Experimental flow field investigations of a film cooling hole featuring an orifice

This paper presents the flow field downstream of a film cooling hole geometry featuring orifice, referred to as nozzle hole, on a flat plate using PIV. The experiments were performed with blowing ratios from 0.5 to 2.0, density ratio of 1.0 and mainstream Reynolds number of 115,000. Velocity fields and vorticity fields of nozzle hole jet are compared with that of cylindrical hole jet. The results indicate that nozzle hole jet features double-decker vortices structure, resulting in vortices canceling out and significant reduction in CRVP strength. The streamwise vorticity of nozzle hole jet averages a drop of 55% at low blowing ratio 0.5 in comparison to cylindrical case. At high blowing ratio from 1.0, 1.5 and 2.0, the average drop is 30%–40%. A round jet bulk is observed to merge from the two legs of a typical kidney-shaped jet and the merged jet brings better coverage over the surface. In addition, it is found that CRVP strength might not have strong impact on jet lift-off but influences jet-mainstream mix characteristics.     

双叉排孔脉动射流气膜冷却效率的实验与数值模拟研究

采用高精度红外热像仪测量了平板绝热气膜冷却效率,比较了双叉排孔和单排孔气膜冷却效率,分析了吹风比(M=0.65,1.0,1.5)和脉动频率(St=0,0.01,0.015,0.025)以及孔间作用对气膜冷却效率的影响,结合数值计算得到的瞬态流场和温度场分析了脉动射流气膜冷却下的流动传热机理。结果表明：在稳态射流工况下,单排孔的气膜冷却效率随着吹风比的增加而减小,双叉排孔的气膜冷却效率却随着吹风比的增加而增大;在脉动射流时,单排和双叉排孔的气膜冷却效率在低吹风比下低于稳态射流,在高吹风比下,脉动射流对气膜冷却效率的影响减小,且低频脉动射流气膜冷却效率略高于稳态射流。     

Mixing augmentation induced by the combination of the oblique shock wave and secondary recirculation jet in a supersonic crossflow

The mixing process of fuel-air in the supersonic crossflow is a pivotal technology for the scramjet engine. In this paper, numerical simulation of the transverse sonic hydrogen jet into a supersonic Mach 3 crossflow with the mixing augmentation strategy induced by the combination of the oblique shock wave and secondary recirculation jet has been carried out. Detailed flow field structures, hydrogen mass fraction distributions, vortex structures, heat flux and some parameters have been explored in order to investigate its mixing enhancement mechanism. Results of the three-dimensional Reynolds-average Navier-Stokes (RANS) equations coupled with the two-equation shear stress transport (SST) κ-ω turbulence model show that the combined strategy of the oblique shock wave and secondary recirculation jet device can effectively improve the mixing speed and mixing efficiency with little total pressure loss. Also, the secondary recirculation jet device can reduce the peak of the heat flux effectively. In this study, the case with the single bleed hole owns the best effect with improving the mixing efficiency by 82.75% locally and reducing the maximum heat flux by 15.24% respectively。     

Numerical investigation of the flow field and heat transfer characteristics for upstream continuous and truncated ribs

To verify the applicability of upstream ribs in film cooling, the present numerical study examines heat transfer characteristics and flow field for ribs located upstream of the film hole. Five ribs including bilaterally truncated ribs, centrally truncated ribs, and continuous ribs are explored with the smooth case at two blowing ratios and fixed crossflow Reynolds number. The results show that the film cooling effectiveness of cases with ribs outperforms the case without rib at a low blowing ratio. Centrally truncated ribs and continuous ribs provide superior cooling effectiveness than bilaterally truncated ribs and smooth cases. The introduction of ribs makes the distribution of the heat transfer coefficient (HTC) uneven after the hole. Among these, centrally truncated ribs increased the HTC, while bilaterally truncated ribs reduce the HTC in the far hole area at a high blowing ratio. It is found that anti-kidney-shaped vortex pairs are generated between two adjacent jets for centrally truncated rib cases, while they are generated in front of the jets for bilaterally truncated rib cases. For continuous rib, the impingement of the mainstream gas on the jet leads to a reduction in strength of the kidney-shaped vortex, which allows the coolant to form a better coverage.     

