Blowing Ratio Research Articles

Effects of coolant supply arrangement on double wall cooling: Hot-side effusion performance and cold-side Nusselt numbers at different initial blowing ratios

The present study considers the effects of different coolant supply arrangements on surface thermal performance for both sides of a double-wall cooled effusion plate. Supply arrangements include a cross flow only arrangement, an impingement jet array arrangement, and a combination cross flow and impingement jet array arrangement. A favorable streamwise pressure gradient is provided by a main flow passage contraction ratio CR of 4. For main flow Reynolds numbers Rems,avg of 222,000 to 233,000, four different combination values of crossflow Reynolds number and impingement Reynolds number are tested, which are associated with four different values of initial blowing ratio BR. For upstream locations along the hot-side of the effusion plate, the addition of cross flow to impingement cooling (employed with the combination cooling arrangement) often seems to degrade coolant distributions, as evidenced by adiabatic film cooling values for the cross flow/impingement combination configuration, which are often substantially lower than impingement only arrangement values. For larger x/de values, the cross flow/impingement combination behaves in a manner which is similar to the impingement only arrangement. When compared at a particular BR value, cold-side measurements show that line-averaged Nusselt number are generally highest for the impingement only configuration, and lowest for the cross flow only arrangement. Nusselt number data associated with the cross flow/impingement combination generally increase as BR increases, whereas the cross flow only data increase only slightly with initial blowing ratio.

Effect and optimization of backward hole parameters on film cooling performance by Taguchi method

In recent years, increasing inlet temperature of gas turbines has far exceeded the melting point of the metal materials. Film cooling technology has widely been used to protect gas turbine blades from erosion of the high-temperature gases. The film cooling performance can be improved by optimization of the hole configurations. Results show that the backward injection hole has a smaller exit momentum and a thinner velocity boundary layer near the wall compared to the forward hole. For the backward hole, high blowing ratio is beneficial to improve the film cooling effectiveness. It was found that the overall average film cooling effectiveness for the backward hole increases by 677% at a blowing ratio of 1.5 compared to that for the forward hole. In addition, the coupling effects of hole length, inclination angle and blowing ratio on the film cooling effectiveness were investigated based on the Taguchi method. A new scheme of three-factor four-level orthogonal calculations was designed. It is found that the inclination angle has the greatest effect on the film cooling effectiveness of the backward hole. When the blowing ratio is 2.0, the backward hole with a hole length of 3D and an inclination angle of 35° is the optimal cooling hole configuration.

Experimental Study on Heat Transfer of Leading Edge Film-Cooling With Counter-Inclined Cylindrical and Laid-Back Holes

AbstractThe heat transfer coefficient of counterinclined film holes fed by different intake structures on the turbine vane leading edge (LE) model is experimentally investigated in this paper. A semicylinder model is adopted to model the vane leading edge, which is arranged with one single row of film holes per side, which are located from the stagnation at a 15-deg angle. The four leading edge models, which are the combinations of the hole-shapes (cylindrical hole and laid-back hole) and intake structures (plenum and impingement), are tested at four blowing ratios M. The contours of the heat transfer coefficient, which are characterized by the Frössling number Fr, since it includes the Reynold number effect, are acquired by the transient measurement technique based on double thermochromic liquid-crystals (LCs). The lateral-averaged Fr of the nonfilm-cooled model is provided by using the same experimental platform with an identical main-flow condition. It is then compared with the published data, which indicates the reliability of the present transient measurement techniques. The results illustrate that a core region with a higher heat transfer appears in the hole-exit downstream, and its distribution is slightly skewed to the inclination direction of the film holes. The shape of the high heat transfer region gradually inclines in the spanwise direction as M increases. The heat transfer in the region where the jet core flows through is relatively low, while the jet edge region is relatively high. The effect of impingement leads to the outflow of each hole becoming increasingly uniform, which can reduce the difference in the heat transfer between the region where the jet core flows through and the jet edge. The heat transfer strength may increase due to the intense turbulence caused by the introduction of the impingement. Compared with the cylindrical hole, the laid-back hole has a spanwise expansion feature, which makes the shape of the high heat transfer region wider in the spanwise direction and increases the heat transfer level. Additionally, the magnitude of the enhancement increases with an increasing M.

