Effects of Wall Curvature on the Dynamics of an Impinging Jet and Resulting Heat Transfer (original) (raw)

Aerodynamic and heat transfer analysis of a impinging jet on a concave surface

International Journal of Thermal Sciences, 2017

Impinging jets are often used in applications requiring important localized cooling. For example, this technique is commonly used to reduce blade temperatures inside gas turbines. When the jet impacts the inner surface of the leading edge of a gas turbine blade, this geometrical configuration is similar to a jet impinging a concave surface inside a cavity. Previous studies have shown that for a certain range of geometrical and dynamic parameters, a jet injected in a cavity may sometime become unstable which is characterized by an oscillatory or flapping movement of the flow within the cavity. The objectives of this study are to investigate some features of this behaviour from velocity fields inside the cavity, pressure coefficients and Nusselt number distributions on the concave surface. This was accomplished using unsteady numerical simulations of a laminar flow at different Reynolds numbers for six different cavity configurations. Furthermore, PIV measurements were realized for some of the configurations in order to validate the numerical results. The results show that for four geometrical configurations, the flow entered in an oscillatory movement inside the cavity. This behaviour can be related to the difference in pressure between the output channels and the main vortex structure present inside the cavity. However, no clear link has been established between the frequency of the oscillatory flow and the geometrical parameters used.

Nonlinear flow and heat transfer dynamics of impinging jets onto slightly-curved surfaces

Applied Thermal Engineering, 2007

The nonlinear flow and heat transfer characteristics for a slot-jet impinging on slightly-curved surfaces are experimentally studied here. The effects of curved surface geometry and jet Reynolds number on the jet velocity distribution and circumferential Nusselt numbers are examined. Two different slightly-curved surface geometries of convex and concave are used as target surfaces. The nozzle geometry is a rectangular slot, and the dimensionless nozzle-to-surface distance equals to L * = 8. The constant heat fluxes are accordingly applied to the surfaces to obtain an impingement cooling by the air jet at ambient temperature. The measurements are made for the jet Reynolds numbers of Re = 8617, Re = 13 350 and Re = 15 415 for both curved surfaces. The velocity distributions of issuing jet from the nozzle exit to the target surface are obtained by a highly sensitive hot-wire anemometer. The T-type thermocouples are used to measure local temperatures of both the air jet and the plates. Two-dimensional velocity measurements show that the surfaces are remained out of the potential core region for all Re tested here. New correlations for local, stagnation point, and average Nusselt numbers as a function of jet Reynolds number and dimensionless circumferential distance are reported. The correlations reveal that the impinging cooling rate is higher with the concave surface and increase with increasing Re.

Impinging jet cooling on concave surfaces

Aiche Journal, 2004

The numerical modeling of jet impingement cooling onto a semicircular concave surface is reported. The performance of two-equation turbulence models (such as the k–ε model) is evaluated vs. the Reynolds stress model proposed. The Reynolds-averaged momentum and energy equations are solved together with equations for the turbulence models, using a fully unstructured control volume method and a second-order high-resolution differencing scheme. Variations of jet Reynolds numbers of the spacing between the nozzle and the concave surface, as well as of the distance from the stagnation point in the circumferential direction, are considered. The predicted results are validated against experimental data. The developed approach yields low-cost and accurate predictions of processes where jet impingement cooling is involved. It can assist the design of relevant applications, with relative ease, especially in view of the enhanced heat transfer encountered in the concave surface jet impingement. © 2004 American Institute of Chemical Engineers AIChE J, 50: 1672–1683, 2004

Heat transfer and flow analysis of jet impingement on concave surfaces

Applied Thermal Engineering, 2015

The current study evaluates the performance of three turbulence models in predicting the heat transfer and flow physics of jet impingement on concave surface. Two of the applied models are zero-equation subgrid-scale (SGS) models which belong to large eddy simulation (LES), namely RAST and dynamic Smagorinsky model (DSM), and the third one is RNG k-ǫ Reynolds Averaged Navier-Stokes (RANS) model. These models are utilized to analyze the heat transfer for two cases: (1) jet impingement on a curved surface with different jet-to-surface distances (2) jet impingement on a heated circular cylinder with varying nozzle-to-surface distance at two different Reynolds numbers. The predicted results are compared with the available experimental data in the literature. The findings revealed that RAST and DSM predictions are in better agreement with experiment than RNG k-ǫ ones. It is also concluded that at higher jet-to-surface ratios, all three models produced almost similar results, proving that the heat transfer distribution and the flow are more affected by the jet-to-surface distance than the magnitude of Reynolds number.

NUMERICAL SIMULATION OF TURBINE BLADE COOLING VIA JET IMPINGEMENT

Various industrial applications use jet impingement against surface to provide an effective mode of heat transfer. Its application includes, but not limited to, heat treatment, thermal management of optical surfaces for defogging, cooling of critical machinery structures, and rocket launcher cooling. In this study, numerical analysis of various heat transfer configurations of jet impingement on a semi-circular surface is carried out. These configurations were compared on the basis of effective heat transfer by achieving higher Nusselt number and lower surface temperature as convection heat is becoming the dominant phenomenon. The numerical model was developed for considering the application of a uniform heat flux on a curved surface subjected to jet flow that simulating an internal channel under cooling. The results found to be in agreement with the literature experimental data. To gain more insight on the underlining physics of the flow, a sensitivity analysis on the jet impingement configuration and flow conditions were conducted and was demonstrated to the inner cooling of the 1 st stage gas turbine blade.

