Experimental investigation on flow patterns and absorption height in a vertical bubble absorber with R124-NMP pair and comparison for R124-DMAC pair

Applied Thermal Engineering 168 (2020) 114842

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Experimental investigation on flow patterns and absorption height in a vertical bubble absorber with R124-NMP pair and comparison for R124DMAC pair

T

Wei Wang, Shiming Xu , Xi Wu, Dongxu Jin, Mengnan Jiang ⁎

Key Laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of Education, School of Energy and Power, Dalian University of Technology, Dalian 116023, China

HIGHLIGHTS

typical flow patterns of R124-NMP bubble absorption process are captured. • Three DAH changes almost linearly as the dimensionless operation parameters vary. • The AH of R124-NMP is lower than that of R124-DMAC under the same conditions. • The • Correlations of AH and DAH were developed to predict the bubble absorption height. ARTICLE INFO

ABSTRACT

Keywords: R124-NMP Visualization Flow pattern Absorption height R124-DMAC

A visual experimental research was conducted to investigate the flow patterns and absorption height (AH) in a vertical bubble absorption tube employing R124-NMP as working fluid. The effects of various operation parameters, solution inlet mass concentration, temperature and volumetric flow rate, vapor volumetric flow rate, absorption pressure and orifice diameter, on the flow patterns and AH were examined. The key operation parameters were all converted into dimensionless numbers and the influences of dimensionless operation parameters on dimensionless absorption height (DAH) were studied. The results showed that the AH increases with increasing solution inlet mass concentration and temperature, vapor volumetric flow rate and orifice diameter and decreases with increasing solution volumetric flow rate and absorption pressure. DAH exhibits nearly linear change along with dimensionless operation parameters. Comparisons between the flow patterns and AH of R124-NMP and R124-DMAC were performed. The compared results illustrated that employing R124NMP as working fluid can shrink the vertical bubble absorber size. Finally, the experimental correlations of AH and DAH were developed with ± 20% and ± 15% error bands, respectively. The correlation of DAH maybe used to predict absorption height in the design of bubble absorber employing refrigerant-organic absorbent as working fluid.

1. Introduction Vapor absorption refrigeration system (VARS) is considered to be a very promising and effective technique by using low grade energy, which is also considered to be a great substitute for vapor compression cycle employed in vehicle air-conditioning systems [1–5]. In consideration of dimension and performance, the absorber is regarded as one critical component in the VARS. Failing film and bubble are two major absorption modes in VARS. However, the falling film absorber is unsuitable for harsh vehicle working conditions, for example, tossing

⁎

up and down, slanting and accelerating or decelerating. In comparison with the falling film model, the bubble absorption process makes the refrigerant vapor be always surrounded by absorption solution [6,7]. That not only improves the bubble absorber performance [8,9] but also makes the bubble absorber be suitable for the precarious conditions. Utilizing the vehicle waste heat for refrigeration, the bubble absorber presents a wide application foreground. The conventional working fluids, LiBr-H2O and NH3-H2O, have their own drawbacks and they are unseemly for VARS in vehicles. (1) Selecting LiBr-H2O as working medium, the VARS operates at high vacuum pressure which results in

Corresponding author. E-mail address: [email protected] (S. Xu).

https://doi.org/10.1016/j.applthermaleng.2019.114842 Received 23 March 2019; Received in revised form 14 November 2019; Accepted 23 December 2019 Available online 24 December 2019 1359-4311/ © 2019 Elsevier Ltd. All rights reserved.

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Nomenclature

q S t T u x

Abbreviations ACHRC AH COP CR DAH DMAC DMEDEG DMETEG DMF GBA GC NMP R124 R134a R22 VARS Re VF

Absorption compression hybrid refrigeration cycle Absorption height Coefficient of performance Circulation ratio Dimensionless absorption height N, N-Dimethylacetamide Dimethylether diethylene glycol Dimethylether tetraethylene glycol N, N-Dimethylformamide Glass bubble absorber Gas chromatography N-Methyl-2-Pyrrolidone 1-chloro-l, 2, 2, 2,-tetrafluoroethane 1, 1, 1, 2-tetrafluoroethane Chlorodifluoromethane Vapor absorption refrigeration system Reynolds number Volumetric flow

Greek letters

µ

Range of instrument (–) Range method coefficient (–) Density (kg/m3) Thickness (mm) Difference (–) Dynamic viscosity (Pa s) Dimensionless (–) Precision grade of instrument (–)

Subscripts A B c cw in ine o s v

Nomenclature D d h L M Max Min n p

Volumetric flow Rate (L/h) Standard deviation (–) Temperature (°C) Thermodynamic temperature (K) Standard uncertainty (–) Mass fraction of refrigerant (–)

Inner diameter of inside glass tube (mm) Diameter (mm) Absorption height (mm) Length of the absorber (mm) Molecular weight (–) Maximal Minimal Simple sizes (1, 2, 3) Pressure (kPa)

Method of type A Method of type B Critical Cooling water Inlet Inner Orifice Solution Vapor

Superscripts sat

larger device dimension and weight. It is also very difficult to keep the high vacuum pressure for VARS in vehicles. (2) Selecting NH3-H2O as working medium, the VARS cooled by air operates at very high pressure and ammonia used as refrigerant has toxicity [10] and flammability. Additionally, a rectifier should be set in VARS because of the low boiling point difference between the two pure components. That also results in larger device dimension and weight. Therefore, the bubble absorption research for VARS in vehicles focuses mainly on organic working pairs due to their nontoxicity and lower flammability. In order to study the influences of vapor and liquid properties on the volumetric mass transfer coefficient, K. S. Sujatha et al. [11] set up a model of vertical bubble absorption process selecting R22-DMF, R22DMAC, R22-DMETEG, R22-DMEDEG and R22-NMP as working mediums. One correlation for volumetric mass transfer coefficient was developed. Selecting R22-DMF as working pair [12], they numerically investigated the heat and mass transfer characteristics and performance of a vertical tubular bubble absorber under co-current condition and proposed a correlation for mass transfer coefficient. Then they conducted an experimental investigation [13] on the R22-DMF vertical tubular bubble absorber. The experimental results were analyzed by contrast with the numerical model results [12]. M. Suresh and A. Mani [14] made a model to research the heat and mass transfer process of R134a-DMF bubble absorber. The results revealed that raising the vapor mass flow rate can enhance the heat and mass transfer performance and absorption efficiency of the bubble absorber. Then they conducted experimental investigations on bubble characteristics [15] and heat and mass transfer characteristics [16] in a glass bubble absorber (GBA) employing R134a-DMF as working fluid. Finally, they made an experimental research on heat and mass transfer

