Jason E. Dees

Turbine Airfoil Net Heat Flux Reduction With Cylindrical Holes Embedded in a Transverse Trench

Film cooling adiabatic effectiveness and heat transfer coefficients for cylindrical holes embedded in a 1d transverse trench on the suction side of a simulated turbine vane were investigated to determine the net heat flux reduction. For reference, measurements were also conducted with standard inclined, cylindrical holes. Heat transfer coefficients were determined with and without upstream heating to isolate the hydrodynamic effects of the trench and to investigate the effects of the thermal approach boundary layer. Also, the effects of a tripped versus an untripped boundary layer were explored. For both the cylindrical holes and the trench, heat transfer augmentation was much greater for the untripped approach flow. A further increase in heat transfer augmentation was caused by use of upstream heating, with as much as a 180% augmentation for the trench. The tripped approach flow led to much lower heat transfer augmentation than the untipped case. The net heat flux reduction for the trench was found to be significantly higher than for the row of cylindrical holes.

Heat Transfer Augmentation Downstream of Rows of Various Dimple Geometries on the Suction Side of a Gas Turbine Airfoil

ABSTRACT Heat transfer coefficients were measured downstream of a row of shaped film cooling holes as well as elliptical, diffuser, and teardrop shaped dimples simulating depressions due to film coolant holes of different shapes. These features were placed on the suction side of a simulated gas turbine vane. The dimples were used as approximations to film cooling holes after the heat transfer levels downstream of active fan shaped film cooling holes was found to be independent of film cooling. The effects of the dimples were tested with varying approach boundary layers, freestream turbulence intensity, and Reynolds numbers. For the case of an untripped (transitional) approach boundary layer, all dimple shapes caused approximately a factor of two increase in heat transfer coefficient relative to the smooth baseline condition due to the dimples effectively causing boundary layer transition downstream. The exact augmentation varied depending on the dimple geometry: diffuser shapes causing the largest augmentation and teardrop shapes causing the lowest augmentation. For tripped (turbulent boundary layer) approach conditions, the dimple shapes all caused the same 20% augmentation relative to the smooth tripped baseline. The already turbulent nature of the tripped approach flow reduces the effect that the dimples have on the downstream heat transfer coefficient.

A Detailed Uncertainty Analysis of Adiabatic Film Cooling Effectiveness Measurements Using Pressure Sensitive Paint

Pressure sensitive paint (PSP) can be a powerful tool in measuring the adiabatic film cooling effectiveness. There are two distinct sources of error for this measurement technique; the ability to experimentally obtain the data and the validity of the heat and mass transfer analogy for the problem being studied. This paper will assess the experimental aspect of this PSP measurement specifically for film cooling applications.Experiments are conducted in an effort to quantifiably bound expected errors associated with temperature non-uniformities in testing and photo-degradation effects. Results show that if careful experimental procedures are put in place, both of these effects can be maintained to have less than 0.022 impact on effectiveness.Through accurate semi-in-situ calibration down to 4% atmospheric pressure, the near-hole distribution of effectiveness is measured with high accuracy. PSP calibrations are performed for multiple coupons, over multiple days. In addition, to reach a partial pressure of 0 the calibration vessel was purged of all air by flowing CO2.The primary contribution of this paper lies in the uncertainty analysis performed on the PSP measurement technique. A thorough uncertainty analysis is conducted and described, in order to completely understand the presented measurements and any shortcomings of the PSP technique. This quantification results in larger, albeit more realistic, values of uncertainty, and helps provide a better understanding of film cooling effectiveness measurements taken using the PSP technique. The presented uncertainty analysis takes into account all random error sources associated with sampling and calibration, from intensities to effectiveness.Adiabatic film cooling effectiveness measurements are obtained for a single row of film cooling holes inclined at 20 degrees, with CO2 used as coolant. Data is obtained for six blowing ratios. Maps of uncertainty corresponding to each effectiveness profile are available for each test case. These maps show that the uncertainty varies spatially over the test surface, high effectiveness corresponds to low uncertainty. The noise floors can be as high as 0.04 at effectiveness levels of 0. Day-to-day repeatability is presented for each blowing ratio and shows that laterally averaged effectiveness data is repeatable within 0.02 effectiveness.Copyright

Overall and Adiabatic Effectiveness Values on a Scaled Up, Simulated Gas Turbine Vane: Part II—Numerical Simulations

