AN EXPERIMENTAL INVESTIGATION OF HIGH-FLUX FREE CONVECTION HEAT TRANSFER TO WATER UP TO NEAR-CRITICAL CONDITIONS

Abstract

An investigation was made to increase the basic knowledge of nucleate and film boiling heat transfer to fluids up to their critical pressures. An extensive literature survey of the subject indicated a scarcity of consistent data and an absence of proven methods for analysis. This situation led to the design and construction of experimental apparatus suitable for measuring rates of free convection heat transfer and associated temperature differences for fluid conditions up to 4000 psi and 800 deg F, with provision for visual observation. The apparatus incorporated a stainless steel system for containing the fluid. Ten-mil-diameter cylindrical and 1/8-in.-high vertical-plate platinum test sections were placed in the fluid inside a pressure vessel, 5 in. in inside diameter and 21 in. high. The pressure and temperature of the fluid envirorment were controlled by constant-volume heating of the fluid in the pressure vessel with the aid of seven zones of radiant guard heaters located between the external wall of the pressure vessel and the insulation. Each of these zones was controlled by a differential-thermocouple-sensed circuit that automatically kept the pressure vessel at a prescribed uniform temperature. Fluid pressure was measured with a deadweight tester and a 0-4000-psi Heise gauge. Fluid temperature was measured with a calibrated resistance thermometer and Mueller bridge, purchased specifically for that purpose. The heat flux in the test section was obtained from electrical measurements of the directcurrent power supplied for Joule heating. The temperature of the test section was measured by means of a resistance thermometer. The system was outgassed and filled under a vacuum with water that was deionized, degassed, and deionized again. The quality of the water was maintained during an experiment with the aid of a thermalsiphon deionizing loop which was connected to the system. The results were tabulated and plotted in terms of heat flux and temperature difference between the heated surface and the bulk fluid (water). Nucleate boiling results are included for pressures of 14.7, 1300, 2400, 2800, 2900, 3000, cluded for pressures of 2400, 2800, 2900, 3000, 3100, 3150, and 3180 psia. Results also included for free convection to water at pressures of 3200, 3250, 3300, 3400, 3625, and 3925 psia. The nucleate and film boiling results were correlated and compared with other data. At a particular nucleate boiling heat flux, the heat transfer coefficient was observed to increase with pressure up to about 2800 psia. The characteristics associated with transition film boiling vanished above the critical pressure according to the plotted results and the experimental behavior. In fact, for pressures above the critical, a trend toward the characteristics associated with single-phase free convection to a constant-property fluid was observed. A large convective flow of the fluid and large density gradients accompanied by considerable optical distortion were observed within plus or minus 400 psi of the critical pressure. Near the critical pressure, sheets of fluid appeared to be passing the field of observation. (auth)