Saturated

performances of R134a-DMF in one compact bubble absorber [17] of VARS. Then they developed one correlation for volumetric mass transfer coefficient for the R134a-DMF bubble absorber. M. Suresh and A. Mani [15] conducted an experimental investigation to observe the bubble behavior in a glass bubble absorber (GBA) employing R134a-DMF as working pair. Meanwhile, the influences of solution inlet concentration and vapor flow rate on bubble (R134a) characteristics in bubble absorption process were studied. Bubble departure diameters and shape were recorded in still and flowing solution. Finally, they presented a dimensionless correlation to predict the bubble departure diameter. For taking advantage of waste heat from vehicle engine, the absorption–compression hybrid refrigeration cycle (ACHRC) [7,18] employing R124-DMAC as working pairs was proposed by Xu. Under aircooled conditions, the refrigerant, R124, has a relatively small condensing pressure which leads to a relatively low generating temperature [7]. The air-cooled VAR system employing R124 and organic absorbent as working fluid can also obtain a good cycle performance (CR and COP). Additionally, the combination of R124 and organic absorbent DMAC or NMP has low toxicity and low flammability. For purpose of enhancing the efficiency of the ACHRC and shortening the size of equipment, Xu conducted a visualization investigation [6] on the bubble absorption process using R124-DMAC as working fluid in a vertical GBA. The influences of the important operating parameters on flow patterns and AH were investigated and a correlation for AH was suggested [19,20]. The effects of the critical parameters on the heat and mass transfer performances were studied and a correlation for volumetric mass transfer coefficient was developed [21]. Then Xu conducted an experimental study on the heat and mass transfer 2

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experimental results are converted into dimensionless numbers to explore the relationship of dimensionless numbers. And then, a comparison of flow patterns and AH between R124-NMP and R124-DMAC [6] is performed to analyze the possibility of miniaturization of bubble absorber. Finally, experimental correlations of AH and DAH are developed for designing vertical bubble absorber.

Table 1 Basic thermal properties [23,24] for pure substances, R124, NMP and DMAC. Material

Formula

M

tc/ °C

pc/kPa

Normal boiling point/°C

R124 NMP DMAC

CHClFCF3 C5H9NO C4H9NO

136.5 99.13 87.12

122.2 453.1 382.4

3574 4596 4211

−11.0 203.0 165.0

2. R124-NMP bubble absorption experiment

performances of R124-DMAC in a vertical copper bubble absorber. Correlations for heat and mass transfer coefficients were developed [22]. To further reduce the equipment size, R124-NMP is suggested to be used in ACHRC. Xu [23] conducted an experimental research on the vapor-liquid equilibrium of R124-NMP binary mixture and depicted the p-t-x relationship. The results showed that the solubility of R124 in R124-NMP solution is higher than that in R124-DMAC solution under the same conditions. Table 1 listed the thermal properties [23,24] of R124, DMAC and NMP. The data in Table 1 indicated that the binary mixture, R124-NMP, was considered to be a potential working fluid to apply in ACHRC. Nevertheless, the investigations of the flow patterns and AH of the bubble absorption process employing R124-NMP as working fluid were vacant. The primary objectives of this study are first to observe and obtain the bubble flow behavior of R124-NMP in a vertical bubble absorption tube by visual experiments, and to explore the influences of key operation parameters on flow patterns and AH. Meanwhile, the

2.1. Experimental system In order to observe flow patterns and measure AH in bubble absorption process of R124-NMP, a vertical glass bubble absorber (GBA) and the experimental platform have been employed, which resemble that in our previous works [6,19,20]. Fig. 1 depicts the principle drawing of the visual experiment system. The system is comprised of R124-NMP solution loop, cooling water loop, hot water loop, and one data measurement and collection system. Practical picture of the vertical bubble absorption experimental platform is given in Fig. 2. The R124-NMP solution loop is constituted of the GBA, the strong/ weak solution containers, a high temperature thermostatic water bath, a solution pump and a solution heat exchanger. The weak solution in the weak solution container is first preheated to a given absorption temperature by the solution heat exchanger in hot water loop. And then it is pumped to the GBA to absorb R124 vapor which comes from the refrigerant tank. The absorption heat is transferred to the cooling water

Table 2 Details of main devices and instruments. Instruments

Model/Material

Accuracy

Range

Remarks

Solution flow meter Vapor flow meter Cooling water flow meter Pressure gauge Thermocouples Solution pump Cooling water pump Heat water pump Gas chromatography Glass bubble absorber

Glass Rota meter LZB-4 Glass Rota meter LZB-6 Glass Rota meter LZB-10 Vacuum manometer Y-60 WZP Pt-100 MDG-H2 A220 ORS 25-8 ORS 25-8 HP4890A Glass

4% 2.5% 2.5% 2.5% ± (0.15 + 0.002|t|) °C – – – ≤0.02 mV/15 min –

1–10 L/h 60–600 L/h 10–100 L/h −0.1–0.15 MPa −50 to 200 °C 1.8 L/min 4 bar 1L/s 8 m 1L/s 8 m – 640 mm

Calibration Calibration Calibration Calibration Calibration Magnet gear pump – – TCD detector Inside tube: dine = 14 mm δ = 3 mm Outside tube: dine = 36 mm

Fig. 1. Schematic diagram of visual experiment system with vertical glass bubble absorber 1: Glass bubble absorber, 2: Gas chromatography (GC), 3: Strong solution container, 4: Weak solution container, 5: Solution pump, 6: Liquid volumetric flow meter, 7: Solution heat exchanger, 8: Vapor volumetric flow meter, 9: Refrigerant tank, 10: High-speed camera, 11: Data collection system, 12: Pressure reducing valve, 13: Liquid six-way valve, 14: Needle valve, 15: High temperature thermostatic water bath, 16: Heating water pump, 17: Low temperature thermostatic water bath, 18: Cooling water pump.

3

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Table 3 Experimental conditions of bubble absorbing and maximal uncertainty. Experimental variables

Variable ranges

Max. combined standard uncertainty

Absolute absorption pressure (kPa) Refrigerant mass fraction in inlet solution (x) Solution inlet temperature (°C) Solution volume flow rate (L/h) Vapor volume flow rate (L/h) Nozzle orifice diameter (mm) Cooling water temperature (°C) Cooling water volume flow rate (L/h)

165, 175, 180 40%-50%

4.1 0.18%

36, 46, 54 2, 4, 6, 8 140–560 1, 2.8 33 44

0.38 0.23 8.81 0.12 0.27 1.46

nozzle exit to the position where bubbles disappeared. The measurement criterion of AH is: observing three sets of the complete dynamic absorption processes under a given absorption condition, the positions of bubble disappearing are recorded. The average of values measured three times is regarded as the AH. It should be mentioned that the position of bubble disappearing is the location where bubbles reduce to a tiny dot. In the dynamic absorption process, some tiny bubbles can be observed to flow out of the GBA, which is caused by the purity of refrigerant. The experimental parameters and image data in each experimental process are all recorded by the computer. When the initial weak solution is used up, the experiments are completed. Turning on the valve set between the two solution tanks makes the strong solution flow totally into the weak solution container. Then turn off the valve and adjust the solution in the weak solution container to a new given concentration. The next group of experiments can be started.

