Blowing Ratio Research Articles

Investigation on cooling effectiveness and aerodynamic loss of a turbine cascade with film cooling

This paper describes the numerical study on film cooling effectiveness and aerodynamic loss due to coolant and main stream mixing for a turbine guide vane. The effects of blowing ratio, mainstream Mach number, surface curvature on the cooling effectiveness and mixing loss were studied and discussed. The numerical results show that the distributions of film cooling effectiveness on the suction surface and pressure surface at the same blowing ratio (BR) are different due to local surface curvature and pressure gradient. The aerodynamic loss features for film holes on the pressure surface are also different from film holes on the suction surface.

Numerical study on film cooling and convective heat transfer characteristics in the cutback region of turbine blade trailing edge

Gas turbine blade trailing edge is easy to burn out under the exposure of high-temperature gas due to its thin shape. The cooling of this area is an important task in gas turbine blade design. The structure design and analysis of trailing edge is critical because of the complexity of geometry, arrangement of cooling channels, design requirement of strength, and the working condition of high heat flux. In the present paper, a 3-D model of the trailing edge cooling channel is constructed and both structures with and without land are numerically investigated at different blowing ratio. The distributions of film cooling effectiveness and convective heat transfer coefficient on cutback and land surface are analyzed, respectively. According to the results, it is obtained that the distributions of film cooling effectiveness and convective heat transfer coefficient both show the symmetrical characteristics as a result of the periodic structure of the trailing edge. The increase of blowing ratio significantly improves the film cooling effectiveness and convective heat transfer coefficient on the cutback surface, which is beneficial to the cooling of trailing edge. It is also found that the land structure is advantageous for enhancing the streamwise film cooling effectiveness of the trailing edge surface while the film cooling effectiveness on the land surface remains at a low level. Convective heat transfer coefficient exhibits a strong dependency with the blowing ratio, which suggests that film cooling effectiveness and convective heat transfer coefficient must be both considered and analyzed in the design of trailing edge cooling structure.

Shroud Cooling with Blade Rotation Using Discrete Holes and an Upstream Slot

Shroud film cooling above the blade row in a low-speed rotating turbine facility is investigated with coolant injected through an upstream slot in combination with injection through angled discrete holes on the shroud. Measurements of the film-cooling effectiveness are performed with coolant injection from the slot alone, from discrete-shroud holes alone, and from combined slot and discrete-shroud holes. Blowing ratios for the discrete holes are in the range of 1.0 to 3.0, whereas those from the slot are in the range of 0.5 to 2.0. The tests are performed at a scaled-down design rotation speed of 550 rpm using liquid crystal thermography. The results for the slot-only cooling tests show increasing cooling effectiveness up to a blowing ratio of 1.25, followed by decreasing effectiveness due to jet liftoff at the higher blowing ratios. When compared on the basis of blowing ratios, slot cooling provides a higher area-averaged film-cooling effectiveness up to a blowing ratio of 2.4, after which the shroud hole cooling for this configuration provides a higher area-averaged effectiveness. The results for the combined cooling show improvement in the area-averaged film-cooling effectiveness for all blowing ratios studied over the individual cooling configuration results at the same blowing ratios. With combined cooling, there is better penetration of the coolant further downstream of the region covered by the slot-only and discrete-hole-only configurations.

Film cooling from a cylindrical hole with parallel auxiliary holes influences

ABSTRACTIn the present paper, numerical analysis has been performed to investigate the film-cooling performance of a cylindrical hole, cylindrical hole with upstream parallel auxiliary holes, cylindrical hole with downstream parallel auxiliary holes, and a cylindrical hole with both upstream and downstream parallel auxiliary holes. Four kinds of film-cooling holes are compared through Reynolds-averaged Navier–Stokes (RANS) analysis for four blowing ratios: 0.5, 1.0, 1.5, and 2.0. The result shows that the auxiliary holes enhance the film-cooling effectiveness. The cylindrical hole with upstream auxiliary holes is better for protecting the text surface than the cylindrical hole with downstream auxiliary holes. In general, the hole with both upstream and downstream parallel auxiliary holes shows the most effective film protection compared with the other three cases tested in this work, especially in blow ratios 0.5 and 1.0.

