Scale formation on heat transfer surfaces results in the build-up of deposits (products of heat and mass transfer processes) which act as additional thermal resistance, thereby leading to the degradation of equipment heat transfer performance. Under most conditions fouling is more severe during sub-cooled boiling heat transfer, mainly due to the mechanisms which govern the bubble formation and detachment process. Therefore in response, the current project aims to investigate the boiling enhancement characteristics and fouling mitigation potential of a three-phase (vapour-liquid-solid) circulating fluidised bed heat exchanger (CFBHX). The design of the three-phase boiling system is predicated on a combination of multiphase fluidisation and flow boiling heat transfer. Experiments are performed at atmospheric pressure, and distilled water is used as the working fluid. The three-phase test unit consist of a glass riser column with a minichannel of square cross sectional area 21.5 mm x 11mm, height 1000mm, fitted with an electrically heated cartridge heater rod of 8mm diameter x 730mm length. The set-up employs stainless steel particles as the solid phase. A systematic experimental study is made to understand the influence of particles and particle size on the boiling heat transfer behaviour of the three-phase CFBHX. The effect of operating parameters such as heat flux and superficial velocity are also investigated. In the riser column, the use of transparent glass walls is of major significance as it provides a means of observing, and studying (via the use of flow visualisation techniques) the complex multiphase flow system. Results from our experimental work show that higher heat transfer coefficients are achieved in (vapour-liquid-solid) three-phase flow boiling, compared with (vapour-liquid) two-phase flow boiling. The observed enhancement becomes more pronounced for progressively larger diameter particles. Based on both an analysis of the mechanisms governing three-phase boiling heat transfer, and the extension of existing two-phase flow boiling (and liquid-solid fluidised bed) heat transfer models, a boiling heat transfer correlation has been derived for the prediction of heat transfer in our vapour-liquid-solid circulating fluidised bed system. A favourable agreement between the derived three-phase boiling correlation and our empirically obtained results has also been duly demonstrated.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:641089 |
| Date | January 2006 |
| Creators | Arumemi-Ikhide, Michael |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/11509 |
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