Stagnation Point Nusselt Number Research Articles

Flow dynamics and heat transfer characteristics of air/water mist jet impingement using Eulerian-Eulerian approach

The present numerical study focuses on a two-phase Eulerian-Eulerian approach to model the flow structure and heat transfer characteristics of an air/water mist jet emanating from a coaxial jet nozzle and impinging on an isothermal heated flat plate. A detailed parametric numerical study has been performed to investigate the effect of various operating parameters like air Reynolds number (2400-9142), mist Reynolds number(1.87–8.68), mist loading fraction (0 % - 0.96 %), droplet diameter (1 μm – 60 μm), and non-dimensional nozzle to plate spacing (10–42) on the flow structure and heat transfer characteristics of air/water mist jet. The present numerical results are in agreement with the experimental results, within 16 % accuracy. The heat transfer increases with an increase in the air Reynolds number or mist loading fraction. The increase in air Reynolds number provides better heat transfer enhancement as high as 94.5 % compared to 27.6 % in the case of an increment in the mist loading fraction. Additionally, the effect of droplet diameter on the flow structure and heat transfer characteristics of air/water mist jet is analyzed at depth. The increase in nozzle-to-plate distance from 10 to 42 leads to a decrease in stagnation point Nusselt number of the order of 31 %. Further, correlations have been proposed to predict the Nusselt number at the stagnation point, stagnation region, and average Nusselt number for the target plate.

Heat Transfer Intensification of a Confined Impinging Air Jet Via a Guiding Baffle

Abstract The heat transfer intensification of a confined impinging jet was achieved using a rough surface or pin fins as well as using modified nozzles such as chamfering, chevron, sweeping, swirling, etc. In this work, an enhanced cooling process utilizing a single confined air jet impinged on a flat plate using a guiding baffle is implemented. The impacts of Reynolds number (Re) ranged from 500 to 5000, guiding baffle diameter-to-nozzle diameter (D/d) of 2, 4, and 6, and guiding baffle height-to-nozzle to impinging plate distance (h/H) of 1/3, 1/2, and 3/4 on the cooling process are studied. The distributions of surface temperature are acquired experimentally using a thermal infrared camera. As well, the local Nusselt number (Nu), stagnation point Nusselt number (Nust), average Nusselt number (Nu¯), and average Nusselt number ratio (Nu¯r) are evaluated. The results reveal that the enhancement of heat transfer is achieved due to installing a guiding baffle with a D/d of 2 for all values of baffle height and Reynolds number. In addition, the Nu¯r is increased with increasing the Re in the range from 500 to 2500, then it is decreased by a further increase in Re. Moreover, based on the experimental results, an empirical correlation is proposed to compute the Nu¯ depending on Re, D/d, and h/H with a ±2.65% standard deviation.

Stagnation Point Heat Transfer to an Axisymmetric Impinging Jet at Transition to Turbulence

Abstract A round air jet issuing from a long straight pipe and impinging perpendicularly onto a heated flat plate was investigated experimentally. The Reynolds number (Re) covered the fully laminar, transitional, and fully turbulent regimes—Re = 850–15,400. The main focus of this investigation was the transition regime, which occurred at Re = 2250–3010. Various measurements were recorded during the experiments using a hot-wire anemometer, an infrared thermometer, and a thermopile heat flux sensor; the mass transfer was measured using the naphthalene sublimation technique. The stagnation point heat transfer was correlated to the laminar and turbulent regimes in the form of the stagnation point Nusselt number, Nu0 = CRemPr0.4, where the exponent m = 0.50 and 0.55, respectively. The Nu0–Re relationship exhibited nonmonotonic behavior (decrease) in the transition regime. Two counteracting mechanisms occur during transition—jet core shortening and an increase in velocity fluctuation; the former reduces Nu0, whereas the latter increases it.

