Influence of different circumferential heat flux distributions on flow boiling of R245fa in a circular horizontal tube

Abstract

Direct steam (and vapour) generation in parabolic trough collectors remains practically untapped due to inherent multi-physics challenges. Such difficulties include an understanding of the effect the non-uniform solar flux has on the boiling flow within absorber tubes. For this purpose, steady-state flow boiling of R245fa was experimentally investigated in a 17.12 mm diameter horizontal stainless-steel tube, at a saturation temperature of 40 °C, and mass fluxes of 125 kg/m2s to300 kg/m2s. Various circumferential heat flux distributions were considered at the same total heat rate, including fully uniform heating, bottom half heating, top half heating, side half heating, and a representative parabolic trough collector heating profile. Locally applied heat fluxes ranged from 0.38 kW/m2 to 30.48 kW/m2. It was found that the transition between flow patterns were not affected by the heat flux distribution. A stochastic inverse model based on Bayesian inference was developed to resolve the heat transfer coefficients. The heating profile significantly altered the heat transfer performance in terms of the local and average heat transfer coefficients. Relative to uniform heating, the average heat transfer coefficients ranged between approximately a 144 % enhancement to a 40 % reduction, depending on the heat flux distribution, mass flux, and vapour quality. The parabolic trough collector heating profile had the best average heat transfer coefficient for most conditions, whereas uniform heating demonstrated superior heat transfer coefficients for highly convective dominant flow. Locally resolved heat transfer coefficients identified the dominance of nucleate boiling at the tube’s lower circumference. Existing predictive methods from literature were found unsuitable when applied at the local level. Further investigation is required to determine the relative dominance of convective and/or nucleate boiling at different circumferential positions.