Nonlinear flow and heat transfer dynamics of a slot jet impinging on a slightly curved concave surface

The nonlinear flow and heat transfer characteristics for a slot jet impinging on a slightly curved concave surface are experimentally studied here. The effects of jet Reynolds number on the jet velocity distribution and circumferential Nusselt numbers are examined. 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 surface to obtain an impingement cooling by the air jet at ambient temperature. The measurements are made for the jet Reynolds numbers of 8617, 13 350 and 15 415. New correlations for local, stagnation point, and average Nusselt numbers as a function of jet Reynolds number and dimensionless circumferential distance are reported.     

Milliscale confined impinging slot jets: Laminar heat transfer characteristics for an isothermal flat plate

Heat transfer and overall visualized flow characteristics of confined, laminar milli-scale slot jets are investigated, as they impinge upon an isothermal flat target plate, with a fully-developed profile at the nozzle exit. The effects of Reynolds number Re and normalized nozzle-to-plate distance ratio H/B are investigated for Re = 120–200, H/B = 2–10, and B = 1.0 mm, with a nozzle aspect ratio of y/B = 50. Instantaneous visualizations of overall slot jet flow structure show unsteady lateral distortions of jet columns at experimental conditions corresponding to the presence of continuous sinusoidal oscillations. Also apparent in flow visualization sequences are smoke signatures associated with instantaneous vortex structures which form as secondary flows develop in fluid which, initially, is just adjacent to the jet column. For each Reynolds number considered, local stagnation region Nusselt numbers Nu_o decrease dramatically as H/B increases to become greater than 7.2–13.2, as the Reynolds number is maintained constant at a value from 200 to 120, changes which occur just as continuous sinusoidal oscillation of the jet column begins to develop. The further development of continuous sinusoidal oscillating motion results in an approximate collapse of stagnation region Nusselt numbers measured at different Re and H/B values. When surface thermal boundary condition data are compared, the constant surface temperature data are generally higher than the constant surface heat flux data near the stagnation location, and lower at locations where x/B is greater than 1–2. The constant surface temperature data also show relatively low values with only very small changes with x/B, for x/B values which are greater than about 5.0. As such, the results illustrate the sensitivity of Nusselt numbers for laminar boundary layer and laminar slot jet flows to thermal boundary condition, as well as the restrictions on near-wall temperature gradients which result from a constant surface temperature thermal boundary condition.     

A numerical investigation on the fluid flow and heat transfer in the confined impinging slot jet in the low Reynolds number region for different channel heights

The numerical solution is obtained for unsteady two-dimensional fluid flow and heat transfer in a confined impinging slot jet using the finite volume method. In order to consider the effect of Reynolds number and height ratio on the flow and temperature fields in the channel, the numerical simulations were performed for different Reynolds numbers of 50–500 and different height ratios of 2–5. The critical Reynolds number, beyond which the flow and thermal fields change their state from steady to unsteady, depends on the Reynolds number and height ratio. The unsteadiness gives a big impact on the flow and temperature fields and as a result the pressure coefficient, skin friction coefficient and Nusselt number in the unsteady region show different characteristics from those in the steady region.     

Numerical study of confined slot jet impinging on porous metallic foam heat sink

This study numerically investigates the impinging cooling of porous metallic foam heat sink. The analyzed parameters ranges comprise ε = 0.93/10 PPI Aluminum foam, L/W = 20, Pr = 0.7, H/W = 2–8, and Re = 100–40,000. The simulation results exhibit that when the Re is low (such as Re = 100), the Nu_max occurs at the stagnation point (i.e. X = 0). However, when the Reynolds number increases, the Nu_max would move downwards, i.e. the narrowest part between the recirculation zone and the heating surface. Besides, the extent to which the inlet thermal boundary condition influences the prediction accuracy of the Nusselt number increases with a decreasing H/W and forced convective effect. The application ranges of H/W and Re that the effect of the inlet thermal boundary condition can be neglected are proposed. Lastly, comparing our results with those in other studies reveals that the heat transfer performance of the Aluminum foam heat sink is 2–3 times as large as that without it. The thermal resistance is also 30% less than that of the plate fin heat sink for the same volumetric flow rate and the 5.3 mm jet nozzle width. Therefore, the porous Aluminum foam heat sink enhances the heat transfer performance of impinging cooling.     

Experimental and numerical investigation of natural convection effects on confined impinging jet heat transfer

The influence of natural convection on the local and average heat transfer at increasing temperature differences between the jet and the target plate from confined impinging jets has been experimentally and numerically investigated. Local Nusselt numbers were obtained numerically for jet Reynolds numbers in the range of 250–1000, and jet to target spacings of 2, 4, 8, 12 jet diameters at various modified Grashof numbers, to determine the effect of buoyancy induced natural convection. To determine the overall effect of natural convection on the average heat transfer, experiments have been conducted for Re numbers in the range 250–5000 and dimensionless jet to target spacing 2, 4, 6, 8, and 12 at increasing modified Grashof numbers. It has been determined that buoyancy induced natural convection might have opposing or assisting influence on local heat transfer at different locations of the target plate. It has also been shown that especially at low jet inlet velocities the average heat transfer coefficient at the highest modified Grashof number, where the natural convection is effective, is higher than the value corresponding to the lowest Grashof number at which buoyancy effects are negligible, by as much as 37%.     

