Endwall Heat Transfer Coefficients Research Articles

Experimental Study on the Endwall Aerothermal Performance of Turbine Cascades in a Novel Transient Test Facility

Abstract Linear turbine cascades have been widely used to conduct fundamental and applied investigations, but only a few transient test facilities with the linear turbine cascade are available in the open literature. A novel transient test facility was presented in this paper, including detailed design and structure, and various experimental measurements (aerodynamic and thermodynamic) were conducted to verify the transient test facility’s capability. This test facility mainly includes a main air supply line (bypass line and test line), a coolant supply line, a test section, and a control system (heater and various valves). The linear cascade holds up six equally spaced cascade profiles, forming five completed cascade passages, and the center passage is used for aerodynamic and thermodynamic measurements. The aerodynamic loss and heat transfer performance were measured at various flow conditions (incidence angle, Ma, and blowing ratio (BR)). The endwall heat transfer coefficient and film cooling effectiveness were estimated by adopting a dual linear regression technique. Results indicate that the mianstream pressure and temperature present a desired step change, and remain relatively steady in the test window. The magnitudes of endwall heat transfer significantly increase as the Maex increases from 0.2 to 0.5, but the thermal load distributions remain the same. The BR is a key parameter for endwall film cooling performance, and the optimal film cooling coverage is acquired at the critical value of BR. Both insufficient and excessive coolant flowrate can result in undesirable endwall film cooling coverage, and may cause unnecessary consumption of the coolant flow.

Thermal-hydraulic performance of additively manufactured lattices for gas turbine blade trailing edge cooling

• Thermal-hydraulic performance of four lattice frame materials, namely, Octet, Tetrakaidecahedron, Face diagonal-cube, and Cube is studied for turbine edge cooling application. • Face diagonal-cube provided the highest heat transfer coefficient and pressure drop values amongst the investigated samples at similar porosity values and unit cell size. • Cube topology yielded the lowest flow losses and consequently highest thermal–hydraulic performance of ∼ 2.78 at the lowest investigated Reynolds number. Gas turbine blade trailing edge cooling is challenging because of the stringent geometrical constraints driven by aerodynamics and thermal stresses. The blade topology becomes significantly thin towards the trailing edge which leaves a narrow room for the construction of internal cooling channels and also makes this section susceptible to failure due to thermal stresses. Conventionally, pin-fins are employed in the internal cooling channels of the trailing edge because they provide high levels of heat transfer coefficient values and also structural integrity. Much of the past studies on pin fins have focused on their relative arrangements, heights, crossflow scheme, etc., with an aim to enhance the endwall heat transfer coefficient as well as leverage the conjugate heat dissipation capabilities of these “extended surfaces”. Lattices on the other hand, have been investigated to a lesser degree, and some studies can be found for blade mid-chord region, however, with their manufacturability challenges at that time, the concepts did not gain traction. Lattices are far more complex topologies with superior local heat transfer characteristics as well as high conjugate heat dissipation capabilities due to large wetted surface area. With the recent advancements in metal additive manufacturing, complex topologies such as lattices can be revisited due to high chances of their realization now. To this end, we are presenting our study on lattices additively manufactured in 420 Stainless Steel (thermal conductivity nearly twice of Inconel 718), where four different unit cell topologies, (a) Octet, (b) Tetrakaidecahedron, (c) Face diagonal-cube, and (d) Cube, were printed using Binder Jetting technology. The aim of this study is to characterize the conjugate heat transfer capabilities of these lattices manufactured in 420 Stainless Steel prior to conduct similar study with Inconel 718, due to relatively simpler additive manufacturing route of Binder jetting (420 stainless steel) compared to Direct Metal Laser Sintering (Inconel 718). The intended design porosity of the samples was 0.886 and the coupons had single unit cell (edge size of 10 mm) in the thickness. The steady-state experiments were conducted to evaluate the effective thermal conductivity and forced convective heat transfer performance. For forced convection, the experiments were conducted for a wide range of Reynolds numbers with air as the working fluid. Overall heat transfer coefficient, pressure drop, Nusselt number and friction factor enhancement level and pumping power requirements of these structures are presented. The implications of the porosity variation due to manufacturing process on the interpretation of relative performance trends is discussed. The effective thermal conductivity was independent of the topology for considered porosity value. Face diagonal-cube yielded the highest heat transfer and pressure drop for the investigated Reynolds number range. However, Cube provided the best overall thermal hydraulic performance value of 1.94 to 2.78 for the investigated range of Reynolds number. At fixed pumping power conditions, Cube was capable of providing the highest heat transfer coefficient.

