title: Heat Transfer Regimes during Charging of Pressure Vessels with Compressed Gas at Low Reynolds Numbers
Creators: name: Heath, Melissa
Publisher: name: Griffith University
Year: 2018
Subjects: subject: Heat transfer
subject: Pressure vessels
subject: Compressed gas
subject: Low Reynolds numbers
subject: Clean energy sources

description: Over the past decade signi cant interest in heat transfer during lling of pressure vessels has arisen due to overheating and under- lling when utilizing compressed hydrogen gas as a practical storage solution for clean energy sources. The problem is most clearly evident when hydrogen is stored as compressed gas for use in the automotive industry since carbon bre reinforced plastic (CFRP) vessels are used for their low weight. The low thermal conductivity of the plastic component of the vessel wall has an insulating e ect on the stored gas. During lling, the gas undergoes heating due to compression work, particularly during lling rates comparable to that of a traditional vehicle. As the CFRP material can be damaged by overheating, it is important to be able to predict the temperature of the gas, and the temperature distribution in the vessel wall. Key to this is understanding the heat transfer between the gas and the wall during lling. While signi cant progress has been made with larger vessels, few studies have considered the lower Reynolds number regimes. The aim of the PhD is to ll the gap in the literature for lower Reynolds number regimes by providing knowledge and fundamental data required for accurately predicting heat transfer during lling of gas pressure vessels. The methodologies being used include experimental work, numerical simulation and analytical methods. The thesis addresses understanding the heat transfer by experimentally producing benchmark quality temperature measurements for validation. Experiments have been undertaken which provide detailed measurements of thermal elds and heat transfer and the thermal response in the vessel. Computational uid dynamics (CFD) is used to determine local Nusselt numbers and understand the development of boundary layers within the vessel. An analytical solution has been developed by neglecting convection, essentially treating the vessel as a solid with and evenly distributed heat source. This model has provided an upper limit on the temperature rise within the vessel and lower limit on the heat transfer. The developed analytical solution elucidates the e ect of near wall compression heating and provides a theoretical limit for the minimum Nusselt number and maximum gas temperature rise in the vessel. The minimum Nusselt number limit is approached in the case of low Reynolds number. Taking the limit of small Reynolds number for a vessel being lled with compressed gas, the energy equation was found to asymptote to the unsteady heat conduction equation with heat generation and variable density. This equation was solved analytically for cylindrical and spherical geometry. Assuming the density changes linearly with time, a solution is obtained which can become identical in form to the constant density solution if the Fourier number is de ned using a log-mean density rather than the instantaneous density. For a constant energy input rate the Nusselt number was found to approach a constant value after an initial transient period. At such 'steady state' conditions, the Nusselt number based on the diameter for cylinders with aspect ratios larger than 1 rapidly approaches an integer solution of NuD = 8. For cylinders with aspect ratios less than 1, the Nusselt number based on the cylinder length (height) characteristic dimension rapidly approaches NuL = 6. It is shown experimentally and numerically that during compression lling, the heat transfer asymptotically approaches this analytical solution at low Reynolds numbers. Four distinct regimes were identi ed during the lling of the vessel. At the very beginning of lling, heat transfer is dominated by heat conduction and can be described by the analytical solution. Following this there is a period dominated by forced convection from the jet of gas entering the vessel. A signi cant temperature di erence and increased density of the gas causes natural convection currents to develop as the Rayleigh number increases. Both free and forced convection play an important role for a period of time which can be described as a mixed convection regime. As the lling rate and jet penetration into the ow eld decays forced convection e ects diminish leaving only free convection which continues after gas has stopped entering the vessel until thermal equilibrium is reached.