Film cooling adiabatic effectiveness measurements of pressure side trailing edge cooling configurations

Nowadays total inlet temperature of gas turbine is far above the permissible metal temperature; as a consequence, advanced cooling techniques must be applied to protect from thermal stresses, oxidation and corrosion the components located in the high pressure stages, such as the blade trailing edge. A suitable design of the cooling system for the trailing edge has to cope with geometric constraints and aerodynamic demands; state-of-the-art of cooling concepts often use film cooling on blade pressure side: the air taken from last compressor stages is ejected through discrete holes or slots to provide a cold layer between hot mainstream and the blade surface. With the goal of ensuring a satisfactory lifetime of blades, the design of efficient trailing edge film cooling schemes and, moreover, the possibility to check carefully their behavior, are hence necessary to guarantee an appropriate metal temperature distribution. For this purpose an experimental survey was carried out to investigate the film covering performance of different pressure side trailing edge cooling systems for turbine blades. The experimental test section consists of a scaled-up trailing edge model installed in an open loop suction type test rig. Measurements of adiabatic effectiveness distributions were carried out on three trailing edge cooling system configurations. The baseline geometry is composed by inclined slots separated by elongated pedestals; the second geometry shares the same cutback configuration, with an additional row of circular film cooling holes located upstream; the third model is equipped with three rows of in-line film cooling holes. Experiments have been performed at nearly ambient conditions imposing several blowing ratio values and using carbon dioxide as coolant in order to reproduce a density ratio close to the engine conditions (DR=1.52). To extend the validity of the survey a comparison between adiabatic effectiveness measurements and a prediction by correlative approach was performed to compare the experimental results with 1D methodologies.

Effects of side hole position and blowing ratio on sister hole film cooling performance in a flat plate

Three kinds of sister hole and one single cylindrical hole were experimentally studied in order to analyze the effects of side hole position and blowing ratio on film cooling performance in a flat plate model by applying Thermochromic Liquid Crystal (TLC) technique. The blowing ratio varied from 0.3 to 2.5 and the density ratio of coolant to mainstream was 1.05. The Reynolds number based on the velocity of mainstream and the diameter of the main hole was fixed at 3400. Compared to the base hole, the film cooling performance of all sister holes showed obvious improvement at all blowing ratios. The middle stream sister hole and downstream sister hole each demonstrated good film cooling performance at all blowing ratios, while the upstream sister hole performs well only at a lower blowing ratio. A numerical calculation is also performed to better analyze the experimental results and determine the relationships among CRVP intensity, flow structure, and film cooling effectiveness at a low blowing ratio of M = 0.5 and high blowing ratio of M = 2.0. The presence of side holes can restrain the CRVP intensity of the main hole and reduce the coolant lift-off, improving the film coverage and film cooling effectiveness. The downstream sister hole can perform best in reducing the CRVP intensity.

Heat Transfer to an Actively Cooled Shroud With Blade Rotation

An experimental study of the shroud heat transfer behavior and the effectiveness of shroud cooling are undertaken in a single-stage turbine at low rotation speeds. The shroud consists of a periodic distribution of laterally oriented cooling holes that are angled at 45 deg to the shroud surface in a repeating circumferential pattern and has five unique hole pitches in the axial direction. Measurements of the normalized Nusselt number and film cooling effectiveness are done using liquid crystal thermography. These measurements are reported for the no-coolant case and nominal blowing ratios (BRs) of 1.0, 1.5, 2.0, 2.5, and 3.0. The tests are performed at an inflow Reynolds number of 17,500 corresponding to a scaled down design rotation speed of 550 rpm, and two off-design speeds imposed by a motor: (1) a rotation speed below the design speed (400 rpm) and (2) a rotation speed above the design speed (700 rpm). The results at the design speed show that increasing the BR increases the area-averaged film cooling effectiveness, while the Nu/Nu0 in the shroud hole region decreases. As the rotor speed is changed from the design speed, the high Nu/Nu0 region migrates on the shroud surface. This migration affects the coolant coverage in the shroud hole region resulting in increased coolant coverage at below-design rotation speeds and decreased coolant coverage at above-design rotation speeds. At all rotation speeds, as the BR increases, the area-averaged film cooling effectiveness in the shroud hole region increases. Decreasing the circumferential shroud coolant hole spacing changes the lateral heat transfer profile from a periodic sinusoidal distribution for a shroud hole spacing of P/D = 10.4 to a more even distribution for a smaller shroud hole spacing (P/D = 4.8).