The Effects of Axisymmetric Convergent Contouring and Blowing Ratio on Endwall Film Cooling and Vane Pressure Side Surface Phantom Cooling Performance

Abstract To reduce the intensity of endwall secondary flow, the axisymmetric convergent contoured endwall is commonly designed in the first nozzle guide vane (NGV) passage of the real gas turbine engines. This endwall contouring can obviously alter flow field near endwall and affect the coolant flow through the upstream double-row discrete film holes. This leads to a significant influence on the endwall film cooling performance, vane surface phantom cooling, and vane passage aerodynamic performance. In this paper, a detailed numerical investigation on the endwall film cooling and vane pressure side surface phantom cooling was performed, at the simulated realistic gas turbine operating conditions (high inlet freestream turbulence level of 16%, exit Mach number of 0.85, and exit Reynolds number of 1.7 × 106). Based on a double coolant temperature model, a novel numerical method for the predictions of adiabatic wall film cooling effectiveness was proposed. This numerical method was validated by comparing the predicted results with experimental data of endwall Nusselt number, endwall film cooling effectiveness, and flow visualization near endwall. The results indicate that the present numerical method can accurately predict endwall thermal load distributions, endwall film cooling distributions, and vane surface phantom cooling distributions. The endwall heat transfer coefficient, endwall film cooling effectiveness, phantom cooling effectiveness of the vane pressure side surface, and total pressure loss coefficients (TPLC) were predicted and compared for two endwall contouring shapes (flat endwall and axisymmetric convergent contoured endwall) at three different blowing ratios (low blowing ratio of BR = 1.0, design blowing ratio of BR = 2.5, and high blowing ratio of BR = 3.5) with a constant density ratio of DR = 1.2, based on the present novel numerical method. Results show that the axisymmetric convergent endwall contouring leads to a slight enhancement (maximum enhancement level less than 20%) of endwall heat transfer in the entire vane passage (0 < x < 0.65Cx). The axisymmetric convergent endwall contouring has a significantly desired effect on endwall film cooling performance (maximum enhancement level of 67%), phantom cooling performance of the vane pressure side surface (maximum increase level approximately 100%), and aerodynamic loss (maximum reduction level of 1.45%) for all blowing ratio cases, but the benefit enhancement level is obviously affected by the blowing ratio values. This suggests that the optimum of endwall contouring shapes is an effective technical way to improve endwall film cooling performance and decrease the depletion of coolant; the coupled effects of the appropriate axisymmetric convergent endwall contouring and the optimum blowing ratio should be considered in the design process of advance endwall cooling schemes.

Endwall heat transfer characteristics of octahedron family lattice-frame materials

Recent studies on traditional metal foam thick blocks suggest that much of the foam volume remains unused in heat dissipation and that the thinner configurations are more efficient in forced convection scenarios. Single cell thick lattice-frame configurations are attractive solutions to the problem of unused foam volume and can also be additively manufactured through Direct Metal Laser Sintering process while also offering superior load bearing capabilities. Due to the inherent nature of such single cell thick configurations, their unit cell topologies' footprint at the enclosing walls has significant effect in both local heat transfer coefficient distribution and net overall heat dissipation. This study presents detailed endwall heat transfer coefficient measurements for lattice-frame structure of Octet unit cell topology and compares its thermal performance with two other members of the Octahedron family, viz. Octa and V-Octet. Developing heat transfer was measured through transient liquid crystal thermography at the endwalls of a unity aspect ratio duct featuring five consecutive interconnected unit cells at a Reynolds number of 37,200. Relative to a smooth channel, the cell-based averaged Nusselt numbers for Octet, Octa and V-Octet were 2.96, 2.78 and 3.05 times, respectively.

