Elevated Freestream Turbulence Research Articles

Elevated freestream turbulence effects on heat transfer for a gas turbine vane

High freestream turbulence levels have been shown to greatly augment the heat transfer along a gas turbine airfoil, particularly for the first stage nozzle guide vane. For this study, augmentations in convective heat transfer have been measured for a first stage turbine vane in the stagnation region, along the mid-span, and along the platform resulting from an approach freestream turbulence level of 19.5%. In addition to quantifying surface heat transfer, boundary layer measurements have been made to better understand high freestream turbulence effects. Although there are a number of correlations that have been developed for scaling freestream turbulence augmentations to heat transfer, the results of this study indicate that these correlations are not successful in predicting heat transfer for various regions along a turbine vane.

Measurements in a Turbine Cascade Flow Under Ultra Low Reynolds Number Conditions

With the new generation of gas turbine engines, low Reynolds number flows have become increasingly important. Designers must properly account for transition from laminar to turbulent flow and separation of the flow from the suction surface, which is strongly dependent upon transition. Of interest to industry are Reynolds numbers based upon suction surface length and flow exit velocity below 150,000 and as low as 25,000. In this paper, the extreme low end of this Reynolds number range is documented by way of pressure distributions, loss coefficients, and identification of separation zones. Reynolds numbers of 25,000 and 50,000 and with 1 percent and 8-9 percent turbulence intensity of the approach flow (free-stream turbulence intensity, FSTI) were investigated. At 25,000 Reynolds number and low FSTI, the suction surface displayed a strong and steady separation region. Raising the turbulence intensity resulted in a very unsteady separation region of nearly the same size on the suction surface. Vortex generators were added to the suction surface, but they appeared to do very little at this Reynolds number. At the higher Reynolds number of 50,000, the low-FSTI case was strongly separated on the downstream portion of the suction surface. The separation zone was eliminated when the turbulence level was increased to 8-9 percent. Vortex generators were added to the suction surface of the low-FSTI case. In this instance, the vortices were able to provide the mixing needed to re-establish flow attachment. This paper shows that massive separation at very low Reynolds numbers (25,000) is persistent, in spite of elevated FSTI and added vortices. However, at a higher Reynolds number, there is opportunity for flow reattachment either with elevated free-stream turbulence or with added vortices. This may be the first documentation of flow behavior at such low Reynolds numbers. Although it is undesirable to operate under these conditions, it is important to know what to expect and how performance may be improved if such conditions are unavoidable.

Detailed Boundary Layer Measurements on a Turbine Stator Vane at Elevated Freestream Turbulence Levels

High freestream turbulence levels have been shown to greatly augment the heat transfer on a gas turbine airfoil. To better understand these effects, this study has examined the effects elevated freestream turbulence levels have on the boundary layer development along a stator vane airfoil. Low freestream turbulence measurements (0.6 percent) were performed as a baseline for comparison to measurements at combustor simulated turbulence levels (19.5 percent). A two-component LDV system was used for detailed boundary layer measurements of both the mean and fluctuating velocities on the pressure and suction surfaces. Although the mean velocity profiles appeared to be more consistent with laminar profiles, large velocity fluctuations were measured in the boundary layer along the pressure side at the high freestream turbulence conditions. Along the suction side, transition occurred further upstream due to freestream turbulence.

Conditionally sampled flow and thermal behavior of a transitional boundary layer at elevated free-stream turbulence

Abstract In the elevated FSTI condition, the laminar flow is highly disturbed. Linear instability amplification, which would take place under low-disturbance conditions triggered by infinitesimal disturbances, is bypassed, hence the term “bypass transition”. Nonlinear instability, triggered by finite-amplitude disturbance, dominates in bypass transition. Due to the highly disturbed condition, the turbulent and non-turbulent parts of the transitional flow become difficult to separate. Because of this, the conventional conditional techniques for such sampling encounter difficulty and large uncertainty. A modified method was made. This modified method was convenient to apply and was theoretically verified. Use of the turbulence shear stress, uv , as the criterion function was found superior to the use of u ′ for separating the turbulent and non-turbulent signals in elevated FSTI conditions. The conditionally sampled results for FSTI, ranging from 0.5% to 6.4%, indicated that u ′, v ′, t ′, and ut are high in the non-turbulent part of the high FSTI cases contrary to the low FSTI cases, whereas ( uv and vt ) are low in the non-turbulent part, similar to the low FSTI cases. This implies that although the velocity trace of the non-turbulent part was hardly distinguishable from the turbulent part in high FSTI situations, the flow and thermal transport mechanisms of the non-turbulent part were distinctively different from the turbulent part. Evaluation of each part (turbulent and non-turbulent) is presented in detail in this paper.

