Flow characteristics of rectangular underexpanded impinging jets

In this paper, the flow fields of underexpanded impinging jet issued from rectangular nozzles of aspect ratio 1, 3 and 5 are numerically and experimentally studied. Two dimensional temperature and pressure distributions are measured by using infrared camera and the combination of a pressure scanning device and a stepping motor, respectively. The variation of the stagnation pressure on the impinging plate reveals that a hysteretic phenomenon exists during the increasing and decreasing of the pressure ratio for the aspect ratio of 3.0 and 5.0. It is also found that the nozzle of aspect ratio 1.0 caused the largest total pressure loss pc / p0 = 0.27 at the pressure ratio of p0 /pb = 6.5, where pc is the stagnation center pressure on the wall, p0 the upstream stagnation pressure, pb the ambient pressure. The other two nozzles showed that the pressure loss pc /p0 =0.52 and 0.55 were achieved by the nozzles of the aspect ratio 3,0 and 5.0, respectively. The comparison between the calculations and experiments is fairly good, showing the three dimensional streamlines and structures of the shock waves in the jets. However, the hysteresis of the pressure variations observed in the experiments between the pressure ratio of 3.5 and 4.5 cannot be confirmed in the calculations.     

旋进射流冲击平板的传热实验研究

对旋进射流冲击平板时的传热进行了实验研究。通过在圆筒套管内设置一块孔板构成旋进射流喷嘴,得到了持续稳定的旋进射流。对旋进射流的流动特性作了研究,给出了旋进射流的频率与尺寸、Re的关系。用两种不同孔径的旋进射流冲击一块加热平板,并与普通的射流冲击传热作对比。结果表明,由于旋进射流与流体混合作用加剧而大大地降低了流速,使得强化传热的效果减弱,这种趋势在驻点附近尤为明显。     

Effect of tabs on impinging heat transfer

The present work experimentally investigates the effect of vortex generators, in the form of small tabs projecting normally into the flow at the nozzle exit, on the fluid flow and heat transfer characteristics of an axisymmetric impinging air jet in the subcritical Reynolds number range. With this comes the expectation of a large eddy structure variation and the possibility of active control. Local heat transfer and static pressure were measured on a target plate for a round air jet issuing from a circular nozzle with rectangular tabs whose numbers and lengths changed at a constant nozzle‐to‐plate gap (L/d = 8) and jet Reynolds number (Re = 34,000). The main results are the following: When two tabs were set at the exit of the circular nozzle, Cpw and Nu profiles flatten in the direction of the tab setting. In the case of three tabs, however, among both Cpw and Nu profiles a concentric profile is found, as well as in the case without any tabs. © 2001 Scripta Technica, Heat Trans Asian Res, 30(7): 561–570, 2001     

Effect of Pressure Gradient on Heat Transfer of Impinging Submerged Circular Jets

An experimental and numerical study have been carried out to investigate the distribution of radial local heat transfer coefficients of impinging submerged circular jets. Good agreement is achieved between the experimental results and the predicted value. Results show that the outer peak usually occurs at the radial location of r/d= 1.8~2.0, in which transition from laminar to turbulence happens resulting from disappeared pressure gradient abruptly, and that the inner peak appears rigidly at r/d=0.5, where the boundary layer has a minimum thickness because of elevating pressure gradient.     

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.     

Experimental and Three—Dimensional Numerical Study on Under—Expanded Impinging Jets

IntroductionUnder-expanded impinging jets have attracted theinterest of many researchers not only because they havepotentially engineering applications such as surfacecooling devicesl'], and plasma spray coating['], alsobecause they are not fully understood yet[' 5]. Goldsteinet al. confirmed that the stagnation temperature near thestagnation region on the impinging plate is larger thanthat in the settling chamber despite no heat added to theflow during its process from the settling chamber to…     

Experimental and Numerical Study of Twin Underexpanded Impinging Jets

In this paper, the dual underexpanded impinging jets are experimentally and numerically studied. The experiments were performed by measuring the unsteady and averaged wall static pressures and by visualizing density fields using schlieren method. Numerical calculations were also conducted by solving unsteady three dimensional compressible Navier-Stokes equations with Baldwin-Lomax turbulence model. The main parameters for the dual jets are the non-dimensional distance between the two nozzle centers HID covering 1.5, 2.0, the nozzle to plate separation LID 2.0, 3.0,4.0 and 5.0 and the pressure ratio defined by po/pb 1.0~6.0, where D is the diameter of each nozzle exit, p0 the stagnation pressure and pb the back pressure. It is found that the agreement between the experiments and the calculations is good. The fountain flow at the middle of the two jets is observed both in the experiments and the calculation. According to FFT analysis of the experiments for the twin jets, relatively low frequency (up to 5     

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     

Heat transfer and fluid flow characteristics in a swirling impinging jet

An experimental study on heat transfer and fluid flow has been carried out for a swirling round impinging jet. A thermosensitive liquid crystal sheet was used for the heat transfer measurements and the three velocity components were measured with LDV in the stagnation region for cases where the Swirl number Sw = 0.0, 0.22, and 0.45 at the Reynolds number Re = 8100. The formation of recirculation flow due to a swirl near the impinging wall was found to deteriorate the heat transfer coefficient in the stagnation region and results in a more uniform distribution of the Nusselt number with an increasing Swirl number. The heat transfer mechanism of the swirling impinging jet is discussed based on the flow characteristics of the mean velocities and turbulence quantities. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(5): 324–335, 2005; Published online in Wiley InterScience ( www.interscience. wiley.com ). DOI 10.1002/htj.20068     

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.     