Effects of Longitudinal Vortices on Turbulent Junction Flow Heat Transfer

Turbulent junction flow is a vortex system upstream of a blockage embedded in a turbulent boundary layer, which leads to augmented heat transfer near the blockage. Vortex legs wrap around the blockage and travel downstream as longitudinal vortices. In multistage axial turbines or multiple row tube heat exchangers, these longitudinal vortices can collide with junction flow vortices further downstream. Furthermore, these devices often augment mixing and increase heat transfer with high freestream turbulence. A comprehensive study on how longitudinal vortices affect junction flow heat transfer for different Reynolds numbers, freestream turbulence values, and longitudinal vortex configurations was performed. Longitudinal vortices were generated upstream of a symmetric airfoil using delta winglet vortex generators. Spatially resolved endwall heat transfer coefficients upstream and around the junction were measured using infrared thermography. Time-averaged flowfield measurements of the incoming longitudinal vortices were taken using stereo particle image velocimetry. Longitudinal vortices provided limited heat transfer augmentation around the junction and did not disrupt the time-averaged horseshoe vortex. These interactions were identical at high turbulence, but longitudinal vortices were less effective at augmenting heat transfer, likely due to an increase in vortex wandering. Vortex wandering also led to a broadening in lateral heat transfer profiles.

Measurements of detailed heat transfer characteristics on a high-pressure turbine vane endwall with coolant injection

Detailed heat transfer characteristics over high-pressure turbine endwall surfaces with coolant injection were experimentally studied in this paper. Heat transfer experiments were conducted in a properly scaled-up endwall model for an engine-representative oncoming flow profile with a high freestream turbulence intensity of 9.8% at an exit Reynolds number of 0.76 × 106 and an exit Mach number of 0.37. Cooling for the endwall was accomplished by upstream purge flow from a slot and film coolant from three axial rows of discrete film-cooling holes within the vane passage on the endwall. An engine-like coolant-to-mainstream density ratio of 1.70 was achieved by using cooled foreign gas. High-resolution measurements of the endwall heat transfer coefficients were conducted by using a steady, high-resolution infrared thermography technique based on the superposition principle for purge flow blowing ratios of 0.1–0.45 and film coolant injection blowing ratios of 1.4–3.6, respectively. In particular, the effects of the cooling geometries without blowing were considered to simulate hole blockage due to deposition. Results showed that local heat transfer coefficients near the film-cooling hole exits were significantly enhanced due to strong interactions between film coolant injection and mainstream flows, but those downstream of the slot were inhibited, somewhat, by purge flow, resulting from the suppression of purge flow on horseshoe vortex at the vane leading edge. The inhibiting effects of purge flow on the endwall heat transfer are more prominent when purge flow and the film coolant are injected simultaneously. For the discrete holes, higher coolant injection rates generate increased heat transfer levels. Because ingestion and re-injection of mainstream flows through the slot or the film-cooling holes without blowing, the presence of both cooling geometries causes heat transfer enhancement, and the enhancement by the film-cooling holes is higher than that by the slot. The detailed heat transfer measurements in this paper can provide useful thermal boundaries for an optimized endwall film-cooling design that will be investigated in a follow-up study.

Effect of gas turbine endwall misalignment with slashface leakage on the blade endwall aerothermal performance

Effect of the slashface leakage on the gas turbine blade endwall film cooling and heat transfer performance under three endwall misalignment modes (aligned, cascade and dam) is numerically investigated using the method of solving the three-dimensional Reynolds-Averaged Navier-Stokes (RANS) equations with the SST k-ω turbulence model. The numerical calculations are conducted with four coolant momentum flux ratios (I) of 0.48, 0.96, 1.42 and 1.87 under three different endwall misalignment modes. The computational results indicate that for the aligned mode, the fore part endwall (z/Cax < 0.4) cooling effectiveness reaches the maximum when I = 0.96 and the coolant coverage on the back part endwall moves upstream with the increasing coolant momentum. The main high Nu regions concentrate at the back part endwall and the small high heat transfer region at z/Cax = 0.2 increases the endwall heat transfer coefficient by 8% and 3% for I = 1.42 and 1.87, respectively. As for the cascade misalignment mode, the cooling effectiveness of the fore part endwall decreases sharply with the increasing I. There are mainly two high Nu regions at the fore and middle (0.4 < z/Cax < 0.7) part endwall caused by misalignment induced vortex. There is also a low Nu region near z/Cax = 0.9 caused by horseshoe vortex separation, and it can decrease the endwall heat transfer coefficient by 10%. For the dam misalignment mode, the cooling effectiveness is lower but distributes more evenly obstructed by the dam. The horseshoe vortex reattaches on the back part endwall (z/Cax > 0.7) and causes a high Nu region near z/Cax = 0.9 that increases the heat transfer coefficient by 20%, which also leads to a cooling protection failure region. Another severe high Nu region at the middle part of the endwall is brought by the separation vortex reattachment.

