阵列射流冲击冷却流场与温度场的数值模拟

采用数值模拟方法对冲击冷却的流动和传热过程进行了三维数值研究。特别研究了在冲击孔叉排方式下,相邻孔间距、冲击距离以及射流入口雷诺数对冲击表面冷却流动传热特性的影响规律。     

Heat Transfer Characteristics of Cooling High Temperature Steel Plate by Single Round Jet Impingement

The heat transfer characteristics of a single round air jet impingement on a high temperature steel plate were examined experimentally using a single‐point temperature measurement method, incorporated with solving the inverse heat conduction problem. During the experiments, the temperature of the steel plate varied from 1073 K to 373 K, the Reynolds number was set to 27,000, the nozzle to plate spacing was set to 4. The results indicated that the radial distribution of the local Nusselt number is bell‐shaped at the initial stage of the transient cooling process. As the cooling process continues, the local Nusselt numbers decrease and a second peak occurs at r/D = 2. The area averaged Nusselt number are in accordance with the correlation proposed by Hofmann and Martin at first and then decrease significantly, but this trend is not obvious at r/D > 10.     

The Impingement Heat Transfer Data of Inclined Jet in Cooling Applications: A Review

On the impingement heat transfer data, the experimental studies of air and liquid jets impingement to the flat surfaces were collected and critically reviewed. The oblique impingements of both single circular and planar slot jets were considered in particular. The review focused on the surface where the jet impingement cooling technique was utilized. The nozzle exit Reynolds numbers based on the hydraulic diameter varied in the range of 1,500–52,000. The oblique angles relative to the plane surf...     

Recent Trends in Computation of Turbulent Jet Impingement Heat Transfer

A review of the current status of computation of turbulent impinging jet heat transfer is presented. It starts with a brief introduction to flow and heat transfer characteristics of jet impinging flows considering the simplest jet impinging geometry: normal impingement of a single jet into a flat surface. Subsequently, a review of recent computational studies related to the same geometry is presented. The effects of different subgrid scale models, boundary conditions, numerical schemes, grid distribution, and size of the computational domain adopted in various large eddy simulations of this flow configuration are reviewed in detail. A review of direct numerical simulation of the same geometry is also presented. Further, some recent attempts in Reynolds-averaged Navier–Stokes modeling of impinging flows are also reviewed. A review of computation of other complex impinging flows is also presented. The review concludes with a listing of some important findings and future directions in the computation of impinging flows.     

阵列射流冲击冷却换热特性的数值研究

运用数值计算的方法对不同流动取向的多排孔冲击射流冷却特性进行了三维模拟,并对有初始横向流的多排孔冲击射流冷却特性进行了数值研究,揭示出射流雷诺数、流动方向、初始横向流对冲击冷却传热特性的影响规律。结果表明:研究范围内,射流雷诺数越大,冲击靶面换热效果越好;冲击腔室两端都设为出口时努赛尔数峰值所对应的射流驻点区向下游偏移最小且换热效果最好;当横流雷诺数与射流雷诺数之比大于0.5之后,有横流时的冲击射流冷却局部努赛尔数比无横流时有较为显著下降。     

Investigation of the Conjugate Heat Transfer and Flow Field for a Flat Plate with Combined Film and Impingement Cooling

Film cooling combined with internal impingement cooling is one of the most effective technologies to protect the gas turbine vanes and blades from the hot gas. In this study, conjugate heat transfer CFD study was undertaken for a flat plate with combined film cooling and impingement cooling. An experiment on conjugate heat transfer of a flat plate with combined film and impingement cooling was performed to validate the code. Then the effects of several parameters including Biot number, blowing r...     

Numerical Investigation of Jet Impingement Cooling of a Flat Plate with Carbon Dioxide at Supercritical Pressures

Confined round jet impingement cooling of a flat plate at constant heat flux with carbon dioxide at supercritical pressures was investigated numerically. The pressure ranged from 7.8 to 10.0 MPa, which is greater than the critical pressure of carbon dioxide, 7.38 MPa. The inlet temperature varied from 270 to 320 K and the heat flux ranged from 0.6 to 1.6 MW/m². The shear-stress transport turbulence model was used and the numerical model was validated by comparison with experimental results for jet impingement heating with hot water at supercritical pressures. Radial conduction in the jet impingement plate was also considered. The sharp variations of the thermal-physical properties of the fluid near the pseudocritical point significantly influence heat transfer on the target wall. For a given heat flux, the high specific heat near the wall for the proper inlet temperature and pressure maximizes the average heat transfer coefficient. For a given inlet temperature, the heat transfer coefficient remains almost unchanged with increasing surface heat flux at first and then decreases rapidly as the heat flux becomes higher due to the combined effects of the thinner high specific heat layer and the smaller thermal conductivity at higher temperature.     

