Flow boiling heat transfer, pressure drop and flow patterns of the environmentally friendly refrigerant R1234yf for cooling avionics

Efficient cooling technology is a major breakthrough in the race to develop integrated avionics. Although airborne vapor compression systems (VCSs) are widely used in aircraft, they are only suitable for cooling antifreeze, which is also used to exchange heat with avionics. To this end, integrating avionics with VCSs will not only reduce the weight and the size of the cooling system but also enables heat dissipation via flow boiling of refrigerant in the cold plate.

Presently, R134a is the common working medium in most airborne VCSs. However, it has a higher global warming potential, making it unsuitable for future related applications. With the increasing need to protect the environment, using refrigerants with low global warming and ozone-depletion potential has become an urgent issue. Environmentally friendly refrigerants include natural media like CO₂, hydrocarbons (HCs) and hydrofluoroolefins (HFOs). In particular, a type of HFOs, namely R1234yf, has drawn research attention owing to its significantly low global warming potential.

Generally, convective and nucleate boiling are the main factors influencing the flow boiling heat transfer characteristics of R1234yf. However, the contributions of these mechanisms are still not clear due to inconsistent experimental results involving flow patterns, heat transfer coefficients (HTCs) and frictional pressure drops (FPDs). While flow boiling technology is a promising approach for solving the heat dissipation challenge in avionics, there are limited studies on the flow boiling behavior of environmentally friendly refrigerants. Therefore, further investigation to enrich the understanding of flow boiling mechanisms, correlations and characteristics of such refrigerants is highly desirable.

Herein, Yu Xu, Zihao Yan and Ling Li from Nanjing University of Aeronautics and Astronautics conducted horizontal flow boiling experiments for cooling avionics using refrigerant R1234yf. Flow boiling HTCs, FPDs and flow patterns in a 1.88 mm circular copper minichannel were investigated under heat fluxes, saturated pressures, mass fluxes and vapor qualities in the ranges of 40–65 kW m^-2, 0.6–0.8 MPa, 400–870 kg m^-2s^-1 and 0.05–1, respectively. Furthermore, experiments using R134a were conducted under the same conditions for comparison purposes. The work is currently published in the journal, Applied Thermal Engineering.

The authors identified five typical flow patterns, namely, annular, throat-annular, slug, churn-annular and annular-mist flows, under the present conditions, with no major difference between the R134a and R1234yf flow patterns. Convective and nucleate boiling dominated the regions in the higher and lower vapor quality, respectively, with a vapor quality threshold of about 0.4. Both R134a and R1234yf reported an increase in HTC with an increase in mass flux, saturation pressure and heat flux. In contrast, the FPD remained constant with heat flux but increased with increasing mass flux and decreased with increasing saturation pressure.

Although the flow boiling characteristics of R1234yf exhibited similar trends to those of R134a, the HTC and FPD of R1234yf were approximately 10% and 20%, respectively, smaller than that of R134a. The differences in HTC and FPD were mainly due to the differences in their thermophysical properties.

In summary, Yu Xu et al. investigated the flow boiling HTCs, FPDs and flow patterns of R1234yf. By comparing the experimental results with 12 and 10 existing correlations of two-phase FPD and HTC, respectively, new correlations with mean absolute deviations of 9.3% and 6.3% for flow boiling HTC and FPD of the R1234yf were proposed. In a joint statement to Advances in Engineering, the authors noted that their findings would contribute to designing environmentally friendly and high-performance refrigerant R1234yf for cooling avionics.