Local heat/mass transfer measurements on effusion plates in impingement/effusion cooling with rotation

This paper describes an investigation of the local heat/mass transfer for rotating impingement/effusion cooling. A study was conducted of parameters such as jet orientation and surface geometry. An experiment using the naphthalene sublimation method provided the local heat/mass transfer coefficients on the effusion plate. The heat/mass transfer distributions for the axial orientation were similar to those for the stationary cases, while the trailing orientation produced different Sherwood number features, with divided high Sh regions and one low Sh region around the stagnation area. The concave surface provided better and more uniform heat/mass transfer than the flat surface.     

Heat/mass transfer in rotating impingement/effusion cooling with rib turbulators

A detailed measurement of the heat/mass transfer coefficients on the ribbed surfaces for the rotating impingement/effusion cooling system has been conducted. Three different jet orientations (front, leading, and trailing) were investigated at the same rotating speed and impinging jet Reynolds number of 3000. A naphthalene sublimation method was used to obtain local heat/mass transfer coefficients. Regardless of rib turbulators, the leading and trailing orientations lead to totally changed heat/mass transfer distributions due to the jet deflection, while the Sh distributions of the front orientation were similar to those of the stationary case. For leading and trailing orientations, the influence of crossflow, which deflected wall jets, decreased due to the blockage effect of the rib turbulators. Therefore, the wall jets spread more widely and the interaction between adjacent wall jets along spanwise direction became stronger, enhancing the heat/mass transfer compared to that on smooth surface.     

Experimental investigation on the leading edge film cooling of cylindrical and laid-back holes with different hole pitches

Experimental investigation has been performed to study the film cooling performances of cylindrical holes and laid-back holes on the turbine blade leading edge. Four test models are measured for four blowing ratios to investigate the influences of film hole shape and hole pitch on the film cooling performances Film cooling effectiveness and heat transfer coefficient have been obtained using a transient heat transfer measurement technique with double thermochromic liquid crystals. As the blowing ratio increases, the trajectory of jets deviates to the spanwise direction and lifts off gradually. However, more area can benefit from the film protection under large blowing ratio, while the is also higher. The basic distribution features of heat transfer coefficients are similar for all the four models. Heat transfer coefficient in the region where the jet core flows through is relatively lower, while in the jet edge region is relatively higher. For the models with small hole pitch, the laid-back holes only give better film coverage performance than the cylindrical holes under large blowing ratio. For the models with large hole pitch, the advantage of laid-back holes in film cooling effectiveness is more obvious in the upstream region relative to the cylindrical holes. For the cylindrical hole model and the laid-back hole model with the same hole pitch, heat transfer coefficients are nearly the same with each other under the same blowing ratios. Compared with the models with large hole pitch, the laterally averaged film cooling effectiveness and heat transfer coefficient are larger for the models with small hole pitch because of larger proportion of film covering area and strong heat transfer region.     

阵列射流冲击冷却传热特性的数值研究

以涡轮叶片冷却技术为背景,采用带转捩的剪切应力输运(Transition SST)模型对阵列射流冲击冷却的传热特性进行数值模拟,分析了冲击Re、冲击间距、初始横向流和冲击孔排列方式的影响规律。结果表明:冲击间距对靶面平均Nu的影响存在最优值,在所计算的范围内,Zn/d=2时平均Nu最大;在冲击孔排列方式影响中,当冲击间距Zn/d≤2时,顺排孔冲击冷却传热效果优于错排,而当Zn/d≥3时,错排孔冷却传热效果优于顺排。     

