An Investigation of Boiling Heat Transfer in Fibrous Porous Media

Abstract

Boiling phenomena are investigated in the presence of porous media composed of ceramic fibers ranging in diameter from 3.0 to 18.5 im for system pressures ranging from 0.18 to 0.28 MPa. Bed porosities range from 0.935 to 0.95, average pore diameters range from 30 to 250 ;m, and permeabilities range from 2 x 10-11 to 6 x 10-9 m 2. For surface temperatures up to 400°C, measured boiling curves for water in the different fiber beds show maximum heat fluxes on the order of 25 W/cm 2. The boiling curves exhibit two distinct regimes-a nucleate-type regime and a constant heat flux regime. In the nucleate-type regime, heat flux is directly related to wall superheat, and the slope is directly related to the average pore diameter of the bed. In the constant flux regime, heat flux is independent of wall superheat, as heat transfer is governed by the rate at which the fiber bed supplies water to the heater surface. The transition region can exhibit a peak heat flux and associated rapid escalation in surface temperature similar to burnout in classical pool boiling. Correlations based on dimensional analysis are presented for both regimes. Data in the constant flux regime are fitted to ± 35%, but data in the nucleate-type regime are only fitted to ± 175%.

Nucleate Pool Boiling Heat Transfer of Refrigerants Using Coated Surfaces

“…Especially, many of the R-22 alternatives azeotropic refrigerant blends and to predict heat transfer coefficients of near azeotropic mixtures, a precise nucleate boiling correlation for pure refrigerants is required immediately. Therefore, a new correlation is developed based upon the present data of three refrigerants following Rudemiller and Lindsay approach [26]. Nucleate pool boiling heat transfer is affected by imposed heat flux, surface specifications, wall superheat, density of liquid and vapor, latent heat of evaporation, characteristic length, other thermophysical properties and their relationship may be summarized in Eq.…”