Experimental study on the effect of orientation on flow boiling using R134a in a mini-channel evaporator

The influence of gravity on flow boiling poses a challenge to the development of two-phase thermal systems for space applications. The present study investigates the heat transfer coefficient of flow boiling in a mini-channel evaporator in both horizontal and vertically downward flow directions to observe the effect of gravity. The evaporator has 23 channels that are 624 μm in width and 923 μm in depth. The experiments show that nucleate boiling dominates the heat transfer. The heat transfer and flow patterns for the two directions differ when the mass flow rate is less than 264.3 kg/m2 s and the heat flux is less than 3.0 W/cm2. The experimental results are compared with five general correlations. The outlet Froude number is more suitable for correlating the results than existing gravity-independent criteria. Gravity may influence the heat transfer through instabilities and changes in the flow patterns.