Transient Thermochromic Liquid Crystal Research Articles

Influence of corrugated jet plates on internal heat transfer and flow dynamics of double-wall cooling: experimental-numerical approach

In the present study, combined experimental and numerical investigations were carried out to analyze the fluid flow dynamics and heat transfer of impinging jets in a corrugated double-wall structure for exhaust nozzle cooling operating at high temperatures. The sinusoidal wavy plate, with multi-jet holes positioned at its trough, served as the jet plate, creating a staggered configuration with respect to the effusion holes. The transient thermochromic liquid crystal (TLC) method was employed to determine the local distribution of the Nusslet number on the flat effusion plate in a wind tunnel at small Reynolds numbers (ranging from 450 to 2700 based on the jet hole diameter (d)). In the numerical part of the study, the k-ω SST turbulence model verified with the TLC data was used to solve the RANS equations. The effects of jet-to-plate spacing (Have/d = 1.6–3.6), corrugated plate amplitude (A/d = 0.2–0.8), effusion hole diameter (de/d = 0.6–1.5), and Reynolds numbers were examined in detail. Our results showed that the introduction of the corrugated structure in the jet plate altered the high-velocity region pattern within the jet hole, leading to a 16 %-81 % reduction in jet core length compared to the flat jet plate. At a relatively high wave amplitude (A/d ≥ 0.5), the vortex structure underwent significant changes, inducing a higher level of turbulence kinetic energy in the near-wall region of the target chamber. This led to enhanced heat transfer near the stagnation and effusion hole regions compared to the flat double-wall cooling. The effusion hole diameter exerted a minimal effect on the overall Nusselt number of the corrugated double-wall cooling. At very low Reynolds numbers (Rej 〈900), the flow and heat transfer characteristics in corrugated double-wall cooling resembled those observed in flat double-wall cooling.

Investigation on the Flow and Heat Transfer Characteristics in a Multi-Pass Channel Using Magnetic Resonance Velocimetry and Transient Thermochromic Liquid Crystal

Abstract The three-dimensional flow field in multi-pass channels with and without ribs was measured by magnetic resonance velocimetry (MRV), while heat transfer performance on the endwall of channels in the same geometry was investigated using transient thermochromic liquid crystal (TLC) technique. The evolution of comprehensive three-dimensional flow field and their correlation with local heat transfer enhancement on end wall of multi-pass channels with and without ribs were revealed as a whole picture. Results show that the flow characteristics in the right-angle bend as well as the second pass are dramatically different for the smooth and ribbed channels, resulting in totally different features of heat transfer distribution on the end wall in those two channels. For the smooth channel, strong dean vortices form around the bend region near the outer wall where heat transfer is enhanced substantially. For the ribbed channel, no dean vortex but complex three-dimensional flow presents around bends. Heat transfer downstream of ribs close to the reattachment regions is strengthened noticeably. Comparison between velocity and heat transfer results suggest that one of the principle mechanisms driving heat transfer enhancement is both endwall directed velocity for smooth and ribbed channels, even though they are induced by different flow structures.

Experimental and numerical investigation of jet impingement cooling onto a rib roughened concave internal passage for leading edge cooling of a gas turbine blade

