Numerical Analyses Of Flow And Heat Transfer Characteristics For Several Typical Fins

Enhanced fins are widely used in various heat exchanger applications,which include the evaporators and condensers used in air conditioners, theautomobile radiators, and more. Researches on such fins are importantbecause they can not only promote the development of the heat transferenhancing techniques, but also bring the economical benefits. In this thesis,the flow and heat transfer characteristics for several typical fins werenumerically analyzed.Four different models, including the steady (time-independent) laminar,the unsteady (time-dependent) laminar, the steady turbulent, and the unsteadyturbulent flow model, were used to predict the heat transfer and flow behaviorin multi-cycle corrugated channels. The results support that the laminar flowmodel can be used for Reynolds numbers (Re) below 600 while the turbulentflow model should be adopted when the Reynolds number exceeds 600. Theunsteady and the steady laminar flow model yield almost the same Nusseltnumber (Nu) and apparent fanning friction factor (f_app) for Re ≦ 600, and sodo the unsteady and the steady turbulent flow model for Re > 600. Therefore,it is appropriate to use the steady model to predict the Nu and f_app of acorrugated channel. Further, for laminar flow in a multi-cycle corrugatedchannel, the upstream cycle yields higher Nu than the downstream cycle;forturbulent flow in the same channel, however, the downstream cycle giveshigher Nu than the upstream cycle.Three new fins were obtained by slitting a wavy fin. Numericalsimulations were conducted to investigate the slitting effects on the flow andheat transfer performance of the resulting slit-wavy fin geometries. Theresults show that slits can help remove the transverse vortices and promotemixing of the fluid, leading to an improved thermal/hydraulic performance.Further, the best performance can be achieved by making the slitting angle ofthe slit-wavy fin equal to the corrugated angle of the original wavy fin.Performances of the plain fin, the fins with punched delta winglets andthe louver fin used in a flat-tube heat exchanger application were numericallypredicted and compared, with the plain fin serving as the baseline fin. It isfound that the enhancement level of the louver fin is considerably higher thanthat of the fin with winglets. The j to f factor ratio of the louver fin issmaller than that of the fin with winglets, but the j to f1/3 ratio of the former isgreater than that of the latter, so the louver fin is superior to the fin withwinglets, if the j/f1/3 is used as the performance evaluation criterion. Tonumerically predict the thermal-hydraulic performance of a flat-tube heatexchanger, it is appropriate to take only half the basic fin-tube configurationand perform simulations with application of the symmetric boundarycondition.The heat transfer coefficient (h) and fin efficiency (η) for the plain finand louver fin used in a flat-tube heat exchanger application are calculatedusing two different data reduction methods. For both fins, the heat transfercoefficient obtained by integrating the local h over the entire heat transfersurface is higher than that corresponding to the fin efficiency equation for astraight fin. For the louver fin, the fin efficiency obtained from the finefficiency equation is higher than that calculated from the fin efficiencyequation for a straight fin. For the plain fin, the difference between the twoefficiencies is less than 1%.