Experimental study on critical heat flux and thermal instability under hypergravity

The critical heat fluxes (CHF) for R245fa flowing inside a horizontal microchannel with diameter of 0.94 mm and heated length of 170 mm under different gravity levels were experimentally studied. Taking the thermal oscillation parameters occurring in the post-dryout region as the research object, which differed from the parameter oscillations during the occurrence of CHF in past literature, and revealing the formation mechanism of thermal oscillations when dryout type CHF occurred, as well as the impact of gravity level on thermal oscillations. The experimental parameters are: mass fluxes of G = 200 and 300 kg/m2 s, inlet pressure of Pin = 211.72 kPa, inlet subcooling of ΔTsub = 5–10 K, and nominal hypergravity of an = 1–1.53 g. The results show that CHF increases with the increase in mass flux, inlet subcooling, and gravity level, which is mainly related to the rates of liquid film evaporation, mainstream droplet entrainment, and droplet deposition onto the wall. The process of post-dryout occurs simultaneously with thermal instability. In addition, the increase in gravity level helps to alleviate the heat transfer deterioration at the test section outlet and suppresses the oscillations in wall temperature and inlet pressure.