Estimation of the local convective heat transfer coefficients of low frequency two-phase pulsating impingement jets using the IHCP

ABSTRACT In this paper, a numerical-experimental investigation was performed for estimation of the local convective heat transfer coefficients of a two-phase pulsating heated jet impinging a cold disk-shaped mass. By solving the energy equation, temperature history of thermocouples was obtained and then using the “temperatures measured” at locations of the sensors and the “temperatures calculated” from the numerical solution of heat equation and then minimizing the sum of squared errors by the conjugate gradient method (CGM), the convective heat transfer coefficients are estimated using the nonlinear inverse heat conduction procedure (IHCP). To compare the heat transfer characteristics of the pulsating, steady, bubbly, and liquid-only jets, the heat transfer coefficients were estimated with respect to the square pulse frequency produced by a solenoid valve in the range of and the gas volume fraction in the range of . In the case of single-phase jet, results show that flow pulsation increases Nusselt number up to at the stagnation point and for farther radial distances as compared to the steady jet. Also, for steady jet, increasing Reynolds number resulted in a increase in the stagnation point Nusselt number and a increase at farther radial distances.

Heat transfer coefficients during the impingement cooling with the use of synthetic jet

The paper presents experimental studies on heat transfer phenomena which occurs during the impingement cooling with the use of synthetic jet. The synthetic jet was created by a special actuator consisting in a plexiglass cavity and a loudspeaker. An air was a working fluid. The effect of synthetic jet actuator geometry and supply parameters on heat transfer enhancement has been investigated. The experiment was performed with the use of classical thermal balance method. Heat transfer coefficients at a stagnation point have been obtained. Measurements were carried out in the range of excitation frequency f = 5 ÷ 600 Hz and of root mean square voltage E = 1, 2, 3, 4, 6 V. Changes of actuator geometry included: orifice diameters d = 9, 15, 24 mm, orifice thickness t = 3, 5, 20 mm and cavity depth H = 20, 40, 60 mm. On the basis of measurements results the correlations for the stagnation point Nusselt number of an axisymmetric impinging synthetic jet characterized by large Reynolds numbers have been obtained. Nusselt number has been determined in the function on scaling parameters: Reynolds number, jet-to-surface spacing x/d and the dimensionless stroke length L0. Reynolds numbers were ranging from 3600 to 22950, x/d = 1 ÷ 20 and L0 = 0.84 ÷ 170.5. The scaling parameters have significantly extended the range of research so far. The obtained Nusselt number correlations provide heat transfer calculations at large Reynolds numbers and are expected to have practical applications.

Effect of number of round jets on impingement heat transfer from a heated cylinder

The details of an experimental study carried out on a single jet and multiple jets emerging from round nozzles of diameter, d and impinging over a uniformly heated cylinder of diameter, D are presented. The number of jets are varied from one to six in the parametric study. In the multi-jet configurations, the jets are arranged circumferentially around the heated cylinder. The range of Reynolds number, Red considered is 5000 – 20,000. The curvature ratio, D/d is varied from 5.1 to 20.4 and the nozzle-to-cylinder spacing, H/d is varied from 2 to 12. The present results show that the Nusselt number at the stagnation point (Nustag) is not affected when the number of jets is increased. However, in multi-jet configurations with four and six jets, the interaction between the wall jets increases heat transfer in the fountain regions. The number of round jets required for the uniform cooling increases with increase in the curvature of the target cylinder, D/d. Based on the experimental study, correlations are given for the stagnation point Nusselt number (Nustag) and average Nusselt number (Nuavg).

Fluid dynamics and heat transfer in the wake of a sphere

Direct numerical simulation and large-eddy simulation have been performed for a heated sphere at Reynolds numbers of Re=1000 and Re=104, respectively. The Prandtl number for both simulations has been Pr=0.7. Measurements of the local and average Nusselt number are performed and compared with literature available experimental results. Average and front stagnation point Nusselt numbers increase with the Reynolds number, while the minimum value moves towards the sphere apex as the flow enters the sub-critical regime. Differences in both viscous and thermal boundary layers are observed, while the shape factor at Reynolds number Re=104 behaves similarly to that observed in circular cylinders at comparable Reynolds numbers. It is shown that as the Reynolds number increases, the increase in turbulent kinetic energy promotes the entrainment of irrotational flow thus enhancing the temperature mixing in the zone. The near wake, between 5 ≤ x/D ≤ 15, spreads at a faster rate at Re=1000 with a slope close to x/D1/2, while at Re=104 it follows a trend close to x/D1/3.

