Endwall Heat Transfer Research Articles

A computational investigation of the effect of surface roughness on heat transfer on the stator endwall of an axial turbine

A numerical investigation of the effect of stochastic surface roughness on vane endwall heat transfer was conducted. The effect of equivalent sand grain roughness height was explored and compared with available experimental data. Steady-state computations using ANSYS CFX 14.0 in conjunction with the shear stress transport turbulence model were performed. Computations were conducted for fully turbulent flow conditions, since this best reproduces the conditions for the corresponding measurements. Roughness measurements were conducted at different locations along the vane passage. Exploration of these measurements indicated roughness Reynolds number values from the transitional and fully rough regime. The roughness model supplied in CFX was applied to explore the impact of surface roughness on heat transfer. Numerical heat transfer results in the vane passage were determined from a set of computations at the same operating point consisting of an adiabatic and a heat flux calculation. Calculations were conducted with a systematic variation of equivalent sand grain roughness heights and compared with experimental data. Results are presented for smooth and rough wall calculations at two different flow conditions.

Infrared thermography to study endwall cooling and heat transfer in turbine stator vane passages using the auxiliary wall method and comparison to numerical simulations

Experiments using the auxiliary wall method and infrared thermography allow to study film cooling and heat transfer in turbomachinery research with high spatial resolution. Using heater foils and pulse width modulation, an aluminium body is heated to constant wall temperature, controlled by thermocouples. The heat flux is then determined across a low conductivity layer of Ethylene-Tetrafluoroethylene (ETFE), whereby 1-D conduction is assumed. Setting the base body to several quasi-steady state wall temperatures allows to deduce adiabatic wall temperatures and heat transfer coefficients. Given a coolant and main flow with different temperature, cooling effectiveness can be calculated, using a superposition approach. Experiments in the linear cascade test rig of the Institute for Gas Turbines and Aerospace Propulsion have been performed to study the effect of hub side coolant injection on the endwall heat transfer of a turbine stator row. The quality of results is examined through extensive data analysis, accompanied by a numerical simulation of the experiment.

On the optimization of 3D-flow and heat transfer by using the Ice Formation Method: Vane endwall heat transfer

Technical flow devices are usually optimized by using numerical algorithms that analyze a multitude of design variants for the flow restraining geometry until an optimum solution is found. In this paper, we present an alternative approach: the Ice Formation Method (IFM). We show the application of this method to a turbine vane endwall in order to optimize the endwall with respect to heat transfer. On the basis of a flat endwall, we first created ice-contoured endwalls in our water channel test facility. We then analyzed these contours using numerical simulations with air to approve them for the working medium of gas turbines. Results show that the IFM reliably creates contours with reduced heat transfer coefficients (HTC). In this vein, we created one contour that reduces the average HTC by 5.2% compared to the baseline. Analyzing this contour revealed that it adapts to the flow field by breaking up the near endwall vortices and thus reducing secondary flows. We found that this reduction in HTC results from the natural character of the IFM to reduce entropy production rates due to heat conduction.

High Resolution Heat Transfer Measurements on the Stator Endwall of an Axial Turbine

In order to continue increasing the efficiency of gas turbines, an important effort is made on the thermal management of the turbine stage. In particular, understanding and accurately estimating the thermal loads in a vane passage is of primary interest to engine designers looking to optimize the cooling requirements and ensure the integrity of the components. This paper focuses on the measurement of endwall heat transfer in a vane passage with a three-dimensional (3D) airfoil shape and cylindrical endwalls. It also presents a comparison with predictions performed using an in-house developed Reynolds-Averaged Navier–Stokes (RANS) solver featuring a specific treatment of the numerical smoothing using a flow adaptive scheme. The measurements have been performed in a steady state axial turbine facility on a novel platform developed for heat transfer measurements and integrated to the nozzle guide vane (NGV) row of the turbine. A quasi-isothermal boundary condition is used to obtain both the heat transfer coefficient and the adiabatic wall temperature within a single measurement day. The surface temperature is measured using infrared thermography through small view ports. The infrared camera is mounted on a robot arm with six degrees of freedom to provide high resolution surface temperature and a full coverage of the vane passage. The paper presents results from experiments with two different flow conditions obtained by varying the mass flow through the turbine: measurements at the design point (ReCax=7.2×105) and at a reduced mass flow rate (ReCax=5.2×105). The heat transfer quantities, namely the heat transfer coefficient and the adiabatic wall temperature, are derived from measurements at 14 different isothermal temperatures. The experimental data are supplemented with numerical predictions that are deduced from a set of adiabatic and diabatic simulations. In addition, the predicted flow field in the passage is used to highlight the link between the heat transfer patterns measured and the vortical structures present in the passage.