A study on the heat and mass transfer properties of multiple pulsating impinging jets

In order to explore the potential effect of unsteady intermittent pulsations on the heat and mass transfer rate of multiple impinging jets, a numerical study is performed on a two-dimensional pulsating impinging jet array under large temperature differences between jet flows and impingement wall when the thermo-physical properties can change significantly in the flow domain. Computational fluid dynamic approach is used to simulate the flow and thermal fields of multiple pulsating impinging jets. The numerical results indicate a significant heat transfer enhancement due to intermittent pulsation over a wide range of conditions. The oscillatory flow periodically alters the flow patterns in contrast to steady jets, which can eliminate the formation of a static stagnation point and enhance the local Nusselt number along the impingement wall between adjacent jets. Examination of the velocity field shows that the instantaneous heat transfer rate on the target surface is highly dependent on the hydrodynamic and thermal boundary layer development with time.     

Heat transfer and pressure loss characteristics of very compact heat sinks with plate fins cooled by jets

High-performance and very compact heat sinks have been developed for effective cooling of VLSIs with high heat-generation densities. Their heat transfer and pressure loss characteristics in air-jet cooling have been experimentally studied. The highly compact heat sinks were plate-fin arrays with a very small fin pitch of 0.4–2.0 mm. The rectangular jet nozzle width that gave the highest cooling performance was 30 to 40% of the streamwise length of the heat sinks. The influence of fin height on heat transfer became weak when the ratio of the height to the thickness of the fin exceeded approximately 35. When the air flow rate was constant, the thermal conductance increased as the fin pitch decreased. For a constant fin pitch, heat sinks with smaller fin thickness showed larger thermal conductance at a given blower power consumption. In our experimental range, the heat dissipation rate per unit heat sink volume increased as the base plate area of the heat sink became small. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(6): 399–414, 1998     

圆形冲击射流传热性能的实验研究

应用萘升华传质/传热比拟技术,对单个圆形射流在不同喷嘴到被冲击表面距离(1≤H/D≤12),在7×103≤R e≤1.9×104时,进行了局部传质/传热实验;研究了不同喷嘴到被冲击表面距离和不同R e对单个圆形射流局部换热特性的影响。单个圆形射流局部传热系数随着R e的增加而大幅度增加,R e是影响局部换热系数的主要因素。在同一R e下局部换热系数沿轴向非单调变化,在驻点处当H/D≌6时换热系数达到峰值;H/D<6时,局部换热系数沿径向有两个峰值;随H/D的增加,中心区局部N u减小,但影响范围变大。     

Simultaneous visualization of flow field and evaluation of local heat transfer by transitional impinging jets

A combined approach has been employed to characterize the flow field and local heat transfer in jet impingement configurations, featuring a mass transfer experiment and a digital visualization technique. A jet velocity range is spanned to ensure flow regime transition.The well-known heat/mass transfer analogy has been used to infer on the local heat exchange on a infinite plate. In this experiment, a naphthalene film is ablated from a disk, due to jet exposure. Automated contact measurements of the variation of film depth in the stagnation region and beyond have been performed. From the local naphthalene loss rate the local heat transfer is then inferred. Coherent structures are created both at the interface between free jet and quiescent medium and upon impingement at plate, and need to be visualized in the vicinity of stagnation. To this end a particle image velocimetry system is exploited to extract the two components velocity instantaneous information.Ablation measurements confirm the non-monotonic progress of local heat transfer for small nozzle-to-plate spacings. The visualizations evidence that local heat transfer is strongly influenced by impingement structures: the maximum heat transfer coefficient offset which can be detected is due, even for laminar or transitional jet, to large-scale toroidal vortices impacting on the plate.     

Experimental study on heat-transfer characteristics of circular water jet impinging on high-temperature stainless steel plate

Water jet impinging heat transfer is widely used in steel-making industry, nucleate power plants, and many other applications. In this study, an experimental study was carried out in an industry-scale facility. In this test facility, an insulating material was added to simplify the heat-transfer process. The data filter method was used to remove the noise in temperature measurements. Using inverse heat conduction analysis, surface heat flux was obtained, and the inverse heat conduction method was verified. The relative error between the calculated and measured temperature at the bottom surface of the test plate was less than 5%, and the relative error of the inner temperature of the test plate was less than 0.35%. Increasing the surface roughness provided better heat transfer in the stagnation region, which is in agreement with the reported results in the literature. By determining the surface temperature of the test plate, the boiling heat-transfer mode can be identified.     