Fig. 2. The hot water loop consists of the heating water pump, the high temperature thermostatic water bath and a solution heat exchanger. It is employed to heat the solution loop through the solution heat exchanger to adjust the weak solution inlet temperature. The cooling water loop comprises the low temperature thermostatic water bath and a cooling water pump. The data measurement and collection system consists of various valves, volumetric flow meters, thermocouples, pressure gauges, a GC, a high speed camera and a computer. The measurement points of temperature (T), pressure (P) and volumetric flow rate (VF) are shown in Fig. 1. The details of main devices and instruments are listed in Table 2. Each measuring instrument is pre-calibrated before experiment.

2.2.2. Experimental conditions The experimental conditions set in this work are listed in Table 3. Absorption pressure is set as 165 kPa which is the saturation pressure of R124 refrigerant evaporation at 5 °C which represents normal evaporation temperature in vehicle air-condition system.

which flows cyclically in the glass annulus under the condition of countercurrent. Finally, the strong solution leaves the absorber and flows into the strong solution container. Photo of visual experiment system with vertical glass bubble absorber.

2.3. Standard uncertainty analysis The standard uncertainty analysis was conducted employing the method presented by Ma and Wang [25]. In order to make the measured results more precise and reliable, each measured parameters and AH was recorded three times under the same experimental conditions.

2.2. Experimental procedures 2.2.1. Experimental procedures Firstly, the weak solution container is filled with the R124-NMP weak solution with initial mass concentration of 40%. Then the solution pump starts up and transports the weak solution to the GBA. The solution flow rate is modulated by a liquid volumetric flow meter installed at the solution pump outlet. The solution heat exchanger and one needle valve installed at the GBA outlet are employed to regulate the absorption temperature and pressure, respectively. The cooling water pump runs to push the cooling water circulating in the GBA annulus at a constant flow rate. When the given experimental conditions are meeting, the pressure reducing valve is opened, the R124 vapor from refrigerant tank is injected into a nozzle set at the bottom of GBA and bubbles form in it. The R124-NMP weak solution in GBA starts to absorb the R124 vapor. Then the strong solution forms and enters into the strong solution container. The solution concentration at the GBA outlet is tested three times using a GC though area correction normalization method. The average concentration is recorded as the strong solution concentration. The flow patterns and AH are acquired employing the high speed camera setting 1/125 s as shutter speed. The definition of AH in Ref. [6] is employed in this work. Absorption height (AH) is defined as the distance from the

Table 4 Expressions of dimensionless operation parameters and DAH. Dimensionless numbers

Expressions

Vapor Reynolds (Re) number

Re v =

Solution Re number

Res =

Orifice diameter Absorption pressure

v qv × 10 2 9 d 0 µv q s s × 10 2 9 Dµs d0

d0 =

D 1

p=

p

Absorption concentration

x=

Absorption temperature

xs, in sat 2 xs, in

T=

Absorption height (DAH)

h=

1

p0

Ts, in

h

(Ts, in + T ) 3 T02

L

p0 represents the saturated pressure when the temperature of refrigerant is equal to 0 °C. 2 xssat , in represent the saturated mass concentration at the same temperature and pressure as the inlet absorption solution. 3 T0 represents the ambient temperature and it is set as 293.15 K. T represents the solution temperature difference between the absorber tube inlet and outlet. 4

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Taking absorption pressure for an example, the method of standard certainties of measurements was introduced. The sample standard deviation, S, can be calculated by the range method when the numbers of measurement parameter is less than 5.

S (p) =

pmax

pmin

(1)

where p is the measured value, represents the range method coefficient. While the simple size is three, the range method coefficient, , is equal to 1.69 [25]. Then the standard uncertainty evaluated by type A can be obtained as:

uA (p) =

S (p) n

(2)

The standard uncertainty evaluated by type B can be obtained as:

uB (p) =

× 3

(3)

Fig. 3. Photo of flow patterns captured by experiment in R124-NMP bubble absorption process [Operation conditions: (p = 165 kPa, x = 45%, ts,in = 54 °C, qs = 4 L/h, qv = 400 L/h, tcw = 33 °C, qcw = 44 L/h, do = 2.8 mm)].

where and are the precision grade and range of the measured instrument, respectively. The rectangular distribution is supposed, and then the coverage factor is 3 . So, the combined standard certainty of p in the range of this work is:

u (p ) =

uA 2 (p) + uB 2 (p)

flow, the slug flow with well-shaped Taylor bubbles appears in the middle part of absorption tube. Taylor bubble is formed by aggregation of many small bubbles which can’t timely be absorbed in the churn flow. The reason is that the rising velocity of small bubbles is higher than that of the absorption solution but lower than that of the trailing bubble, which leads to small bubbles merging to form a big Taylor bubble. The vapor in Taylor bubbles are continuously absorbed by the solution with them rising in the GBA and their volume reduces. Finally, the bubble flow is formed when the bubble diameter is less than the GBA inner diameter. The bubbles can be completely absorbed at the GBA exit when the refrigerant vapor flow rate is less than or equal to the given flow rate.

(4)

The maximal standard uncertainties of measured operating parameters are summarized and listed in Table 3. The standard uncertainties of measured AH are given in the absorption height plots. In order to obtain a low standard uncertainty [10], the thermal resistances of bubble absorption process are calculated employing the method provided by Garimella S [5,10]. The average resistance ratio is 1.87 (with min = 0.55 and max = 4.11) in the present work. It indicates that the experiments are designed such that the heat transfer resistance from convection on water and conduction through glass is significantly lower than the heat transfer resistance on the absorption solution side. Thus, a low standard uncertainty can be obtained.