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

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.     

Hydrodynamics of a turbulent slot jet flow impinging on a concave surface

This study investigates hydrodynamic characteristics of a slot jet flow impinging on a concave surface experimentally and numerically. Six different concave plates with varying surface curvature and a flat plate are used. Air is used as the impinging coolant. In the experimental work, the slot nozzle used was specially designed with a sixth degree polynomial in order to provide a uniform velocity profile at its exit. The experiments were carried out for the jet Reynolds numbers in the range of 3000 < Re < 12500, the dimensionless nozzle-to-surface distance range of 1 ≤ H/W ≤ 14 for dimensionless value of the curvature of impinging surfaces in the range of R/L = 0.5, 0.5125, 0.566, 0.725, and 1.3. The pressure coefficient, C_p, for each test case was obtained across dimensionless arc length, s/W. Numerical computations were performed by using the k-ε turbulence model with enhanced wall functions for the concave plate with R/L = 0.725 and for the flat plate. The numerical results showed a reasonable agreement with the experimental data.     

Effects of geometric parameters on confined impinging jet heat transfer

The objective of this work is to carry out a numerical investigation to examine the effects of geometric parameters on the confined impinging jet heat transfer. Parameters such as Nusselt number, Reynolds number, H/W have been studied. Nozzle width H ranges from 0.6 mm to 2 mm, and nozzle-to-plate spacing W ranges from 0.5 mm to 10 mm. The jet flow is in the range of laminar flow with Reynolds number from 26.8 to 1000. This paper presents distributions of target surface temperature, local and average Nusselt number on the target plate. Pressure drop for different H/W is also obtained. This study can provide useful information to the application of impinging jet heat transfer in industry.     

A numerical study on the fluid flow and heat transfer in the confined jet flow in the presence of magnetic field

The present study numerically investigates two-dimensional fluid flow and heat transfer in the confined jet flow in the presence of applied magnetic field. For the purpose of controlling vortex shedding and heat transfer, numerical simulations to calculate the fluid flow and heat transfer in the confined jet are performed for different Reynolds numbers in the absence and presence of magnetic fields and for different Prandtl numbers of 0.02 (liquid metal), 0.7 (air) and 7 (water) in the range of 0 N 0.05, where N is the Stuart number (interaction parameter) which is the ratio of electromagnetic force to inertia force. The present study reports the detailed information of flow and thermal quantities in the channel at different Stuart numbers. As the intensity of applied magnetic fields increases, the vortex shedding formed in the channel becomes weaker and the oscillating amplitude of impinging jet decreases. The flow and thermal fields become the steady state if the Stuart number is greater than the critical value. Thus the pressure coefficients and Nusselt number at the stagnation point representing the fluid flow and heat transfer characteristics also vary as a function of Stuart number.     

Flow field and heat transfer characteristics in an oblique slot jet impinging on a flat plate

An experimental study was performed to determine the effects of inclination of an impinging two dimensional slot jet on the heat transfer from a flat plate. Local Nusselt numbers and surface pressure distributions were determined depending on inclination angle, jet-to-plate spacing and Reynolds number. The results showed that the location of maximum heat transfer was mainly due to the angle of inclination. As the inclination angle increases, the location of the maximum heat transfer shifts towards the uphill side of the plate and the value of the maximum Nusselt number gradually increases at lower jet-to-plate spacings.     

Heat transfer characteristics of a slot jet impinging on a semi-circular convex surface

Surface heat transfer characteristics of a heated slot jet impinging on a semi-circular convex surface have been investigated by using the transient heating liquid crystal technique. Free jet velocity, turbulence and temperature characteristics have been determined by using a combination of an X-wire and a cold wire anemometry. The parametric effects of jet Reynolds number (Re_W) ranging from 5600 to 13,200 and the dimensionless slot nozzle-to-impingement surface distance (Y/W) ranges from 2 to 10 on the local circumferential heat transfer have been studied. Local circumferential Nusselt number (Nu_S) decreases with increasing the dimensionless circumferential distance (S/W) from its maximum value at the stagnation point up to S/W=3.1. The transition in the wall jet from laminar to turbulent flow was completed by about 3.3?S/W?4.2 which coincided with a secondary peak in heat transfer. Correlations of local and average Nusselt numbers with Re_W, Y/W and S/W have been established for the stagnation point and the circumferential distribution. The rate of decay of average circumferential Nusselt numbers around the semi-circular convex surface is much faster than that which occurs laterally along the flat surface. As Y/W increases, the effect of surface curvature becomes apparent and the difference between the flat surface correlation and the convex surface becomes more pronounced.     

