Free convection flow and heat transfer in pipe exposed to cooling
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One of the challenges with thermal insulation design in subsea equipment is to minimize the heat loss through cold spots during production shut down. Cold spots are system components where insulation is difficult to implement, resulting in an insulation discontinuity which creates by nature a thermal bridge. It is difficult to avoid cold spots or thermal bridges in items like sensors, valves, connectors and supporting structures. These areas of reduced or no insulation are referred to as cold spots. Heat is drained faster through these spots, resulting in an increased local fluid density resulting in an internal fluid flow due to gravity and accelerated cooldown. This natural convection flow is important for both heat loss and internal distribution of the temperature. This thesis is presenting both experimental work and modelling work. A series of cool down tests and Computational Fluid Dynamics (CFD) simulations of these tests are presented. These tests and simulations were carried out in order to understand the flow physics involved in heat exchange processes caused by free convection flow in pipe exposed to cooling. Inclination of the pipe relative to the direction of gravity and temperature difference between cooling water and internal pipe water are the two main parameters investigated in this study. The experimental heat removal and temperature field is discussed and further interpreted by means of computational fluid dynamics. For prediction of the evolvement of the local temperature and heat flow, selection of an appropriate turbulence model is critical. Hence, different models and wall functions are investigated. The predicted temperature profiles and heat extraction rates are compared to the experiments for the selected turbulence models. Our main conclusions, supported by our experimental and CFD results, include: (i) Heat transfer from a localized cold spot in an inclined pipe is most efficient when the pipe orientation is close to horizontal. As the pipe becomes more and more inclined the heat transfer (heat extraction) is reduced and the flow becomes more unstable. The results indicate that heat transfer from cold surfaces, where surface normal vector being normal to the gravity vector or surface normal vector facing down, is very efficient, and if possible, both should be avoided in order to minimize local formation of wax or hydrates (crystals) which may take place and could create flow blockage during start-up. (ii) The flow in the selected geometry is complex. In the present work, large eddy simulation (LES) and RANS (Standard k __ ) turbulence model are compared to our measurements. Consistent with experimental observation, strong unsteadiness was clearly observed in the results of both models; however, the LES model achieved significantly better agreement with temperature and heat transfer measurements than the k __ turbulence model. Based on the results obtained in this work, further application of LES to flows of industrial complexity is recommended. (iii) On the cold spot, developing boundary layers are not in equilibrium with the outer flow, making it difficult to use wall functions.