4.2. Influence of vapor/solution flow rates on flow pattern and absorption height

3. Dimensionless operation parameters and DAH In this work, the solution volumetric flow rate, inlet concentration and temperature, vapor volumetric flow rate, absorption pressure and orifice diameter are considered as the key operation parameters. They are all converted into dimensionless numbers which are listed in Table 4:

Fig. 4 illustrates the effect of refrigerant vapor flow rate on flow patterns and AH in the GBA. It can be found from Fig. 4 that with the increase of R124 vapor flow rate, the two-phase flow patterns in the GBA gradually change from one type of flow pattern (bubble flow) to three types of flow patterns (churn, slug and bubble flows), and the AH increases correspondingly. The R124 vapor ejected from the nozzle outlet will be quickly absorbed by R124-NMP solution in the GBA when the vapor flow rate is low. So, only bubble flow can be observed [Fig. 4(a)]. When the vapor flow rate is less than or equal to 320 L/h, the flow pattern still keeps bubble flow although the diameters of bubbles obviously increase as the vapor flow rate rises gradually [Fig. 4(b) and (c)]. When the vapor flow rate is larger than 320 L/h, with the vapor flow rate increasing, churn flow will appears but slug flow cannot be observed clearly [Fig. 4(d)]. As the vapor flow rate increases continuously, three types of flow patterns will coexist [Fig. 4(e)]. The churn flow forms at the nozzle outlet, then the flow pattern shifts from churn flow to slug flow with well-shaped Taylor bubble, and finally it evolves to bubble flow until to disappear with the vapor being absorbed by solution. However, when the vapor flow rate is larger than 560 L/h, the slug flow does not disappear showing incomplete absorption in the GBA [Fig. 4(f)]. Fig. 5 illustrates the influence of solution flow rate on flow patterns and AH. It can be found from the figure that the slug flow shorts gradually until to disappear in the GBA when the vapor flow rate keeps 400 L/h and the solution flow rate increases from 2 L/h to 8 L/h [from Fig. 5(a) to (d)]. The AH also decreases as the solution flow rate

4. Experimental results and discussion 4.1. The flow patterns captured from the experiments Bubble absorption is an efficient heat and mass transfer process which mainly presents complicated vapor-liquid two phase flow patterns in the GBA. The flow patterns in the GBA can be generally observed churn flow, slug flow and bubble flow shown as Fig. 3. In the figure, the whole GBA is equally divided into three sections from the inlet to the outlet, which is illustrated in Fig. 3 from left to right. The length of each section is 210 mm. In addition, the Fig. 3 is processed with gray scale by Photoshop software. For the convenience of comparison, all the photographs in bubble absorption process captured by the high speed camera in this paper are addressed employing the processing method as shown in Fig. 3. Fig. 3 shows that the churn flow is formed at the nozzle outlet and approximately occupies one of third height of the GBA. The churn flow regime is merely an entrance effect in the absorption tube because the high speed vapor at the nozzle outlet disturbs to absorption solution strongly. So, the height of churn flow relates mainly to the vapor volumetric flow rate and the nozzle orifice diameter. Following churn 5

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Fig. 4. Photo of two-phase flow pattern variations in the GBA [Operation conditions: (p = 165 kPa, x = 40%, ts,in = 54 °C, qs = 4 L/h, tcw = 33 °C, qcw = 44 L/h, do = 2.8 mm) qv = 160L/h (a), 240 L/h (b), 320 L/h (c), 400 L/h (d), 480 L/h (e) 560 L/h (f)].

Fig. 5. Photo of two-phase flow pattern variations in GBA [Operation conditions: (p = 165 kPa, x = 40%, ts,in = 46 °C, qv = 400 L/h, tcw = 33 °C, qcw = 44 L/h, do = 2.8 mm) qs = 2 L/h (a), 4 L/h (b), 6 L/h (c), 8 L/h (d)].

increases. The reason is that the solution average mass fraction will decrease with the increase of solution flow rate when the refrigerant vapor flow rate keeps constant. That enhances the mass transfer impetus between the vapor and absorption solution. Therefore, the vapor is rapidly absorbed by the absorption solution. Based on the visual experiment, it can be found that both vapor and solution flow rates have a demonstrable effect on flow pattern distribution and AH in the GBA. The variations of AH versus to the vapor and solution flow rates are plotted in Fig. 6 (a) and (b) by the experimental data. Fig. 6(a) and (b) presents that the AH increases as the refrigerant vapor flow rate rises, but decreases as solution flow rate rises. Fig. 6(c) and (d) presents the effects of vapor Reynolds number (Rev) and solution Reynolds number (Res) on dimensionless absorption height (DAH). Fig. 6(c) and (d) depicts that both Rev and Res had an effect on the DAH. Res had a much lower effect compared to Rev. The effect of Rev and Res is very straight forward. The DAH increased approximately linearly with the Rev increasing but decreased nearly linearly with the Res increasing.

patterns in Fig. 7(b) increases obviously with increase in solution inlet mass concentration and the length of slug itself also increases. A superficial explanation for this phenomenon can be given that the R124 vapor isn’t timely absorbed with increase in solution inlet mass concentration. Fig. 7 also depicts that the AH obviously increases as the solution inlet mass concentration increases owing to the increased height of each flow pattern. Based on the visual observations, the experimental data of the influence of solution inlet mass concentration on AH is charted in Fig. 8. Fig. 8 (a) shows the variations of AH with respect to the vapor volumetric flow rate at different concentrations of absorption solution. The figure illustrates that the AH increases with the solution inlet mass concentration rising. Fig. 8(b) displays the effect of dimensionless absorption concentration ( x ) on DAH at different Revs. At the same Rev, the DAH increased nearly linearly with the x increasing. The partial pressure of refrigerant R124 vapor in the absorption solution increases as the solution inlet mass concentration rises. That causes a reduction in mass transfer impetus between the refrigerant vapor and the absorption solution. Then the AH increases. Fig. 8 (a) also shows that when the solution inlet mass concentration is improved to 45%, the AH reaches to the maximum at the vapor flow rate of 400L/h.

4.3. The effect of solution inlet concentration on flow patterns and AH Fig. 7 presents the influence of solution inlet mass concentration on flow patterns and AH at solution inlet temperature and volumetric flow rate of 54 °C and 4 L/h, respectively, vapor volumetric flow rate of 480 L/h, absorption pressure of 165 kPa and orifice diameter of 2.8 mm. As shown in Fig. 7, the solution inlet mass concentration has a significant influence on flow patterns and AH. Churn, slug and bubble flow patterns are all observed and recorded in Fig. 7(a) and (b). Compared with Fig. 7(a), the height of each kind of the three bubble flow

4.4. The effect of absorption pressure on flow patterns and AH Fig. 9 depicts the effect of absorption pressure on flow patterns and AH at solution inlet mass concentration, temperature and volumetric flow rate of 40%, 54 °C and 4 L/h, respectively, vapor volumetric flow rate of 480 L/h and orifice diameter of 2.8 mm. As shown in Fig. 9, the absorption pressure has a noticeable effect on flow patterns and AH. When the absorption pressure is set as 165 kPa, the churn flow pattern 6

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Fig. 6. a, b: The AH versus vapor volumetric flow rate at different absorption solution volumetric flow rates (p = 165 kPa, x = 40%, ts,in = 46 °C/54 °C, tcw = 33 °C, qcw = 44L/h, do = 2.8 mm) c, d: The effect of Rev and Res on DAH.

