Jason E. Albert

Net Heat Flux Reduction and Overall Effectiveness for a Turbine Blade Leading Edge

The external cooling performance of a film cooled turbine airfoil can be quantified as a net reduction in heat transfer relative to the turbine airfoil without film cooling. This quantification is generally accomplished by using measurements of the adiabatic effectiveness and the change in heat transfer coefficients (hf /h0 ) for the film cooled surface to determine the net heat flux reduction (Δqr ). Although measurement of Δqr for laboratory models give an indication of the ultimate film cooling performance, this does not show how much the surface temperature of the airfoil is reduced by film cooling. Measurement of scaled surface temperatures can be accomplished by using laboratory models constructed so that the Biot number is matched with that of the actual airfoil. These measurements provide a scaled temperature distribution on the airfoil that is referred to as the overall effectiveness, φ. For the current study, measurements of Δqr and φ have been made for a simulated turbine blade leading edge. The simulated leading edge incorporated shaped coolant holes, and had three rows of coolant holes. Improvements due to the shaped holes were determined by comparisons with previously measured round hole configurations. Spatially distributed hf /h0 show increases of 5% to 15% for M = 1.0 and 10% to 30% for M = 2.0. Results show that local variation in Δqr much greater than variation in φ, but laterally averaged Δqr distributions are reasonable predictors of the laterally averaged φ distributions.Copyright

Adiabatic and Overall Effectiveness for a Film Cooled Blade

Laboratory studies of film cooling performance for turbine section airfoils typically quantify adiabatic effectiveness and occasionally the heat transfer coefficient for the film cooling configuration. In this study the normalized airfoil metal surface temperatures are obtained directly by using a test model that has a material conductivity scaled to the external and internal heat transfer coefficients so that the Biot number for the model is similar to that for the actual airfoil. These results provide an experimental test case of the conjugate heat transfer involved in turbine airfoil cooling. In this study, conventional adiabatic effectiveness and the overall cooling effectiveness (normalized surface temperature for the matched Biot model) were measured for a generic blade leading edge using three rows of shaped holes. Distinct differences were found between the adiabatic effectiveness and overall cooling effectiveness. Also included is a practical application of this experimental method for which the degradation of overall cooling effectiveness due to a plugged cooling hole is examined.Copyright

Measurements of Adiabatic Film and Overall Cooling Effectiveness on a Turbine Vane Pressure Side With a Trench

Film cooling performance is typically quantified by separating the external convective heat transfer from the other components of the conjugate heat transfer that occurs in turbine airfoils. However, it is also valuable to assess the conjugate heat transfer in terms of the overall cooling effectiveness, which is a parameter of importance to airfoil designers. In the current study, adiabatic film effectiveness and overall cooling effectiveness values were measured for the pressure side of a simplified turbine vane model with three rows of showerhead cooling at the leading edge and one row of body film cooling holes on the pressure side. This was done by utilizing two geometrically identical models made from different materials. Adiabatic film effectiveness was measured using a very low thermal conductivity material, and the overall cooling effectiveness was measured using a material with a higher thermal conductivity selected such that the Biot number of the model matched that of a turbine vane at engine conditions. The theoretical basis for this matched-Biot number modeling technique is discussed in some detail. Additionally, two designs of pressure side body film cooling holes were considered in this study: a standard design of straight, cylindrical holes and an advanced design of “trenched” cooling holes in which the hole exits were situated in a recessed, transverse trench. This study was performed using engine representative flow conditions, including a coolant-to-mainstream density ratio of DR = 1.4 and a mainstream turbulence intensity of Tu = 20%. The results of this study show that adiabatic film and overall cooling effectiveness increase with blowing ratio for the showerhead and pressure side trenched holes. Performance decreases with blowing ratio for the standard holes due to coolant jet separation from the surface. Both body film designs have similar performance at a lower blowing ratio when the standard hole coolant jets remain attached. Far downstream of the cooling holes both designs perform similarly because film effectiveness decays more rapidly for the trenched holes.Copyright

Experimental Simulation of Contaminant Deposition on a Film Cooled Turbine Airfoil Leading Edge

