Turbulent flow and heat transfer characteristics of a two-dimensional oblique plate impinging jet

Turbulent flow and heat transfer characteristics of a two-dimensional oblique plate impinging jet (OPIJ) were experimentally investigated. The local heat transfer coefficients were measured using thermochromic liquid crystals. The jet mean velocity and turbulent intensity profiles were also measured along the plate. The jet Reynolds number (Re, based on the nozzle width) ranged from 10, 000 to 35,000, the nozzle-to-plate distance (H/B) from 2 to 16, and the oblique angle (α) from 60 to 90 degree. It has been found that the stagnation point shifted toward the minor flow region as the oblique angle decreased and the position of the stagnation point nearly coincided with that of the maximum turbulent intensity. It has also been observed that the local Nusselt numbers in the minor flow region were larger than those in the major flow region for the same distance along the plate mainly due to the higher levels in turbulent intensity caused by more active mixing of the jet flow.     

Effect of cross-flow velocity on flow and heat transfer characteristics of impinging jet with low jet-to-plate distance

An effect of cross-flow velocity on flow and heat transfer characteristics of impinging jet in the case of low jet-to-plate distance at H = 2D was experimentally and numerically investigated. In the experiments, the air jet from orifice impingement on the wall of wind tunnel while a cross-flow was simultaneously induced normal to the jet flow. The jet velocity was fixed while the cross-flow velocity was varied corresponding to velocity ratios (jet velocity/cross-flow velocity) VR = 3, 5 and 7. The temperature distribution on an impinged surface was visualized by using thermochromic liquid crystal sheet (TLCs), and Nusselt number distribution was evaluated by using image processing method. The flow pattern on impingement surface was visualized by using oil film technique. The numerical simulation was carried out for a better understanding of the jet flow in the cross-flow. The results show that Nusselt number peak shifts downstream and the Nusselt number peak increases with increasing cross-flow velocity.     

The effect of ambient air condition on heat transfer of hot steel plate cooled by an impinging water jet

It has been observed that the cooling capacity of an impinging water jet is affected by the seasonal conditions in large-scale steel manufacturing processes. To confirm this phenomenon, cooling experiments utilizing a hot steel plate cooled by a laminar jet were conducted for two initial ambient air temperatures (10°C and 40°C) in a closed chamber, performing an inverse heat conduction method for quantitative comparison. This study reveals that the cooling capacity at an air temperature of 10°C is lower than the heat extracted at 40°C. The amount of total extracted heat at 10°C is 15% less than at 40°C. These results indicate the quantity of water vapor, absorbed until saturation, affects the mechanism of boiling heat transfer.     

Heat transfer of an evaporating liquid on a horizontal plate

We consider a horizontal static liquid layer on a planar solid boundary. The layer is evaporating when the plate is heated. Vapor recoil and thermo-capillary are discussed along with the effect of mass loss and vapor convection due to evaporating liquid and non-equilibrium thermodynamic effects. These coupled systems of equations are reduced to a single evolution equation for the local thickness of the liquid layer by using a long-wave asymptotics. The partial differential equation is solved numerically.     

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.     

An experimental study on the distribution of the drop size and velocity in asymmetric impinging jet sprays

Distributions of the drop size and velocity in an asymmetric impinging jet are investigated by injecting water and a sodium carbonate (Na₂CO₃) solution, which simulates the mixing process in impinging jet sprays of liquid oxidizer and liquid fuel for liquid propellants. The liquid sheet formed from the impinging jet is visualized and the drop size distributions are obtained by using image processing for the visualized images. The drop size distribution of the asymmetric impinging jets is fitted to the Rosin-Rammler distribution function. The obtained drop size distributions according to the azimuth angle in the impinging jet are compared with the theoretical predictions of previous research. The experimental results of the drop size distributions are located between the two curves obtained from the theoretical predictions by treating each jet in the asymmetric impinging jets as an independent wall-impinging jet. PIV images using a double-exposure method were processed to obtain the drop velocity vector in the impinging jets. Whether the drops shedding from the edge of the asymmetric impinging jets occurs radially or tangentially is also investigated from the PIV results.     

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.

     

Heat transfer characteristics of a short helical plate in a horizontal circular tube

Journal of Mechanical Science and Technology - Enhancement techniques based on artificial roughness are used in numerous applications of heat exchangers. Heat exchange devices are essential...     