Thermal Management of a Transonic Turbine: Leakage Flow and Endwall Contouring Effects

Comparison of heat transfer performance of a nonaxisymmetric contoured endwall to a planar baseline endwall in the presence of leakage flow through a stator-rotor rim seal interface is reported in this paper. Heat transfer experiments were performed on a high turning turbine airfoil passage at a transonic exit Mach number at different leakages to mainstream mass flow ratios. The contoured endwall geometry was optimized for secondary flow aerodynamic losses. Transient infrared thermography was used to measure the endwall surface temperature. Simultaneous calculation of the heat transfer coefficient (HTC) and adiabatic cooling effectiveness (ETA) was performed assuming one-dimensional semi-infinite transient conduction. Results show nominal reduction in the area-averaged HTC but significantly higher coolant film coverage using the contoured endwall compared to baseline endwall at different levels of coolant MFR. Interestingly, significant reduction in the area-averaged heat transfer coefficient was observed for cases without coolant flow in the presence of a purge slot when compared with a solid cascade without any cooling features. The backward-facing step seemed to have a first-order impact on endwall heat transfer coefficient distribution compared to coolant blowing cases. The contoured endwall also showed major improvement in net heat flux reduction (combining the effects of the HTC and ETA) especially near the suction side of the platform.

Experimental investigation of heat transfer characteristics on turbine endwall with full coverage film cooling

Transient heat transfer measurement by Thermochromic Liquid Crystal (TLC) is applied in this paper to investigate heat transfer characteristics of the nozzle endwall with full coverage film cooling. The endwall heat transfer coefficient (h) with different blowing ratios (M) ranging from 0.7 to 4.0 at a constant mainstream Reynolds Number 1.63×105 is measured and analysed. The experiment results show that the heat transfer characteristics of the endwall are significantly affected by the film cooling. However, this effect changes with blowing ratio and varies in different regions of the endwall. At low M (M=0.7), coolant jets get into the boundary layer of the secondary flow, and enhance heat transfer, but lead to highly uneven h on the endwall. With the increase of M, coolant jets start to detach from the endwall while the turbulent mixing begins to enhance the heat transfer of the endwall downstream. Therefore h on the endwall first decreases and then increases with M. However, different turning points (the cases of M = 1 or M = 1.5) and increasing trends with M (staying constant or increasing monotonically) appear in different regions of the endwall.

Flow structure and surface heat transfer from a turbine component endwall contoured using the ice formation method

Steady-state flow structure and surface heat transfer characteristics are numerically predicted for a turbine vane passage, which is contoured using the ice formation method, using ANSYS® Fluent version 12.1. Utilized are an SST turbulence closure model with a low-Reynolds formulation, along with a pressure-based approach to solve the governing equations with the SIMPLE algorithm. The solution domain is spatially discretized with a hybrid grid, created with the commercial grid generator CENTAUR™. The endwall contour shape is obtained using an IFM or ice formation method, which relies upon natural processes wherein energy dissipation and entropy production are minimized. This ice-contouring is imposed only within the vane passage, with endwall transition regions just upstream and just downstream. Particular attention is devoted to the intricate and detailed flow structural variations which are present near the endwall, both through the vane passage and downstream of the vanes. Considered are surface oil flow visualization distributions, vortex distributions, flow pathline distributions, and variations of the z-component of velocity, including their effects of ice-contoured flow structure on surface heat transfer coefficients, relative to a flat, baseline endwall for a turbine flow passage with air flow for a vane chord Reynolds number of 49,900. Results show that contouring significantly alters the endwall heat transfer coefficient distribution, with local values which are both lower and higher compared to baseline results. Such changes are strongly linked to the altered flow field due to the contouring, including changes to passage vortex advection and development.

