Publisher Summary This chapter presents a discussion on jet impingement heat transfer. The chapter describes the applications and physics of the flow and heat transfer phenomena, available empirical correlations and values they predict, and numerical simulation techniques and results of impinging jet devices for heat transfer. The relative strengths and drawbacks of the Reynolds stress model, algebraic stress models, shear stress transport, and v 2 f turbulence models for impinging jet flow and heat transfer are compared in the chapter. The chapter provides select model equations as well as quantitative assessments of model errors and judgments of model suitability. The review of recent impinging jet research publications identified a series of engineering research tasks that are important for improving the design and resulting performance of impinging jets: (1) clearly resolve the physical mechanisms by which multiple peaks occur in the transfer coefficient profiles, and clarify which mechanism(s) dominate in various geometries and Reynolds number regimes, (2) develop a turbulence model, and associated wall treatment if necessary, that reliably and efficiently provides time-averaged transfer coefficients, (3) develop alternate nozzle and installation geometries that provide higher efficiency, meaning improved Nu profiles at either a set flow or set blower power, and (4) further explore the effects of jet interference in jet array geometries, both experimentally and numerically. This includes improved design of exit pathways for spent flow in array installations.

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Jet Impingement Heat Transfer: Physics, Correlations, and Numerical Modeling

A robust near-wall elliptic-relaxation eddy-viscosity turbulence model for CFD

Near-wall behavior of RANS turbulence models and implications for wall functions

Numerical study of turbulent heat transfer in confined and unconfined impinging jets

In order to gain a better insight into flow, vortical and turbulence structure and their correlation with the local heat transfer in impinging flows, we performed large-eddy simulations (LES) of a round normally impinging jet issuing from a long pipe at Reynolds number Re = 20000 at the orifice-to-plate distance H = 2D, where D is the jet-nozzle diameter. This configuration was chosen to match previous experiments in which several phenomena have been detected, but the underlying physics remained obscure because of limitations in the measuring techniques applied. The instantaneous velocity and temperature fields, generated by the LES, revealed interesting time and spatial dynamics of the vorticity and eddy structures and their imprints on the target wall, characterized by tilting and breaking of the edge ring vortices before impingement, flapping, precessing, splitting and pairing of the stagnation point/line, local unsteady separation and flow reversal at the onset of radial jet spreading, streaks pairing and branching in the near-wall region of the radial jets, and others. The LES data provided also a basis for plausible explanations of some of the experimentally detected statistically-averaged flow features such as double peaks in the Nusselt number and the negative production of turbulence energy in the stagnation region. The simulations, performed with an in-house unstructured finite-volume code T-FlowS, using second-order-accuracy discretization schemes for space and time and the dynamic subgrid-scale stress/flux model for unresolved motion, showed large sensitivity of the results to the grid resolution especially in the wall vicinity, suggesting care must be taken in interpreting LES results in impinging flows.

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Vortical structures and heat transfer in a round impinging jet

Prediction of heat transfer in an axisymmetric turbulent jet impinging on a flat plate

A second moment closure (SMC) computation is compared in detail with the direct numerical simulation (DNS) data of Le et al. (1997) for the backstep flow at Re = 5100 in an attempt to understand why the intensity of the backflow and, consequently, the friction coefficient in the recirculation bubble are under-estimated. The data show that this recirculation bubble is far from being laminar except in the very near wall layer. A novel “differential a priori” procedure was used, in which the full transport equation for one isolated component of the Reynolds stress tensor was solved using DNS data as input. Conclusions are then different from what would have been deduced by comparing a full simulation to a DNS. In particular, the e-equation, usually blamed for faults in model predictions, has been found to give excellent results in this case. In fact, the main problem comes from the uv -equation which predicts a too high turbulent force. A modification, by including the gradients of mean flow in the transport model, has then been attempted and has cured 50 percent of the backflow discrepancy.

A Procedure for Using DNS Databases

Cooling or heating of a flat plate by an impinging jet, due to its many applications, has been widely studied. Recent experimental data concerning more complex geometries has become available. In this study, the cooling of a heated pedestal mounted on a flat plate, a configuration which is closer to the one met in some engineering applications (e.g., cooling of electronic components), has been numerically simulated. The normal velocity relaxation turbulence model (V2F model) in an axisymmetric geometry has been adopted. Results have been obtained for a range of jet Reynolds numbers and jet-to-pedestal distances. Comparison of the predicted heat transfer coefficient with experiments has shown a very good agreement. For comparison, results have also been obtained with the widely used κ – e turbulence model and the agreement with the data is poor.

S. Parneix

Papers

Numerical study of turbulent heat transfer in confined and unconfined impinging jets

Prediction of heat transfer in an axisymmetric turbulent jet impinging on a flat plate

A Procedure for Using DNS Databases

Predictions of turbulent heat transfer in an axisymmetric jet impinging on a heated pedestal