Andrew C. Nix

Experimental Measurements and Modeling of the Effects of Large-Scale Freestream Turbulence on Heat Transfer

The influence of freestream turbulence representative of the flow downstream of a modern gas turbine combustor and first stage vane on turbine blade heat transfer has been measured and analytically modeled in a linear, transonic turbine cascade. High intensity, large length-scale freestream turbulence was generated using a passive turbulence-generating grid to simulate the turbulence generated in modern combustors after passing through the first stage vane row. The grid produced freestream turbulence with intensity of approximately 10–12% and an integral length scale of 2 cm (Λx /c = 0.15) near the entrance of the cascade passages. Mean heat transfer results with high turbulence showed an increase in heat transfer coefficient over the baseline low turbulence case of approximately 8% on the suction surface of the blade, with increases on the pressure surface of approximately 17%. Time-resolved surface heat transfer and passage velocity measurements demonstrate strong coherence in velocity and heat flux at a frequency correlating with the most energetic eddies in the turbulence flow field (the integral length-scale). An analytical model was developed to predict increases in surface heat transfer due to freestream turbulence based on local measurements of turbulent velocity fluctuations and length-scale. The model was shown to predict measured increases in heat flux on both blade surfaces in the current data. The model also successfully predicted the increases in heat transfer measured in other work in the literature, encompassing different geometries (flat plate, cylinder, turbine vane and turbine blade) and boundary layer conditions.Copyright

Experimental measurements and modeling of the effects of large-scale freestream turbulence on heat transfer

The influence of freestream turbulence representative of the flow downstream of a modern gas turbine combustor and first stage vane on turbine blade heat transfer has been measured and analytically modeled in a linear, transonic turbine cascade. High-intensity, large length-scale freestream turbulence was generated using a passive turbulence-generating grid to simulate the turbulence generated in modern combustors after passing through the first stage vane row. The grid produced freestream turbulence with intensity of approximately 10-12% and an integral length scale of 2 cm (Λ x /c=0.15) near the entrance of the cascade passages. Mean heat transfer results with high turbulence showed an increase in heat transfer coefficient over the baseline low turbulence case of approximately 8% on the suction surface of the blade, with increases on the pressure surface of approximately 17%. Time-resolved surface heat transfer and passage velocity measurements demonstrate strong coherence in velocity and heat flux at a frequency correlating with the most energetic eddies in the turbulence flow field (the integral length scale). An analytical model was developed to predict increases in surface heat transfer due to freestream turbulence based on local measurements of turbulent velocity fluctuations and length scale. The model was shown to predict measured increases in heat flux on both blade surfaces in the current data. The model also successfully predicted the increases in heat transfer measured in other work in the literature, encompassing different geometries (flat plate, cylinder turbine vane, and turbine blade) and boundary layer conditions.

High Intensity, Large Length-Scale Freestream Turbulence Generation in a Transonic Turbine Cascade

Heat transfer predictions in gas turbine engines have focused on cooling techniques and on the effects of various flow phenomena. The effects of wakes, passing shock waves and freestream turbulence have all been of primary interest to researchers. The focus of the work presented in this paper is to develop a turbulence grid capable of generating high intensity, large-scale turbulence for use in experimental heat transfer measurements in a transonic facility. The grid is desired to produce freestream turbulence characteristic of the flow exiting the combustor of advanced gas turbine engines. A number of techniques are discussed in this paper to generate high intensity, large length-scale turbulence for a transonic facility. Ultimately, the passive grid design chosen is capable of producing freestream turbulence with intensity of approximately 10–12% near the entrance of the cascade passages with an integral length-scale of 2 cm.Copyright

Boundary-Layer Equation-Based Wall Model for Large-Eddy Simulation of Turbulent Flows with Wall Heat Transfer

A new thermal boundary-layer model is developed that alleviates the stringent near-wall grid resolution requirement in large-eddy simulations of turbulent flows with wall heat transfer. The model is based on solving the turbulent temperature boundary-layer equation to determine the temperature profile in the near-wall region. The near-wall temperature profile is used to compute the instantaneous wall heat flux, which replaces the temperature (Dirichlet) wall boundary condition specified a priori. This approach is analogous to the wall stress model developed by Balaras et al. [1], in which the instantaneous wall shear stresses replace the no-slip wall boundary conditions. Three benchmark turbulent flows are studied using coarse-grid large-eddy simulations coupled with the new thermal wall model: (1) a fully developed turbulent channel flow with a heated top wall, (2) a backward-facing step flow with a heated bottom wall, and (3) an impinging jet on a heated circular plate. Large-eddy simulations are performed using the commercial code CFD-ACE+, with the localized dynamic subgrid kinetic energy model of Kim and Menon [2] providing the subgrid stresses. Turbulence statistics are compared with benchmark data from direct numerical simulations and experiments and also with data from resolved large-eddy simulations (i.e., near-wall y + ≈ 1). Excellent agreement among the data is obtained.

