Experimental CHF for low-GWP fluids and R134a. Effect of the L_h/D ratio at low and high mass velocities

摘要

New saturated critical heat flux (CHF) experiments for refrigerants flowing into a multi-microchannel heat sink are presented in this paper. These tests integrate an existing database (Mastrullo et al., 2016) by adding new fluids and operating conditions. The test section consists of an aluminum heat sink heated from the bottom and made up of seven rectangular microchannels, each of them 35 mm long and 2 mm wide. The possible heights are 1.0 and 0.5 mm, whereas the heated lengths are 25 and 35 mm, thus obtaining three different L_h/D ratio of 19, 27 and 44, respectively. R134a and the low-GWP refrigerants R1234ze, R1234yf and R32 are used as working fluids. The mass flux was varied from 146 to 1504 kg/ (m~2 s) and tests were performed at saturation temperatures from 24.7 to 75.5 ℃. With a new operative definition of critical heat flux, the experimental results show that CHF is strongly enhanced with higher mass fluxes, while it is only slightly influenced by the saturation temperature. The effect of L_h/D is a CHF reduction at low mass velocities (G < 500 kg/(m~2 s)), while the trend is reversed for high mass velocities. Finally, the agreement of the present database with some of the most quoted CHF correlations is also tested. The comparison results show that the correlation of Callizo (2010) best fits the whole database and also the low-G data, whereas the predictive methods of Ong and Thome (2011) and Wojtan et al. (2006) work better at higher mass fluxes.