Experimental Heat Transfer Study of Endwall in a Linear Cascade with IR Thermography

This abstract presents an endwall heat transfer experimental data of air flow going through outlet guide vanes (OGVs) situated in a low speed linear cascade. The measurement technique for this experiment was infrared thermography. In order to calculate the heat transfer coefficient (HTC) on the endwall, it has been used an instrumented window with a controlled constant temperature in one side of a 5 millimeter Plexiglass in order to generate high temperature gradients and, therefore, by measuring the surface temperature one the other side of the Plexiglass, it is calculated the HTC. Due to the fact that Plexiglass material has not good optical properties at infrared spectrum, it has been used a thin layer of black paint (10-12 μm) which has high emissivity (0.973) in the range of temperature that we are working. The Reynolds number for this experiment is 300000 in on and off-design configuration of the OGVs (on-design 25° and off-design cases are 40° and-25° incident angle). Furthermore, the on-design case is run at two different Reynolds number, 300000 and 450000. During this experiments it can be seen how changing the inlet angle to the OGVs produces significant differences on the heat transfer along the endwall. The main objective for this investigation is to study the heat transfer along the endwall of a linear cascade so that it would be a well-defined test case for CFD validation.

EXPERIMENTAL ANALYSIS OF MIXED CONVECTION HEAT TRANSFER ON HEAT SINK OF LED LAMP

The device made of fan and pin-fin heat sink should be a powerful heat sink for LED lamp. This study used transient liquid crystal experimentation to measure the end-wall heat transfer coefficient of linearly arrayed square pin array in the rectangular channel, and discussed the influence of longitudinal spacing on heat transfer. The relative transverse spacing (XT = ST/d) was set as 3, and the relative longitudinal spacing (XL = SL/d = 1.88−5) and the Reynolds number (Re = 11047−17937) were changed. Considering the endwall area, the average Nusselt number with square pin was 1.46−2.58 times of that without square pin, and the square pin array of XL = 3.75 had the maximum end-wall heat transfer gain.

Convective Heat Transfer and Pressure Loss in Rectangular Ducts With Inclined Pin-Fin on a Wavy Endwall

Local endwall heat transfer characteristics and overall pressure loss of normal and inclined pin fins arrayed in rectangular ducts with flat and wavy endwalls have been investigated to improve the cooling efficiency of jet engine combustor liners. The detailed time-mean local Nusselt number profiles were measured using a naphthalene sublimation method based on the heat/mass transfer analogy. Four kinds of angled pin fins (−45, 0, and +45 deg with a flat endwall, and −45 deg with a wavy endwall) were tested and compared with each other. As a result, the average heat transfer coefficient on the flat endwall of normal pin fins was higher than that of the angled pin fins. The average heat transfer coefficient of −45-deg inclined pin fins with a wavy endwall is the same or a little higher than the heat transfer coefficient of those with a flat endwall; however, the pressure loss of the −45-deg inclined pin fins with a wavy endwall is less than the pressure loss of those with a flat endwall. Corresponding numerical simulations using large eddy simulation (LES) with the mixed time scale (MTS) model have been also conducted at Red = 1000 for fully developed regions, and the results have shown good quantitative agreement with mass transfer experiments. It can be concluded that wavy endwalls can realize better heat transfer with less pressure loss as long as the aim consists in enhancing endwall heat transfer in inclined pin-fin channels.

