Point Of Maximum Heat Transfer Research Articles

On the impingement of heat transfer using swirled air jets

Experimental analyses were conducted under identical experimental conditions on the heat transfer between a constant heat flux flat plate and a round air jet for both conventional and swirled jets. Vane-type swirl generators inserted at the nozzle exit were used to produce a swirl. The experimental measurements were performed at a fixed Reynolds number value (Re = 23,000) calculated on the jet tube’s inside diameter. A comparison between conventional air jets and swirl jets with swirl numbers of S = 0, 0.19, 0.42, and 0.72 was presented for the different nozzle-to-plate spacings Z/D = 2, 6, and 10. The results show that heat transfer to the plate decreases when the nozzle-to-plate distance increases. In addition, increasing the swirl number S increases heat transfer uniformity but decreases global heat transfer. At the low plate-to-jet distance Z/D = 2, the point of maximum heat transfer is shifted to a radial position depending on the swirl number. Also, for both Z/D = 6 and 10, the stagnation point and stagnation region heat-transfer enhances only for swirl numbers 0 and 0.19.

Numerical Simulation of Oblique Air Jet Impingement on a Heated Flat Plate

Jet impingement cooling has been studied extensively as this finds applications in the areas of reactor safety, electronic cooling, etc. Here, the convective heat transfer process between the air jet impingement on a uniformly heated inclined flat plate is studied numerically. In this numerical study, 3D simulations are carried out using commercial CFD code to investigate the effect of angle of inclination of plate, Reynolds number, and distance between the nozzle exit and the plate on the heat transfer characteristics. V2F model has been used to model turbulence for various nozzle–plate distance and Reynolds number. It can be concluded that V2F model predicts the Nusselt number variation on the plate satisfactorily. It is observed that point of maximum heat transfer is at the stagnation point in case of vertical jet impinging on a horizontal plate, while it shifts away from the point of impingement for the case of a vertical jet impinging on an inclined flat surface. The shift is toward the “compression side” or the “uphill side” of the air jet. The results are validated with experimental data from the literature. Detailed analysis of local heat transfer coefficients, velocity contours, temperature contours, and Nusselt number variations on the flat plate is presented.

Bond graph aided thermal modelling of twin-tube shock absorber

Keeping in view the role of heat generation and dissipation, the aim of this work is to develop a thermal model for the twin-tube hydraulic shock absorber using bond graph approach. In this paper, experiment is also conducted on a rear shock absorber of a commercial passenger sedan. The temperature at the upper and lower part of the shock absorber and the force applied on the shock absorber are measured. Simulation results are compared with the experimental results to validate the model. Both the experimental and simulation results show that the volume flow rate of hydraulic fluid is more in the upper chamber than that in the lower chamber. If the surface temperature and hydraulic fluid temperature become equal, external work given to the shock absorber acts as a single entity transferring heat to the surroundings. This point marks the point of maximum heat transfer to the surroundings.

Forced Convection Heat Transfer of Newtonian Fluids from a Backward‐Facing Step: Effects of Expansion Ratio, Reynolds, and Prandtl Numbers

This study involves the numerical solution of the laminar heat transfer in a separating and reattaching flow by simulating the flow and heat transfer downstream of a backward‐facing step. The in‐house finite volume code has been implemented employing a hybrid differencing scheme and the SIMPLE algorithm for the pressure–velocity coupling. Three principal parameters governing heat transfer in this geometry, that is channel expansion ratio (ER), Reynolds number (Re), and Prandtl number (Pr), are systematically varied in the range ER = 1.111 to 2, Re = 1 to 200, and Pr = 0.71 to 100, and the simple correlations between these parameters have been elucidated. A series of important findings have been established by analyzing the results some of which are: (1) there is an associated shifting of the point of maximum heat transfer with respect to the flow‐reattachment point with gradually decreasing the values of ER and (2) the heat transfer enhancement increases with the increase in Pr number as a result of the compression of the thermal boundary layer and the maximum Nusselt number varies as .

