Surface Heat Transfer Distributions Research Articles

The Influence of Turbulence and Reynolds Number on Endwall Heat Transfer in a Vane Cascade

Abstract Endwall heat transfer measurements have been acquired in a vane cascade over a range of turbulence conditions and Reynolds numbers using an array of small commercial infrared (IR) cameras. The linear cascade was tested over five inlet turbulence conditions ranging from low turbulence (0.7%) to high turbulence (17.4%) and three exit chord Reynolds numbers ranging from 500,000 to 2,000,000. The small commercial IR cameras made by Therm-App have a resolution of 384 by 288 pixels and were connected to individual smartphones to record the images. The cascade was modified with small zinc selenide windows to provide IR access for the cameras. The five cameras were calibrated against a constant temperature test plate and the output images were adjusted for the fisheye effect and thermal droop at the edges. The large-scale low-speed cascade, used in the endwall heat transfer study, was configured in a four-vane three full passage arrangement. The vane design includes a large leading and aft loading. This same cascade has been used in the acquisition of vane surface heat transfer distributions, vane suction surface heat transfer visualizations, and vane surface film cooling distributions. This paper includes comparisons with two large eddy simulation calculations, which were conducted prior to the acquisition of the heat transfer data. The influence of the secondary flows on the endwall heat transfer distributions, including the leading edge horseshoe vortex system, is particularly visible at lower turbulence levels and lower Reynolds numbers. However, at higher turbulence levels, the influence of secondary flows is less visible but the influence of Reynolds number and turbulence on transition can be discerned.

Laminar and turbulent flow development study in a rectangular duct with 180° sharp bend by using stereo particle image velocimetry and liquid crystal thermography measurements

This work presents a detailed insight into the flow progression and surface heat transfer distribution across the sharp 180° bend of a two-pass rectangular duct for laminar (Re = 800) and turbulent (Re = 6500) in-flow conditions. Stereoscopic particle image velocimetry (stereo PIV) as well as two-dimensional and two-component PIV measurements and liquid crystal thermography techniques are appropriately used for flow and heat transfer characterization across the complete sharp 180° bend. The centrifugal instabilities arise due to the sharp bend, which induces the secondary flows in the form of counter-rotating vortex pairs commonly known as Dean vortices. These secondary vortices play a significant role in the localized laminar–turbulent transition and turbulence augmentations for laminar and turbulent inflow conditions. Subsequently, quantitative analysis shows that complete 180° turning of flow resulted in intense augmentation of spatially averaged turbulence quantities. Root mean square (RMS) fluctuations in the transverse direction V¯T|rms increase by 298% and 186% for respective flow conditions. Augmentation of ∼ 287% (laminar) and 260% (turbulent) in the wall-normal RMS fluctuations (V¯N|rms) are observed. These augments in transverse and wall-normal velocity fluctuations result in a very sharp amplification of spatially averaged turbulent kinetic energy (k¯), that is, 1825% for inlet laminar and 928% for inlet turbulent flow regimes.

Experimental study of turbulent flow heat transfer and pressure loss over surfaces with dense micro-depth dimples under viscous sublayer

Presented are experimentally-measured heat transfer and pressure loss characteristics for dimpled surfaces, placed along one surface of a channel, with different ratios of dimple depth to channel height δ/H, and for Reynolds numbers ranging from 10,000 to 70,000. With the same relative dimple spacing to dimple print diameter, and the same ratio of dimple depth to dimple print diameter of δ/d = 0.20, experimental results for arrays of spherical indentation dimples with micro depths of δ = 0.6 and 1.0 mm (δ/H = 0.03 and 0.05) are compared with thermal characteristics for arrays of larger dimples with δ = 4.0 mm and δ/H = 0.20. Local surface heat transfer distributions are measured using transient liquid crystal thermography. Additional steady-state experiments are also employed to provide overall heat transfer enhancement and friction factor augmentation values. Results indicate that Nusselt number ratios and friction factor ratios of a turbulent flow in the channel are dependent upon the ratio of dimple micro depth to channel height, which are different findings from previous studies with larger-depth dimples in the cooling channels. As such, the present data for micro-depth dimples with ratios of dimple depth to channel height δ/H of 0.03 and 0.05 are firstly provided. The importance of a critical Reynolds number is demonstrated by the results, which is determined when the ratio of viscous sublayer layer thickness to the dimple depth is equal to unity, and has been ignored in the previous dimpled channel heat transfer design. Nusselt number enhancements for the micro dimples are generally lower at lower Reynolds numbers, but increase as the Reynolds number becomes larger and approaches the critical Reynolds number. Nusselt number enhancements are then approximately invariant with Reynolds number, when Reynolds number then exceeds the critical value.

