No abstract

The phenomenon of nature convection boiling on the outer surface of a heated hemispherical vessel surrounded by a thermal insulation structure was investigated experimentally. The objectives were to observe the behavior of the boiling-induced two-phase motion in the annular gap between the vessel and the insulation structure and to determine the flow effect on the rate of boiling. High-speed photographic records revealed the presence of violent cyclic ejection of the vapor masses generated by boiling on the vessel outer surface which resulted in a buoyancy-driven, upward, concurrent two-phase flow through the channel. When boiling was taking place at high heat flux levels, the flow was found to be three-dimensional with swirl formation. Measurements of the local boiling heat fluxes and the local wall superheats were made under steady-state boiling conditions covering the entire range of nucleate boiling with special emphasis on the high-heat-flux regime.

The process of two-phase natural convection in an annular channel formed between a heated hemispherical vessel and a surrounding thermal insulation structure is studied theoretically and experimentally. The flow, being driven by buoyancy, is induced entirely by the boiling process taking place on the downward-facing side of the channel, i.e., on the outer surface of the heated vessel. Results of the flow analysis and the experimental observations indicate that there is a strong interaction between the induced two-phase motion and the boiling process. While the mass flow rate of the induced flow is a strong function of the wall heat flux, the rate of boiling is significant affected by the resulting flow. Relative to the case without thermal insulation, a higher boiling heat transfer is obtained in the present case, evidently due to an enhanced mass flow rate induced by the boiling process with the presence of a thermal insulation structure.