Experimental Investigation of Heat Transfer for a Jet Impinging Obliquely on a Flat Surface

An experimental study investigated the effects of the inclination of an impinging circular jet on heat transfer from a horizontal flat plate. Local Nusselt number distribution was determine depending on inclination angle, jet-to-plate distance, and Reynolds number. The results show that the maximum heat transfer point moves toward the uphill side of the plate and that the maximum heat transfer decreases as the inclination angle decreases. The correlations were conducted to predict maximum and local Nusselt number as a function of Re, θ, L/D, and x/D for three specific regions.

Heat transfer from a flat surface to an inclined impinging jet

An experimental study was carried out to investigate the effect of the inclination jet on convection heat transfer to horizontal flat plate. Local heat transfer determined as a function is of three parameters including inclination angle of the air jet relative to the plate in range of 90° ≤ θ ≤ 45°, jet-to-plate spacing in range of 2 ≤ L/D ≤ 8 and Reynolds number in range of 1,500 ≤ Re ≤ 30,000. The results show that the maximum heat transfer point moves towards the uphill side of the plate and the maximum heat transfer decreases as the inclination angle decreases. The correlations were conducted to predict maximum and local Nusselt number as a function of Re, θ, L/D, and x/D for a specific three regions.

Heat transfer of a row of three butane/air flame jets impinging on a flat plate

Experiments were performed to investigate the heat transfer characteristics of a row of three premixed, laminar, butane/air flame jets impinging on a water-cooled flat plate. The between-jet interference was found to reduce the heat transfer rate in the jet-to-jet interacting zone due to the depressed combustion. The interference became stronger when the jet-to-jet spacing and/or the nozzle-to-plate distance were/was small. The positive pressure existed in the between-jet interacting zone caused the asymmetric flame and heat transfer distribution of the side jet. The meeting point of the spreading wall jets of the central and the side jets did not occur at the midpoint of the neighboring jets, but at a location shifted slightly outwards. The maximum local heat flux and the maximum area-averaged heat flux occurred at a moderate nozzle-to-plate distance of 5d with a moderate jet-to-jet spacing of 5d. The lowest area-averaged heat flux was produced when both the jet-to-jet spacing and the nozzle-to-plate distance were small. Comparing with a single jet under the same experimental conditions, the heat transfer rates in both the stagnation point and the maximum heat transfer point were shown to be enhanced in a row of three-jet-impingement system. The present study provided detailed information on the heat transfer characteristics of a row of three in-line impinging flame jets, which had rarely been reported in previous study.

Heat transfer studies of the oblique impingement of round jets upon a curved surface

The effect of jet inclination of the local heat transfer under an obliquely impinging round air jet striking on isothermal circular cylinder is experimentally investigated. The circumferential heat transfer distribution as well as axial Nusselt number is measured. The considered parameters are jet Reynolds number in range of 3800–40,000, and jet inclination angle, ranging from 90 to 20. The experiments are carried out for nozzle sizes, d=3, 5 and 7 mm, and separation distance from 7 to 30 of the nozzle diameter. The output results indicated that the point of maximum heat transfer along the x-axis is shifted upstream and the local heat transfer distribution changed as a function of jet inclination. The magnitude of the shift was found to be significantly higher than that observe for a flat plate. The increasing inclination caused increasing asymmetry around the point of maximum heat transfer, with the upstream side of heat transfer profile dropping off more rapidly than the downstream side. Correlations of both the magnitude and shift of maximum heat transfer point are presented. The surface average heat transfer rate is calculated and compared with the normal impingement.

The effect of inclination on the heat transfer between a flat surface and an impinging two-dimensional air jet

Abstract An experimental study was performed to determine the effect of the inclination of an impinging two-dimensional air jet on the heat transfer from a uniformly heated flat plate. The impingement surface was a stainless steel plate of the same width as the jet nozzle. Local Nusselt numbers were determined as a function of three parameters: (a) inclination angle of the air jet relative to the plate in the range of 90–40°, (b) nozzle exit-to-plate spacing ( z / D ) in the range of 4–12 and (c) Reynolds number based on the hydraulic diameter of the slot nozzle in the range of 4000–12 000 (corresponding to an exit jet velocity from 6.3 to 18.7 m/s). The results are presented in the form of graphs showing the variation of the local Nusselt number as a function of these parameters. The region of maximum heat transfer shifts towards the uphill side of the plate and the maximum Nusselt number decreases as the inclination angle decreases. The location of the maximum heat transfer region appears to fall between 0 and 3 D uphill from the geometrical impingement point, and was found to be insensitive to the Reynolds number in the range used in this study. For low values of inclination angle, the local Nusselt number on the uphill side from the maximum heat transfer point was insensitive to jet exit-to-plate spacing. Correlations are proposed to predict the local Nusselt number as a function of x / D , z / D , θ and Re .