Full Surface Heat Transfer Characteristics of Stator Ventilation Duct of a Turbine Generator

Turbine generators operate with complex cooling systems due to the challenge in controlling the peak temperature of the stator bar caused by Ohm loss, which is unavoidable. Therefore, it is important to characterize and quantify the thermal performance of the cooling system. The focus of the present research is to investigate the heat transfer and pressure loss characteristics of a typical cooling system, the so-called stator ventilation duct. A real scale model was built at its operating conditions for the present study. The direction of cooling air was varied to consider its operation condition, so that there are: (1) outward flow; and (2) inward flow cases. In addition, the effect of (3) cross flow (inward with cross flow case) was also studied. The transient heat transfer method using thermochromic liquid crystals is implemented to measure full surface heat transfer distribution. A series of computational fluid dynamics (CFD) analyses were also conducted to support the observation from the experiment. For the outward flow case, the results suggest that the average Nusselt numbers of the 2nd and 3rd ducts are at maximum 100% and 30% higher, respectively, than the inward flow case. The trend was similar with the effect of cross flow. The CFD results were in good agreement with the experimental data.

Aerothermal Characteristics of Solid and Slitted Pentagonal Rib Turbulators

This work is an experimental study of detailed aerothermal characteristics inside a duct carrying an array of solid and permeable pentagonal ribs with a parallel and inclined slit, mounted on the bottom wall. The rib height-to-hydraulic diameter ratio, the rib pitch-to-height ratio, and the open area ratio fixed during experiments are 0.125%, 12%, and 25%, respectively. The heat transfer coefficient (HTC) distribution is mapped by using transient liquid crystal thermography (LCT), while the detailed flow measurements are performed by using particle image velocimetry (PIV). The primary focus of the study is to assess the influence of inter-rib region flow characteristics on the local heat transfer fields. The heat transfer and friction factor measurements are evaluated along with thermohydraulic performances at different Reynolds numbers, i.e., 26,160, 42,500, and 58,850. Performance indexes show that the pentagonal ribs with the inclined-slit are superior to other configurations from both perspective. Aerothermal features within inter-rib region were elucidated by analyzing the time-averaged streamlines, mean velocities, fluctuation statistics, vorticity, turbulent kinetic energy (TKE) budget terms, and local and spanwise-averaged Nusselt number as well as augmentation Nusselt numbers. Critical flow structures and coherent structures were identified, which illustrate about different flow dynamic processes. The flow emanating out of the inclined-slit pentagonal rib significantly affects the magnitude of streamwise velocity, fluctuation statistics, vorticity, and TKE budget terms at the downstream corner, whereas the dissipation term of TKE budget correlates well with the surface heat transfer distribution.

Natural convection heat transfer around a horizontal circular cylinder near an isothermal vertical wall

Natural convection heat transfer from a single horizontal cylinder near an isothermal wall is experimentally investigated in the environmental chamber. When cylinder is heated the plume rising from the heated cylinder interacts with the wall and significantly affects the surface heat transfer distribution. Thus, the distance between horizontal cylinder and side wall can influence the heat transfer. The heat transfer from heated cylinder with different distances was compared. It was found that the influence of the wall on the convection heat transfer from the circular cylinder began when the distance between the wall and cylinder is less than 80 mm.

