Thermochromic Liquid Crystal Technique Research Articles

Investigation on the flow and heat transfer characteristics in a multi-pass channel using MRV and transient TLC

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 whole field three-dimensional velocity distribution resolved by MRV was used to reveal the flow mechanism driving heat transfer enhancement in deferent regions of multi-pass channels with and without ribs. 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 channel, 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, flow separates and reattaches downstream of ribs and heat transfer close to the reattachment regions is strengthened noticeably. Comparison between velocity and heat transfer results suggest that the principle mechanisms driving heat transfer enhancement are both endwall directed velocity, even though they are induced by different flow structures for smooth and ribbed channel.

A new perspective on internal turbine blade cooling by perforated blockages and pin-fins

Abstract The cooling configuration, sequentially combining perforated blockages (forming blockage jets) and a pin-fin array inside the trailing-edge of a turbine blade has been perceived unsuitable due to the presumed inferior thermal performance (η < 1.0). In the present study, we provide a new perspective on this particular cooling configuration, based on fluidic mechanisms, newly established in a better representative setup for blockage jets aligned with pin-fins, accounting for relevant heat transfer surfaces. To this end, heat transfer on the blockage, pin-fin, and end-wall surfaces was measured at a selected Reynolds number of ReD = 26,000 using a thermochromic liquid crystal technique. Flow field mapping by particle image velocimetry and oil-dye flow visualization were supplementally performed. We demonstrate, contrary to previous studies that the thermal performance of the blockage pin-fin configuration can be e.g., η = 1.35 if the blockages and pin-fins are arranged to maximize both elements' thermofluidic advantages. Our data further suggests that unlike conventional pin-fin configurations subjected to uniform coolant stream, the blockage pin-fin configuration can offer a better performance with fewer pin-fin rows used.

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.

Investigation on Heat Transfer Distribution of Rotating Two-Pass Smooth and Ribbed Channels Under Aero-Engine Simulated Conditions

Abstract Rotating significantly alters the internal cooling of turbine rotor blades by induced Coriolis force and buoyancy force, whose effects are characterized by the nondimensional rotation number (Ro) and buoyancy parameter (Bo). The present work was carried out in a new experimental rig of rotor blade internal cooling to obtain detailed heat transfer distributions when the three nondimensional criterion numbers (i.e., Re, Ro, and Bo) are similar to aero-engine operating conditions. Smooth and ribbed two-pass internal cooling channels with a 180-deg tip turn are investigated. The hydraulic diameter is 25.4 mm (1 in.), and the aspect ratio is 2:1. The Reynolds number is fixed at 25,000, with the maximum Ro and Bo of 0.316 and 0.272, respectively. The steady-state thermochromic liquid crystal (TLC) technique is used to measure detailed heat transfer distributions in the channel. Steady-state RANS simulations are also employed to resolve the flow characteristics. The effects of rotation on the flow and heat transfer characteristics are studied in this paper. The results show effects of rotation on the heat transfer distribution present apparent spatial discrepancy, especially around the bend region. The significant difference in the influence of rotation is witnessed in the smooth and the ribbed channels.

Experimental study on the melting performance of magnetic NEPCMs embedded in metal foam subjected to a non-uniform magnetic field

This experimental study aims to evaluate the melting performance of non-magnetic and magnetic nano-enhanced phase change materials (NEPCMs) composited with metal foam inside a cavity heated from the left in a non-uniform magnetic field via thermochromic liquid crystal (TLC) visualization. The evolutions of solid-liquid interface and mushy zone during melting are captured by TLC technique and temperature distributions are recorded by ten thermocouples. The effects of particles concentrations of NEPCM (0 wt%, 1 wt%, 3 wt%, 5 wt%), filling arrangements of metal foam (filling height, porosity gradient direction) and directions of magnetic field (left, right) are studied with emphasis on the heat transfer and energy storage characteristics including liquid fraction, energy stored and Nusselt number. The results show that comparing to pure PCM, the 3 wt% Al 2 O 3 NEPCM maximizes the enhancement of melting performance for decreasing complete melting time as 29.32% and increasing energy storage as 0.26%. For partial porous filling, as the filling height increases, the melting rate firstly increases then decreases, but energy stored shows the opposite trend. The filling height of 3/4h is considered as the optimal filling height with 43.51% decreasing of complete melting time and only 6.81% decreasing of energy stored. For gradient porosity filling, positive gradient has faster melting rate (62.16% amplification), but negative gradient has less energy loss (only 6.30% reduction). Under a non-uniform magnetic field, the convection of magnetic NEPCM is greatly suppressed by the Kelvin force, with 6.76% and 19.59% increasing of complete melting time and 0.07% reduction and 2.45% growth of energy stored for left and right magnet cases, respectively. • Conduct experiments to explore the effect of magnetic field on NEPCM melting performance. • Use TLC technique to capture evolution of melting front and mushy zone. • Gradient porosities of full filling can improve the melting performance but decrease energy storage. • Kelvin force suppresses the convection of magnetic NEPCM and reduces the width of mushy zone.