LES of rotational effects on film cooling effectiveness and heat transfer coefficient in a gas turbine blade with one row of air film injection

The effects of rotation on film cooling effectiveness and heat transfer coefficient distributions on the suction and pressure surfaces of a gas turbine blade were numerically simulated using large eddy simulation method. The flow is in the low Mach number (incompressible) regime. The suction (convex) side has simple angled cylindrical film-cooling holes; the pressure (concave) side has compound angled cylindrical film-cooling holes. The blowing ratio (BR) is 0.5 on both the suction and pressure sides and the inlet Reynolds number based on the axial chord is 3 × 105. The effects of turbine rotating conditions on the film cooling effectiveness and heat transfer coefficient distributions were investigated at four rotating speeds of 0 rpm, 125 rpm, 250 rpm, and 500 rpm. Air was injected through one row of film holes each on the pressure and suction surfaces. The commercial code STAR-CCM+ was used in the prediction. The results indicate that film cooling effectiveness increases with an increase in the rotating speed. Higher turbine rotating speeds show more local film cooling effectiveness spread on pressure surface. The film cooling effectiveness and Nusselt number distributions were presented along with the discussions on the influences of rotating speed. Results showed that the rotation promotes an earlier boundary layer transition and increases the transition length on the suction surface.

Simulations of Multiphase Particle Deposition on a Gas Turbine Endwall With Impingement and Film Cooling

Replacing natural gas fuels with coal-derived syngas in industrial gas turbines can lead to molten particle deposition on the turbine components. The deposition of the particles, which originate from impurities in the syngas fuels, can increase surface roughness and obstruct film cooling holes. These deposition effects increase heat transfer to the components and degrade the performance of cooling mechanisms, which are critical for maintaining component life. The current experimental study dynamically simulated molten particle deposition on a conducting blade endwall with the injection of molten wax. The key nondimensional parameters for modeling of conjugate heat transfer and deposition were replicated in the experiment. The endwall was cooled with internal impingement jet cooling and film cooling. Increasing blowing ratio mitigated some deposition at the film cooling hole exits and in areas of coolest endwall temperatures. After deposition, the external surface temperatures and internal endwall temperatures were measured and found to be warmer than the endwall temperatures measured before deposition. Although the deposition helps insulate the endwall from the mainstream, the roughness effects of the deposition counteract the insulating effect by decreasing the benefit of film cooling and by increasing external heat transfer coefficients.

Film cooling sensitivity of laidback fanshape holes to variations in exit configuration and mainstream turbulence intensity

Experimental measurements and numerical simulations have been performed to investigate the sensitivity of film cooling of laidback fanshape hole to the variations in hole exit configuration and mainstream turbulence intensity. The measurements are conducted on a flat plate model with low-speed, incompressible mainstream. Film cooling performances of two laidback fanshape hole configurations are measured under four blowing ratios using transient liquid crystal measurement technique which can process the nonuniform initial wall temperature. Hole exit corner shape is changed from sharp corner to round corner to simulate the effect of real manufacturing and/or deposition on the exit configuration of ideal laidback fanshape hole. The lateral film coverage as well as the film cooling effectiveness of laidback fanshape hole with round corner exit are reduced due to the smaller lateral expansion caused by round corner shape, especially under low turbulence condition. Large mainstream turbulence intensity can enhance the lateral diffusion of film jets which is beneficial to film cooing of laidback fanshape hole with round corner under larger blowing ratios. For the heat transfer coefficient distributions of the two film hole configurations, variation in film hole exit configuration has very small influence on both the spatial features and the averaged values under the same mainstream turbulence conditions. However, the variation in mainstream turbulence intensity has notable influences on the heat transfer coefficient distributions for both film hole configurations, because the enhancement effect of film jets on heat transfer and the strength of counter-rotating vortex pair are different between high and low mainstream turbulence conditions. The discharge coefficients of laidback fanshape hole configurations are not sensitive to the variations in hole exit configuration and mainstream turbulence intensity under the same blowing ratio.