Natural Convection Heat Transfer Around a Horizontal Circular Cylinder for the Case of Constant Surface Temperature of the Cylinder Near a Vertical Wall

Natural convection heat transfer from a single horizontal cylinder near an isothermal wall is experimentally investigated in the environmental chamber. The cylinder is heated with constant temperature, the rising plum from the heated cylinder interacts with the side wall and significantly affects the surface heat transfer distribution. Thus, the distance between horizontal cylinder and side wall can influence the heat transfer. The heat transfer from heated cylinder with different distances were compared. Results are obtained for Ra = 104. It was found that natural convection heat transfer from horizontal cylinder depends on the distance between wall. Also, a heat transfer correlation has been proposed for a single horizontal cylinder near a wall and is compared with earlier work. The results indicate that the vertical wall affect the average Nusselt number at the cylinder. Therefore, the vertical wall has a positive effect on the Nusselt number at the cylinder.

Heat transfer enhancement to an array of synthetic air jets by an induced crossflow

Jet vectoring and crossflow enhanced heat transfer to a modular synthetic air jet array consisting of six individually controllable parallel slot jets is investigated. When applied in a multi-jet array, jet vectoring can be implemented to operate as an adaptive, modular heat exchanger capable of dynamically targeting hot spots and enhancing local cooling. Time averaged surface heat transfer distributions are presented for varying Reynolds number, stroke length and axial spacing. It is shown that crossflow is achievable for an array of jets; however vectoring performance is dictated by inter-jet phase delay and limited by axial spacing and stroke length. All operating parameters showed increased levels of heat transfer are produced by operating the array out of phase. In particular, at small axial spacings this is attributed to crossflow generated by the out of phase jet pulsation.

Convective heat transfer for multiple rows of impinging air jets with small jet-to-jet spacing in a semi-confined channel

The effects of jet-to-jet pitch ratios in both streamwise and spanwise directions (xn/d and yn/d, ranged from 2 to 5), jet-to-target spacing ratio (zn/d, ranged from 2 to 4) and jet array arrangement (inline and staggered) on the jet impingement heat transfer behaviors are experimentally investigated under the jet Reynolds number ranged from 5000 to 25,000. Special attention is paid to examine the dense-array jet impingement behaviors inside a semi-confined channel with one-directional spent air outflow where the jet-induced crossflow is more serious. The detailed surface heat transfer distribution under the densest-array jet impingement inside the semi-confined channel is behaved as the character of a channel-like flow due to strong adjacent jet interference. The strongest jet impingement heat transfer is achieved for the case with xn/d=yn/d=3 among the presented jet-to-jet pitch ratios under the same impingement Reynolds number. While evaluating on the base of the same mass flow rate of coolant per unit area of cooled surface, the heat transfer performance of multiple-jets with xn/d=yn/d=5 is obviously superior to that of denser multiple-jets array. The uniformity coefficient is not significantly affected by the jet-to-jet pitch investigated in the present study. Besides, the heat transfer performance under zn/d=2 is confirmed to be superior to the other jet-to-target spacing ratios for the dense array of multiple jets. For the multiple-jets with xn/d=yn/d=3, the local area-averaged row Nusselt number corresponding to the front rows under staggered array is somewhat higher than that under the inline array. From the fourth row on, the jet-induced crossflow effect behaves more obviously for the staggered array. While for the multiple-jets with xn/d=yn/d=5, the heat transfer for the staggered array seems to be somewhat superior to that of inline array.

Experimental investigation on surface heat transfer characteristics of hypersonic two-dimensional rearward-facing step flow

Hypersonic rearward-facing step flow is one of the basic flow problems in the design of engine for endo-atmospheric hypersonic vehicle, including thermal protection, and aero-optical correction for infrared imaging window of hypersonic interceptors, etc. To know the characteristics of hypersonic rearward-facing step flow is of vital importance in improving the performances of vehicles, and understanding the basis of the flow. This paper investigates the characteristics of a two-dimensional hypersonic rearward-facing step flow, measures the surface heat transfer coefficient and the surface static pressure downstream the step, and compares the results with the values predicted using the hypersonic boundary layer theory. And the results are demonstrated by the flow structure visualization using NPLS (nano-based planar laser scattering) technique. It is concluded that for the hypersonic two-dimensional rearward-facing step flow, the surface heat transfer distribution can be determined directly by the boundary layer edge parameters at the step; and the viscous effect dominates the flow characteristic in the separation and reattachment region; whole in the far-field region downstream the step, the heat transfer coefficient approaches an asymptotic value that may be equal to the turbulent flat plate value. Furthermore, the boundary layer structure may depend on the ratio of boundary layer thickness to the height of step. It is concluded that, when studying the problem of hypersonic rearward-facing step using CFD (computational fluid dynamics) technology, choosing an appropriate turbulent model is needed.

