Experiments on dry-out during flow boiling in a round minichannel

Temperature measurements during flow boiling of R134a in a 0.96 mm single circular channel are reported in order to provide a criterion for the determination of the critical conditions in the channel. The flow boiling heat transfer is obtained by using a secondary fluid; the wall temperature displays larger fluctuations in the zone where dryout occurs. These temperature fluctuations in the wall denote the presence of a liquid film drying up at the wall with some kind of an oscillating process. These temperature fluctuations never appear during condensation tests, neither are present during flow boiling at low vapour qualities. The fluctuations also disappear in the post-critical condition zone. Experimental values of dryout quality measured with the above method are reported in this paper at mass velocity ranging between 300 and 600 kg m-2s-1. In the practical applications of flow boiling, the dryout quality is a key parameter in the two-phase systems for cooling of devices, both for ground and microgravity applications. The test conditions reported here refer to relatively high mass velocities, and are obtained at earth gravity. Nevertheless, since the critical heat flux differences between the two gravity environments decrease with increasing velocity, the present data may also be used for inertia dominated systems at low g.