Darius D. Sanders

Predicting Separation and Transitional Flow in Turbine Blades at Low Reynolds Numbers

There is increasing interest in design methods and performance prediction for aircraft engine turbines operating at low Reynolds numbers. In this regime, boundary layer separation may be more likely to occur in the turbine flow passages. For accurate CFD predictions of the flow, correct modeling of laminar-turbulent boundary layer transition is essential to capture the details of the flow. To investigate possible improvements in model fidelity, CFD models were created for the flow over two low pressure turbine blade designs. A new three-equation eddy-viscosity type turbulent transitional flow model originally developed by Walters and Leylek was employed for the current RANS CFD calculations. Previous studies demonstrated the ability of this model to accurately predict separation and boundary layer transition characteristics of low Reynolds number flows. The present research tested the capability of CFD with the Walters and Leylek turbulent transitional flow model to predict the boundary layer behavior and performance of two different turbine cascade configurations. Flows over the Pack-B turbine blade airfoil and the midspan section of a typical low pressure turbine (TLPT) blade were simulated over a Reynolds number range of 15,000–100,000, and predictions were compared to experimental cascade results. The turbulent transitional flow model sensitivity to turbulent flow parameters was investigated and showed a strong dependence on free-stream turbulence intensity with a second order effect of turbulent length scale. Focusing on the calculation of the total pressure loss coefficients to judge performance, the CFD simulation incorporating Walters and Leylek’s turbulent transitional flow model produced adequate prediction of the Reynolds number performance for the TLPT blade cascade geometry. Furthermore, the correct qualitative flow response to separated shear was observed for the Pack-B blade airfoil. Significant improvements in performance predictions were shown over predictions of conventional RANS turbulence models that cannot adequately model boundary layer transition.

OPERATION OF A PLASMA TORCH FOR SUPERSONIC COMBUSTION APPLICATIONS WITH A SIMULATED CRACKED JP-7 FEEDSTOCK

Research conducted at Virginia Tech has examined plasma torch operational characteristics using a feedstock gas of mixed hydrocarbons representing a cracked JP-7 surrogate. The tests are part of a program to examine the torch as an igniter and flame-holder for hydrocarbon-fueled SCRAMJET engines. Previous research has shown that the plasma torch has promise as a robust igniter and flame-holder for gaseous fuels such as methane, ethylene and propylene when combined with an aeroramp to assist with the combustion process. The present investigation tested the plasma torch with a feedstock mixture of hydrocarbons that simulates thermally cracked JP-7 jet fuel. This simulation of a cracked hydrocarbon fuel was studied to initiate planned work with liquid hydrocarbon fuel, which is of interest for today’s aerospace vehicles. The cracked JP-7 surrogate used consists of a 15/25/60 mixture of methane/ethane/ethylene. The research results include torch operational characteristics such as temperature and emission spectroscopy within the combustion plume, as well as the power supplied to the torch over a range of mass flow rates. Filtered still photographs of the emissions plume were studied to aid torch plume diagnostics. Other observations made are coking and erosion of the anode, which will help determine the potential lifespan of the torch using cracked JP-7 fuel. The results show successful operation over a range of powers with simulated cracked JP-7 feedstock flows. Measured spectra, current, and voltage are compared with similar results for other hydrocarbon feedstock gases. The torch operating on the JP-7 surrogate feedstock appears to be a satisfactory device for ignition, flame-holding, and combustion enhancement of cracked hydrocarbons in supersonic combustion.

A Mixing Plane Model Investigation of Separation and Transitional Flow at Low Reynolds Numbers in a Multistage Low Pressure Turbine

