The work commences with a review of the literature on turbulent forced convection heat transfer to supercritical pressure fluids flowing in pipes. Only in the more recent work is any mention made of the effects of free convection on the heat transfer. An extensive programme of experiments is carried out with supercritical pressure carbon dioxide flowing both upwards and downwards in a 1.9 cm bore resistance heated test section. The test section is part of a closed loop system in which mass flow rate, wall heat flux and bulk inlet temperature can be varied systematically. Locally impaired wall temperature profiles occur in upflow whereas downflow profiles appear well behaved. The difference between them is the result of buoyancy effects. The influence of small variations in the test section wall thickness is investigated with regard to large circumferential non-uniformities of wall temperature. Downward flow is identified as being pseudo fully developed but nevertheless dependent upon wall and bulk temperature conditions. The trends in upward flow are recognised by examining the relative effects of free and forced convection. Comparison of upward and downward flow experimental data for supercritical water and carbon dioxide using ideas 01: .. non-dimensional similarity lead to the conclusion that although heat transfer is locally impaired in up~low in large bore pipes when compared with downflow, both upward and downward flow in small bore pipes produce worse heat transfer than in large bore pipes. Theory is produced in .which the influence of free convection on the non-dimensional shear stress in the boundary layer of an upward flow is included. Processing of the data produced in the present experiments and from published data of other workers using other fluids enables locally impaired heat transfer in upflow to be identified as a product of the effect of buoyancy in reducing the shear stress in this region. This method is applicable if buoyancy effects are sufficiently large relative to forced flow effects. Finally, it is shown that for a turbulent flow of air in a pipe, better heat transfer can be obtained for the case of downflow rather than upflow under the influence of variable density alone. The absolute shear stress level in each case relative to the other, rather than the non-dimensional level, is the quantity which is of greater importance in the understanding of these results.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:636797 |
| Date | January 1972 |
| Creators | Weinberg, Robert Saul |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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