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Influence of the flashing phenomenon on the boiling curve of refrigerant R134a in minichannels
International Journal of Heat and Mass Transfer 53 (2010) 1036–1043
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International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt
Influence of the flashing phenomenon on the boiling curve of refrigerant R134a in minichannels Krzysztof Dutkowski * Koszalin University of Technology, Division of Heat and Refrigeration Engineering, ul. Racławicka 15-17, 75-620 Koszalin, Poland
a r t i c l e
i n f o
Article history: Received 23 September 2009 Accepted 27 October 2009 Available online 27 November 2009 Keywords: Minichannel Experimental investigations Boiling curve Flashing
a b s t r a c t The results of experimental investigations of heat transfer during the flow of R134a in a minichannel are presented here. The experimental investigations were conducted using a minichannel with a total length of 500 mm and 1.68 mm internal diameter. The heated length of the minichannel was 200 mm, the total mass flow rate of the refrigerant (wq) = 200–450 kg/m2 s, the inlet subcooling DTs = 5–15 K, and the heat flux density q = 1.7–60.3 kW/m2. The results of experimental investigations are presented as a boiling curve. The phenomenon known as the zero boiling crisis and the influence of the flashing phenomenon on the boiling curve show the importance of these elements on heat transfer in single- and two-phase systems. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction At present, there is a widespread interest in compact heat exchangers. The small sizes of these elements results in a very small internal cross-section of a channel. In accordance with the classification included in the paper by Kandlikar [9], these channels can be called ‘‘minichannels” (3 mm > dh > 200 lm) or ‘‘microchannels” (200 lm > dh > 10 lm). The use of small-sized channels has the following advantages: the small dimensions of the device, a high effectiveness, a small volume of working medium, and low financial and environmental costs. There are numerous papers dedicated to the general aspects of heat transfer and pressure drop, especially with regard to twophase flow in minichannels. However, it is necessary to focus on details, such as the start of the boiling process, the critical heat flux, the flashing, the boiling curve, and the phenomena involved in the zero boiling crisis (boiling hysteresis). The classical boiling curve (Fig. 1) is a curve that represents the influence of the heat flux q on the difference of temperatures DT = Tw–Tf, where Tw is the temperature of the heated wall and Tf is the temperature of the fluid. The temperature of the fluid is equal to the temperature of saturation Ts while the process of boiling lasts. When q = 0, the temperature of the wall is equal to the temperature of the fluid (DT = 0). The increase of q results in an increase of DT. This means that the temperature of the wall is greater than the temperature of the liquid. The process can clearly be described by a straight line and is presented in Fig. 1 as section 0-B. * Tel.: +48 94 3478228; fax: +48 94 3426753. E-mail address: [email protected]. 0017-9310/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijheatmasstransfer.2009.11.007
At point B, the temperature of the wall is much higher than the temperature of the liquid. In addition, the temperature of the wall is higher than the saturation temperature, where this is a metastable condition. At a certain point, the boiling process will occur. Vapor bubbles constitute heat sinks, which result in a reduction of the temperature of the wall. Therefore, with a constant value of q, a sudden drop of DT is observed (this is represented as the line from point B to point C). This phenomenon is known as a zero boiling crisis, TOS – temperature overshoot or nucleation hysteresis (Poniewski and Thome [18]). A further increase of q results in an increase of the number of bubble sites. The difference of temperature DT is slowly increasing. This continues until the critical heat flux (CHF), at which a burn-out can occur. During the reduction of the heat flux, the process does not progress in the same manner. The DT values during the reduction of the heat flux are different from those when the heat flux was being increased. The boiling process lasts until point A. The above-mentioned processes presented in Fig. 1 are known as the boiling hysteresis. The flashing phenomenon in minichannels occurs when the pressure at some cross-section of the minichannel drops below the vapor pressure of the fluid being passed, and the whole volume of the liquid in this section becomes superheated. The unique characteristic of the flashing processes is a withdrawal of the latent heat of evaporation from the internal energy of the superheated liquid (Hahne and Barthau [5]). In that cross-section, vapor is suddenly generated in an explosive manner (Eliasa and Chambre, [4]). When flashing occurs, bubbles are formed in the flow stream. These bubbles carry large amounts of energy. The present paper is focused on the experimental investigation of the influence of the flashing phenomena on the boiling curve.
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Nomenclature d dh L p q Tf Tin
Ts Tw Dp DT DTs (wq)
diameter (m) hydraulic diameter (m) length (m) pressure (Pa) heat flux (W m2) local fluid temperature (K) inlet temperature (K)
D
q
C
q kr
B
A
ΔT= T w – T f
O
Fig. 1. Part of classical boiling curve.
The R134a refrigerant was used as the working fluid because of its widespread application in refrigeration and air conditioning systems. The refrigerant flowed through a stainless steel pipe with an internal diameter of 1.68 mm and total length of 500 mm. Ten thermocouple sensors were set on a 200 mm electrically heated section in even intervals (18.2 mm). Different characteristics were prepared for each location. This allowed for the determination of the place where flashing started and its influence on the boiling curve. 2. Literature review Lists of results in the form of boiling curves in minichannels occur ever more frequently in publications. This is interesting that some authors did not observe a zero boiling crisis in minichannels and the ONB moment on the boiling curve was determined based on a boiling curve trend change. This is also interesting that sometimes in minichannels before incipience of the boiling process the flashing phenomenon is observed. Hsieh et al. [8] conducted experimental investigations of the R-407C boiling process inside an annular gap. The hydraulic diameters of the channel were 4 and 2 mm. The experiments used mass flow rates (wq) = 300–600 kg/m2 s, a heat flux of up to q = 45 kW/m2, and a system pressure of p = 776 kPa or p = 899 kPa. The authors did not note any zero boiling crisis phenomena in any of the examined cases, while the ONB (Onset Nucleate Boiling) moment was represented by a change in trend of the boiling curve. Lee and Garimella [12] tested the boiling process in parallel minichannels with a depth of 400 lm and a width from 102 to 997 lm. Using deionized water that was heated in various configurations of minichannels with a heat flux of q = 10–340 W/cm2, a boiling process was produced. The authors did not observe a zero boiling crisis, and the ONB moment on the boiling curve was also determined based on a boiling curve trend change.
local saturation temperature (K) local temperature of the wall (K) pressure drop (Pa) difference of temperatures, DT = Tw–Tf (K) inlet subcooling, DTs = Ts–Tin (K) mass flux (kg m2 s1)
There are additional papers dedicated to minichannels where the authors did not observe the zero boiling crisis phenomena. The following papers need to be mentioned in this regard: Bergles and Kandlikar [1], Sumith et al. [23], Ohta [16], Owhaib et al. [17], Bertsch et al. [2], Lee and Mudawar [10], Prodanovic et al. [19]. On the other hand, there are papers that prove that the zero boiling crisis in minichannels is equally as common as in conventional channels. Experimental investigations of heat sinks with different geometries of parallel minichannels conducted by the team of Harirchian and Garimella [6] with the use of a dielectric fluid, Fluorinert FC-77, confirmed the existence of the zero boiling crisis phenomenon. This phenomenon occurred for every mass flow rate, and also for every heat sink geometry used. The largest temperature drop of 8 K was observed for the smallest flow rate (250 kg/ m2 s). Hetsroni et al. [7] observed the zero boiling phenomenon in an annular gap filled with water and surfactants with various concentrations. The measured temperature of the wall drop was approximately 18 K. It is interesting to note that this process was observed only when the gap was filled with a degraded surfactant solution. Martin-Callizo et al. [14,15] conducted research with subcooled R-134a refrigerant in vertical pipes with internal diameters of 0.83 mm, 1.22 mm and 1.70 mm. The authors investigated the influence of the heat flux, q = 1–26 kW/m2, mass flux, (wq) = 300–700 kg/m2 s, inlet subcooling, DTs = 5–15 K, system pressure, p = 7.70–0.17 bar, and channel diameter, on the subcooled boiling heat transfer. The authors demonstrate the existence of a boiling hysteresis, and the temperature drop at the boiling moment reached 18 K. Lie and Lin [13] investigated a subcooled flow boiling heat transfer of R-134a in a narrow (1.0 and 2.0 mm) annular duct. For each investigated mass flow rate (wq) = 200–300 kg/m2 s and heat flux q = 0–55 kW/m2, a drop in the temperature of the wall in ONB was observed. The greatest drop of 18 K occurred when (wq) = 200 kg/m2 s, gap = 1 mm, Tsat = 15 °C and DTs = 13 °C. Perhaps the most spectacular temperature drop was observed by Yen et al. [25]. HCFC123 and FC72 were used as working fluids. The investigations, which were carried out by the authors in 0.19, 0.3, 0.51 mm internal diameter pipes, allowed them to obtain a 70 K overheating of the channel wall before the boiling process started. This result corresponds to the following experimental conditions: HCFC123, d = 0.19 mm, q = 2.4 kW/m2, (wq) = 145 kg/m2 s. Numerous papers provide information about the possibility of the occurrence of the flashing phenomenon (Bergles and Kandlikar [1], Lee and Mudawar [11], Revellin et al. [21], Schneider et al. [22]). However, there are no publications that discuss this issue more extensively. One of the few papers is a paper by Warrier et al. [24]. The authors assume that a two-phase flow starts when a rapid increase of the heat transfer coefficient occurs, and the local vapor quality should also take the flashing phenomenon into account.