Measurements of skin friction and heat transfer beneath an impinging slot jet

An experimental study was performed to measure skin friction and surface heat transfer for an orthogonal slot jet impinging on a flat plate. Surface pressure, total pressure, velocity profiles, skin friction and temperature distributions were obtained within twenty nozzle widths of jet centerline at a range of 20,000<Rej<36,000. Stanton gauge, and oil film methods were used to measure skin friction. Multiple micro-thermocouples attached to an iso-flux surface were utilized to acquire the target plate temperature distribution. The majority of the experimental data was taken at a nozzle-to-plate spacing of four nozzle widths where the jet was still within its potential core length. The local surface heat transfer and skin friction distributions showed local extrema that were not coincident, and whose behavior do not appear to be entirely consistent with hypotheses that attribute the secondary peak to laminar-to-turbulent transition of the surface layer. It is believed that these off center peaks occur as a consequence of enhanced turbulent momentum and heat transport in the wall-normal direction.

Flow regime characterisation of an impinging axisymmetric synthetic jet

Impinging synthetic jets have excellent potential for energy-efficient local cooling in confined geometries. For a given geometry, synthetic jet flows are mainly characterised by the Reynolds number and the ratio of stroke length to a geometric length scale. The flow field of an impinging synthetic jet and the corresponding surface heat transfer distribution are strongly dependent on the dimensionless stroke length, yet few studies have investigated the flow field dependence for a wide range of stroke lengths. Therefore, the aim of this paper is to identify the various flow regimes as a function of stroke length. The experimental approach combines high speed particle image velocimetry and single point hot wire anemometry, and investigates an axisymmetric synthetic air jet impinging onto a smooth planar surface for a wide range of stroke length (3<L0/D<32) and nozzle-to-surface spacing (2<H/D<16). Since the Reynolds number effect is better understood, most of the presented results are for a single Reynolds number (Re=1500). Four free synthetic jet flow morphology regimes are identified based on threshold values for the stroke length L0/D, which are in good agreement with previously published findings for an impulsively started jet flow. Furthermore, four impinging synthetic jet flow regimes are identified based on threshold values for the ratio of normalised stroke length to nozzle-to-surface spacing (L0−2D)/H, which are in good agreement with previously published thresholds for stagnation point heat transfer regimes.

The effect of global cross flows on the flow field and local heat transfer performance of miniature centrifugal fans

Centrifugal fans are often integrated into thermal management solutions for a range of applications. Consequently, centrifugal fan designs can be subjected to varying environmental conditions, many of which can alter fan performance characteristics and ultimately influence the heat transfer performance of the cooling solution. Global cross flows are a commonly encountered practical operating condition, particularly in the cooling of electronics. Air-cooled electronic enclosures often incorporate miniature centrifugal fans to maintain reliable component operating temperatures at a local level, while larger system level fans are used to simultaneously control the ambient temperature within the enclosure. This type of operating condition has been investigated by introducing a uniform crossing air flow above a centrifugal fan inlet. Two scaled miniature centrifugal fan designs were selected to fundamentally assess the influence on local velocity field and heat transfer performance. This was achieved experimentally using Particle Image Velocimetry, and a combined infrared and heated-thin-foil technique developed for the accurate measurement of local heat transfer coefficients. the introduction of a crossing air flow above the fan inlet indirectly reduced both the local and global thermal performance of the centrifugal fan, and the resultant distorted inflow shifted the surface heat transfer distribution at the fan outlet from an axisymmetric to asymmetric profile. However, strategic positioning of components relative to a centrifugal fan can maintain the average component heat transfer coefficient at a similar level to a case without any cross flow. Results also indicate issues associated with the implementation of miniature centrifugal fan designs into crossing air flow environments, with reductions in thermal performance of over 30% observed.