Flow separation with increased losses is known to occur when low pressure turbine (LPT) blades are operated at high altitudes with a reduced inlet Reynolds number. Under these conditions, boundary layer separation is more likely to be present within the flowfield of the LPT stages due to thickening of the boundary layers and an increase in the portion of the airfoil experiencing laminar flow. More accurate CFD predictions are needed in order to improve design methods and performance prediction for LPT stages operating at low Reynolds numbers. Steady flow CFD simulations of multistage LPT flow were completed at nominal and high altitude conditions with the conventional Spalart-Allmaras turbulence model. This model was used in combination with a mixing plane model for the simulation of flow through domains with one or more regions in relative rotational motion. Flow visualizations were completed using surface flow and streamline calculations to help identify vortical structures present within the flowfield. Also, the total pressure loss coefficient was calculated for each blade row. Qualitative comparisons indicate that the simulated high altitude condition had an increase in the amount of separated flow present within the flowfield compared to the nominal altitude condition. This can be attributed to the reduction in the inlet Reynolds number. Initial investigations with a recently-developed three-equation eddy-viscosity type turbulent transitional flow model are also reported. Comparisons of flow predictions for the 1st turbine stage with the two models revealed that large vortices predicted with the Spalart-Allmaras model were not present, and the wake loss coefficient was significantly lower with the three-equation turbulence model. Based on these and previous results, the CFD with the three-equation model is considered to have potential to provide improved prediction of separation and transitional flow in low Reynolds number turbine applications.

An Investigation of Controlled Oscillations in a Plasma Torch for Combustion Enhancement

An oscillating plasma torch was investigated as a device to produce an oscillating shock and resulting dynamic input to the supersonic combustion process. Several experiments were performed and the results were analyzed. The aim of the research was to thoroughly investigate the oscillation behavior of the plasma torch with the plan of controlling the oscillation at chosen frequencies. A modulating power system for dynamic control of the plasma torch oscillation was designed and tested in quiescent conditions (no flow), Mach 2.4 cold supersonic flow, and Mach 2 heated supersonic flow conditions. The oscillating plasma torch used nitrogen feedstock and was operated over a frequency range of 2Hz-4kHz. A dynamic torch model using the hybrid Mayr-Cassie electric arc model was developed to predict the plasma torch electric arc response at appropriate frequencies for interaction with supersonic combustion. In quiescent conditions, the dynamic response of the plasma torch power system and plasma jet were characterized using signal processing techniques and high speed video imaging. High speed Schlieren images were used to determine the behavior of the oscillating plasma jet in Mach 2.4 cross flow and its influence on the induced shock structure. The nitrogen-and air-fed torches were integrated with a flush-walled 4-hole aerodynamic ramp injector using hydrogen and hydrocarbon fuels, and was tested at the University of Virginia Aerospace Research Lab (ARL) in heated Mach 2 supersonic flow. Unsteady pressure variations from the oscillating shock produced by the plasma torch were recorded and measurements of the static pressure of the combustion produced by the steady and oscillating plasma torch were obtained. The oscillating torch system performed well over a range of different flow conditions. It will enable active control input to the combustion process. The controllable unsteady blockage can provide a type of shock interaction that has been shown to increase turbulence and mixing augmentation.

Turbulence Model Comparisons for Mixing Plane Simulations of a Multistage Low Pressure Turbine Operating at Low Reynolds Numbers

There has been a need for improved flow prediction methods for low pressure turbine (LPT) blades operating at high altitudes with a reduced inlet Reynolds number. These conditions present an increased amount of laminar-to-turbulent transitional flow within the boundary layers on the LPT blade surfaces. Also, boundary layer separation is more likely to occur within the flowfield of the LPT stages due to the lower freestream velocities in the regions of adverse pressure gradients on the suction surfaces. More accurate predictions of aerodynamic losses due to low Reynolds effects are needed for CFD to provide more accurate input to the design process for LPT stages operating at high altitudes. Steady flow CFD simulations of flow in a multistage LPT geometry were completed at nominal and high altitude conditions with the conventional Spalart-Allmaras turbulence model and a recentlydeveloped three-equation eddy-viscosity type transitional flow model. These models were used in combination with a mixing plane model for the simulation of flow through a three stage low pressure turbine. Flow visualizations were completed using surface flow and streamline calculations to help identify vortical structures present within the flowfield. Also, the total pressure loss coefficient was calculated for each blade row. Qualitative comparisons indicate the amount and the location of the flow separation differed significantly depending on the chosen turbulence model. Overall, the high altitude condition had an increased amount of separated flow compared to the nominal altitude condition resulting in an increase in the loss coefficient. The altitude effect on the laminar-to-turbulent transition location was studied using the three-equation model. The model provided a more detailed understanding of the aerodynamic loss mechanisms present in low Reynolds number flows, since it accounted for transitional boundary layer flow effects. Based on the these results, the CFD using the three-equation model has the potential to be a more effective method for turbine flow prediction at low Reynolds numbers compared to conventional RANS turbulence models.