DRYOUT DURING FLOW BOILING IN A SINGLE CIRCULAR MINICHANNEL: EXPERIMENTATION AND MODELLING

摘要

This paper presents an experimental investigation on the dryout during flow boiling of R134a and R32 inside a 0.96 mm diameter single circular minichannel. In the present tests, the test channel is not electrically heated; instead, the flow boiling is achieved by means of a secondary fluid (water). Therefore, the heat flux is not uniform in the channel since the temperature of the water varies. The onset of dryout is detected by means of the standard deviation of the temperature readings in the wall. The wall temperature in fact displays larger fluctuations in the zone where dryout occurs, which are related to the presence of a liquid film drying up at the wall with some kind of an oscillating process. These temperature fluctuations are detected by means of the standard deviation in the wall temperature. These temperature fluctuations never appear during condensation tests, neither are present during flow boiling at low vapor qualities. The fluctuations also disappear in the post-dryout zone. Experimental values of dryout quality measured with the above method are reported in this paper at mass velocity ranging between 100 and 700 kg m~(-2)V~(-1) for R134a and between 200 and 900 kg m~(-2)V~(-1) for R32. Since the heat flux is not uniform along the channel, each dryout point is characterized by its own boiling story. Nevertheless, an average value of heat flux can be defined in the channel, with the purpose of comparing it to critical heat flux values in uniformly heated channels. Present experimental data has been compared against some models available in the literature, which provide either the critical heat flux or the dryout quality in microchannels.