Effects Of High Free-stream Turbulence Research Articles

On the Influence of High Turbulence on the Convective Heat Flux on the High-Pressure Turbine Vane LS89

High-pressure turbine vanes and blades are subjected to a turbulent combustor flow affecting the heat transfer and boundary layer transition, hence, the temperature distribution. The accurate prediction of the temperature distribution is crucial for a reliable design and cooling implementation. Engine-representative measurements are hence mandatory for improving design tools. Recently, convective heat transfer measurements were conducted on a high-pressure turbine inlet guide vane (VKI LS89 airfoil) in the Isentropic Compression Tube (CT-2) facility at the von Karman Institute. This contribution focuses on the effect of high freestream turbulence generated by a new turbulence grid allowing a range of turbulence intensities in excess of 10% with representative length scales of the order of 1–2 cm. Three cases with varying turbulence levels are discussed in this paper. The different flow conditions are exit isentropic Mach numbers of 0.70–0.97, Reynolds numbers of 0.53 × 106 and 1.15 × 106 and a constant temperature ratio equal to 1.36. The heat transfer distributions along the vane suction side indicate a clear link between boundary layer transition and the stream-wise pressure gradients even at high levels of freestream turbulence intensity. Differences are put in evidence in the dynamics of the transition development. Future developments will focus also on the contribution of the other flow parameters under high turbulence. Heat transfer predictions from the boundary layer code TEXSTAN and Reynolds-Averaged Navier–Stokes code elsA (ensemble logiciel pour la simulation en Aérodynamique) are additionally compared to the experiments. Inherent difficulties associated with high turbulence modelling are clear from this early numerical work.

Effect of High Freestream Turbulence on Flowfields of Shaped Film Cooling Holes

Shaped film cooling holes have become a standard geometry for protecting gas turbine components. Few studies, however, have reported flowfield measurements for moderately expanded shaped holes and even fewer have reported on the effects of high freestream turbulence intensity relevant to gas turbine airfoils. This study presents detailed flowfield and adiabatic effectiveness measurements for a shaped hole at freestream turbulence intensities of 0.5% and 13%. Test conditions included blowing ratios of 1.5 and 3 at a density ratio of 1.5. Measured flowfields revealed a counter-rotating vortex pair (CRVP) and high jet penetration into the mainstream at the blowing ratio of 3. Elevated freestream turbulence had a minimal effect on mean velocities and rather acted by increasing turbulence intensity around the coolant jet, resulting in increased lateral spreading of coolant.

A Model for Simulation of Turbulent Flow With High Free Stream Turbulence Implemented in OpenFOAM®

A low-Reynolds number k-ε model for simulation of turbulent flow with high free stream turbulence is developed which can successfully predict turbulent kinetic energy profiles, skin friction coefficient, and Stanton number under high free stream turbulence. Modifications incorporating the effects of free stream velocity and length scale are applied. These include an additional term in turbulent kinetic energy transport equation, as well as reformulation of the coefficient in turbulent viscosity equation. The present model is implemented in OpenFOAM CFD code and applied together with other well-known versions of low-Reynolds number k-ε model in flow and heat transfer calculations in a flat plate turbulent boundary layer. Three different test cases based on the initial values of the free stream turbulence intensity (1%, 6.53%, and 25.7%) are considered and models predictions are compared with available experimental data. Results indicate that almost all low-Reynolds number k-ε models, including the present model, give reasonably good results for low free stream turbulence intensity case (1%). However, deviations between current k-ε models predictions and data become larger as turbulence intensity increases. Turbulent kinetic energy levels obtained from these models for very high turbulence intensity (25.7%) show as much as 100% underprediction while skin friction coefficient and Stanton number are overpredicted by more than 70%. Applying the present modifications, predictions of skin friction coefficient, and Stanton number improve considerably (only 15% and 8% deviations in average for very high free stream turbulence intensity). Turbulent kinetic energy levels are vastly improved within the boundary layer as well. It seems like the new developed model can capture the physics of the high free stream turbulence effects.

