Experimental study of heat transfer in He-I and He-II under stationary heat load

Abstract

to know the critical heat flux qcr and its corresponding thermal head ATcr so as to prevent passage of the superconductor to the normal state. However, data for performing reliable thermal calculations associated with heat transfer in He-I and, expecially He-ll are still inadequate. We made an experimental study of heat transfer from a copper specimen to He-I and He-ll under a stationary heat load. The specimen was made in the form of a 0.005  0.I  0.I m plate of annealed M3 copper whose surface was machined. A heater was mounted on one side of the plate; the surface temperature of the opposite side of the plate was measured by three carbon resistors and the results of the measurements were recorded by a digital voltmeter. The experiments were performed on a horizontally placed plate facing the heater downward. The relative e error in temperature measurement was 2% at 4.2~ and 3% at 2~ A description of the experimental unit and some data are given in [3]. In Fig. la the experimental values of the heat flux from the specimen q to He-I (curve I) and the integral values of the heat transfer coefficient ~ (curve 3 constructed by the equation ~ = q/AT) have been plotted against the thermal head AT = Tw -- Ts (here T w is the wall temperature and T s is the saturation temperature). Typical heat exchange regions (convective, developed bubble boiling, transient, and steady film boiling) occur on experimental curve I of boiling in He-l. For the investigated, relatively massive, copper specimen the first critical density of heat flux qcrl ~8000 W/m = was noted at a thermal head ATcr1~ 0.9~ With the passage into the film boiling region the heat transfer coefficient diminishes, as is also evidenced by the course of curve 3. In [5, 6] less massive specimens were investigated and the first boiling crisis was noted at a critical heat flux density qcr1~ 8000-10,000 W/m 2 and the thermal head ATcrl ~ 0.7-1~ Typical heat exchange regions (Kapitsa resistance, nonfilm boiling, transient, and film boiling) are noted on experimental curve i (Fig. ib) by analogy with the data of [9]. It can be seen that for the investigated specimen a critical heat flux density qcr =2500 W/m 2 was observed at a temperature head ATcr =0.32~ The values obtained can be explained by the experimental conditions (a relatively massive specimen, a rough machined heat-exchanging surface) and, of course, by the mechanism of heat transfer in an He-ll medium. At high heat