Experimental Investigation on the Adiabatic Film Effectiveness for Counter-Inclined Simple and Laid-Back Film-Holes of Leading Edge

The adiabatic film effectiveness η of the counter-inclined film-holes fed by varying internal coolant intake on the turbine vane leading edge model was experimentally investigated. A semi-cylinder model was adopted to model the vane leading edge which was arranged with two-row holes, which located at ±15° on both sides. The four Leading edge model with the combinations of hole-shape (simple holes and laid-back holes) and intake structure (plenum and impingement) were tested under four blowing ratios M of 0.5, 1.0, 1.5, and 2.0. The η contours were obtained by the transient measurement technique based on double thermochromic liquid-crystals. The results present that the η is sensitive to the M for the four studied leading edge cases. The addition of impingement enhances the η for the two studied holes. The film jets make the coolant-flow closed to the target surface, resulting in higher η under lower M. The core with higher η appears in the downstream area of hole-exit. The η enhancement can be provided to almost the identical level by adding the impingement-holes and improving the hole-exit shaping in most areas. With increasing M, the jets with stronger exit normal momentum penetrate into the main-flow. The impingement addition may be a more effective program to upgrade the η relatively to the exit shaping under larger M. Besides, the laid-back holes with impingement case produce the highest film cooling performance among the four cases, providing great potential in the leading edge especially under larger M.

Computational Analysis on Gas Turbine Blade by Hole Modified for Optimization the Effectiveness

In order to reduce the operating costs of the engine, turbine designers must also increase the life of their components. However, high gas temperatures throughout the engine require more cooling air or better cooling efficiency to protect the parts from thermal damage. This study presents numerical research on cooling holes. Research focused on aerodynamics and thermal aspects of shallow whole angle. The numerical simulation is performed based on Reynolds Averaged Navier-Stokes (RANS) equations with SST turbulence model by using CFX. A modification has been done in the normal injection hole of 35°, by injecting the cold fluid at different blowing ratio, providing a significant change in the shape of holes which later we found in our numerical investigation giving good quality of film cooling effectiveness.

Analysis of upstream, double-row, cylindrical holes on primary and secondary effects of endwall flow and film cooling

Flow features and film cooling performance of five configurations of double-row, cylindrical holes, upstream of an E3 vane, in a linear cascade are numerically investigated. This simulation is completed using a verified turbulence model at four blowing ratios (M = 0.5, 1.0, 1.5, 2.0). The first three configurations have two rows of cylindrical holes, each row with the same compound angle (β=-45°, 0° or 45°), while the other two have two rows with opposite compound angles (β=-45°, 45° and β=45°, -45°), which are also referred to as double-jet film cooling (DJFC) holes. The primary effects on the downstream endwall and the secondary effects on the nearby airfoil of the cooled passage are analyzed and discussed in detail. Results show that at low blowing ratios the movement of the coolant is denominated by the interaction between the jets and vortices resulting in similar film coverage on both the endwall and airfoil. The effect of vortices is reduced at high blowing ratios. It is also shown that the movement of the coolant is determined by the initial velocity direction, as well as the film cooling configuration.