EFFECT OF JET TO PLATE SPACING ON FILM COOLING PERFORMANCE IN A COMBINED IMPINGEMENT AND FILM COOLING ARRANGEMENT

The influence of jet to plate spacing on conjugate heat transfer from a flat plate with combined impingement and film cooling has been studied both numerically and experimentally. The flow configuration consists of a matrix of impingement holes and multiple staggered rows of cylindrical film holes. One side of the plate is impinged with air jets, while the other side is exposed to hot mainstream as well as the vented air from the film cooling holes. The effect of increasing jet to plate ratio (H/D = 0.4, 1.2, and 2.0) on film cooling flow structure and heat transfer is analyzed under conjugate boundary conditions. The flow and temperature patterns are predicted in the entire computational domain; namely, the plenum chamber, film holes, and mainstream-coolant interaction zone. The mainstream Reynolds number is varied from 49,050 to 130,800 while keeping the jet Reynolds number constant at 825 and the blowing ratio ranging from 0.6 to 1.6. The velocity, turbulence, and temperature distributions at the exit of the film hole together define the film coverage on the interaction surface. The thermochromic liquid crystal (TLC) technique is used to evaluate the surface temperature of the plate and validate the computed values of the surface effectiveness. Both measured and computed effectiveness distributions are found to be comparable. Increase in the H/D values beyond a certain limit causes decrease in film effectiveness on the interaction surface, especially downstream of the film hole. The influence of H/D is noticed for all the three blowing ratios investigated, albeit to varying degree.

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 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.

Local Heat Transfer Measurements for an Impinging Synthetic Jet

Abstract Research results demonstrate the heat transfer effectiveness of an impinging synthetic jet toward cooling a plane normal to it. The utility of the synthetic jet lies in that the supply of coolant comes from the device itself as an alternating jetting flow that emerges from a plenum followed by a sink flow that returns to that same plenum. Experiments reported herein were conducted with the synthetic jet driven by an oscillating diaphragm powered by a rotating cam to expel fluid from the plenum out of a single hole, then return it through the same hole. The frequency of diaphragm oscillation and the distance from the synthetic jet's orifice to the surface being cooled are varied in the test program to determine their effects on cooling performance. A numerical study agrees with the results given by the experiment and flow visualization utilizing a smoke generator supports the data and numerical results. The local, time-average Nusselt numbers were measured in the experiment using the thermochromic liquid crystal technique and air as coolant. The color display of each test case was recorded with a fisheye camera. In the case of the highest frequency and shortest distance from orifice to cooled plate, a Nusselt number of nearly 40 was achieved within the central region of the cooled plate when the Reynolds number based upon jet maximum velocity and orifice diameter was 7500 and the distance from the orifice to cooled plate was 3.2 orifice diameters.

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.

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.

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.

Heat Transfer in a Rib Turbulated Pin Fin Array for Trailing Edge Cooling

Abstract Ribs were added into a pin fin array for a uniquely new cooling concept enabled through additive manufacturing. Heat transfer measurements are obtained using the thermochromic liquid crystal technique in a trapezoidal duct with pin fins and rib turbulators. Three pin blockage ratios and four rib heights at Reynolds numbers between 40,000 and 106,000 were tested. The Nusselt number augmentation is generally higher at the longer base of the trapezoidal duct. The same high heat transfer trend is seen at the columns closer to the longer base of the trapezoidal duct than on the shorter base. Through the length of the duct, the flow shifts from the nose region to the larger opening on the opposite wall. Also, it is observed that increasing the blockage ratio as well as increasing the rib height has a positive impact on heat transfer as ribs act as additional extended surfaces and alter the near-wall flow field. The heat transfer augmentation of pins and ribs is found to not be equal to the sum of both: Both heat transfer mechanisms are highly non-linear, thus cannot be superimposed. The observed heat transfer augmentation of the combined cases exceeded over the rib and pin only cases by up to 100%, but the weighted friction factor also doubled. The combination of ribs and pins is an excellent concept to achieve more uniform cooling over an array at higher levels when pressure drop is not of concern.