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In addition, during the experimental investigations of the liquid nitrogen boiling process conducted by Qi et al. [20] in minipipes with internal diameters of 0.531, 0.834, 1.042 and 1.931 mm, the existence of the zero boiling crisis phenomenon was noted. The investigations were carried out mass flow rates between (wq) = 440–3000 kg/m2 s and heat fluxes between q = 5.09–21.39 W/cm2. It was noted that a drop in the temperature of the wall at the ONB moment reached 5.0 K. The authors also analyzed the influence of the flashing phenomenon on the local vapor quality. The increase of the vapor quality was 18% when this phenomenon was taken into account. It is characteristic that, in each of the papers mentioned above, the authors speak about two zones: the single-phase flow zone and the two-phase flow zone. There are two cases: – the absence of the zero boiling crisis phenomenon – in this case, the single-phase flow zone on the boiling curve finishes when the trend of the curve changes; – the zero boiling crisis phenomenon occurs – the single-phase flow occurs until the moment at which a rapid temperature drop occurs. This moment is considered to be the start of the boiling process and incipience of a two-phase structure. This notion is also reported in the papers where the flashing phenomenon was observed. Therefore we should ask. Before incipience of the boiling process when the flashing phenomenon is observed have we singlephase flow or two-phase flow? The aim of this paper is to present experimental boiling curves. The influence of the flashing phenomenon on their course constitutes an important aspect of the paper. Another issue is to show that the area before the incipience of boiling, considered in all of the papers as a single-phase flow area, can be a two-phase flow area (indicating the presence of flashing).
3. Experimental apparatus and procedure 3.1. The test loop A schematic of the test loop is presented in Fig. 2. The liquid of the R134a refrigerant pumped from tank 12 by pump 2 flowed through filter 3 and precooler 4. Next, the Coriolis mass-flowmeter _ ¼ 0—20kg=h, accuracy of 0.075% of measured 5 was installed (m value). A liquid with a known flow rate was fed to heat exchangers 6, 7 and 8. We were able to use these exchangers alternatively, which allowed one to achieve the proper refrigerant conditions on the test section inlet 1. The parameters of the refrigerant (pressure and temperature) were measured at the heat exchangers inlet (p1, T1) and outlet (p2, T2). The designed set of heat exchangers enabled experimental tests of the heat transfer and pressure drop during a single-phase flow of the refrigerant, subcooled flow boiling or saturated flow boiling of refrigerant. The refrigerant (with suitable outflow parameters) was directed to the test section. The following parameters were measured: the pressure in the initial cross section of the tested part of the minichannel pin, the pressure drop Dp on the tested part of the minichannel, the profile of the minichannel wall temperature T1T10, temperature on the inlet Tin and outlet Tout of the tested part, and the supplied electric power Pel. All of the voltage signals of the measured values were connected with data acquisition system 18, which cooperates with computer 17. 3.2. The test section The test section was made from a stainless steel pipe 1 with a total length of 500 mm and an internal diameter of 1.68 mm. The surface roughness was obtained using scanning electron microscope (SEM). The average roughness of inner surface of minichannel is 0.65 lm from five measurements. The pipe was divided into
Fig. 2. Experimental set-up. 1 – minichannel, 2 – pump (with instrumentation), 3 – filter, 4 – precooler, 5 – flowmeter, 6 – preheater, 7 – cooler no. 1 (cooling with water), 8 – cooler no. 2 (cooling with refrigerant), 9 – additional cooling system, 10 – valves, 11 – condenser (cooled with water), 12 – tank of refrigerant, 13 – spare tank of refrigerant, 14 – system of electric heating of measuring section, 15 – pressure sensors, 16 – differential pressure sensor, 17 – computer, 18 – data acquisition system.
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several sections. The first one, which was 150 mm long, constituted a hydraulic stabilization area. The second one (300 mm) was the proper test part of minichannel, and the third one, which was 50 mm long, was the outlet section. The proper test section was electrically heated over a length of 200 mm. Using a high voltage adjustable transformer, a high voltage shunt of 150 A/60 mV and voltmeters, it was possible to change the heat flux and to measure the parameters required for its determination. On the measuring section, 10K type thermocouples and a wire with a diameter of / = 0.2 mm were installed. Using these sensors, the temperature of the external surface of the minichannel wall was measured. Right before and after the heated area, the same two thermocouples were additionally fixed to the pipe surface in order to measure the refrigerant’s temperature on the inlet and the outlet of the test section. Before the sensors were installed, their individual experimental thermo-electric characteristics were prepared in relation to the precise thermometer with an accuracy of 0.1 °C. Holes were made in the pipe 50 mm before and after the heated section. These made it possible to measure the working fluid pressure and pressure drop on the minichannel’s tested part. For the measurement of the pressure, a piezoresistant sensor (manufactured by Endress + Hauser) with a transducer was used (Deltabar S PMD75). The measuring range of this device was 0–4 MPa, and its accuracy was 0.075. The measurement of the pressure drop was conducted with a differential pressure transducer (Deltabar S PMD75) with a basic measuring range of 0–1.6 MPa and an accuracy of 0.075. The uncertainty of the pressure and the pressure drop itself were ±3 kPa and ±1.2 kPa, respectively. The whole test section was insulated in order to create nearly adiabatic conditions. The total heat losses to the environment were assessed to be smaller than 5% of the supplied electric power. 3.3. The test procedure All of the data was collected once steady state conditions were achieved. The required mass flow rate was set first. The refrigerant, with a pre-defined flow rate, was directed to one of three heat exchangers in the preparation section. The selection of the exchanger depended of the expected value of the refrigerant’s temperature at the minichannel inlet. The value of the refrigerant’s saturation temperature depended on the working pressure of the refrigerant in the minichannel. This value could be changed with a valve installed behind the test section. When the mass flow rate, pressure, pressure drop and the refrigerant’s temperature were constant, data registration began. The averaged values from several dozen measurements were the basis for further calculations. It should be mentioned that oscillations of the every abovementioned parameters were registered in some cases. After these measurements, the registration was conducted under quasi-steady state conditions. The experiments were conducted using R134a refrigerant under the following range of parameters: d = 1.68 mm, (wq) = 260– 2144 kg/m2 s, q = 1.7–60.3 kW/m2, Tin = 13.1–23.5 °C. 3.4. Data reduction The heat transfer rate to the refrigerant in the minichannel is obtained based on the net power input and the total internal surface area of the minichannel. The total power input is calculated by subtracting the heat loss due to electrical power
q¼
qel qloss : A
ð1Þ
It is accepted that the temperature of the wall on the internal side of the minichannel is equal to the temperature of the minichannel surface measured directly on the external side. It is assumed that the liquid temperature changes linearly along the length of the channel. In the single-phase flow, it is calculated from the heat balance while, in the two-phase flow region, it corresponds to the saturation temperature as it is a result of the local pressure inside the minichannel:
( Tf ¼
pdL — single-phase region T in þ q mc _ p
T s ¼ f ðps Þ — two-phase region
:
ð2Þ
The following was assumed in formula (2): Tin is the temperature of the liquid measured at the inlet to the heated minichannel section, L is the distance measured from the beginning of the heated section, _ is the refrigerant’s flow (kg/s), cp is the specific heat of the liquid, m Ts is the local saturation temperature which is dependent on saturation pressure – ps. 4. Results and discussion 4.1. Boiling curves without the flashing phenomenon Fig. 3 presents boiling characteristics of refrigerant R134a that were determined for the mass flow rate (wq) = 450 kg/(m2 s), with a liquid subcooling DTs = 10 K. The liquid subcooling is the difference of saturation temperature Ts and inflowing liquid Tin (DTs = Ts Tin). Fig. 3a shows the dependence of the local temperature difference DT (DT = Tw Tf) on the heat flux density q. Tw denotes the local temperature of the wall. Because ten thermocouples were placed on the heated part of the minichannel, it was possible to determine the difference in temperature DT = f(L) along ten cross-sections. In the first phase of heating, the difference in temperature DT between each measuring cross-section of length L had the same value. An increase of the heat flux density q resulted in the growth of DT. This corresponds to the heating of a liquid during forced convection, which characterizes section O–B in Fig. 1. For the parameters of the experiment given in Fig. 3, it is observed that a single-phase liquid flow in a minichannel occurs at q = 22.4 kW/ m2 (each thermocouple confirms the trend of the process in compliance with section O–B). A further growth of the heat flux caused an increased difference of the temperature DT while this process is in the meta-stable equilibrium of the system. Any disturbances of the meta-stable equilibrium lead to the initiation of the zero boiling crisis, which was accompanied by a rapid drop of the minichannel wall temperature. For the example presented here, this took place when the heat flux was q = 27 kW/m2. The measurement of the temperature showed that the boiling process occurred initially only on a part of the minichannel length, and covered those cross-sections that were located above 91 mm from the inlet cross-section of the minichannel. It was observed that, in the part of the measuring section where the five first temperature sensors were installed (18.2–91 mm), there was no zero boiling crisis effect. This means that a single-phase liquid flow continued there. An increase of the heat flux to q = 36.6 kW/m2 caused the zero boiling crisis to cover also this part of the minichannel, and was accompanied by a drop of the wall temperature. Only then was almost the whole volume of the pipe minichannel filled with boiling fluid. It should be noted that the temperature recorded by the first sensor, installed at a distance of 18.2 mm from the inlet cross-section relative to the section heated, confirmed a singlephase flow of the R134a liquid in this part of the length of the minichannel. During the zero boiling crisis, the boiling front moved in the direction contrary to the liquid flow in the pipe minichannel. This type of boiling front movement also occurs in conventional
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Fig. 3. Results of the experimental investigation: R134a, d = 1.68 mm, (wq) = 450 kg/(m2 s), DTs = Ts Tin = 10 K; q = 0–36.6 kW/m2. (a) heat flux q versus temperature difference DT = Tw Tf, (b) local heat transfer coefficient a on the length of the test section, (c) wall temperature Tw on length L of the test section and (d) refrigerant temperature Tf on length L of the test section.