Heat Transfer and Film Cooling on a Contoured Blade Endwall With Platform Gap Leakage

Turbine blade components in an engine are typically designed with gaps between parts due to manufacturing, assembly, and operational considerations. Coolant is provided to these gaps to limit the ingestion of hot combustion gases. The interaction of the gaps, their leakage flows, and the complex vortical flow at the endwall of a turbine blade can significantly impact endwall heat transfer coefficients and the effectiveness of the leakage flow in providing localized cooling. In particular, a platform gap through the passage, representing the mating interface between adjacent blades in a wheel, has been shown to have a significant effect. Other important turbine blade features present in the engine environment are nonaxisymmetric contouring of the endwall, and an upstream rim seal with a gaspath cavity, which can reduce and increase endwall vortical flow, respectively. To understand the platform gap leakage effect in this environment, measurements of endwall heat transfer, and film cooling effectiveness were performed in a scaled blade cascade with a nonaxisymmetric contour in the passage. A rim seal with a cavity, representing the overlap interface between a stator and rotor, was included upstream of the blades and a nominal purge flowrate of 0.75% of the mainstream was supplied to the rim seal. The results indicated that the endwall heat transfer coefficients increased as the platform gap net leakage increased from 0% to 0.6% of the mainstream flowrate, but net heat flux to the endwall was reduced due to high cooling effectiveness of the leakage flow.

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.

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.

Evaluation of Wall Heat Transfer in Blade Trailing-Edge Cooling Passage

Model configurations of turbine blade trailing-edge internal cooling passage with staggered elliptic pin-fins in streamwise and spanwise are adopted for numerical investigation using computational fluid dynamics (CFD). Grid refinement study is performed at first to identify a baseline mesh, followed by validation study of passage total pressure loss, which gives 2% and 4% discrepancies respectively for two chosen configurations in comparison with experimental measurements. Further investigations are focused on evaluation of wall heat transfer coefficient (HTC) of both pin-fin and end walls, and it is found that CFD predicted pin-fin wall HTC are generally in good agreement with test data for the streamwise staggered elliptic pin-fins, but not the spanwise staggered elliptic pin-fins in which some discrepancies occur. CFD predicted end wall HTC have shown reasonable good agreement for the first three rows, but discrepancies seen in downstream rows are around a factor of 2-3. A ratio of averaged pin-fin and end walls HTC is estimated 1.3-1.5, close to that from a circular pin-fin configuration that has 1.8-2.1. Further study should focus on improving end wall HTC predictions, probably through a conjugate heat transfer model.

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.

Heat Transfer Measurements in a First-Stage Nozzle Cascade Having Endwall Contouring: Misalignment and Leakage Studies

This work supports new gas turbine designs for improved performance by evaluating endwall heat transfer rates in a cascade that is representative of a first-stage stator passage and incorporates endwall assembly features and leakage. Assembly features, such as gaps in the endwall and misalignment of those gaps, disrupt the endwall boundary layer, typically leading to enhanced heat transfer rates. Leakage flows through such gaps within the passage can also affect endwall boundary layers and may induce additional secondary flows and vortex structures in the passage near the endwall. The present paper documents leakage flow and misalignment effects on the endwall heat transfer coefficients within a passage which has one axially contoured and one straight endwall. In particular, features associated with the combustor-to-turbine transition piece and the assembly joint on the vane platform are addressed.

Turbulent Flow and Endwall Heat Transfer Analysis in a 908 Turning Duct and Comparisonswith Measured Data: Part II: Influence of Secondary Flow, Vorticity, Turbulent Kinetic Energy, and Thermal Boundary Conditions on Endwall Heat Transfer

