Laidback Hole Research Articles

Experimental and numerical investigations on film cooling characteristics and hole shape improvement of turbine vane at one engine inoperative condition

This study investigates the film cooling characteristics of four cooling schemes of a gas turbine vane at various mass flux ratios (8.0%, 8.5% and 9.0%) using the Pressure Sensitive Paint (PSP) technology. The improved cooling scheme involves the alteration from cylindrical holes to laid-back fan-shaped holes and laid-back holes on the pressure surface and the leading edge. Considering the complex fluctuations of the real engine operating environment, the one engine inoperative (OEI) situation is numerically simulated to explore the turbine vane's cooling performance at varying operating conditions. Experimental and numerical results indicate significantly enhanced film cooling effectiveness of vane with shaped holes (improved cooling schemes) compared to the cylindrical hole (baseline), particularly at higher mass flux ratio (MFR = 9.0%). At three OEI conditions, the film coverage of improved cooling schemes is significant compared to the baseline on the pressure side. When the OEI condition is increased from 100% to 120%, the area-averaged cooling effectiveness of the shaped hole vane decreases by 4.3%, due to the interaction between the freestream and lift-off cooling jet.

AN EXPERIMENTAL STUDY OF THE COOLING EFFICIENCY ON CURVED SURFACES WITH DIFFERENT SHAPED HOLES

Improving the film-cooling efficiency is one of the main goals of heat transfer in modern industrial applications. To follow the actual requirements to film-cooling systems, different hole geometries including fan-shaped, laidback, and complex shapes were earlier developed and studied. However, most appropriate studies were performed on flat surfaces whereas the actual geometry of protected walls has curved surfaces, e.g., gas turbine nozzle vanes. The present paper reports experimental results of comparative studies of the film-cooling efficiency for curved surfaces with five different hole geometries: cylindrical, short and long fan-shaped, laidback, and joint fan-shaped and laidback holes. Each experimental model had 11 holes located in a row perpendicular to the main stream direction and was studied using the IR-thermography technique. The experimental results show that the choice of the most efficient cooling hole shape also strongly depends on the blowing ratio value, which was changed in the range 0.5 ≤ m ≤ 1.5 during these studies. For cylindrical holes, the film separation takes place at m > 0.7. Meanwhile, short fan-shaped and laidback holes provide the film separation at m > 1. The highest values of the cooling efficiency were obtained for long fan-shaped holes in the blowing ratio range 0.5 ≤ m ≤ 1. Under mentioned conditions, long fan-shaped holes' efficiency is at least 35# higher compared to others. However, studies at m >1 showed that joint fanshaped and laidback holes provide maximal cooling efficiency due to the almost tangential-to-surface direction of the secondary air flow blown from these holes. The obtained results can be further applied in designing new cooling systems for the protection of the most thermally stressed elements of industrial applications.

Film cooling comparison of shaped holes among the pressure surface, the suction surface and the leading edge of turbine vane

The present study employed commercial computational fluid dynamics software ANSYS 2019R3 to explore the adiabatic film cooling effectiveness and the net heat flux reduction (NHFR) for the comparison of the five selected shaped holes and conventional cylindrical holes between the pressure surface, the suction surface and the leading edge. Amo4ng the shape parameters of shaped holes, the lateral divergence angle (β) and the forward divergence angle (δ) were fixed as 12° and 7° in all shaped holes structures, respectively. The others varied with different regions of the turbine vane. Results showed that different holes fit different positions of vanes. On the suction surface, laidback holes performed the worst net heat flux reduction in most blowing ratios conditions, which indicated the forward divergence angle was not conducive to the flow field and heat transfer characteristics on the suction surface. Whereas, the lateral divergence angle was beneficial to the film cooling and heat transfer characteristics. Laidback fan-shaped holes performed the best adiabatic film cooling effectiveness, but once simultaneously thinking about the heat transfer, fan-shaped holes performed better in net heat flux reduction due to less vortices at holes exit. On the leading edge, the divergence angle towards the upper wall (the lateral divergence angle of spanwise expansion holes) of vanes was not conducive to steady flow. And conical holes performed best, which indicated that coolant from holes with axial divergence angles (the lateral divergence angle in axial direction of conical holes) under the influence of mainstream impact could perform higher film cooling effectiveness and more stable flow fields. On the pressure surface, holes had a lateral divergence angle in the direction of vane height, which was conducive to increasing the coolant coverage area and improving the ability to attach to the pressure surface. Additionally, the laidback hole case was observed the lowest net heat flux reduction when the blowing ratio was less than 2, which revealed that holes that expanded only in flow direction was not conducive to film cooling and heat transfer characteristics.