channels (Bohdal [3]). The zero boiling crisis was recorded twice, and the front of nucleation occurred in the range of L = 91–109.2 mm from the inlet cross-section of the minichannel for q = 27.0 kW/m2, and then in the range L = 18.2–36.4 mm for q = 36.6 kW/m2. For the given experimental conditions, this was the maximum value of the heat flux. After this point, the heat flux was decreased. With a decreasing heat flux, the characteristic points of the boiling curve moved and connected, one by one, to the straight line of the single-phase liquid flow. The distribution of the local heat transfer coefficient (for selected values q = const) during a single-phase liquid flow and during boiling is presented in Fig. 3b. A clear increase of the heat transfer coefficient value is observed in the boiling region. The changes of the channel wall temperature during a singlephase liquid flow and after the incipience of boiling (for q = 27 kW/m2 and q = 36.6 kW/m2) are presented in Fig. 3c. Fig. 3d presents the distribution of the refrigerant temperature along the minichannel. A linear growth of the refrigerant temperature occurs in the region where it does not exceed the saturation temperature. The only one exception occurred for L = 182 mm and q = 36.6 kW/ m2. Fig. 4 presents the results of experimental investigations of boiling R134a. The mass flow rate was (wq) = 450 (kg/m2 s), and the subcooling of the liquid was DTs = 15 K. Boiling curves of type q = f(DT) are presented in Fig. 4a. With these parameters, the zero boiling crisis occurred once, when the heat flux value of q = 22.8 kW/m2 was exceeded. For a heat flux of q = 26.1 kW/m2, the state of the refrigerant was in a meta-stable range. After the
initiation of the boiling process, there was a drop of the wall temperature by over 15 K (for L = 182 mm). According to the reading of the first thermocouple (L = 18.2 mm), the states of the refrigerant were on the straight line O–B (from Fig. 1), which signified a single-phase flow of the liquid R134a refrigerant in this part of the minichannel. An increase of the heat flux above q = 35.8 kW/m2 shifted the front boiling location to before the first thermocouple. At a heat flux of q = 42.3 kW/m2, the boiling process covered the whole minichannel length. This was the largest heat flux value in this part of the experiment for this case of experiment. A decrease of the heat flux resulted in a change of the refrigerant’s states according to line D–A (Fig. 1), which was different for each cross-section on the minichannel length. While for a value of q = 14.8 kW/m2 a boiling process occurred in a part of the minichannel (decrease of heat flux), only single-phase heating of the liquid occurred during the increase of the heat flux. Fig. 4b presents a distribution of the local heat transfer coefficient for the subsequent values of the heat flux until the moment of boiling initiation with q = 26.1 kW/m2. Additionally, a dependence of the local heat transfer coefficient for the case when q = 42.3 kW/m2 was presented. At that heat flux, the boiling front moved and also covered the cross-section where the first thermocouple was installed (at a distance of L = 18.2 mm from the beginning of heated zone). The distributions of the wall temperature and the refrigerant temperature along the minichannel length for various levels of heat flux are presented in Fig. 4c and d, respectively.
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Fig. 4. Results of the experimental investigations: R134a, d = 1.68 mm, (wq) = 450 kg/(m2 s), DTs = Ts Tin = 15 K, q = 0 – 42.3 kW/m2. (a) heat flux q versus temperature difference DT = Tw Tf, (b) local heat transfer coefficient a on the length of the test section, (c) wall temperature Tw on a length L of the test section and (d) refrigerant temperature Tf on a length L of the test section.
4.2. Boiling curve with the flashing phenomenon Two further investigated cases are illustrated in Figs. 5 and 6. They show the heating of R134a refrigerant in a minichannel with the same internal diameter as in experiments described above, but under conditions of flashing. The results demonstrated that this phenomenon has an influence not only on the heat exchange during a single-phase liquid flow, but also on flow boiling. Fig. 5 presents the results of experimental investigations under the following conditions: (wq) = 280 kg/(m2 s), DTs = 4 K. During the increase of the heat flux, the refrigerant’s states should occur in the same manner as for the previously discussed cases (in the graphical interpretation, this corresponds to the straight line O– B in Fig. 1). It appears that the points that represent the location of these states on the diagram depart clearly from the straight line O–B. First of all, this results concerns those states of the refrigerant in which the temperature was measured by thermocouples that were placed at the end of the minichannel. For the higher values of the heat flux, the deviation concerned the cross-section situated more and more closely to the inlet cross-section. This can be explained by a drop of the refrigerant temperature on the minichannel length, which causes a growth of the temperature difference DT = Tw Tf. This is clearly visible in Fig. 5d, which presents the distribution of the refrigerant temperature on the minichannel length for various values of the heat flux. This undoubtedly is the result of the occurrence of the flashing phenomenon, which initially (for a heat flux q = 15.7 kW/m2) covered only the two last cross-sections of the minichannel located nearest to the outlet cross-section and moved in the direction opposite
to the refrigerant outlet. For conventional channels, it is considered that, along the straight line O–B (Fig. 1), heat transfer is realized during a single-phase forced convection. In the case when the flashing phenomenon takes place, there is in fact a two-phase structure of refrigerant (however, this is not classical flow boiling). Under these experimental conditions, the value of the heat flux q = 54.4 kW/m2 can be treated as the highest one at which boiling has not yet occurred. Its increase of up to 55.1 kW/m2 resulted in a meta-stable condition. Sufficient overheating of the wall occurred, and a boiling process with its corresponding zero crisis was initiated. New temperatures of the wall were established, as in Fig. 5c. As a result, a drop of the temperature difference DT occurred. In addition, a rapid growth of the intensity of the heat transfer process (Fig. 5b) serves to confirm the occurrence of flow boiling. It is characterized by a growth of the local heat transfer coefficient. The value of this coefficient significantly exceeds the values obtained in forced convection conditions during a singlephase flow and a flow with flashing. The technical potential of the experimental set-up did not allow for tests to be conducted for a heat flux growth above q = 55.1 kW/m2. In addition, tests were conducted where the heat flux density was decreased. A characteristic feature of the boiling process in a flow demonstrating the flashing phenomenon is overlapping states of the refrigerant in each cross-section of the minichannel. In practice, this means that, for each value of heat flux density q that occurred under the conditions of boiling with the flashing phenomenon, the temperature difference DT = Tw Tf had the same value in all cross-sections on the whole length of the pipe minichan-
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60.0
14.0
a
q[kW/m 2]
50.0
10.0
40.0 18,2 [mm] 36,4 [mm] 54,6 [mm] 72,8 [mm] 91,0 [mm] 109,2 [mm] 127,4 [mm] 145,6 [mm] 163,8 [mm] 182,0 [mm]
30.0
20.0
10.0
0.0
b
α [kW/m2K]
12.0
q [kW/m2] 5,2 11,0 15,7 21,3 26,2 33,6 43,4 54,4 55,1 20,4
8.0 6.0 4.0 2.0
Tw -T f [ °C]
0
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25
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0.0
L [mm]
0
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40
60
80
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23.5
c
Tw [ °C]
d
Tf [°C]
23.0
45.0
22.5 40.0
q [kW/m2] 5,2 11,0 15,7 21,3 26,2 33,6 43,4 54,4 55,1 20,4
35.0
30.0
25.0
q [kW/m2 ] 5,2
22.0
11,0 15,7
21.5
21,3 26,2
21.0
33,6 43,4
20.5
54,4 55,1
20.0 L [mm]
20.0 0
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19.5
20,4
L [mm]
0
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Fig. 5. Results of the experimental investigations: R134a, d = 1.68 mm, (wq) = 280 kg/(m2 s), DTs = Ts Tin = 4 K; q = 0–55.1 kW/m2, flashing. (a) heat flux q versus temperature difference DT = Tw Tf, (b) local heat transfer coefficient a on the length of the test section, (c) wall temperature Tw on a length L of the test section and (d) refrigerant temperature Tf on a length L of the test section.
nel. This can be explained by a significant growth of vapor bubbles, which make the flow more homogeneous by making it turbulent. Another case (Fig. 6) concerns the flow of R134a (also with the accompaniment of the flashing phenomenon). This occurred during the flow of the refrigerant in a minichannel that was already at a heat flux of q = 9.4 kW/m2, and covered only the two last cross-sections of the minichannel (situated near the outflow cross-section: Fig. 6d). At this moment a two-phase structure of the flow occurred. A further increase of the heat flux resulted in the growth of the region of flow with flashing. Similarly, as in the case presented in Fig. 5, this resulted in a deviation of the points from a straight line (O–B, Fig. 1). These points illustrate the refrigerant states situated in the flashing region. In this case, the maximum value of the heat flux at which no boiling in the flow had occurred was q = 19.1 kW/m2. The meta-stable states occurred at q = 25.3 kW/m.2. A growth of the wall temperature was noted, and when the refrigerant at the wall reached sufficient overheating, the boiling process commenced together with a rapid drop of the wall temperature. It can be seen from the boiling curve (Fig. 6a) that, in those cross-sections of the minichannel that were situated at the nearest distance of the inlet cross-section (L = 18.2 mm and 36.4 mm), there was a departure from the remaining boiling lines. This proves that boiling had already occurred but that flashing existed in further cross-sections (above 36.4 mm from the inlet cross-section). For q = 35 kW/m2, flashing also covered the cross-section of the channel at a distance of L = 36.4 mm, which was the maximum value of the heat flux density in this experiment.