Considered is thermal protection of the concave surface within an internal passage, which models the leading edge cavity of vanes or blades of stationary or aero engine gas turbines. Included along the internal surface are different arrangements of V-shaped ribs to further augment surface cooling heat transfer rates. Impingement jets are employed for the cooling, which are configured so that associated cooling air exits the cavity through a crossflow outlet and an array of staggered film cooling holes. The curved part of the asymmetric, concave target plate features a constant radius and connects to straight lines on the pressure and suction sides. Addressed are H/Djet values of 2.7 and 4.0, and Reynolds number Re values of 18,000, 24,000, and 30,000. A maximum crossflow configuration is utilized wherein the coolant flow exits the cooling passage through the film cooling holes and from one end of the passage, as the opposite end is blocked. Different surface configurations are investigated, including 4 V-ribs in four different positions, 8 V-ribs in one position, and a smooth surface of the leading edge cavity. Each V-rib geometry consists of a regular V-rib pointing upstream. A transient thermochromic liquid crystal (TLC) technique is employed to obtain spatially-resolved surface heat transfer data on the internal wall of the leading edge cavity. Resulting experimental heat transfer data are used to validate and verify CFD simulations. Employed for this purpose are steady-state 3D RANS simulations obtained using the OpenFOAM Version 7 numerical code with a kω-shear-stress-transport (SST) turbulence model. Experimentally-measured surface Nusselt number results indicate significant local enhancements adjacent to rib wakes, provided local crossflow velocity values are substantial. Local surface Nusselt number enhancements thus depend upon rib position, but local surface heat transfer increases are most strongly tied to magnitudes of local crossflow velocity. In contrast to these experimental results, CFD simulation results do not show significant variations as rib position is altered. The best thermal performance is associated with 4 V-rib arrangements in two positions, and 8 V-ribs in one position.

Experimental investigations on the heat transfer characteristic of impingement/swirl cooling structures inside turbine blade leading edge

Experimental investigations were performed to study the heat transfer characteristics of impingement/swirl cooling structures inside a turbine leading edge (LE) as measured using the transient thermochromic liquid crystal (TLC) technique. The considered Reynolds numbers (Re) were 10,000, 20,000, and 30,000, and the effects of Re and the ratio of the impingement cooling hole offset distance to the impingement cooling hole diameter (e/d) on the heat transfer performance of the impingement/swirl cooling structures inside the blade LE were analyzed. The effects of the coolant outflow mode were also considered. The results show that the heat transfer intensity of the high heat transfer region of impingement/swirl cooling structures increases with Re. The e/d on the local Nusselt number (Nu) distributions for the two shooting areas are similar under different Re conditions. Changes in e/d significantly influence the heat transfer characteristics. As e/d increases from −1.5 to 1.5, the heat transfer effect and thermal uniformity of the LE inner wall chamber improved. Comparative analyses with the experimental results of the LE model without film cooling holes show that the thermal uniformity of the LE model significantly improves when including film cooling holes.

Study on convective heat transfer characteristics of inclined jet impinging cylindrical target surface in the confined space

• Heat transfer of cylinder surface with flow confinement is experimentally tested. • The effects of jet angle and Reynolds number on heat transfer have been studied. • The mechanism of heat transfer for different jet angles is analyzed in detail. In this study, the convective heat transfer characteristics of a cylinder with flow confinement under the impingement of a circumferentially arranged turbulent inclined circular air jet array are experimentally studied. This jet configuration can be effectively applied in the cooling/heating of rotating machinery, such as cooling of the steam turbine shaft, cement rotary kiln cooling, and textile drying. The purpose of the study is to elucidate the effect of the jet angle ( θ ) and the jet Reynolds number ( Re d ) on the heat transfer behaviors of the cylindrical target surface. The jet Reynolds number varies from 20,000 to 35,000, and three different jet angles ( θ =20°,30°,45°) are adopted. The local Nusselt number distribution of the region of interest on the cylindrical target surface is experimentally measured by applying the transient thermochromic liquid crystal technique. Numerical studies are conducted to obtain details of flow patterns in the confined space, so as to analyze the convective heat transfer mechanisms. The results show that for small jet angles ( θ =20°,30°), due to the intense circumferential cross-flow, the overall distribution of the Nusselt number is relatively uniform. As the jet angle increases to 45 °, the Nusselt number in the middle of the target surface increases significantly because of the formation of the stagnation region. As the jet angle increases from 30 ° to 45 °, the heat transfer uniformity on the cylindrical surface decreases significantly. The maximum difference of the variation coefficient of the Nusselt number is 26.6% at Re d = 35 , 000 . As the jet Reynolds number increases, the local Nusselt number on the cylindrical target surface increases as a whole. The average Nusselt number with Re d = 35 , 000 increases by 64.3%-74.9% compared with the cases of Re d = 20 , 000 .