Numerical analysis and design optimization on full coverage film-cooling for turbine guided vane

Based on numerical simulations, the heat transfer and flow field of a turbine vane are analyzed and the film cooling is improved. The optimization objective is increasing the overall cooling effectiveness with cascade pressure loss factors staying almost unchanged. Thus, cylindrical film holes were replaced by laidback holes and V-crater holes. To analyze the effect of structural adjustment on the vane, pressure distributions, mass flow distributions, and heat transfer coefficients were investigated for internal and external cooling systems. To explain the advantages of shaped holes over cylindrical holes, the flow mechanisms, film superposition, discharge coefficients, blow ratios, and film cooling effectiveness were compared. Meanwhile, the influence of mass flow ratios and mainstream Reynolds numbers was analyzed. After optimization, at the design condition, the overall cooling effectiveness increased by 4.19% on the pressure surface and 1.78% on the suction surface. However, the cascade pressure loss factor increased by 0.26% only.

Numerical Analysis of Film Cooling of Gas Turbine Blades using Shaped Holes

Film cooling of gas turbine blades is essential as increasing the combustor exit temperature and gas turbine inlet temperature improves the performance and efficiency of gas turbine. The focus of this numerical investigation is to investigate the film cooling performance of the laidback fan shaped holes. The study is carried out at different blowing ratios namely, M = 0.60, 1.25 and 1.50. All the comparison is made with the baseline cylindrical hole. Jet lift-off is found to be suppressed in the laidback hole in comparison with cylindrical hole. This suppression of jet lift-off by the laidback hole helps in ensuring the coolant to the closer to the bottom plate of the gas turbine blade thereby reducing the temperature observed on the gas turbine blade. This trend is observed to be increasing with blowing ratios. Moreover, the coolant spread is also found to be better in laidback fan shaped hole in comparisons with baseline cylindrical hole. Based on the temperature distributions patterns observed on the gas turbine blade, it can be concluded that laidback fan shaped hole provides a better lateral spreading of the ejected coolant and improved film cooling.

Boundary-condition models of film-cooling holes for large-eddy simulation of turbine vanes

In many industrial applications, the mechanical integrity of a surface operating under large thermal loads is ensured by injecting a cold fluid through a series of hole along the surface, forming a thin film of cool fluid shielding the solid surface from external heat. An accurate prediction of the heat transfer provided by these so-called film-cooling systems is crucial to ensure the durability of the cooled surfaces. However, the large-eddy simulation of film-cooling systems is complex and expensive because the in-hole flow must be meshed and simulated. To address this difficulty, the modelling of the film-cooling jet by mean of a dedicated boundary condition has recently been proposed. This paper investigates several potential improvements for this type of model in four geometries: an inclined cylindrical hole, a fanshaped hole and two fanshaped laidback holes. The analysis focuses on the comparison of a spatially uniform injection to a model taking into account velocity and temperature spatial variations at the hole exit. The study also compares a non-turbulent injection to a model with synthetic turbulence injection. The comparisons are first performed using a fine mesh to validate the approach, then using a coarse mesh representative of a mesh that could be used to simulate a cooled nozzle guide vane. The results show that both spatial inhomogeneity and turbulence injection significantly improve the cooling effectiveness predictions in a wide variety of cases. The spatial inhomogeneity is especially crucial for the near-hole behaviour of the flow while turbulence injection is particularly important when the destabilisation of the jet by the crossflow is not sufficient to immediately trigger transition. Using a very coarse mesh, the turbulent mixing is observed to be underestimated with all examined boundary-condition models and the behaviour of the jet is not correctly described for some of the configurations investigated. Although not sufficient, non-negligible improvements are nevertheless obtained with an inhomogeneous turbulent injection compared to the baseline uniform model.

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.

Improvement in Film Cooling Effectiveness Using Single and Double Rows of Holes With Adverse Compound Angle Orientations

Abstract Three-dimensional Reynolds-averaged Navier–Stokes equations with shear stress transport turbulence model are used to analyze the film cooling effectiveness on a flat plate having single row of film hole involving cylindrical hole (CH) and laidback hole (LBH). The CH and LBH are inclined at 35 deg to the surface with a compound angle (β) orientation ranging from favorable to adverse inclination (i.e., β = 0–180 deg) and examined at high and low blowing ratios (M = 1.25 and 0.60). CH with an adverse compound angle of 135 deg gives the highest area-averaged film cooling effectiveness in comparison with LBH configuration. Also, CH β = 135 deg film hole shows a higher lateral coolant spread. Later, double jet film cooling (DJFC) concept is studied for this CH. In all the cases, the first hole compound angle is fixed as 135 deg, and the second hole angle is varied from 135 deg to 315 deg. At high blowing ratio, the dual jet cylindrical hole (DJCH) with β = 135 deg, 315 deg gives a higher area-averaged film cooling effectiveness by around 66.50% compared to baseline CH β = 0 deg. On comparing all CH, LBH, and DJCH cases, the highest area-averaged film cooling effectiveness is obtained by CH configuration with β = 135 deg. Hence, the CH with its adverse compound angle (β = 135 deg) orientation could be an appropriate film cooling configuration for gas turbine blade cooling.

