Influence Of Buoyancy On Heat Transfer Characteristics Of Kerosene Flowing In Tiny Tubes Under Supercritical Pressure

This paper studies the effects of buoyancy on heat transfer of kerosene flowing in tiny rectangular tubes under supercritical pressure through experimental research and numerical simulation combined with theoretical analysis.For kerosene flowing in tubes,temperature of fluid in the near wall region is higher than the mainstream,while the density of near wall region is lower.Difference of density will cause buoyancy because of gravity,thus influencing the heat transfer characteristics.Experiments were done to study the influence of buoyancy on heat transfer of kerosene flowing in horizontal tiny rectangular tubes under supercritical pressure.Temperature of the top wall is higher than the bottom wall while the heat transfer coefficient is lower.This indicates that the low temperature fluid of high density gathers at the bottom region of the tube while the high temperature fluid of low density gathers at the top region of the tube.The relative difference of heat transfer coefficient between top wall and bottom wall decreases with the rise of mass flow rate and increases slowly with the rise of pressure while changes a little with the increase of heat flux.By comparing buoyancy with inertia force and taking into consideration of the properties of kerosene,influence of different parameters on buoyancy's effect on heat transfer are theoretical analysed.Experiments were also done to investigate the influence of buoyancy on heat transfer in tubes of different sizes.Results indicate that the relative difference of heat transfer coefficient between top wall and bottom wall increases with the channel size and geometric aspect ratio.Experiments were done to study the influence of buoyancy on heat transfer of kerosene flowing in vertical tiny square tubes under supercritical pressure.For upward flow,the direction of buoyancy and flow direction is identical while in downward flow is opposite.Heat transfer coefficient in downward flow is higher than upward flow because of buoyancy.The relative difference of heat transfer coefficient between downward and upward flow increases with the increase of channel size.The flow and heat transfer characteristics of kerosene flowing in horizontal tiny rectangular tubes under supercritical pressure are numerically investigated.Results indicate that buoyancy will cause secondary flow and the intensity of secondary flow first increases and then decreases along the flow direction.The velocity peak is in the lower side of the tube center axis.The distribution of temperature,density and velocity asymmetry first increases and then decreases along the flow direction.Numerical simulation are also done to investigate the influence of different parameters on buoyancy's effect on heat transfer.Investigation of geometric aspect ratio and size are studied in horizontal tubes.Intensity of the secondary flow increases with the inrcease of geometric aspect ratio or channel size.The influence of buoyancy on heat transfer characteristics of supercritical kerosene flowing in vertical tiny square tubes are also numerically investigated.