B. A. Jubran

A Numerical Study on Improving Large Angle Film Cooling Performance through the Use of Sister Holes

The present study evaluates a novel sister hole cooling technique using large inclination angle cylindrical holes simulated numerically. Here, a 55° inclination angle has been applied to simulate more realistic turbine conditions. Two sister holes bound the primary injection hole and are shifted slightly downstream to promote flow adhesion. As a means of determining the validity of the technique, adiabatic effectiveness and vortex flow structures were evaluated at four blowing ratios: 0.2, 0.5, 1.0, and 1.5. The results indicate that the sister hole technique dramatically reduces the primary kidney vortex pair offering significant improvements in effectiveness at all blowing ratios.

Computational Investigation of Film Cooling from Trenched Holes Near the Leading Edge of a Turbine Blade

Computational results are presented for a row of coolant injection holes on each side of a symmetrical turbine blade model near the leading edge. Four film cooling configurations, (1) a cylindrical film hole, (2) a forward diffused film hole, (3) a cylindrical film hole within a transverse slot, and (4) a forward-diffused film hole within a transverse slot, are used. Also, the effect of slot width is investigated. All simulations are at a fixed density ratio of 1.0, a blowing ratio of 0.5, and a pitch-to-diameter ratio of 5.0. Computational solutions of the steady, Reynolds-averaged Navier-Stokes equations are obtained using a finite-volume method. It is found that the shape of the hole and the integration of the holes with a continuous slot can significantly affect the film cooling flow over the protected surface. Also, it is found that when the slot width is almost equal to the elliptic footprint major axis, where the film hole exit is located, the shaped hole within the slot has the highest cooling effectiveness, but in other slot widths the advantage of this kind of hole configuration disappears.

Reynolds Stress Transport Modeling of Film Cooling at the Leading Edge of a Symmetrical Turbine Blade Model

This paper investigates the performance of the SSG (Speziale, Sarkar, and Gatski) Reynolds Stress Model for the prediction of film cooling at the leading edge of a symmetrical turbine blade model using the CFX 5.7.1 package from ANSYS, Inc. Using a finite-volume method, the performance of the selected turbulence model is compared to that of the standard k−ϵ model. The test case blade model is symmetric and has one injection row of discrete cylindrical holes on each side near the leading edge. Numerical simulations are conducted for three different blowing ratios; film cooling effectiveness contours on the blade surface and lateral averaged adiabatic film cooling effectiveness are presented and compared with available measurements. The computations with the standard k−ϵ model reproduce the well-known underpredicted lateral spreading of the jet, and, consequently, lower values of the lateral averaged adiabatic film cooling effectiveness has been obtained. On the other hand, the second order Reynolds Stress Model yields reasonably good agreement with measurement data. In addition to validation data, several longitudinal and transversal contours and vector planes are reproduced and clearly underscore the anisotropic turbulent field occurring in the present shower head film cooling configuration.

A Numerical Study on Increasing Film Cooling Effectiveness Through the Use of Sister Holes

Film cooling has been the primary focus of turbine blade cooling research for the past half century. However, as engines become more powerful, more effective non-traditional means of cooling become necessary. The current study branches out into a new scheme for film cooling; sister holes. The geometry of the current work makes use of three cylindrical holes inclined at 35° to the horizontal: one primary injectant hole bound by two sister holes. Numerical simulations were run with blowing ratios varying from M = 0.2 to M = 1.5, using the realizable k-e turbulence model with near-wall modeling. The results were analyzed for both adiabatic thermal effectiveness as well as vortex production due to flow mixing. In general, sister holes offer significant advantages in thermal protection over their single hole counterparts both laterally and along the centre-line, particularly in regions close to the hole. Simulations showed that the laterally averaged adiabatic thermal effectiveness increased by a factor of 1.35 for M = 0.2 up to a factor of 1.62 for M = 1.5. Similarly, the centre-line effectiveness increased by a factor of 1.22 at M = 0.2 up to a factor of 1.68 at M = 1.5. These benefits are heavily weighted by the near-hole region; however, increases are evident throughout the computational domain. This sister hole technique offers significant advantages with minimal penalties, making it a valuable candidate for future blade cooling applications.Copyright

Influence of Trenched Shaped Holes on Turbine Blade Leading Edge Film Cooling

Computational results are presented for a row of coolant injection holes on each side of a high-pressure turbine blade near the leading edge. Seven hole configurations have been used to show the effect of various diffusion shaped holes and their trenching on film cooling effectiveness: (1) cylindrical film hole; (2) forward diffused film hole; (3) trenched forward diffused film hole; (4) conically flared film hole; (5) trenched conically flared film hole; (6) laterally diffused film hole; and (7) trenched laterally diffused film hole. Computational solutions of the steady, Reynolds-averaged Navier–Stokes equations are obtained using a finite-volume method. Results show that the main effect of trenching is the reduction of jet lifting off from the blade surface and so the prevention of sudden lowering of cooling effectiveness after the injection location. Moreover, hole trenching has more effect on film cooling flow on the suction side than on the pressure side. Also, the trenched laterally diffused shaped hole has the highest laterally averaged effectiveness on both the suction side and the pressure side of the blade.