旋转对气冷涡轮内部流场影响的PIV测量

采用PIV测速技术分别对旋转和不旋转两种情况下的气冷涡轮内部流场进行实验测量,研究旋转对气冷涡轮内部流场的影响。同时改变吹风比(M=1.5,2),研究不同射流吹风比对涡轮流场的影响。实验结果表明,冷却孔射流下游附近存在明显的尾迹区域。旋转情况下涡轮内部流场中存在的离心力、哥氏力的作用使射流与主流的掺混流场结构改变。与静止涡轮叶栅流场相比,旋转对叶片压力面侧流场的影响明显大于吸力面。同时,吹风比增大使射流与主流掺混流场区域以及射流尾迹区的范围扩大。     

Computational prediction of a slightly heated turbulent rectangular jet discharged into a narrow channel crossflow using two different turbulence models

A computational investigation of three-dimensional mean flow field resulting due to the interaction of a rectangular heated jet issuing into a narrow channel crossflow has been reported in the present paper. The jet discharge slot spans more than 55% of the crossflow channel bed, leaving a small clearance between the jet edge and sidewalls. Such flow configurations are encountered in several industrial processes such as mixing product streams, drying product streams, etc. The objective of the present work was to carry out a detailed investigation of the mean flow field and flow structure, which could not be obtained in a similar two-dimensional experimental work reported in the literature. The commercial code FLUENT 6.2.16 based on the finite volume method was used to predict the mean flow and temperature fields for the jet to crossflow velocity ratio (R) = 6. Two different turbulence models, namely, Reynolds-stress transport model (RSTM) and the standard k–ε model, were used for the computations. Different terms of the Reynolds-stress transport equation were modeled based on the proposals in the literature that are appropriate for the important flow features of the present configuration. Important flow features predicted by the two models, such as the formation of different vortical structures and their effects on the flow field are discussed. Some predicted results are compared with the available experimental data reported in the literature. The predicted mean and turbulent flow properties are shown to be in good agreement with the experimental data. However, the performance of RSTM is found to be better than that of the standard k–ε model.     

Heat Transfer Enhancement by Jet Impingement on a Flat Surface with Detached-Ribs under Cross-flow Conditions

In this article, the heat transfer augmentation on a flat surface with jet impingement on axisymmetric detached ribs is numerically investigated. Both single and multiple jet impingement with and without crossflow interaction is investigated. Numerical simulations are done using Reynold-averaged Navier–Stokes (RANS) equations with a shear stress transport (SST) model with the commercial CFD code ANSYS-CFX. The influence of jet Reynolds number (7000 ≤ Re _j  ≤ 78,000), blowing ratio (5.8 ≤ M ≤ 11.5), and jet-outlet-to-target wall distance (2 ≤ H/D ≤ 6) are examined. Results show that the heat transfer is enhanced on the target wall with detached ribs in single and multiple jet impingement at moderate crossflow speeds.     

Liquid jet in a subsonic gaseous crossflow: Recent progress and remaining challenges

This article reviews published literature on the characteristics of a liquid jet injected transversally into a subsonic gaseous crossflow. The review covers the following aspects: (і) liquid jet primary breakup regimes, (іі) liquid jet trajectory and penetration, (ііі) liquid jet breakup length, and (іv) droplets features and formation mechanisms. The focus is on analyzing the role of different prominent parameters which include gaseous and liquid properties, and liquid injector geometry. The review revealed that gas Weber number plays a crucial role in defining non-turbulent primary breakup regimes, while liquid jet Weber number is of great importance for the transition to turbulent primary breakup. Jet-to-crossflow momentum flux ratio is the most important parameter for predicting the trajectory, penetration, and breakup length of a liquid jet in a crossflow. The characteristics of droplets disintegrated during the primary breakup are mostly influenced by the nozzle exit conditions, whereas the characteristics of droplets produced via the secondary breakup are strongly dependent on the velocity of cross airflow. Although the review revealed that substantial progress has been made in understanding this complex two-phase flow phenomenon, there still remain several shortcomings which require further research.     

