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。     

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。     

大型轴类工件喷气冷却流动和传热特性的数值模拟

以某喷气冷却装置为参考,对缝隙喷气冷却大型轴类工件展开研究.通过数值模拟研究了具有两相对缝隙的大型轴类工件(直径D=1 000～3 000 mm)喷气冷却装置内的漉动传热特性,对比数值计算结果与实验数据验证了模型预测的准确性,探讨了大型喷气冷却装置内部气体的流动特点,分析了双缝隙喷气冷却轴类工件的传热规律,获得了轴类工...     

涡轮端壁流动传热与冷却性能研究进展

随着进口燃气温度的不断增大和燃烧室出口温度的均匀化,燃气涡轮端壁承受极高的热负荷。涡轮端壁处的复杂流动结构使端壁部分区域冷却困难,容易造成端壁烧蚀从而降低涡轮气动性能且威胁涡轮的安全运行。为了提高涡轮的冷却和气动性能,需要深入分析端壁附近的流动结构和传热冷却特性。本文以端壁冷却为出发点,对燃气涡轮的气动传热和冷却技术的发展进行总结分析,结合实验和数值计算结果,对端壁流动传热和冷却相关的先进实验和数值研究结果进行分析讨论。在此基础上,对涡轮端壁的先进冷却技术和非轴对称端壁下冷却结构优化进行了展望。     

车用内燃机冷却系的流动与传热仿真

本提出了车用内燃机冷却系的流动与传热仿真方法。采用一维的方法研究了车用内燃机冷却系的流动问题；采用集总参数法研究了车用内燃机冷却系的传热问题；将车用内燃机冷却系的流动问题与传热问题耦合起来作为一个系统，进行整体研究，建立了内燃机冷却系的流动与传热问题的整体模型。编制了计算程序。对某型坦克内燃机冷却系的流动与传热进行了实例计算，仿真结果与试验值符合较好。     

燃气轮机高温叶片内部冷却技术概述

综述了燃气轮机典型的内部冷却结构和设计手段.通过文献分析,提出内部冷却典型强化换热手段包括带肋通道冷却、扰流柱冷却和冲击冷却,重要设计手段包括一维管网和三维数值优化.管网计算基于实验总结的经验公式,计算效率高,关键在于动量方程的求解算法;三维数值优化可以降低设计对人工经验的依赖,关键在于合理选择优化目标和优化算法.分析表明,旋流等新的强化传热形式、微型冷却等新的叶片冷却模式、旋转和真实通道几何对内部冷却详细传热和流场的影响都将得到进一步研究,叶片设计技术将由人工经验性的设计逐渐向计算机自动化方向发展.     

简讯

不同内部冷却结构的燃气涡轮叶片试验和数值分析据《ASME Journal of Turbomachinery》2011年1月刊报道,将油漆喷入水中,利用流动显像试验完成了流动分析,以评定主要的几何结构修改对气膜冷却燃气涡轮转子叶片内部传热的影响。同时,采用SST紊流模型并利用ANSYS CFX软件数值模拟叶片内部的流动和传热。     

扰流肋柱在流动方向排列密度对矩形通道表面传热影响的数值研究

运用数值计算的方法将流动方向扰流圆柱排列密度对涡轮叶片尾缘冷却通道中流动传热的影响进行了三维数值研究。研究了流动雷诺数、流动方向圆柱排列密度对肋柱扰流矩形通道表面传热影响的规律。计算结果表明:在研究范围内,肋柱表面的平均Nu均随着Re的增大而增大。在Re相同的情况下,随X/D取值的增大,肋柱表面平均Nu有所减小。Nu在通道进口附近逐渐增加,然后达到充分发展值。传热在迎向流动方向的圆柱侧较强,在流动向背侧表面传热较弱。沿圆柱高度方向在中部传热较强。     

轧钢碳套炉辊冷却系统耦合传热数值模拟研究

以高温环境下的炉辊为研究对象,建立了炉辊内气体流动与传热的物理和数学模型,利用FLUENT软件进行流固耦合传热数值模拟.研究分析结果表明,通过有限差分计算所得的温度场分布与实际情况基本吻合,从而验证了模型的有效性.在此基础上,对不同冷却气体参数及辊子结构下的通气冷却效果进行了分析,为气冷辊的优化设计提供依据.     

