The effects of the compressibility of the subsonic flow on heat transfer characteristics in a microscale impinging slot jet

We experimentally investigated the effects of both the compressibility and nozzle width on the local heat transfer distribution of microscale unconfmed slot jets impinging on a uniformly heated flat plate. We made heat transfer measurements under the following experimental conditions; Reynolds numbers of Re = 4000~10000, Mach numbers of Ma = 0.13~0.68, nozzle-to-plate distances of H/B = 3~25, lateral distances of x/B = 0~25, and nozzle widths of B = 300~700 μm having a nozzle aspect ratio of y/B = 30. A thermal infrared imaging technique was used to measure the impingement plate temperature. The experimental results show that for all tested Re and H/B values at a nozzle width of B = 300 μm, the Nusselt number maximum occurred nearly at the stagnation point and then monotonically decreased along the downstream. However, at B = 500 and 700 μm, the maximum Nusselt number point shifted toward x/B ≈ 1.5~2.0. And the Nusselt number increased, as x/B increased, from the stagnation point to the shifted maximum point and monotonically decreased afterward. This shifted maximum point may be attributable to vortex rings promoting sudden flow acceleration and entrainment of surrounding air moving along the jet axis. For the same Reynolds number, the Nusselt number in the stagnation region increased as the nozzle width increased due to a momentum increase of the jet flow caused by the formation of vortices. And, the Nusselt numbers for the smallest nozzle width of B = 300 μm (or highest Mach number at a given Reynolds number) at all H/B and Reynolds numbers tested significantly deviated from those for B = 500 and 700 μm in the downstream region corresponding to x/B > 5, suggesting that the compressibility, when it is high, can affect the heat transfer in the downstream region.

     

An experimental study on the flow and heat transfer characteristics of an impinging jet

The flow and heat transfer characteristics of an impinging jet is investigated in two major stages. The first stage is about the investigation of the three dimensional mean flow and the turbulent flow quantities in free jet, stagnation and wall jet region. After a complete documentation of the flow field, the convective heat transfer coefficient distributions on the impingement plate are presented, during the second stage of the study. Heat transfer experiments using the new hue-capturing technique result in high resolution wall heating rate distributions. The technique is fully automated using a true color image processing system. The present heat transfer results are discussed in detail in terms of the flow characteristics. The measurements from the new method are compared with conventional heat flux sensors located on the same model. These heat transfer distributions are also compared with other studies available from the literature. The new non-intrusive heat transfer method is highly effective in obtaining high resolution heat transfer maps with good accuracy.     

A naphthalene sublimation study on heat/mass transfer for flow over a flat plate

It is important to completely understand heat/mass transfer from a flat plate because it is a basic element of heat/mass transfer. In the present study, local heat/mass transfer coefficient is obtained for two flow conditions to investigate the effect of boundary layer using the naphthalene sublimation technique. Obtained local heat/mass transfer coefficient is converted to dimensionless parameters such as Sherwood number, Stanton number and Colburnj-factor. These also are compared with correlations of laminar and turbulent heat/mass transfer from a flat plate. According to experimental results, local Sherwood number and local Stanton number are in much better agreement with the correlation of turbulent region rather than laminar region, which means analogy between heat/mass transfer and momentum transfer is more suitable for turbulent boundary layer. But average Sherwood number and average Colburnj-factor representing analogy between heat/mass transfer and momentum transfer are consistent with the correlation of laminar boundary layer as well as turbulent boundary layer.     

缩放管强化传热机理分析

应用数值模拟方法对光滑圆管与缩放管内的空气湍流对流传热在相同条件下进行了计算,对缩放管与光滑圆管内的对流传热系数、速度模量分布、径向速度分布、湍流强度的对比表明：缩放管内空气对流传热系数的提高,是近壁区域流体扰动增强产生了较大的流动径向速度与湍流强度的结果。表面传热系数、近壁区径向速度、近壁区湍流强度三者沿流向周期性变化的规律一致。缩放管强化传热的机理分析指出,强化气体传热的粗糙高度应位于流动过渡区。     