A Parametric Study on the Effect of Sister Hole Location on Active Film Cooling Flow Control

This paper presents an investigation on the effect of sister holes on film cooling. The proposed technique surrounds a primary injection hole by two or four smaller sister holes to actively maintain flow adhesion along the surface of the blade. A numerical evaluation using the realizable k-e turbulence model led to the determination that the use of sister holes significantly improves adiabatic effectiveness by countering the primary vortical flow structure. Research was performed to determine the optimal hole configuration, arriving at the conclusion that placing sister holes slightly downstream of the primary injection hole improves the near-hole effectiveness, while placing sister holes slightly upstream of the primary hole improves downstream effectiveness. On the whole, the sister hole approach to film cooling was found to offer viable improvements over standard cooling regimes.Copyright

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.

Film Cooling From Short Holes With Sister Hole Influence

This paper reports a computational analysis on the effect of sister hole control on film cooling from short holes. The proposed method includes surrounding a primary injection hole by two or four smaller sister holes to actively maintain flow adhesion along the surface of the blade. A numerical study using the realizable k-e turbulence model led to the determination that the use of sister holes significantly improves adiabatic effectiveness by countering the primary vortical flow structure. Research was carried out to determine the optimum hole configuration, arriving at the conclusion that placing sister holes slightly downstream of the primary injection hole improves the near-hole effectiveness, while placing sister holes slightly upstream of the primary hole improves downstream effectiveness. Similar results were found in evaluating both long and short hole geometries with a significantly less coherent flow field arising from the short hole. However, on the whole, the sister hole approach to film cooling was found to offer viable improvements over standard cooling regimes.Copyright

A Numerical Study of an Impingement Array Inside a Three Dimensional Turbine Vane

A simplified impingement high pressure turbine vane is modeled and solved via Fluent. A relatively flat section of the vane is fitted with 15 0.51mm diameter impingement holes — 5 rows of 3 jets. Results are then compared to known experimental data. Two different turbulence models are used to study this preliminary configuration: K-omega SST and the RNG k-epsilon model. The jet exit Reynolds numbers, cross flow velocity, and the average and local heat transfer distribution are analyzed with varying Reynolds numbers and jet to target spacing. It is observed that the static pressure decreases across the vane with the cross flow velocity increasing towards the trailing edge exit, thereby uniformly increasing the jet exit velocity at each row. Forced convection is seen in the downstream rows in-between span-wise jets due to high cross flow velocities. All numerical results were capable of replicating the higher heat transfer obtained with a higher Reynolds number, and conversely, a lower heat transfer with an increase in jet to target spacing. In its entirety, validating against all correlations, the RNG model obtained an average deviation of 15.7%, while the K-omega SST yielded only 7.8%.Copyright

Film Cooling From Circular and Elliptical Exit Shaped Holes With Sister Hole Influence

In traditional film cooling configuration, coolant is injected through a cylindrical pipe with an inclined angle (0<α<90), which results in an elliptical exit shaped hole (EESH) at the blade surface. The present study makes use of an elliptical injection coolant pipe that leads to a circular exit shaped hole (CESH). The film cooling effectiveness and the associated flow for both cases of circular and elliptical shaped holes are numerically investigated. A comparison between the predicted results and the available experimental results from the literature for blowing ratios of M = 0.5 and 1, clearly indicated a better agreement with the experimental results when the realizable k-e model was used. Further, the results indicate that the circular exit shaped hole improves the centerline and laterally averaged adiabatic effectiveness, particularly, at a higher bowing ratio of 1. The analysis of the vortex generation downstream of the jet for both exit shaped holes, shows a considerable decrease in the jet lift-off where the coolant flow tends to adhere more to the surface and hence, provides a better film cooling protection for the circular exit shaped hole, in comparison with the common elliptical exit shaped hole. The influence of sister holes on film cooling performance tends to be more effective with circular exit shaped hole than that with elliptical exit shaped hole.Copyright