Heat Transfer Coefficients Over a Flat Surface With Air and CO2 Injection Through Compound Angle Holes Using a Transient Liquid Crystal Image Method

This paper presents the detailed heat transfer coefficients over a flat surface with one row of injection holes inclined streamwise at 35 deg for three blowing ratios (M = 0.5–2.0). Three compound angles of 0, 45, and 90 deg with air (D.R. = 0.98) and CO2 (D.R. = 1.46) as coolants were tested at an elevated free-stream turbulence condition (Tu ≈ 8.5 percent). The experimental technique involves a liquid crystal coating on the test surface. Two related transient tests obtained detailed heat transfer coefficients and film effectiveness distributions. Heat transfer coefficients increase with increasing blowing ratio for a constant density ratio, but decrease with increasing density ratio for a constant blowing ratio. Heat transfer coefficients increase for both coolants over the test surface as the compound angle increases from 0 to 90 deg. The detailed heat transfer coefficients obtained using the transient liquid crystal technique, particularly in the near-hole region, will provide a better understanding of the film cooling process in gas turbine components.

Film Effectiveness Over a Flat Surface With Air and CO2 Injection Through Compound Angle Holes Using a Transient Liquid Crystal Image Method

This paper presents detailed film effectiveness distributions over a flat surface with one row of injection holes inclined streamwise at 35 deg for three blowing ratios (M = 0.5, 1.0, 2.0). Three compound angles of 0, 45, and 90 deg with air (D.R. = 0.98) and CO2 (D. R. = 1.46) as coolants are tested at an elevated free-stream turbulence condition (Tu ≈ 8.5 percent). A transient liquid crystal technique is used to measure local heat transfer coefficients and film effectiveness. Detailed film effectiveness results are presented near and around film injection holes. Compound angle injection provides higher film effectiveness than simple angle injection for both coolants. Higher density injectant produces higher effectiveness for simple injection. However, lower density coolant produces higher effectiveness for a large compound angle of 90 deg. The detailed film effectiveness obtained using the transient liquid crystal technique, particularly in the near-hole region, provided a better understanding of the film cooling process in gas turbine components.

The Influence of Large-Scale, High-Intensity Turbulence on Vane Aerodynamic Losses, Wake Growth, and the Exit Turbulence Parameters

An experimental research program was undertaken to examine the influence of large-scale high-intensity turbulence on vane exit losses, wake growth, and exit turbulence characteristics. The experiment was conducted in a four-vane linear cascade at an exit Reynolds number of 800,000 based on chord length and an exit Mach number of 0.27. Exit measurements were made for four inlet turbulence conditions including a low-turbulence case (Tu ≈ 1 percent), a grid-generated turbulence case (Tu ≈ 7.5. percent) and two levels of large-scale turbulence generated with a mock combustor (Tu ≈ 12 and 8 percent). Exit total pressure surveys were taken at two locations to quantify total pressure losses. The suction surface boundary layer was also traversed to determine losses due to boundary layer growth. Losses occurred in the core of the flow for the elevated turbulence cases. The elevated free-stream turbulence was found to have a significant effect on wake growth. Generally, the wakes subjected to elevated free-stream turbulence were broader and had smaller peak velocity deficits. Reynolds stress profiles exhibited asymmetry in peak amplitudes about the wake centerline, which are attributable to differences in the evolution of the boundary layers on the pressure and suction surfaces of the vanes. The overall level of turbulence and dissipation inside the wakes and in the free stream was determined to document the rotor inlet boundary conditions. This is useful information for assessing rotor heat transfer and aerodynamics. Eddy diffusivities and mixing lengths were estimated using X-wire measurements of turbulent shear stress. The free-stream turbulence was found to strongly affect eddy diffusivities, and thus wake mixing. At the last measuring position, the average eddy diffusivity in the wake of the high-turbulence close combustor configuration (Tu ≈ 12) was three times that of the low turbulence wake.

96/05685 Quantitative electron spectroscopic analysis of the surface chemistry of bituminous coal

The combined effects of elevated free-stream turbulence intensity (FSTI) and streamwise acceleration on flow and thermal structures in transitional boundary layers were investigated experimentally on a heated flat plate with elevated FSTI levels ranging from 3 to 7%. The acceleration strength, based on the parameter, K [≡ (ν/U∞2)dU∞/dx], rangedfrom 0.39 × 10−6 to 4.1 × 10−6. A three-wire probe was used to measure detailed momentum and thermal boundary layer structures, including the streamwise and cross-stream velocity fluctuations, temperature fluctuation, Reynolds stresses, Reynolds heat fluxes, eddy viscosity, turbulent thermal diffusivity, and turbulent Prandtl number. The results show that FSTI has a greater effect than streamwise acceleration on flow and thermal structures in transitional boundary layers. However, the effect of streamwise acceleration on increasing the transition length is significant. Furthermore, streamwise acceleration suppresses the magnitude of uv and slows down the uv transition process relative to u′ even when elevated FSTI seems to have a dominant effect. Acceleration is also shown to suppress ϵH more than ϵM in the transitional region.