Effect of Mesh Topologies on Wall Heat Transfer and Pressure Loss Prediction of a Blade Coolant Passage

This paper studies the effect of mesh topologies such as hybrid and structured meshes on the evaluation of wall heat transfer coefficient (HTC) and pressure loss of a blade cooling passage. An experimental model is chosen; it has five-row of stream wise staggered elliptical pin-fin fitted inside a 10owedge-shape duct and one-row of fillet circular pin-fin in the exit region. Simulations consider two types; i.e. warm test with isothermal wall condition and cold test with adiabatic wall condition respectively, in order to evaluate flow and thermal characteristics such as HTC and pressure loss. Further simulations are carried out by varying Re number, wall surfaces roughness, inlet turbulence intensity and turbulence models. It was found that for unstructured or structured mesh with proper near wall and middle passage grid resolutions, CFD predicted HTC and pressure loss are in good agreement with available experimental data. The wall surface roughness is found to have significant impact on HTC, simulations produce results in better agreement with experimental measurements. Simulation results also confirm that inlet turbulence intensity and turbulence model have insignificant effect of predicting the pin-fin wall and end wall heat transfer coefficient.

Detailed Measurements of End-Wall Heat Transfer Coefficients of Square Pin-Fin Heat Sink for LED Lamp at Forced Convective State

The device made of fan and pin-fin heat sink should be a powerful heat sink for LED lamp. This study used transient liquid crystal experimentation to measure the end-wall heat transfer coefficient of linearly arrayed square pin array in the rectangular channel, and discussed the influence of axial spacing on heat transfer. The air was used as operating fluid, and the square pin size was 8 mm (d) × 8 mm (d) × 64 mm (Hf), arrayed in a 240 mm (L) × 120 mm (W) × 64 mm (H) rectangular channel. The relative lateral spacing (XT=ST/d) was set as 3, and the relative axial spacing (XL=SL/d=1.88~5) and the Reynolds number (Re=11047~17937) were changed. Considering the end-wall area, the average Nusselt number with square pin was 1.46~2.58 times of that without square pin, and the square pin array of XL= 3.75 had the maximum end-wall heat transfer gain.

Experimental Study of Endwall Heat Transfer in a Linear Cascade

The endwall heat transfer characteristics of forced flow past outlet guide vanes (OGVs) in a linear cascade have been investigated by using a liquid crystal thermography (LCT) method. Due to the special design of an OGV profile, the focus of this study is emphasized on the heat transfer patterns around the leading part of a vane. The Reynolds number is kept constant at 260,000. Two attack angles of the vane are considered. For α = 0°, the vane obstructs the incident flow like a bluff body and a remarkable flow separation phenomenon was noticed. For α = 30°, the vane is more "streamlined" with respect to the incoming flow and no obvious flow separation was observed. In general, the endwall heat transfer for α = 0° is higher than that for α = 30°.

HP Vane Aerodynamics and Heat Transfer in the Presence of Aggressive Inlet Swirl

Modern lean burn combustors now employ aggressive swirlers to enhance fuel-air mixing and improve flame stability. The flow at combustor exit can therefore have high residual swirl. A good deal of research concerning the flow within the combustor is available in open literature. The impact of swirl on the aerodynamic and heat transfer characteristics of an HP turbine stage is not well understood, however. A combustor swirl simulator has been designed and commissioned in the Oxford Turbine Research Facility (OTRF), previously located at QinetiQ, Farnborough UK. The swirl simulator is capable of generating an engine-representative combustor exit swirl pattern. At the turbine inlet plane, yaw and pitch angles of over ±40 deg have been simulated. The turbine research facility used for the study is an engine scale, short duration, rotating transonic turbine, in which the nondimensional parameters for aerodynamics and heat transfer are matched to engine conditions. The research turbine was the unshrouded MT1 design. By design, the center of the vortex from the swirl simulator can be clocked to any circumferential position with respect to HP vane, and the vortex-to-vane count ratio is 1:2. For the current investigation, the clocking position was such that the vortex center was aligned with the vane leading edge (every second vane). Both the aligned vane and the adjacent vane were characterized. This paper presents measurements of HP vane surface and end wall heat transfer for the two vane positions. The results are compared with measurements conducted without swirl. The vane surface pressure distributions are also presented. The experimental measurements are compared with full-stage three-dimensional unsteady numerical predictions obtained using the Rolls Royce in-house code Hydra. The aerodynamic and heat transfer characterization presented in this paper is the first of its kind, and it is hoped to give some insight into the significant changes in the vane flow and heat transfer that occur in the current generation of low NOx combustors. The findings not only have implications for the vane aerodynamic design, but also for the cooling system design.