Turbulent mixing flow characteristics of solid-cone type diesel spray

The intermittent spray characteristics of the single-hole diesel nozzle (d_n=0.32 mm) used in the fuel injection system of heavy-duty diesel engines were experimentally investigated. The mean velocity and turbulent characteristics of the diesel spray injected intermittently into the still ambient were measured by using a 2-D PDPA (phase Doppler particle analyzer). The gradient of spray half-width linearly increased with time from the start of injection, and it approximated to 0.04 at the end of the injection. The axial mean velocity of the fuel spray measured along the radial direction was similar to that of the free air jet within R/b=1.0-1.5 regardless of elapsing time, and its non-dimensional distribution corresponds to the theoretical velocity distributions suggested by Hinze in the downstream of the spray flow fields. The turbulent intensity of the axial velocity components measured along the radial direction represented the 20-30% of the Ū_cl and tended to decrease in the outer region. The turbulent intensity in the trailing edge was higher than that in the leading edge.     

Comparative study of a turbulent wall-attaching offset jet and a plane wall jet

The flow field characteristics of a two-dimensional wall-attaching offset jet (WAOJ) are experimentally investigated by comparing with those of a turbulent plane wall jet(PWJ). The mean velocity, the turbulent stresses and triple velocity correlations are measured with a split film probe and anX wire probe. Even with the strong influence of the suction pressure field in the recirculation bubble at the lower corner, it is found that the WAOJ in the wall jet region has a close similarity with the PWJ. Especially, the decay of maximum velocity and the upper jet spread along the maximum velocity line of the WAOJ are virtually the same as those of the PWJ. The mean velocity profile of the WAOJ attains similarity after the jet impingement onto the lower plate. However the profiles of second and third-order moments of fluctuating velocities vary rapidly before the impingement and then relax very slowly to the similarity profiles of the PWJ.     

Effect of guide wall on jet impingement cooling in blade leading edge channel

The characteristics of fluid flow and heat transfer, which are affected by the guide wall in a jet impinged leading edge channel, have been investigated numerically using three-dimensional Reynolds-averaged Navier–Stokes analysis via the shear stress transport turbulence model and gamma theta transitional turbulence model. A constant wall heat flux condition has been applied to the leading edge surface. The jet-to-surface distance is constant, which is three times that of the jet diameter. The arrangement of the guide wall near the jet hole is set as a variable. Results presented in this study include the Nusselt number contour, velocity vector, streamline with velocity, and local Nusselt number distribution along the central line on the leading edge surface. The average Nusselt number and average pressure loss between jet nozzle and channel exit are calculated to assess the thermal performance. The application of the guide wall is aimed at improving heat transfer uniformity on the leading edge surface. Results indicated that the streamwise guide wall ensures the vertical jet impingement flow intensity and prevents the flow after impingement to reflux into jet flow. Thus, a combined rectangular guide wall benefits the average heat transfer, thermal performance and heat transfer distribution uniformity.     

Effects of combustor-level high inlet turbulence on the endwall flow and heat/mass transfer of a high-turning turbine rotor cascade

Experimental data are presented which describe the effects of a combustor-level high free-stream turbulence on the near-wall flow structure and heat/mass transfer on the endwall of a linear high-turning turbine rotor cascade. The endwall flow structure is visualized by employing the partial- and total-coverage oil-film technique, and heat/mass transfer rate is measured by the naphthalene sublimation method. A turbulence generator is designed to provide a highly-turbulent flow which has free-stream turbulence intensity and integral length scale of 14.7% and 80mm, respectively, at the cascade entrance. The surface flow visualizations show that the high free-stream turbulence has little effect on the attachment line, but alters the separation line noticeably. Under high free-stream turbulence, the incoming near-wall flow upstream of the adjacent separation lines collides more obliquely with the suction surface. A weaker lift-up force arising from this more oblique collision results in the narrower suction-side corner vortex area in the high turbulence case. The high free-stream turbulence enhances the heat/mass transfer in the central area of the turbine passage, but only a slight augmentation is found in the endwall regions adjacent to the leading and trailing edges. Therefore, the high free-stream turbulence makes the endwall heat load more uniform. It is also observed that the heat/mass transfers along the locus of the pressure-side leg of the leading-edge horseshoe vortex and along the suctionside corner are influenced most strongly by the high free-stream turbulence. In this study, the endwall surface is classified into seven different regions based on the local heat/mass transfer distribution, and the effects of the high free-stream turbulence on the local heat/mass transfer in each region are discussed in detail.     

