Experimental and numerical investigation of electronics equipment cooling with impinging jets

Flow field and heat transfer characteristics of impinging jets depend on many parameters including, jet to plate distance, turbulence in the jet, jet exit geometry and the temperature difference between the jet and the target plate. In the first part of this study, the effect of temperature difference between the jet and the target plate on local and average heat transfer characteristics of impinging jets has been numerically and experimentally investigated. In the parametric study, Re number and dimensionless jet to plate distance were varried from 250 to 10000, and dimensionless jet to plate distance from 2 to 12 respectively. It has been shown that, as a result of increasing temperature difference, average heat transfer rate could be enhanced up to 35% at low jet exit velocities. In addition, buoyancy induced flow might have assisting or opposing effect on local heat transfer depending on the position on the target plate. Along the target plate, Nu number changes nonlinearly from the impinging point onwards. For small jet diameters, it is difficult to v correctly determine this change by using thermocouples. In order to determine local heat transfer rates for different jet exit geometries, experiments have been conducted for 9 different jet exit geometries including circular, elliptic and rectangular jets by using liquid crystal thermography temperature measurement technique. As a result, it has been shown that elliptic and rectangular jets could be used as a passive enhancement mechanism, especially in the stagnation region. Flow field characteristics of circular and elliptic impinging jets have been determined by detailed LDA and pressure measurements as a function of dimensionless jet to plate distance. Axis switch-over phenomena and shorter potential core length of eliptic jets, compared to circular jets, have been mapped. Analysis of velocity and turbulence data in the flow field together with the results of the parametric numerical study for the determination of maximum heat transfer rate location, showed that heat transfer is linked to flow acceleration on the target plate and turbulence in the jet. Due to their simple geometry and complex flow field charateristics impinging jets are very suitable for testing different turbulence models. Low Re number, Lam-Bramhorst k-? model has been tested to model circular, elliptic and rectangular jets. Model was not appropriate to determine the heat transfer rate at every point on the target plate but was able to predict the general profile change. Key Words : Impinging jet, LDA, liquid crystal, heat transfer