Full-Coverage Film Cooling: Heat Transfer Coefficients and Film Effectiveness for a Sparse Hole Array at Different Blowing Ratios and Contraction Ratios

The present experimental investigation considers a full coverage film cooling arrangement with different streamwise static pressure gradients. The film cooling holes in adjacent streamwise rows are staggered with respect to each other, with sharp edges and streamwise inclination angles of 20 deg with respect to the liner surface. Data are provided for turbulent film cooling, contraction ratios of 1 and 4, blowing ratios (BRs) (at the test section entrance) of 2.0, 5.0, and 10.0, a coolant Reynolds number of 12,000, freestream temperatures from 75 °C to 115 °C, a film hole diameter of 7 mm, and density ratios from 1.15 to 1.25. Nondimensional streamwise and spanwise film cooling hole spacings, X/D and Y/D, are 18 and 5, respectively. Data illustrating the effects of contraction ratio, BR, and streamwise location on local, line-averaged, and spatially averaged adiabatic film effectiveness data; and on local, line-averaged and spatially averaged heat transfer coefficient data are presented. Varying BR values are present along the length of the contraction passage, which contains the cooling hole arrangement, when contraction ratio is 4. Dependence on BR indicates important influences of coolant concentration and distribution. For example, line-averaged and spatially averaged adiabatic effectiveness data show vastly different changes with BR for the configurations with contraction ratios of 1 and 4. In addition, much larger effectiveness alterations are present as BR changes from 2.0 to 10.0, when significant acceleration is present and Cr = 4 (in comparison with the Cr = 1 data).

Optimal Arrangement of Combined-Hole for Improving Film Cooling Effectiveness

Film cooling technique is widely used to protect the components from being destroyed by hot mainstream in a modern gas turbine. Combining round-holes is a promising way of improving film cooling effectiveness. A DoE (design of experiment) simulation of 396 cases focusing on the arrangement of the combined-hole with double holes for improving film cooling performance is carried out in this work, and the influence of an aerodynamic parameter, blowing ratio is considered as well. The dimensionless lateral distance (PoD) and compound angle (CA) of the double holes have relative influence on the film cooling performance of the combined-hole unit. At the low blowing ratio, increasing symmetrical compound angle (SCA) has positive influence on the area-average effectiveness (EFF) of the combined-hole. But at the intermediate and large blowing ratio, the influence of SCA on the area-average EFF depends on the range of PoD. At the small PoD, the area-average EFF ascends basically along SCA axis. However, the area-average EFF first ascends and subsequently descends along SCA axis at the large PoD. Asymmetrical compound angle (ACA) is also considered to fit the antikidney vortexes produced in the combined-hole film cooling compared to their ideal schematic. However, the film cooling effect of the cases with ACA is not as good as expected. The area-average EFF of ACA cases locates in the level between that of the adjacent SCA cases. The optimal arrangement of combined-hole unit for improving film cooling effectiveness is relative to the local flow field. The optimal arrangement of PoD and CA for improving the combined-hole film cooling performance is different at different blowing ratios.

Numerical Computation and Optimization of Turbine Blade Film Cooling

The effect of film cooling parameters on the cooling effectiveness of an actual turbine blade is studied numerically. Film cooling parameters such as the hole shape, holes distribution, blowing ratio, streamwise angle, and spanwise angle are investigated to select the appropriate cooling parameters. Unstructured finite volume technique is used to solve the steady, three-dimensional, and compressible Navier-Stokes equations. Using one cooling holes array indicates that the average overall film cooling effectiveness is enhanced by decreasing the streamwise angle for high blowing ratio on the suction side of the turbine blade. The film cooling effectiveness is enhanced on the pressure side for a blowing ratio of unity. In addition, the cooling effectiveness increases by increasing the lateral and forward diffusion angles.The computations reveal that the efficiency of cooling is decreased at the leading edge due to the large surface curvature of the blade. The presence of compound shape (spanwise angle) enhanced the film cooling effectiveness on the two sides. Multistagger cooling hole arrays are investigated and the results indicate that five-stagger cooling arrays on the pressure side and three-stagger cooling arrays on the suction side with LFDCA-9.3-14.6 hole shape are enough to have good cooling of the two sides using 2.17% bleed air of the engine.