Experimental investigation on convective heat transfer of inclined jets impinging on the rotating cylindrical surface in the confined space

This paper presents an experimental study on the convective heat transfer characteristics of a circumferentially arranged oblique jet array impinging on the rotating cylindrical surface in the confined space. This cooling structure can be used for the cooling of rotating equipment, such as the cooling of rotating shafts of steam turbines, the quenching of metal materials, and the cooling of cement rotary kilns. The heat transfer coefficient on the rotating cylindrical surface is obtained with the transient thermochromic liquid crystal (TLC) method. Local and circumferential average heat transfer characteristics, overall heat transfer uniformity, and heat transfer capacity are analyzed comprehensively. The effects of rotational Reynolds number (Rer), jet Reynolds number (Rej), and jet angle (θ) on the heat transfer behaviors are investigated where the rotational Reynolds number is varied from 105,115 to 420,460 and the jet Reynolds number is varied from 20,000 to 35,000 at three different jet angles (θ=20∘,30∘,45∘). The experimental results show that with the increase of the rotational Reynolds number, the overall circumferential average Nusselt number presents a trend of first decreasing and then increasing, mainly depending on the relative velocity of the circumferential velocity of the main flow and the tangential velocity of the rotating cylindrical surface. The average Nusselt number of the cylindrical surface decreases first and then increases gradually with the increase of the rotational Reynolds number and jet Reynolds number. As the jet angle increases from 20∘ to 45∘, the average Nusselt number increases gradually.

Experimental and numerical study on an enhanced swirl cooling with convergent tube wall and local dimple arrangements

This paper experimentally and numerically investigates an enhanced swirl cooling structure, featuring multiple connected convergent swirl tubes and dimpled wall. The dimpled swirl tube adopts a novel local dimpled wall scheme, only arranging dimple arrays under the tangential jets, which takes advantage of the heat transfer potential of the individual dimple. A counterpart multistage convergent swirl tube without dimples is also studied as a comparison baseline to clarify the action of local dimple arrays on swirling flow. The spatially resolved heat transfer data are measured by applying transient thermochromic liquid crystal (TLC) technique. Four channel Reynolds numbers (Re = 20,000 to 50,000) are tested based on the maximum tube parameters. The results show that the enhanced dimpled swirl tube performs a synergistic reinforcement of the high-speed swirling flow over the dimple arrays, which produces many local high Nusselt number patterns. Under the individual injection slot, the upstream dimples have a better enhanced heat transfer performance. For the studied working conditions, the globally averaged Nusselt number of the dimpled swirl tube can be increased by 6.0%–11.5% in comparison with the baseline convergent swirl tube. Besides, the dimpled wall also suppresses the swirl intensity, which leads to the weakening of wall-fluid shear stress action and the disappearance of downstream vortex breakdown, and consequently reduces the pressure loss by 2.0%–10.2%. Based on the well-validated CFD results, the dimpled wall under incoming jets redistributes the contribution of the near-wall turbulence and the forced convection due to the near-wall flow motion to the heat transfer enhancement in the swirl tube, which achieve an ideal cooling performance of high heat transfer in combination with low pressure loss. This provides a new inspiration for the design and optimization of swirl cooling device.

Experimental and numerical study of swirl impingement cooling for turbine blade leading edge with internal ridged wall and film extraction holes