Heat Transfer Enhancement of Spray Cooling on Flat Aluminum Tube Heat Exchanger

This study provides an experimental analysis on the heat transfer performance of a flat aluminum tube microchannel heat exchanger with spray cooling. The effects of water spraying rate, airflow rate, and relative humidity were investigated. The test results show that the heat transfer performance increased with increasing the water spraying rate but without the penalty of increased flow resistance at low spray conditions. This effect is further enhanced by increasing the water spraying rate. However, when the spraying rate is high, part of the nonevaporated drops attached to the fin surface and formed a liquid film, which caused the flow passage to become narrower. Further increase in the spraying rate resulted in part of the flow passages being blocked by the nonevaporated water drops and caused a region of poor heat transfer. The friction coefficient jumped drastically at this condition. This phenomenon deceased gradually with increasing airflow rate. High inlet air humidity resulted in the water accumulation phenomenon appearing at lower water spraying rates. The evaporative cooling effect decreased and flow friction increased. The test results just described show that the water spray is able to significantly improve the air-side heat transfer performance. The optimum spray rate for each airflow rate must be carefully determined.     

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 Model with Two Heat Transfer Coefficients Along a M ultiperforated Plate-Application to Combustion Chamber Wall Cooling

Walls' cooling of aeronautic propeller combustion chamber is performed with the injection, through the combustion chamber wall, of a part of the air coming from compressors placed upstream. Measurements of the wall thermal fields are made by infrared thermography along the injection wall. This injection wall is pierced by 9 rows of 8 holes (α=90°) in staggered configuration(p/D=s/D=6). We propose a model using two heat transfer coefficients to represent the convective exchanges. The results are non-dimensioned and presented in comparison with the case without holes. The use of this model allows us to define 4 zones. Those 4 zones exist for the 5 blowing rates.     

Large Eddy Simulation of Turbulent Slot Jet Impingement Heat Transfer at Small Nozzle-to-Plate Spacing

We present fluid flow and heat transfer of a slot jet impingement heat transfer at a small value of the nozzle-to-plate spacing at which a secondary peak in the Nusselt number is observed. Large eddy simulation has been performed with a finite-volume-based computational fluid dynamics code and using a dynamic Smagorinsky model. The optimum domain size and grid for large eddy simulation (LES) have been produced based on LES computations on a coarse mesh and Reynolds-averaged Navier–Stokes-based computations. Two inflow conditions, namely, using the vortex method and no perturbations, were compared. The present LES results, using the vortex method, capture the secondary peak in the Nusselt number better as compared to the case with no perturbations. Results show that mean velocity profile in the stagnation region deviates from the standard law of the wall. Further, large-scale vortical structures were observed near the location of the secondary Nusselt number peak. Increases in both the streamwise and wall normal turbulence fluctuations are observed near the secondary peak in the Nusselt number. The secondary peak in Nusselt number is found to be associated with the combined effect of flow acceleration and an increase in the turbulence kinetic energy.     

Evaluation of Jet Impingement,Spray and Microchannel Chip Cooling Options for High Heat Flux Removal

Thermal management for high heat flux removal from microelectronic chips is gaining critical importance in many earth-based and space-based systems. Heat fluxes greater than 1 MW/m² (100 W/cm²) have already been realized in high-end server applications, while cooling needs in next generation chips and advanced systems such as high-power electronics and electrical systems, pulsed power weapons systems, solid-state sensors, and phased-array radars are expected to reach 5–10 MW/m² (500–1000 W/cm²). After evaluating the contributions from different thermal resistances in the chip-to-ambient thermal path, this paper presents a critical review and research recommendations for three prominent contending technologies: jet impingement, spray cooling, and microchannel heat sinks.     

Heat Transfer Characteristics during Mist Cooling on a Heated Cylinder

Mist cooling is expected to contribute to better steel products because it enables uniform and moderate cooling in steel-making processes. In this study, experimental data were obtained to understand the mist cooling process in a high-temperature cylinder. The wetting phenomenon on the cylinder surface was also observed with an installed CCD camera. Comparing these results with those from previous investigations, we found that our cooling curve corresponded to two distinct heat transfer regimes in mist cooling. A simplified model from the analysis was introduced to simulate the cooling curve in each heat transfer regime. The estimated cooling curves by this model agreed with the experimental data in one regime and had a similarity in the other regime.     

Experimental Study of Unsteady Local Heat Transfer for Impinging Miniature Jet

An experimental study was performed to characterize local and average heat transfer coefficients on a horizontal disk impinged by a circular liquid jet. The radial distribution of local heat flux and surface temperature were determined under unsteady-state conditions. The inverse heat conduction problem was solved using the responses of sensors located inside the experimental disk. It was shown that the distribution of the surface heat flux and the surface temperature was dependent on the radial location and the time. The average Nusselt number was seen to be independent of the nozzle-to-heat exchange surface spacing. It increased with the Reynolds number and the nozzle diameter.     