When the heat flux was decreased, the parameters measured indicated that boiling still existed, and that the flashing phenomenon covered a decreasing volume of the minichannel. Just as when the heat flux was being increased, the readings of subsequent thermocouples placed in area covered with flashing departed from the straight line O–B (Fig. 1). Once the heat flux decreased, in the inversely sequence, the readings of the thermocouples departed from the common trend and indicate the disappearance of the flashing phenomenon. 5. Conclusions
1. The experimental investigations conducted with the R134a refrigerant flowing in a heated minichannel with an internal diameter of 1.68 mm enabled the presentation of boiling curves. A description of the boiling curves, known for conventional channels, proved to be insufficient for a minichannel. 2. The mechanism of the flow boiling process in minichannels is substantially different from the one that has already been well recognized with regard to conventional channels. The large drop of the pressure in the flow has a special impact on this mechanism. 3. The so-called flashing phenomenon, which is usually left out in an analysis of boiling in conventional channels and consists of the presence of a two-phase structure that occurs as a result of the pressure lowering (below the saturation pressure) due to the large flow resistances in the minichannel, plays a significant role and has an impact on the boiling curves.
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7,0
a
q[kW/m 2 ]
35.0
q [kW/m2]
30.0 25.0 20.0 15.0
18,2 [mm] 36,4 [mm]
5,0
54,6 [mm] 72,8 [mm] 91,0 [mm]
4,0
109,2 [mm] 127,4 [mm] 145,6 [mm]
3,0
163,8 [mm] 182,0 [mm]
3,1 4,5 7,1 9,4 12,2 15,1 19,1
2,0
10.0
25,3
1,0
5.0 0.0
b
α [kW/m2K]
6,0
Tw -T f [ °C]
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5
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0,0
L [mm]
0
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c
Tw [ °C]
d
Tf [ °C]
27.0
45.0 q [kW/m2 ]
26.0
3,1
40.0
25.0
4,5
q [kW/m2]
7,1
3,1
24.0
9,4
4,5
12,2
35.0
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21.0 25.0
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L [mm]
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Fig. 6. Results of the experimental investigations: R134a, d = 1.68 mm, (wq) = 525 kg/(m2 s), DTs = Ts Tin = 5 K, q = 0– 5.3 kW/m2, flashing. (a) heat flux versus temperature difference DT = Tw Tf, (b) local heat transfer coefficient a on the length of the test section, (c) wall temperature Tw on length L of the test section and (d) refrigerant temperature Tf on length L of the test section.
4. The paper presents and discusses characteristic cases of boiling curves in a pipe minichannel with the flashing phenomenon taken into account.
References [1] A.E. Bergles, S.G. Kandlikar, Critical heat flux in microchannels: experimental issues and guidelines for measurement, in: First International Conference on Microchannels and Minichannels, New York, 2003. [2] S.S. Bertsch, E.A. Groll, S.V. Garimella, Refrigerant flow boiling heat transfer in parallel microchannels as a function of local vapor quality, Int. J. Heat Mass Transfer 51 (2008) 4775–4787. [3] T. Bohdal, Refrigerants Bubble Boiling Phenomena (in Polish), Monograph, Publication of Koszalin University of Technology, Koszalin, 2001. [4] E. Eliasa, P.L. Chambre, Bubble transport in flashing flow, Int. J. Multiphase Flow 26 (2000) 191–206. [5] E. Hahne, G. Barthau, Evaporation waves in flashing processes, Int. J. Multiphase Flow 26 (2000) 531–547. [6] T. Harirchian, S.V. Garimella, Microchannel size effects on local flow boiling heat transfer to a dielectric fluid, Int. J. Heat Mass Transfer 51 (2008) 3724–3735. [7] G. Hetsroni, A. Mosyak, E. Pogrebnyak, Z. Segal, Natural convection boiling of water and surfactants in narrow horizontal annular channels, Int. J. Multiphase Flow 33 (2007) 469–483. [8] F.C. Hsieh, K.W. Li, Y.M. Lie, C.A. Chen, T.F. Lin, Saturated flow boiling heat transfer of R-407C and associated bubble characteristics in a narrow annular duct, Int. J. Heat Mass Transfer 51 (2008) 3763–3775. [9] S.G. Kandlikar, Microchannels and minichannels – history, terminology, classification and current research needs, in: First International Conference on Microchannels and Minichannels, New York, 2003. [10] J. Lee, I. Mudawar, Fluid flow heat transfer characteristics of low temperature two-phase micro-channel heat sinks – Part 2. Subcooled boiling pressure drop and heat transfer, Int. J. Heat Mass Transfer 51 (2008) 4327–4341. [11] J. Lee, I. Mudawar, Two-phase flow in high-heat-flux micro-channel heat sink for refrigeration cooling applications: Part I – pressure drop characteristics, Int. J. Heat Mass Transfer 48 (2005) 928–940.
[12] P.-S. Lee, S.V. Garimella, Saturated flow boiling heat transfer and pressure drop in silicon microchannel arrays, Int. J. Heat Mass Transfer 51 (2008) 789–806. [13] Y.M. Lie, T.F. Lin, Subcooled flow boiling heat transfer and associated bubble characteristics of R-134a in a narrow annular duct, Int. J. Heat Mass Transfer 49 (2006) 2077–2089. [14] C. Martin-Callizo, W. Owhaib, B. Palm, Subcooled flow boiling of R-134a in a vertical channel of small diameter, in: 6th World Conference on Experimental Heat Transfer, Fluid Mechanics, and Thermodynamics, Matsushima, Miyagi, Japan, 2005. [15] C. Martin-Callizo, B. Palm, W. Owhaib, Subcooled flow boiling of R-134a in vertical channels of small diameter, Int. J. Multiphase Flow 33 (2007) 822–832. [16] H. Ohta, Boiling and two-phase flow in channels with extremely small diameters – review of Japanese research, in: Second International Conference on Microchannels and Minichannels, New York, 2004. [17] W. Owhaib, C. Martin-Callizo, B. Palm, Evaporative heat transfer in vertical circular microchannels, Appl. Therm. Eng. 24 (2004) 1241–1253. [18] M.E. Poniewski, J.R. Thome, Nucleate Boiling on Micro-Structured Surfaces. Heat Transfer Research, Inc. (HTRI), 150 Venture Drive, College Station, TX 77845, USA, 2008. [19] V. Prodanovic, D. Fraser, M. Salcudean, On the transition from partial to fully developed subcooled flow boiling, Int. J. Heat Mass Transfer 45 (2002) 4727– 4738. [20] S.L. Qi, P. Zhang, R.Z. Wang, L.X. Xu, Flow boiling of liquid nitrogen in microtubes: part I – the onset of nucleate boiling two-phase flow instability and two-phase flow pressure drop, Int. J. Heat Mass Transfer 50 (2007) 4999–5016. [21] R. Revellin, V. Dupont, T. Ursenbachr, J.R. Thome, I. Zun, Characterization of diabatic two-phase flows in microchannels: flow parameter results for R-134a in a 0.5 mm channel, Int. J. Multiphase Flow 32 (2006) 755–774. [22] B. Schneider, A. Kosa, Y. Peles, Hydrodynamic cavitation and boiling in refrigerant (R-123) flow inside microchannels, Int. J. Heat Mass Transfer 50 (2007) 2838–2854. [23] B. Sumith, F. Kaminaga, K. Matsumura, Saturated flow boiling of water in a vertical small diameter tube, Exp. Therm. Fluid Sci. 27 (2003) 789–801. [24] G.R. Warrier, V.K. Dhir, L.A. Momoda, Heat transfer and pressure drop in narrow rectangular channels, Exp. Therm. Fluid Sci. 26 (2002) 53–64. [25] T.-H. Yen, N. Kasagi, Y. Suzuki, Forced convective boiling heat transfer in microtubes at low mass and heat fluxes, Int. J. Multiphase Flow 29 (2003) 1771–1792.