Numerical simulation of flow and heat transfer from slot jets impinging on a cylindrical convex surface

Flow and heat transfer characteristics of slot jets impingement to a cylindrical convex surface are numerically investigated.Suitable turbulence models have been determined through comparison with the experimental data.Flow structures are described and impingement heat transfer characteristics are discussed.The effects of Re,H/B and D/B on single-slot jets impingement heat transfer are analyzed and heat transfer characteristics of multiple-slot jets are investigated.The results show that:Gas flows along the convex surface and boundary layer separation occurs in both single and multiple-slot jets impingement.A maximum stagnation Nu appears at H/B=8 and the local Nu decreases with increasing H/B in the region far away from the stagnation.The Nu in the stagnation region decreases with increasing D/B but the Nu is nearly the same in the region far away from the stagnation.Pressure gradient is an important factor on heat transfer enhancement.Correlations of the Num for single-slot,double-slot and quadric-slot jets impinging on a convex surface are obtained.It indicates the effects of Re and D/B on Num could become more important in less slot jets impingement.     

Experimental study on boiling heat transfer with an impinging jet on a hot block

Previous studies on boiling heat transfer by impinging jets were mainly concerned with the impinging point by using small heat transfer surfaces of about 20 mm. An experimental study was made of the boiling heat transfer to an impinging water jet on a massive hot block. The upward heating surface was made of copper, its diameter and the nozzle diameter being 80 and 2.2 mm, respectively. The velocity of the impinging jet was varied between 0.6 and 2.1 m/s. Saturated water impinged normally on the heating surface, flowed radially, and subsequently dispersed into the atmosphere. It is clarified in the present study that heat transfer characteristics vary with the temperature of the heat transfer surface, and also with the distance from the impinging point. © 1999 Scripta Technica, Heat Trans Asian Res, 28(5): 418–427, 1999     

Evaporation under an obliquely impinging laminar ducted slot jet — a numerical model

Results of a numerical study of the flow, heat and mass transfer under a semi-confined laminar slot jet are presented for oblique impingement into a crossflowing stream. The specific case studied is that of evaporation of water from the isothermal wet impingement surface into a relatively dry air stream.     

Estimation of heat flux on the surface of an initially hot cylinder cooled by a laminar confined impinging jet

In this study, an inverse algorithm based on the conjugate gradient method and the discrepancy principle is applied to estimate the unknown space-and time-dependent heat flux at the surface of an initially hot cylinder cooled by a laminar confined slot impinging jet from the knowledge of temperature measurements taken on the cylinder’s surface. It is assumed that no prior information is available on the functional form of the unknown heat flux; hence the procedure is classified as the function estimation in inverse calculation. The temperature data obtained from the direct problem are used to simulate the temperature measurements, and the effect of the errors in these measurements upon the precision of the estimated results is also considered. The results show that an excellent estimation on the space-and time-dependent heat flux can be obtained even the distributions of thermal properties inside the cylinder is unknown.     

Height effect on heat transfer characteristics of sintered porous blocks with a confined slot jet

This work numerically investigates the heat transfer of a sintered porous block under a confined slot air jet. The width of the jet nozzle (W) is 5 mm; the ratio of the porous block length to the jet nozzle width (L/W) is 12, and the Prandtl number (Pr) is 0.7. Variable parameters are the ratio of the porous block height to the jet nozzle width (H/W) and the Reynolds number (Re). The findings reveal that the cooling performance with the sintered porous block was better than that with an aluminum foam block, and was at least 5.8 times as large as that without it. The Nusselt number increased as the H/W fell. The effect of Reynolds number on the heat transfer was negligible at Re ≤ 1000 but considerable at Re > 1000.     

Experimental investigation of the flow and heat transfer of an impinging jet under acoustic excitation

Classic and high speed particle image velocimetry and infrared thermography are used to investigate the behavior of a round jet impinging on a flat plate for a Reynolds number 28,000, for orifice-to-plate distances of 3 or 5 nozzle diameters and for two different nozzles, a contraction and a long tube. The contraction nozzle reveals a different heat transfer distribution on the impinging plate compared to the long tube case. The jet can be excited by a loudspeaker at Strouhal numbers 0.26, 0.51 and 0.79. This acoustic forcing changes the jet structure, modifying annular vortex rings in the shear layer of the jet and increasing the turbulent values. The heat transfer is therefore modified, resulting in an increase of the Nusselt number near the jet axis and an alleviation or a shift of the secondary peak.     

Electrochemical mass transfer between an impinging jet and a rotating disk in a confined system

This paper presents the mass transfer results from an impinging liquid jet to a rotating disk. The mass transfer coefficients were measured using the electrochemical limiting diffusion current technique (ELDCT). Rotational Reynolds number (Re_r) in the range of 3.4 × 10⁴–1.2 × 10⁵, jet Reynolds number (Re_j) 1.7 × 10⁴–5.3 × 10⁴ and non-dimensional jet-to-disk spacing (H/d) 2–8 were taken into consideration as parameters. It was found that the jet impingement resulted in a substantial enhancement in the mass transfer compared to the case of the rotating disk without jet.