Numerical Study of Jet Impingement Cooling on a Smooth on 3-D Convex Surface

2020

Jet impingement cooling is an alternative method to force a fluid flowing to a target surface in order to remove heat from it. Many applications are implementing this system such as electronic device coolers and cooler for turbine blades. The significance of this application is it can increase the rate of heat transfer towards the fluid. A set of simulations was run in ANSYS Fluent on 2D axis-symmetry of round nozzle tip with a convex target surface. After validation, it was found that the best viscous model that can be used throughout the simulation was a standard k-epsilon model. Reynold number, nozzle tip to target surface distance, L/d, and target surface radius, r/d are the parameters considered. The ranges of the parameters are 5000 Re100000, 1L/d10and 8.86r/d20. The numerical results were validated against the experimental data. The ACFD method was applied to derive an equation that correlates the average Nusselt number to the parameters. The validity of the equation is teste...

Jet impingement heat transfer on a flat plate at a constant wall temperature

International Journal of Thermal Sciences, 2008

Gas-to-wall heat transfer configuration for a round air jet impinging on a circular flat plate is investigated experimentally to derive an average Nusselt number correlation. The impingement plate is placed at the bottom of a large adiabatic enclosure, and its temperature is imposed, by external circulation of a coolant. The simultaneous measurements of mass flow rate and characteristic temperatures (hot jet, cold wall, enclosure outlet) permit the determination of the average wall heat transfer coefficient, through an enthalpic balance of the enclosure. The jet Reynolds number, nozzle diameter D and nozzle-to-plate distance H are varied. These experimental measurements are compared with the results of a numerical CFD modelling. Simulations under constant wall heat flux conditions are compared to local Nusselt number distributions as given by the current literature, which validates the use of the Shear Stress Transport (SST) k-ω turbulence model for this problem. Simulated Nusselt numbers obtained at a constant wall temperature are found lower than under uniform heat flux conditions. Measurements and simulation results, at a constant wall temperature, are in good agreement. An average Nusselt number correlation is proposed for jet impingement heat transfer calculations under constant wall temperature conditions, as a function of the jet Reynolds number Re j (10 000 Re j 30 000), the geometrical parameters R/D, H/D (3 R/D 10; 2 H/D 6), and the dimensionless viscosity ratio μ j /μ w (1.1 μ j /μ w 1.4).

Heat transfer in a turbulent slot jet flow impinging on concave surfaces

International Communications in Heat and Mass Transfer, 2013

An experimental and numerical study is conducted to investigate turbulent slot jet impingement cooling characteristics on concave plates with varying surface curvature. Air is used as the impingement coolant. In the experimental work, a slot nozzle specially designed with a sixth degree polynomial in order to provide a uniform exit velocity profile was used. The experiments were carried out for the jet Reynolds numbers in the range of 3423 ≤ Re ≤ 9485, the dimensionless nozzle-to-surface distance range of 1 ≤ H/W ≤ 14 for dimensionless values of the curvature of impinging surfaces in the range of R/L = 0.5, 0.725, and 1.3 and a flat impingement surface. Constant heat flux was applied on the plates. Numerical computations were performed using the k-ε turbulence model with enhanced wall functions. For the ranges of the governing parameters studied, the stagnation, and local and average Nusselt numbers have been obtained both experimentally and numerically. The numerical results showed a reasonable agreement with the experimental data.

Numerical Simulation of Heat Transfer in an Axisymmetric Turbulent Jet Impinging on a Flat Plate

2007

A computational study of the impingement of a thermally turbulent jet on a solid plate, using k e model, is reported. The possibility of improving the heat transfer is carried out according to the characteristic parameters of the interaction jet-wall. The close zone solid wall required a particular treatment using an economic method known as "wall functions". The numerical resolution of the equations is carried out using the finite volume method. For a fixed nozzle–plate distance, the influence of the Reynolds number on the stagnation point heat transfer is investigated. Good agreement with experimental results is observed. The influence of the nozzle–plate distance on the stagnation point Nusselt number is also discussed.

Heat Transfer to an Obliquely Impinging Air Jet

The current research is concerned with the measurement of convective heat transfer to an impinging air jet for a range of test parameters which include Reynolds numbers, (Re) of 10000 and 20000; nozzle to impingement surface distance, (H/D) from 0.5 to 2, and angle of impingement, (α) from 45 • to 90 • (normal impingement). Both time-averaged and fluctuating heat transfer is investigated. In this range of low nozzle to impingement surface distances, the wall jet undegoes transition from laminar to turbulent. The transitional boundary layer is identified from the time-averaged heat transfer profiles. A flow structure initiates in the shear layer of the free jet and then impacts on the plate and moves along the wall jet. The corresponding fluctuating heat transfer is reported. It is shown that the flow structure grows initially as it moves radially from the stagnation point and eventually fades with further increasing radial position as the boundary layer becomes fully turbulent.