Contents lists available at ScienceDirect
International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt
Influence of the flashing phenomenon on the boiling curve of refrigerant R134a in minichannels Krzysztof Dutkowski * Koszalin University of Technology, Division of Heat and Refrigeration Engineering, ul. Racławicka 15-17, 75-620 Koszalin, Poland
a r t i c l e
i n f o
Article history: Received 23 September 2009 Accepted 27 October 2009 Available online 27 November 2009 Keywords: Minichannel Experimental investigations Boiling curve Flashing
a b s t r a c t The results of experimental investigations of heat transfer during the flow of R134a in a minichannel are presented here. The experimental investigations were conducted using a minichannel with a total length of 500 mm and 1.68 mm internal diameter. The heated length of the minichannel was 200 mm, the total mass flow rate of the refrigerant (wq) = 200–450 kg/m2 s, the inlet subcooling DTs = 5–15 K, and the heat flux density q = 1.7–60.3 kW/m2. The results of experimental investigations are presented as a boiling curve. The phenomenon known as the zero boiling crisis and the influence of the flashing phenomenon on the boiling curve show the importance of these elements on heat transfer in single- and two-phase systems. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction At present, there is a widespread interest in compact heat exchangers. The small sizes of these elements results in a very small internal cross-section of a channel. In accordance with the classification included in the paper by Kandlikar [9], these channels can be called ‘‘minichannels” (3 mm > dh > 200 lm) or ‘‘microchannels” (200 lm > dh > 10 lm). The use of small-sized channels has the following advantages: the small dimensions of the device, a high effectiveness, a small volume of working medium, and low financial and environmental costs. There are numerous papers dedicated to the general aspects of heat transfer and pressure drop, especially with regard to twophase flow in minichannels. However, it is necessary to focus on details, such as the start of the boiling process, the critical heat flux, the flashing, the boiling curve, and the phenomena involved in the zero boiling crisis (boiling hysteresis). The classical boiling curve (Fig. 1) is a curve that represents the influence of the heat flux q on the difference of temperatures DT = Tw–Tf, where Tw is the temperature of the heated wall and Tf is the temperature of the fluid. The temperature of the fluid is equal to the temperature of saturation Ts while the process of boiling lasts. When q = 0, the temperature of the wall is equal to the temperature of the fluid (DT = 0). The increase of q results in an increase of DT. This means that the temperature of the wall is greater than the temperature of the liquid. The process can clearly be described by a straight line and is presented in Fig. 1 as section 0-B. * Tel.: +48 94 3478228; fax: +48 94 3426753. E-mail address: [email protected]. 0017-9310/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijheatmasstransfer.2009.11.007
At point B, the temperature of the wall is much higher than the temperature of the liquid. In addition, the temperature of the wall is higher than the saturation temperature, where this is a metastable condition. At a certain point, the boiling process will occur. Vapor bubbles constitute heat sinks, which result in a reduction of the temperature of the wall. Therefore, with a constant value of q, a sudden drop of DT is observed (this is represented as the line from point B to point C). This phenomenon is known as a zero boiling crisis, TOS – temperature overshoot or nucleation hysteresis (Poniewski and Thome [18]). A further increase of q results in an increase of the number of bubble sites. The difference of temperature DT is slowly increasing. This continues until the critical heat flux (CHF), at which a burn-out can occur. During the reduction of the heat flux, the process does not progress in the same manner. The DT values during the reduction of the heat flux are different from those when the heat flux was being increased. The boiling process lasts until point A. The above-mentioned processes presented in Fig. 1 are known as the boiling hysteresis. The flashing phenomenon in minichannels occurs when the pressure at some cross-section of the minichannel drops below the vapor pressure of the fluid being passed, and the whole volume of the liquid in this section becomes superheated. The unique characteristic of the flashing processes is a withdrawal of the latent heat of evaporation from the internal energy of the superheated liquid (Hahne and Barthau [5]). In that cross-section, vapor is suddenly generated in an explosive manner (Eliasa and Chambre, [4]). When flashing occurs, bubbles are formed in the flow stream. These bubbles carry large amounts of energy. The present paper is focused on the experimental investigation of the influence of the flashing phenomena on the boiling curve.
K. Dutkowski / International Journal of Heat and Mass Transfer 53 (2010) 1036–1043
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Nomenclature d dh L p q Tf Tin
Ts Tw Dp DT DTs (wq)
diameter (m) hydraulic diameter (m) length (m) pressure (Pa) heat flux (W m2) local fluid temperature (K) inlet temperature (K)
D
q
C
q kr
B
A
ΔT= T w – T f
O
Fig. 1. Part of classical boiling curve.
The R134a refrigerant was used as the working fluid because of its widespread application in refrigeration and air conditioning systems. The refrigerant flowed through a stainless steel pipe with an internal diameter of 1.68 mm and total length of 500 mm. Ten thermocouple sensors were set on a 200 mm electrically heated section in even intervals (18.2 mm). Different characteristics were prepared for each location. This allowed for the determination of the place where flashing started and its influence on the boiling curve. 2. Literature review Lists of results in the form of boiling curves in minichannels occur ever more frequently in publications. This is interesting that some authors did not observe a zero boiling crisis in minichannels and the ONB moment on the boiling curve was determined based on a boiling curve trend change. This is also interesting that sometimes in minichannels before incipience of the boiling process the flashing phenomenon is observed. Hsieh et al. [8] conducted experimental investigations of the R-407C boiling process inside an annular gap. The hydraulic diameters of the channel were 4 and 2 mm. The experiments used mass flow rates (wq) = 300–600 kg/m2 s, a heat flux of up to q = 45 kW/m2, and a system pressure of p = 776 kPa or p = 899 kPa. The authors did not note any zero boiling crisis phenomena in any of the examined cases, while the ONB (Onset Nucleate Boiling) moment was represented by a change in trend of the boiling curve. Lee and Garimella [12] tested the boiling process in parallel minichannels with a depth of 400 lm and a width from 102 to 997 lm. Using deionized water that was heated in various configurations of minichannels with a heat flux of q = 10–340 W/cm2, a boiling process was produced. The authors did not observe a zero boiling crisis, and the ONB moment on the boiling curve was also determined based on a boiling curve trend change.
local saturation temperature (K) local temperature of the wall (K) pressure drop (Pa) difference of temperatures, DT = Tw–Tf (K) inlet subcooling, DTs = Ts–Tin (K) mass flux (kg m2 s1)
There are additional papers dedicated to minichannels where the authors did not observe the zero boiling crisis phenomena. The following papers need to be mentioned in this regard: Bergles and Kandlikar [1], Sumith et al. [23], Ohta [16], Owhaib et al. [17], Bertsch et al. [2], Lee and Mudawar [10], Prodanovic et al. [19]. On the other hand, there are papers that prove that the zero boiling crisis in minichannels is equally as common as in conventional channels. Experimental investigations of heat sinks with different geometries of parallel minichannels conducted by the team of Harirchian and Garimella [6] with the use of a dielectric fluid, Fluorinert FC-77, confirmed the existence of the zero boiling crisis phenomenon. This phenomenon occurred for every mass flow rate, and also for every heat sink geometry used. The largest temperature drop of 8 K was observed for the smallest flow rate (250 kg/ m2 s). Hetsroni et al. [7] observed the zero boiling phenomenon in an annular gap filled with water and surfactants with various concentrations. The measured temperature of the wall drop was approximately 18 K. It is interesting to note that this process was observed only when the gap was filled with a degraded surfactant solution. Martin-Callizo et al. [14,15] conducted research with subcooled R-134a refrigerant in vertical pipes with internal diameters of 0.83 mm, 1.22 mm and 1.70 mm. The authors investigated the influence of the heat flux, q = 1–26 kW/m2, mass flux, (wq) = 300–700 kg/m2 s, inlet subcooling, DTs = 5–15 K, system pressure, p = 7.70–0.17 bar, and channel diameter, on the subcooled boiling heat transfer. The authors demonstrate the existence of a boiling hysteresis, and the temperature drop at the boiling moment reached 18 K. Lie and Lin [13] investigated a subcooled flow boiling heat transfer of R-134a in a narrow (1.0 and 2.0 mm) annular duct. For each investigated mass flow rate (wq) = 200–300 kg/m2 s and heat flux q = 0–55 kW/m2, a drop in the temperature of the wall in ONB was observed. The greatest drop of 18 K occurred when (wq) = 200 kg/m2 s, gap = 1 mm, Tsat = 15 °C and DTs = 13 °C. Perhaps the most spectacular temperature drop was observed by Yen et al. [25]. HCFC123 and FC72 were used as working fluids. The investigations, which were carried out by the authors in 0.19, 0.3, 0.51 mm internal diameter pipes, allowed them to obtain a 70 K overheating of the channel wall before the boiling process started. This result corresponds to the following experimental conditions: HCFC123, d = 0.19 mm, q = 2.4 kW/m2, (wq) = 145 kg/m2 s. Numerous papers provide information about the possibility of the occurrence of the flashing phenomenon (Bergles and Kandlikar [1], Lee and Mudawar [11], Revellin et al. [21], Schneider et al. [22]). However, there are no publications that discuss this issue more extensively. One of the few papers is a paper by Warrier et al. [24]. The authors assume that a two-phase flow starts when a rapid increase of the heat transfer coefficient occurs, and the local vapor quality should also take the flashing phenomenon into account.