Conjugate heat transfer (CHT) calculation techniques for the heat transfer analysis of high-pressure turbines (HPT) have been developed during the past few years. Thus, it has become possible to take into account the coupling of the film, internal cooling, external gas flow and the metal diffusive heat transfer. The coupling problem may become extremely important in regions such as the airfoil leading edge and the vicinity of film hole breakout region where heat fluxes and thermal gradients are high. This article presents the results obtained using fully coupled 3D CHT simulations of a simplified film-cooled leading edge model and a NASA C3X vane with suction side film cooling. The results for the two cases are compared against experimental data obtained at University of Texas at Austin. The numerical simulations were conducted using the k-ω turbulence model. The leading edge model overall effectiveness predictions are in good agreement with the experiments, especially in the low blowing ratio range (1≤M≤2). For the C3X vane, the CHT results tend to underpredict the midspan and laterally averaged effectiveness due to film liftoff. However, the quantitative agreement is still reasonably good. The different levels of overall effectiveness agreement found between all cases are discussed.Copyright

The Effect of Internal Crossflow on the Adiabatic Effectiveness of Compound Angle Film Cooling Holes

In gas turbine engines, film cooling holes are often fed by an internal crossflow, with flow normal to the direction of the external flow around the airfoil. Many experimental studies have used a quiescent plenum to feed model film cooling holes and thus do not account for the effects of internal crossflow. In this study, an experimental flat plate facility was constructed to study the effects of internal crossflow on a row of cylindrical compound angle film cooling holes. There are relatively few studies available in literature that focus on the effects of crossflow on film cooling performance, with no studies examining the effects of internal crossflow on film cooling with round, compound angled holes. A crossflow channel allowed for coolant to flow alternately in either direction perpendicular to the mainstream flow. Experimental conditions were scaled to match realistic turbine engine conditions at low speeds. Cylindrical compound angle film cooling holes were operated at blowing ratios ranging from 0.5 to 2.0 and at a density ratio (DR) of 1.5. The results from the crossflow experiments were compared to a baseline plenum-fed configuration. This study showed that significantly greater adiabatic effectiveness was achieved for crossflow counter to the direction of coolant injection.

The Effects of Conjugate Heat Transfer on the Thermal Field Above a Film Cooled Wall

Common gas turbine heat transfer analysis methods rely on the assumption that the driving temperature for heat transfer to a film cooled wall can be approximated by the adiabatic wall temperature. This assumption implies that the gas temperature above a film cooled adiabatic wall is representative of the overlying gas temperature on a film cooled conducting wall. This assumption has never been evaluated experimentally. In order for the adiabatic wall temperature as driving temperature for heat transfer assumption to be valid, the developing thermal boundary layer that exists above a conducting wall must not significantly affect the overriding gas temperature. In this paper, thermal fields above conducting and adiabatic walls of identical geometry and at the same experimental conditions were measured. These measurements allow for a direct comparison of the thermal fields above each wall in order to determine the validity of the adiabatic wall temperature as driving temperature for heat transfer assumption. In cases where the film cooling jet was detached, a very clear effect of the developing thermal boundary layer on the gas temperature above the wall was measured. In this case, the temperatures above the wall were clearly not well represented by the adiabatic wall temperature. For cases where the film cooling jet remained attached, differences in the thermal fields above the adiabatic and conducting wall were small, indicating a very thin thermal boundary layer existed beneath the coolant jet.Copyright

Effects of Regular and Random Roughness on the Heat Transfer and Skin Friction Coefficient on the Suction Side of a Gas Turbine Vane

Skin friction coefficients and heat transfer coefficients are measured for a range of regular and random roughnesses on the suction side of a simulated gas turbine vane. The skin friction coefficients are calculated using boundary layer data and the momentum integral method. High resolution surface temperature data measured with an IR camera yield local heat transfer values. 80 grit, 50 grit, 36 grit, and 20 grit sandpapers along with a regular array of conical roughness elements are tested. Measured skin friction coefficient data show that the conical roughness array behaves very similar to the 50 grit, 36 grit, and 20 grit sandpapers in terms of the effect of the roughness on the hydrodynamic boundary layer. In terms of heat transfer, the conical roughness array is most similar to the 80 grit sandpaper, which are both lower than the roughest sandpapers tested. These data show that the particular regular array of roughness elements tested has fundamentally different behavior than randomly rough surfaces for this position on the simulated turbine vane. In addition, this difference is in the opposite direction as seen in previous experimental studies. In order to draw a more general conclusion about the nature of random and regular roughness, a parametric study of regular roughness arrays should be performed.

