Comparative experimental procedures for measuring the local heat transfer coefficient during flow boiling in a microchannel

Most of the microchannel flow boiling experiments are performed using electrical heaters to promote the boiling process. However, there are many heat exchangers applications (e.g. automotive, refrigeration, air-conditioning) in which the vaporization heat is provided by a secondary fluid (water, air...). The paper presents a comparative analysis between two experimental techniques that can be adopted when the heat flux is not imposed by Joule effect heating and hence its direct measurement from electrical parameters is not possible. The two methods are applied to the measurement of the local heat transfer coefficient during flow boiling inside a 0.96 mm diameter microchannel. The first method is the one proposed by Del Col et al. (2013), the second one is presented here for the first time. A key advantage of the new technique is the fact that the heat transfer coefficients can be measured in the dryout zone as well. The two methods are used to reduce new experimental data of flow boiling of R1234ze(E), which is a fluorinated propene isomer and a possible low-GVVP refrigerant to replace R134a in some air conditioning and refrigeration applications. The flow boiling heat transfer coefficient strongly depends on the heat flux, with little effect of vapor quality and mass velocity. Only the adoption of the new technique allows to investigate the post dryout region as well.