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In addition, during the experimental investigations of the liquid nitrogen boiling process conducted by Qi et al. [20] in minipipes with internal diameters of 0.531, 0.834, 1.042 and 1.931 mm, the existence of the zero boiling crisis phenomenon was noted. The investigations were carried out mass flow rates between (wq) = 440–3000 kg/m2 s and heat fluxes between q = 5.09–21.39 W/cm2. It was noted that a drop in the temperature of the wall at the ONB moment reached 5.0 K. The authors also analyzed the influence of the flashing phenomenon on the local vapor quality. The increase of the vapor quality was 18% when this phenomenon was taken into account. It is characteristic that, in each of the papers mentioned above, the authors speak about two zones: the single-phase flow zone and the two-phase flow zone. There are two cases: – the absence of the zero boiling crisis phenomenon – in this case, the single-phase flow zone on the boiling curve finishes when the trend of the curve changes; – the zero boiling crisis phenomenon occurs – the single-phase flow occurs until the moment at which a rapid temperature drop occurs. This moment is considered to be the start of the boiling process and incipience of a two-phase structure. This notion is also reported in the papers where the flashing phenomenon was observed. Therefore we should ask. Before incipience of the boiling process when the flashing phenomenon is observed have we singlephase flow or two-phase flow? The aim of this paper is to present experimental boiling curves. The influence of the flashing phenomenon on their course constitutes an important aspect of the paper. Another issue is to show that the area before the incipience of boiling, considered in all of the papers as a single-phase flow area, can be a two-phase flow area (indicating the presence of flashing).
3. Experimental apparatus and procedure 3.1. The test loop A schematic of the test loop is presented in Fig. 2. The liquid of the R134a refrigerant pumped from tank 12 by pump 2 flowed through filter 3 and precooler 4. Next, the Coriolis mass-flowmeter _ ¼ 0—20kg=h, accuracy of 0.075% of measured 5 was installed (m value). A liquid with a known flow rate was fed to heat exchangers 6, 7 and 8. We were able to use these exchangers alternatively, which allowed one to achieve the proper refrigerant conditions on the test section inlet 1. The parameters of the refrigerant (pressure and temperature) were measured at the heat exchangers inlet (p1, T1) and outlet (p2, T2). The designed set of heat exchangers enabled experimental tests of the heat transfer and pressure drop during a single-phase flow of the refrigerant, subcooled flow boiling or saturated flow boiling of refrigerant. The refrigerant (with suitable outflow parameters) was directed to the test section. The following parameters were measured: the pressure in the initial cross section of the tested part of the minichannel pin, the pressure drop Dp on the tested part of the minichannel, the profile of the minichannel wall temperature T1T10, temperature on the inlet Tin and outlet Tout of the tested part, and the supplied electric power Pel. All of the voltage signals of the measured values were connected with data acquisition system 18, which cooperates with computer 17. 3.2. The test section The test section was made from a stainless steel pipe 1 with a total length of 500 mm and an internal diameter of 1.68 mm. The surface roughness was obtained using scanning electron microscope (SEM). The average roughness of inner surface of minichannel is 0.65 lm from five measurements. The pipe was divided into
Fig. 2. Experimental set-up. 1 – minichannel, 2 – pump (with instrumentation), 3 – filter, 4 – precooler, 5 – flowmeter, 6 – preheater, 7 – cooler no. 1 (cooling with water), 8 – cooler no. 2 (cooling with refrigerant), 9 – additional cooling system, 10 – valves, 11 – condenser (cooled with water), 12 – tank of refrigerant, 13 – spare tank of refrigerant, 14 – system of electric heating of measuring section, 15 – pressure sensors, 16 – differential pressure sensor, 17 – computer, 18 – data acquisition system.
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several sections. The first one, which was 150 mm long, constituted a hydraulic stabilization area. The second one (300 mm) was the proper test part of minichannel, and the third one, which was 50 mm long, was the outlet section. The proper test section was electrically heated over a length of 200 mm. Using a high voltage adjustable transformer, a high voltage shunt of 150 A/60 mV and voltmeters, it was possible to change the heat flux and to measure the parameters required for its determination. On the measuring section, 10K type thermocouples and a wire with a diameter of / = 0.2 mm were installed. Using these sensors, the temperature of the external surface of the minichannel wall was measured. Right before and after the heated area, the same two thermocouples were additionally fixed to the pipe surface in order to measure the refrigerant’s temperature on the inlet and the outlet of the test section. Before the sensors were installed, their individual experimental thermo-electric characteristics were prepared in relation to the precise thermometer with an accuracy of 0.1 °C. Holes were made in the pipe 50 mm before and after the heated section. These made it possible to measure the working fluid pressure and pressure drop on the minichannel’s tested part. For the measurement of the pressure, a piezoresistant sensor (manufactured by Endress + Hauser) with a transducer was used (Deltabar S PMD75). The measuring range of this device was 0–4 MPa, and its accuracy was 0.075. The measurement of the pressure drop was conducted with a differential pressure transducer (Deltabar S PMD75) with a basic measuring range of 0–1.6 MPa and an accuracy of 0.075. The uncertainty of the pressure and the pressure drop itself were ±3 kPa and ±1.2 kPa, respectively. The whole test section was insulated in order to create nearly adiabatic conditions. The total heat losses to the environment were assessed to be smaller than 5% of the supplied electric power. 3.3. The test procedure All of the data was collected once steady state conditions were achieved. The required mass flow rate was set first. The refrigerant, with a pre-defined flow rate, was directed to one of three heat exchangers in the preparation section. The selection of the exchanger depended of the expected value of the refrigerant’s temperature at the minichannel inlet. The value of the refrigerant’s saturation temperature depended on the working pressure of the refrigerant in the minichannel. This value could be changed with a valve installed behind the test section. When the mass flow rate, pressure, pressure drop and the refrigerant’s temperature were constant, data registration began. The averaged values from several dozen measurements were the basis for further calculations. It should be mentioned that oscillations of the every abovementioned parameters were registered in some cases. After these measurements, the registration was conducted under quasi-steady state conditions. The experiments were conducted using R134a refrigerant under the following range of parameters: d = 1.68 mm, (wq) = 260– 2144 kg/m2 s, q = 1.7–60.3 kW/m2, Tin = 13.1–23.5 °C. 3.4. Data reduction The heat transfer rate to the refrigerant in the minichannel is obtained based on the net power input and the total internal surface area of the minichannel. The total power input is calculated by subtracting the heat loss due to electrical power
q¼
qel qloss : A
ð1Þ
It is accepted that the temperature of the wall on the internal side of the minichannel is equal to the temperature of the minichannel surface measured directly on the external side. It is assumed that the liquid temperature changes linearly along the length of the channel. In the single-phase flow, it is calculated from the heat balance while, in the two-phase flow region, it corresponds to the saturation temperature as it is a result of the local pressure inside the minichannel:
( Tf ¼
pdL — single-phase region T in þ q mc _ p
T s ¼ f ðps Þ — two-phase region
:
ð2Þ
The following was assumed in formula (2): Tin is the temperature of the liquid measured at the inlet to the heated minichannel section, L is the distance measured from the beginning of the heated section, _ is the refrigerant’s flow (kg/s), cp is the specific heat of the liquid, m Ts is the local saturation temperature which is dependent on saturation pressure – ps. 4. Results and discussion 4.1. Boiling curves without the flashing phenomenon Fig. 3 presents boiling characteristics of refrigerant R134a that were determined for the mass flow rate (wq) = 450 kg/(m2 s), with a liquid subcooling DTs = 10 K. The liquid subcooling is the difference of saturation temperature Ts and inflowing liquid Tin (DTs = Ts Tin). Fig. 3a shows the dependence of the local temperature difference DT (DT = Tw Tf) on the heat flux density q. Tw denotes the local temperature of the wall. Because ten thermocouples were placed on the heated part of the minichannel, it was possible to determine the difference in temperature DT = f(L) along ten cross-sections. In the first phase of heating, the difference in temperature DT between each measuring cross-section of length L had the same value. An increase of the heat flux density q resulted in the growth of DT. This corresponds to the heating of a liquid during forced convection, which characterizes section O–B in Fig. 1. For the parameters of the experiment given in Fig. 3, it is observed that a single-phase liquid flow in a minichannel occurs at q = 22.4 kW/ m2 (each thermocouple confirms the trend of the process in compliance with section O–B). A further growth of the heat flux caused an increased difference of the temperature DT while this process is in the meta-stable equilibrium of the system. Any disturbances of the meta-stable equilibrium lead to the initiation of the zero boiling crisis, which was accompanied by a rapid drop of the minichannel wall temperature. For the example presented here, this took place when the heat flux was q = 27 kW/m2. The measurement of the temperature showed that the boiling process occurred initially only on a part of the minichannel length, and covered those cross-sections that were located above 91 mm from the inlet cross-section of the minichannel. It was observed that, in the part of the measuring section where the five first temperature sensors were installed (18.2–91 mm), there was no zero boiling crisis effect. This means that a single-phase liquid flow continued there. An increase of the heat flux to q = 36.6 kW/m2 caused the zero boiling crisis to cover also this part of the minichannel, and was accompanied by a drop of the wall temperature. Only then was almost the whole volume of the pipe minichannel filled with boiling fluid. It should be noted that the temperature recorded by the first sensor, installed at a distance of 18.2 mm from the inlet cross-section relative to the section heated, confirmed a singlephase flow of the R134a liquid in this part of the length of the minichannel. During the zero boiling crisis, the boiling front moved in the direction contrary to the liquid flow in the pipe minichannel. This type of boiling front movement also occurs in conventional
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40.0
12.0
a
q[kW/m2 ]
35.0
b
α [kW/m 2 K]
10.0
30.0
18,2 [mm] 36,4 [mm]
8.0
54,6 [mm]
25.0
q [kW/m2]
72,8 [mm]
1,7
91,0 [mm]
4,7
6.0
109,2 [mm]
20.0
8,9
127,4 [mm]
14,9
145,6 [mm]
15.0
163,8 [mm]
18,6
4.0
182,0 [mm]
22,4 27,0
10.0
36,6
2.0 5.0 0.0
Tw -T f [ °C]
0
5
10
15
20
25
30
0.0 35
L [mm]
0
20
40
60
80
100
120
140
160
180
45.0
60.0 Tw [°C]
Tf [0C]
c
d
40.0
50.0
40.0
q [kW/m2]
35.0 q [kW/m2]
1,7
1,7
4,7
30.0
4,7
30.0
8,9
8,9 14,9
14,9 18,6
20.0
22,4
18,6
25.0
22,4 27,0
27,0 36,6
10.0
0.0
200
L [mm]
0
20
40
60
80
100
120
140
160
180
200
36,6
20.0
15.0
L [mm]
0
20
40
60
80
100
120
140
160
180
200
Fig. 3. Results of the experimental investigation: R134a, d = 1.68 mm, (wq) = 450 kg/(m2 s), DTs = Ts Tin = 10 K; q = 0–36.6 kW/m2. (a) heat flux q versus temperature difference DT = Tw Tf, (b) local heat transfer coefficient a on the length of the test section, (c) wall temperature Tw on length L of the test section and (d) refrigerant temperature Tf on length L of the test section.
