Abstract
In this work, entropy generation analysis is applied to characterize and optimize a turbulent impinging jet on a heated solid surface. In particular, the influence of plate inclinations and Reynolds numbers on the turbulent heat and fluid flow properties and its impact on the thermodynamic performance of such flow arrangements are numerically investigated. For this purpose, novel model equations are derived in the frame of Large Eddy Simulation (LES) that allows calculation of local entropy generation rates in a post-processing phase including the effect of unresolved subgrid-scale irreversibilities. From this LES-based study, distinctive features of heat and flow dynamics of the impinging fluid are detected and optimal operating designs for jet impingement cooling are identified. It turned out that (1) the location of the stagnation point and that of the maximal Nusselt number differ in the case of plate inclination; (2) predominantly the impinged wall acts as a strong source of irreversibility; and (3) a flow arrangement with a jet impinging normally on the heated surface allows the most efficient use of energy which is associated with lowest exergy lost. Furthermore, it is found that increasing the Reynolds number intensifies the heat transfer and upgrades the second law efficiency of such thermal systems. Thereby, the thermal efficiency enhancement can overwhelm the frictional exergy loss.
Highlights
Jet impingement cooling has been widely used as means of heat transfer equipment in a variety of engineering applications such as cooling of gas turbine blades or electronic components
We present novel model equations that allow calculation of local entropy generation rates in the post-processing phase of Large Eddy Simulation (LES) for turbulent heat and fluid flows
Since the entropy generation is directly linked to the dissipation of energy which is predominantly a subgrid-scale process in the context of LES, appropriate closures are of utmost importance once dealing with the entropy balance equation in which flow and heat transport processes contribute to the entropy production
Summary
Jet impingement cooling has been widely used as means of heat transfer equipment in a variety of engineering applications such as cooling of gas turbine blades or electronic components. The large number of operating parameters, complex heat and fluid flow dynamics and turbulent flow conditions (typically in the range of Re = 4000–80,000 [1]) impede a complete description of jet impingement cooling and make the optimization of such thermal devices very difficult. In this respect, it was shown by Ries et al [9] that an analysis based on entropy generation is a promising approach to provide a deeper insight into the complex physical mechanisms of impinging flows and to localize possible irreversibilities within such thermal devices. A detailed evaluation of the proposed, novel model equations to calculate local entropy generation rates using LES are provided in the Appendix C for the sake of completeness
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