Transfert de chaleur au voisinage du point de recollement en aval d'une marche descendante

It has been noticed, although not yet explained, that in recirculating flows the heat transfer coefficient, which is maximum in the reattachment zone, does not correspond in position with the mean reattachment point. We propose an explanation based on impact (main component of velocity V < 0) and sweeping (main component of velocity U > 0) phenomena which are both induced by the vortical structures interacting with the heated wall. This approach uses information given by a numerical code based on the finite volume technique, as well as an analysis of the time evolution of the reattachment point X r and the point of maximum heat transfer X max . This analysis is supported by the time series and the profiles of the correlations relative to the velocities and temperature variables.

Heat and mass transfer in a separated flow region for high Prandtl and Schmidt numbers under pulsatile conditions

Heat and mass transfer phenomena were studied in the sudden expansion region of a pipe under steady and pulsatile conditions. The Prandtl number was varied from 100 to 12 000 and the flow was characterized for both uniform and parabolic entrance velocity profiles. A uniform velocity profile was used for pulsatile flow. It was found that heat transfer in the recirculation region was maximal near the area where wall shear was minimal. Blunting of the inlet profile caused the point of maximum heat transfer to move upstream. There was a nonlinear effect of Prandtl number on heat transfer which plateaued for Pr > 10 3. The wall shear rate in the separation zone varied markedly with pulsatile flows, but the wall heat transfer remained relatively constant. The time-averaged pulsatile heat transfer at the wall was approximately the same as with steady flow with the mean Reynolds number. However, the isotherms within the pulsatile flow were markedly different from steady flow. The results demonstrate the complexity of separation flows and identify characteristic regions of high and low heat/mass transfer for high Prandtl/Schmidt pulsatile flow.

HEAT TRANSFER AND FLUID FLOW OF TURBULENT NATURAL CONVECTION ALONG A VERTICAL FLAT PLATE WITH A BACKWARD-FACING STEP

Heat transfer and fluid flow of turbulent natural convection along a vertical flat plate with a backward-facing step were investigated experimentally. The effect of step height on heat transfer was classified through a series of heat transfer measurements. The wall temperature and fluid flow were then visualized using a liquid-crystal sheet and water-soluble fluorescent paint in order to investigate the separation and reattachment of fluid flow. It was revealed that the characteristic low-temperature patterns appear on the backward heat transfer plate and that they play a significant role in heat transfer enhancement. These patterns are generated by the penetration of the low-temperature fluid lumps having three-dimensional and unstable structures near the wall. It was further found that the maximum heat transfer point is not consistent with the reattachment point of the separated flow and that these points shift downstream with increasing step height.

Fluid flow and heat transfer in a laminar radial impinging jet

From the solution of full Navier—Stokes and energy equations, the development of the flow field and heat transfer characteristics in a radial jet reattachment flow have been analysed. The influence of Reynolds number of re‐attachment length for the case of steady laminar flows has been determined. However, beyond a Reynolds number of 250, the flow field becomes unsteady and has been found to have a periodic nature. This periodic flow has been found to persist up to a Reynolds number of 750. The periodicity has been characterized by the Strouhal number which shows a slight but continuous variation with Reynolds number around a value of 0.12. The point of maximum heat transfer is within the re‐attachment zone in the range of Reynolds numbers studied.

Experimental Study on Heat-Transfer Enhancement in Turbulent Natural Convection along a Vertical Flat Plate. 2nd Report, Effect of Backward-Facing Step.

