Investigations of Heat Transfer During Subcooled Flow Boiling of Refrigerants in Minichannels

Companies producing heat exchangers with instrumentation, particularly in the mini-and microscale are dynamically developing. This applies to manufacturers of compact heat exchangers and miniature pumps, for systems which precisely dose chemical reagents. A particular field of interest is the need to discharge or supply an increasing density of heat fluxes generated in electronic circuits. Recently, several studies were carried out on the developed boiling of refrigerants in minichannels. Substantially less studies were conducted on sub-cooled boiling. Therefore, authors undertook experimental investigations in this field. This paper presents the results of experimental investigations of sub-cooled flow boiling in minichannels. The local heat transfer coefficient was determined during measurements of sub-cooled flow boiling of R134a and R404A in 12 horizontal pipes with the internal diameters of: 0.45 divided by 2.30 mm. The local heat transfer coefficient was obtained for heat fluxes up to 90 kW/m(2) with average mass fluxes as follows: 358, 438, 558, 789, 940, 1401 kg/(m(2)s) and 361, 440, 570, 811, 939, 1395 kg/(m(2)s) for R134a and R404A refrigerants respectively. Influence of selected parameters on local heat transfer coefficient was presented. Value of the local heat transfer coefficient alpha in the range of subcooled boiling rapidly increases along with increase of the degree of dryness x. Evident impact of heat flux density q on the local heat transfer coefficient alpha was observed. With increasing heat flux density value of local heat transfer coefficient increases. Influence of mass flux density (w rho) on the value of local heat transfer coefficient alpha, at a constant stream of heat inflowing to reagent q is negligibly small. No relevant influence of the type of refrigerant (R134a or R404A) on the local heat transfer coefficient during subcooled boiling was observed.