Modeling of Wake-Induced Transition in Linear Low-Pressure Turbine Cascades

An unsteady Reynolds-averaged Navier-Stokes (URANS) strategy is applied to the problem of wake-induced transition at high freestream turbulence on the suction side of two blades representative of those used in low-pressure turbines. Experimentally, the blades are arranged in high-aspect-ratio linear cascades, with upstream circular bars generating passing wakes, and two-dimensional flow conditions are, thus, assumed. The strategy combines an explicit algebraic Reynolds-stress turbulence model with transition-specific modifications targeted at capturing the effects of high freestream turbulence and of pretransitional laminar fluctuations. Close attention is paid to numerical accuracy, and grids of up to 140,000 cells are used in combination with 800 time steps per pitchwise traverse to resolve small-scale features in the blade boundary layers that are associated with the unsteady interaction. The computational results demonstrate that the combined model returns a good representation of the response of the suction-side boundary layer to the passing wakes in both blades. Specifically, in the boundary layer of one of the two blades, the wakes are observed to cause a periodic upstream shift in the transition onset and, thus, correspondingly periodic attachment and calming. In the other, no separation occurs, and the wakes are shown to produce a significant periodic reduction in shape factor and increase in skin friction in the blade boundary layer, again as a consequence of the upstream shift in the transition location.

Turbine Blade Cooling Studies at Texas A&M University: 1980-2004

Gas turbines are used extensively for aircraft propulsion, land-based power generation, and industrial applications. Developments in turbine cooling technology play a critical role in increasing the thermal efficiency and power output of advanced high-temperature gas turbine engines. Gas turbine blades are cooled internally by passing the coolant through several rib-enhanced serpentine passages to remove heat conducted from the outside surface. External cooling of turbine blades by film cooling is achieved by injecting relatively cooler air from the internal coolant passages out of the blade surface to form a protective layer between the blade surface and hot gas-path flow. The most important research contributions on turbine blade cooling studies at Texas A&M University's Turbine Heat Transfer Laboratory from 1980 to 2004 are summarized. For turbine blade internal cooling, the focus is on the effect of rotation on rotor blade coolant passage heat transfer with rib turbulators, pin fins, dimples, and impinging jets. For turbine blade external cooling, the focus is on unsteady high freestream turbulence effects on film-cooling performance with a special emphasis on turbine blade edge region heat transfer and cooling problems.

Effects of combustor-level high inlet turbulence on the endwall flow and heat/mass transfer of a high-turning turbine rotor cascade

Experimental data are presented which describe the effects of a combustor-level high free-stream turbulence on the near-wall flow structure and heat/mass transfer on the endwall of a linear high-turning turbine rotor cascade. The endwall flow structure is visualized by employing the partial- and total-coverage oil-film technique, and heat/mass transfer rate is measured by the naphthalene sublimation method. A turbulence generator is designed to provide a highly-turbulent flow which has free-stream turbulence intensity and integral length scale of 14.7% and 80mm, respectively, at the cascade entrance. The surface flow visualizations show that the high free-stream turbulence has little effect on the attachment line, but alters the separation line noticeably. Under high free-stream turbulence, the incoming near-wall flow upstream of the adjacent separation lines collides more obliquely with the suction surface. A weaker lift-up force arising from this more oblique collision results in the narrower suction-side corner vortex area in the high turbulence case. The high free-stream turbulence enhances the heat/mass transfer in the central area of the turbine passage, but only a slight augmentation is found in the endwall regions adjacent to the leading and trailing edges. Therefore, the high free-stream turbulence makes the endwall heat load more uniform. It is also observed that the heat/mass transfers along the locus of the pressure-side leg of the leading-edge horseshoe vortex and along the suctionside corner are influenced most strongly by the high free-stream turbulence. In this study, the endwall surface is classified into seven different regions based on the local heat/mass transfer distribution, and the effects of the high free-stream turbulence on the local heat/mass transfer in each region are discussed in detail.

