Boiling flow, Heat transfer, Pressure drop, Microfin tube, Small-diameter tube, R1234ze(E)
Currently, the development of high-performance and compact heat exchangers with small- diameter tubes having a hydraulic diameter of less than 5 mm is needed in order to improve the performance of the heat exchanger and to reduce the refrigerant charge for air-conditioning systems. The effects of surface tension and shear stress on boiling heat transfer and flow characteristics become dominant as the tube diameter decreases. In addition, these effects are different from those in conventional-diameter tubes. It is necessary to clarify boiling heat transfer and pressure drop to facilitate the design of evaporators. Furthermore, low global warming potential refrigerants such as HFOs have been attracting attention. However, only limited research is available on the boiling heat transfer and pressure drop of HFO refrigerants in small-diameter microfin tubes. This study experimentally investigated the boiling heat transfer and pressure drop of R1234ze(E) in a horizontal small-diameter microfin tube having 2.5 mm outer diameter and 2.1 mm equivalent diameter. The boiling heat transfer and pressure drop were measured in a mass velocity range of 100–400 kg/(m2s) and heat flux range of 5–20 kW/m2 at a saturation temperature of 15°C. The boiling heat transfer coefficient at the mass velocity of 200 kg/(m2s) exhibited the highest value at the dominant region of tin liquid film evaporation heat transfer. The measured boiling heat transfer coefficient agreed well with previous correlations in only the dominant region of forced convection evaporation. The frictional pressure drop increased with increasing mass velocity and vapor quality. The measured pressure drop agreed well with previous correlations for conventional-diameter microfin tubes.