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。     

Fluid flow characteristics and convective heat transfer improvement on a gas turbine blade

The flow field features and heat transfer enhancement are investigated on a gas turbine blade by applying the jet impingement cooling method. The distribution of the flow field and the Nusselt number (Nu) was determined on the targeted surface in the cooling channel. The injection holes of different shapes, such as circular, square, and rectangular were considered. The Reynolds numbers (Re) of the airflow in the range of 2000–5000 and aspect ratios of 0.5–2 were particularly focused. The flow vortices and recirculation in the cooling channel and their influence on the heat transfer enhancement were analyzed in detail under different airflow and geometric conditions. Decreasing the ratio of the distance between jet-to-target plate to the diameter of the jet orifice (H/d) increased the heat transfer rate and produced high-intensity vortices and recirculation zones. It was noticed that the formation and generation of vortices and recirculation have important effects on the convective heat transfer rate at the impingement surface. Local Nusselt number, formation of complex vortices, and airflow recirculation in the cooling channel decreased with the increase in the distance between the jet hole and the targeted surface. It was found that with the increase in the Reynolds number of the jet, heat transfer between cold airflow and the targeted surface increased. Moreover, it was observed that the cooling performance of the round and square jet holes was better than the rectangular holes.     

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.     

Local heat/mass transfer and flow characteristics of array impinging jets with effusion holes ejecting spent air

The present study investigates the effects of spent air flows with and without effusion holes on heat/mass transfer on a target plate for array impinging jets. For a conventional type of array impinging jets without effusion holes, the spent air of the injected jets forms a cross-flow within the confined space and affects significantly the downstream jet flow. The injection plate of array impinging jets is modified having effusion holes to prevent the cross-flow of the spent air where the spent air is discharged through the effusion holes after impingement on the target plate. A naphthalene sublimation method is employed to determine local heat/mass transfer coefficients on the target plate using a heat and mass transfer analogy. The flow patterns of the array impinging jets are calculated numerically and compared for the cases without and with the effusion holes. For small gap distances, heat/mass transfer coefficients without effusion holes are very non-uniform due to the strong effects of cross-flow and re-entrainments of spent air. However, uniform distributions and enhancements of heat/mass transfer coefficients are obtained by installing the effusion holes. For large gap distances, the effect of cross-flow is weak and the distributions and levels of heat/mass transfer coefficients are similar for both cases.     

Jet impingement heat transfer – Part II: A temporal investigation of heat transfer and local fluid velocities

Impinging jets are a means of achieving high heat transfer coefficients both locally and on an area averaged basis. The temporal nature of both the fluid flow and heat transfer has been investigated for Reynolds numbers from 10,000 to 30,000 and non-dimensional surface to jet exit distance, H/D, from 0.5 to 8. At the impingement surface simultaneous acquisition of both local heat flux and local velocity signal has facilitated a comprehensive analysis of the effect that fluid flow has on the heat transfer. Results are presented in the form of surface heat transfer and fluid velocity signal spectra, and coherence and phase difference between the corresponding velocity and heat flux signals. It has been shown that the evolution of vortices with distance from the jet exit has an influence on the magnitude of the heat transfer coefficient in the wall jet.     

The effect of flow field and turbulence on heat transfer characteristics of confined circular and elliptic impinging jets

The flow field of confined circular and elliptic jets was studied experimentally with a Laser Doppler Anemometry (LDA) system. In addition, heat transfer characteristics were numerically investigated. Experiments were conducted with a circular jet and an elliptic jet of aspect ratio four, jet to target spacings of 2 and 6 jet diameters, and Reynolds number 10 000. The toroidal recirculation pattern was observed in the outflow region for both geometries at dimensionless jet to plate distance 2. Higher spreading rates in the minor axis direction of the elliptic jet have also been mapped. Along the target plate, different boundary layer profiles were obtained for circular and elliptic jets at H/d=2, but profiles became similar when dimensionless jet to plate distance was increased to 6. Positions of maximum radial and axial velocities and turbulence intensities have been determined for both geometries. For the confined circular and elliptic jet geometries, analysis of flow field measurements and numerical heat transfer results showed that inner peaks in local heat transfer closely relate to turbulence intensities in the jet and radial flow acceleration along the wall. Differences between the circular and elliptic jet, in terms of flow field and heat transfer characteristics, reduced with increase in the jet to plate distance.     