发散孔纵向波纹隔热屏气膜冷却特性研究

对燃烧室内开有发散孔的纵向波纹隔热屏进行了数值模拟。研究隔热屏的四种结构参数开孔率、波纹板高度、气膜孔直径和冷却通道高度的改变对隔热屏冷却效果的影响。研究表明：在气膜孔出流总量相同的情况下,3％开孔率比6％开孔率的隔热屏平均冷却效率较高;汉纹板高度对隔热屏冷却效果影响较大,波纹板无量纲高度B=1％时的隔热屏平均冷却效率最高;冷却通道高度和气膜孔直径对隔热屏冷却效果影响较小,冷却通道高度只影响隔热屏前段的冷却效率,发散孔气膜孔直径的大小则对隔热屏冷却效率几乎没有影响。     

Single-phase hybrid micro-channel/micro-jet impingement cooling

A new hybrid cooling scheme is proposed for high-flux thermal management of electronic and power devices. This scheme combines the cooling benefits of micro-channel flow and micro-jet impingement with those of indirect refrigeration cooling. Experiments were performed to assess single-phase cooling performance using HFE 7100 as working fluid. Excellent predictions were achieved using the standard k–ε model. The proposed cooling scheme is shown to involve complex interactions of impinging jets with micro-channel flow. Increasing jet velocity allows jets to penetrate the micro-channel flow toward the heated surface, especially in shallow micro-channels, greatly decreasing wall temperature. Despite the relatively poor thermophysical properties of HFE 7100, the proposed cooling scheme facilitated the dissipation of 304.9 W/cm² without phase change; further improvement is possible by increasing jet velocity and/or decreasing coolant temperature. In addition to the numerical predictions, a superpositioning technique is introduced that partitions the heat transfer surface into zones that are each dominated by a different heat transfer mechanism, and assigning a different heat transfer coefficient value to each zone. Using this technique, a new correlation is developed that fits the data with a mean absolute error of 6.04%.     

Influence of shaped injection holes on turbine blade leading edge film cooling

To improve the film cooling performance by shaped injection holes for the turbine blade leading edge region, we have investigated the flow characteristics of the turbine blade leading edge film cooling using five different cylindrical body models with various injection holes, which are a baseline cylindrical hole, two laidback (spanwise-diffused) holes, and two tear-drop shaped (spanwise- and streamwise-diffused) holes, respectively. Mainstream Reynolds number based on the cylinder diameter was 7.1 × 10⁴ and the mainstream turbulence intensities were about 0.2%. The effect of injectant flow rates was studied for various blowing ratios of 0.7, 1.0, 1.3 and 1.7, respectively. The density ratio in the present study is nominally equal to one. Detailed temperature distributions of the cylindrical body surfaces are visualized by means of an infrared thermography (IRT). Results show that the conventional cylindrical holes have poor film cooling performance compared to the shaped holes. Particularly, it can be concluded that the laidback hole (Shape D) provides better film cooling performance than the other holes and the broader region of high effectiveness is formed with fairly uniform distribution.     

Experimental study of impingement spray cooling for high power devices

An experimental study of a closed-loop impingement spray cooling system to cool a 1 kW 6U electronic test card has been conducted. The system uses R134a as working fluid in a modified refrigeration cycle. The spray from four vapor assisted nozzles is arranged to cover a large ratio of the heated area of the card. Investigations are currently focused on effects of mass flow rate, nozzle inlet pressure and spray chamber pressure. Experimental results are promising with a stable average temperature of around 23 °C being maintained at the heated surface, and maximum temperature variation of about 2 °C under suitable operating conditions. Heat transfer coefficients up to 5596 W/m² K can be achieved with heat flux input around 50,000 W/m² in this study. It is found that cooling performance improved with increasing mass flow rate, nozzle inlet pressure and spray chamber pressure, whereas uniformity of the heated surface temperature can only be improved with higher mass flow rate and nozzle inlet pressure. The mechanisms for the enhanced performance are also presented.     

