Increase In Film Cooling Effectiveness Research Articles

Experimental and numerical study on the effects of shaped multi-holes on the effectiveness of film cooling

In the present work, experimental and numerical study has been performed to investigate the effects of a novel film-cooling design with shaped multi-holes on the film-cooling effectiveness over a flat plate. A single cylindrical hole with 11.1 mm diameter has been replaced by multi-holes (14 holes with 2.97 mm diameter) with elliptical and fan-shaped configurations. The exit area, length to diameter ratio (L/D = 4.0) and injection angle (35°) of multi-holes are selected identical to the single cylindrical hole. The multi-holes were machined with a fixed spacing pitch (33.3 mm) between the centers of two adjacent holes. The surface temperature of the test plate was measured by an infrared camera. The experimental studies were performed at blowing ratios of 0.5 and 2. The numerical results based on the steady RANS with realizable k–e turbulence model and enhanced wall treatment were capable to predict the influence of the shape of multi-hole configurations on overall adiabatic film-cooling effectiveness. The experimental and numerical results showed that replacing a single hole with a shaped multi-hole leads to a considerable increase in film-cooling effectiveness in both axial and lateral directions. According to the results at blowing ratio of 2.0, the elliptical and fan-shaped configurations provide a higher area-averaged film-cooling effectiveness by 75% and 248.2% in comparison with the single hole, respectively.

LES of rotating film-cooling performance in a 1-1/2 turbine stage

The large eddy simulation method was employed to investigate the film-cooling performance in a low-speed rotor blade of a 1-1/2 turbine stage. The rotor blade height and axial chord length were 99 mm and 124.3 mm, respectively. Two rows of film holes were placed on the rotor blade surface, one each on the pressure and suction surfaces. Each row had three cylindrical film holes with a diameter of 4 mm and a tangential injection angle of 28° on the pressure side and 36° on the suction side. The Reynolds number was fixed at Re=1.92×105 and the coolant-to-mainstream density ratio (DR) was about 2.0. Simulations were carried out for three different rotating speeds of 1800, 2100, and 2400 rpm with the blowing ratio (BR) varying from 0.3 to 3.0. The commercial CFD code STAR-CCM+ was used to run the simulations using the WALE subgrid-scale model for modelling the turbulence. The results show that on the pressure side, the film coverage and film-cooling effectiveness decrease with increasing rotation number (Ro) and increase with increasing blowing ratio (BR). A higher Ro and lower BR result in a stronger film deflection. The film injection with higher BR produces better film attachment. The film deflects centrifugally where the deflection becomes greater with increasing Ro. On the suction side, the film coverage and film-cooling effectiveness increase with increasing either Ro or BR and a centripetal deflection of the film is observed. The deflection of the film path could be amplified by either increasing the Ro at a constant BR or decreasing the BR at a constant Ro. Increasing the rotation weakens the film deflection towards the hub on the suction surface. Overall, it was found that both rotation number and blowing ratio play significant roles in determining the film-cooling effectiveness distributions of the rotor blade surface.

Unsteady Wake and Coolant Density Effects on Turbine Blade Film Cooling Using Pressure Sensitive Paint Technique

The effect of an unsteady stator wake (simulated by wake rods mounted on a spoke-wheel wake generator) on the modeled rotor blade is studied using the pressure sensitive paint (PSP) mass-transfer analogy method. Emphasis of the current study is on the midspan region of the blade. 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 blade also has radial shower-head leading edge film-cooling holes. Strouhal numbers studied range from 0 to 0.36; the exit Reynolds number based on the axial chord is 530,000. Blowing ratios range from 0.5 to 2.0 on the suction side and 0.5 to 4.0 on the pressure side. Density ratios studied range from 1.0 to 2.5, to simulate actual engine conditions. The convex suction surface experiences film-cooling jet lift-off at higher blowing ratios, resulting in low effectiveness values. The film coolant is found to reattach downstream on the concave pressure surface, increasing effectiveness at higher blowing ratios. Results show deterioration in film-cooling effectiveness due to increased local turbulence caused by the unsteady wake, especially on the suction side. Results also show a monotonic increase in film-cooling effectiveness on increasing the coolant to mainstream density ratio.

Effect of Upstream Step on Flat Plate Film-Cooling Effectiveness Using PSP

The effect of a step positioned upstream of a row of film-cooling holes on the film-cooling effectiveness is studied systematically using the steady state pressure sensitive paint technique. The upstream step effect is studied on four separate hole geometries: simple angled (axial angle of 30 deg) and compound angled (axial angle of 30 deg and compound angle of 45 deg) and cylindrical and fan-shaped film-cooling holes. Each plate considered has seven holes, each hole 4 mm in diameter. The plates with cylindrical holes have a spacing of 3 diameters (12 mm) between the centers of two consecutive holes while the fan-shaped holes have a spacing of 3.75 diameters (15 mm). Three different step heights (12.5%d, 25%d, and 37.5%d) are studied. The effect of the width of the step is also studied; the distance of the step upstream of the hole and the positioning of the step downstream of the film-cooling hole. Four separate blowing ratios are reported for all tests: M=0.3, M=0.6, M=1.0, and M=1.5. All studies have been conducted with a mainstream of 25 m/s velocity at an ambient temperature of 22°C. Results indicate an increase in film-cooling effectiveness in the region near the hole due to the upstream step for all the plates considered. This increase due to the step is found to be most significant in the case of compound angled cylindrical holes and least significant in the case of simple angled fan-shaped holes.

