Fluid Flow And Heat Transfer In Cross-Corrugated Triangular Channels

Due to the increased requirements in people's indoor comfort, air conditioning systemshave been widely used. Energy consumption for air conditionings is very large. The total heatexchanger, which can effectively recovery sensible and latent heat from ventilation air, hasattracted much attention. Up until now, plate fin total heat exchanger is the most popular onein the market. Besides membrane materials, the structures have significant impacts on thefluid flow and heat transfer for the plate fin heat exchanger.Cross corrugated triangular duct heat exchanger is better because of its high strength.Further, the flow is enchanted by the periodic expansion and contraction in the flowingchannels, which gives rise to heat transfer coefficients. Therefore the cross corrugatedtriangular duct heat exchanger is quite applicable to air conditioning systemss. Althoughthere are many reports on cross corrugated triangular ducts, they are mostly only focused onnumerical simulation. This paper addresses this problem, by the combination of experimentaland numerical studies. In addition, field synergy theory is employed to analyze the numericalresults. These works would provide a theoretical basis for engineering design. The mainworks are summarized as following:(1) A test rig for heat transfer in cross corrugated triangular ducts is set up. The frictionfactor and heat transfer coefficient are tested under different Reynolds numbers, includedvarious angles and various heat flux boundary conditions. The variations of these parametersare analyzed.Further, a test rig for fluid flow in the cross corrugated triangular ducts is set up. Tostudy the flow characteristics inside the channels, three velocity components are measuredusing IFA300high speed hot wire anemometer. Spectrum analysis is carried out for velocitycomponents. The velocity vectors fields in the different plane are plotted by the Tecplotsoftware. The turbulence intensity contours and the turbulence kinetic energy contours arealso plotted. The results show that the flow in the cross corrugated triangular ducts becomesturbulent easily. When Re is275, the flow has become transitional. When Re is above1000,the flow has become turbulent.(2) A mathematical model for cross corrugated triangular ducts is developed and solvedby Fluent software. The established model is validated by the experimental data. It has beenfound that the results obtained by the low Reynold number k ε turbulence model fit theexperimental results best. Compared to the conjugated wall surface, the Nusselt numbersobtained for an ideal wall surface increase35%. The velocity vectors, the temperature contours and the turbulence kinetic energy contours inside channels are given. Further, thedeviations between the simulated and the experimental data are analyzed.(3) Comparisons between the heat transfer parameters in the cross corrugated triangularducts and those in other shapes tubed or parallel plate channels are made. Compared to roundtubes, elliptical tubes, square tubes and triangular tube ducts, the friction factor in thecross corrugated triangular ducts is relatively larger. However, the integrated performances ofheat transfer and resistance are better. It is because that fluid flow is enhanced by theclockwise vortexes and the complex secondary flows appearing in triangular cross regions.Compared to parallel plate channels, corrugated plate channels, herringbone corrugatedchannels, the friction factor for the cross corrugated triangular ducts is moderate but the heattransfer is relatively higher. Considering application prospect, processing cost, heat transferenhancement and resistance for total heat exchanger, the cross corrugated triangular duct is anideal choice.(4) For included angles (α)45°,60°,90°and120°, the experimental and numerical datain the cross corrugated triangular ducts are investigated. It is found that the fluid flow andheat transfer are seriously influenced by the included angles. Besides the analyses of themacroscopic parameters such as friction factor, Nusselt number, j factor and PEC values,macro level parameters are also disclosed with temperature and velocity contours. It is foundthat the cross corrugated triangular duct with90°included angle has the best heat transferperformance. For included angles45°,60°and120°, the average Nusselt number (NuD), jfactor and PEC value decrease30%~75%,14%~76%and24%~73%, respectively.(5) To disclose the heat transfer enhancement mechanism in the cross corrugatedtriangular duct, the field synergies between velocity gradient, temperature gradient andpressure gradient are investigated. Effects of the included angles on the fluid flow and heattransfer are discussed by analyzing the contours ofβ,θandγlocal synergy angles withdifferent included angles. Further, the contours of the local synergy angles under differentwall conditions are analyzed and compared. To validate the performances of the macroscopicheat transfer parameters such as fD,NuD,j and PEC, the volume average synergy angles andthe integral mean synergy angles are calculated and analyzed. The results are used to disclosethe variations of the macroscopic heat transfer parameters. It can be found that when Re is2000, the field averaged synergy angle (βVOL) for cross corrugated triangular duct with90°included angle is74.1°, which is1~2.3°less than that for other included angles. The value is14.1°less than that for a straight tube and14.5°less than that for a parallel flat plates duct.