Flow Boiling In Different Flow Regimes Under Transient Heat Flux

摘要: Flow boiling has great potential of becoming the prevalent method in various heat transfer applications, because it combines a high heat transfer rate with a relatively uniform surface temperature. The majority of the previous studies on flow boiling heat transfer applied a constant heat flux as boundary condition. However, many of the possible applications are subject to heat fluxes that vary with time, for example the cooling of electronics dissipating a non-constant heat flux or the heating of the working fluid in a concentrated solar power system which is dependent on the irradiation of the sun. Therefore, this study focusses on the effect of transient heat fluxes, more specifically a square waveform profile transient heat flux, on flow boiling of R245fa in a horizontal tube with an inner diameter of 17.12mm. The heat flux was applied by a constantan resistance wire wrapped evenly around the outside of the test tube. During each test, the average dissipated power was 650W, corresponding to an inner wall heat flux of 12.09kW/m². The oscillation amplitude tested was 30% of this 650W average heat dissipation and the oscillation periods considered were 10s, 20s and 30s. Three different combinations of mass flux and vapor quality led to three different flow regimes that were tested: annular, slug and intermittent flow. It could be concluded from the data that the annular flow regime exhibited the highest heat transfer coefficients. Furthermore, it seemed to depend on the flow regime whether the transient heating case resulted in an improved or declined heat transfer compared with constant heating.