新型单圆锥体热沉单孔射流散热数值模拟*

射流冷却是高热流密度换热的有效方法之一,其热沉形状是影响换热的关键因素。提出一种新型圆锥体热沉,应用RNG k-ε湍流模型,对新型单圆锥体热沉进行单孔水冷散热数值模拟,并进行试验验证。结果表明：单圆锥体热沉的散热效果明显强于常规平板热沉,前者的射流流体域比后者多一个转折区,转折区内存在二次冲击,使换热得到强化。雷诺数越大,圆锥体热沉散热性能越优异;在不同锥角(15°~60°)的单圆锥体热沉传热模拟中,当锥角在45°左右时,热沉表面的平均努塞尔数数最高;圆锥体底面直径d₁和射流孔直径d比值在1~3范围内,数在比值为2.5左右达到最大值;射流高度H与射流孔直径d比值在5左右时,数趋于最大。     

Evaporation heat transfer and pressure drop characteristics of r-134a in the oblong shell and plate heat exchanger

The evaporation heat transfer coefficienthr and frictional pressure drop δp_f of refrigerant R-134a flowing in the oblong shell and plate heat exchanger were investigated experimentally in this study. Four vertical counterflow channels were formed in the oblong shell and plate heat exchanger by four plates of geometry with a corrugated sinusoid shape of a 45° chevron angle. Upflow of refrigerant R-134a boils in two channels receiving heat from downflow of hot water in other channels. The effects of the refrigerant mass flux, average heat flux, refrigerant saturation temperature and vapor quality of R- 134a were explored in detail. Similar to the case of a plate heat exchanger, even at a very low Reynolds number, the flow in the oblong shell and plate heat exchanger remains turbulent. The results indicate that the evaporation heat transfer coefficienthr and pressure drop Δp_f increase with the vapor quality. A rise in the refrigerant mass flux causes an increase in theh _r and Δp_f. But the effect of the average heat flux does not show significant effect on the h_r and Δp_f. Finally, at a higher saturation temperature, both theh _r and Δp_f are found to be lower. The empirical correlations are also provided for the measured heat transfer coefficient and pressure drop in terms of the Nusselt number and friction factor.     

新型钢板桩液力自旋式喷嘴设计及流场分析

针对钢板桩遇密实土层难以沉桩的问题,设计了一种新型液力自旋式喷嘴结构。基于流体力学理论,利用Fluent软件,通过改变斜射流口的偏心距、斜射流口与水平面的夹角等结构参数,得到了该喷嘴内腔的压力、流速及旋转体转矩的变化规律。结果表明:随着偏心距的增大,旋转体转矩增大;斜射流口的出口平均流速在小偏心距变化微小,在大偏心距逐渐减小;随着水平夹角的增大,旋转体转矩减小;斜射流口的出口平均流速随着水平夹角的增大先增大后减小,且随着偏心距的增大其峰值点逐渐后移。     

某数字T/R 组件液冷冷板的改进设计

文中针对某数字T/R组件出现的局部过热问题,对原有液冷冷板进行改进设计。组件加工工艺由传统的铣削加工改为一体压铸成型,冷板流道改为铜管嵌装结构,并引入微通道散热技术,分别设计直齿微通道散热模块和菱形柱微通道散热模块。测试结果表明:微通道散热模块对散热效果改善明显,可解决局部过热问题;引入微通道散热模块后冷板流阻有所增加,但在允许范围内;菱形柱微通道散热模块由于边界层重新发展和二次流的产生,散热效果比直齿微通道散热模块好,热流密度越高,改善效果越明显。文中数字T/R组件最终选用直齿微通道散热模块的流道结构。菱形柱微通道散热技术可在更高热流密度的情况下应用。     

Heat/mass transfer characteristics in the near-tip region on a turbine blade surface under combustor-level high inlet turbulence