A Preliminary Numerical Study on the Effect of High Freestream Turbulence on Anti-Vortex Film Cooling Design at High Blowing Ratio

Researchers at NASA Glenn Research Center have developed and investigated a novel film cooling design, the anti-vortex hole (AVH), which has been shown to cancel or counter the vorticity generated by conventional holes and increase film effectiveness at high blowing ratios and low turbulence levels. This paper presents preliminary CFD results on the film effectiveness and net heat flux reduction at high blowing ratio and elevated freestream turbulence levels for the adjacent AVH. Baseline cases at low turbulence levels of 5% intensity and length scale of Λx /dm = 1 with a nominal blowing ratio of 2 and a density ratios of 1 and 2 were compared to previous results reported by Heidmann [1]. Higher freestream turbulence cases were investigated with a turbulence intensity and length scale of 10% and Λx /dm = 1 and 3, respectively. Results showed that higher freestream turbulence improves adiabatic effectiveness for the AVH design.Copyright

Effects of shock wave passing on turbine blade heat transfer in a transonic cascade

The progression of shock waves through a transonic turbine cascade and their effects on blade surface heat transfer are reported. Shadowgraph flow visualization was used to track the progression of moving shock waves through a linear turbine cascade and measurements of surface heat flux were made using Heat Flux Microsensors. Simultaneous surface static pressure measurements were made using Kulite pressure transducers. A shock tube and shock shaper configuration was used to produce a roughly planar, moving shock wave which was introduced into the cascade along the leading edge of the blades. The resulting heat flux and pressure traces were correlated to the progression of the shock wave through the cascade. The heat transfer increase resulting from shock passing averaged over 200 (is (typical blade passing period) was found to be a maximum of 60% on the-suetien surface near the leading edge.

Regulated Gaseous Emissions from In-use High Horsepower Drilling and Hydraulic Fracturing Engines

Unconventional well development is an energy intensive process, which relies heavily on diesel fuel to power high-horsepower engines. To reduce emissions and fuel costs, and increase natural gas utilization, industry has employed a limited number of dual fuel compression-ignited and dedicated natural gas spark-ignited engines. However, little in-use data are available for conventional engines or these new technologies. We measured regulated gaseous emissions from engines servicing the unconventional natural gas well development industry to understand better their in-use characteristics such that insight into real world emissions factors could be developed for use by researchers, regulators, or industry. Data collection efforts were limited by low utilization of these new technologies, therefore these data may not be representative of the current distribution of engines either nationally or by shale play. Emissions and fuel consumption were collected from two drilling engines operating as Tier 2 diesel only and dual fuel, two drilling engines that were dedicated natural gas, and two hydraulic fracturing engines operated as diesel only and dual fuel. Emissions for diesel only operation were below Tier 2 certification standards for carbon monoxide and non-methane hydrocarbon plus oxides of nitrogen. Dual fuel engines require use of oxidation catalysts to reduce carbon monoxide and non-methane hydrocarbon emissions resulting from this mode of combustion. For dual fuel engines with diesel oxidation catalysts, carbon monoxide emissions were reduced below Tier 2 diesel only standards by an order of magnitude. Dual fuel operation showed varied effects on non-methane hydrocarbon plus oxides of nitrogen emissions depending on configuration. These variations were mainly driven by some technologies increasing or decreasing oxides of nitrogen emissions. One dual fuel drilling engine failed to meet Tier 2 standards, as it did not include a diesel oxidation catalyst. Of the two dedicated natural engines tested, one had a failed catalyst and did not meet off-road standards for spark-ignited engines; however, emissions from the engine with the properly functioning catalyst were well below standards. Dedicated natural gas engines also demonstrated potential to meet Tier 2 carbon monoxide regulations while producing significantly lower oxides of nitrogen emissions than diesel only or dual fuel engines.

