Abstract
ABSTRACT A series of experimental studies on the fluid flow and heat transfer characteristics from non-compact/compact PPF heat sinks at unconfined and confined condition by using different cooling methods have been performed. Two types of cooling methods such as natural convection and forced convection due to channel flow are employed in the present study. The relevant parameters influencing fluid flow and heat transfer performance in natural convection and forced convection studies are listed, respectively. They are: (1) natural convection－the steady-state Grashof number (GrS) and ratio of heat sink height to channel height (H/HC). The ranges of these parameters studied are GrS=6.42x105~1.64x106 and H/HC=0.47-1.0; (2) forced convection－the steady-state Grashof number (GrS), ratio of heat sink height to channel height (H/HC)and Reynolds number (ReD). The ranges of these parameters studied are GrS =1.66x105~1.05x106 (or qC = 2486.56W/m2 ~ 15676.90W/m2), H/HC=0.47~1.0 and ReD=7356~30767. Their effects on fluid flow and heat transfer characteristics in natural convection and forced convection have been systematically explored. In the hydrodynamic aspect for confined compact heat sinks in channel flow, the fluid flow characteristics including the spanwise velocity distribution, local turbulence intensity distribution, and pressure drop are investigated. From the results, the spanwise dimensionless velocity distributions are not significantly affected by Reynolds number. Besides, for all the cases of non-compact heat sinks, the turbulence intensities at all measuring locations are less than 7.5% for the cases with 0.47<H/HC< 1 and 1.98m/s<Ui<8.02m/s. A theoretical model to effectively predict the velocity and pressure drop for partially-confined heat sinks has been successfully developed. The results show good agreement between the predicted and the experimental data. For unconfined/confined compact heat sinks in natural convection, the transient/steady-state local and average heat transfer characteristics are studied. The transient/steady-state local and average Nusselt number increases with increasing GrS, but decreasing with H/HC. In addition, two new correlations of steady-state average Nusselt numbers in terms of relevant influencing parameters for unconfined and confined compact PPF heat sinks in natural convection are proposed, respectively. For confined compact heat sinks in forced convection, the transient/steady-state local and average heat transfer characteristics are successively explored. The transient/steady-state local and average Nusselt numbers increase with increasing GrS, H/HC ratio or ReD. Similar trend can be found for the cases at different heat sink fin and base materials. In addition, a new correlation of steady-state average Nusselt number in terms of relevant influencing parameters for confined compact PPF heat sinks in forced convection is proposed. According to the results in ANOVA, a sensitivity analysis for the design factors is performed. The accuracies of the quadratic RSM models for both thermal resistance and pressure drop have been verified by comparing the predicted response values to the actual experimental data. The Response Surface Methodology is applied to establish analytical models of the thermal resistance and pressure drop constraints in terms of the key design factors with a CCD experimental design. By employing the Sequential Quadratic Programming technique, a series of constrained optimal designs can be efficiently performed. The numerical optimization results for four cases under different constraints are obtained, and the comparisons between these predicted optimal designs and those measured by the experimental data are made with a satisfactory agreement.