Heat transfer and Fluid flow of turbulent natural convection with a backward-step were investigated experimentally, and the effect of step size on heat transfer was discussed. The wall temperature and the fluid flow were visualized using a liquid crystal and fluorescent paint to clarify the behavior of heat transfer coefficients. It was realized from a series of experiments that low-temperature streaks appear on the backward-heat-transfer plate and that they a significant role in heat-transfer enhancement. These patterns are generated by low-temperature fluid lumps, which possess three-dimensional and unstable structures. Moreover, the maximum heat-transfer point is not consistent with the reattachment point of the separation flow, and these points shift downstream with increasing step size H.

Evaporation of a liquid droplet on a hot plate

The evaporation of a small liquid droplet impinging on a hot stainless steel plate is investigated. The liquids include water, various pure and mixed hydrocarbon fuels with drop sizes ranging from 0.07 to 1.8 mm. The wall temperatures varying from 63 to 605°C cover the entire spectrum of heat transfer characteristics from film evaporation (below boiling temperature) to spheroidal vaporization (above Leidenfrost temperature). A strobe-video visual system is used to record the transient process and to measure the droplet lifetime. Qualitatively, the droplet lifetime curve as a function of wall temperature is similar for all liquids and all drop sizes. The maximum heat transfer rate occurs at 50–60°C above the boiling temperature for all pure liquids and the Leidenfrost heat transfer rate occurs at about 120°C for pure fuels and 180°C for water above the boiling temperature. The maximum evaporation rate can significantly exceed the burning rate of fuel droplets. Visual observations show that beyond the maximum heat transfer point, larger droplets levitate above the surface whereas smaller droplets bounce up and down to many diameters above the surface. Thus for the smallest droplets beyond the Leidenfrost temperature, the droplet lifetime actually increases with wall temperature which is in contrast to the larger droplets. The maximum heat transfer rate is independent of drop sizes for all fuels and shows a small decrease with increasing drop sizes in the case of water. At the Leidenfrost temperature, the data show that the heat transfer rate is maximum at a droplet diameter of approximately 0.5 mm.

The effect of inclination on local heat transfer under an axisymmetric, free liquid jet

The effect of jet inclination on the local heat transfer under an obliquely impinging, round, free liquid jet striking a constant heat flux surface is investigated experimentally. The problem parameters investigated are jet Reynolds number in the range 6600–52000, and jet inclination, ranging from 40 to 90 deg, measured from the horizontal. Experiments are carried out for nozzle sizes, d = 4.6 and 9.3 mm. It is found that the point of maximum heat transfer along the x-axis (the line of intersection of the jet inclination plane with the impingement surface) is shifted upstream (with respect to the jet flow) as a function of jet inclination with a maximum observed shift of 0.5 nozzle diameters. In addition, it is found that the shape of local Nusselt number profiles along the x-axis changed as the jet is inclined. One of these changes is a sharpening of the peak in the profile at the point of maximum heat transfer. Another change is an increasing asymmetry around the point of maximum heat transfer with the upstream side of the profile dropping off more rapidly than the downstream side. Some measurements are taken to characterize the full two-dimensional heat transfer distribution under the inclined jet. Correlations are developed for the radial variation of local heat transfer in the plane of nozzle inclination.

Heat Transfer to Turbulent Swirling Flow Through a Sudden Axisymmetric Expansion

Experimental data are presented for local heat transfer rates in the tube downstream of an abrupt 2:1 expansion. Water, with a nominal inlet Prandtl number of 6, was used as the working fluid. In the upstream tube, the Reynolds number was varied from 30,000 to 100,000 and the swirl number was varied from zero to 1.2. A uniform wall heat flux boundary condition was employed, which resulted in wall-to-bulk fluid temperatures ranging from 14° C to 50°C. Plots of local Nusselt numbers show a sharply peaked behavior at the point of maximum heat transfer, with increasing swirl greatly exaggerating the peaking. As swirl incressed from zero to its maximum value, the location of peak Nusselt numbers was observed to shift from 8.0 to 1.5 step heights downstream of the expansion. This upstream movement of the maximum Nusselt number was accompanied by an increase in its magnitude from 3 to 9.5 times larger than fully developed tube flow values. For all cases, the location of maximum heat transfer occurred upstream of the flow reattachment point.