Theory of Non-Dimensional Groups in Film Effectiveness Studies

Abstract The desire to improve gas turbines has led to a significant body of research concerning film cooling optimization. The open literature contains many studies considering the impact on film cooling performance of both geometrical factors (hole shape, hole separation, hole inclination, row separation, etc.) and physical influences (effect of density ratio (DR), momentum flux ratio, etc.). Film cooling performance (typically film effectiveness, under either adiabatic or diabatic conditions) is almost universally presented as a function of one or more of three commonly used non-dimensional groups: blowing—or local mass flux—ratio, density ratio, and momentum flux ratio. Despite the abundance of papers in this field, there is some confusion in the literature about the best way of presenting such data. Indeed, the very existence of a discussion on this topic points to lack of clarity. In fact, the three non-dimensional groups in common use (blowing ratio (BR), density ratio, and momentum flux ratio) are not entirely independent of each other making aspects of this discussion rather meaningless, and there is at least one further independent group of significance that is rarely discussed in the literature (specific heat capacity flux ratio). The purpose of this paper is to bring clarity to this issue of correct scaling of film cooling data. We show that the film effectiveness is a function of 11 (additional) non-dimensional groups. Of these, seven can be regarded as boundary conditions for the main flow path and should be matched where complete similarity is required. The remaining four non-dimensional groups relate specifically to the introduction of film cooling. These can be cast in numerous ways, but we show that the following forms allow clear physical interpretation: the momentum flux ratio, the blowing ratio, the temperature ratio (TR), and the heat capacity flux ratio. Two of these parameters are in common use, a third is rarely discussed, and the fourth is not discussed in the literature. To understand the physical mechanisms that lead to each of these groups being independently important for scaling, we isolate the contribution of each to the overall thermal field with a parametric numerical study using 3D Reynolds-averaged Navier–Stokes (RANS) and large eddy simulations (LES). The results and physical interpretation are discussed.

Turbine Vane Endwall Film Cooling Effectiveness of Different Purge Slot Configurations in a Linear Cascade

Abstract This study is concerned with the film cooling effectiveness of the flow issuing from the gap between the nozzle guide vane (NGV) and the transition duct on the NGV endwall, i.e., the purge slot. Different slot widths, positions, and injection angles were examined in order to represent changes due to thermal expansion as well as design modifications. Apart from these geometric variations, different blowing ratios (BRs) and density ratios (DRs) were realized to investigate the effects of the interaction between secondary flow and film cooling effectiveness. The experimental tests were performed in a linear scale-1 cascade equipped with four highly loaded turbine vanes at the Institute of Fluid Mechanics and Fluid Machinery of the University of Kaiserslautern. The mainstream flow parameters were, with a Reynolds number of 300,000 and a Mach number (outlet) of 0.6, set to meet real engine conditions. By using various flow conditioners, periodic flow was obtained in the region of interest (ROI). The adiabatic film cooling effectiveness was determined using the pressure sensitive paint (PSP) technique. In this context, nitrogen and carbon dioxide were used as tracer gases realizing two different density ratios DR = 1.0 and 1.6. The investigation was conducted for a broad range of blowing ratios with 0.25 ≤ BR ≤ 1.50. In combination with 10 geometry variations and the aforementioned blowing and density ratio variations, 100 single operating points were investigated. For a better understanding of the coolant distribution, the secondary flows on the endwall were visualized by oil dye. The measurement results will be discussed based on the areal distribution of film cooling effectiveness, its lateral spanwise, as well as its area average. The results will provide a better insight into various parametric effects of gap variations on turbine vane endwall film cooling performance—notably under realistic engine conditions.

A Parametric Investigation of Vane Showerhead Film Cooling by Pressure-Sensitive Paint Technique

Abstract In this study, a parametric analysis of the thermal performance of a nozzle vane cascade with a showerhead cooling system made of four rows of cylindrical holes was carried out by using the pressure-sensitive paint (PSP) technique. Coolant-to-mainstream blowing ratio (BR), density ratio (DR), main flow isentropic exit Mach number (Ma2is), and turbulence intensity level (Tu1) were the considered parameters. The cascade was tested in an atmospheric wind tunnel at Ma2is values ranging from 0.2 to 0.6, with an inlet turbulence intensity level of 1.6% and 9%, at variable injection conditions of BR = 2.0, 3.0, 4.0. Moreover, the influence of the DR on the leading-edge film-cooling performance was investigated: the testing was carried out at DR = 1.0, using nitrogen as foreign gas, and DR = 1.5, with carbon dioxide serving as a coolant. In the near-hole region, higher BR and Ma2is resulted in higher effectiveness, while higher mainstream turbulence intensity reduced the thermal coverage in between the rows of holes, whatever the BR is. Further downstream along the vane pressure side, the effectiveness was negatively affected by rising the BR but positively influenced by lowering the mainstream turbulence intensity. Moreover, a decrease in the DR caused a reduction in the film-cooling performance, whose extent depends on the injection condition.