This paper presents a comparative experimental and numerical study of the internal heat transfer and pressure loss of swirl impingement cooling for turbine blade leading edge with internal ridges and film extraction holes. The leading edge cooling study includes a typical impingement cooling with centerline normal jets, a swirl impingement cooling with offset jets, and a swirl impingement cooling with offset jets and ridged wall. The normal jets are arranged at the impingement plate centerline, and the offset jets adopt a staggered arrangement alternatively on the lateral sides of the impingement plate. The ridged concave wall features two small semi-circular ridges symmetrically on opposite sides under the offset jets. Three rows of film holes are arranged symmetrically on the leading edge cavity, and another two rows of film holes are arranged on the opposite cavity wall behind. Transient thermochromic liquid crystal (TLC) experiments are conducted to obtain the detailed heat transfer distributions on the internal surface of the leading edge cavities with different cooling configurations. The results show that in the swirl impingement cooling, the swirling flow induced by the offset jets impinging onto the concave wall improves the streamwise heat transfer uniformity on the leading edge internal wall, which overcomes the limitation of large heat transfer non-uniformness between jet stagnation region and surrounding regions in typical normal jet impingement cooling. In the swirl impingement cooling with ridged wall, the near-wall jet interacts with the ridges on the concave wall and develops more widely along the spanwise directions, which greatly improves the spatial heat transfer uniformity. In addition, the ridges on the wall can also induce near-wall jets to separate from the wall and attach to the opposite concave wall, which produces low-velocity recirculation in the leading edge cavity and improves the capability and uniformity of coolant outflow through the film holes. Compared to the normal jet impingement cooling, the heat transfer of swirl impingement cooling with smooth and ridged wall can be increased respectively by 10.8%–14.0% and 10.9%–23.6% for the jet Reynold number from 20,000 to 50,000. In addition, the pressure loss is also reduced slightly by less than 5% in the swirl impingement cooling. As supplements to the experiments, numerical computations provide more insights into the turbulent flow structure in the leading edge cooling configurations.

Heat transfer characteristics in a trailing-edge slot cooling surface with outward protrusions

ABSTRACT A novel turbulated cutback structure is designed for trailing-edge cooling, which has smooth-transition protrusions on the cutback surface. Experimental investigations are carried out on the effects of the protrusion configuration and the blowing ratio M on adiabatic cooling effectiveness and convective heat transfer coefficient of the cutback surface using transient thermochromic liquid crystal technology and steady pressure-sensitive paint technology as measuring methods. As M increases, the comprehensive heat transfer performance (heat flux ratio) first increases and then decreases, reaching a peak at M = 1.00. The smooth-transition protrusions effectively enhance the comprehensive heat transfer performance by 12.4 ~ 36.8% of the cutback surface.

Experimental and Numerical Investigations of Extended Jet Effects on Multiple-Jet Impingement Heat Transfer Characteristics

Abstract A jet impingement system equipped with extended jet holes was experimentally evaluated in this study. To reproduce a strong crossflow condition, the jet impingement configuration featured a multiple-jet array of 16 × 5 rows in the streamwise and pitchwise directions, respectively, and the crossflow was exhausted from one open side of the impingement channel only. The transient thermochromic liquid crystal (TLC) technique was used to measure heat transfer on the target surface for different extended lengths of 1.0–2.5 jet hole diameters with Reynolds numbers from 1.0 × 104 to 3.0 × 104. To provide complementary flow physics for elaborating the heat transfer patterns observed from the experiments, well-validated numerical simulations were carried out. Comparisons with a baseline jet hole configuration showed that the extended jet holes helped to significantly improve the heat transfer levels as well as to generate a more uniform distribution pattern by suppressing the crossflow. Despite an aerodynamic penalty, the extended jet holes provided much higher heat transfer levels at equal pumping power consumption. The flow fields obtained by the numerical simulations revealed that the jets issued from the extended jet holes were straighter and had less mixing with the crossflow, resulting in a higher jet momentum impinging onto the target. Most importantly, it was found that the dominated flow mechanism of the extended jet holes was to prevent the jets from being redirected by the crossflow in strong crossflow conditions, rather than reducing the jet-to-target distance.

A Monte Carlo approach to evaluate the local measurement uncertainty in transient heat transfer experiments using liquid crystal thermography

This work presents an improved estimation of the local measurement uncertainty of transient heat transfer experiments using Thermochromic Liquid Crystals (TLC). Additional signal analysis steps and a Monte Carlo (MC) approach are added to the evaluation routine. As a result, the local uncertainty in the indication time of the maximum green intensity is determined. All other input quantities for the heat transfer coefficient calculation are provided with their characteristic distribution function introducing additional uncertainties. As a final result, lower and upper boundaries for a confidence interval with a user defined confidence level for the heat transfer coefficient are calculated locally for single pixels. The new method is applied to own measurement data from a transient TLC experiment. The local measurement uncertainty is compared to the traditional approach based on error propagation. Hence, the advantages of taking the non-linearity and the local uncertainty in the peak detection into consideration are shown.