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.     

Erosive wear of ductile metals by a particle-laden high velocity liquid jet

A liquid-solid particle jet impingement flow apparatus is described and experimental measurements are reported for the accelerated erosion of copper, aluminum and mild steel sheet metal by coal suspensions in kerosene and Al₂O₃ and SiC suspensions in water. Slurry velocities of up to 130 ft s^?1 (40 m s^?1) and impingement angles of 15°–90° were investigated. The maximum particle concentration used was 40 wt.%. For high velocity the results of this work show two erosion maxima; these are found at impingement angles of 90° and 40°. However, in corresponding gas-solid particle investigations maximum erosion occurs at approximately 20°. In this work both particle concentration and composition were varied. A polynomial regression technique was used to calculate empirical and semitheoretical correlation constants.     

平板传热对气膜冷却的影响

对不同平板厚度的圆柱孔平板气膜冷却模型进行了数值研究,其中冷却空气入射角均为35°,湍流模型采用Standardk-ε湍流模型。所研究的模型分为单孔模型和排孔模型,板的厚度分为4mm、2mm及0mm。文章的重点在于分析平板的传热对气膜冷却效果的影响,研究结果显示气膜冷却效率沿主流方向逐渐降低,当吹风比为0.8时冷却效率最高,平板内部导热对气膜冷却效率及平面温度分布有较大影响。     

Free surface planar liquid jet impingement on a moving surface: interfacial flow and heat transfer characteristics

Journal of Mechanical Science and Technology - Understanding heat transfer characteristics of sheet metal is of practical importance in sheet metal rolling operation to ensure strength and quality...     

Entropy analysis of flow and heat transfer caused by a moving plate with thermal radiation

This study examines the effects of thermal radiation on entropy generation in flow and heat transfer caused by a moving plate. The equations that govern the flow and heat transfer phenomenon are solved numerically. Velocity and temperature profiles are obtained for the parameters involved in the problem. The expressions for the entropy generation number and the Bejan number are obtained based on the profiles. Graphs for velocity, temperature, the entropy generation number, and the Bejan number are plotted and discussed qualitatively.     

Design and performance of a high velocity air–sand jet impingement erosion facility

This paper describes the design, construction and capabilities of a high velocity air–sand erosion rig. It has been designed with the aid of computational fluid dynamics to approximately simulate the erosion conditions often experienced by subsea choke valves used in the offshore oil industry. It has also been designed to evaluate the erosion performance of CVD diamond coatings at sonic velocity. The rig is of the gas-blast design in which solid particles, typically sand 60–660 μm in size, are injected into a high velocity air stream and accelerated down a 16-mm-diameter tube, 1 m in length, before striking the sample under test. Tests can be carried out with particle velocities of up to 340 m/s under a wide range of sand fluxes, impact angles and standoff distances. The results of pressure, velocity and sand flux calibration work are described. In addition, preliminary experimental data on tests carried out on mild steel, bulk and sprayed tungsten carbide are also presented. The flexibility of the air–sand rig allows the erosion behaviour of materials to be studied under a wide range of conditions.     

Heat transfer enhancement in cold plate based on FVM method and field synergy theory

Journal of Mechanical Science and Technology - To optimize the overall heat dissipation performance of the straight channel of a cold plate for lithium battery in vehicles, we used the wavy channel...     

Estimation of the performance of a narrow porous journal bearing with hydrodynamic turbulent lubrication considering slip velocity and arbitrary wall thickness

A closed-form mathematical analysis is presented for the hydrodynamic lubrication of a 360° short porous metal journal bearing with arbitrary wall thickness which is press fitted in a solid housing and works with a turbulent film of newtonian lubricant. A new pressure equation is used. The bearing is assumed to be narrow, and therefore circumferential flow of the lubricant in the clearance region is negligible in comparison with that in the axial direction which makes the governing differential equation simpler to solve. However, this simplification is not applicable to darcian flow in the porous matrix so that a three-dimensional Laplace equation is required to describe the continuity of flow in the pores. The film curvature is included by retaining terms containing $\text{C}\text{R}{}_1$ in the expression for film thickness. The curvature of the permeable bearing matrix, which allows it to have an arbitrary wall thickness, is taken into account by a direct approach. Infinite Fourier series and their orthogonal properties are utilized for the determination of the turbulent hydrodynamic pressure distribution from which the load-carrying capacity and attitude angle are calculated. All the results of interest are simple and fully analytical in nature permitting easy and economical calculation of numerical data over a very wide range of parameters.     