A Detailed Uncertainty Analysis of Adiabatic Film Cooling Effectiveness Measurements Using Pressure-Sensitive Paint

Pressure-sensitive paint (PSP) can be a powerful tool in measuring the adiabatic film cooling effectiveness. There are two distinct sources of error for this measurement technique: the ability to experimentally obtain the data and the validity of the heat and mass transfer analogy for the problem being studied. This paper will assess the experimental aspect of this PSP measurement specifically for film cooling applications. Experiments are conducted in an effort to quantifiably bound expected errors associated with temperature nonuniformities in testing and photodegradation effects. Results show that if careful experimental procedures are put in place, both of these effects can be maintained to have less than 0.022 impact on effectiveness. Through accurate semi in situ calibration down to 4% atmospheric pressure, the near-hole distribution of effectiveness is measured with high accuracy. PSP calibrations are performed for multiple coupons, over multiple days. In addition, to reach a partial pressure of zero the calibration vessel was purged of all air by flowing CO2. The primary contribution of this paper lies in the uncertainty analysis performed on the PSP measurement technique. A thorough uncertainty analysis is conducted and described, in order to completely understand the presented measurements and any shortcomings of the PSP technique. This quantification results in larger, albeit more realistic, values of uncertainty and helps provide a better understanding of film cooling effectiveness measurements taken using the PSP technique. The presented uncertainty analysis takes into account all random error sources associated with sampling and calibration, from intensities to effectiveness. Adiabatic film cooling effectiveness measurements are obtained for a single row of film cooling holes inclined at 20 deg, with CO2 used as coolant. Data are obtained for six blowing ratios. Maps of uncertainty corresponding to each effectiveness profile are available for each test case. These maps show that the uncertainty varies spatially over the test surface and high effectiveness corresponds to low uncertainty. The noise floors can be as high as 0.04 at effectiveness levels of 0. Day-to-day repeatability is presented for each blowing ratio and shows that laterally averaged effectiveness data are repeatable within 0.02 effectiveness.

Experimental Measurements and Computational Predictions for an Internally Cooled Simulated Turbine Vane

In this study the conjugate heat transfer effects for an internally cooled vane were studied experimentally and computationally. Experimentally, a large scale model vane was used with an internal cooling configuration characteristic of real gas turbine airfoils. The cooling configuration employed consisted of a U-bend channel for cooling the leading edge region of the airfoil and a radial channel for cooling the middle third of the vane. The thermal conductivity of the solid was specially selected so that the Biot number for the model matched typical engine conditions. This ensured that scaled nondimensional surface temperatures for the model were representative of those in the first stage of a high pressure turbine. The performance of the internal cooling circuit was quantified experimentally for internal flow Reynolds numbers ranging from 10,000 to 40,000. The external surface temperature distribution was mapped over the entire vane surface. Additional measurements, including internal surface temperature measurements as well as coolant inlet and exit temperatures, were conducted. Comparisons between the experimental measurements and computational predictions of external heat transfer coefficient are presented.

Near-Hole Thermal Field Measurements for Round Compound Angle Film Cooling Holes Fed by Cross-Flow

The effectiveness of film cooling from short cooling holes, scaled to engine conditions, has been shown to be dependent on the nature of the internal coolant feed. A common method of supplying coolant to film cooling holes in engine components is through an internal cross-flow, which causes skewed effectiveness profiles on the surface of film cooled parts. For round axial holes, this effect causes coolant jets to more effectively spread across the surface. Additionally, for compound angle round holes, the direction of the cross-flow relative to the direction of injection has a substantial effect on film cooling effectiveness. A cross-flow directed counter to the span-wise direction of coolant injection has previously been shown to cause greater lateral jet spreading than cross-flow directed in-line with the span-wise injection direction. To better understand the phenomena responsible for the improved coolant spreading, two-dimensional thermal field profiles were measured downstream of compound angle film cooling holes fed by an internal cross-flow. A smooth-walled rectangular channel was used to produce an internal cross-flow in both a counter and in-line flow direction. Thermal field cross-section data was collected at three stream-wise locations: 0.7, 3.4, and 8.8 diameters downstream of the holes. Blowing ratios of 0.75 and 1.00 were studied at a density ratio of 1.5. Experiments were performed in a low speed recirculating wind tunnel at high mainstream turbulence with a thick approach boundary layer relative to the film cooling holes. It was found that the improved lateral spreading observed in the coolant jets fed by a counter cross-flow occurred due to the formation of a bulge on the downstream side of the jet.Copyright