Experimental investigations of a double‐decker jet impingement/film structure's cooling effectiveness

Experiments were carried out to investigate the cooling coefficient of a laminar double‐decker jet impingement/film structure. Blowing rate M, the ratio of the jet impingement distance to the diameter of the jet hole H/D, the ratio of the distance between the jet hole and film hole to the diameter of the jet hole P/D were changed to study the rules of local cooling coefficient varying with these parameters. All the results show that a higher cooling coefficient was achieved under larger blowing rate conditions. There exists a proper range of H/D and P/D, which can result in a maximum cooling coefficient. It is verified that when H/D=0.5 and P/D=4, the maximum cooling coefficient can be achieved. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res, 37(4): 232–239, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20199     

Experimental investigation and performance analysis on a solar adsorption cooling system with/without heat storage

A solar adsorption cooling system which can be switched between a system with heat storage and a system without heat storage was designed. In the system with heat storage, a heat storage water tank was employed as the link between the solar collector circulation and the hot water circulation for the adsorption chillers. However, the heat storage water tank was isolated in the system without heat storage, and the hot water was directly circulated between the solar collector arrays and the adsorption chillers. It was found that the inlet and outlet temperatures for the solar collector arrays and the adsorption chillers in the system without heat storage were more fluctuant than those of the system with heat storage. Also found was that the system with heat storage operated stably because of the regulating effect by the heat storage water tank. However, under otherwise similar conditions, the cooling effect of the system without heat storage was similar to that of the system with heat storage. Compared with the system with heat storage, the system without heat storage has the advantages of higher solar collecting efficiency as well as higher electrical COP.     

异型孔阵对气膜冷却效果的数值研究

对不同孔型在不同的吹风比下的冷却效率进行数值模拟,计算结果表明:收缩进气可以强化孔内的对流传热;扩张出气可以使冷气出口的速度降低,气膜覆盖面更广,提高气膜的覆盖效果。在小吹风比下,孔内的对流传热在总的冷却效果中占主导地位,此时缩扩孔的冷却效果最好,而随着吹风比的增大,气膜覆盖所起的作用在增大,当吹风比增大到一定程度时,扩张孔的冷却效果将具有更好的表现。     

Experimental study of impingement cooling by heat sinks with thin longitudinal fins for LSI packages

This paper reports on the impingement cooling characteristics of a heat sink with thin longitudinal fins of 0.2 mm thickness, which are spaced with a fin-pitch in the range 0.5 mm to 2.0 mm. The air cooling of the heat sink comes from a slot-shaped orifice positioned above the heat-sink center. The breadth of the gap between the fin tops and the inlet orifice is in the range 0 mm to 10 mm. The thermal resistance of the thin longitudinal fins used is about 50% to 57% that of the thick longitudinal fins now in commercial use. The cooling performance of the thin-plate fins is almost the same as that of optimally arranged pin-fins with the same total surface area. A maximum value of six times the heat transfer rate of a single flat plate having the same base area was observed for the thin-plate fins. A comparison of cooling performance between impingement and channel flow systems was conducted. The performance of impingement cooling systems is almost unaffected by the breadth of the gap between the fin tops and the inlet orifice (or, for channel cooling, the upper wall). On the other hand, the performance of channel-cooling systems decreases significantly as the gap widens. © 1997 Scripta Technica, Inc. Heat Trans Jpn Res, 25(7): 449–459, 1996     

“冲击-气膜”复合式结构冷却效果数值研究

分析了六种不同的"冲击-气膜"复合式冷却结构,将其应用在燃气轮机涡轮导向器叶片中弦区并对其内部流体的流动和换热进行了数值模拟.计算条件采用某燃气轮机的典型工况,流体物性参数随温度变化.将不同"冲击-气膜"复合式冷却结构的计算结果进行对比得出:冲击孔与气膜孔在展向的排列形式对冷却效果有较大影响,叉排明显优于顺排;随着冲击孔的后移,冷却气体对腔内壁的覆盖面积逐渐减小,冷却效果逐渐降低,流阻逐渐增大;在来自冲击冷却和气膜冷却多种影响因素的共同作用下,气膜孔角度和所在面曲率对冷却效果和流阻的影响被大幅度削弱.     