Optimization of a Confined Jet Geometry to Improve Film Cooling Performance Using Response Surface Methodology (RSM)

This study investigates the interrelated parameters affecting heat transfer from a hot gas flowing on a flat plate while cool air is injected adjacent to the flat plate. The cool air forms an air blanket that shield the flat plate from the hot gas flow. The cool air is blown from a confined jet and is simulated using a two-dimensional numerical model under three variable parameters; namely, blowing ratio, jet angle and density ratio. The interrelations between these parameters are evaluated to properly understand their effects on heat transfer. The analyses are conducted using ANSYS-Fluent, and the performance of the air blanket is reported using local and average adiabatic film cooling effectiveness (AFCE). The interrelation between these parameters and the AFCE is established through a statistical method known as response surface methodology (RSM). The RSM model shows that the AFCE has a second order relation with the blowing ratio and a first order relation with both jet angle and density ratio. Also, it is found that the highest average AFCE is reached at an injection angle of 30 degree, a density ratio of 1.2 and a blowing ratio of 1.8.

Numerical investigation of particle deposition in film-cooled blade leading edge

This study numerically investigates film-cooling performance and particle trajectories in AGTB (two rows of cylindrical holes equipped on suction side (SS) and pressure side (PS) of the leading edge, respectively) turbine cascade. Particle deposition on a turbine blade is analyzed by calculations of capture efficiency and impact efficiency. The turbulent flow is modeled by the Realizable k-ε turbulence model, and the discrete phase model (DPM) with user-defined functions (UDFs) is used to simulate the particle motions. An invasion efficiency is proposed to analyze the possibility of particle invasion into the film hole. Comparisons of various particles with diameters of 1 µm, 5 µm, 10 µm, 20 µm, and 50 µm, respectively, are conducted for four blowing ratios (0.53, 0.93, 1.31, 1.63) and three inlet flow angles (123°, 133°, and 143°). It is observed that with a small inlet flow angle and a large blowing ratio, the capture efficiency on the PS decreases. It is found that smaller particle size results in lower invasion efficiency, and larger particles are more likely to invade into the film-cooling hole especially at a low blowing ratio.

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.

Double wall cooling of an effusion plate with cross flow and impingement jet combination internal cooling: Comparisons of main flow contraction ratio effects

The present study considers comparisons of hot-side effusion plate results for a mainstream flow passage with CR = 1 and for a mainstream flow passage with CR = 4. The coolant supply arrangement for both CR contraction ratio values includes simultaneous use of cross flow and an impingement jet array. For the effusion cooled/hot surface, presented are spatially-resolved distributions of surface adiabatic film cooling effectiveness, and surface heat transfer coefficients (measured using infrared thermography). These results are given for main flow Reynolds numbers Rems of 89,900 to 95,800. For this main flow Reynolds number range, four different combination values of crossflow Reynolds number and impingement Reynolds number are tested for each CR value, which are associated with four different values of initial blowing ratio BR. With this arrangement, crossflow Reynolds number is varied, as impingement jet Reynolds number is approximately constant. The resulting variations of surface adiabatic film cooling effectiveness and surface heat transfer coefficients are due to three competing phenomena, whose relative influences change with x/de location. These include increased turbulent mixing and transport result from effusion coolant jets, decreased magnitudes of local blowing ratio with streamwise location, and significant streamwise acceleration, which induces local boundary layer re-laminarization. When the CR = 4 arrangement is employed, these phenomena result in dramatic decreases of local and line-averaged heat transfer coefficients, and local and line-averaged adiabatic film effectiveness magnitudes, with streamwise development, for x/de > 60. Resulting values are then significantly lower than when CR = 1, provided comparisons are made at a particular streamwise location for each value of initial blowing ratio BR.

