On Numerical Investigation of Non-dimensional Constant Representing the Occurrence of Secondary Peaks in the Nusselt Distribution Curves

The Study of heat transfer augmentation in micro scale and electronic packaging systems are some of the paramount areas of the impending universe. In such systems the cooling of the heat sinks are generally achieved through the impingement of air jet. Assuming the air jet to be continues & incompressible fluid, the heat dissipation rate over the target surface seems to either uniform or well characterized in radial direction. In order to study the characteristic of heat dissipation rate, a graph of Nusselt number versus radial distance over the target surface is plotted. This is done at various injection and geometric parameters. The resulting trend of Nusselt profile manifests a gradual decrement in its magnitude with increase in the radial distance from the stagnation point. This is the property of characterized Nusselt distribution profile. In order to study the Nusselt distribution profile in more accurate sense, the present research takes an effort in numerically simulating the present geometry. A 2-D axis-symmetric geometric model is being proposed which consists of air (fluid) and aluminum (solid) as two vital computational domains. The computational simulations are being carried out using an appropriate turbulence model, in order to correctly predict the most accurate flow regime. This also enables the recording of necessary heat interactions. As far as the transition and intermediacy in the flow structure which occurs at target surface is concerned, prediction of accurate flow profile using commercial turbulence model becomes difficult. In order to predict these vital phenomenon, SST turbulence model along with Gamma – Theta transitional model is solved simultaneously in a commercial CFX solver. At lower nozzle - target spacing and higher impinging velocity, Nusselt distribution curve was observed to posses some unpredicted and localized secondary rise. The occurrence of such secondary peaks increases the magnitude of area average heat transfer rate. As per the survey, very less light is observed in the research areas of determining the exact cause and the intervening range for the occurrence of such peaks. Looking into this issue the current research focuses over the determination of the critical constant and its magnitude within which these peaks exists. The non dimensional constant representing the critical magnitude is defined as a ratio of diameter based Reynolds number with nozzle - target spacing (Z/d). While the corresponding critical magnitude was approximately investigated to be 6000. Not only that the present work takes an initiative in mapping the velocity contours at different values of this non dimensional constant. This enables a physical understanding of the flow profile at different impinging condition. A very contiguous observation of such contours demonstrates the occurrence of a transition region in the flow regime to be responsible for the origin of such peaks