Heat transfer mechanism of a swirling impinging jet in a stagnation region

Heat transfer characteristics of a swirling impinging jet have been experimentally examined using a combined particle image velocimetry (PIV) and laser‐induced fluorescence (LIF) technique for simultaneous measurement of velocity and temperature fields. The present study shows that the radial width of the jet stretches with increasing swirl intensity, and that the stretching phenomenon contributes to the maximum local heat transfer coefficient. At the stagnation region, the flow near the heated surface is mixed intermittently by reverse flows toward upstream, and spatial distributions of temperature are correlated with instantaneous velocity vector maps. The dynamic behavior of recirculation zones, attributed to swirl number Sw and impinging distance, mainly determines the turbulent heat transfer at the stagnation region. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(8): 663–673, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10120     

Visualization of flow and heat transfer characteristics for swirling impinging jet

Flow and heat transfer characteristics of swirling impinging jet (SIJ) were studied experimentally at constant nozzle-to-plate distance of L = 4D. The swirling jet is generated by inserting twisted tapes within a pipe nozzle. Effects of swirl on the impinged surface are investigated at twist ratios (y/W) of ∞ (straight tape), 3.64, 2.27, 1.82, and 1.52. The flow patterns of the free swirling jet and the swirling impinging jet were visualized by mixing dye with the jet flow. Distributions of temperature and convective heat transfer coefficient on the impinged surface were measured with thermochromic liquid crystal (TLC) sheet and image processing technique. Additionally, an oil film technique was performed as a complementary technique for flow visualization on the impinged surface. The experimental results reveal that there appear to be two peaks of heat transfer in the jet impingement region. The heat transfer enhancements in jet impingement region can be achieved at a low twist ratio of 3.64 which corresponds to the swirl number of 0.4.     

Heat transfer from a plate impinging swirl jet

Heat transfer and flow visualization experiments were conducted to investigate the performance of swirling and multi‐channel impinging jets and compare the results with those of a multi‐channel impinging jet (MCIJ) and conventional impinging jet (CIJ) for the present work at the same conditions. Swirling impinging jets (SIJs) employed the fixed blade lengths of 12.3 mm with four blades at the exit of the housing tube to divert the air flow through four narrow channels with a desired swirl angle (θ of 22.5, 41 and 50°). The MCIJ jet had the same dimensions as the SIJs, except that the narrow channels in the solid insert were vertical (θ=0°). The local and surface average Nusselt numbers of MCIJ were generally higher than those of the CIJ and SIJs. The SIJs, however, demonstrated significant improvement in radial uniformity of heat transfer compared to the MCIJ and CIJ. In the region of 2.7?X/D?0 for H/D=8 and Re=20 000, the average Nusselt number for the MCIJ was 11, 33, 72 and 98 per cent higher than that of the CIJ, θ=22.5, θ=41 and θ=50°, respectively. Copyright © 2002 John Wiley & Sons, Ltd.     

Experimental study and theoretical analysis of local heat transfer distribution between smooth flat surface and impinging air jet from a circular straight pipe nozzle

An experimental investigation is performed to study the effect of jet-to-plate spacing and 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 12,000 and 28,000 and jet-to-plate spacing between 0.5 and 8 nozzle diameters. The local heat transfer characteristics are estimated using thermal images obtained by infrared thermal imaging technique. Measurements for the static wall pressure distribution due to impinging jet at different jet-to-plate spacing are made. The local heat transfer distributions are analyzed based on theoretical predictions and experimental results of the fluid flow characteristics in the various regions of jet impingement. The heat transfer at the stagnation point is analyzed from the static wall pressure distribution. Semi-analytical solution for heat transfer in the stagnation region is obtained assuming an axisymmetric laminar boundary layer with favourable pressure gradient. The heat transfer in the wall jet region is studied considering fluid flow over a flat plate of constant heat flux. However, heat transfers in the transition region are explained from reported fluid dynamic behaviour in this region. Correlations for the local Nusselt numbers in different regions are obtained and compared with experimental results.     

A numerical study of the unsteady flow and heat transfer in a transitional confined slot jet impinging on an isothermal surface

A numerical finite-difference approach was used to compute the steady and unsteady flow and heat transfer due to a confined two-dimensional slot jet impinging on an isothermal plate. The jet Reynolds number was varied from Re=250 to 750 for a Prandtl number of 0.7 and a fixed jet-to-plate spacing of H/W=5. The flow was found to become unsteady at a Reynolds number between 585 and 610. In the steady regime, the stagnation Nusselt number increased monotonically with Reynolds number, and the distribution of heat transfer in the wall jet region was influenced by flow separation caused by re-entrainment of the spent flow back into the jet. At a supercritical Reynolds number of 750 the flow was unsteady and the net effect in the time mean was that the area-averaged heat transfer coefficient was higher compared to what it would have been in the absence of jet unsteady effects. The unsteady jet exhibited a dominant frequency that corresponded to the formation of shear layer vortices at the jet exit. Asymmetry in the formation of the vortex sheets caused deformation or buckling of the jet that induced a low-frequency lateral jet “flapping” instability. The heat transfer responds to both effects and leads to a broadening of the cooled area.     