A significant challenge of utilizing coal-derived synthetic fuels for gas turbine engines is mitigating the adverse effects of fuel-born contaminant deposits on film cooled turbine surfaces. A new experimental technique has been developed that simulates the key physical, but not the chemical, aspects of coal ash deposition on film cooled turbine airfoil leading edges in order to better understand the interaction between film cooling and deposition and to produce improved film cooling designs. In this large-scale wind tunnel facility, the depositing contaminants were modeled with atomized molten wax droplets sized to match the Stokes numbers of coal ash particles in the engine conditions. The sticking mechanism of the molten contaminants to the turbine surfaces was modeled by ensuring the wax droplets remained somewhat molten when they arrived at the cooled model surface. The airfoil model and wax deposits had thermal conductivities such that they matched the Biot numbers of clean and fouled turbine airfoils at engine conditions. The behavior of the deposit growth was controlled by adjusting the mainstream, coolant, and wax solidification temperatures. Simulated deposits were created for a range of test durations, film cooling blowing ratios, and controlling temperatures. Inspection of the resulting deposits revealed aspects of the flow field that augment and suppress deposition. Deposit thickness was found to increase in time until an equilibrium thickness was attained. Blowing ratio and the difference between mainstream and wax solidification temperatures strongly affected characteristics of the deposits. Model surface temperatures greatly reduced under the deposits as they developed.

Experimental Simulation of Contaminant Deposition on a Film Cooled Turbine Vane Pressure Side With a Trench

An important issue in the use of coal- or biomass-derived synthetic gaseous (syngas) fuels is the deposition of contaminants on film cooled turbine surfaces, which alter cooling and aerodynamic performance and increase material degradation. The current study applied a new experimental technique that simulated the key physical aspects of contaminant deposition on a film cooled turbine vane. The depositing contaminants were modeled in a wind tunnel facility with a spray of molten wax droplets of a size range that matched the Stokes number of the contaminant particles in engine conditions. Most experiments were performed using a vane model with a thermal conductivity selected such that the model had the same Biot number of an actual engine airfoil, resulting in a cooler surface temperature. Some experiments were performed using an approximately adiabatic model for comparison. The film cooling design consisted of three rows of showerhead cooling at the leading edge and one row of body film cooling holes on the pressure side. Two designs of pressure side body film cooling holes were considered: a standard design of straight, cylindrical holes and an advanced design of “trenched” cooling holes in which the hole exits were situated in a recessed, transverse trench. The results showed thin deposits formed in the trench, with the thickest deposits on its downstream wall between coolant jets. Adiabatic film effectiveness levels were essentially unchanged by the presence of deposits for either film configuration. Deposit formation was strongly influenced by the model surface temperature with cooler surfaces inhibiting deposition. There was evidence of a threshold surface temperature above which deposits became significantly thicker.Copyright

Experimental simulation of contaminant deposition on a film cooled turbine airfoil leading edge

A significant challenge of utilizing coal-derived synthetic fuels for gas turbine engines is mitigating the adverse effects of fuel-born contaminant depositions on film cooled turbine surfaces. A new experimental technique has been developed that simulates the key physical, but not the chemical, aspects of coal ash deposition on film cooled turbine airfoil leading edges in order to better understand the interaction between film cooling and deposition and to produce improved film cooling designs. In this large-scale wind tunnel facility, the depositing contaminants were simulated by atomized molten wax droplets sized to match the Stokes numbers of coal ash particles in the engine conditions. The sticking mechanism of the molten contaminants to the turbine surfaces was simulated by ensuring the wax droplets remained somewhat molten when they arrived at the cooled model surface. The model and wax deposits had thermal conductivities such that they matched the Biot numbers of clean and fouled turbine airfoils at engine conditions. The behavior of the deposition growth was controlled by adjusting the mainstream, coolant, and wax solidification temperatures. Simulated depositions were created for a range of test durations, film cooling blowing ratios, and controlling temperatures. Inspection of the resulting depositions revealed aspects of the flow field that augment and suppress deposition. Deposition thickness was found to increase in time until a quasi-steady thickness was attained. Blowing ratio and the difference between mainstream and wax solidification temperatures strongly affected characteristics of the depositions. Model surface temperatures greatly reduced under the depositions as they developed.Copyright