Jacqueline Biancon Copetti

Flow Boiling Heat Transfer Characteristics of R600a in Multiport Minichannel

ABSTRACT Evaporators of small and medium refrigeration systems, as in commercial and automobile air conditioning applications, are being studied to develop more compact and lighter equipment, that reaches good thermal performance and reliability, with low pressure drop. In this way, evaporators are being designed with small channels and materials like aluminum. Moreover, different refrigerants are being tested to substitute for hydrofluorocarbon (HFC) refrigerants, with different operational temperatures and pressures. Some of them, like hydrocarbons, although they present advantages with respect to their thermodynamic and transport properties, should be used with small charge in the system due to their flammability. This work presents the results of an experimental study to characterize the flow boiling of the refrigerant R600a (isobutane) in a multiport aluminum extruded tube with 7 parallel minichannels of 1.47 mm hydraulic diameter. The effects of mass velocity, heat flux, and vapor quality on heat transfer were investigated for constant saturation temperature and pressure. Heat fluxes in the range from 5 to 30 kW m−2, mass velocities set to discrete values in the range of 50 to 200 kg m−2 s−1, and saturation temperature of 20°C were considered. It was verified a significant effect of heat flux. Moreover, some images of flow patterns, in different conditions, are presented, and the main patterns identified were slug, intermittent, and annular.

Boiling of R-134a in horizontal mini tube

This work presents the results of an experimental study carried out with R-134a to characterize heat transfer and pressure drop during flow boiling in a horizontal smooth tube of 2.6 mm ID. The experimental tests included (i) heat fluxes in the range from 10 to 100 kW/m2, (ii) the refrigerant mass velocities set to discrete values in the range of 240-930 kg/m2s and (iii) saturation temperatures of 12 and 22°C. The study analyzed the heat transfer through the local heat transfer coefficient along the flow and pressure drop, under the variation of these different parameters. It was possible to observe the significant influence of heat flux on the heat transfer coefficient and mass velocity on the pressure drop, and the effects of saturation temperature. Moreover, it was possible to produce a complete database.