Numerical study of transient conjugate heat transfer of a turbulent impinging jet

This study presents the numerical study of transient conjugate heat transfer in a high turbulence air jet impinging over a flat circular disk. The numerical simulation of transient, two-dimensional cylindrical coordinate, turbulent flow and heat transfer is adopted to test the accuracy of the theoretical model. The turbulent governing equations are resolved by the control-volume based finite-difference method with a power-low scheme, and the well-known low-Re κ–ω turbulence model to describe the turbulent structure. The SIMPLE algorithm is adopted to solve the pressure–velocity coupling. The parameters studied include turbulent flow Reynolds number (Re = 16,100–29,600), heated temperature of a circular disk (T_h = 373 K) or heat flux (q″ = 63–189 kW/m²), and orifice to heat-source spacing (H/D = 4–10). The numerical results of the transient impinging process indicate that the jet Reynolds number has a significant effect on the hydrodynamics and heat transfer, particularly in the stagnation region of an impinging jet. High turbulence values lead to greater heat transfer coefficients in the stagnation region and cause a bypass of the laminar-to-turbulent transition region in the wall jet region. Induced turbulence from the environment around the jet also influences the variation of the stagnation heat transfer. The modeling approach used here effectively captures both the stagnation region behavior and the transition to turbulence, thus forming the basis of a reliable turbulence model.     

Numerical Analysis of Jet Impingement Heat Transfer at High Jet Reynolds Number and Large Temperature Difference

Jet impingement heat transfer from a round gas jet to a flat wall was investigated numerically for a ratio of 2 between the jet inlet to wall distance and the jet inlet diameter. The influence of turbulence intensity at the jet inlet and choice of turbulence model on the wall heat transfer was investigated at a jet Reynolds number of 1.66 × 10⁵ and a temperature difference between jet inlet and wall of 1600 K. The focus was on the convective heat transfer contribution as thermal radiation was not included in the investigation. A considerable influence of the turbulence intensity at the jet inlet was observed in the stagnation region, where the wall heat flux increased by a factor of almost 3 when increasing the turbulence intensity from 1.5% to 10%. The choice of turbulence model also influenced the heat transfer predictions significantly, especially in the stagnation region, where differences of up to about 100% were observed. Furthermore, the variation in stagnation point heat transfer was examined for jet Reynolds numbers in the range from 1.10 × 10⁵ to 6.64 × 10⁵. Based on the investigations, a correlation is suggested between the stagnation point Nusselt number, the jet Reynolds number, and the turbulence intensity at the jet inlet for impinging jet flows at high jet Reynolds numbers.     

Heat transfer enhancement of round impinging jet by orifice nozzle (Effects of contraction area ratio)

The effects of the nozzle contraction ratio on the flow and heat transfer characteristics of an orifice impinging jet were investigated in this experiment. The nozzle diameter was d_o=10.0 mm=const., and the contraction area ratio CR=(d_o/d_i)², where d_i is the inner pipe diameter was varied from CR=1.00 to 0.11 and the nozzle‐plate distance was varied from H/d_o=2.0 to 5.0. The nozzle Reynolds number was Re=1.5×10⁴=const. The flow characteristics were clarified by measuring the pressure and velocity distributions on the plate and flow visualization. The Nusselt number obtained from measuring the temperature distribution on the plate of an orifice impinging jet with a CR of 0.11 and 0.69 were respectively larger by 19% and 9% than those of a pipe impinging jet (CR=1.00), because the centerline velocity of the orifice jet was larger than that of the pipe jet. Under the same operation power, an orifice impinging jet has improved heat transfer characteristics. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20225     

Heat transfer from a turbulent swirling inverse diffusion flame to a flat surface

The heat transfer characteristics of a turbulent and swirling inverse diffusion flame (IDF) impinging vertically normal to a flat surface were investigated experimentally. The heat flux was measured by a heat flux sensor, Vatell HFM-6D/H. The effects of Reynolds number, overall equivalence ratio, nozzle-to-surface distance H and swirl number on the heat flux distributions were examined. The comparison of heat transfer of impinging IDFs with and without swirl was also conducted.The experimental results showed that the swirling effect influences the local heat flux in three ways. (1) The heat transfer at the stagnation point is severely suppressed. (2) The peak of local heat flux dwells at a radial distance from the stagnation point. (3) The radial position of peak local heat flux shifts farther away from the stagnation point with increasing H. There exists an optimum value of H at which the heat transfer to the target surface is the maximum and the optimum H increases with increasing Ф while the Reynolds number and the swirl number are unchanged.The comparison of IDFs revealed that the swirling IDF has more complete combustion and thus it is accompanied by higher heat transfer rates at small H at which there exists a cool core in the case of the non-swirling IDF. The IDF, however, has worse heat transfer at higher H where the non-swirling IDF achieves complete combustion while the swirling IDF has been cooled by the entrained ambient air.Upon comparing the swirling and non-swirling IDFs at the same Re and Ф, their respective optimum H showed an unfavorable effect of swirl on the overall heat transfer rate which has a reduction of up to 25% in the swirling IDF compared with the non-swirling IDF.     

