Hybrid RANS-LES of Shaped Hole Film Cooling on an Adiabatic Flat Plate at Low Reynolds Number

Hybrid RANS-LES methods offer a means of reducing computational cost and setup time to simulate transitional flows. Several methods are evaluated in ANSYS CFX, including Scale-Adaptive Simulation (SAS), Shielded Detached Eddy Simulation (SDES), Stress-Blended Eddy Simulation (SBES), and Zonal Large Eddy Simulation (ZLES), along with a no-model laminar simulation. Each is used to simulate an adiabatic flat plate film cooling experiment of a shaped hole at low Reynolds number. Adiabatic effectiveness is calculated for Blowing Ratio (BR) = 1.5 and Density Ratio (DR) = 1.5. The ZLES method and laminar simulation most accurately match experimental lateral-average adiabatic effectiveness along the streamwise direction from the trailing edge of the hole to 35 hole diameters downstream of the hole (X/D = 0 to X/D = 35), with RMS deviations of 5.1% and 4.2%, and maximum deviations of 8% and 11%, respectively. The accuracy of these models is attributed to the resolution of turbulent structures in not only the mixing region but in the upstream boundary layer as well, where the other methods utilize RANS and do not switch to LES.

Analysis of Film Cooling Effectiveness on Shaped Hole and Antivortex Hole

Film cooling is introduction of a secondary fluid （coolant or injected fluid） at one or more discrete locations along a surface exposed to a high temperature environment to protect that surface not only in the immediate region of injection but also downstream region. This paper numerically investigated the film cooling effectiveness on two types of hole geometries which are cut-shaped hole and antivortex hole. The 3D computational geometries are modeled with a single 30 deg angled hole on a flat surface. The different blowing ratios of 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5,5 and k-Epsilon turbulence model are used in this study. A two dimensional distribution of film cooling effectiveness in the downstream region of the cooling hole is performed. A comparison of spanwise averaged effectiveness is also performed in the field starts from center point of hole to X/D=-30.

Experimental study on supersonic film cooling on the surface of a blunt body in hypersonic flow

The experimental study focuses on the heat flux on a double cone blunt body in the presence of tangential-slot super- sonic injection into hypersonic flow. The tests are conducted in a contoured axisymmetric nozzle with Mach numbers of 7.3 and 8.1, and the total temperature is about 900 K. The injection Mach number is 3.2, and total temperature is 300 K. A constant voltage circuit is developed to supply the temperature detectors instead of the normally used constant current circuit. The schlieren photographs are presented additionally to visualize the flow and help analyze the pressure relationship between the cooling flow and the main flow. The dependence of the film-cooling effectiveness on flow parameters, i.e. the blow ratio, the convective Mach number, and the attack angle, is determined. A semi-empirical formula is tested by the present data, and is improved for a better correlation.

Numerical Modeling and Analysis of Grooved Surface Applied to Film Cooling

In order to improve the efficiency of film cooling, numerical investigation was carried out to study the effects of different film-cooled plates on surface heat transfer. Both grooved and non-grooved surfaces were concerned. The modeling was per- formed using Fluent software with the adoption of Shear-Stress Transport （SST） k-ωmodel as the turbulence closure. The coolant was supplied by a single film cooling hole with an inclination angle of 30°. The Mach numbers for the coolant flow and the mainstream flow were fixed at 0 and 0.6, respectively. At three blowing ratios of 0.5, 1.0 and 1.5, the aerodynamic behaviour of the mixing process as well as the heat transfer performance of the film cooling were presented. The numerical results were validated using experimental data extracted from a benchmark test. Good agreements between numerical results and the ex- perimental data were observed. For the film cooling efficiency, it shows that both local and laterally averaged cooling effectiveness can be improved by the non-smooth surface at different blowing ratios. Using the grooved surface, the turbulence intensity upon the plate can be reduced notably, and the mixing between the two flows is weakened due to the reduced turbu lence level. The results indicate that the cooling effectiveness of film cooling can be enhanced by applying the grooved surface.