Heat transfer and flow characteristics of turbulent slot jet impingement on plane and ribbed surfaces

A computational study is carried out to assess the suitability of various RANS based turbulence models for slot jet impingement on flat and ribbed surfaces with various values of Reynolds number and jet to plate spacing. The com-puted results are compared with the reported experimental data. It was observed that none of the turbulence models considered predicted the heat transfer data accurately. However, some models predicted the experimental data with good trends, e.g., secondary peak and several spikes in Nusselt number for ribbed surface, with a precise computation of the stagnation point Nusselt number. Further, the effects of slot width, rib pitch and jet to ribbed surface spacing were investigated for jet impingement on a ribbed surface. It was observed that the local Nusselt number increased with slot width and rib to plate spacing. It was also observed that increasing Reynolds number had a positive effect on the local heat transfer. With increasing rib pitch the local Nusselt number increased near the stagnation zone but de-creased downstream. The observed flow pattern was different for jet impingement on a ribbed surface than that on a flat surface.

Particle image velocimetry and infrared thermography of turbulent jet impingement on an oscillating surface

Jet impingement is widely used for forced-convection heat transfer applications and knowledge about its flow structure and heat transfer rate on a static surface are well established. However, the characteristics of jet impingement on an oscillating surface are relatively unknown. This study experimentally investigates the effect of surface oscillation on the fluid dynamics and heat transfer of an unconfined turbulent impinging jet. The Reynolds numbers of the axisymmetric jet are 5000 and 10,000, based on the jet-nozzle exit diameter, and the surface is placed at nominal standoff distances of 2 and 5 diameters from the jet-nozzle exit. The surface oscillates in a direction parallel to the jet axis at frequencies of 20 Hz and 50 Hz and at a peak-to-peak displacement amplitude of 0.2 times the jet-nozzle exit diameter. The phase-average and mean flow characteristics at six phases through the surface oscillation cycle, and the steady-state mean heat transfer rate at the oscillating surface, are determined using particle image velocimetry and infrared thermography respectively. These are analyzed and compared with the mean flow and heat transfer characteristics for jet impingement on a static surface. Surface oscillation directly affects the mean axial jet velocities and thence the mean radial velocities, and this effect is greater at locations in the flow-field closer to the surface. This gives rise to lower mean axial and radial strain rates in the impingement region and lower turbulence intensities in the wall-jet region when compared with those for a static surface. The frictional interaction between the impinging jet and oscillating surface induces higher surface temperatures than those on a static surface. These factors reduce the heat transfer rate for jet impingement on an oscillating surface when compared with that on a static surface. The reduction is greater in the impingement region than in the wall-jet region with the stagnation point Nusselt number for an oscillating surface being lower by a maximum value of 15%. Overall, for the range of parameters considered in this study, these findings suggest that surface oscillation in jet impingement weakens the transport phenomena capabilities from those present in the case of a static surface.