Fundamental character of forced convection heat transfer to the endwall surface of a gas turbine passage can be simulated by using a 90° turning duct. Elevated operating temperatures in gas turbines require a thorough understanding of the turbulent thermal transport process in the three-dimensional end-wall boundary layers. The current study uses an in-house developed three-dimensional viscous flow solver to computationally investigate the heat transfer character near the endwall surfaces. Extensive heat transfer experiments also illuminate the local heat transfer features near the endwall surface and form a baseline data set to evaluate the computational method used. Present experimental effort at Re=342,190 employs a prescribed heat flux method to measure convective heat transfer coefficients on the end-wall surface. Local wall temperatures are measured with liquid crystal thermography. Local convective heat flux is determined by solving the electric potential equation with constant uniform potential that is applied along the inlet and exit boundaries of the endwall. The viscous flow and heat transfer computation algorithms are based on the same methods presented in Part I. The influence of the two primary counter-rotating vortices developing as a result of the balance of inertia and pressure forces on the measured and computed endwall heat transfer coefficients is demonstrated. It is shown that the local heat transfer on the endwall surface is closely related to the structure of the three-dimensional mean flow and the associated turbulent flow field. A comparison of the molecular/ turbulent heat diffusion, wall shear stress, visualization of local turbulent kinetic energy are a few of the tasks that can be performed using a numerical approach in a relatively time efficient manner. The current numerical approach is capable of visualizing the near wall features that can not be easily measured in the flow field near the end-wall surfaces.

Heat Transfer and Flowfield Measurements in the Leading Edge Region of a Stator Vane Endwall

The leading edge region of a first-stage stator vane experiences high heat transfer rates, especially near the endwall, making it very important to get a better understanding of the formation of the leading edge vortex. In order to improve numerical predictions of the complex endwall flow, benchmark quality experimental data are required. To this purpose, this study documents the endwall heat transfer and static pressure coefficient distribution of a modern stator vane for two different exit Reynolds numbers (Reex = 6 × 105 and 1.2 × 106). In addition, laser-Doppler velocimeter measurements of all three components of the mean and fluctuating velocities are presented for a plane in the leading edge region. Results indicate that the endwall heat transfer, pressure distribution, and flowfield characteristics change with Reynolds number. The endwall pressure distributions show that lower pressure coefficients occur at higher Reynolds numbers due to secondary flows. The stronger secondary flows cause enhanced heat transfer near the trailing edge of the vane at the higher Reynolds number. On the other hand, the mean velocity, turbulent kinetic energy, and vorticity results indicate that leading edge vortex is stronger and more turbulent at the lower Reynolds number. The Reynolds number also has an effect on the location of the separation point, which moves closer to the stator vane at lower Reynolds numbers.

Passage Flow Structure and Its Influence on Endwall Heat Transfer in a 90 deg Turning Duct: Mean Flow and High Resolution Endwall Heat Transfer Experiments

Three-dimensional measurements of the mean velocity field have been made in a square-cross-sectional, strongly curved, 90 deg turbulent duct flow. The mean radius to duct width ratio was 2.3. The study was performed as part of an overall investigation of the physics of endwall convective heat transfer. All three components of the velocity vector and the static and total pressure fields were measured using a five-hole probe at four duct cross sections: inlet, 0, 45, and 90 deg. Preliminary turbulence measurements using a single sensor hot wire at the inlet cross section were also obtained to provide streamwise fluctuation levels through the boundary layer. The endwall heat transfer coefficient distribution was determined using a steady-state measurement technique and liquid crystal thermography. A high-resolution heat transfer map of the endwall surface from far upstream of the curve through the 90 deg cross section is presented. The three-dimensional velocity field measurements indicate that a highly symmetric, strong secondary flow develops in the duct with a significant transfer of streamwise momentum to the transverse directions. The cross-stream vorticity components within the measurement plane were estimated using the five-hole probe data and an inviscid from of the incompressible momentum equation. The development of the total and static pressure fields, the three-dimensional mean velocity field, and all three components of the vorticity field are discussed. The endwall heat transfer distribution is interpreted with respect to the measured mean flow quantities.

A study on fluid flow and heat transfer around the circular cylinder located on a flat plate in crossflow

The present study is concerned with the heat transfer enhancement associated with a symmetrical or asymmetrical horseshoe vortex in front of and around the circular cylinder centered between the side walls of a wind tunnel. The static pressure measurements and the flow visualization in front of and around cylinders have been performed to determine the existence of horseshoe vortex. The hue-capturing method using the thermochromatic liquid crystals with great spatial resolution was used to obtain the local information of the endwall heat transfer coefficients. In case of one cylinder, the convective heat transfer coefficients of the region where the horseshoe vortex exists are larger than those of any other region. In case of two cylinders with tandem arrangement, the heat transfer rate of gap spacing (d/D= 1.5) is higher than that of gap spacings (d/D=2.0 or 2.5).