Numerical and Experimental Examination of Turbulent Mixing of Heated Jet in Crossflow

The injection of fully developed turbulent heated air from a tube into a cooler turbulent duct flow is examined as a analogy to film-cooled turbine blades. An implicit large-eddy simulation numerical model is applied such that tube and duct turbulence inflow effects are emulated using a divergence-free synthetic eddy method. For direct comparison, a hot-wire experiment is conducted within the Engine Research Building (ERB) test cell SW-6 at the NASA John H. Glenn Research Center. Excellent agreement is obtained for these numerical and experimental results as related to the velocity, temperature, and heat flux for a blowing ratio of 1.2 and by involving a 36 K temperature difference. The relative effect on the solutions of the tube and duct inflow turbulence is systematically evaluated. The impact of inherent low-pass filtering of temperature measurements and probe wire offset on the experimental results are addressed. The validity of the gradient diffusion hypothesis, which is fundamental to Reynolds-averaged Navier–Stokes models, is evaluated.

A numerical study of anti-vortex film-cooling holes designs in a 1-1/2 turbine stage using LES

The primary focus of the present study is to investigate the impact of anti-vortex holes design on the film-cooling performance in a film-cooled rotor blade model using the large eddy simulation method (LES). One row of the film holes was positioned on the pressure surface of the rotor blade. This row had three cylindrical holes (the main hole in the present study) with a diameter (D) of 4 mm and a tangential injection angle of 28 deg. Each main hole supplemented with the addition of two symmetrical side holes (anti-vortex holes), which branch out from the same main hole. Three positions for the anti-vortex side holes were considered; namely: upstream to the outlet of the main hole; in line with the main hole; and downstream of the main hole. The Reynolds number was fixed at Re = 1.92 × 105 and the speed of the rotor blade was taken to be 1800 rpm. The blowing ratio varied from 1.0 to 5.0 and the density ratio of coolant to mainstream was 2.0. Compared to the base hole, the film cooling performance of the all anti-vortex cases showed obvious improvement at all blowing ratios. The middle stream side holes and downstream side holes each demonstrated good film cooling performance at all blowing ratios, while the upstream side holes perform well only at a lower blowing ratio. The presence of side holes can restrain the CRVP (counter rotating vortex pairs) intensity of the main hole and reduce the coolant lift-off, improving the film coverage and film cooling effectiveness. The downstream side holes can perform better in reducing the CRVP intensity.

Vortex dynamics based analysis of internal crossflow effect on film cooling performance

Film cooling is an important technology in protecting the turbine blade of modern aero-engine and gas turbine. Besides the widely studied parameters including the cooling hole geometry and blowing ratio, film cooling effectiveness is also affected by the crossflow in the internal cooling channel, which causes off-design inflow directions of the cooling hole. In-hole counter-rotating vortex pair is observed which accounts for the effect of internal crossflow on the film cooling. The current paper numerically studies the effect of internal channel flow on the film cooling performance by comparing the results of three configurations. A theoretical model about the boundary layer deformation is proposed to explain the formation of in-hole vortices based on the vortex dynamics, and two comparative cases with slip and no-slip channel wall respectively are used to highlight the importance of internal channel flow boundary conditions. By comparing adiabatic film cooling effectiveness under varying blowing ratios and internal crossflow velocities, it is shown that internal crossflow plays an important role in film cooling performance. Enhanced cooling effectiveness can be obtained at internal co-flow conditions where in-hole vortices has the opposite direction with downstream kidney vortices. Due to the competitive effects of distorted velocity profile and in-hole vortices, an optimal velocity of internal crossflow can be found at given blowing ratio and the optimal velocity magnitude slightly increases with increasing blowing ratio.