channels (Bohdal [3]). The zero boiling crisis was recorded twice, and the front of nucleation occurred in the range of L = 91–109.2 mm from the inlet cross-section of the minichannel for q = 27.0 kW/m2, and then in the range L = 18.2–36.4 mm for q = 36.6 kW/m2. For the given experimental conditions, this was the maximum value of the heat flux. After this point, the heat flux was decreased. With a decreasing heat flux, the characteristic points of the boiling curve moved and connected, one by one, to the straight line of the single-phase liquid flow. The distribution of the local heat transfer coefficient (for selected values q = const) during a single-phase liquid flow and during boiling is presented in Fig. 3b. A clear increase of the heat transfer coefficient value is observed in the boiling region. The changes of the channel wall temperature during a singlephase liquid flow and after the incipience of boiling (for q = 27 kW/m2 and q = 36.6 kW/m2) are presented in Fig. 3c. Fig. 3d presents the distribution of the refrigerant temperature along the minichannel. A linear growth of the refrigerant temperature occurs in the region where it does not exceed the saturation temperature. The only one exception occurred for L = 182 mm and q = 36.6 kW/ m2. Fig. 4 presents the results of experimental investigations of boiling R134a. The mass flow rate was (wq) = 450 (kg/m2 s), and the subcooling of the liquid was DTs = 15 K. Boiling curves of type q = f(DT) are presented in Fig. 4a. With these parameters, the zero boiling crisis occurred once, when the heat flux value of q = 22.8 kW/m2 was exceeded. For a heat flux of q = 26.1 kW/m2, the state of the refrigerant was in a meta-stable range. After the
initiation of the boiling process, there was a drop of the wall temperature by over 15 K (for L = 182 mm). According to the reading of the first thermocouple (L = 18.2 mm), the states of the refrigerant were on the straight line O–B (from Fig. 1), which signified a single-phase flow of the liquid R134a refrigerant in this part of the minichannel. An increase of the heat flux above q = 35.8 kW/m2 shifted the front boiling location to before the first thermocouple. At a heat flux of q = 42.3 kW/m2, the boiling process covered the whole minichannel length. This was the largest heat flux value in this part of the experiment for this case of experiment. A decrease of the heat flux resulted in a change of the refrigerant’s states according to line D–A (Fig. 1), which was different for each cross-section on the minichannel length. While for a value of q = 14.8 kW/m2 a boiling process occurred in a part of the minichannel (decrease of heat flux), only single-phase heating of the liquid occurred during the increase of the heat flux. Fig. 4b presents a distribution of the local heat transfer coefficient for the subsequent values of the heat flux until the moment of boiling initiation with q = 26.1 kW/m2. Additionally, a dependence of the local heat transfer coefficient for the case when q = 42.3 kW/m2 was presented. At that heat flux, the boiling front moved and also covered the cross-section where the first thermocouple was installed (at a distance of L = 18.2 mm from the beginning of heated zone). The distributions of the wall temperature and the refrigerant temperature along the minichannel length for various levels of heat flux are presented in Fig. 4c and d, respectively.
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10.0
a
q[kW/m 2]
40.0
b
α [kW/m2K]
9.0 8.0
35.0 30.0
18,2 [mm] 36,4 [mm] 54,6 [mm] 72,8 [mm] 91,0 [mm] 109,2 [mm] 127,4 [mm]
25.0 20.0 15.0
145,6 [mm] 163,8 [mm] 182,0 [mm]
10.0
7.0 6.0
q [kW/m2] 2,8 6,5 11,6 14,8 17,9 20,2 22,8 26,1 42,3
5.0 4.0 3.0 2.0
5.0 Tw -T f [°C]
0.0 0
5
10
15
20
25
30
35
40
1.0 L [mm]
0.0
0
20
40
60
80
100
120
140
160
180
200
45.0
60.0
d
Tf [°C]
c
Tw [°C]
40.0
50.0 35.0 q [kW/m2 ]
30.0
40.0
2,8 6,5 11,6 14,8 17,9 20,2 22,8 26,1 42,3
q [kW/m2] 2,8 6,5 11,6 14,8 17,9 20,2 22,8 26,1 42,3
30.0
20.0
25.0 20.0 15.0 10.0
10.0 5.0 0.0
L [mm]
0
20
40
60
80
100
120
140
160
180
200
0.0
L [mm]
0
20
40
60
80
100
120
140
160
180
200
Fig. 4. Results of the experimental investigations: R134a, d = 1.68 mm, (wq) = 450 kg/(m2 s), DTs = Ts Tin = 15 K, q = 0 – 42.3 kW/m2. (a) heat flux q versus temperature difference DT = Tw Tf, (b) local heat transfer coefficient a on the length of the test section, (c) wall temperature Tw on a length L of the test section and (d) refrigerant temperature Tf on a length L of the test section.
4.2. Boiling curve with the flashing phenomenon Two further investigated cases are illustrated in Figs. 5 and 6. They show the heating of R134a refrigerant in a minichannel with the same internal diameter as in experiments described above, but under conditions of flashing. The results demonstrated that this phenomenon has an influence not only on the heat exchange during a single-phase liquid flow, but also on flow boiling. Fig. 5 presents the results of experimental investigations under the following conditions: (wq) = 280 kg/(m2 s), DTs = 4 K. During the increase of the heat flux, the refrigerant’s states should occur in the same manner as for the previously discussed cases (in the graphical interpretation, this corresponds to the straight line O– B in Fig. 1). It appears that the points that represent the location of these states on the diagram depart clearly from the straight line O–B. First of all, this results concerns those states of the refrigerant in which the temperature was measured by thermocouples that were placed at the end of the minichannel. For the higher values of the heat flux, the deviation concerned the cross-section situated more and more closely to the inlet cross-section. This can be explained by a drop of the refrigerant temperature on the minichannel length, which causes a growth of the temperature difference DT = Tw Tf. This is clearly visible in Fig. 5d, which presents the distribution of the refrigerant temperature on the minichannel length for various values of the heat flux. This undoubtedly is the result of the occurrence of the flashing phenomenon, which initially (for a heat flux q = 15.7 kW/m2) covered only the two last cross-sections of the minichannel located nearest to the outlet cross-section and moved in the direction opposite
to the refrigerant outlet. For conventional channels, it is considered that, along the straight line O–B (Fig. 1), heat transfer is realized during a single-phase forced convection. In the case when the flashing phenomenon takes place, there is in fact a two-phase structure of refrigerant (however, this is not classical flow boiling). Under these experimental conditions, the value of the heat flux q = 54.4 kW/m2 can be treated as the highest one at which boiling has not yet occurred. Its increase of up to 55.1 kW/m2 resulted in a meta-stable condition. Sufficient overheating of the wall occurred, and a boiling process with its corresponding zero crisis was initiated. New temperatures of the wall were established, as in Fig. 5c. As a result, a drop of the temperature difference DT occurred. In addition, a rapid growth of the intensity of the heat transfer process (Fig. 5b) serves to confirm the occurrence of flow boiling. It is characterized by a growth of the local heat transfer coefficient. The value of this coefficient significantly exceeds the values obtained in forced convection conditions during a singlephase flow and a flow with flashing. The technical potential of the experimental set-up did not allow for tests to be conducted for a heat flux growth above q = 55.1 kW/m2. In addition, tests were conducted where the heat flux density was decreased. A characteristic feature of the boiling process in a flow demonstrating the flashing phenomenon is overlapping states of the refrigerant in each cross-section of the minichannel. In practice, this means that, for each value of heat flux density q that occurred under the conditions of boiling with the flashing phenomenon, the temperature difference DT = Tw Tf had the same value in all cross-sections on the whole length of the pipe minichan-
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60.0
14.0
a
q[kW/m 2]
50.0
10.0
40.0 18,2 [mm] 36,4 [mm] 54,6 [mm] 72,8 [mm] 91,0 [mm] 109,2 [mm] 127,4 [mm] 145,6 [mm] 163,8 [mm] 182,0 [mm]
30.0
20.0
10.0
0.0
b
α [kW/m2K]
12.0
q [kW/m2] 5,2 11,0 15,7 21,3 26,2 33,6 43,4 54,4 55,1 20,4
8.0 6.0 4.0 2.0
Tw -T f [ °C]
0
5
10
15
20
25
50.0
0.0
L [mm]
0
20
40
60
80
100
120
140
160
180
200
23.5
c
Tw [ °C]
d
Tf [°C]
23.0
45.0
22.5 40.0
q [kW/m2] 5,2 11,0 15,7 21,3 26,2 33,6 43,4 54,4 55,1 20,4
35.0
30.0
25.0
q [kW/m2 ] 5,2
22.0
11,0 15,7
21.5
21,3 26,2
21.0
33,6 43,4
20.5
54,4 55,1
20.0 L [mm]
20.0 0
20
40
60
80
100
120
140
160
180
200
19.5
20,4
L [mm]
0
20
40
60
80
100
120
140
160
180
200
Fig. 5. Results of the experimental investigations: R134a, d = 1.68 mm, (wq) = 280 kg/(m2 s), DTs = Ts Tin = 4 K; q = 0–55.1 kW/m2, flashing. (a) heat flux q versus temperature difference DT = Tw Tf, (b) local heat transfer coefficient a on the length of the test section, (c) wall temperature Tw on a length L of the test section and (d) refrigerant temperature Tf on a length L of the test section.
