Experimental study of impinging heat transfer along rib-roughened walls by using transient liquid crystal technique

The objective of this work is to examine the detailed heat transfer coefficient distributions over a ribbed surface under impingement of in-line and staggered jet arrays by using a liquid crystal thermograph technique. In-line and staggered jet arrays with different exit flow orientations were considered. Three jet-to-target spacing Z of 3, 6 and 9 with in-line and staggered jet arrays were considered at jet Reynolds numbers of Re = 1500, 3000 and 4500 with three different exit flow orientations. In addition, the effects of rib configuration on the heat transfer distributions were discussed in detail. Results show that the local heat transfer rates over the ribbed surface are characterized by obvious periodic-type variation of Nusselt number distributions. The downstream peaks are diminished for increasing cross flow effect. Compared to the results without ribs, the heat transfer over the ribbed surface may be enhanced or retarded. Whereas, among the test angled-rib arrangements, the best heat transfer performance is obtained with a surface with 45° angled ribs.     

Flow and heat transfer studies of multijet impingement cooling for different configurations: A review

Increasing the gas turbine engine's turbine intake temperature has long been a potential strategy for increasing the specific work output of the engine. However, the melting temperature of the turbine blades and vane material limits the maximum intake temperature. As a result, internal and external cooling techniques are commonly used to maintain the vane material in a safe condition. This study provided an overview of internal impingement cooling to highlight the significance of geometrical variations, such as flat plate, curve plate, and actual vanes. It was observed that flat and curved plate impingement heat transfer studies were reported extensively, whereas limited studies were found on the conjugate effects on airfoil surfaces. The importance of conjugate heat transfer studies and their impact has recently been described in the literature. In most of the literature, a wide range of instruments, such as Laser Doppler Velocimeter, Particle Image Velocimeter, liquid crystal sheets, and so forth, were used for experimental investigations. According to most studies, the local value of internal surface temperature and heat transfer coefficient are vital factors of local flow behavior. Jet-to-jet spacing, jet-to-plate spacing, jet hole diameter, and jet Reynolds numbers played a crucial role in both numerical and experimental analyses. Different geometric variations strongly influence flow behavior. Therefore, the usual method for determining interior temperature distributions and heat transfer coefficients by considering generalized geometries like the flat and curved plate may not produce accurate conjugate solutions. Most of the computational studies on the flat and curved plate indicate the usage of κ−ω shear stress transport and κ–ε realizable model to predict the heat transfer coefficient.     

Flow and heat transfer of confined impingement jets cooling using a 3-D transient liquid crystal scheme

It has been shown that the heat transfer coefficients obtained from using the 1-D transient liquid crystal scheme are higher than those obtained from employing the 3-D scheme when surface heat transfer is highly nonuniform such as on a hot surface subject to jet impingement cooling. This is due to the fact that 1-D method does not include the lateral heat flows induced by local temperature gradients. The objective of this study is to provide a new database of heat transfer coefficient distribution on the jet impingement target surface in the confined cavity by employing a 3-D transient liquid crystal scheme. The study is performed with an 8 × 11 array of confined impinging jets with Reynolds numbers ranging from 1039 to 5175. The 1-D results are higher than the 3-D results with the local maximum and minimum heat transfer values being overvalued by about 15–20% and the overall heat transfer by approximately 12%. In addition, hot-film measurements of the flow structure are conducted to gain insight into the effects of cross-flow on heat transfer behavior. The surface mapping of heat transfer coefficient demonstrates a change from columnar pattern to a horizontal pattern and switching back to the columnar pattern as Reynolds number increased consecutively. This pattern switching is thought to be caused by the competition between jet penetration and the cross-flow buffering effect. A nonuniformity index is defined to provide a quantitative measure for cooling effectiveness for various cases. The results indicate that increased cross-flow degrades the heat transfer performance but increase uniformity.     