Instantaneous and time-averaged flow structures around a blunt double-cone with or without supersonic film cooling visualized via nano-tracer planar laser scattering

In a Mach 3.8 wind tunnel, both instantaneous and time-averaged flow structures of different scales around a blunt double-cone with or without supersonic film cooling were visualized via nano-tracer planar laser scattering （NPLS）, which has a high spatiotemporal resolution. Three experimental cases with different injection mass flux rates were carried out. Many typical flow structures were clearly shown, such as shock waves, expansion fans, shear layers, mixing layers, and turbulent boundary layers. The analysis of two NPLS images with an interval of 5 us revealed the temporal evolution characteristics of flow structures. With matched pressures, the laminar length of the mixing layer was longer than that in the case with a larger mass flux rate, but the full covered region was shorter. Structures like K-H （Kelvin-Helmholtz） vortices were clearly seen in both flows. Without injection, the flow was similar to the supersonic flow over a backward- facing step, and the structures were relatively simpler, and there was a longer laminar region. Large scale structures such as hairpin vortices were visualized. In addition, the results were compared in part with the schlieren images captured by others under similar conditions.

Experiment on Adiabatic Film Cooling Effectiveness in Front Zone of Effusion Cooling Configuration

Experimental investigation is performed to investigate the cooling characteristics in the front zone of effusion configuration. Effects of blowing ratio,multi-hole arrangement mode,hole-to-hole pitch and jet orientation angle on the adiabatic film cooling effectiveness are concentrated on. The results show that the film layer displays an obvious"developing"feature in the front zone of effusion cooling scheme,for either the staggered or inline multi-hole arrangement. The varying gradient of the laterally-averaged adiabatic cooling effectiveness along the streamwise direction is greater for the staggered arrangement than that for the inline arrangement. The holes array arranged in staggered mode with small hole-tohole pitches is in favor of obtaining developed film coverage layer rapidly.

Large Eddy Simulation of the Effects of Plasma Actuation Strength on Film Cooling Efficiency

In this article, numerical investigation of the effects of different plasma actuation strengths on the film cooling flow characteristics has been conducted using large eddy simulation （LES）. For this numerical research, the plasma actuator is placed downstream of the trailing edge of the film cooling hole and a phenomenological model is employed to provide the electric field generated by it, resulting in the body forces. Our results show that as the plasma actuation strength grows larger, under the downward effect of the plasma actuation, the jet trajectory near the cooling hole stays closer to the wall and the recirculation region observably reduces in size. Meanwhile, the momentum injection effect of the plasma actuation also actively alters the distributions of the velocity components downstream of the cooling hole. Consequently, the influence of the plasma actuation strength on the Reynolds stress downstream of the cooling hole is remarkable. Furthermore, the plasma actuation weakens the strength of the kidney shaped vortex and prevents the jet from lifting off the wall. Therefore, with the increase of the strength of the plasma actuation, the coolant core stays closer to the wall and tends to split into two distinct regions. So the centerline film cooling efficiency is enhanced, and it is increased by 55% at most when the plasma actuation strength is 10.

Numerical investigation of unsteady mixing mechanism in plate film cooling

A large-scale large eddy simulation in high performance personal computer clusters is carried out to present unsteady mixing mechanism of film cooling and the development of films. Simulation cases include a single-hole plate with the inclined angle of 30° and blowing ratio of 0.5, and a single-row plate with hole-spacing of 1.5D and 2D （diameters of the hole）. According to the massive simulation results, some new unsteady phenomena of gas films are found. The vortex system is changed in different position with the development of film cooling with the time marching the process of a single-row plate film cooling. Due to the mutual interference effects including mutual exclusion, a certain periodic sloshing and mutual fusion, and the structures of a variety of vortices change between parallel gas films. Macroscopic flow structures and heat transfer behaviors are obtained based on 20 million grids and Reynolds number of 28600.

Interpretation of gas-film cooling against aero-thermal heating for high-speed vehicles

The possible application of the film-cooling technique against aero-thermal heating for surfaces of high-speed flying vehicles is discussed. The technique has been widely used in the heat protection of gas turbine blades. It is shown in this paper that, by applying this technique to high-speed flying vehicles, the working principle is fundamentally different. Numerical simulations for two model problems axe performed to support the argument. Besides the heat protection, the appreciable drag reduction is found to be another favorable effect. For the second model problem, i.e., the gas cooling for an optical window on a sphere cone, the hydrodynamic instability of the film is studied by the linear stability analysis to observe possible occurrence of laminar-turbulent transition.