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.     

Fluid flow and heat transfer characteristics of cone orifice jet (effects of cone angle)

The use of a jet from an orifice nozzle with a saddle‐backed‐shape velocity profile and a contracted flow at the nozzle exit may improve the heat transfer characteristics on an impingement plate because of its larger centerline velocity. However, it requires more power to operate than a common nozzle because of its higher flow resistance. We therefore initially considered the use of a cone orifice nozzle to obtain better heat transfer performance as well as to decrease the flow resistance. We examined the effects of the cone angle α on the cone orifice free jet flow and heat transfer characteristics of the impinging jet. We compared two nozzles: a pipe nozzle and a quadrant nozzle. The first one provides a velocity profile of a fully developed turbulent pipe flow, and the second has a uniform velocity profile at the nozzle exit. We observed a significant enhancement of the heat transfer characteristics of the cone orifice jets at Re=1.5×10⁴. Using the cone orifice impinging jets enhanced the heat transfer rates as compared to the quadrant jet, even when the jets were supplied with the same operational power as the pipe jet. For instance, a maximum enhancement up to approximately 22% at r/d_o?0.5 is observed for α=15^°. In addition, an increase of approximately 7% is attained as compared to when the pipe jet was used. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20243     

Asymmetric entrainment effect on the local surface temperature of a flat plate heated by an obliquely impinging two-dimensional jet

The flow and temperature fields caused by a two-dimensional heating air jet obliquely impinging on a flat plate are experimentally characterized. Whilst the jet flow is discharged at Re_Dh = 8.2 × 10³ based on the hydraulic diameter of the orifice, D_h, and the jet exit-to-plate spacing (separation distance) is fixed at 8D_h, the impingement angle (inclination) is systematically decreased from 90° (normal impinging) to 30° (oblique impinging). A separate experiment is carried out for a two-dimensional cooling jet obliquely impinging on a heated plate (constant heat flux). The results demonstrate that the response of local surface temperature to plate inclination behaves in a completely different manner. For impinging jet cooling, the inclination (from normal impinging position) reduces the local effective temperature values at corresponding points about actual stagnation point, inclusive of it. For impinging jet heating, the inclination causes, conversely, an increase in local surface temperature including the stagnation point temperature. However, the shifting of the actual stagnation point towards the uphill side of the plate is consistently observed for both hot and cold jet cases. This newly found feature for an obliquely impinging jet is attributed to the combined effects of asymmetric entrainment and momentum redistribution (i.e., thickening/thinning of hydraulic boundary layers on each side of the plate with respect to the actual stagnation point).     

Flow structure,wall pressure and heat transfer characteristics of impinging annular jet with/without steady swirling

The detailed impinging annular jet characteristics with/without induced swirling motion are experimentally studied. Obtained flow and wall pressure data confirm the classical observation for conventional annular impinging jet, with the flow exhibiting simple round impinging jet-like features at sufficiently large separation distances. In contrast, no such transition occurs in the presence of swirling. At short and intermediate separation distances, the swirling annular jet causes more non-uniform distributions of wall pressure and heat transfer on the impingement plate compared to the conventional annular jet. However, the reverse holds at large separation distances.     

Heat transfer of impinging air and liquid nitrogen mist jet onto superheated flat surface

This experimental study performs the detailed heat transfer measurements of an impinging air-liquid nitrogen mist jet onto a superheated flat surface at atmospheric pressure with reference to the design of an instant freezing facility. A selection of experimental results illustrates the interacting effects of jet Reynolds number, mass flow ratio of air to liquid nitrogen flows and separation distance on the spatial distributions of heat transfer over the impinging surface. Mechanism associated with phase change of impacting droplets generates an enhanced and uniformly distributed heat transfer region centered on the stagnation point. A narrow oval-ring region encapsulating the enhanced core transits heat transfer from the wetting regime of complete evaporation to the non-wetting rebound regime. Stagnation heat transfer augmentation factor in the range of 1.2-2.8 times of the air-jet level is achieved. An empirical correlation based on the experimental data, which is physically consistent, has been developed to permit the evaluation of stagnation heat transfer.     