Three-Dimensional RANS Prediction of Gas-Side Heat Transfer Coefficients on Turbine Blade and Endwall

This paper presents a study using 3D computational fluid dynamics (CFD) based on Reynolds-averaged Navier-Stokes (RANS) equations to predict turbine gas-side heat transfer coefficients (HTC) on the entire airfoil and endwall. The CFD results at different spanwise sections and endwall have been compared with the flat-plate turbulent boundary layer correlation and with the data in a NASA turbine rotor passage with strong secondary flows, under three different flow conditions. The enhancement effects of secondary flow vortices on the blade surface and endwall heat transfer rate have been examined in detail. Analyses were conducted for the impact of Reynolds number and exit Mach number on heat transfer. The SST, k-ɛ, V2F, and realizable k-ɛ turbulence models have been assessed. The classical log-law wall-functions have been found to be comparable to the wall-integration methods but with much reduced sensitivity to inlet turbulence conditions. The migration of hot gas was simulated with a radial profile of inlet temperature. CFD results for mid-span HTCs of two other airfoils were also compared with test data. Overall, results are encouraging and indicate improved HTC and temperature predictions from 3D CFD could help optimize the design of turbine cooling schemes.

Heat Transfer for the Film-Cooled Vane of a 1-1/2 Stage High-Pressure Transonic Turbine—Part I: Experimental Configuration and Data Review With Inlet Temperature Profile Effects

This paper investigates the vane airfoil and inner endwall heat transfer for a full-scale turbine stage operating at design corrected conditions under the influence of different vane inlet temperature profiles and vane cooling flow rates. The turbine stage is a modern 3D design consisting of a cooled high-pressure vane, an un-cooled high-pressure rotor, and a low-pressure vane. Inlet temperature profiles (uniform, radial, and hot streaks) are created by a passive heat exchanger and can be made circumferentially uniform to within ±5% of the bulk average inlet temperature when desired. The high-pressure vane has full cooling coverage on both the airfoil surface and the inner and outer endwalls. Two circuits supply coolant to the vane, and a third circuit supplies coolant to the rotor purge cavity. All of the cooling circuits are independently controlled. Measurements are performed using double-sided heat-flux gauges located at four spans of the vane airfoil surface and throughout the inner endwall region. Analysis of the heat transfer measured for the uncooled downstream blade row has been reported previously. Part I of this paper describes the operating conditions and data reduction techniques utilized in this analysis, including a novel application of a traditional statistical method to assign confidence limits to measurements in the absence of repeat runs. The impact of Stanton number definition is discussed while analyzing inlet temperature profile shape effects. Comparison of the present data (Build 2) to the data obtained for an uncooled vane (Build 1) clearly illustrates the impact of the cooling flow and its relative effects on both the endwall and airfoils. Measurements obtained for the cooled hardware without cooling applied agree well with the solid airfoil for the airfoil pressure surface but not for the suction surface. Differences on the suction surface are due to flow being ingested on the pressure surface and reinjected on the suction surface when coolant is not supplied for Build 2. Part II of the paper continues this discussion by describing the influence of overall cooling level variation and the influence of the vane trailing edge cooling on the vane heat transfer measurements.