신경회로망기법을 사용한 원통형 막냉각 홀의 최적설계

This study presents a numerical procedure to optimize the shape of cylindrical cooling hole to enhance film-cooling effectiveness. The RBNN method is used as an optimization technique with Reynolds-averaged Navier-Stokes analysis of fluid flow and heat transfer with shear stress transport turbulent model. The hole length-to-diameter ratio and injection angle are chosen as design variables and film-cooling effectiveness is considered as objective function which is to be maximized. Twelve training points are obtained by Latin Hypercube Sampling for two design variables. In the sensitivity analysis, it is found that the objective function is more sensitive to the injection angle of hole than the hole length-to diameter ratio. Optimum shape gives considerable increase in film-cooling effectiveness.

Effects of injection angle orientation on concave and convex surfaces film cooling

The present study employs a transient liquid crystal thermography to measure film cooling performance over constant curvature of concave and convex surfaces. This work investigates detailed distributions of both film cooling effectiveness and heat transfer coefficient on concave and convex surfaces with one row of injection holes inclined stream-wise at 35° at four blowing ratios (0.5, 1.0, 1.5 and 2.0) on four test pieces with different hole configurations. All test models have a row of discrete holes with a stream-wise injection angle ( γ of 35° and a pitch-to-diameter ratio ( P/ d) of 3. The current work examines four different injection configurations, one with simple and three with 8° forward-expanded holes. Three compound angles of 0, 45 and 90° with air ( ρ c/ ρ m = 0.98) as coolants are tested under the mainstream Reynolds number ( Re d) of 2300 on concave surface, and 1700 on convex surface. Measured results of the concave surface show that both the span-wise averaged heat transfer coefficient and film cooling effectiveness increase with blowing ratios for all tested models. Higher heat transfer levels induced by large flow disturbance of compound-angle injection also lead to poorer overall film cooling performance, especially at high blowing ratio and large span-wise injection angle. Present results show that the best surface protection on the concave surface over the widest range of M can be provided by the forward-expanded holes with β = 0° (Model-B), followed by the forward-expanded holes with β = 45° (Model-C). Convex surface results show that the compound-angle injection indicates increases in both film cooling effectiveness and heat transfer at moderate and high blowing ratios. The forward-expanded hole with simple-angle injection provides the best film performance because of high film cooling effectiveness and low heat transfer coefficient at blowing ratio of 0.5.

A Transient Method Using Liquid Crystal for Film Cooling Over a Convex surface

In order to explore the effect of blowing ratio on film cooling over a convex surface, the present study adopts the transient liquid crystal thermography for the film cooling measurement on a straight circular hole configuration. The test piece has a strength of curvature (2r/D) of 92.5, pitch to diameter ratio (P/D) of 3 and streamwise injection angle (γ) of 35∘ All measurements were conducted under the mainstream Reynolds number (Red) of 1700 with turbulence intensity (Tu) of 3.8%, and the density ratio between coolant and mainstream (ρc/ρm) is 0.98. In current study, the effect of blowing ratio (M) on film cooling performance is investigated by varying the range of blowing ratio from 0.5 to 2.0. Two transient tests of different injection flow temperature were conducted to obtain both detailed heat transfer coefficient and film cooling effectiveness distributions of measured region. The present measured results show that both the spanwise averaged heat transfer coefficient and film cooling effectiveness increase with decreased blowing ratio.

Effect of Unsteady Wake on Detailed Heat Transfer Coefficient and Film Effectiveness Distributions for a Gas Turbine Blade

Unsteady wake effects on detailed heat transfer coefficient and film cooling effectiveness distributions from a gas turbine blade with film cooling are obtained using a transient liquid crystal technique. Tests were performed on a five-blade linear cascade at a axial chord Reynolds number of 5.3 × 105 at cascade exit. Upstream unsteady wakes are simulated using a spoke-wheel type wake generator. The test blade has three rows of film holes on the leading edge and two rows each on the pressure and suction surfaces. Air and CO2 were used as coolants to simulate different coolant-to-mainstream density ratio effect. Coolant blowing ratio for air injection is varied from 0.8 to 1.2 and is varied from 0.4 to 1.2 for CO2. Results show that Nusselt numbers for a film-cooled blade are much higher compared to a blade without film injection. Particularly, film injection causes earlier boundary layer transition on the suction surface. Unsteady wakes slightly enhance Nusselt numbers but significantly reduce film cooling effectiveness on a film-cooled blade compared with a film-cooled blade without wakes. Nusselt numbers increase slightly but film cooling effectiveness increase significantly with an increase in blowing ratio for CO2 injection. Higher density coolant (CO2) provides higher effectiveness at higher blowing ratios (M = 1.2) whereas lower density coolant (Air) provides higher effectiveness at lower blowing ratios (M = 0.8).

Flat-Plate Film Cooling With Steam Injection Through One Row and Two Rows of Inclined Holes

Experiments have been performed to investigate the film-cooling characteristics with steam injection through one row (7 tubes) and two rows (13 tubes) of holes, inclined at an angle of 35 deg, over a flat plate. The spacing between the holes as well as the distance between the rows is 2 1/2 hole diameters. Data have been obtained for both steam and air film-cooling effectiveness at different axial and lateral locations downstream of the injection holes. The blowing rate M varied from 0.2 to 1.5. In the case of one-row injection, the results show that the film-cooling effectiveness with steam injection is about 50 to 100 percent higher than that with air injection at downstream locations, depending upon the blowing rate; however, the increase in film-cooling effectiveness is reduced near the injection hole region at high blowing rates. In the case of two-row injection, the laterally averaged film cooling effectiveness η can be correlated with the two-dimensional film-cooling parameter ξ. The η with steam injection is about 80 to 100 percent higher than that with air injection at low blowing rates and/or at downstream locations (ξ ≥ 15). However, the increase in η with steam injection is reduced near the injection hole region and/or at high blowing rates (ξ ≤ 15).