Large-eddy simulation of the effect of distributed plasma forcing on the wake of a circular cylinder

We conducted large-eddy simulation (LES) to investigate the effect of segmented plasma forcing on the wake of a circular cylinder, at a subcritical Reynolds number of 4700. The action of the plasma actuators was simulated by a body-force model developed by Shyy et al. [1]. The main objective of this study was to investigate the changes in the three-dimensionality of the wake, and the nature of separation on the cylinder surface, with the increasing power of segmented forcing. By observing the three-dimensional (3D) structures, and the separating streamlines close to the cylinder surface, we aim to provide more insight on the experimental findings of Bhattacharya and Gregory [2], which contained only two-dimensional (2D) velocity field data. To that end, we used segmented actuators with a spatial wavelength of 5d (d= cylinder diameter), and two separate blowing ratios of 0.2 and 0.7. At the lower blowing ratio (BR=0.2), spanwise vortex shedding was not canceled, although waviness developed in the spanwise vortices. However, there was no considerable difference in the separation angles between two adjacent spanwise locations. Forcing with higher blowing ratio (BR=0.7) completely disrupted the vortex shedding infront of the plasma region. The separation line on the surface of the cylinder developed significant periodicity leading to a spanwise variation of formation length. We show that such spanwise variation created spatially locked streamwise vortices which disrupted regular vortex shedding.

Effect of Casing Movement on Flow and Heat Transfer Characteristics of Grooved Blade Tip

Using numerical simulation method, the influence of casing movement on the flow and heat transfer characteristics in the groove tip has analysed, and the influence of rotation speed and blowing ratio was also considered. The results show that the casing movement reduces the casing temperature at the top of the rib and the Nu peak number at the bottom of the groove, it also reduces the mixing loss from the interaction of coolant and leakage flow. The higher the casing motion speed, the lower the average Nu on the wall surface and the smaller the mixing loss in the gap. Large blowing ratio cooling jet can effectively improve wall heat transfer.

A Detailed Study of Row-Trenched Holes at the Combustor Exit on Film-Cooling Effectiveness

Abstract To analyse the effects of cylindrical- and row-trenched cooling holes with an alignment angle of 90 degrees on the film-cooling effectiveness near the combustor end wall surface at a blowing ratio of 3.18, the current research was done. This research included a 3D representation of a Pratt and Whitney gas turbine engine, which was simulated and analysed with a commercial finite volume package FLUENT 6.2.26. The analysis was done with Reynolds-averaged Navier–Stokes turbulence model on internal cooling passages. This combustor simulator was combined with the interaction of two rows of dilution jets, which were staggered in the streamwise direction and aligned in the spanwise direction. In comparison with the baseline case of cooling holes, using row-trenched hole near the end wall surface increased the film-cooling effectiveness 44% in average.

Film cooling of cylindrical holes on turbine blade suction side near leading edge

The influences of blowing ratio, rotational speed, Reynolds number, and injection angle on film cooling were experimentally studied on turbine blade suction side near leading edge. One row of round holes is located at streamwise location of 13.5% on suction side. Three injection angles, 30°, 45°, and 60°, were tested. Investigations were conducted with three rotational speeds, 300 rpm, 400 rpm, and 500 rpm with blowing ratio varying from 0.5 to 3.0. Reynolds numbers corresponding to rotational speeds are 20,300, 27,200, and 33,800, respectively. Steady-state Thermochromic Liquid Crystal thermography was adopted to measure the adiabatic film cooling effectiveness. The pressure distributions and the second flow of the whole fluid domain were numerical simulated. Results showed that the blade mid-span has the best film coverage. Film cooling effectiveness descends with the increasing blowing ratio. Rotation can deflect the film trajectories toward the tip of the blade. The corresponding Reynolds number increases with the increase of rotational speed, which improves the film cooling performance. The coolant ejects with a smaller injection angle and a smaller blowing ratio can produce weaker inward film defections and so a better film coverage.