Investigation of boiling behavior in periodic narrowed–expanded mini channel

In engineering applications, heat transfer and two-phase flow studies in mini-channels are maintained in a large scale, especially depending on the development of cooling systems. Two-phase flow control in mini-channels is a great precaution for designing a more efficient system for optimum conditions and dimensions. As known, the improvement of heat transfer in single and two-phase flows is achieved by two methods as active and passive. In this study, the boiling phenomenon was investigated in a rectangular mini channel that periodically narrowed-expanded its geometry. R-134a was used as working fluid. Mini channels were manufactured to be of three different types: H/R = 1 (flat), H/R = 0,8 and H/R = 0,5. The hydraulic diameter (Dh) of the mini channel is 1,83 mm. Experiments have been carried out in three different mass fluxes (G = 100 – 200 – 400 kg/m2s) of the refrigerant, 0 – 180 kW/m2 of heat fluxes (q). The working pressure was fixed at 3 and 4 bar. During the experiments, the inlet temperature of the mini – channel of the refrigerant was kept constant at -1°C. Simultaneously with temperature and pressure measurements, flow boiling regimes are visualized with a high – speed camera. Experimental results have shown that heating surface area has a significant effect on the boiling phenomenon. In the narrowing – expanding channel geometry, the minimum resistance size in the maximum heat transfer surface area is more suitable. The heat transfer coefficient at the high value of the refrigerant rate is high and the outlet quality is low. Key words: mini channel, boiling, narrowed expanded, R-134a