Jet Impingement Heat Transfer Characteristics with Variable Extended Jet Holes under Strong Crossflow Conditions

In this paper, detailed flow patterns and heat transfer characteristics of a jet impingement system with extended jet holes are experimentally and numerically studied. The jet holes in the jet plate present an inline array of 16 × 5 rows in the streamwise (i.e., the crossflow direction) and spanwise directions, where the streamwise and spanwise distances between adjacent holes, which are normalized by the jet hole diameter (xn/d and yn/d), are 8 and 5, respectively. The jets impinge onto a smooth target plate with a normalized distance (zn/d) of 3.5 apart from the jet plate. The jet holes are extended by inserting stainless tubes throughout the jet holes and the extended lengths are varied in a range of 1.0d–2.5d, depending on the jet position in the streamwise direction. The experimental data is obtained by using the transient thermochromic liquid crystal (TLC) technique for wide operating jet Reynolds numbers of (1.0 × 104)–(3.0 × 104). The numerical simulations are well-validated using the experimental data and provide further insight into the flow physics within the jet impingement system. Comparisons with a traditional baseline jet impingement scheme show that the extended jet holes generate much higher local heat transfer levels and provide more uniform heat transfer distributions over the target plate, resulting in the highest improvement of approximately 36% in the Nusselt number. Although the extended jet hole configuration requires a higher pumping power to drive the flow through the impingement system, the gain of heat transfer prevails over the penalty of flow losses. At the same pumping power consumption, the extended jet hole design also has more than 10% higher heat transfer than the baseline scheme.

Cooling characteristics of the trailing-edge slot downstream from internal multi-ribbed channel

To investigate the cooling characteristics of the trailing-edge slot fed by the coolant channel with the straight ribs arranged at small pitch, the effects of the straight rib height and blowing ratio on the adiabatic cooling effectiveness and convective heat transfer coefficient of expanded cutback surface were studied by using the steady pressure sensitive paint technology and transient thermochromic liquid crystal technology, respectively. The cutback discharge coefficient and cooling performance under the multi-ribbed channel were analyzed and discussed, then the heat flux ratio was introduced to evaluate the optimal cooling structure. The experimental results showed that the discharge coefficient of trailing-edge slot increases slightly with the increase of blowing ratio, which is opposite to the increase of rib-height. At low blowing ratio (M≤0.50), the increase of the rib-height significantly strengthens the degree of jet disarray. As the coolant core region upstream of the slot-exit changes, a region with low adiabatic cooling effectiveness appears in the downstream. At the excessive blowing ratio(M=0.75), the adiabatic cooling effectiveness of the ribbed case with low height shows an abnormal trend opposite to the increase of blowing ratio. As the flow loss (based on the discharge coefficient) of the large height case is intensified, the instability of the cold vortex is strengthened, which cancels out the dominance of the flow field. The case with multi-ribbed channel can significantly improve the heat transfer performance of the region near the slot-exit. At large blowing ratio(M≥1.00), the high heat transfer coefficient area near the slot-exit develops into a core region with the high heat transfer coefficient, which can be extended to the downstream of the flow direction for the low height case. The optimal case with rib height of h/H=0.3 can improve the overall cooling effectiveness of the cutback surface by 20∼50%.

Analytical Thermal Boundary Condition for Quasi-Transient Simulation of Internal Flow Experiments