EXPERIMENTAL STUDY ON FILM COOLING EFFECTIVENESS AND HEAT TRANSFER COEFFICIENT BY NAPHTHALENE SUBLIMATION

Based on the linearity of the heat transfer coefficient and dimensionless temperature ratio, and analogy between heat and mass transfer, an experimental study on film cooling characteristic was carried out with a novel naphthalene sublimation. In general, cooling effectiveness distributions have been obtained on adiabatic surface, but separate testing on a non adiabatic wall is required to determine the heat transfer coefficient distribution. This novel naphthalene sublimation, which is different from previous heat transfer or analogy method, can measure both cooling effectiveness and heat transfer coefficient together with the same test section. The results show well by being compared with other references.

Experimental Study of Heat Transfer and Film Cooling Performance of Upstream Ejected Coolant on a Turbine Endwall

An upstream coolant injection that is different from the known leakage flow was introduced to protect the turbine endwall.This coolant is ejected tangentially from a row of cylindrical holes situated at the side of a backward-facing step.In this experiment,the effects of mass flow ratio and leakage slot width on the endwall heat transfer characteristics were investigated.The dimensionless heat transfer coefficient(Nu)and adiabatic film cooling effectiveness(η)on an axisymmetric turbine endwall were measured by the stable-state thermochromic liquid crystal(TLC)technique and the pressure sensitive paint(PSP)technique,respectively.Three mass flow ratios(MFR)of 0.64%,0.85%,and 1.07%,as well as two leakage slot widths(W)of 3.93 mm,and 7.86 mm were considered.Results indicate that the injection film suppresses the strength of the passage vortex,which leads to the coolant covering almost the entire endwall.This result is more evident for the higher MFR cases,meanwhile,the corresponding averaged film cooling effectiveness is increased with the enhancement of the MFR.However,the case with a higher MFR produces a higher heat transfer coefficient distribution,especially in the region close to the leakage slot edge.Besides,when the W is lower,the endwall presents a higherηand a lower Nu for all the cases,which can guide the optimal design of the endwall.

Stage identification and process optimization for fast drilling EDM of film cooling holes using KBSI method

Fast drilling electrical discharge machining(EDM)is widely used in the manufacture of film cooling holes of turbine blades.However,due to the various hole orientations and severe electrode wear,it is relatively intricate to accurately and timely identify the critical moments such as breakout,hole completion in the drilling process,and adjust the machining strategy properly.Existing breakout detection and hole completion determination methods are not suitable for the high-efficiency and fully automatic production of film cooling holes,for they almost all depend on preset thresholds or training data and become less appropriate when machining condition changes.As the breakout and hole completion detection problems can be abstracted to an online stage identification problem,in this paper,a kurtosis-based stage identification(KBSI)method,which uses a novel normalized kurtosis to denote the recent changing trends of gap voltage signals,is developed for online stage identification.The identification accuracy and generalization ability of the KBSI method have been verified in various machining conditions.To improve the overall machining efficiency,the influence of servo control parameters on machining efficiency of each machining stage was analyzed experimentally,and a new stage-wise adaptive control strategy was then proposed to dynamically adjust the servo control parameters according to the online identification results.The performance of the new strategy is evaluated by drilling film cooling holes at different hole orientations.Experimental results show that with the new control strategy,machining efficiency and the machining quality can be significantly improved.

Investigation of Interacting Mechanism between Film Cooling and Internal Cooling Structures of Turbine Blade

This paper presents three-dimensional numerical simulations with the established realizable k-εmodel to clarify the underlying and interacting mechanisms between the film cooling and the internal cooling.On the one hand,the effects of three different internal cooling channels,i.e.,smooth channel,continuous ribbed channel,and truncated ribbed channel,on the film cooling effectiveness and the discharge coefficients are investigated.On the other hand,the influences of three different film cooling holes,i.e.,cylindrical hole,two elliptical holes and two circular-to-elliptical holes,on the heat transfer performances and pressure loss of the internal cooling channel are revealed.Especially,the suction effects of the film cooling holes are analyzed through setting up baselines with only internal cooling channels.Results show that the placement of ribs in the internal channel has different influences on the film cooling effectiveness with respect to different hole shapes depending on the blowing ratio.The discharge coefficient of the film hole can be improved by introducing ribs to the internal channel.Suction of film hole is helpful for enhancing the heat transfer performance and reducing the pressure loss of the internal channel.Besides,ribs instead of the suction effect of film hole play a major role to enhance the heat transfer performance in the internal cooling channel.