Numerical investigation of the effectiveness of effusion cooling for plane multi-layer systems with different base-materials

Within Collaborative Research Center (SFB) 561 “Thermally Highly Loaded, Porous and Cooled Multi-Layer Systems for Combined Cycle Power Plants” at RWTH Aachen University, an effusion-cooled multi-layer plate configuration is investigated numerically by the application of a three-dimensional in-house fluid flow and heat transfer solver, CHTflow. CHTflow is a conjugate code, which yields information on the temperature distribution in the solid body. This enables a detailed discussion of the effects of a change in materials. The geometrical set-up and the fluid flow conditions derive from modern gas turbine combustion chambers and bladings. Within the SFB, two different multi-layer systems, one consisting of substrate made of CMSX-4 (a singlecrystal super-alloy), anMCrAlY-bondoat and a ZrO₂ thermal barrier coating (TBC), and the other consisting of a NiAlalloy and a graded bondcoat/TBC, have been investigated. The grading will increase the life-span of the TBC as it can better compensate the different thermal expansion coefficients of different materials. The main focus in this study is on the different substrate materials, because the thermal conductivity of the NiAl is considerably higher than that of CMSX-4, which leads to different temperature profiles in the components.     

Experimental investigation on a DI dual fuel engine with hydrogen injection

Over the past two decades considerable efforts have been undertaken to develop and introduce new alternative fuels for the conventional gasoline and diesel. Many alternative fuels, both liquid and gaseous, have been experimented and some have even been commercialized such as ethanol, natural gas, etc. Hydrogen has been considered as an excellent fuel to replace the petroleum‐based fuels due to its clean burning characteristics. In the present experimental investigation, hydrogen was injected in the intake manifold and diesel fuel was injected inside the engine cylinder in the conventional manner. Hydrogen injection parameters such as injection timing, injection duration and quantity of hydrogen injected were optimized based on the performance and emission characteristics. Exhaust gas recirculation (EGR) technique was adopted to reduce the oxides of nitrogen emission. From the results it was observed that for hydrogen diesel dual fuel (DF) engine, the optimal operating parameters for hydrogen injection were start of injection at gas exchange top dead centre with injection duration of 30° crank angle with the hydrogen flow rate of 7.5 litres per minute (lpm). With EGR the optimized condition was found to be 20% for the entire load. The brake thermal efficiency with 20% EGR increases by 16% at 75% load as compared with diesel, while at full load it reduces by 8% due to the recirculation of exhaust gases that results in a reduction of intake oxygen concentration compared with part load. NO_X emission decreases by five and half times, while other emissions increase by 1.4 times as compared with DF engine. Copyright © 2008 John Wiley & Sons, Ltd.     

Numerical investigation of the effectiveness of effusion cooling for plane multi-layer systems with different base-materials

Within Collaborative Research Center (SFB) 561 “Thermally Highly Loaded, Porous and Cooled Multi-Layer Systems for Combined Cycle Power Plants” at RWTH Aachen University, an effusion-cooled multi-layer plate configuration is investigated numerically by the application of a three-dimensional in-house fluid flow and heat transfer solver, CHTflow. CHTflow is a conjugate code, which yields information on the temperature distribution in the solid body. This enables a detailed discussion of the effects of a change in materials. The geometrical set-up and the fluid flow conditions derive from modern gas turbine combustion chambers and bladings. Within the SFB, two different multi-layer systems, one consisting of substrate made of CMSX-4 (a single-crystal super-alloy), an MCrAlY-bondoat and a ZrO₂ thermal barrier coating (TBC), and the other consisting of a NiAl-alloy and a graded bondcoat/TBC, have been investigated. The grading will increase the life-span of the TBC as it can better compensate the different thermal expansion coefficients of different materials. The main focus in this study is on the different substrate materials, because the thermal conductivity of the NiAl is considerably higher than that of CMSX-4, which leads to different temperature profiles in the components. The numerical grid for the simulations contains the coolant supply (plenum), the solid body for the conjugate calculations, and the main flow area on the plate. The effusion-cooling is realized by finest drilled shaped holes with a diameter of 0.2mm. The investigation is concentrated on a cooling hole geometry with a laterally widened fan-shaped outlet, contoured throughout, and one without lateral widening that is only shaped in the TBC-region of the system. Two blowing ratios, M=0.28 and M=0.48, are investigated, both for a hot gas Mach number of 0.25. The results for the lower blowing ratio and the fully contoured hole are discussed as well as those of the higher blowing ratio and the non-laterally widened hole. These represent two characteristic cases.     

