [21] X. Fang, A new correlation of flow boiling heat transfer coefficients based on R134a data, Int. J. Heat Mass Transfer 66 (2013) 279–283. [22] W. Li, Z. Wu, A general correlation for evaporative heat transfer in micro/minichannels, Int. J. Heat Mass Transfer 53 (2010) 1778–1787. [23] M.M. Mahmoud, T.G. Karayiannis, Heat transfer correlation for flow boiling in small to micro tubes, Int. J. Heat Mass Transfer 66 (2013) 553–574. [24] Y. Xu, X. Fang, G. Li, D. Li, Y. Yuan, An experimental study of flow boiling heat transfer of R134a and evaluation of existing correlations, Int. J. Heat Mass Transfer 92 (2016) 1143–1157. [25] T.W. Lim, J.U. Park, Prediction of the transition criterion to annular flow in twophase flow pattern of refrigerants, JSME Int. J. Ser. B 46 (2003) 618–624. [26] R. Balakrishnan, L.M. Dhasan, S. Rajagopal, Flow boiling heat transfer coefficient of R-134a/R-290/R-600a mixture in smooth horizontal tubes using varied heat flux method, Appl. Therm. Eng. 29 (2009) 1778–1785. [27] B. Raja, D.M. Lal, R. Saravanan, Stratified flow boiling heat transfer study of a HFC/HC refrigerant mixture in smooth horizontal tubes, Heat Mass Transfer 46 (2010) 323–331. [28] B.R. Fu, P.H. Lin, M.S. Tsou, C. Pan, Flow pattern maps and transition criteria for flow boiling of binary mixtures in a diverging microchannel, Int. J. Heat Mass Transfer 55 (2012) 1754–1763. [29] R.J. Moffat, Describing the uncertainties in experimental results, Exp. Therm. Fluid Sci. 1 (1988) 3–17. [30] L. Yin, L. Jia, P. Guan, D. Liu, Experimental investigation on bubble confinement and elongation in microchannel flow boiling, Exp. Therm. Fluid Sci. 54 (2014) 290–296. [31] G. Hetsroni, A. Mosyak, E. Pogrebnyak, Z. Segal, Explosive boiling of water in parallel micro-channels, Int. J. Multiphase Flow 31 (2005) 371–392. [32] D. Bogojevic, K. Sefiane, G. Duursma, A.J. Walton, Bubble dynamics and flow boiling instabilities in microchannels, Int. J. Heat Mass Transfer 58 (2013) 663– 675. [33] F. Dittus, L. Boelter, Heat transfer in automobile radiators of the tubular type, Univ. Calif. Publ. Eng. 2 (1930) 443–461. [34] V. Gnielinski, New equations for heat and mass transfer in turbulent pipe and channel flows, Int. Chem. Eng. (1976) 403–406. [35] L. Yin, L. Jia, P. Guan, F. Liu, An experimental investigation on the confined and elongated bubbles in subcooled flow boiling in a single microchannel, J. Therm. Sci. 21 (6) (2012) 549–556. [36] S. Saisorn, J. Kaew, Flow pattern and heat transfer characteristics of R-134a refrigerant during flow boiling in a horizontal circular mini-channel, Int. J. Heat Mass Transfer 53 (2010) 4023–4038. [37] C. Keepaiboon, S. Wongwises, Two-phase flow patterns and heat transfer characteristics of R134a refrigerant during flow boiling in a single rectangular micro-channel, Exp. Therm. Fluid Sci. 66 (2015) 36–45. [38] Y. Fujita, Q. Bai, Critical heat flux of binary mixtures in pool boiling and its correlation in terms of Marangoni number, Int. J. Refrig. 20 (8) (1997) 616– 622. [39] W.R. Mcgillis, V.P. Carey, W.R. Mcgillis, V.P. Carey, On the role of Marangoni effects on the critical heat flux for pool boiling of binary mixtures, J. Heat Transfer-Trans. ASME 118 (1996) 103–109. [40] M.M. Mahmoud, T.G. Karayiannis, A statistical correlation for flow boiling heat transfer in micro tubes, in: Proceedings of the Third European Conference on Microfluidics–Microfluidics, Springer, Heidelberg, 2012, pp. 3–5.