Multiple orifice synthetic jet for improvement in impingement heat transfer

Synthetic jet is potentially useful for cooling of electronic components and its utility has been investigated in previous studies. Synthetic jet will become further attractive if additional cooling can be obtained without a corresponding increase in the input power. In this context, we explore the use of multiple orifice single-cavity synthetic jet employed in direct impingement mode of cooling. Experiments are conducted for different configurations with a center orifice surrounded by multiple satellite orifices. The Reynolds number is in the range of 1000–2600 while the normalized axial distance is varied in the range of 1–30 in this study. The maximum heat transfer coefficient with multiple orifice synthetic jet is approximately 12 times that of the natural heat transfer coefficient and up to 30% more as compared to that obtained with a conventional single orifice jet. Interestingly, the average Nusselt number gets maximized at two axial distances-the two peaks can be of comparable magnitude. The appearance, location and magnitude of the two peaks depend on the number of satellite orifices and the pitch circle radius on which the satellite holes lie. It is proposed that a transition in flow behavior from multiple-jet to a combined-jet occurs, which leads to the appearance of this additional peak. The additional peak (at the smaller axial distance) can be utilized in the design of cooling solutions for compact devices. The input power reduces slightly in the multi-orifice case with respect to the conventional design. The average velocity at the surface is also obtained with the help of hot-wire anemometry. The use of multiple orifice synthetic jet does not appear to have been explored earlier and the results are expected to be useful in several practical applications.     

Experimental investigation on impingement/effusion cooling with short normal injection holes

An experimental investigation on cooling performances of integrally impingement/effusion cooling configurations with film cooling holes angled normal to the mainstream flow is conducted. The adiabatic film cooling effectiveness and the overall cooling effectiveness are measured on a polycarbonate test plate and a stainless steel plate respectively. Effects of the blowing ratio (ranged from 0.6 to 2.4), multi-hole arrangement (inline and staggered), hole-to-hole pitch ratio (ranged from 3 to 5) and jet-to-target spacing ratio (ranged from 2 to 4) on the cooling performance are examined. In addition, jet impingement heat transfer is measured to evaluate the dense array jet impingement behaviors with local extraction of coolant via effusion holes. A new parameter named corrected blowing ratio is introduced in the present to evaluate the cooling effectiveness for different effusion or impingement–effusion configurations under a given quantity of cooling air. In an integrally impingement–effusion cooling configuration, multiple jet impingement with local extraction of coolant via effusion holes is able to produce higher overall heat transfer under lower jet-to-target spacing and denser jet array. The action of additional jet impingement heat transfer on improving overall cooling performance is highly dependant on the blowing ratio, multi-hole arrangement and jet-to-target spacing, which seem to be behaved superior in the situations where the film cooling effect isolating the wall surface from the hot mainstream is weak. For an integrally impingement–effusion cooling configuration, the densest hole-to-hole array is favorable in the situations where the coolant mass flow rate per unit area of cooled surface is low. As the coolant mass flow rate per unit area of cooled surface increases, the hole-to-hole pitches could be gradually enlarged to make effective utilization of array jet impingement.     

The role of jet inlet geometry in impinging jet heat transfer,modeling and experiments

Effects of jet inlet geometry and aspect ratio on local and average heat transfer characteristics of totally nine confined impinging jets have been investigated experimentally using thermochromic liquid crystals and numerically by using a 3-D low Reynolds number k–? model. Experimental study by using liquid crystals for temperature measurement was conducted for three different jet exit geometries (circular, elliptic, rectangular). In addition, simulations were performed at the same mass flow rate for totally nine jet exit geometries including circular, elliptic and rectangular jets with different aspect ratios for dimensionless jet to plate distances 2, 6, and 12.As the aspect ratio of equal cross-sectional area elliptic and rectangular jets increases, heat transfer enhancement in the stagnation region was obtained. As a result higher aspect ratio jets can be used as a passive enhancement technique for localized heating or cooling especially at small jet to plate distances. Wall jet region comprises very large portion of the impinging plate under study and generally lower heat transfer rates were attained for higher aspect ratio jets in this region especially at small jet to plate distances. Therefore as the aspect ratio increases, lower average heat transfer rates were acquired. The effect of aspect ratio on local and average heat transfer decreases with increasing jet to plate distance. Even though the mass flow rate is the same, heat transfer rate of rectangular jets were reduced with increasing the cross-sectional area. With increasing jet to plate distance very similar heat transfer characteristics were observed along the major and minor axis directions.