Study on flow and heat transfer of multiple impingement jets

Multiple jets are often used to obtain high heat transfer in a wide area. Heat and flow behavior of multiple jets show more complicated characteristics due to the existence of the interactions between adjoining jets, and between jets and spent flows. To clarify the influence of the flow behavior on heat transfer fields, the time and spatial heat transfer coefficients over the impingement plate were measured using an infrared radiometer when four jets impinged to the impingement plate for various jets arrangements and separating distances from jet exit to the impingement plate. The flow fields were also visualized by means of a water table. The heat transfer characteristics are made clear from the results of the instantaneous heat transfer contour or temperature fluctuation values. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(6): 419–431, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20073     

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

The nonlinear flow and heat transfer characteristics for a slot-jet impinging on slightly-curved surfaces are experimentally studied here. The effects of curved surface geometry and jet Reynolds number on the jet velocity distribution and circumferential Nusselt numbers are examined. Two different slightly-curved surface geometries of convex and concave are used as target surfaces. The nozzle geometry is a rectangular slot, and the dimensionless nozzle-to-surface distance equals to L¹ = 8. The constant heat fluxes are accordingly applied to the surfaces to obtain an impingement cooling by the air jet at ambient temperature. The measurements are made for the jet Reynolds numbers of Re = 8617, Re = 13 350 and Re = 15 415 for both curved surfaces. The velocity distributions of issuing jet from the nozzle exit to the target surface are obtained by a highly sensitive hot-wire anemometer. The T-type thermocouples are used to measure local temperatures of both the air jet and the plates. Two-dimensional velocity measurements show that the surfaces are remained out of the potential core region for all Re tested here. New correlations for local, stagnation point, and average Nusselt numbers as a function of jet Reynolds number and dimensionless circumferential distance are reported. The correlations reveal that the impinging cooling rate is higher with the concave surface and increase with increasing Re.     

Local heat transfer characteristics of array impinging jets from elongated orifices

The aim of this research is to enhance the heat transfer on an impinged surface under an impinging jet array by minimizing a cross-flow effect. Conventional round orifices (aspect ratio, AR = 1) are substituted by the elongated orifices with aspect ratio AR = 4 and 8 with the same jet exit area. Two types of orifice arrangements; in-line and staggered arrays are compared. The experimental investigation was carried out at constant distance from orifice plate to impinged surface H = 2D_E (D_E is equivalent diameter of orifice). The heat transfer characteristic was visualized using thermochromic liquid crystal sheet (TLCs) and the Nusselt number distribution was evaluated by an image processing technique. The flow characteristic on the impinged surface was also visualized by oil film technique. The results show that the cross-flow in a case of the jets issued from the orifices with AR = 4 is considerably less significant than that in cases of the ones delivered from the orifices with AR = 1 and 8. At Reynolds number of 13,400, the Nusselt numbers for the jet arrays issued from the elongated orifices with AR = 4 with in-line and staggered arrangements are respectively 6.04% and 12.52% higher than those for the case of AR = 1.     

Study of heat transfer for a pair of rectangular jets impinging on an inclined surface

Critical design parameters in jet impingement heat transfer like nozzle hydraulic diameter, jet angle and velocity, physical properties of the fluid, and nozzle-to-target plane spacing are the subject. This paper identifies the dominant fluid-thermal characteristics of a pair of rectangular air jets impinging on an inclined surface. Heat transfer modes and flow characteristics are studied with eight different Reynolds numbers ranging from 500 to 20 000. Local and average Nusselt numbers are evaluated with two different boundary conditions on three specified lines located on the inclined surface. The correlation between stagnation Nusselt number and Reynolds number is presented. Turbulent intensity and wall y⁺ distributions are compared on three lines parallel to the incline. The effect of jet impingement angle on local and average Nusselt number is also documented. Finally, a correlation between the average Nusselt number, nozzle exit Reynolds number and the jet angle is documented.     

Local heat transfer and recovery factor with impinging free-surface circular jets of transformer oil

Measurements were made to investigate the local behavior of the recovery factor and the heat transfer coefficient with free-surface circular jets. The experiments were performed with transformer oil jets impinging on a vertical constant-heat-flux surface from small pipe and orifice nozzles of 1 mm diameter in the ranges of Re = 183–2600 and Pr = 82–337. Large values of recovery factor over 20 were recorded with medium jet velocity about 20 m s⁻¹. Radial distribution of the recovery factor was determined and expressed in empirical equations. The heat transfer coefficient at stagnation point was found to be nearly independent of nozzle-to-plate spacing, but proportional to the square root of the jet Reynolds number. Profiles of local heat transfer coefficients were obtained and correlated. Based on the local measurements, integral average heat transfer coefficients were obtained and correlated.