Unveiling impacts of heat flux distribution on forced convection in horizontal evacuated tubes

Integration of evacuated tubes with solar thermal desalination can significantly enhance thermal efficiency and water production. To provide optimization strategies for such desalination systems, 3D numerical simulations were performed to analyze the influence of the heat flux distribution on forced convection in horizontal evacuated tubes in terms of pressure drop, secondary flow, temperature distribution, and Nu. The improved method considered multi-layer wall heat transfer, fluid-solid coupling heat transfer, and temperature dependent fluid properties. Non-uniform heat fluxes significantly affected the friction pressure drop in the laminar regime, but were less pronounced in the transition and turbulent regimes. At low Re, the backward non-uniform heat flux strengthened the secondary flow while the forward non-uniform heat flux suppressed it. The rapid warming of the wall temperature occurred over a short distance due to the difference in thermal conductivity between the fluid and solid domains. The non-uniformity of heat flux distribution moderately affected average fluid temperature. The influence of heat flux distribution on secondary flow and average temperature weakened as Re increased. Furthermore, a correction factor was introduced to quantify the impact of heat flux distribution on Nu. For forward and backward non-uniform heat fluxes, it assumes values of 0.94 and 0.86 respectively.