Heat/mass transfer characteristics on the near-tip blade surface under combustor-level high inlet turbulence have been investigated within a high-turning turbine rotor passage by using the naphthalene sublimation technique. The inlet turbulence intensity and length scale are 14.7% and 80 mm, respectively. The tip gap-to-chord ratio is changed to beh/c = 0.74, 1.47, and 2.94 percents. Increasingh/c results not only in higher heat/mass transfer in the pressure-side tip region but also in more convective transport on the pressure surface even far away from the tip edge. Severe heat/mass transfer is always observed in the suction-side tip-leakage flow region which can be divided into two distinct high transport regions. There is a local maximum of heat/mass transfer along the trailing-edge centerline. This arises from the interaction of a tip-leakage vortex with a trailing-edge vortex shedding. Comparisons of the present data forh/c = 2.94 percents with the previous low turbulence one show that there is a large discrepancy of heat/mass transfer in the pressure-side near-tip area, which diminishes with departing from the tip edge. The suction-side heat/mass transfer in the tip-leakage flow region is less influenced by the high inlet turbulence than that at the mid-span. The leading-edge heat/mass transfer under the high inlet turbulence is always higher than that in the low turbulence case, while there is no big difference in the trailing-edge heat/mass transfer between the two cases.     

An experimental approach to turbulent heat transfer using a symmetric expanded plane channel

The flow in a symmetric expansion plane channel is known to deflect to one side of a channel even at a low Reynolds number due to the Coanda effect. Details of flow structure have been investigated by various authors; however, there have been a few works conducted in the area of on heat transfer. This paper presents experimental results of turbulent heat transfer in separated and reattached flows in a symmetric expansion plane channel. Experiments were conducted using a low-speed open-circuit wind tunnel. The step H was 20 mm high and 200 mm wide, with an expansion ratio of 2.0. The Reynolds number based on the uniform flow velocity at step and H was varied from 5,000 to 35,000, respectively. The mean and turbulent fluctuating velocities were measured using mainly two types of split film probe. A cold single wire probe was used for measuring the mean and turbulent fluctuating temperatures. It was found that the local Nusselt number profile was considerably different on the upper and lower walls due to the Coanda effect, which is was caused by instability between the upper and lower separated shear layers. Empirical formulae for the maximum Nusselt number in the reattachment region are hereby proposed for the upper and lower walls, respectively. The two formulae are well correlated with the previous general formula proposed. The location of maximum Nusselt number is found to be very close to the flow reattachment point. Details of the velocity and temperature fields were clarified and their correlations with the heat transfer characteristics described above investigated. Furthermore, the wavelet transformation methodology was employed to study instantaneous flow and temperature behaviors, which exhibited its usefulness in the study of the present complicated flow and temperature fields.     

Heat/mass transfer measurement on concave surface in rotating jet impingement

The objective of this paper is to investigate the heat/mass transfer characteristics on a concave surface for rotating impinging jets. The jet with Reynolds number of 5,000 is applied to the concave surface and the flat surface, respectively. The rotating experiments have been carried out at the rotating speed of 560RPM which is corresponding to Ro number of 0.075. The two jet orientation (front and trailing orientation) are considered. Detailed heat/mass transfer coefficients on the target plate were measured using a naphthalene sublimation method. The result indicates that the rotation leads to change in local heat/mass transfer distributions and the slight increase in the Sh level. The front orientation induces asymmetric Sh distributions, whereas the trailing orientation shows the shifted heat/mass transfer feature due to rotation-induced flow behavior. The crossflow effect on heat/mass transfer is also observed as the streamwise direction increases. Compared to flat surface, the heat/mass transfer on the concave surface is enhanced with increasing the spanwise direction due to the curvature effect, providing the higher averaged Sh value. It is proved that the difference of surface geometry affects somewhat the local and averaged heat/mass transfer regardless of rotation condition. This paper was presented at the 9^th Asian International Conference on Fluid Machinery (AICFM9), Jeju, Korea, October 16–19, 2007.     

新型人字形组合板式换热器及导流区的数值模拟

基于计算流体动力学(Computational fluid dynamics,CFD)方法,运用SOLIDWORKS三维建模软件和FLUENT软件对一种新型人字形组合板式换热器及四种新型导流区进行研究。分析了不同设计参数的新型人字形组合板式换热器换热系数和压降,得到优化后的设计参数;从流型、压降、出口流体均匀分配程度等方面对市面上常见的2种导流区进行分析,在此基础上提出4种新型导流区,通过数值模拟研究其导流效果。结果发现:新型人字形组合板式换热器J型板较市面上常见的M型板的综合性能更优。当组合板中大波纹倾角板片(②板片)的倾角为60°,波纹截距为18 mm、16 mm或者14 mm中某一值时,随着组合板中小波纹倾角板片(①板片)波纹倾角增大,无论是冷流体还是热流体,表面传热系数和压降均增大。当①板片的波纹倾角为30°或40°,②板片的波纹截距变化对组合板片的性能影响不大;当1板片的波纹倾角为50°,②板片的波纹截距变化对组合板片的性能影响较大。导流区的设计对流体分布情况起主要影响作用。市面上常见的某种常规型导流区导流效果差,压降大,某公司M系列巧克力块型的导流区导流效果较好,压降小。4种新型导流区比常规型导流区的导流效果更好,压降介于常规型和巧克力块型之间,其中新型导流区Ⅲ的导流效果最好,压降很小。     