Flowfield Measurements in a Single Row of Low Aspect Ratio Pin Fins

Pin-fin arrays are commonly used as compact heat exchangers for cooling the trailing edge of gas turbine airfoils. While much research has been devoted to the heat transfer characteristics of various pin-fin configurations, little work has been done to investigate the flowfield in pin-fin arrays. Such information may allow for further optimization of pin-fin configurations. A new pin-fin facility at large scale has been constructed to allow optical access for the use of nonintrusive measurement techniques such as laser Doppler velocimetry and time-resolved, digital particle image velocimetry. Using these techniques, the flow through a single row of pin fins having a height-to-diameter ratio of 2 and span-to-diameter ratio of 2.5 was investigated. Results showed that the length of the wake region decreased with increasing Reynolds number. At higher Reynolds numbers, Kármán vortices developed closer to the pin fins than for single, infinitely long cylinders. Transverse fluctuations correlated well with endwall heat transfer indicating that the Kármán vortices play a key role in energy transport.

Influence of flow injection angle on a leading-edge horseshoe vortex

Junction flows that develop at the base of protruding obstructions occur in many applications. An unsteady horseshoe vortex is formed as a component of these junction flows, which increases the local heat transfer on the associated endwall. Augmenting this junction flow can be achieved through the injection of fluid upstream of the obstruction. This experimental study evaluated the effects of injection angle for a two-dimensional slot placed upstream of a vane leading-edge with four injection angles of 90°, 65°, 45°, and 30°. Results showed that high momentum injection increased the endwall heat transfer at each slot angle while low momentum injection resulted in a relatively lower augmentation of endwall heat transfer. A leading-edge vortex turning into the endwall was formed at the junction in the stagnation plane for high momentum injection at 90° and 65° while a leading-edge vortex turning away from the wall was formed for 45° and 30° injection. For low momentum injection, a vortex turning into the endwall was formed at all injection angles.

Identifying Inefficiencies in Unsteady Pin Fin Heat Transfer Using Orthogonal Decomposition

Internal cooling of the trailing edge region in a gas turbine blade is typically achieved with an array of pin fins. In order to better understand the effectiveness of this configuration, high performance computations are performed on cylindrical pin fins with a spanwise distance to fin diameter ratio of 2 and height over fin diameter ratio of one. For validation purposes, the flow Reynolds number based on hydraulic channel diameter and bulk velocity (Re = 12,800) was set to match experiments available in the open literature. Simulations included a URANS and LES on a single row of pin fins where the URANS domain was 1 pin wide versus the LES with 3 pins. The resulting time-dependent flow field was analyzed using a variation of bi-orthogonal decomposition (BOD), where the correlation matrices were built using the internal energy in addition to the three velocity components. This enables a detailed comparison of URANS and LES to assess the URANS modeling assumptions as well as a flow decomposition with respect to the flow structure’s influence on surface heat transfer. This analysis shows low order modes which do not contribute to turbulent heat flux, but instead increase the heat exchanger’s global inefficiency. In the URANS study, the forth mode showed the first nonzero temperature basis function, which means that a considerable amount of energy is contained in flow structures that do not contribute to increasing endwall heat transfer. In the LES, the first non zero temperature basis function was the seventh mode. Both orthogonal basis function sets were evaluated with respect to each mode’s contribution to turbulent heat exchange with the surface. This analysis showed that there exists one distinct, high energy mode that contributes to wall heat flux, whereas all others do not. Modifying this mode could potentially be used to improve the heat exchanger’s efficiency with respect to pressure loss.

Endwall convective heat transfer for bluff bodies

The endwall heat transfer characteristics of forced flow past bluff bodies have been investigated using liquid crystal thermography (LCT). The bluff body is placed in a rectangular channel with both its ends attached to the endwalls. The Reynolds number varies from 50,000 to 100,000. In this study, a single bluff body and two bluff bodies arranged in tandem are considered. Due to the formation of horseshoe vortices, the heat transfer is enhanced appreciably for both cases. However, for the case of two bluff bodies in tandem, it is found that the presence of the second bluff body decreases the heat transfer as compared to the case of a single bluff body. In addition, the results show that the heat transfer exhibits Reynolds number similarity. For a single bluff body, the Nusselt number profiles collapse well when the data are scaled by Re0.55; for two bluff bodies arranged in tandem, the heat transfer scaling is changed to Re0.51, indicating that the power index of Reynolds number is flow dependent.