shown in Fig. 9 that the churn flow and slug flow occupy nearly twothird of the absorption tube in Fig. 9(a) while they only occupy a little more than one-third of the absorption tube in Fig. 9(b). The entire phenomenon directly results in the decreased AH with absorption pressure changing from 165 kPa to 180 kPa. Based on the still photographs and experimental movies, the experimental data of the noticeable influence of absorption pressure on AH is plotted in Fig. 10. Fig. 10 (a) shows the changes of AH versus to the vapor volumetric flow rate under different absorption pressures. It can be found from the figure that the absorption pressure affects significantly on the absorption height. The AH will decrease obviously with the absorption pressure rising. When the vapor volumetric flow rate is 480 L/h and the absorption pressure is 165 kPa, the measured AH is 630 mm which equals just to the bubble absorption tube length. However, under other operating parameters being equal, the AH is only 540 mm with the solution flow rate of 4 L/h and 490 mm with the solution flow rate of 6 L/h when the absorption pressure reaches to 180 kPa. Fig. 10 (b) depicts the variations of DAH with respect to p under different Revs. It shows that the DAH decreases approximately in straight line with the increasing of p at the same Rev. The reason is that: when the

Fig. 7. Photo of two-phase flow pattern comparisons in the GBA [Operation conditions: (p = 165 kPa, ts,in = 54 °C, qs = 4L/h, qv = 480L/h, tcw = 33 °C, qcw = 44L/h, do = 2.8 mm) x = 40% (a), 45% (b)].

occupies almost a third of absorption tube as presented in Fig. 9(a) and the slug flow pattern also occupies almost a third of absorption tube. As the absorption pressure is raised up to 180 kPa, the location of vapor slugs generated is obviously advanced and the churn flow pattern occupies only one-sixth of the absorption tube as shown in Fig. 9(b). It is 7

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Fig. 8. a: The AH versus vapor volumetric flow rate at different inlet mass concentrations and volumetric flow rates of absorption solution (p = 165 kPa, ts,in = 54 °C, tcw = 33 °C, qcw = 44 L/h, do = 2.8 mm) b: The effect of x on DAH at different Revs.

Fig. 9. Photo of two-phase flow pattern comparisons in the GBA [Operation conditions: (x = 40%, ts,in = 54 °C, qs = 4 L/h, qv = 480L/h, tcw = 33 °C, qcw = 44L/h, do = 2.8 mm) p = 165 kPa (a), 180 kPa (b)].

Fig. 11. Photo of two-phase flow pattern comparisons in the GBA [Operation conditions: (p = 165 kPa, x = 40%, ts,in = 54 °C, qs = 4 L/h, qv = 480 L/h, tcw = 33 °C, qcw = 44 L/h) do = 1 mm (a), 2.8 mm (b)].

Fig. 10. a: The AH versus vapor volumetric flow rate at different absorption pressures and volumetric flow rates of absorption solution (x = 40%, ts,in = 54 °C, tcw = 33 °C, qcw = 44 L/h, do = 2.8 mm) b: The effect of p on DAH at different Revs.

8

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absorption pressure rises under the same inlet temperature, the saturated concentration of the R124-NMP absorption solution will also increase. The increasing of the saturated concentration of the R124-NMP absorption solution and the constant solution inlet mass concentration lead to the mass transfer impetus between the vapor and the absorption solution augment. Then the AH decreases.

makes the AH lessen. In order to see the impact intuitively, the experimental data of the effect of orifice diameter on AH is plotted in Fig. 12. Fig. 12 (a) presents the variations of AH versus to the vapor volumetric flow rate at orifice diameters of 1 mm and 2.8 mm. From the figure, it can be found that the orifice diameter affects the AH significantly. As can be seen, the Fig. 12 resembles the Fig. 10 (a) in the tendency of absorption height. The AH will decrease obviously with the orifice diameter reducing. Fig. 12 (b) reflects the variations of DAH with respect to Rev at different dimensionless orifice diameters ( d0 ). It depicts that at the same vapor flow rate, reducing dimensionless orifice diameter results in a steep increase in nozzle inlet Rev. The higher nozzle inlet Rev brings tremendous disturbances which is beneficial for absorbing refrigerant vapor. Therefore, the smaller the dimensionless orifice diameter is, the lower the DAH is. Fig. 12(a) depicts that when the vapor volumetric flow rate is 480 L/h and the orifice diameter is 2.8 mm, the measured AH was 630 mm which equals just to the length of absorption tube. With further increase in the vapor volumetric flow rate, the R124 vapor can’t be absolutely absorbed and departs from absorption tube in the form of bubble or slug. However, under other operating parameters being equal, the AH is 530 mm with solution flow rate of 4 L/h and 500 mm with solution flow rate of 6 L/h when the orifice diameter decreases to 1 mm. As seen in Fig. 12(a), when the orifice diameter is 1 mm, the AH reaches up to the length of absorption tube at the vapor volumetric flow rate is 560 L/h. Only from the view of reducing absorption height, smaller orifice diameter is beneficial for bubble absorber. For VARS, when the vapor flow rate keeps constant, reducing the orifice diameter leads to an increase of flow resistance. That results in either the absorption pressure decreasing when the evaporation pressure keeps constant, or the evaporation pressure increasing when the absorption pressure keeps constant. For the former situation, it will reduce the solution concentration of bubble absorber outlet and the bubble absorber capacity. For the latter situation, it will bring a deterioration of the refrigeration effect. Both those two will reduce the COP of VARS. Therefore, the suitable orifice diameter should be selected in the design of a bubble absorber.

4.5. The effect of orifice diameter on flow patterns and AH For the purpose of exploring the effect of orifice diameter on flow patterns and AH, two different orifice diameters, 1 mm and 2.8 mm, are selected in this work. Fig. 11 depicts two sets of photographs which are captured under the conditions which solution inlet mass concentration, temperature and volumetric flow rate, vapor volumetric flow rate, absorption pressure, are set as 40%, 54 °C, 4 L/h, 480 L/h and 165 kPa respectively, while the orifice diameters are 1 mm (Fig. 11(a)) and 2.8 mm (Fig. 11(b)), respectively. As illustrated in Fig. 11, the orifice diameter has a distinct influence on flow patterns and AH. When the orifice diameter is 2.8 mm (Fig. 11(b)), the three flow patterns are observed. Similar to the Fig. 11(b), the three typical flow patterns are also observed when the orifice diameter is replaced with 1 mm shown in Fig. 11(a). Comparing Fig. 11(a) with Fig. 11(b), as the orifice diameter changing from 2.8 mm to 1 mm, the conspicuous variations can be observed. Firstly, the length of churn flow pattern increases obviously and the first half of churn flow is mainly consisted of massive tiny bubbles. Secondly, the slug flow pattern has a shorter slug tail and the shorter slug tail is quickly absorbed which indicates a higher absorption rate, and then the slug flow converts into bubble flow. Finally, the AH greatly reduces. A plausible explanation for the changing of flow patterns and reducing of AH can be given considering the violent vapor disturbance and the mass transfer area between R124 vapor and absorption solution. When the vapor volumetric flow rate remains constant and the orifice diameter decreases, the vapor speed at the nozzle exit suddenly increases. That can generate a stronger vapor disturbance which makes the length of churn flow pattern extend and also makes the vapor column at the nozzle exit be broken up into massive tiny bubbles as can be observed in Fig. 11 (a). It extremely enlarges the contact area between the vapor and absorption solution. In another word, the mass transfer area between the vapor and absorption solution increases. The slug flow has a shorter slug tail which can be attributed to the excellent absorption effect in churn flow. The combined role of the strengthen vapor disturbance and enlarged mass transfer area