Investigation on the Film Cooling Performance of Diffuser Shaped Holes with Different Inclination Angles

AbstractFilm cooling performances of laidback hole and laidback fan-shaped hole have been investigated using transient liquid crystal measurement technique. For film cooling of laidback hole, the increase of inclination angle reduces the film cooling effectiveness under small blowing ratio, while produces better film coverage and higher film cooling effectiveness under larger blowing ratios. Heat transfer coefficient is higher in the upstream region of laidback hole film cooling with large inclination angle, while laidback hole with small inclination angle has relatively higher heat transfer coefficient in the downstream region. The increase of inclination angle reduces the film cooling effectiveness and heat transfer coefficient of laidback fan-shaped hole film cooling, especially in the upstream region. Increase of inclination angle is beneficial to the thermal protection of laidback hole film cooling under large blowing ratio. Film hole with large inclination angle has larger discharge coefficient due to smaller aerodynamic loss in the hole.

Experimental investigation on the leading edge film cooling of cylindrical and laid-back holes with different radial angles

Film cooling performances of the cylindrical film holes and the laid-back film holes on the turbine blade leading edge model are investigated in this paper. Experimental measurements have been carried out to investigate the influence of the inclined angle in the spanwise direction (i.e. radial angle for a blade in the engine) on the film cooling performances of these two kinds of holes. Three rows of holes are arranged in a semi-cylinder model which is used to model the blade leading edge. Two inclined angles and three blowing ratios are tested. Transient heat transfer measurement technique with double thermochromic liquid crystals is employed in the present experiment. The results show that the trajectory of the film jets in the leading edge region deviates from the mainstream direction to the spanwise direction gradually as the blowing ratio increases. Under large blowing ratio, more area can benefit from the film protection and the film cooling effectiveness distribution is more uniform than those under small blowing ratio, while the heat transfer coefficient is also higher. The basic distribution features of heat transfer coefficient are similar for all the tested models. The heat transfer coefficient in the region where the jet core flows through is relatively lower, while the heat transfer coefficient in the jet edge region is relatively higher. Compared with the cylindrical holes, the jets from the laid-back holes have better film coverage and meanwhile make more area have relatively higher heat transfer coefficient, especially under large blowing ratio. Under the same blowing ratio, the jets from film holes with small radial angle can attach on the wall surface better and give higher film cooling effectiveness in the region close to the hole exit than the film holes with large radial angle, while they also produce relatively higher heat transfer coefficient.

Experimental investigation on the leading edge film cooling of cylindrical and laid-back holes with different hole pitches

Experimental investigation has been performed to study the film cooling performances of cylindrical holes and laid-back holes on the turbine blade leading edge. Four test models are measured for four blowing ratios to investigate the influences of film hole shape and hole pitch on the film cooling performances Film cooling effectiveness and heat transfer coefficient have been obtained using a transient heat transfer measurement technique with double thermochromic liquid crystals. As the blowing ratio increases, the trajectory of jets deviates to the spanwise direction and lifts off gradually. However, more area can benefit from the film protection under large blowing ratio, while the is also higher. The basic distribution features of heat transfer coefficients are similar for all the four models. Heat transfer coefficient in the region where the jet core flows through is relatively lower, while in the jet edge region is relatively higher. For the models with small hole pitch, the laid-back holes only give better film coverage performance than the cylindrical holes under large blowing ratio. For the models with large hole pitch, the advantage of laid-back holes in film cooling effectiveness is more obvious in the upstream region relative to the cylindrical holes. For the cylindrical hole model and the laid-back hole model with the same hole pitch, heat transfer coefficients are nearly the same with each other under the same blowing ratios. Compared with the models with large hole pitch, the laterally averaged film cooling effectiveness and heat transfer coefficient are larger for the models with small hole pitch because of larger proportion of film covering area and strong heat transfer region.

Influence of shaped injection holes on turbine blade leading edge film cooling

To improve the film cooling performance by shaped injection holes for the turbine blade leading edge region, we have investigated the flow characteristics of the turbine blade leading edge film cooling using five different cylindrical body models with various injection holes, which are a baseline cylindrical hole, two laidback (spanwise-diffused) holes, and two tear-drop shaped (spanwise- and streamwise-diffused) holes, respectively. Mainstream Reynolds number based on the cylinder diameter was 7.1 × 10 4 and the mainstream turbulence intensities were about 0.2%. The effect of injectant flow rates was studied for various blowing ratios of 0.7, 1.0, 1.3 and 1.7, respectively. The density ratio in the present study is nominally equal to one. Detailed temperature distributions of the cylindrical body surfaces are visualized by means of an infrared thermography (IRT). Results show that the conventional cylindrical holes have poor film cooling performance compared to the shaped holes. Particularly, it can be concluded that the laidback hole (Shape D) provides better film cooling performance than the other holes and the broader region of high effectiveness is formed with fairly uniform distribution.