Analysis of Unsteady Tip and Endwall Heat Transfer in a Highly Loaded Transonic Turbine Stage

In a previous study, vane-rotor shock interactions and heat transfer on the rotor blade of a highly loaded transonic turbine stage were simulated. The geometry consists of a high pressure turbine vane and a downstream rotor blade. This study focuses on the physics of flow and heat transfer in the rotor tip, casing, and hub regions. The simulation was performed using the unsteady Reynolds-averaged Navier–Stokes code MSU-TURBO. A low Reynolds number k-ε model was utilized to model turbulence. The rotor blade in question has a tip gap height of 2.1% of the blade height. The Reynolds number of the flow is approximately 3×106/m. Unsteadiness was observed at the tip surface that results in intermittent “hot spots.” It is demonstrated that unsteadiness in the tip gap is governed by inviscid effects due to high speed flow and is not strongly dependent on pressure ratio across the tip gap contrary to published observations that have primarily dealt with subsonic tip flows. The high relative Mach numbers in the tip gap lead to a choking of the leakage flow that translates to a relative attenuation of losses at higher loading. The efficacy of new tip geometry is discussed to minimize heat flux at the tip while maintaining choked conditions. In addition, an explanation is provided that shows the mechanism behind the rise in stagnation temperature on the casing to values above the absolute total temperature at the inlet. It is concluded that even in steady (in a computational sense) mode, work transfer to the near tip fluid occurs due to relative shearing by the casing. This is believed to be the first such explanation of the work transfer phenomenon in the open literature. The difference in pattern between steady and time-averaged heat fluxes at the hub is also explained.

Experimental Study of Surface Roughness Effects on a Turbine Airfoil in a Linear Cascade— Part I: External Heat Transfer

Abstract The present experimental study is part of a comprehensive heat transfer analysis on a highly loaded low pressure turbine blade and endwall with varying surface roughness. Whereas a former paper (Lorenz et al., 2009, “An Experimental Study of Airfoil and Endwall Heat Transfer in a Linear Turbine Blade Cascade—Secondary Flow and Surface Roughness Effects,” International Symposium on Heat Transfer in Gas Turbine Systems, Aug. 9–14, Antalya, Turkey) focused on full span heat transfer of a smooth airfoil and surface roughness effects on the endwall, in this work further measurements at the airfoil midspan with different deterministic surface roughness are considered. Part I investigates the external heat transfer enhancement due to rough surfaces, whereas part II focuses on surface roughness effects on aerodynamic losses. A set of different arrays of deterministic roughness is investigated in these experiments, varying the height and eccentricity of the roughness elements, showing the combined influence of roughness height and anisotropy of the rough surfaces on laminar to turbulent transition and the turbulent boundary layer as well as boundary layer separation on the pressure and suction side. It is shown that, besides the known effect of roughness height, eccentricity of roughness plays a major role in the onset of transition and the turbulent heat transfer. The experiments are conducted at several freestream turbulence levels (Tu1=1.4–10.1%) and different Reynolds numbers.

Effects of an Axisymmetric Contoured Endwall on a Nozzle Guide Vane: Convective Heat Transfer Measurements

Heat transfer is a critical factor in the durability of gas turbine components, particularly in the first vane. An axisymmetric contour is sometimes used to contract the cross sectional area from the combustor to the first stage vane in the turbine. Such contouring can lead to significant changes in the endwall flows, thereby altering the heat transfer. This paper investigates the effect of axisymmetric contouring on the endwall heat transfer of a nozzle guide vane. Heat transfer measurements are performed on the endwalls of a planar and contoured passage whereby one endwall is modified with a linear slope in the case of the contoured passage. Included in this study is upstream leakage flow issuing from a slot normal to the inlet axis. Each of the endwalls within the contoured passage presents a unique flow field. For the contoured passage, the flat endwall is subject to an increased acceleration through the area contraction, while the contoured endwall includes both increased acceleration and a turning of streamlines due to the slope. Results indicate heat transfer is reduced on both endwalls of the contoured passage relative to the planar passage. In the case of all endwalls, increasing leakage mass flow rate leads to an increase in heat transfer near the suction side of the vane leading edge.