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Numerical Study on Heat Transfer and Fluid Flow Characteristics of Composite Fin

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Tutor: LvYanLi
School: Zhengzhou Institute of Light Industry
Course: Refrigeration and Cryogenic Engineering
Keywords: composite fin,semi-dimpled split fin,enhanced heat transfer,field synergyprincip
CLC: TK172
Type: Master's thesis
Year:  2012
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Abstract:
Fin-and-tube heat exchangers are widely used in various heat transfer equipments, suchas power engineering, petroleum and chemical engineering, air conditioning and refrigerationsystem. To this heat exchanger, air-side thermal resistance generally comprises over70%ofthe total thermal resistance. Improving the air-side heat transfer capacity of heat exchangers, itcan be energy-saving of run process. In this paper, three kinds of fins are proposed and theirsheat transfer and fluid flow characteristics are invertigated based on the flied synergyprinciple and entransy dissipation principle.Firstly, the slit composite fin is presented based on the advantages of longitudinal vortexgenerator and slit. The performances of heat transfer and fluid flow are investigated bynumerical simulation. Fundamental principles of enchanced heat transfer of the slit compositefin lie in two aspects:(i) some recalculating eddies develop behind the X-shaped slit andtriangle winglet, which can increase the disturbances of the fluid flow;(ii) the interruptedsurfaces of slits make the boundary layer thinner, so the high average heat transfer coefficientcan be achieved. Compared with plain fin, fin with delta winglet and slit fin, the heat transferof slit composite fin can improve76%~90%,49%~82%,6%~36%, respectively. The overallperformance of composite fin is better than the other three kinds of fins. By applying on thefield synergy principle and entransy dissipation principle to analyze the fins, the conclusionsshow that the composite fin can improve the synergy of temperature gradient and velocityfields, and decrease equivalent thermal.Secondly, the heat transfer and fluid flow characteristics of wave composite fin are alsoinvestigated by numerical simulation, which originates from longitudinal vortex generator andwave fin. Several different structures are proposed, which include case1(front wave and reardelta winglet composite fin with the same tube diameter), case2(front delta winglet and rear wave composite fin with the same tube diameter), case3(front wave and rear delta wingletcomposite fin with first row tube in smaller size and second row tube in larger size) andcase4(front wave and rear delta winglet composite fin with first row tube in larger size andsecond row tube in smaller size). The results show that the flow resistance of the wavecomposite fin is lower and heat transfer coefficient is higher than wave fin. The overallperformance of case1is better than case2. For hybrid tube diameter wave composite fins, theflow resistance of case3exhibits observably lower, which is more effective than case4onoverall performance.Finally, the semi-dimpled split fin is proposed and its heat transfer and fluid flowcharacteristics is studied to understand the physical mechanism. The results indicate that thedimple generates the downstream longitudinal vortices by analyzing the flow field. Comparedwith the plain fin, the semi-dimpled split fin can increase the heat transfer and overallperformance by24.06%~67.73%and7.55%~23.88%, respectively. The factors of thesemi-dimpled split fin are analyzed based on the orthogonal experiment design method. Theresults show fin pitch has significant effect on the heat transfer and flow characteristics, theinfluence is dwindle in the order of the arrangement of split-dimple, the dimple radius and theopening direction of split-dimple. When Re¡Ü1521, the overall performance of slit fin is betterthan that of optimization; While Re>1521, the overall performance of optimization is betterthan that of slit fin. So the semi-dimpled split fin is more suitable for high inlet velocity heatexchanger.
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