Measurement of detailed heat transfer along rib-roughened surface under arrays of impinging elliptic jets

The objective of the present study is to examine the detailed heat transfer coefficient distributions over a ribbed-surface under impingement of elliptic jet arrays using a liquid crystal thermograph technique. Both continuous and broken V-shaped-rib configurations with different exit flow orientations were considered. To examine the angled rib effects, three angled ribs were discussed under jet-to-plate spacing Z = 3 for different Reynolds numbers. Measured results show that the local heat transfer rates over the ribbed-surface are characterized by obvious periodic-type variation of Nusselt number distributions. The downstream peaks are diminished for increasing crossflow effect. Compared to the results without ribs, the heat transfer over the ribbed-surface may be enhanced or retarded. Whereas, among the test angled-rib arrangements, the best heat transfer performance is obtained with a surface with 45° V-shape ribs. In addition, the surface with continuous ribs provides a better impingement heat transfer than that with broken ribs.     

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。     

Design optimization of rib-roughened channel to enhance turbulent heat transfer

This work presents a numerical procedure to optimize the shape of two-dimensional channel with periodic ribs mounted on both of the walls to enhance turbulent heat transfer. The response surface method is used as an optimization technique with Reynolds-averaged Navier-Stokes analysis of flow and heat transfer. Standard k-ε turbulence model is used as a turbulence closure. Computational results for overall heat transfer rate show good agreements with experimental data. The width-to-height ratio of the rib, rib height-to-channel height ratio, pitch-to-rib height ratio and distance between opposite ribs to rib pitch ratio are chosen as design variables. The objective function is defined as a linear combination of heat-transfer and friction-loss related terms with weighting factor. D-optimal design is used to reduce the data points, and, with only 36 points, reliable response surface is obtained. Optimum shapes of the channel have been obtained in the range from 0.0 to 0.1 of weighting factor. In the weighting factor range where designer’s goal is shifted to reduction of pressure loss, both of pitch-to-rib height ratio and distance between opposite ribs to rib pitch ratio reach almost constant values.     

Heat transfer characteristics in steam-cooled rectangular channels with two opposite rib-roughened walls

An experimental study of heat transfer characteristics in steam-cooled rectangular channels with two opposite rib-roughened walls for Reynolds number (Re) in the range of 10,000–80,000 was conducted. To simulate the actual geometry and heat transfer structure of turbine blade/vane internal cooling passage, each of the test channels was made by welding four stainless steel plates. The pitch-to-rib height ratio p/e was kept at 10 and the channel length L was kept at 1000 mm. The channel aspect ratios (W/H) were 0.25, 0.5 and 1, respectively. The channel blockage ratios were 0.047 for W/H = 1, 0.5 and 0.078 for W/H = 0.25. We have found that the average Nusselt number (Nu) for the channel with α = 45° is about 15–25 percent higher than that for the channel with α = 60°. For the channels with W/H = 0.5 or 1, the average Nu decreases along the rib axes because of the rib-induced secondary flow that moves from the left-hand side to the right-hand side. In addition, based on the heat transfer results, we have developed the semi-empirical correlations for the two test channels with the highest and the lowest heat transfer. The correlations can be used in the design of the internal cooling passage of new-generation steam-cooled gas turbine blade/vane.     

Shape optimization of rib-roughened surface to enhance turbulent heat transfer

This work presents an investigation on numerical optimization technique coupled with Reynolds-averaged Navier-Stokes analysis of flow and heat transfer for design of rib-roughened surface in case of single surface roughened in two-dimensional channel. Standard k-ε model is used as a turbulence closure. The objective function is defined as a function of heat transfer coefficient and friction drag coefficient with weighting factor. And, the ratio of width-to-height of the rib and the ratio of pitch-to-height are selected as design variables. Three different weighting factors and two sets of initial values of the design variables in each case are tested. In case of double design variable, the histories of design variables and objective function are complicated, and the number of iterations is very sensitive to the initial values of design variables. However, overall performance of the optimization process is proved quite reliable.     