URANS study of pulsed hydrogen jet characteristics and mixing enhancement in supersonic crossflow

In this paper, three-dimensional pulsed hydrogen jet in supersonic crossflow (PJISC) is investigated by the unsteady Reynolds Averaged Navier-Stokes (URANS) simulations with the k-ω shear stress transport (SST) turbulence model. The numerical validation and mesh resolution have been carried out against experiment firstly. The effects of the pulsed frequency and amplitude on the jet flow field and mixing performance in supersonic cross-flow are all addressed. It significantly changes the distribution of the hydrogen jet flow by comparing with the steady jet in supersonic crossflow. The fuel jet penetration, mixing efficiency, decay rate of the maximum hydrogen mass fraction and total pressure losses are used to quantitatively analyze the mixing performance. The mixing of fuel and incoming air flow is enhanced by the pulsed jet, especially for the case of 50 kHz, which is the optimal pulsed frequency while considering the effects of jet excitation frequency in the present simulations. The decay rate of the maximum mass fraction of hydrogen in the far field downstream is related to the frequency of the pulse jet. Moreover, the pulsed frequency and amplitude have little effects on the total pressure recovery coefficient for the cases studied in the present simulations.     

涡轮叶片前缘气膜冷却的数值研究

对前缘带有2排冷却孔的高压涡轮叶栅进行了气膜冷却数值计算,在吹风比分别为0、0.7、1.1、1.5的情况下得到了叶片型面的静压分布,并将计算结果与实验数据进行了对比研究.此外,还详细分析了冷却孔出口附近区域的流场特性.结果表明,吹风比对叶片型面的压力分布影响不大,只是在冷却孔附近有较为明显的变化.同时,由于冷气射流的注入,在冷却孔后出现了一对旋转方向相反的肾形涡,此时增大吹风比不会对旋涡产生明显的影响,但对旋涡下游流场的影响比较明显.     

Numerical Investigation of Confined Multiple-Jet Impingement Cooling Over A Flat Plate at Different Crossflow Orientaions

This study investigates the fluid flow and heat transfer characteristics of round jet arrays impinging orthogonally on a flat-plate with confined walls at different crossflow orientations. A computational fluid dynamic technique based on a control volume method is used to compute the detailed Nusselt number distributions on the flat plate. This is achieved by solving the steady-state three-dimensional incompressible Reynolds-averaged Navier-Stoke's equations. The Reynolds stress turbulence quantities are determined by a realizable κ-ε turbulence model with an enhancement near-wall treatment. Numerical computations are performed for two types of arrangements in round jet arrays, both inline and staggered, and three different crossflow directions, parallel, hybrid, and counter. The jet Reynolds numbers ranging from 2,440 to 14,640 and three different jet-to-plate spacing ratios (Zn/d_j) of 1, 3, and 6 are investigated in this study. Results show that the flow exit crossflow direction would significantly affect the developing jet flow fields and Nusselt number distributions on the target flat-plate. Area-averaged Nusselt number increases with an increase of jet Reynolds number. Of all the cases tested, the highest average Nusselt numbers were obtained for the case with inline jets and hybrid crossflow orientation. The thermal performance of impingement multiple jets is enhanced when the value of Zn/d_j decreases from 6 to 3. Results show that further reducing the value of Zn/d_j to 1 creates a significant nonuniform distribution in local Nusselt number over the target plate regardless of the crossflow orientations. This study also provides a correlation of the area-averaged Nusselt number with the jet Reynolds number for both inline and staggered jet arrays.     

