NATURAL-CONVECTION HEAT-TRANSFER FROM A VERTICAL PLATE .1. ENHANCEMENT WITH GAS INJECTION

A natural convection heat transfer experiment in mercury was conducted with gas injection in a vertical enclosure heated on one face at constant heat flux and cooled on the opposite face. Nitrogen gas bubbles were injected from a row of hypodermic tubes facing upwards along the face of the heated plate. The range of the applied heat flux was 370 less-than-or-equal-to q'' less-than-or-equal-to 16000 W m-2, which corresponds to a modified Boussinesq number range of 10(5) less-than-or-equal-to Bo(x)* less-than-or-equal-to 10(9). The gas injection rate range was 0.9 less-than-or-equal-to Q(g) less-than-or-equal-to 9.2 cm3 s-1. Local heat transfer and void fraction measurements were made with thermocouple and double-conductivity probes, respectively. The measured heat transfer coefficient at low heat fluxes with gas injection, where the free convective flow was mostly laminar along the heated plate, was enhanced at least two-fold. In comparison, at higher heat fluxes, where the flow was predominantly turbulent, gas injection enhanced the heat transfer coefficient very little even at the highest injection rate. The overall enhancement in the heat transfer coefficient due to the bubble-induced turbulence is a collective action of both the induced liquid motion by the bubbles and the turbulence created in their trails. Void fraction measurements at both low and high heat fluxes indicated a three layer boundary layer structure distinct from the thermal boundary layer. The layers are: a single-phase inner layer next to the heated wall, a two-component layer where most of the bubbles reside, and an outer single-phase layer extending to the core of the cell. An analysis of our mercury heat transfer data with zero gas injection indicates that the transition to turbulence occurs at a Rayleigh number of about 2 x 10(7) rather than 10(9), the accepted value.