Admissible heat fluxes during cooling of a horizontal weakly-inclined surface from below

Abstract

resistance heater. The plate width is b = 0. I m. The experiments were conducted with 18 combinations of the plate length l = 0.3 and 0.5 m and plate thickness ~ = 0.2, 1, and 2 ram and slope angles/3 of up to 9.4 and 13.3" with respect to the horizontal plane. Sheets of thermal insulation consisting of 10 nun thick teflon and 2 mm thick paronite were pressed down again.q the plate. The heat flux through the insulation was equal to qtop/(tsurf -- ta) = 30 W/(m2.K). This construction and the current leads were secured to a trough-shaped framework -- a 1 mm thick plate. The lateral edges of the upside down "trough" were located below the heat-releasing surface (~z = 10 mm) and prevented ma~ transfer through the side boundaries. The plate was placed in a horizontal pressure-bearing cylindrical vessel with dimensions of 219 x I0 mm and 1.6 m long. The vessel was 2-2ched to an external circulation loop, through which the water in the vessel was heated and the pressure was compensated. The elements of the vessel and circulation pump which came into contact with water were made of stainless steel, with the exception of the sheets of the thermal insulation and the copper current leads. The temperature of the plate was measured with 12 KTMS cable chromel-alumel thermocouples separated and pressed to the top surface of the plate through a 10 -2 mm thick layer of mica. The thermocouple outputs through holes drilled in the thermal insulation and the framework as well as the faces were waterproofed with organosilicon lacquer. The water temperature (tw) under the heat releasing surface and the outer surface of the vessel was also measured along the bottom, middle, and top geueratrices. Furthermore, the pressure in the vessel and the electrical power supplied to the plate were fixed. The experiments were conducted in the following range of parameters: pressure p = 0.102-7.5 MPa; t w = 20195"C; tsurf = 40-268"C; At = tsurf -- t w = 20-170"C; and, the specific heat flux to the heat-releasing surface q = 8-186 kW/m 2. For ~ 3 ~ as a result of temperature stratification of the water, only the indications t~ of the surface thermocouples at the top of the plate were used. The heat-transfer coefficients were equal to c~ = q/At = 200-1150 W/(m2-K). The experimental data were analyzed in criteria characteristic for heat transfer by natural convection GrPr = (ql3h,2)/~AtPr; Nu = all),. The physical constants were determined at t = 0.5 (t w + tsurf). The range of variation of the criteria was GrPr = (2-950)-10 l~ Nu = 92-700. Figure la displays plots of such an analysis for two limiting slope angles of the plate 0 and 13.3 ~ A satisfactory generalization of the experimental data (the deviation of 95 % of the points from the generalizing lines is less than + 10%) was achieved with the relation