Abstract
In order to clarify the effect of a buoyancy force on conduction-convection coupled heat transfer in a hollow cylinder,the flow and thermal characteristics were analyzed using an RNG k-εturbulence model.The Reynolds number was fixed at 1.014 ×106,and the Rayleigh number varied from 1.122 × 1010 to 1.088 × 1011.Results have shown that,when considering the effect of an opposed buoyancy force,increasing the Rayleigh number has a positive impact on the rate of change and uniformity of the cylinder temperature.The temperature distributions along the axial and circumferential directions are similar for different Rayleigh numbers,but extreme values differ.Along the axial direction,the maximum temperature is obtained at the interface between the variable-diameter part and the constant-diameter part.The maximum dimensionless temperature value decreases to 0.12 when the Rayleigh number increases to 1.088 × 1011.Along the circumfer-ential direction,the temperature distribution is affected by the buoyancy force,which results in the temperature of the upper part being higher than that of the lower part.After non-dimensionalization of the temperature and time,a correlation was proposed to illustrate the tran-sient heat transfer process quantitatively.The standard deviation of the maximum relative temper-ature,representing the temperature uniformity,was also calculated.It was found that the difference in the direction of the buoyancy force made a huge difference.Compared with the opposed buoy-ancy force,the maximum dimensionless temperature is almost two times higher with an assisted buoyancy force.Similarly,the heat transfer coefficient with an assisted buoyancy force is half of that with an opposed buoyancy force.Overall,an assisted buoyancy force plays a negative role in terms of thermal characteristics.The flow field around the hollow cylinder was also illustrated to reveal the mechanism of the buoyancy force on magnitude and direction aspects.
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