An experimental investigation of the heat transfer in a ribbed triangular cooling channel

One of the most challenging aspects of gas turbine cooling is the cooling of the first stages of turbine blades. Here the highest external heat load is seen at the leading edge of the blade. The present study investigates the internal cooling in a triangular channel with a rounded edge as a model of a leading edge cooling channel for a gas turbine blade. A transient liquid crystal method is used to measure the heat transfer. Experimental results are reported for a number of new 3D rib configurations for Reynolds numbers between 50 000 and 200 000. From the experimental results it has been found that 60 deg. ribs provide in general higher heat transfer enhancements than 45 deg. ribs. However, this results in extremely high friction factors for the 60 deg. ribs. Taking the local and mean distributions of the heat transfer coefficients (as well as the increase in friction factors) into consideration, it was found that the most promising rib arrangement for leading edge cooling is a 3D rib with 45 deg. angle and double-sided fully overlapped ribs in the arc area. These ribs provide uniform heat transfer in the arc area as well as a high level of the heat transfer coefficients in the channel. The resulting friction factors are in an acceptable range for these ribs.     

Heat transfer characteristics of synthetic jet impingement cooling

Synthetic jet is a novel flow technique which synthesizes stagnant air to form a jet, and is potentially useful for cooling applications. The impingement heat transfer characteristics of a synthetic jet are studied in this work. Toward that end, the behavior of the average heat transfer coefficient of the impinged heated surface with variation in the axial distance between the jet and the heated surface is measured. In addition, radial distribution of mean and rms velocity and static pressure are also measured. The experiments are conducted for a wide range of input parameters: the Reynolds number (Re) is in the range of 1500–4200, the ratio of the axial distance between the heated surface and the jet to the jet orifice diameter is in the range of 0–25, and the length of the orifice plate to the orifice diameter varies between 8 and 22 in this study. The maximum heat transfer coefficient with the synthetic jet is found to be upto 11 times more than the heat transfer coefficient for natural convection. The behavior of average Nusselt number is found to be similar to that obtained for a continuous jet. The exponent of maximum Nusselt number with Re varies between 0.6 and 1.4 in the present experiments, depending on the size of the enclosure. A direct comparison with a continuous jet is also made and their performances are found to be comparable under similar set of conditions. Such detailed heat transfer results with a synthetic jet have not been reported earlier and are expected to be useful for cooling of electronics and other devices.     

Single-phase and two-phase heat transfer characteristics of low temperature hybrid micro-channel/micro-jet impingement cooling module

This study examines the single-phase and two-phase cooling performance of a hybrid micro-channel/micro-jet impingement cooling scheme using HFE 7100 as working fluid. This scheme consists of supplying coolant from a series of jets that deposit liquid into the micro-channels. A single-phase numerical scheme that utilizes the k–ε turbulent model and a method for determining the extent of the laminarized wall layer shows very good predictions of measured wall temperatures. It is shown jet velocity has a profound influence on single-phase cooling performance. High jet velocities enable jet fluid to penetrate the axial micro-channel flow and produce a strong impingement effect at the wall. On the other hand, the influence of jets at low jet velocities is greatly compromised compared to the micro-channel flow. During nucleate boiling, vapor layer development along the micro-channel in the hybrid module is fundamentally different from that encountered in conventional micro-channels. Here, subcooled jet fluid produces repeated regions of bubble growth followed by bubble collapse, rather than the continuous growth common to conventional micro-channel flow. By reducing void fraction along the micro-channel, the hybrid scheme contributes greater wall temperature uniformity. Increasing subcooling and/or flow rate delay the onset of boiling to higher heat fluxes and higher wall temperatures, and also increase critical heat flux considerably. A nucleate boiling heat transfer coefficient correlation is developed that fits the present data with a mean absolute error of 6.10%.     