Numerical Investigation of Adiabatic Film Cooling Effectiveness over a Flat Plate Model with Cylindrical Holes

Film cooling is one of the cooling techniques used in gas turbines to protect the blades from high temperature. The efficiency of gas turbine can be increased by increasing the inlet gas temperature. The present study aims at numerical investigation of adiabatic film cooling effectiveness over the cylindrical hole exit flat plate test model. The hole inclination angle of 20° with stream wise direction is considered. The test model has 35 film cooling holes arranged in two rows in a staggered configuration with the coolant hole length to diameter ratio of 5.4 and pitch to diameter ratio of 4. During the experimental analysis, the adiabatic film cooling effectiveness is found at five different blowing ratios in the range of 0.5 to 2.5 with the coolant to mainstream density ratio at 1.6. The laterally averaged film cooling effectiveness is calculated along the stream wise direction at these blowing ratios. The realizable k-epsilon turbulence model with enhanced wall function is used to solve the flow field. The CFD obtained results are validated with the experimental results at a blowing ratio of 1.0. Further, the CFD analysis is done for other blowing ratios in the range of 0.5 to 4.0 at density ratios of 1.6, 1.8 and 2.0. From the CFD results, the laterally averaged film cooling effectiveness is found to be increasing with the increase in blowing ratio up to 1.5 immediately downstream of the coolant holes. For the blowing ratios of 2.0 to 4, the adiabatic film cooling effectiveness is less immediately downstream of coolant holes up to x/d=15 and then increases with increase in the blowing ratios considered.

Analyzes of Film Cooling from Cylindrical and Row Trenched Holes with Alignment Angle of 90 Degrees at Low Blowing Ratio

The current study was conducted to analyze the effects of cylindrical and row trenched cooling holes with alignment angle of 90 degrees at blowing ratio, BR = 1.25 on the film cooling effectiveness near the end wall surface of a combustor simulator. In the current research a three dimensional representation of Pratt and Whitney gas turbine engine was simulated and analyzed with a commercial finite volume package FLUENT 6.2.26. This study has been performed with Reynolds-averaged Navier-Stokes turbulence model (RANS) on internal cooling passages. This combustor simulator combined the interaction of two rows of dilution jets, which were staggered in the stream wise direction and aligned in the span wise arrangement, with that of film cooling along the combustor liner walls. The findings of the study declared that with using the row trenched holes near the end wall surface, film cooling effectiveness is increased three times compared to the cooling performance of baseline case.

Parametric Study on Anti-Vortex Film Cooling Hole Arrangements

Film cooling has been extensively used to provide thermal protection for the external surface of the gas turbine blades. Numerous number of film cooling holes designs and arrangements have been introduced. The main motivation of these designs and arrangements are to reduce the lift-off effect cause by the counter rotating vortices (CRVP) produce by cylindrical cooling hole. One of the efforts is the introduction of newly found anti-vortex film cooling design. The present study focuses on anti-vortex holes arrangement consists of a main hole and pair of smaller holes. All three holes share a common inlet with the outlet of the smaller holes varies base on it relative position towards the main hole. Three anti-vortex holes arrangements have been considered; downstream anti-vortex hole arrangement (DAV), lateral anti-vortex hole arrangement (LAV), and upstream anti-vortex hole arrangement (UAV). In addition, a single hole (SH) film cooling has also been considered as the baseline. The investigation make used of ANSYS CFX software ver. 14. The investigations are made through Reynolds Average Navier Stokes analyses with the application of shear k-ε turbulence model. The results show that the anti-vortex designs produce significant improvement in term of film cooling effectiveness and distribution. The LAV arrangement shows the best film cooling effectiveness distribution among all considered cases and is consistent for all blowing ratios (BR). The results also unveil the formation of new vortex pair on both side of the primary hole CRVP. Interaction between the new vortices and the main CRVP structure reduce the lift off explaining the increased lateral film effectiveness.

Numerical Assessment of the Film Cooling Through Novel Sister-Shaped Single-Hole Schemes

In the present paper, a numerical investigation is conducted on film cooling performance from novel sister-shaped single-hole schemes. Based on the sister hole film cooling technique, shaped holes are formed by merging discrete sister holes to a primary hole. Simulations are performed at four blowing ratios of 0.25, 0.5, 1, and 1.5. The novel-shaped holes resulted in a significant reduction in the jet liftoff effect in comparison with a cylindrical and a forward-diffused shaped hole. Moreover, film cooling effectiveness is notably increased at the high blowing ratios of 1 and 1.5.