Experimental investigation of impingement heat transfer on a flat and dimpled plate with different crossflow schemes

A nine-by-nine jet array impinging on a flat and dimpled plate at Reynolds numbers from 15,000 to 35,000 has been studied by the transient liquid crystal method. The distance between the impingement plate and target plate is adjusted to be 3, 4 and 5 jet diameters. Three jet-induced crossflow schemes, referred as minimum, medium and maximum crossflow correspondingly, have been measured. The local air jet temperature is measured at several positions on the impingement plate to account for an appropriate reference temperature of the heat transfer coefficient. The heat transfer results of the dimpled plate are compared with those of the flat plate. The best heat transfer performance is obtained with the minimum crossflow and narrow jet-to-plate spacing no matter on a flat or dimpled plate. The presence of dimples on the target plate produce higher heat transfer coefficients than the flat plate for maximum and minimum crossflow.     

Influence of secondary hole injection angle on enhancement of film cooling effectiveness with horn-shaped or cylindrical primary holes

Abstract

Film cooling with primary and secondary hole injection is numerically investigated. Effects of primary hole shape and secondary hole injection angle are documented. Each primary hole, either cylindrical or laterally diffused, has two secondary, cylindrical holes located symmetrically about it. Adding secondary holes improves cooling performance. Five cases of different secondary hole injection configuration are analyzed. With a cylindrical primary hole, increasing secondary hole inclination angle provides better cooling; outwardly inclining the secondary holes shows continued improvement. With horn-shaped primary holes, smaller secondary hole inclination angles provide higher cooling at lower blowing ratios; larger secondary hole inclination angles provide higher cooling at higher blowing ratios, and compound-angle secondary hole injection shows no improvement over parallel hole injection.     

Film cooling performance of a row of dual-fanned holes at various injection angles

Film cooling performance about a row of dual-fanned holes with injection angles of 30°, 60 ° and 90° were experimentally investigated at blowing ratios of 1.0 and 2.0. Dual-fanned hole is a novel shaped hole which has both inlet expansion and outlet expansion. A transient thermochromic liquid crystal technique was used to reveal the local values of film cooling effectiveness and heat transfer coefficient. The results show that injection angles have strong influence on the two dimensional distributions of film cooling effectiveness and heat transfer coefficient. For the small injection angle of 30 degree and small blowing ratio of 1.0, there is only a narrow spanwise region covered with film. The increase of injection angle and blowing ratio both leads to the enhanced spanwise film diffusion, but reduced local cooling ability far away from the hole. Injection angles have comprehensive influence on the averaged film cooling effectiveness for various x/d locations. As injection angles are 30 and 60 degree, two bands of high heat transfer coefficients are found in mixing region of the gas and coolant. As injection angle increases to 90 degree, the mixing leads to the enhanced heat transfer region near the film hole. The averaged heat transfer coefficient increases with the increase of injection angle.     

夏季降温的太阳房实验分析

对建造在南宁市西郊的一座被动式太阳房在夏季运行时的实验数据进行了分析，其屋顶集热表面在夏季夜间能产生一定的致冷效果。所制取的冷空气不但在夜间可以对该太阳房起到降温的作用，并且可以储藏起来以供白天降温使用。