Experimental Study on the Film Cooling Characteristics of Three Complex Tip Structures

The turbine blades of aircrafts must be properly cooled to prevent engine failure.Thus,to investigate the influence of the tip structure on the film cooling effect,pressure-sensitive paint test technology was used to determine the adiabatic film cooling effectiveness in this study.The experiment was completed in a cascade comprising three straight blades.The effects of the blowing ratio,density ratio,tip clearance,and tip structure on film cooling efficiency were analyzed.The experimental results demonstrated that,as the blowing ratio increased,the film coverage area and film cooling efficiency increased under most experimental conditions.However,the film cooling efficiency was found to initially increase,and subsequently decrease,as the blowing ratio increased.The respective influences of the density ratio and tip clearance on the film cooling efficiency were found to be significant.The density ratio experiments revealed that a high-density ratio can result in better film coverage than the low-density-ratio air.The tip clearance experimental results indicated that a small tip clearance promotes an increase in film cooling efficiency;this is because the small tip clearance negatively affects the main stream leakage flow,which can reduce the film coverage area.Under the conditions of the Base case 2 configuration,a blowing ratio of 2.1,and a tip clearance of 0.6%h,the average film cooling efficiency of the blade tip was 0.22.Among the three blade tip structures applied in this study,Base case 2 demonstrated higher film cooling efficiency than the other two blade tip structures under the conditions of the same blowing ratio,tip clearance,and density ratio.

Effects of plasma actuation and hole configuration on film cooling performance

In this paper,plasma actuators are arranged asymmetrically downstream the wall to improve film cooling performance.Effects of blowing ratio,hole configuration and applied voltage on flow characteristics and film cooling effectiveness were investigated numerically on a flat plate.Results show that highest film cooling effectiveness distribution is obtained both in the spanwise and streamwise directions under blowing ratio of 0.5.Average wall film cooling effectiveness of cylindrical hole increases by 251.9%under blowing ratio of 0.5 compared to that under blowing ratio of 1.5.The scale of the counter rotating vortex pairs(CRVP)from fan shaped hole and sister hole are significantly reduced compared to that from cylindrical hole.The console hole has an anti-counter rotating vortex pair(Anti-CRVP),which weakens the entrainment of the CRVP to the coolant air near the wall.Compared with the cylindrical hole,average wall film cooling effectivenesses for fan shaped hole,sister hole and console hole increase by 73.1%,97.5%and 119.9%.The adherent performance of the coolant air is enhanced after applying plasma actuator.The aerodynamic actuation of the plasma results in the rebound of the fluid close to the wall at 24 kV applied voltage.Average wall film cooling effectiveness of the console hole at 12 kV applied voltage is 10.6%higher than that without plasma.

Recent advances in film cooling enhancement:A review

Film cooling is an indispensable scheme in the design of highly-efficient cooling configurations to satisfy the thermal protection requirement of turbine hot section components.During the last few decades,vast efforts have been paid on the discrete-hole film cooling enhancement.In this paper,some of the recent literatures related to the passive strategies(such as shaped film cooling holes,upstream ramps,shallow trenches,mesh-fed slots)and the active strategies(such as the use of pulsation modulating device or plasma actuator)for film cooling enhancement are surveyed,with the aim at presenting an updated overview about the state of the art in advanced film cooling.In addition,some challenging issues are also outlined to motivate further investigations in such a broad topic.