An experimental investigation of secondary flow characteristics in a linear turbine cascade with upstream converging slot-holes using TR-PIV

To study the time-mean and temporal characteristics of secondary flow within a linear GE-E3 high pressure turbine cascade, a planar Time-Resolved Particle Image Velocimetry (TR-PIV) system is used. In the double-passage cascade, a row of six converging slot-holes is placed upstream of center blade to generate film cooling effect, and different turbulence grids are replaced to create various free-stream turbulence (Tuin) levels. In this experiment, the time-mean characteristics of secondary flow, the fast switch process of unsteady leading edge horseshoe vortex (LEHV), and the temporal characteristics of corner vortices (CVs) are completely exhibited by the TR-PIV technique. The influences of the upstream coolant injection and Tuin level on the flow characteristics of LEHV and passage vortex (PV) are discussed. The discussion reveals that: (1) in the case of no coolant injection, a high Tuin level slightly moves the LEHV toward the blade, changes the shape of the PV, increases the fluctuations of the LEHV and PV, and reduces the frequency of the LEHV switch process; (2) at various Tuin levels, the coolant injections suppress the formation of the LEHV, and the LEHV disappears at a high coolant-to-mainstream blowing ratio (BR) of 1.5; (3) a high BR of 1.5 can greatly weaken the PV at various Tuin levels, and relative to the case of low Tuin, the high Tuin level induces a larger reduction; (4) for a low BR, at various Tuin levels, a slight change in the LEHV results in a distinct difference of the PV characteristics; (5) for a high BR, since the LEHV disappears, the Tuin effect on the secondary flow characteristics is slight.

Measurement of Film Effectiveness for Cylindrical and Row Trenched Cooling Holes at Different Blowing Ratios

This study was performed to investigate the effects of cylindrical and row trenched cooling holes with alignment angle of 0° and 90° at different blowing ratios on the film-cooling performance adjacent the endwall surface of a combustor simulator. The film-cooling blowing ratios varied from 1.25 to 3.18. In this study, a three-dimensional representation of a Pratt and Whitney gas turbine engine was simulated and analyzed with a commercial finite volume package FLUENT 6.2.26. The analysis has been carried out 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. Film cooling was placed along the combustor liner walls. In comparison with the baseline case of cooling holes, the application of row trenched hole near the endwall surface doubled the performance of film-cooling effectiveness.

An experimental study of density ratio effects on the film cooling injection from discrete holes by using PIV and PSP techniques

An experimental study was conducted to investigate the performance of film cooling injection from a row of circular holes spaced laterally across a flat test plate. While a high-resolution Particle Image Velocimetry (PIV) system was used to conduct detailed flow field measurements to quantify the dynamic mixing process between the coolant jet stream and the mainstream flows, Pressure Sensitive Paint (PSP) technique was used to map the corresponding adiabatic film cooling effectiveness on the surface of interest based on a mass-flux analog to traditional temperature-based cooling effectiveness measurements. The cooling effectiveness data of the present study were compared quantitatively against those derived directly from the temperature-based measurements under the same or comparable test conditions in order to validate the reliability of the PSP technique. The effects of the coolant-to-mainstream density ratio (DR) on the film cooling effectiveness were investigated by performing experiments at fixed blowing ratios by using either Nitrogen (DR=0.97) or CO2 (DR=1.53) as the coolant streams for the PSP measurements. An accompanying analysis of scaling quantities, such as the coolant-to-mainstream momentum flux ratio, I, and bulk coolant-to-mainstream velocity ratio, VR, in addition to the most-commonly used blowing ratio (i.e., the coolant-to-mainstream mass flux ratio), M, was also conducted to illuminate the extent to which the flow scenario can be described using purely kinematic or dynamic means. It was found that the scaling quantities that give more weight to density ratio (i.e., blowing ratio, M, and then momentum ratio, I) have more success to collapse measurement data from varying density ratio of the coolant flows for the cases with relatively low coolant flow rates, while the coolant-to-mainstream bulk velocity ratio, VR, may be used with some success to scale the film cooling effectiveness for the cases with higher coolant flow rates.