Transient thermochromic liquid crystal (TLC) experiments can provide high-fidelity spatially resolved heat transfer data for complex geometries, particularly where infrared techniques cannot be applied. One challenge when applying transient methods to internal geometries is the local definition of the driving gas temperature. The transient nature of the streamwise driving gas temperature profile has led to comparisons with steady-state computational fluid dynamics being questioned. This paper explores simulating the temporal behavior of transient TLC experiments directly. A novel technique is developed to account for differences in the gas and solid time scales, where surface temperature is calculated at each spatial location analytically from the surface heat flux, using an impulse response method assuming one dimensional, semi-infinite conduction. Postprocessing of the simulated surface temperature history is performed using the same method as experimentally, allowing for direct comparison. This analytical thermal boundary condition (ATBC) is applied to simulate a transient TLC experiment of a stationary superscaled rib turbulated internal cooling passage in a gas turbine engine. Traditional steady-state simulations were also performed with constant temporal and spatial temperature boundary conditions. Results show that calculations using the new ATBC and traditional steady-state method give very similar Nusselt number distributions and mean values in relation to the experimental data, suggesting the larger discrepancy between simulations and the experiments is not the definition of the driving gas temperature. Analysis of transient variation of Nusselt number indicated brief highly localized maximum variations up to 40%, although this was not found to significantly affect the mean values, and passage-averaged values converged to within 0.5% of the final value within 0.2 s.

Experimental and Computational Investigations of a Comb-Like Film-Cooling Scheme

Film-cooling hole-geometry research needs a better understanding in the mechanism of film cooling effectiveness, which was recently shown to depend rather strongly on the counter-rotating vortex pair (CRVP); in this work, a mechanism of the film-cooling heat transfer is proposed. Its mainstream entrainment was aimed to be eliminated by developing a new scheme named comb scheme, which was intended to move CRVP away from mainstream-coolant interface, rather than suppressing CRVP itself. It is investigated experimentally and numerically in this work. The transient thermochromic liquid crystal technique was used in the experimental work, while the Reynolds-averaged Navier-Stokes equations coupled with a realizable κ-ε turbulence model was used to simulate the flow numerically. The results were assessed against open published results hence demonstrating the so called ‘ideal’ performance (film cooling effectiveness = 1) of the new scheme, but with practical structural integrity. The geometric parameter analysis showed that the visualized strong CRVP, where intensity is more than 20, is trapped in the blind slot, hence its impact on mainstream-coolant interface is dropped to approximately 3, and mainstream entrainment is eliminated. It confirms the success of the comb scheme in achieving the high performance of the film-cooling heat transfer mechanism.

Experimental and Numerical Heat Transfer Investigation of Impingement Jet Nozzle Position in Concave Double-Wall Cooling Structures

In this paper, experimental and numerical study has been carried out to investigate impingement cooling with a row of five circular jets, varied between target positions on a realistic leading edge region of gas turbine blade geometry. Experimental data is collected from a transient thermochromic liquid crystal measurement technique at the target surface. Numerical study was conducted with the geometry using commercial computational fluid dynamics software to analyze the fluid flow. The unique aims of the study are to observe the effects of variation in jet location, and those specific to realistic target and nozzle geometries. Distributions of local and average Nusselt number show that a location targeting the concave surface at 90° demonstrates an overall higher heat transfer coefficient, especially in the stagnation region, and toward the airfoil sides, with significantly fewer swirls. The experiment was performed with the following parameters: distance from nozzle to target of 1.7 to 2.1 jet diameters, pitch between jets of 4.4 jet diameters, and concave target diameter of 8.0 jet diameters. The jet Reynolds number range during this test was 20,000 − 40,000. A standard flat target plate impingement test is also experimentally conducted and compared against existing literature for method validation.

Experimental study on film cooling and heat transfer characteristics of a twisted vane with staggered counter-inclined film-hole and laid-back-shaped-hole

The present study first proposes the counter-inclined film-hole with the staggered arrangement applied on the vane leading edge. The film cooling effectiveness (η) distribution of a twisted vane with the new cooling scheme of the leading edge and laid-back-shaped-holes is measured by the pressure Sensitive Paint (PSP) technique in a fan-shaped wind tunnel. The transient Thermochromic Liquid Crystal (TLC) technique is used to measure the heat transfer coefficient (h). The effects of density ratios and mass flow ratios (blowing ratios) are studied. The experiments are carried out at four mass flow ratios 12%, 9%, 8%, and 7.5% and three density ratios 1.0, 1.5, and 2.0. The pressure in the mid-span of the vane is measured to understand the η and h distributions. The results indicate that the increase rate of the span-wise averaged η upstream of the pressure surface is more prominent with the density ratio increasing from 1.0 to 1.5, and the maximum is about 86.9%; whereas increasing the density ratio from 1.5 to 2.0 significantly improves the span-wise averaged η of the suction surface. Increasing mass flow rates of the trailing and leading cavities has positive effects on the η of the whole vane surface except for the leading edge near the end wall at the density ratio 1.5; whereas it only increases the η on the pressure surface at the density ratio 1. Increasing the mass flow ratio of the leading cavity from 7% to 10% increases the h ratio of the whole cane, and it enlarges the high h ratio of leading edge the mid-chord regions at the suction side. The span-wise averaged heat flux ratio in the region between film hole rows at the density ratio 1.5 is lower than that at the density ratio 1.

