Prethodno priopćenje
Effect of pressure on free convection heat transfer from a horizontal cylinder at constant wall temperature
Mozaffar Ali Mehrabian
; Mechanical Engineering Department, Shahid Bahonar University of Kerman, Kerman, N
Sažetak
Free convection heat transfer from a horizontal solid cylinder is examined by experimental methods when the pressure of the surrounding air is varied. The surface temperature of the cylinder is maintained at a uniform temperature using the electric current. The cylindrical element is suspended horizontally in a steel pressure vessel. The vessel is charged with air at a wide range of pressures. The objective of this paper is to show that as the pressure approaches zero, the free convective heat transfer approaches a limiting non-zero value. The surface temperature of the element is measured by a thermocouple at the mid point. The element is sufficiently remote from the walls of the vessel to give a substantially free convection. The temperature of the atmosphere in which the element is suspended is taken as equal to that of the vessel and is measured by a thermocouple in the vessel wall. Experimental measurements are carried out at a number of pressures when the voltage and current (electric power)
are held constant. It is observed that the data of temperature difference between the element and the vessel as well as the data of mean temperature with respect to the fourth root of pressure fall approximately on a straight line. This, supports the idea of extrapolating the temperature difference to near zero pressure. The method of extrapolation to near zero pressure is repeated for a variety of electric power inputs and the data of temperature difference at near zero pressure are obtained. It is also observed that at a given power input, the temperature difference between the element and the vessel decreases as the pressure is increased. The method of extrapolation is extended to high pressures to find the pressure at which the temperature difference between the element and the vessel approaches zero at a given power input. The data are then compared with standard convective heat transfer correlations in terms of dimensionless groups.
Ključne riječi
heat transfer, heated cylinder, radiation, Nusselt number, pressure
Hrčak ID:
316074
URI
Datum izdavanja:
30.6.2003.
Posjeta: 38 *