周期性冲击射流流动对传热特性的影响分析

采用经过实验验证的数值模型研究了周期性射流冲击下的流场对传热强化的影响。根据不同波形（正弦波、三角形波和矩形波）规律变化的射流对平板的冲击会产生不同的传热特性,研究得到了滞止点处的温度、传热系数和湍流强度随时间变化的规律。研究结果表明：当正弦波射流、三角形波射流的信号处于上升阶段时,湍流强度在稍有延迟后会产生一个瞬时的增强,可对强化传热起到促进作用,但在它们随后的波形变化中湍流强度仅有缓慢的升降,矩形波射流在信号发生阶跃变化时,会产生湍流强度的脉冲增强,尤其是在信号跃降时产生的瞬时脉冲增强比信号跃升时产生的脉冲增强更大,可有效强化传热;远离滞止点的流场的周期性波动仍然存在,但幅值大大减小,矩形波射流的平均速度大于其他波形射流的速度。     

Investigation on the determination of flow direction using two parallel cylindrical hot film sensors

Measurement of instantaneous air flow velocity with high frequency can be carried out by using a hot wire anemometer (HWA). HWA works on the basis of heat transfer rate from hot wire to the fluid flow, therefore directional identification of the air flow using hot wire anemometer is a difficult task. By using two parallel cylindrical hot film sensors a probe was built. By considering the wake and heat effect of the upstream sensor on the downstream sensor, direction of the air flow can be identified. In this work, the wake and heat effect resulting from the upstream sensor to the velocity measurement, by the downstream sensor was studied. This measured velocity is dependent of the following factors namely; air velocity, upstream sensor overheat ratio, distance between the two sensors and turbulence intensity of the flow. As a result it was found that the manufactured probe with sensor distance of 1 mm apart is capable of measuring reverse flow measurements of up to 20 m/s for a moderate turbulent flow.     

An experimental study on heat transfer characteristics with turbulent swirling flow using uniform heat flux in a cylindrical annuli

An experimental study was performed to investigate heat transfer characteristics of turbulent swirling flow in an axisymmetric annuli. The static pressure, the local flow temperature, and the wall temperature with decaying swirl were measured by using tangential inlet conditions and the friction factor and the local Nusselt number were calculated for Re=30000-70000. The local Nusselt number was compared with that obtained from the Dittus-Boelter equation with swirl and without swirl. The results showed that the swirl enhances the heat transfer at the inlet and the outlet of the test tube.     

A numerical study of impingement heat transfer in a confined circular jet

Heat transfer characteristics of a submerged circular jet impingement with a confined plate was studied numerically. The continuity, momentum and energy equations were solved simultaneously. FIDAP, a finite element code, was used to formulate and solve the matrix equations for fluid elements. The effects of channel height and Reynolds number on the local Nusselt number were considered in the range of H=0.5–1.5 and Re=100–900, respectively. It was found that the channel height influenced strongly on the surface temperature, shear stress and pressure drop. The peak temperature was observed and gradually moved outward to the rim of the heated circular plate with increasing the Reynolds number, which may be related to flow recirculation region in the channel. It is also noted that the pressure drop increased more than the average heat transfer coefficient as the Reynolds number increased. For Pr=7, the Nusselt number was much more dependent on the Reynolds number than the channel height, and the magnitude of the second peak in the Nusselt number distribution increased as the Reynolds number increased. The local Nusselt number calculated based on a mixing-cup temperature was considerably different from that using the inlet nozzle temperature for H=0.5 and Re=100. The present study showed that the local Nusselt number of a confined submerged jet was significantly larger than that of the unconfined free jet which was available in the literature.