Combined effects of elevated free-stream turbulence and streamwise acceleration on flow and thermal structures in transitional boundary layers

The combined effects of elevated free-stream turbulence intensity (FSTI) and streamwise acceleration on flow and thermal structures in transitional boundary layers were investigated experimentally on a heated flat plate with elevated FSTI levels ranging from 3 to 7%. The acceleration strength, based on the parameter, K [≡ (ν/ U ∞ 2)d U ∞/dx] , rangedfrom 0.39 × 10 −6 to 4.1 × 10 −6. A three-wire probe was used to measure detailed momentum and thermal boundary layer structures, including the streamwise and cross-stream velocity fluctuations, temperature fluctuation, Reynolds stresses, Reynolds heat fluxes, eddy viscosity, turbulent thermal diffusivity, and turbulent Prandtl number. The results show that FSTI has a greater effect than streamwise acceleration on flow and thermal structures in transitional boundary layers. However, the effect of streamwise acceleration on increasing the transition length is significant. Furthermore, streamwise acceleration suppresses the magnitude of uv and slows down the uv transition process relative to u′ even when elevated FSTI seems to have a dominant effect. Acceleration is also shown to suppress ϵ H more than ϵ M in the transitional region.

Turbulence Measurements in a Heated, Concave Boundary Layer Under High-Free-Stream Turbulence Conditions

Turbulence measurements for both momentum and heat transfer are taken in a lowvelocity, turbulent boundary layer growing naturally over a concave wall. The experiments are conducted with negligible streamwise acceleration and a nominal freestream turbulence intensity of ∼8 percent. Comparisons are made with data taken in an earlier study in the same test facility but with a 0.6 percent free-stream turbulence intensity. Results show that elevated free-stream turbulence intensity enhances turbulence transport quantities like uv and vt in most of the boundary layer. In contrast to the low-turbulence cases, high levels of transport of momentum are measured outside the boundary layer. Stable, Go¨rtlerlike vortices, present in the flow under low-turbulence conditions, do not form when the free-stream turbulence intensity is elevated. Turbulent Prandtl numbers, Prt, within the log region of the boundary layer over the concave wall increase with streamwise distance to values as high as 1.2. Profiles of Prt suggest that the increase in momentum transport with increased free-stream turbulence intensity precedes the increase in heat transport. Distributions of near-wall mixing length for momentum remain unchanged on the concave wall when free-stream turbulence intensity is elevated. Both for this level of free-stream turbulence and for the lower level, mixing length distributions increase linearly with distance from the wall, following the standard slope. However, when free-stream turbulence intensity is elevated, this linear region extends farther into the boundary layer, indicating the emerging importance of larger eddies in the wake of the boundary layer with the high-turbulence free stream. Because these eddies are damped by the wall, the influence of the wall grows with eddy size.

Effects of Elevated Free-Stream Turbulence on Flow and Thermal Structures in Transitional Boundary Layers

The effects of elevated free-stream turbulence on flow and thermal structures in transitional boundary layers were investigated experimentally on a heated flat plate. Detailed boundary layer measurements using a three-wire probe and wall heat transfer were made with free-stream turbulence intensities of 0.5, 3.8, 5.5, and 6.4 percent, respectively. The onset of transition, transition length, and the turbulent spot formation rate were determined. The statistical results of the streamwise and cross-stream velocity fluctuations, temperature fluctuation, Reynolds stresses, and Reynolds heat fluxes were presented. The eddy viscosity, turbulent thermal diffusivity, and the turbulent Prandtl number were calculated and related physical mechanisms are discussed.

Effect of Periodic Passing Wakes on the Leading Edge Film Cooling Effectiveness. Studies on 90-degree-inclination Cooling Holes.

Detailed studies are conducted on film effectiveness of discrete cooling holes around the leading edge of a blunt body that is subjected to periodically incoming wakes as well as free-stream turbulence with various levels of intensity. The cooling holes have a similar configuration to that of typical turbine blades except for the spanwise inclination angle. Secondary air is heated so that the temperature difference between the primary and secondary air is about 20 K. In this case, the air density ratio of the primary and secondary flows becomes less than unity ; therefore the density ratio encountered in an actual aero-engine cannot be simulated in the present study. A spoped-wheel-type wake generator is used in this study. Moreover, three types of turbulence grids are used to elevate the free-stream turbulence intensity. We adopt three blowing ratios of the secondary flow to the primary flow. For each of the blowing ratios, wake-affected adiabatic wall temperature around the test surface is measured with the thermocouples in the test model. The effect of elevated free-stream turbulence on the wake-affected film effectiveness is also investigated. Flow visualization by use of liquid crystal is performed to obtain qualitative information on the film-effectiveness distributions.