The effect of circumferentially non-uniform heat flux on flow boiling heat transfer in a horizontal tube

Abstract

Flow boiling is an important energy transfer mechanism in several thermodynamic power cycles. In direct steam generation concentrated solar power applications, flow boiling occurs under non-uniform heat flux conditions, such as in horizontal tubes which are predominantly heated from below. This can influence the local and average heat transfer coefficient. In this experimental study the heat transfer characteristics of saturated flow boiling under different circumferentially non-uniform heat fluxes were determined. To demonstrate the influence of a non-uniform heat flux, tests were conducted on R245fa in an 8.5 mm inner diameter tube at a mass fluxes of 200 kg/m²s and 300 kg/m²s, at saturation temperatures of 35 °C and 40 °C, for vapour qualities between 0.2 and 0.7, and for an averaged heat flux of approximately 6.8 kW/m2. Eight non-uniform heat flux distributions, including variations of heating from above, from the side and below, were considered and their results were compared against those of a uniform heat flux distribution. In all cases, the same or comparable total heat rate was maintained. It was found that uniform heat flux conditions exhibited the highest inner wall heat transfer coefficients, irrespective of the mass flux, saturation temperature or vapour quality under consideration. Compared to the uniform heat flux cases, non-uniform heat flux conditions exhibited a significantly reduced average heat transfer which became worse at higher vapour qualities. Depending on the heat flux scenario, heat transfer performances reduced by up to approximately 45%, and for conditions where heating was applied from below, a reduction of approximately 25% was observed. Based on the circumferential local heat transfer coefficients, that were solved via an inverse problem model, it was concluded that the applied heat flux distribution needs more attention and research. The overall pressure drop was not affected by the heat flux condition.