Numerical Investigation of Laminar Forced Convection and Entropy Generation of Nanofluid in a Confined Impinging Slot Jet Using Two-Phase Mixture Model

In this article, computational fluid dynamics (CFD) simulations is used to investigate the volumetric entropy generation and heat transfer on confined impinging slot jet, with a mixture of water and Al2O3 nanoparticles as working fluid. The flow is laminar and a constant temperature is applied on the impingement surface. The governing mass and momentum equations for mixture and dispersed phase and also energy equation for mixture are solved using the finite volume method. This paper studies the effects of different geometric parameters, particle volume concentration and Reynolds number on local and average Nusselt number, stagnation point Nusselt number, entropy generation and stream function contours. The results showed that the intensity and size of the vortex structures depend on jet-to-impingement surface distance ratio (H/W), Reynolds number and particle concentrations. As H/W ratio increases, average and stagnation point Nusselt number decrease due to flow instability. By increasing Reynolds number and volume concentration, average Nusselt number and exergy loss increase due to stretching of the vortex structure in downstream direction. From the CFD results, it is found that a substantial portion of entropy generation occurs at stagnation and wall jet regions.

Experimental Investigation of Heat Transfer Characteristics on a Flat Surface by Circular Air Jet Impingement

An experimental investigation is performed to study the effect of jet to plate spacing and low Reynolds number on the local heat transfer distribution to normally impinging submerged circular air jet on a smooth and flat surface. A single jet from a straight circular nozzle of length to diameter ratio(l/d) of 83 is tested. Reynolds number based on nozzle exit condition is varied between 500 and 8,000 and jet to plate spacing between 0.5 and 8 nozzle diameter. The local het transfer characteristics are obtained using thermal images from infrared thermal imaging technique. It was observed that at lower Reynolds numbers, the effect of jet to plate distances covered during the study on the stagnation point Nusselt numbers is minimal. At all jet to plate distances, the stagnation point Nusselt numbers decrease monotonically with the maximum occurring at a z/d of 0.5 as opposed to the stagnation point Nusselt numbers at high Reynolds numbers which occur around a z/d of 6.

Effects of the stroke length and nozzle-to-plate distance on synthetic jet impingement heat transfer

This study focuses on the combined effect of the nozzle-to-plate distance and of the stroke length on the cooling performances of impinging synthetic jets. Infrared thermography is used as temperature transducer in conjunction with the heated thin foil heat transfer sensor to measure time- and phase-averaged convective heat transfer. All the experiments have been performed at a fixed Reynolds number equal to 5250, while different values of the dimensionless stroke length (L0/D equal to 5, 10 and 20) and nozzle-to-plate distance (H/D between 2 and 10) have been considered. At high L0/D, the heat transfer behaviour resembles that of a continuous impinging jet. It is characterized by a time-averaged stagnation Nusselt number maximum between H/D equal to 4 and 6 and inner and outer ring-shaped regions of Nusselt number maximum at short H/D. These two regions are replaced by a bell-shaped distribution at higher nozzle-to-plate distances. The existence of these regions is clearly observed through the phase-averaged heat transfer measurements. At short H/D, the heat transfer evolution reveals the simultaneous presence of two outer ring-shaped regions. The external outer region is ascribed to the strong coherence of the primary vortex ring, while the internal one is mainly due to the vortex rings generated by the Kelvin–Helmholtz instability along the trailing jet shear layer. At high H/D, the internal outer-ring shaped region disappears because of the weakening of the trailing jet Kelvin–Helmholtz vortex rings. In opposition, at short L0/D, the time-averaged stagnation point Nusselt number is found to have a maximum at lower values of H/D, and no inner ring-shaped region of heat transfer maximum is observed. This region is not present, at low dimensionless stroke lengths, because of the weakness and reduced extent of the trailing jet in the flow field. Indeed, the phase-averaged measurements mainly show the heat transfer caused by the impinging primary vortex ring. Despite the weakness of the trailing jet, the outer ring-shaped region of heat transfer maximum is observed at short H/D because of the presence of a strong primary vortex ring. In addition to that, the dimensionless stroke length and nozzle-to-plate distance also affect the heat transfer fluctuations, which decrease as L0/D decreases and/or H/D increases.

