Heat energy is important to all aspects of life. Various industries including food processing plants, chemical processing plants, thermal power plants, refrigeration and air conditioning equipments, petrochemical plants, etc. are faced with the problems of effective utilization, conservation and recovery of heat. The production of heat exchangers involves huge investments for capital and operation costs. In view of this, it has become important to design heat exchangers that will be efficient and also save energy, cost and materials. Different techniques known as heat transfer enhancement are employed to achieve this. Of these techniques, the tube-insert technology is applied in the present research. No prior work on heat transfer and fluid flow in a tube induced with twisted tape insert with emphasis on cuts with different geometrical shapes but equal area has been reported. Hence, in the present work, heat transfer and fluid flow of water in tubes induced with twisted tape inserts with different-shape-equal-area cuts is investigated numerically. The present studies pay attention to the thermo-hydraulic characteristics of laminar, transitional and turbulent flows of water through different tube designs fitted individually with twisted tape of different design under uniform wall heat flux. The numerical simulation in this work is carried out by using Fluent software. The RANS-based RNG κ-ε model is employed for the turbulent flow because it is found to give a more accurate result than other turbulence models. Since transitional flow is not fully turbulent, the transitional variant of the SST κ-ω model is applied for the simulation of the transitional flow. The analyses quantify the improvement in the heat transfer, friction factor and thermal performance index in each of the tube systems and these results are used to ascertain the system that gives the best performance. Correlations are also proposed for the Nusselt number and friction factor. The results indicates that the superior fluid mixing provided by the alternate-axis triangular cut twisted tape is one of the reasons why it offers heat transfer enhancement and thermal performance factor that is higher than those that are offered by other induced tubes. Interestingly, the enhancement in heat transfer increases as the size of the cuts on the tape and the width of the tape increase but decreases as the pitch of the tape increases. The enhancement of heat transfer affects the start and the end of the transition to turbulent flow. Transition to turbulent flow occurs and ends earliest in the tube system with the highest heat transfer enhancement. Investigation is also performed on the combined forced and free convection heat transfer in an inclined tube for laminar, transitional and turbulent flows. The induced tube is inclined at different angles (15o≤θ≤90o ) with respect to the horizontal. Importantly, the heat transfer enhancement of the tube systems under mixed convection is higher than those under forced convection, and the enhancement for the mixed convection increases as the inclination angle increases.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:643116 |
| Date | January 2015 |
| Creators | Oni, Taiwo Oluwasesan |
| Publisher | University of Glasgow |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://theses.gla.ac.uk/6237/ |
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