Suitability of the k–ω turbulence model for scramjet flowfield simulations

SUMMARYThe suitability of Wilcox's 2006 k– ω turbulence model for scramjet flowfield simulations is demonstrated by validation against five test cases that have flowfields representative of those to be expected in scramjets. The five test cases include a 2D flat plate, an axisymmetric cylinder, a backward‐facing step, the mixing of a pair of coaxial jets and the interaction between a shock wave and turbulent boundary layer. A generally good agreement between the numerical and experimental results is obtained for all test cases. These tests reveal that despite the turbulence model's sensitivity to freestream turbulence properties, the numerically predicted skin friction agrees with experimental data and theoretical correlations to their degree of uncertainty. The tests also confirm the importance of using a y+ value of less than 1 in getting accurate surface heat transfer distributions. In the coaxial jets case, the importance of matching the turbulence intensities at the inflow plane in improving the predictions of the turbulent mixing phenomena is also shown. A review of guidelines with regard to the setting up of grids and specification of freestream turbulence properties for turbulent Reynolds‐averaged Navier–Stokes CFD simulations is also included in this paper. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Transonic Turbine Blade Tip Aerothermal Performance With Different Tip Gaps—Part I: Tip Heat Transfer

A closely combined experimental and computational fluid dynamics (CFD) study on a transonic blade tip aerothermal performance at engine representative Mach and Reynolds numbers (Mexit=1,Reexit=1.27×106) is presented here and its companion paper (Part II). The present paper considers surface heat-transfer distributions on tip surfaces and on suction and pressure-side surfaces (near-tip region). Spatially resolved surface heat-transfer data are measured using infrared thermography and transient techniques within the Oxford University high speed linear cascade research facility. The Rolls-Royce PLC HYDRA suite is employed for numerical predictions for the same tip configuration and flow conditions. The CFD results are generally in good agreement with experimental data and show that the flow over a large portion of the blade tip is supersonic for all three tip gaps investigated. Mach numbers within the tip gap become lower as the tip gap decreases. For the flow regions near the leading edge of the tip gap, surface Nusselt numbers decrease as the tip gap decreases. Opposite trends are observed for the trailing edge region. Several “hot spot” features on blade tip surfaces are attributed to enhanced turbulence thermal diffusion in local regions. Other surface heat-transfer variations are attributed to flow variations induced by shock waves. Flow structure and surface heat-transfer variations are also investigated numerically when a moving casing is present. The inclusion of moving casing leads to notable changes to flow structural characteristics and associated surface heat-transfer variations. However, significant portions of the tip leakage flow remain transonic with clearly identifiable shock wave structures.

The Effects of Freestream Turbulence, Turbulence Length Scale, and Exit Reynolds Number on Turbine Blade Heat Transfer in a Transonic Cascade

This paper experimentally investigates the effect of high freestream turbulence intensity, turbulence length scale, and exit Reynolds number on the surface heat transfer distribution of a turbine blade at realistic engine Mach numbers. Passive turbulence grids were used to generate freestream turbulence levels of 2%, 12%, and 14% at the cascade inlet. The turbulence grids produced length scales normalized by the blade pitches of 0.02, 0.26, and 0.41, respectively. Surface heat transfer measurements were made at the midspan of the blade using thin film gauges. Experiments were performed at the exit Mach numbers of 0.55, 0.78, and 1.03, which represent flow conditions below, near, and above nominal conditions. The exit Mach numbers tested correspond to exit Reynolds numbers of 6×105, 8×105, and 11×105, based on true chord. The experimental results showed that the high freestream turbulence augmented the heat transfer on both the pressure and suction sides of the blade as compared with the low freestream turbulence case. At nominal conditions, exit Mach 0.78, average heat transfer augmentations of 23% and 35% were observed on the pressure side and suction side of the blade, respectively.

Effects of Large Scale High Freestream Turbulence and Exit Reynolds Number on Turbine Vane Heat Transfer in a Transonic Cascade

