Study On The Heat Transfer Enhancement Of High Power Condensers With Multi-louvered Fins

It is widely used of automobile air-conditioners with multi-louvered fins, because of their good heat transfer effect. Thus, multi-louvered fins represent good technique of interrupted extended surface for heat transfer enhancement. The present thesis combine phase change heat transfer cooling system of converters for electricity supply system of locomotive traction, multi-louvered fins are taken the place of trapeziform cross-section plain straight fins. The flow and heat transfer characteristics of larger-size multi-louvered fins are analyzed on the basis of large eddy simulation (LES). And heat transfer enhancement analysis of air-side high power condensers is conduct in order to enhance the whole heat transfer performance of converters'cooling system and realize light type, high efficiency, energy conservation and reduction of pollutant emissions. In addition, it can offer theoretical foundation and application reference for electrical equipment cooling systems with phase change heat transfer.(1) A set of fairly integrated numerical model of multi-louvered fin arrays is established and parameter settings of equivalent heat source boundary conditions, geometrical models, models of flow state description, data reduction method et cetera are particularly analyzed. Comparisons of numerical results with original experimental studies in the literature indicate that, laminar model is settled for the computations of small-size multi-louvered fin arrays. But for larger-size multi-louvered fin arrays at present, Reynolds numbers based on louver pitch are over2000and it is reasonable of employing LES with flat-landing and transitional-region models.(2) Flow and heat transfer performance of larger-size plain straight fins and multi-louvered fins is compared in the light of adequate arguments with numerical model and computational method. It can be obtained that, the integrated performance of multi-louvered fins is superior to that of plain straight fins. When the exterior sizes of the condensers are the same, heat transfer capacity and effectiveness is enhanced over40%by multi-louvered fins, comparing with plain straight fins.(3) Aim at LES calculations of larger-size multi-louvered fin arrays, single parameter effect with louver angles, louver pitch to fin pitch ratios, fin thicknesses, flow lengths and fin heights are discussed. Field synergy principle is introduced to explain the flow and heat transfer characteristics of various geometrical structural parameters. Reasonable parameters related to thermal-hydraulic performance are put forward and new correlations of fanning friction f factor and colburn-j factor are present. It is found that, occupied fractional number of vertical gap between oblique louvers LFA=(Lp/Fp) tanLα can be used to describe the effect law of louver angles and louver pitch to fin pitch ratios. The optimal heat transfer occurs at the range of0.75≤LFA≤0.858. Moreover,6/Fth is used to describe the effect of fin thicknesses. The average total pressure drops are exponential to flow lengths and louver pitch to fin pitch ratios. There are small differences of heat transfer coefficients between different flow lengths. And the average heat transfer coefficients are exponential to louver pitch to fin pitch ratios.(4) In view of dimensionless process, it is obtained from signal-to-noise ratio SN that, when good heat transfer effect is taken into account with various controlling factors'levels, the contribution ratio34.14%of louver pitch to fin pitch ratios is the largest, following with contribution ratio of29.652%,16.293%for fin thicknesses and flow lengths. And louver angles have the least ratio8.364%. When louver angles and louver pitch to fin pitch ratios have a wide span values, contribution ratios from high to low are louver pitch to fin pitch ratios, louver angles, fin thicknesses, flow lengths, fin heights.(5) Several evaluation indexes such as volume good factor, area power factor, minimum pump power method, minimum entropy product rate method, minimum entransy dissipation principle and so on are employed to evaluate the integrated thermal-hydraulic performance of larger-size multi-louvered fins. When it is aim to minimum frontal area, small fin thickness and large louver height to fin height ratio are needed for good performance. Whereas small flow depth and louver pitch to fin pitch ratio are necessary for minimum pump power method. Large flow length, louver pitch to fin pitch ratio and small louver height to fin height ratio take good performance for minimum entropy product rate method. In addition, for minimum entransy dissipation principle, larger flow length, louver pitch to fin pitch ratio and louver height to fin height ratio perform good.(5) Thermal resistance network models are established for boiling-condensation phase change cooling systems. It is demonstrated that, air-side thermal resistance is decreased by58%,36%with multi-louvered fins of240mm,160mm flow lengths comparing to original240mm plain straight fins. The whole performance is improved by multi-louvered fins. The thermal resistance of pool boiling heat transfer section are just lower than the air-side thermal resistance, especially with an occupied fractional number of30-40%for the case with multi-louvered fins. The thermal resistance of flat-tube wall thermal conduction and condensation section can be neglected. Original cooling system with plain straight fins can be settled for with160mm multi-louvered fins, with1/3reduction of volume and weight. When heat transfer capacity changes from lOkW to20kW, plain straight fins cannot meet the heat removing requirements. Multi-louvered fins still can meet higher power cooling.