4.6. The effect of solution inlet temperature on flow patterns and AH Absorption temperature is considered to be an important operating parameter affecting the performance of absorber. The effect of solution

Fig. 12. a: The AH versus vapor volumetric flow rate at different orifice diameters and volumetric flow rates of absorption solution (p = 165 kPa, x = 40%, ts,in = 54 °C, tcw = 33 °C, qcw = 44 L/h) b: DAH versus Rev at different dimensionless orifice diameters. 9

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Fig. 15. Photo of two-phase flow pattern comparisons in the GBA [Operation conditions: (p = 165 kPa, x = 40%, ts,in = 54 °C, qs = 4 L/h, qv = 400 L/h, tcw = 33 °C, qcw = 44 L/h, do = 2.8 mm) Working fluids: R124-DMAC (a), R124-NMP (b)].

Fig. 13. Photo of two-phase flow pattern comparisons in the GBA [Operation conditions: (p = 165 kPa, x = 40%, qs = 4 L/h, qv = 480 L/h, tcw = 33 °C, qcw = 44 L/h, do = 2.8 mm) ts,in = 36 °C (a), 46 °C (b), 54 °C (c)].

vapor in the absorption tube. So the initial position of slug flow pattern is advanced. Meanwhile, the tail of slug flow pattern lengthens. Fig. 13 also depicts that the AH increases as the solution inlet temperature rises. The experimental effect of solution inlet temperature on AH is drawn in Fig. 14. Fig. 14 (a) presents the variations of AH versus to the vapor volumetric flow rate at different inlet solution temperatures. It illustrates that the AH increases with inlet solution temperatures increase when vapor volumetric flow rate and absorption pressure keep constant. Fig. 14 (b) gives the effect of T on DAH at different Revs. When the Rev keeps constant, DAH increases nearly linearly with T increasing. It can be explained from the prospective in vapor-liquid equilibrium. Under the same pressure, the mass concentration of refrigerant, x, will decrease as the temperature increases. As the solution inlet temperature rises, the saturated concentration of absorption solution reduces under the same absorption pressure. That leads to the mass transfer impetus between the vapor and the absorption solution argument. Therefore, the AH increases.

inlet temperature on flow patterns and AH is researched. Fig. 13 displays three compared photographs which are captured under the conditions which solution inlet mass concentration and volumetric flow rate, vapor volumetric flow rate, absorption pressure and orifice diameter are set as 40%, 4 L/h, 480 L/h, 165 kPa and 2.8 mm respectively, while the solution inlet temperatures are 36 °C (Fig. 13(a)), 46 °C (Fig. 13(b)) and 54 °C (Fig. 13(c)), respectively. As shown in Fig. 13, the higher solution inlet temperature isn’t beneficial for solution absorbing refrigerant vapor. The three flow patterns become more and more obvious with solution inlet temperature increasing. When the solution inlet temperature is 36 °C (Fig. 13(a)), following churn flow, unstable slug flow with short tail is formed approaching the end of one-third of absorption tube and the short tail is swiftly absorbed. Then the bubble flow is formed in the intermediate section. When the solution inlet temperature is changed to 46 °C (Fig. 13(b)), the initial position the slug flow forming is lower than that in Fig. 13(a). The short tail is not timely absorbed like Fig. 13(a), therefore, the slug flow with shorter tail is observed in the intermediate section of Fig. 13(b). When the solution inlet temperature is further improved to 54 °C (Fig. 13(c)), the initial height of the slug flow forming is further decreased by comparison with that in Fig. 13(b). The tail of slug flow becomes longer than that in Fig. 13(a) and (b). The reason of the change of flow patterns is that as the solution inlet temperature increases, the R124 vapor cannot be absorbed timely, which leads to the gather of

4.7. The comparison with the flow patterns and AH for R124-DMAC and R124-NMP Fig. 15 depicts two sets of photographs which are captured under the conditions which solution inlet mass concentration, temperature and volumetric flow rate, vapor volumetric flow rate, absorption pressure and orifice diameter are set as 40%, 54 °C, 4 L/h, 400 L/h,

Fig. 14. a: The AH versus vapor volumetric flow rate at different inlet temperatures and volumetric flow rates of absorption solution (p = 165 kPa, x = 40%, tcw = 33 °C, qcw = 44 L/h, do = 2.8 mm, qs = 4L/h) b: The effect of T on DAH at different Revs. 10

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respectively. As can be seen in Fig. 16, the tendency of AH for R124DMAC versus to vapor volumetric flow rate at different solution volumetric flow rate is similar to that of R124-NMP. As the vapor volumetric flow rate increases or the solution volumetric flow rate decreases, the AH for both binary systems increases. It also can be seen from Fig. 16, the AH of R124-NMP working pair is always lower than that of R124DMAC under the same absorption conditions. It indicates that the absorption effect for R124-NMP in vertical bubble absorber is better than that for R124-DMAC under the same working conditions. 5. Correlations of absorption height (AH) and dimensionless absorption height (DAH) 5.1. Correlation of AH Based on the experimental results, the AH correlation is considered including seven parameters which are vapor and solution volumetric flow rate, solution inlet and outlet temperature, orifice diameter, absorption pressure and solution inlet concentration.

Fig. 16. Comparison of AH for R124-NMP and R124-DMAC versus vapor volumetric flow rate at different solution inlet temperatures (p = 165 kPa, x = 40%, tcw = 33 °C, qs = 4 L/h, qcw = 44 L/h, do = 2.8 mm).

a8 4 d a5 pa6 x a7 (t h = a1 qsa2 qva3 tsa, in o s, in s, in + t )

(5)

The coefficients in AH correlation in Eq. (5) were obtained by regression of all the experimental points, using MATLAB software. The following experimental correlation was presented for R124-NMP mixture with ± 20% error band.