Characteristics and mixing enhancement of a self-throttling system in a supersonic flow with transverse injections

A three-dimensional self-throttling system is proposed in a scramjet combustor with transverse fuel jet, and investigated by Reynolds-averaged Navier-Stokes (RANS) simulations with the k-ω SST turbulence model. Numerical validation has been carried out against experiment and LES results. The effects of the jet-to-cross-flow momentum flux ratio and the throttling angle on mixing performance, fuel jet penetration depth and total pressure losses are all addressed. Through the proposed throttling system, the higher pressure upstream of the transverse fuel injection can drive part of the low momentum mainstream air into the downstream lower pressure region. The flow structures and the interactions between the shock waves and boundary layer are significantly changed to improve the mixing performance. The enhancement of mixing efficiency in the self-throttling system is closely related to the magnitude of the jet to crossflow momentum flux ratio, and a smaller throttling angle is found to further improve the mixing. On the other hand, the self-throttling system has a good performance in reducing the total pressure losses.     

Numerical study of film cooling effectiveness of a gas turbine blade under variable blowing ratio and turbulence intensity

The present paper investigates a three-dimensional simulation of film cooling on a C3X turbine blade with a single hole at a suction surface. The Reynolds averaged Navier–Stokes approach with k–ε realizable turbulence model and enhanced wall function are used for the numerical simulation. To simulate the jet flows, the length of the jet input approximately 4.5 times the diameter of the hole is added to the geometry so that the jet outlet flow is closer to the actual condition. The density ratio of the cooling flow to the mainstream flow is assumed about 2. The numerical results in four blowing ratios of 0.5, 0.7, 1.0, and 1.4, and at the low turbulence intensity (0.02%), and high turbulence intensity (12%) are extracted and compared for the turbine blade with a single hole. The results show that the turbulence intensity has a dual effect on the film cooling effectiveness and a higher blowing ratio increases the strength of the jet against the cross-flow. Moreover, it is illustrated that the distribution of the film cooling effectiveness in higher blowing ratios and high turbulence intensity is more uniform than the low blowing ratios and low turbulence intensity.     

Study of Heat Transfer Distribution in a Channel with Inclined Target Surface Cooled by a Single Array of Staggered Impinging Jets

An experimental investigation has been carried out to study the heat transfer characteristics in a channel with a heated target surface inclined at an angle, cooled by a single array of staggered impinging jets. The work encompasses the effect of three feed channel aspect ratios (5, 7, 9) and three exit outflow orientations (coincident with the entry flow, opposed to the entry flow, and both), and three Reynolds numbers (9400, 14,400, 18,800) on heat transfer. Results show that increasing the Reynolds number increases the heat transfer on the inclined target surface. The outflow orientations affect significantly the local heat transfer charactracistrics, through influencing the jet flow together with the crossflow in the impingement channel. The outflow orientation coincident with the entry flow and the outflow from both sides show better averaged Nusselt number values compared to outflow orientation opposed to the entry flow. The inclined surface affects the local Nusselt number distribution especially for the outflow orientation opposing the entry flow at the narrow region of the impingement channel. In general, the feed channel aspect ratio does not affect the Nusselt number distribution, except for outflow coincident with the entry flow. The local Nusselt number for aspect ratio 9 has been found to be greater than the Nusselt number for aspect ratio 5 by 11%. Additionally, for a given jet-orifice plate with staggered holes, the heat transfer is almost the same throughout the target surface for the outflow exiting in both directions.     

Turbulent flow in the distribution header of a PEM fuel cell stack

A numerical investigation of the flowfield in a model distribution header manifold of a polymer electrolyte membrane fuel cell stack is conducted. The computational model simulates two segments of an experimental setup of a pair of model headers which replicate the headers of a fuel cell stack. The model headers consist of an inlet and outlet sections connected with a plate containing an array of holes that replicate the unit cells. The flow structures in the outlet header are rather complex and are the result of the superposition of a series of impinging jets in a confined space in the presence of crossflow. The flow from each hole, which represents an individual cell outlet, enters the outlet header as a jet stream and is subjected to a crossflow. Large Eddy Simulations (LES) are performed for a portion of the outlet header to investigate the complex turbulent flow and related structures under different crossflow conditions, and are complemented by Particle Image Velocimetry (PIV) measurements. The LES results show that two large vortical structures are formed in the header cross-section, with a high-speed round jet from the cell outlet holes forcing a diversion of the crossflow, dividing it into two separate branches. Investigation of the flow restructuring after a blockage of one of the jets is performed. Simulation results using a slot opening for the jet show flow instabilities. The results of this study highlight the unsteady and highly turbulent nature of the flow in the header and provide a characterization of the complex three-dimensional structure of the flow. The flowfield and flow structures may impact the overall pressure drop along the header and the effective cross-sectional area for the flow leaving the header. The observations and insights obtained from the LES simulation and PIV measurements point to the need to further investigate the impact on flow sharing in a stack of the flowfield development in the outlet header.     