This paper experimentally and numerically investigates the effects of large scale high freestream turbulence intensity and exit Reynolds number on the surface heat transfer distribution of a turbine vane in a 2D linear cascade at realistic engine Mach numbers. A passive turbulence grid was used to generate a freestream turbulence level of 16% and integral length scale normalized by the vane pitch of 0.23 at the cascade inlet. The base line turbulence level and integral length scale normalized by the vane pitch at the cascade inlet were measured to be 2% and 0.05, respectively. Surface heat transfer measurements were made at the midspan of the vane using thin film gauges. Experiments were performed at exit Mach numbers of 0.55, 0.75, and 1.01, which represent flow conditions below, near, and above nominal conditions. The exit Mach numbers tested correspond to exit Reynolds numbers of 9×105, 1.05×106, and 1.5×106 based on a vane chord. The experimental results showed that the large scale high freestream turbulence augmented the heat transfer on both the pressure and suction sides of the vane as compared to the low freestream turbulence case and promoted a slightly earlier boundary layer transition on the suction surface for exit Mach 0.55 and 0.75. At nominal conditions, exit Mach 0.75, average heat transfer augmentations of 52% and 25% were observed on the pressure and suction sides of the vane, respectively. An increased Reynolds number was found to induce an earlier boundary layer transition on the vane suction surface and to increase heat transfer levels on the suction and pressure surfaces. On the suction side, the boundary layer transition length was also found to be affected by increase changes in Reynolds number. The experimental results also compared well with analytical correlations and computational fluid dynamics predictions.

Natural convection heat transfer from two horizontal cylinders

Natural convection heat transfer from a single horizontal cylinder and a pair of vertically aligned horizontal cylinders is investigated. Surface heat transfer distributions around the circumference of the cylinders are presented for Rayleigh numbers of 2 × 10 6, 4 × 10 6 and 6 × 10 6 and a range of cylinder spacings of 1.5, 2 and 3 diameters. With a cylinder pairing the lower cylinder is unaffected by the presence of the second cylinder; the same is true of the upper cylinder if the lower one is not heated. However, when both cylinders are heated it has been found that a plume rising from the heated lower cylinder interacts with the upper cylinder and significantly affects the surface heat transfer distribution. Spectral analysis of surface heat transfer signals has established the influence of the plume oscillations on the heat transfer. Thus, when the plume from the lower cylinder oscillates out of phase with the flow around the upper cylinder it increases the mixing and results in enhanced heat transfer.

Experiments on the hypersonic turbulent shock-wave/boundary-layer interaction and the effects of surface roughness

AbstractAn experimental investigation was performed to study the effects of surface roughness on the Mach 8·2 hypersonic turbulent shockwave–boundary-layer interaction characteristics of a deflected control flap configuration. In particular, the surface pressure and heat transfer distribution along a quasi-2D ramp compression corner model was measured for flap angles between 0° and 38°, along with a Schlieren flow visualisation study. It was found that surface roughness, of scale 10% of the hinge-line boundary layer thickness, significantly increased the extent of the interaction, while increasing the magnitude of the peak pressure and heat flux just aft of reattachment. The incipient separation angle for a fully turbulent, Mach 8·2 boundary layer with a hinge line Reynolds number of 1·44 × 106, was estimated at 28-29°, reducing to between 19-22° with the introduction of laminar sub-layer scale surface roughness.

Measurement and Prediction of Heat Transfer Distributions on an Aft-Loaded Vane Subjected to the Influence of Catalytic and Dry Low NOx Combustor Turbulence

Aft-loaded vane designs can have an impact on surface heat transfer distributions by accelerating boundary layers for a greater portion of the suction surface. New combustion systems developed for low emissions have produced substantial changes to the characteristics of inlet turbulence entering nozzle guide vanes. This paper documents heat transfer rates on an aft-loaded vane subject to turbulence generated by mock combustion configurations representative of recently developed catalytic and dry low NOx (DLN) combustors. Four different inlet turbulence conditions with levels ranging up to 21% are documented in this study and vane heat transfer rates are acquired at vane exit chord Reynolds numbers ranging from 500,000 to 2,000,000. Heat transfer distributions show the influence of the turbulence conditions on heat transfer augmentation and transition. Cascade aerodynamics are well documented and match pressure distributions predicted by a commercial computational fluid dynamics (CFD) code for this large-scale low-speed facility. The aft-loaded vane pressure distribution exhibits a minimum value at about 50% arc on the suction surface. This comprehensive vane heat transfer data set is expected to represent an excellent test case for vane heat transfer predictive methods. Predictive comparisons are shown based on a two-dimensional boundary layer code using an algebraic turbulence model for augmentation as well as a transition model.