A Study of Heat Transfer and Heat Flow Asymmetry through Water in the presence of the Density Maximum
Mooney, Priscilla A. (2007) A Study of Heat Transfer and Heat Flow Asymmetry through Water in the presence of the Density Maximum. PhD thesis, National University of Ireland Maynooth.
This thesis is concerned with heat flow asymmetry through composites of water and aqueous solutions. Central to this exploration are the behaviour of heat transfer in the vicinity of the density maximum and the behaviour of the temperature of maximum density of aqueous solutions. Both of these topics are investigated by cooling a rectangular enclosure of the test fluid in a quasi-steady state manner. During cooling, the temperature at select points within the liquid is monitored and the flow of heat at both isothermal walls is measured. As the liquid cools through its density maximum the normal single-cell convection that occurs in the presence of a horizontal temperature gradient changes to a double cell configuration in the vicinity of the density maximum. This transition manifests itself in changes in the horizontal temperature profile across the cavity and in the rate of cooling of the fluid. A measurement technique to study the behaviour of the temperature of maximum density of aqueous solutions is described in this thesis that relies on these changes. These changes are investigated experimentally and numerically. The study of the behaviour of heat transfer and the temperature of maximum density of aqueous solutions revealed that heat transfer is reduced in the vicinity of the density maximum and that the temperature of maximum density of aqueous solutions depends on the nature and concentration of the solute. Both of these results are exploited in the study of heat flow asymmetry through a device that consists of two cubic enclosures side by side; one enclosure contains water with a density maximum at 4oC and the other enclosure contains a saline solution with a density maximum at 2oC. A temperature gradient, which spans both of these temperatures of maximum density, is applied horizontally across the composite system, resulting in different rates of heat transfer through the device depending on the gradient direction. Experiments performed with a 12cm x 6cm x 6cm container yield heat transfer rates of 0.55W and 0.19W depending on the direction of the temperature gradient, resulting in a rectification factor of 65.4%. Asymmetrical heat transfer rates are also found in composite systems of water and solids when the temperature gradient spans the temperature of maximum density of the water. Results from computational fluid dynamics confirm the experimental results, and are used to investigate the influence of such parameters as temperature gradient and container aspect ratio on the rectification factor.
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