Flow characteristics and heat transfer performances of a semi-confined impinging array of jets: effect of nozzle geometry

The flow and heat transfer characteristics of confined jet array impingement with crossflow is investigated. Discrete impingement pressure measurements are used to obtain the jet orifice discharge flow coefficient. Digital particle image velocimetry (DPIV) and flow visualization are used to determine the flow characteristics. Two thermal boundary conditions at the impinging surface are presented: an isothermal surface, and a uniform heat flux, where thermocouple and thermochromic liquid crystal methods were used, respectively, to determine the local heat transfer coefficient. Two nozzle geometries are studied, circular and cusped ellipse. Based on the interaction with the jet impingement at the surface, the crossflow is shown to influence the heat transfer results. The two thermal boundary conditions differ in overall heat transfer correlation with the jet Reynolds number. Detailed velocity data show that the flow development from the cusped ellipse nozzle affects the wall region flow more than the circular nozzle, as influenced by the crossflow interactions. The overall heat transfer for the uniform heat flux boundary condition is found to increase for the cusped ellipse orifice.     

Vortices and heat flux around a wall-mounted cube cooled simultaneously by a jet and a crossflow

Vortex morphology and heat transfer over a wall-mounted heated cube in an in-line array, cooled simultaneously by a crossflow and a normally impinging round jet, have been studied by conjugate large-eddy simulations. The interaction of the two streams and the cubes leads to the formation of complex vortical structures that govern heat removal from the cube surface. The strongest and the most evenly distributed cooling were found on the cube top and the front face. The heat flux on the side faces is lower in the zones where the flow separates, while it increases downstream where a fresh fluid from the crossflow flushes the faces. The separation on the back face of the cube creates an arch vortex, which dictates the heat transfer from that face. Despite its persistence and relative steadiness, significant nonuniformity of the temperature field has been detected on the rear face, characterised by the time meandering of hot spots. Vortex rings, created in the jet shear layer before its impact on the cube, break up upon impingement, leading to the re-establishing of the thermal boundary layer, and the consequent enhancement of heat transfer. The turbulent heat flux and its budget correlate well with the corresponding turbulent stress components.     

Influence of sleeve tube on the flow and heat transfer behavior at a T-junction

The flow structure of a sleeved jet into a main crossflow was experimentally investigated employing particle imaging velocimetry technology and numerically simulated using a CFD code. The jet-to-crossflow velocity ratio, VR, was ranged from 0.5 to 8. Three basic flow patterns were marked, namely attaching jet, lift-off jet and impinging jet as VR gradually increased. The flow in the main duct was characterized by a stream of discharge from the annular space at the rear part of the sleeve near the jet exit, which primarily came from the upstream crossflow. This annulus discharge isolated the leeward wall from the jet fluid and also caused weak local heat transfer in the large momentum deficiency region, and hence could supply an effective protection of the leeward wall from the thermal shock caused by a very cold jet injection.     

旋转对复合角度气膜冷却叶片的数值模拟

采用Realizable k-ε紊流模型并结合SIMPLEC算法,对前缘复合角度α=30°、β=45°,α=90°、β=45°的动叶栅在不同旋转状速度下的气膜冷却效率进行计算。分析了不同转速、吹风比、叶片前缘射流角度对气膜冷却效率的影响。计算结果表明:旋转导致冷却射流向叶顶偏移,转速越高气膜冷却效率越低;高转速时叶盆区域有回流涡旋形成;高吹风比使得冷却射流在吸力面的贴壁性变差;比较两种前缘冷气喷射角度的计算结果可以看出,前缘冷却气流喷射角度较小时的气膜冷却效果较好。     

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.