组合涡发生器在螺旋板式换热器上的强化传热研究

在螺旋板式换热器螺旋通道内设置三角翼和椭圆柱组合涡发生器,利用流体计算软件Fluent进行三维数值模拟。研究了Re为4000～7000内组合涡发生器对通道平均Nu和平均阻力系数f的影响,并应用场协同原理进行了分析。与只加装椭圆柱涡发生器的螺旋板式换热器进行对比,结果表明,纵向涡发生器产生的二次流能改善螺旋通道内的速度场与温度场的协同性,起到强化换热作用。在正三角形排列方式下,组合涡发生器通道的平均Nu比椭圆柱涡发生器的平均Nu增大8.7%,阻力因子f减小23.7%,强化换热的效果较好。     

LDV measurements of turbulent flow behavior of droplets in a two-phase coaxial jet

Coaxial nozzles are frequently utilized for the atomization of liquids in sprays. The performance of a nozzle is generally evaluated by its atomizing characteristics, which are actually governed by the turbulence interactions of two fluids. With this point of view, this experimental study was carried out to investigate the turbulent behavior of the droplets atomized in a two-phase coaxial jet. Air and water have been used as the working fluids, and the measurements have been made by an on-line data acquisition system connected to a two-channel LDV set(DISA, 5W, Argon laser, blue: 488 nm, green: 514.5 nm). In order to generate a two-phase mixing jet, two types of coaxial nozzles (liquid column type, liquid sheet type) were used. For the investigations of the turbulent flow structure of this two-phase mixing jet, the spreading rates, mean and fluctuating components, intermittency factors and the iso-contours of joint probability densities were measured and analyzed. The results from the both types of nozzles did not show remarkable differences in mean and fluctuating velocity distributions, intermittency factors or the iso-joint probability density contours. Since the measurements were made in the fully developed turbulent mixing regions, the mean velocity distribution profiles showed good similarities and agreed well with the semi-empirical curves. The RMS values were represented as high order levels and so were the intermittency factors. The typical development trends of turbulent components ofu′ andv′ for both types were illustrated in the iso-joint probability density contours.     

PIV measurements in a turbulent wall jet over a backward-facing step in a three-dimensional,non-confined channel

A study of a turbulent wall jet over a backward-facing step is especially of interest because it shows a rich phenomenon flow and a mechanism to alter the flow characteristics downstream of the step. However, studies on this flow configuration are rare. In this paper, we considered this flow configuration in a non-confined channel as the specific engineering applications of electrical rotating machines and alternator that can be found in modern wind generators of the power production industry and automobile engines. The turbulent wall jet over a backward-facing step in a non-confined wind tunnel had the jet Reynolds number of 24,100 and the step Reynolds number of 11,900. Particle image velocity (PIV) and stereoscopic PIV measurements were performed along the central plane and several cross-stream planes. Numerical simulation of the test configuration was conducted by solving the three-dimensional Reynolds Averaged Navier–Stokes (RANS) equations with the second-order closure Reynolds stress model (RSM). The mean flow fields and second-order statistical moments from the RSM simulation were compared to results that were obtained through the PIV and stereo-PIV experiments. The mean reattachment length obtained from the current configuration was much shorter than those from the backward-facing step in the plane channel. The stereo-PIV measurements in the cross-stream planes revealed a high three-dimensionality of the flow, a high population of streamwise vortice in the upper region, near the side walls and the corners formed by the side walls and the bottom wall. The obtained results also confirmed the presence of the wall-jet formation on the bottom wall.     

Numerical investigation of NOx reduction in a sudden-expansion combustor with inclined turbulent air jet

Axisymmetric sudden-expansion geometry of a co-flowing methane-air diffusion flame is considered to investigate the effect of air inlet conditions on NO_x formation, flow field and temperature distribution using the k-? turbulence and β-PDF combustion model. The predicted results are in acceptable agreement with the published experimental and numerical data. The obtained results show that increasing air turbulence intensity results in considerable decrease in NO formation. Increasing the inlet angle of the air causes the NO formation to decrease due to raising vorticity strength. As a new index, the mass-averaged integral of vorticity magnitude is introduced to investigate the effect of altering inlet angle of the air on the flow field.