Double circular air jet impingement cooling of a heated circular cylinder

Experimental and numerical investigations were carried out on double circular air jet impingement cooling of a heated circular cylinder to understand flow and heat transfer characteristics. The surface of the heated cylinder was maintained at a constant heat flux condition. In the parametric study, the Reynolds number, Red defined based on the diameter of the nozzle was varied from 10,000 to 25,000. The ratio of the diameter of the nozzle to the diameter of the heated cylinder, d/D was maintained at 0.208. The effect of non-dimensional distance between double the jets (s/d) and the ratio of the non-dimensional distance between the nozzle exit and the circular cylinder surface (h/d) were investigated for the range of 4–20 and 4–16, respectively. No appreciable change was observed in the stagnation point Nusselt number with change in s/d. However, with the increase in s/d, the average Nusselt number over the cylinder surface was also found to increase. In this study, the effect of flow confinement below the heated cylinder on heat transfer was also investigated. Both experimental and numerical studies show that confinement at the bottom of the cylinder increases heat transfer.

Experimental investigation of the relationship between heat transfer rate and number of broken glass particles in tempering process of glass plates

Thermal glass tempering process is based on heating the glass in a furnace and suddenly cooling it down in a cooling unit. The quality of the glass obtained at the end largely depends on the heat transferred from the glass surface. In this study, the glass tempering process was conducted in a specially designed and manufactured glass tempering unit prototype, and the change in the local number of broken glass particles was examined based on the local heat transfer rate. A 6-mm-thick flat glass in 170×170mm dimensions was used in the study. Two single nozzles, 13mm in diameter and 300mm in length that were placed against each other were used for the cooling process. The distance between the nozzle and the glass plate surface was taken as 1⩽H/D⩽10, and the Reynolds number as 40,000⩽Re⩽60,000. In the sudden cooling process conducted according to different H/D ratios and Reynold numbers, the cooling times, and the local, average and stagnation-point Nusselt numbers were obtained. Then, the tempered glasses produced under these conditions were broken apart, and the number of particles was determined. The results showed that the cooling time firstly decreased and then increased with H/D ratio generally, whereas it decreased as the Reynolds number increased. It was also observed that as the average Nusselt number increased, the number of particles also increased. Besides, as the air jet moved away from the stagnation point, the Nusselt number decreased, and concordantly, the number of particles also decreased. It was found that the number of particles was substantially proportional to the heat transfer rate.

Experimental investigation of impingement heat transfer from a round rib-roughened surface

An experimental investigation on impingement heat transfer from a rib-roughened surface is performed. Single, double, and triple rib configurations are tested. The rib is also located at three different positions for a single rib case. The Reynolds number is varied from 10,000 to 50,000 whereas the dimensionless jet-to-surface distance (z/D) is kept constant as 8. Results show that, the Nusselt number decreases just before a rib. After the rib, however, the Nusselt number decreases by a larger amount. Since ribs cause a reincreasing in the Nusselt number after the stagnation point and since the stagnation point Nusselt number is not affected by ribs, they can be used to enhance heat transfer especially for spot cooling applications. It is also obtained that using ribs is more useful for low speed impinging jets, when heat transfer from the whole surface is considered.

Heat transfer characteristics of the jet deflector impinged by underexpanded jets

An experimental investigation is carried out to study the local convective heat transfer between smooth wedge shaped plate and a single round underexpanded impinging jet, using thin metal foil technique and IR camera. In the present study experiments are carried out for five different nozzle pressure ratios in the range of 2.36–5.08. The jet-to-wedge apex distance (z/d) is varied from 1 to 12 nozzle diameters. The influence of the included angle of wedge is studied by carrying out experiments for three apex angles of 31.5°, 63° and 94.5°. Heat transfer coefficient is calculated based on wall temperature. The local Nusselt numbers, stagnation point Nusselt numbers and recovery factor distribution are reported in present study. Shadowgraphs are used to explain distribution of Nusselt number and recovery factor. It is concluded that local Nusselt number and recovery factor are greatly affected by the nozzle pressure ratios but independent of wedge apex angle.

