An experimental study of flow boiling heat transfer of zeotropic mixture R32/R134a in a microchannel heat exchanger

This paper presents experimental data on heat transfer of a binary zeotropic mixture of refrigerants R32/R134a in a microchannel heat exchanger with a high specific surface within a range of parameters that is practically important for the development of cooling systems for microelectronics and space technology. The experiments were carried out in a horizontal heat exchanger with one-sided heating of a copper microchannel plate 20x40 mm, containing 21 rectangular microchannels with a cross-section of 335x930 μm, within the range of mass fluxes from 80 to 250 kg/m2s, and at an absolute pressure in the system ranged from 12 to 14 bar. A zeotropic mixture of refrigerants R32/R134a with a molar concentration of the initial mixture of 65%/35% was used as a working fluid. Experimental data were compared with model-based calculations that take into account the influence of changes in the concentrations of components in the liquid and gas phases.


Introduction
Flow boiling heat transfer in microchannels is a highly efficient thermal management technology that provides cooling of electronic components with high power and heat flux [1]. On the other hand, multicomponent refrigerants are widely used in refrigeration and heat pump applications. Using mixture as working fluid is driven by the inevitable rejection of the widely used HFC refrigerants and the lack of adequate alternative refrigerants. It should be noted that most refrigerant mixtures are zeotropic. It is well known that using a zeotropic mixture as a working fluid instead of a pure fluid causes significant deterioration in heat transfer [2]. Thermal diffusion resistance has a significant effect on heat transfer, especially nucleate boiling. This is due to the higher resistance to mass transfer caused by the temperature slip of the mixture, hence the suppression of nucleate boiling. But taking into account the reasonable operating temperature range of electronic components, zeotropic mixtures of refrigerants have played an important role in the selection of electronic coolants, as far as, can increase the critical heat flux during flow boiling. [3]. Over the past decades, numerous correlations have been developed for calculating heat transfer during flow boiling of a mixture; however, most have been developed based on limited experimental data from one or more research projects. Such correlations usually do not provide accurate predictions for a wide range of operating conditions and require verification [4].
The purpose of the present study is to obtain experimental data on flow boiling heat transfer of a binary zeotropic mixture of R32/R134a refrigerants in a microchannel heat exchanger with a high specific surface area within a range of flow parameters that is practically important for the development of cooling systems for microelectronics and space technology, as well as to compare the  [5] which takes into account the effect of changes in the temperature slip and the concentrations of the liquid and gas phase components on heat transfer.

Experimental equipment and data processing
The experiments were carried out in a horizontal heat exchanger with one-sided heating of a copper microchannel heat sink 20x40 mm, containing 21 rectangular microchannels with a cross-section of 335x930 μm. A zeotropic mixture of R32/R134a refrigerants was used as a working fluid at a molar concentration of components of 65% / 35%, at an absolute pressure in the system ranged from 12 to 14 bar and mass fluxes ranged from 80 to 250 kg/m 2 s. The schematic diagram of the experimental setup and measurement procedure are described in detail in [6].
During experiments, a single-phase mixture with a temperature 1-2 degrees below the boiling point Tb was fed to the inlet of the heat exchanger. Heat transfer coefficient h was determined as here qw is the average wall heat flux; <Tw> is the average wall temperature; Tin and Tout are the inlet and outlet flow temperature.
The measured pressure and temperature at the outlet Tout of the heat exchanger were used to determine the molar concentrations of the components in the liquid phase xi and the gas phase yi. For the calculation, the equation of state of the R32/R134a binary mixture from [7] was used. The change in the composition of the mixture in the liquid phase was used to determine the temperature slip during the experiments. The temperature difference between the dew point Td and the boiling point Tb in dependencies on the R32 molar concentration is present in figure 1. To calculate the thermophysical properties of the vapor and liquid phases of a binary mixture, we used the properties of pure components and the method described in [8]. To calculate the heat transfer of a binary mixture, a model from [5] was used. The heat transfer coefficient was calculated as Here,  is the aspect ratio of channels. The mixture factor Smix was calculated as ( )

Results and Discussion
The obtained dependencies of the average heat transfer coefficient on the average heat flux on the inner wall of the microchannels for mass fluxes of 85 and 200 kg/m 2 s are shown in Fig. 2a. The heat transfer coefficients increase with an increase in the heat flux and are practically independent of the mass flux of the mixture, which indicates that in the case under study, nucleate boiling is the dominant mechanism of heat transfer. At heat fluxes at the walls above 50 kW/m 2 , a decrease in the heat transfer growth with an increase in the heat flux is observed, which indicates the suppression of nucleate boiling in microchannels with an increase in the vapor phase velocity. At a heat flux of 80 kW/m 2 in experiments with a mass flux of 85 kg/m 2 s, a heat transfer crisis was observed, and for a mass flux of 200 kg/m 2 s, a complete suppression of nucleate boiling and a transition to the convective mechanism of heat transfer was observed. The dependence of the average heat transfer coefficient on the output vapor quality presented in Fig. 2b for the data shown in Fig. 2a reveals that a heat transfer crisis occurs when the liquid is completely evaporated in the microchannels. It should be noted that, at a low mass flux of 85 kg/m 2 s, an increase in the heat transfer coefficients is observed before the crisis, which indicates the absence of the development of dry regions due to poor evaporation of the heavy boiling component and a significant contribution of the evaporation of thin films to heat transfer. Comparison of the heat transfer coefficients depending on the heat flux, calculated according to equation (2) for a mass flux of 85 kg/m 2 s is shown in Figure 3a, and one for a mass flux of 200 kg/m 2 s is shown in Figure 3b. The calculation is in good agreement with the experimental data for heat fluxes up to 50 kW/m 2 . At higher heat fluxes, increasing the heat transfer coefficients with the heat flux increasing decreases, and the calculations exceed the experimental data regardless of the mass fluxes, which indicates the need to take into account the suppression of nucleate boiling in calculations.