A Parametric Numerical Study of the Effects of Freestream Turbulence Intensity and Length Scale on Anti-Vortex Film Cooling Design at High Blowing Ratio

An advanced, high-effectiveness film-cooling design, the anti-vortex hole (AVH) has been investigated by several research groups and shown to mitigate or counter the vorticity generated by conventional holes and increase film effectiveness at high blowing ratios and low freestream turbulence levels. [1, 2] The effects of increased turbulence on the AVH geometry were previously investigated and presented by researchers at West Virginia University (WVU), in collaboration with NASA, in a preliminary CFD study [3] on the film effectiveness and net heat flux reduction (NHFR) at high blowing ratio and elevated freestream turbulence levels for the adjacent AVH. The current paper presents the results of an extended numerical parametric study, which attempts to separate the effects of turbulence intensity and length-scale on film cooling effectiveness of the AVH. In the extended study, higher freestream turbulence intensity and larger scale cases were investigated with turbulence intensities of 5, 10 and 20% and length scales based on cooling hole diameter of Λx/dm = 1, 3 and 6. Increasing turbulence intensity was shown to increase the centerline, span-averaged and area-averaged adiabatic film cooling effectiveness. Larger turbulent length scales were shown to have little to no effect on the centerline, span-averaged and area-averaged adiabatic film-cooling effectiveness at lower turbulence levels, but slightly increased effect at the highest turbulence levels investigated.© 2013 ASME

Investigation of Factors That Contribute to Deposition Formation on Turbine Components in a High-Pressure Combustion Facility

Researchers at West Virginia University worked with the U.S. Department of Energy, National Energy Technology Laboratory (NETL) to study particulate deposition in a high-pressure and high-temperature environment. To simulate deposition of particulate from combustion of coal synthesis gas on the pressure side of an Integrated Gasification Combined Cycle (IGCC) turbine first stage vane, angled film-cooled thermal barrier coated (TBC) test articles scaled to turbine flow conditions using Reynolds similarity were subjected to accelerated deposition at a pressure of approximately 4 bar and a gas temperature ranging from 1373–1560K. The effects on deposition rates of five different factors were examined; free stream temperature, impaction angle, blowing ratio, particulate loading, and TBC vs. non-TBC coated surface. As the freestream temperature increased the results showed that the deposition also increased. The amount of deposition increased as the impaction angle increased from 10° to 20°. The effect of blowing ratio (M, mass flux ratio) was examined at M = 0.0, 0.25 and 1.0. As the blowing ratio increased the amount of deposition decreased. The particulate loading was varied from 100 ppmw to 200 ppmw. The amount of deposition increased with the higher particulate loading case; however, coverage on the test article face did not increase significantly. Finally, a comparison test was performed between a TBC coated test article and a bare metal test article. This test showed that more deposition formed on the TBC coated article than the bare metal article. During testing, the deposition that formed on the TBC coated test articles demonstrated a resistance to adhering to the surface once the mainstream temperature was reduced during facility shut down. The results of this work will aid gas turbine manufacturers to better understand and develop mitigations for the five factors studied that cause deposit formations in IGCC engines. This work will also give insight to researchers studying deposition on the methods developed and issues encountered in simulating particulate matter into a high-pressure combustion facility.Copyright

Preliminary Experimental Investigation of the Effects of Particulate Deposition on IGCC Turbine Film-Cooling in a High-Pressure Combustion Facility

Researchers at West Virginia University are working with the U.S. Department of Energy, National Energy Technology Laboratory (NETL) to study the effects of particulate deposition on turbine film cooling in a high pressure and high temperature environment. To simulate deposition on the pressure side of an Integrated Gasification Combined Cycle (IGCC) turbine first stage vane, angled film-cooled test articles with thermal barrier coatings (TBC) are subjected to accelerated deposition at a pressure of approximately 4 atm and a gas temperature of 1100°C. Two different test article geometries were designed, with angles of 10° and 20° to the mainstream flow. Both geometries have straight-cooling holes oriented at a 30° angle to the hot-side surface. A high pressure seeding system was used to generate a particulate concentration of approximately 33.3 ppmw. Particle concentrations of 0.02 ppmw exist in the IGCC hot gas path. An accelerated simulation method was developed to simulate deposition that would occur in 10000 hr of engine operation. Preliminary tests were performed at 4 atm and 1100 °C to validate the deposition process. The results showed more deposition on the 20° test article than the 10° test articles; however no substantial deposition developed on either test article. A lumped mass analysis showed that the fly ash particles dropped below the theoretical sticking temperature as they approached the test article. Deposition was analyzed non-destructively through visual observation and scanning with a scanning laser microscope. Based on the initial test run results, a detailed plan was created to increase the operating temperature of the rig and allow two 3-hour tests to be performed on each of the test articles. Non-destructive testing will be used before, in between and after the runs to examine the evolution of the deposition growth. Following the final run, destructive testing will be used to examine the chemical composition of the deposits and their potential interaction with the TBC. Preliminary work will lead to a future study the would enhance the understanding of particle deposition evolution and examine the effects of deposition on film cooling by performing the tests in a high-pressure and high-temperature environment that is similar to the high-pressure combustion exhaust gas environment of the first stage region in IGCC turbines.Copyright