nel. This can be explained by a significant growth of vapor bubbles, which make the flow more homogeneous by making it turbulent. Another case (Fig. 6) concerns the flow of R134a (also with the accompaniment of the flashing phenomenon). This occurred during the flow of the refrigerant in a minichannel that was already at a heat flux of q = 9.4 kW/m2, and covered only the two last cross-sections of the minichannel (situated near the outflow cross-section: Fig. 6d). At this moment a two-phase structure of the flow occurred. A further increase of the heat flux resulted in the growth of the region of flow with flashing. Similarly, as in the case presented in Fig. 5, this resulted in a deviation of the points from a straight line (O–B, Fig. 1). These points illustrate the refrigerant states situated in the flashing region. In this case, the maximum value of the heat flux at which no boiling in the flow had occurred was q = 19.1 kW/m2. The meta-stable states occurred at q = 25.3 kW/m.2. A growth of the wall temperature was noted, and when the refrigerant at the wall reached sufficient overheating, the boiling process commenced together with a rapid drop of the wall temperature. It can be seen from the boiling curve (Fig. 6a) that, in those cross-sections of the minichannel that were situated at the nearest distance of the inlet cross-section (L = 18.2 mm and 36.4 mm), there was a departure from the remaining boiling lines. This proves that boiling had already occurred but that flashing existed in further cross-sections (above 36.4 mm from the inlet cross-section). For q = 35 kW/m2, flashing also covered the cross-section of the channel at a distance of L = 36.4 mm, which was the maximum value of the heat flux density in this experiment.
When the heat flux was decreased, the parameters measured indicated that boiling still existed, and that the flashing phenomenon covered a decreasing volume of the minichannel. Just as when the heat flux was being increased, the readings of subsequent thermocouples placed in area covered with flashing departed from the straight line O–B (Fig. 1). Once the heat flux decreased, in the inversely sequence, the readings of the thermocouples departed from the common trend and indicate the disappearance of the flashing phenomenon. 5. Conclusions
1. The experimental investigations conducted with the R134a refrigerant flowing in a heated minichannel with an internal diameter of 1.68 mm enabled the presentation of boiling curves. A description of the boiling curves, known for conventional channels, proved to be insufficient for a minichannel. 2. The mechanism of the flow boiling process in minichannels is substantially different from the one that has already been well recognized with regard to conventional channels. The large drop of the pressure in the flow has a special impact on this mechanism. 3. The so-called flashing phenomenon, which is usually left out in an analysis of boiling in conventional channels and consists of the presence of a two-phase structure that occurs as a result of the pressure lowering (below the saturation pressure) due to the large flow resistances in the minichannel, plays a significant role and has an impact on the boiling curves.
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7,0
a
q[kW/m 2 ]
35.0
q [kW/m2]
30.0 25.0 20.0 15.0
18,2 [mm] 36,4 [mm]
5,0
54,6 [mm] 72,8 [mm] 91,0 [mm]
4,0
109,2 [mm] 127,4 [mm] 145,6 [mm]
3,0
163,8 [mm] 182,0 [mm]
3,1 4,5 7,1 9,4 12,2 15,1 19,1
2,0
10.0
25,3
1,0
5.0 0.0
b
α [kW/m2K]
6,0
Tw -T f [ °C]
0
5
10
15
20
25
50.0
0,0
L [mm]
0
20
40
60
80
100
120
140
160
180
200
28.0
c
Tw [ °C]
d
Tf [ °C]
27.0
45.0 q [kW/m2 ]
26.0
3,1
40.0
25.0
4,5
q [kW/m2]
7,1
3,1
24.0
9,4
4,5
12,2
35.0
15,1
7,1
23.0
9,4
19,1 25,3
30.0
12,2
22.0
15,1 19,1 25,3
21.0 25.0
L [mm]
0
20
40
60
80
100
120
140
160
180
200
20.0
L [mm]
0
20
40
60
80
100
120
140
160
180
200
Fig. 6. Results of the experimental investigations: R134a, d = 1.68 mm, (wq) = 525 kg/(m2 s), DTs = Ts Tin = 5 K, q = 0– 5.3 kW/m2, flashing. (a) heat flux versus temperature difference DT = Tw Tf, (b) local heat transfer coefficient a on the length of the test section, (c) wall temperature Tw on length L of the test section and (d) refrigerant temperature Tf on length L of the test section.
4. The paper presents and discusses characteristic cases of boiling curves in a pipe minichannel with the flashing phenomenon taken into account.
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[12] P.-S. Lee, S.V. Garimella, Saturated flow boiling heat transfer and pressure drop in silicon microchannel arrays, Int. J. Heat Mass Transfer 51 (2008) 789–806. [13] Y.M. Lie, T.F. Lin, Subcooled flow boiling heat transfer and associated bubble characteristics of R-134a in a narrow annular duct, Int. J. Heat Mass Transfer 49 (2006) 2077–2089. [14] C. Martin-Callizo, W. Owhaib, B. Palm, Subcooled flow boiling of R-134a in a vertical channel of small diameter, in: 6th World Conference on Experimental Heat Transfer, Fluid Mechanics, and Thermodynamics, Matsushima, Miyagi, Japan, 2005. [15] C. Martin-Callizo, B. Palm, W. Owhaib, Subcooled flow boiling of R-134a in vertical channels of small diameter, Int. J. Multiphase Flow 33 (2007) 822–832. [16] H. Ohta, Boiling and two-phase flow in channels with extremely small diameters – review of Japanese research, in: Second International Conference on Microchannels and Minichannels, New York, 2004. [17] W. Owhaib, C. Martin-Callizo, B. Palm, Evaporative heat transfer in vertical circular microchannels, Appl. Therm. Eng. 24 (2004) 1241–1253. [18] M.E. Poniewski, J.R. Thome, Nucleate Boiling on Micro-Structured Surfaces. Heat Transfer Research, Inc. (HTRI), 150 Venture Drive, College Station, TX 77845, USA, 2008. [19] V. Prodanovic, D. Fraser, M. Salcudean, On the transition from partial to fully developed subcooled flow boiling, Int. J. Heat Mass Transfer 45 (2002) 4727– 4738. [20] S.L. Qi, P. Zhang, R.Z. Wang, L.X. Xu, Flow boiling of liquid nitrogen in microtubes: part I – the onset of nucleate boiling two-phase flow instability and two-phase flow pressure drop, Int. J. Heat Mass Transfer 50 (2007) 4999–5016. [21] R. Revellin, V. Dupont, T. Ursenbachr, J.R. Thome, I. Zun, Characterization of diabatic two-phase flows in microchannels: flow parameter results for R-134a in a 0.5 mm channel, Int. J. Multiphase Flow 32 (2006) 755–774. [22] B. Schneider, A. Kosa, Y. Peles, Hydrodynamic cavitation and boiling in refrigerant (R-123) flow inside microchannels, Int. J. Heat Mass Transfer 50 (2007) 2838–2854. [23] B. Sumith, F. Kaminaga, K. Matsumura, Saturated flow boiling of water in a vertical small diameter tube, Exp. Therm. Fluid Sci. 27 (2003) 789–801. [24] G.R. Warrier, V.K. Dhir, L.A. Momoda, Heat transfer and pressure drop in narrow rectangular channels, Exp. Therm. Fluid Sci. 26 (2002) 53–64. [25] T.-H. Yen, N. Kasagi, Y. Suzuki, Forced convective boiling heat transfer in microtubes at low mass and heat fluxes, Int. J. Multiphase Flow 29 (2003) 1771–1792.