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.     

Three‐dimensional turbulent forced convection around a hot cubic block exposed to a cross‐flow and an impinging jet

A numerical study of a three‐dimensional, turbulent, forced convection flow around a hot cubic block exposed to cross‐flow and an impinging jet is carried out. The standard k‐ε turbulence model is used to study the effects of Reynolds number ratio on the flow and heat transfer. For each value of the Reynolds number of the jet, the Reynolds number ratio is equal to 1, 1.5, and 2. The influence of the channel height and the jet axis location are also examined. The governing equations are solved by using Ansys Fluent software 14.5. Results show that the heat transfer increases with the increase in the Reynolds number ratio. At the top of the cube, better cooling occurs with an increase in the speed of the impinging jet. A reduction in the height of the channel and the displacement of the axis of the jet toward the channel inlet improve the heat transfer. Our simulations are compared with experimental data found in the literature, using different turbulence models.     

Flow field and heat transfer of a two‐dimensional impinging jet disturbed by an elastically suspended circular cylinder

The flow field around a circular cylinder elastically suspended with a cantilever‐type plate spring in the jet impingement region was visualized to investigate the mechanism of the impingement heat transfer. The impingement distance H was kept constant at 3 or 5 times as large as the jet slot width, h = 15 mm.The Reynolds number was fixed at 10,000, or 5000 in the case of flow visualization. The self‐induced periodic swing motion of the cylinder across the jet axis was caused by the interaction between the jet and the elastically suspended cylinder. It was found that this swing motion has direct effects on the flow and heat transfer characteristics of the stagnation region. The ensemble‐averaged values of the flow velocity and its fluctuations depended on the cylinder diameter and the impingement distance. The local Nusselt number in the case of H/h = 3 with the oscillating cylinder of the smallest diameter D = 4 mm was increased to 1.15 times as large as that without the cylinder. The interesting patterns of the intermittency function defined with a certain threshold level of turbulence intensity were obtained under the above experimental conditions. © 2001 Scripta Technica, Heat Trans Asian Res, 30(4): 313–330, 2001     

Streamwise distribution of the recovery factor and the local heat transfer coefficient to an impinging circular air jet

An investigation of the radial distribution of the recovery factor and the local heat transfer for an axisymmetric impinging air jet formed by a smooth nozzle is described. The recovery factor is dependent on the jet nozzle to impingement plate spacing, but is independent of jet Reynolds number. The maximum stagnation region heat transfer occurs at a nozzle to impingement plate spacing of about eight jet diameters. A correlation is obtained for the average heat transfer from the surface.     

The role of jet inlet geometry in impinging jet heat transfer,modeling and experiments

Effects of jet inlet geometry and aspect ratio on local and average heat transfer characteristics of totally nine confined impinging jets have been investigated experimentally using thermochromic liquid crystals and numerically by using a 3-D low Reynolds number k–? model. Experimental study by using liquid crystals for temperature measurement was conducted for three different jet exit geometries (circular, elliptic, rectangular). In addition, simulations were performed at the same mass flow rate for totally nine jet exit geometries including circular, elliptic and rectangular jets with different aspect ratios for dimensionless jet to plate distances 2, 6, and 12.As the aspect ratio of equal cross-sectional area elliptic and rectangular jets increases, heat transfer enhancement in the stagnation region was obtained. As a result higher aspect ratio jets can be used as a passive enhancement technique for localized heating or cooling especially at small jet to plate distances. Wall jet region comprises very large portion of the impinging plate under study and generally lower heat transfer rates were attained for higher aspect ratio jets in this region especially at small jet to plate distances. Therefore as the aspect ratio increases, lower average heat transfer rates were acquired. The effect of aspect ratio on local and average heat transfer decreases with increasing jet to plate distance. Even though the mass flow rate is the same, heat transfer rate of rectangular jets were reduced with increasing the cross-sectional area. With increasing jet to plate distance very similar heat transfer characteristics were observed along the major and minor axis directions.     

Heat transfer enhancement in duct with blunt body inserted close to its wall

This paper shows the effects of clearance length between a body and a duct wall, and duct height on the heat transfer characteristics and flow behavior at a downstream region of the body when a blunt body was set in a parallel plate duct with some distance separating it from the duct wall as a turbulence promoter. For the ratio of clearance to body height, C/D = 0.05–01, the heat transfer was characterized by the reattachment of shear flow separated from the body. Furthermore, the heat transfer depended on both the reattachment flow and the separation vortex at C/D = 0.15–0.2, and the side vortex induced by Karman vortex at C/D = 0.25–0.275 was also observed. We found the reattachment flow gives a superior effect to enhance heat transfer at a low Reynolds number, but at a larger Reynolds number, the side vortex induced by Karman vortex becomes more effective to heat transfer enhancement. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(5): 336–349, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20067