Natural convection in inclined pipes - A new correlation for heat transfer estimations

Abstract

Abstract. Heat intake minimization is one of the main challenges during the design process of cryogenic storage tanks. It is widely known that connection pipes significantly contribute to this residual heat transfer from ambient temperature conditions to the cold inner vessel. A certain pipe inclination can cause a convective flow field within the fluid. This effect usually increases the total heat transfer much more dramatically than anticipated. In several previous papers we discussed the impact of pipe geometry as well as boundary conditions intensively. However, there is no suitable correlation in literature available which could be used to estimate the total heat transfer properly. The large number of experimental data we gained during our investigations allows us to propose a new correlation in order to predict the total heat transfer through an inclined pipe in function of the inclination angle. In this paper we derivate this new correlation and show its application for heat transfer estimations. Several comparisons are carried out against our own measurements as well as literature data.Keywords: Natural convection, inclined pipe, heat transfer, storage vessels INTRODUCTIONA primary concern in the design process of low loss cryogenic storage tanks is the minimization of the overall heat intake in order to increase the dormancy time as much as possible. A major contribution to the overall heat intake is caused by connection pipes mounted between the warm outer vessel and the cold inner vessel. Depending on to the inclination angle γ we can observe a substantial change of the heat transfer characteristic. The inclination angle is defined to be γ = −90 • for the vertical pipe with the cold end down and the warm end up, γ = 0 • for a horizontal configuration and γ = 90 • for a vertical pipe with the warm end down and the cold end up. Typical dimensions for these pipes are several millimeters in diameter and a length lower than one meter. In terms of high-pressure connection pipes a certain wall thickness of several millimeters also need to be taken into account. In case the warm end of the pipe is positioned above the cold end (γ = −90 • ), the fluid inside can be seen as stagnant and therefore no natural convection is present. In this case the amount of transported heat can be calculated from the equations for pure heat conduction. In case the warm end of the pipe is positioned much below or even at the same height as the cold end (γ = 0 • ..90 • ), a natural convective flow field occurs. The total heat flux usually increases substantially. In previous studies [1, 2] we already discussed the theoretical background on natural convection in inclined connection pipes and emphasized the relevance of this topic with a special focus to high-pressure cryogenic storage tanks. In literature only a very low number of references covering this topic is available. Moreover, there are no relevant data or correlations available to appropriately predict the total heat flux except of CFD analysis. Therefore, we see a tremend...

Thermal convection in inclined cylindrical containers

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