Heat transfer characteristics of premixed methane–air flame jet impinging obliquely onto a flat surface

Oblique flame jet impingement finds a wide range of domestic and industrial applications. An experimental investigation is carried out to study the effect of oblique flame jet impingement on the heat transfer characteristics. An asymmetric boundary layer is formed on the impingement surface due to oblique impingement. Steady state technique using one dimensional energy balance across the target plate is employed to determine the local heat transfer coefficient and the reference temperature of the impinging flame jet. The effect of the mixture Reynolds number (400–1200), mixture equivalence ratio (0.8–1.5), burner or plate inclination (0–45°) and burner to plate distance (2–6 times the burner diameter) on the heat transfer characteristics are investigated. It is found that stoichiometric mixture provides optimum Nusselt number distribution and lower burner to plate spacing provides maximum effectiveness for impinging premixed cone flames. Normal impingement resulted smooth heat flux distribution and better thermal performance. Correlations are developed for stagnation point Nusselt number and effectiveness.

Influence of the orifice shape on the local heat transfer distribution and axis switching by compressible jets impinging on flat surface

An experimental investigation is performed to study the effects of the orifice shape and Mach number (M) on the local heat transfer distribution by normally impinging compressible jets. Four different orifice cross-sections namely circular, elliptical, square and triangular are used and jets Mach number is maintained from 0.4 to 1in present study. The heat transfer is measured by thin foil IR technique for different nozzle to plate distances. To calculated Nusselt number, adiabatic wall temperature is used as a reference temperature. The stagnation point Nusselt number is significantly higher for circular orifice as compared to other three shapes while that for the elliptical orifice is minimum. Recovery factor distribution is independent of the Reynolds number and the Mach number. The square, triangular and elliptical orifice respectively undergoes a 45°, 180° and 90° axis switching.

Simulation of a gaseous jet impinging on a convex heated surface – effect of inlet condition

Flow and heat transfer due to a Nitrogen jet at ambient temperature impinging on an isothermally heated convex surface are numerically investigated and compared to experimental measurements obtained from a similar configuration. The temperature difference between the jet and the surface reaches up to 580 K and the nozzle-to-surface distance is equal to the nozzle diameter. On account of the small nozzle-to-surface distance and nozzle length, flow and heat transfer over the impinging surface are highly sensitive to the nozzle inlet conditions; therefore, in this study various inlet conditions are examined and compared. Due to high temperature variation, changes of density are accounted for using a compressible solver instead of constant property approach, widely used in the literature. For the same reason, temperature dependence of fluid properties is included in the solution. The agreement between the solution and experiments is reasonable, and mean velocity profile is found to have significant influence on surface heat transfer (up to 25% difference in stagnation point Nusselt number among different profiles).

Steady interaction of a turbulent plane jet with a rectangular heated cavity

Turbulent heat transfer between a confined jet flowing in a hot rectangular cavity is studied numerically by finite volume method using the k-w SST one point closure turbulence model. The location of the jet inside the cavity is chosen so that the flow is in the non-oscillation regime. The flow structure is described for different jet-to-bottom-wall distances. A parametrical study was conducted to identify the influence of the jet exit location and the Reynolds number on the heat transfer coefficient. The parameters of this study are: the jet exit Reynolds number (Re, 1560< Re <33333), the temperature difference between the cavity heated wall and the jet exit (DT=60?C) and the jet location inside the cavity (Lf, 2? Lf? 10 and Lh 2.5<Lh?10). The Nusselt number increased and attained its maximum value at the stagnation points and then decreased. The flow structure is found in good agreement with the available experimental data. The maximum local heat transfer between the cavity walls and the flow occurs at the potential core end. The ratio between the stagnation point Nusselt numbers of the cavity bottom (NuB0) to the maximum Nusselt number on the lateral cavity wall (NuLmax) decreased with the Reynolds number for all considered impinging distances. For a given lateral confinement, the stagnation Nusselt number of the asymmetrical interaction Lh?10 is almost equal to that of the symmetrical interaction Lh=10.