Flame impingement heat transfer: A review

A review of the literature pertaining to flame impingement heat transfer is presented. Studies related to different modes of heat transfer, flame shapes and flame stabilization are considered. Investigations of previous work for different experimental configurations, operating conditions, burner geometry, separation distance and stagnation target with instrumentation are compared. Studies related to measurement and contributions of non-luminous radiation to overall heat transfer are considered. Modeling and computation of flame impingement heat transfer processes in the recent literature have been reviewed. Semi-analytical and empirical correlations to find the total heat flux are briefly presented. Finally, unresolved issues are mentioned for further research.     

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.     

Flow field and heat transfer analysis of local structure for regenerative cooling panel

Local structure of cooling panel has great effects on the heat transfer in the cooling channel for the scramjet. The problems of flow dead area and mass flow rate non-uniform distribution caused by the local structure effect the cooling effectiveness in the channel seriously. Numerical simulation to the flow field of scramjet cold panel with four different fuel injection island structures respectively has been carried out using the CFD commercial software-CFX in this research. The results reveal that flow dead area has been eliminated and flow field has been improved for the optimized structure. Furthermore, local resistance loss has been decreased and the mass flow rate non-uniform distribution in the channel has been reduced. Based on the optimized results, some suggestions about the local design of cooling panel have been proposed in this research.     

Flow and heat transfer characteristics in fuel cooling channels of a recooling cycle

Developing fuel with higher heat sink is widely carried out to meet the cooling requirement for an air breathing hypersonic vehicle. Especially, a recooling cycle has been newly proposed for an actively cooled scramjet to reduce the fuel flow for cooling. Fuel heat sink (cooling capacity) is repeatedly used to indirectly increase the fuel heat sink in a recooling cycle. The variation of fuel thermal property related with heat transfer and flow as temperature and pressure is added to the one-dimensional analytical fin-type model for rectangular ducts. Heat transfer performance comparison between recooling cycle and regenerative cooling is carried out, and detailed discussion about the variation and influence of heat transfer and flow characteristics caused by the introduction of the recooling process are discussed. Performance comparison between a recooling cycle at supercritical pressure and it at subcritical condition is also investigated.     

阵列射流冲击冷却换热系数的数值研究

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.     

Premixed flame impingement heat transfer with induced swirl

Experiments were performed to study the heat transfer characteristics of a swirling premixed flame impinging vertically normal to a horizontal plate. The effects of Reynolds number (Re), equivalence ratio (Ф) and nozzle-to-plate distance (H) on the heat flux were examined. Comparisons were also made between the heat transfer behaviors of the swirling premixed flame (SPF) with a non-swirling premixed flame (PF) operating under the same conditions.Compared with the PF, the swirling flows in the SPF increase the entrainment of ambient air and induce a faster radial spreading rate of the flame jet. Therefore, the SPF provides a larger heating area and produces a more uniform radial heat flux distribution. For both the SPF and PF, the heat flux increases with Re due to the more complete combustion occurring at higher Re. For the SPF, the heat transfer increases with Ф, while it decreases with Ф for the PF because the stronger entrainment of ambient air in the SPF supports a more complete combustion. A smaller H is required for the maximum heat transfer to occur for the SPF.     

Jet impingement heat transfer – Part I: Mean and root-mean-square heat transfer and velocity distributions

Impinging jets provide a means of achieving high heat transfer coefficients both locally and on an area averaged basis. The current work forms the first stage of a two part investigation of heat transfer distributions from a heated flat surface subject to an impinging air jet for Reynolds numbers from 10,000 to 30,000 and non-dimensional surface to jet exit spacing, H/D, from 0.5 to 8. In the present paper, the relative magnitudes of the local heat transfer coefficients are compared to the fluctuating components and to the mean and root-mean-square local velocity components. It has been shown that at low nozzle to surface spacings (<2 diameters) secondary peaks in the radial heat transfer distributions are due to an abrupt increase in turbulence in the wall jet. In particular the velocity fluctuations normal to the impingement surface have a controlling influence on the enhancement in the wall jet.