Recent Studies in Turbine Blade Cooling

Gas turbines are used extensively for aircraft propulsion, land‐based power generation, and industrial applications. Developments in turbine cooling technology play a critical role in increasing the thermal efficiency and power output of advanced gas turbines. Gas turbine blades are cooled internally by passing the coolant through several rib‐enhanced serpentine passages to remove heat conducted from the outside surface. External cooling of turbine blades by film cooling is achieved by injecting relatively cooler air from the internal coolant passages out of the blade surface in order to form a protective layer between the blade surface and hot gas‐path flow. For internal cooling, this presentation focuses on the effect of rotation on rotor blade coolant passage heat transfer with rib turbulators and impinging jets. The computational flow and heat transfer results are also presented and compared to experimental data using the RANS method with various turbulence models such as k‐ε, and second‐moment closure models. This presentation includes unsteady high free‐stream turbulence effects on film cooling performance with a discussion of detailed heat transfer coef‐ ficient and film‐cooling effectiveness distributions for standard and shaped film‐hole geometry using the newly developed transient liquid crystal image method.

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.

Effect of High Free-Stream Turbulence With Large Length Scale on Blade Heat/Mass Transfer

The naphthalene sublimation technique is used to investigate the influence of high free-stream turbulence with large length scale on the heat/mass transfer from a turbine blade in a highly accelerated linear cascade. The experiments are conducted at four exit Reynolds numbers, ranging from 2.4 × 105 to 7.8 × 105, with free-stream turbulence of 3, 8.5, and 8 percent and corresponding integral length scales of 0.9 cm, 2.6 cm, and 8 cm, respectively. On the suction surface, the heat/mass transfer rate is significantly enhanced by high free-stream turbulence due to an early boundary layer transition. By contrast, the transition occurs very late, and may not occur at very low Reynolds numbers with low free-stream turbulence. In the turbulent boundary layer, lower heat/mass transfer rates are found for the highest free-stream turbulence level with large length scale than for the moderate turbulence levels with relatively small scales. Similar phenomena also occur at the leading edge. However, the effect of turbulence is not as pronounced in the laminar boundary layer.

Characterization and Laboratory Simulation of Turbine Airfoil Surface Roughness and Associated Heat Transfer

The physical characteristics of surface roughness observed on first-stage high-pressure turbine vanes that had been in service for a long period were investigated in this study. Profilometry measurements were utilized to provide details of the surface roughness formed by deposits of foreign materials on different parts of the turbine vane. Typical measures of surface roughness such as centerline average roughness values were shown to be inadequate for characterizing roughness effects. Using a roughness shape parameter originally derived from regular roughness arrays, the turbine airfoil roughness was characterized in terms of equivalent sand-grain roughness in order to develop an appropriate simulation of the surface for laboratory experiments. Two rough surface test plates were designed and fabricated. These test plates were evaluated experimentally to quantify the heat transfer rate for flow conditions similar to that which occurs on the turbine airfoil. Although the roughness levels on the two test plates were different by a factor of two, both surfaces caused similar 50 percent increases in heat transfer rates relative to a smooth surface. The effects of high free-stream turbulence, with turbulence levels from 10 to 17 percent, were also investigated. Combined free-stream turbulence and surface roughness effects were found to be additive, resulting in as much as a 100 percent increase in heat transfer rate.

The Effect of High Free-Stream Turbulence on Film Cooling Effectiveness

This study investigated the adiabatic wall cooling effectiveness of a single row of film cooling holes injecting into a turbulent flat plate boundary layer below a turbulent, zero pressure gradient free stream. Levels of free-stream turbulence (Tu) up to 17.4 percent were generated using a method that simulates conditions at a gas turbine combustor exit. Film cooling was injected from a single row of five 35 deg slant-hole injectors (length/diameter = 3.5, pitch/diameter = 3.0) at blowing ratios from 0.55 to 1.85 and at a nearly constant density ratio (coolant density/free-stream density) of 0.95. Film cooling effectiveness data are presented for Tu levels ranging from 0.9 to 17 percent at a constant free-stream Reynolds number based on injection hole diameter of 19,000. Results show that elevated levels of free-stream turbulence reduce film cooling effectiveness by up to 70 percent in the region directly downstream of the injection hole due to enhanced mixing. At the same time, high free-stream turbulence also produces a 50–100 percent increase in film cooling effectiveness in the region between injection holes. This is due to accelerated spanwise diffusion of the cooling fluid, which also produces an earlier merger of the coolant jets from adjacent holes.