Experiments on film cooling with sonic injection into a supersonic flow

Film cooling experiments with sonic injection were conducted to investigate the effects of the number of the injection holes,the mass flow ratio,and the hole spacing on the film cooling effectiveness.The mainstream was obtained by the hydrogen-oxygen combustion,entering the experimental section at a Mach number of 2.0.The nitrogen with ambient temperature was injected into the experimental section at a sonic speed.The measured mainstream recovery temperature was approximately 910K.The mass flow ratio was regulated by varying the nitrogen injection pressure.The experimental results show that for the investigated cooling surface,the cooling effectiveness increases with the increase in the number of the injection holes with other parameters held constant.For a fixed cooling configuration,the cooling effectiveness increases with the increase in the mass flow ratio.Different from the subsonic film cooling,the optimal mass flow ratio is not observed.When the hole spacing is less than 4,no obvious difference is observed on the cooling effectiveness and lateral uniformity.With the mass flow ratio increasing further,this difference becomes much smaller.The shock wave also has an effect on the cooling effectiveness.Downstream the incident point of the shock wave,the cooling effectiveness is lower than that in the case without the shock wave.

Improvement on shaped-hole film cooling effectiveness by integrating upstream sand-dune-shaped ramps

A numerical investigation and experimental validation is performed to address deeper insights into the combined effect of shaped holes and Sand-Dune-shaped upstream Ramp(SDR)on enhancing the film cooling effectiveness,under a wide blowing ratio range(M=0.25–1.5).Three kinds of holes(Cylindrical Hole(CH),Fan-Shaped Hole(FSH),and Crater-Shaped Hole(CSH))are taken into consideration.The SDR shows an inherent affecting mechanism on the mutual interaction of jet-in-crossflow.It aggravates the lateral spreading of cooling jet and thus improves the film cooling uniformity significantly,regardless of film-hole shape and blowing ratio.When the blowing ratio is beyond 1.0,the combined effect of shaped holes and SDR on improving film cooling effectiveness behaves more significantly.It is suggested that FSH-SDR is a most favorable film cooling scheme.For FSH-SDR case,the spatially-averaged film cooling effectiveness is increased monotonously with the increase of blowing ratio,among the present bowing ratio range.

Numerical investigation of transcritical liquid film cooling in a methane/oxygen rocket engine

Transcritical film cooling was investigated by numerical study in a methane cooled methane/oxygen rocket engine.The respective time-averaged Navier-Stokes equations have been solved for the compressible steady three-dimensional(3-D) flow.The flow field computations were performed using the semi-implicit method for pressure linked equation(SIMPLE) algorithm on several blocks of nonuniform collocated grid.The calculation was conducted over a pressure range of 202 650.0 Pa to 1.2×107 Pa and a temperature range of 120.0 K to 3 568.0 K.Twenty-nine different cases were simulated to calculate the impact of different factors.The results show that mass flow rate,length,diameter,number and diffused or convergence of film jet channel,injection angle and jet array arrangements have great impact on transcritical film cooling effectiveness.Furthermore,shape of the jet holes and jet and crossflow turbulence also affect the wall temperature distribution.Two rows of film arranged in different axial angles and staggered arrangement were proposed as new liquid film arrangement.Different radial angles have impact on the film cooling effectiveness in two row-jets cooled cases.The case of in-line and staggered arrangement are almost the same in the region before the second row of jets,but a staggered arrangement has a higher film cooling effectiveness from the second row of jets.

Experimental investigation of the full coverage film cooling effectiveness of a turbine blade with shaped holes

The film cooling effectiveness of two turbine blades at different turbulence intensities(0.62%and 16.00%)and mass flux ratios(2.91%,5.82%,8.73%and 11.63%)is studied by using the Pressure-Sensitive Paint(PSP)measurement technique.There are a baseline and an improved turbine blade in current work,and their film cooling hole position distribution is the same.But the hole shape on suction surface and pressure surface is changed from cylindrical hole(baseline)to laid-back fan-shaped hole(improved blade).Both blades have 5 rows of cylindrical holes at the leading edge and 4 rows of cooling-holes on the suction surface and the pressure surface.The experimental results show that the film cooling effectiveness of the improved blade is much better than the baseline.The increase in turbulence intensity will reduce the cooling effectiveness on the surface of turbine blade,but the effect of turbulence intensity becomes weaker with an increase in the mass flux ratio.Compared with the multiple rows of cylindrical holes,the cooling effectiveness of shaped holes is more influenced by the turbulence intensity at low mass flux ratio.