165 kPa and 2.8 mm respectively, whilst the working fluids are R124DMAC (Fig. 15(a)) and R124-NMP (Fig. 15(b)), respectively. Comparing Fig. 15(a) with Fig. 15(b), it is obvious that absorption effect of GBA employing R124-NMP as working fluid is much better than that employing R124-DMAC under the same absorption conditions. It mainly reflects in the following aspects, (1) For R124-DMAC (Fig. 15(a)), R124 vapor can’t be absolutely absorbed and slug bubble is observed at the outlet of GBA. Therefore, only the churn and slug flow patterns are captured in GBA. Different from that, for R124-NMP (Fig. 15(b)), R124 vapor can be absolutely absorbed and no vapor is found at the outlet of GBA. Thus, three typical flow patterns can be found in the GBA. (2) The churn flow of R124-DMAC system occupies more than one third of absorption tube height and slug flow occupies the remaining absorption tube. But for R124-NMP system, the churn flow takes up only one ninth of the absorption tube height, thereafter changes to slug flow and finally changes to bubble flow. Compared with R124-DMAC system, the height of churn and slug flow reduces significantly. The above phenomenon occurs because the saturation concentration of R124-NMP solution is higher than that of R124-DMAC solution under the same absorption pressure and temperature, which causes a larger absorption driving potential between the vapor and the R124-NMP solution. When the GBA is feed with the same amount vapor, the R124 vapor can be absorbed timely by R124-NMP solution. That decreases the height of churn and slug flow and thus results in the bubble AH decreasing. For VARS, the refrigerant vapor flow rate depends on the refrigerating capacity. When the refrigerating capacity of VARS is fixed, employing R124-NMP as working fluid instead of R124-DMAC has the following advantages: (1) The solution pump work can be decreased which can be attributed to the reduction of the absorption solution flow rate. Then the COP of system can be improved. (2) The height of the vertical absorption tube can be shortened, which shrinks the size of the vertical bubble absorber. That makes the VARS fit the narrow space to meet the technical requirements for automobile application. The AH using R124-NMP is lower than that using R124-DMAC under the same operating conditions which can be verified by the Fig. 16. In Fig. 16, the AH measured with R124-NMP is compared with that of R124-DMAC taken by the previous work of our laboratory [6]. The comparison is made at the same operating conditions which solution inlet mass concentration and flow rate, absorption pressure and orifice diameter are set as 40%, 4L/h, 0.165 MPa and 2.8 mm

h = 0.02qs

0.19

0.23 qv0.99 ts1.31 p , in do

0.92 x 1.39 (t s, in s, in

+ t)

0.34

(6)

The above experimental correlation of the AH can be employed to predict the bubble AH in the design of R124-NMP bubble absorber. The application ranges of the Eq. (6) are: qs : 2–8 L/h qv : 140–480 L/h xs, in : 40%–50% d 0 : 1–2.8 mm p : 165–180 kPa ts, in : 36–54 °C t : 3–12 °C 5.2. Correlation of DAH From the above discussion of the influence of dimensionless operation parameters on dimensionless absorption height, one DAH correlation is presented:

h = h L = b1Rebv 2Re bs 3

(7)

d 0 b4 pb5 x b6 T b7

The coefficients of the correlation Eq. (7) were also obtained by regression of all experimental points of R124-NMP mixture, using MATLAB software. The following experimental correlation was presented for R124-NMP mixture with ± 15% error band.

h = 8.435 × 10 3Re1.052 Re s 0.262 d01.145 p v

3.434

x 0.49 T 3.886

(8)

It can be seen from Eq. (8) that DAH increases with improvingRe v , d0 , x and T or reducing Res and p. The DAH increases or decreases nearly linearly as the dimensionless operating parameters vary. The reason is that the effect of dimensionless operating parameters on DAH is very straight forward in experimental ranges of operating conditions. The application ranges of Eq. (8) are: Re v : 431.3–3795.3 Res : 70–396 d0 : 0.07–0.2 p: 1.04–1.12 x : 0.69–0.89 T : 1.18–1.29 Fig. 17 shows the experimental correlation of DAH of R124-NMP mixture. It indicates that the dimensionless correlation can predict the DAH of R124-NMP mixture well. Using the presented DAH of R124NMP mixture, the DAH of R124-DMAC mixture was calculated and plotted in Fig. 17. It can be seen that the calculated DAH of R124-DMAC mixture agrees well with experimental results within acceptable error range. In conclusion, the presented correlation of DAH maybe used to predict DAH well in the design of bubble absorber using refrigerantorganic absorbent as working fluid.

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R124-DMAC under the same operating conditions. In another word, under the same absorption conditions, the size of vertical bubble absorber can be reduced by using R124-NMP mixture instead of R124-DMAC. That makes the VARS using R124-NMP fit the narrow space to meet the technical requirements for automobile application. 6. The correlations of AH and DAH are developed for R124-NMP mixture with ± 20% and ± 15% error bands respectively. AH:

h = 0.02qs

0.19

0.23 qv0.99 ts1.31 p , in do

0.92 x 1.39 (t s, in s, in

+ t)

0.34

DAH:

h = 8.435 × 10 3Re1.052 Re s 0.262 d01.145 p v

3.434

x 0.49 T 3.886

The presented correlations of AH and DAH can be used to predict the absorption height for R124-NMP bubble absorption process in a vertical bubble absorber. The correlation of DAH maybe used to predict the absorption height well in the design of vertical bubble absorber employing refrigerant-organic absorbent as working fluid.

Fig. 17. Experimental correlation of the DAH.

6. Conclusions

CRediT authorship contribution statement

In pursuit of reducing the dimension of vertical bubble absorber for better practical utilizations, an experiment is conducted to acquire the flow patterns and AH in the bubble absorption process employing R124-NMP as working fluid. Solution inlet mass concentration, temperature and volumetric flow rate, vapor volumetric flow rate, absorption pressure and orifice diameter are considered as the key absorption parameters and converted into dimensionless operation parameters. The effects of the key absorption parameters on flow patterns and AH are studied. Meanwhile, the effects of dimensionless operation parameters on DAH are researched. Moreover, the flow patterns and AH of R124-NMP are compared with that of R124-DMAC taken by the previous work of our laboratory [6]. Main results can be summarized as follows:

Wei Wang: Writing - original draft, Writing - review & editing, Investigation, Formal analysis. Shiming Xu: Conceptualization, Supervision, Funding acquisition, Resources. Xi Wu: Validation, Supervision, Project administration. Dongxu Jin: Supervision, Software, Data curation, Validation. Mengnan Jiang: Methodology, Resources, Supervision. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements

1. With a lower R124 vapor volumetric flow rate, only bubble flow pattern is observed under certain stable absorption conditions. The R124 vapor can be easily absorbed. With increase in vapor volumetric flow rate, bubble flow pattern is captured only but the diameter of bubble in bubble flow pattern enlarges. With further improving volumetric flow rate, flow patterns have three castes for coexisting and observing: churn, slug, bubble flow patterns. 2. The effects of key absorption parameters on flow patterns focus on the length of per flow pattern. When the variation of key absorption parameters is beneficial for absorbing, the R124 vapor inlet can be timely absorbed and the length of per flow pattern decreases. Conversely, when the variation of key absorption parameters is not beneficial for absorbing, the length of per flow pattern increases. 3. The effects of key absorption parameters on AH are essentially through the effects of key absorption parameters on flow patterns. The AH increases with improving solution inlet mass concentration and temperature, vapor volumetric flow rate and orifice diameter and decreases with improving solution volumetric flow rate and absorption pressure. 4. The variations of DAH with respect to dimensionless operation parameters correspond to the variations of AH with respect to the key operation parameters. The DAH varies almost linearly with the dimensionless operation parameters. The reason is that the effect of dimensionless operation parameters on DAH is very straight forward in experimental ranges of operating conditions. 5. The AH employing R124-NMP mixture evidently decreased varying the vapor/solution volumetric flow rates by comparison with R124DMAC. The compared results illustrated that the performance of vertical bubble absorber using R124-NMP is superior to that using

The authors sincerely acknowledge the financial support from the National Natural Science Foundation of China (51376032, 51776029). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.applthermaleng.2019.114842. References [1] Ying Pan, Shuangchen Ruan, Xu. Gang, et al., Development of the refrigerant airconditioner driven by exhaust heat from automotive engine in large and mediumsized bus, Fluid Machinery 3 (2012) 76–79. [2] A.A. Manzela, S.M. Hanriot, L. Cabezas-Gómez, et al., Using engine exhaust gas as energy source for an absorption refrigeration system, Appl. Energy 87 (4) (2010) 1141–1148. [3] İ. Hilali, M.S. Söylemez, On the optimum sizing of exhaust gas-driven automotive absorption cooling systems, Int. J. Energy Res. 32 (7) (2008) 655–660. [4] S.S. Mathapati, M. Gupta, S. Dalimkar, A study on automobile air-conditioning based on absorption refrigeration system using exhaust heat of a vehicle, Int. J. Eng. Res. General Sci. 2 (4) (2014). [5] D.C. Hoysall, G. Srinivas, Investigation of a serpentine micro-pin fin heat and mass exchanger for absorption systems, Int. J. Refrig (2018). [6] Xu. Shiming, Mengnan Jiang, Hu. Junyong, et al., Visual experimental research on bubble absorption in a vertical tube with R124–DMAC working pair, Exp. Therm Fluid Sci. 74 (2016) 1–10. [7] S. Xu, J. Li, F. Liu, An investigation on the absorption–compression hybrid refrigeration cycle driven by gases and power from vehicle engines, Int. J. Energy Res. 37 (12) (2013) 1428–1439. [8] J. Castro, C. Oliet, I. Rodríguez, et al., Comparison of the performance of falling film and bubble absorbers for air-cooled absorption systems, Int. J. Therm. Sci. 48 (7) (2009) 1355–1366. [9] Y.T. Kang, A. Akisawa, T. Kashiwagi, Analytical investigation of two different

12

Applied Thermal Engineering 168 (2020) 114842

W. Wang, et al.

[10] [11] [12] [13] [14] [15] [16] [17] [18]

absorption modes: falling film and bubble types, Int. J. Refrig. 23 (6) (2000) 430–443. B.M. Fronk, S. Garimella, Condensation of ammonia and high-temperature-glide ammonia/water zeotropic mixtures in minichannels-Part I: Measurements, Int. J. Heat Mass Transf. 101 (2016) 1343–1356. K.S. Sujatha, A. Mani, S.S. Murthy, Analysis of a bubble absorber working with R22 and five organic absorbents, Heat Mass Transf. 32 (4) (1997) 255–259. K.S. Sujatha, A. Mani, S.S. Murthy, Finite element analysis of a bubble absorber, Int. J. Numer. Meth. Heat Fluid Flow 7 (7) (1997) 737–750. K.S. Sujatha, A. Mani, S.S. Murthy, Experiments on a bubble absorber, Int. Commun. Heat Mass Transf. 26 (7) (1999) 975–984. M. Suresh, A. Mani, Heat and mass transfer studies on R134a bubble absorber in R134a/DMF solution based on phenomenological theory, Int. J. Heat Mass Transf. 53 (13–14) (2010) 2813–2825. M. Suresh, A. Mani, Experimental studies on bubble characteristics for R134a–DMF bubble absorber, Exp. Therm Fluid Sci. 39 (39) (2012) 79–89. M. Suresh, A. Mani, Experimental studies on heat and mass transfer characteristics for R134a–DMF bubble absorber, Int. J. Refrig. 35 (4) (2012) 1104–1114. M. Suresh, A. Mani, Heat and mass transfer studies on a compact bubble absorber in R134a-DMF solution based vapor absorption refrigeration system, Int. J. Refrig. 36 (3) (2013) 1004–1014. J. Li, S. Xu, The performance of absorption–compression hybrid refrigeration driven

[19] [20] [21] [22] [23]

[24] [25]

13

by waste heat and power from coach engine, Appl. Therm. Eng. 61 (2) (2013) 747–755. M.N. Jiang, S.M. Xu, J.Y. Hu, W. Wang, X. Wu, Visual experiment on evolution of flow pattern of R124-DMAC bubble absorption process in vertical tube, CIESC J. 66 (7) (2015) 2433–2441. S.M. Xu, M.N. Jiang, J.Y. Hu, W. Wang, X. Wu, Working parameters affect the bubble absorption capacity of R124/DMAC in a vertical tube, J. Tianjin Univ. (Sci. Technol.) 49 (8) (2016) 855–862. M. Jiang, S. Xu, X. Wu, et al., Heat and mass transfer characteristics of R124-DMAC bubble absorption in a vertical tube absorber, Exp. Therm Fluid Sci. (2016). M. Jiang, S. Xu, X. Wu, Experimental investigation for heat and mass transfer characteristics of R124-DMAC bubble absorption in a vertical tubular absorber, Int. J. Heat Mass Transf. 108 (2017) 2198–2210. S. Xu, W. Wang, X. Wu, et al., Measurement and correlation of vapor-liquid equilibrium for R124 (1-chloro-1,2,2,2-tetrafluoroethane)-NMP (N-Methyl-2-pyrrolidone) and R124-DMF (N, N-dimethylformamide) mixtures, J. Chem. Eng. Data (2017). I. Borde, M. Jelinek, N.C. Daltrophe, Working fluids for an absorption system based on R124 (2-chloro-1,1,1,2,-tetrafluoroethane) and organic absorbents, Int. J. Refrig. 20 (4) (1997) 256–266. H. Ma, J.B. Wang, Instrument Accuracy Theory, second ed., Beihang University Press, 2014 (in Chinese).