Heat Transfer Measurements Using Multiple Thermochromic Liquid Crystals in Symmetric Cooling Channels

Abstract The transient thermochromic liquid crystal (TLC) method is applied to determine the distribution of the local heat transfer coefficients using a configuration with parallel cooling channels at an engine-relevant Reynolds number. The rectangular channels with a moderate aspect ratio and a high length-to-diameter ratio are equipped with one-sided oblique ribs with high blockage, which is a promising configuration for turbine near-wall cooling applications. In this arrangement, the three inner channels should experience same flow and thermal conditions. Numerical simulations are performed to substantiate this assumption. The symmetric single channels are sprayed with narrowband TLC with various indication temperatures. Multiple experiments were conducted. All start at ambient conditions before the fluid is heated up to several temperatures between 46 °C and 73 °C. The results show that the determined local heat transfer coefficients and therefore the Nusselt numbers vary significantly for the different experimental conditions especially at locations of high heat transfer coefficient behind the ribs. A simplified procedure with respect to measurement uncertainties is applied to enable an easy and fast valuation of the data quality. This might be used within the data reduction analysis for such experiments directly. The approach is illustrated using the obtained experimental data.

On the Use of High-Aspect-Ratio Film Cooling Scheme

Current film-cooling hole-geometry research is attempting to clarify the mechanism of film cooling effectiveness and its relation to the counter-rotating vortex pair (CRVP), in this paper a mechanism of film cooling heat transfer has been proposed. The coolant expansion in this mechanism is likely to improve film cooling effectiveness. It was characterized by developing a high aspect ratio (AR), hence named high-aspect-ratio (HAR) scheme. Typical AR values investigated experimentally and numerically in this work were 17.5 and 35. The transient thermochromic liquid crystal technique and the steady Reynolds-Averaged Navier-Stokes simulation with realizable κ-epsilon turbulence model were used. The results were assessed against available data hence demonstrating the high performance of the new scheme. The film cooling effectiveness of the full expansion cases is close to 1, achieved the so call “ideal” performance. Meanwhile, CRVP was still very strong, demonstrating that the CRVP did not result in decaying performance in the new scheme. Consequently, the HAR scheme achieved successfully the high performance of the proposed film cooling heat transfer mechanism.

Experimental and numerical investigation of jet impingement cooling onto a concave leading edge of a generic gas turbine blade

This study focuses on an experimental and numerical heat transfer investigation of an impingement jet array with a concave target plate that resembles a turbine blade's leading edge. The array consists of nine circular jets with a diameter of Djet and the target plate has a curvature radius of R=Djet. The target plate also features film cooling holes in a staggered configuration so that each jet is surrounded by three film cooling holes. A Reynolds number of 30,000 is tested, while the separation distance H is altered between H/Djet=2.7 and H/Djet=4. In addition the jets are exposed to a crossflow (cf). The experimental heat transfer data are obtained by using the transient Thermochromic Liquid Crystal (TLC) method. In the Computational Fluid Dynamics (CFD) study 3D steady state Reynolds-Averaged Navier-Stokes (RANS) simulations with the software package OpenFOAM and the kω−shear−stress−transport (SST) turbulence model are performed. The numerical results from OpenFOAM agree well with the experimental data and local flow phenomena are captured correctly. The crossflow decreases the stagnation point heat transfer as well as the overall heat transfer but homogenizes the local heat transfer distribution. A smaller separation distance enhances the crossflow effects and generally increases the heat transfer level.