倾斜射流对移动平板表面紊动和传热特性的影响

采用雷诺应力湍流模型和Simplic算法对半封闭槽道内倾斜射流冲击移动平板的流动和传热特性进行了数值模拟,研究了不同射流角度和不同平板移动速度下平板近壁湍动能和板面努塞尔数的变化.结果表明：射流角度和平板运动速度对平板近壁湍动能和表面努塞尔数值分布影响显著;当入射角与平板运动方向相同时,板速的升高提高了近壁面的湍动能,但是降低了冲击区域的局部努塞尔数值;平板表面的平均努塞尔数值随板速的提高先降低后大幅升高,高速下角度对平板表面的平均传热效果影响较小;当入射角为80°,平板运动方向与入射方向相反且板速和射流速度相同时,在移动平板表面能够获得较佳的紊动和传热效果.     

Numerical Study of Heat Transfer and Separated Flow over Rectangular Obstacle in Jet

This paper is about a separated reattaching flow over a hot rectangular obstacle. Two types of incoming flow are examined in order to show the influence of the external zone of the flow on the reattachment process. It comes about due to a wall jet and a boundary layer. The inner region of these two flows is similar, but their external regions are extremely different. The separating and reattaching flow phenomena are of particular interest in engineering fields such as for an aeronautical application. Wall jet flow over an obstacle occurs in many engineering applications such as environmental discharges, heat exchangers, fluid injection systems, cooling of combustion chamber wall in a gas turbine, automobile design, and others. In electronics cooling, the prediction of the Nusselt number distribution along the obstacles is necessary before the design of the apparatus. For a heated obstacle at a constant temperature, T = 350 K and an aspect ratio of 10 (L = 10 H), the problem parameters are: (a) jet exit Reynolds number (Re) ranged from 1000 to 50000, (b) incoming flow configuration (boundary layer and wall jet). The ratio between the thickness of the nozzle (b) to the obstacle height (H) are examined simultaneously. The formulation is based on the SST k–ω turbulence model. The results show that the increasing of nozzle thickness; enhances the heat transfer and considerably modifies the stagnation point location. The highest incoming flow momentum provides the greatest values of average Nusselt number. Such as the boundary layer case in comparison with the wall jet cases. The average Nusselt number is correlated according to problem parameters .     

Experimental Result on Heat Transfer during Quenching of Hot Steel Plate by Spray Impingement

The present article discusses the experimental results on cooling characteristics of a stationary hot steel plate by spray impingement. The experimental setup consisted of an electrically heated flat stationary steel plate of dimension 120 mm × 120 mm × 4 mm, spray setup, water supply, and air supply unit. The effects of various controlling parameters such as air-water pressures, water flow rate, nozzle tip to target distance and impingement density were determined and analyzed. The cooling rates were computed from the time-dependent temperature history and used to analyze the parametric effects. The results obtained in the study confirmed the higher efficiency of the spray cooling system and the cooling strategy was found advantageous over the conventional cooling methods available in the open literature.     

Fouling Propensity of Modified Heat Transfer Surfaces

Abstract

As much as attention that has been paid to surface treatment as an efficient, and environmentally friendly approach toward fouling mitigation, the characterization of many innovative modified surfaces has become a matter of much debate. The latter is closely associated with the intermolecular interaction energies which would profoundly influence the adhesion of precursors onto the modified surfaces. In this study, based on the extended Derjaguin, Landau, Verwey and Overbeek (DLVO) theory, a new criterion is proposed to predict the propensity of a surface when prone to crystallization fouling or biofouling. Thereafter, the proposed criterion is examined against the present experimental results as well as those from previous studies where the required information for the determination of new criterion is available. The comparison shows that deposit formation onto heat transfer surfaces decreases with increasing the new proposed fouling propensity indicator criterion. Moreover, nearly 75% of the collated crystallization and biological fouling data points are predictable with this criterion and reasons for those that are not in compliance with the proposed criterion are discussed.     

Evaporation Heat Transfer Coefficients of R-446A

ABSTRACT

The present study investigated the evaporation heat transfer coefficients of R-446A, as a low global warming potential alternative refrigerant to R-410A. The evaporation heat transfer coefficients were obtained by measuring the wall temperature of a straight stainless tube and refrigerant pressure. The heat transfer coefficients were measured for the quality range from 0.05 to 0.95, the mass flux from 100 to 400 kg/m²s, heat flux from 10 to 30 kW/m², and saturation temperature from 5 to 10°C. The evaporation heat transfer coefficient of R-410A was verified by comparing the measured evaporation heat transfer coefficient with the value predicted by the existing correlation. The evaporation heat transfer coefficient of R-446A was measured using a proven experimental apparatus. When the heat flux was 10 kW/m², the evaporation heat transfer coefficient of R-446A was always higher than that of R-410A. But, when the heat flux was 30 kW/m², the evaporation heat transfer coefficient of R-446A was measured to be lower than that of R-410A near the dry-out point. The effect of the tube diameter on the R-446A evaporation heat transfer coefficient was negligible. The effect of saturation pressure on the evaporation heat transfer coefficient was prominent in the low quality region where the nucleate boiling was dominant.