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.

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.

Investigation of Endwall Heat Transfer in Staggered Pin Fin Arrays

Abstract Arrays of staggered pin fins are a typical geometry found in the trailing edge region of modern airfoils. If coolant is supplied by bleeding from the mid-section of the airfoil instead of provided through the root, the channel length is insufficiently long to reach a fully developed flow which is commonly found from the fifth row downstream. This present study focuses on the developing section (four rows) of a staggered array with a height-to-diameter ratio of 2 and a spanwise and streamwise spacing of 2.5, respectively. Measurements are conducted at Reynolds numbers of 10,000 and 30,000 based on the maximum velocity and pin diameter. Stereoscopic particle image velocimetry (PIV) is used to describe the flow field and turbulence characteristics in the wake of the first and third row pin. It is found that the dominating vortical structures depend highly on the Reynolds number. A transient thermochromic liquid crystal (TLC) technique is used to obtain local heat transfer coefficients on the endwall which are then discussed in the context with the vortical structures. The structure of the horseshoe vortex and the transient wake shedding behaves differently in the first and third row. The interaction of both vortex systems affects directly the endwall heat transfer. The results are supplemented by a thorough discussion of TLC and PIV uncertainty.

Investigation and numerical simulation on film cooling performance with an anti-vortex hole design: Influences of diameter ratio

This paper extends the development of film cooling for application to gas turbines. Three diameter ratios of secondary hole to primary hole (0.3, 0.5 and 0.7) are constructed in an anti-vortex hole design. Film cooling effectiveness values and heat transfer coefficients are measured by the Thermochromic Liquid Crystal technique. In-flow temperature fields over downstream planes oriented normal to the main flow are measured by traversing a thermocouple. Numerical simulations are made to compute details of the flow structure to describe underlying mechanisms. Three coolant-to-mainstream blowing ratios (0.5, 1.0 and 1.5) and two coolant-to-mainstream mass flow ratios (3.43% and 5.15%) are tested. The anti-vortex hole design reduces the deleterious effects of the kidney vortex pair generated at the emersion point of the coolant jet into the passage, enlarges the film coverage, and finally improves film cooling effectiveness. The anti-vortex hole with larger secondary holes usually provides higher film cooling effectiveness.

Experimental Study of Heat Transfer on the Internal Surfaces of a Double-Wall Structure with Pin Fin Array

The double-wall structure is one of the most effective cooling techniques used in many engineering applications, such as turbine vane/blade, heat exchangers, etc. Heat transfer on the internal surfaces of a double-wall structure was studied at impinging Reynolds numbers ranging from 1 × 104 to 6 × 104 using the transient thermochromic liquid crystal (TLC) technique. The two-dimensional distributions of Nusselt numbers and their averaged values were obtained on the impingement surface, target surface and the pin fin surface. The Nusselt number correlations on the surfaces mentioned above were determined as a function of Reynolds number. The results show that the second peak values of the Nusselt number distribution appear on the target surface at all Reynolds numbers studied in this paper for a short distance of the target surface to impingement surface. This phenomenon becomes significant with the further increase of the Reynolds number. The difference between the Nusselt number at the second peak and the stagnation point decreases with the increasing Reynolds number. The maximal Nusselt number regions on the impingement surface appear at the left and right sides of the pin fins between the two impingement holes. The Nusselt numbers of the pin fin surfaces are highly dependent on their various locations in the double-wall structures. The contributions of the impingement surface, pin fin surface and target surface to the overall heat transfer rate are analyzed. The target surface contributed the largest amount of heat transfer rate with a value of about 62%. The heat transfer contribution is from 18% to 21% for the impingement surface and 16% to 18% for the pin fin surfaces within the studied Reynolds numbers.

Effect of Rotation Number on Heat Transfer Characteristics of a Row of Impinging Jets in Confined Channel

Jet impingement has been applied for internal cooling in gas turbine blades. In this study, heat transfer characteristics of impinging jets from a row of circular orifices were investigated inside a flow channel with rotations. The Reynolds number (Re) based on the jet mean velocity was fixed at 6,700. Whereas, the rotation number (Ro) of a channel was varied from 0 to 0.0099. The jet-to-impingement distance ratio (L/Dj) and jet pitch ratio (P/Dj) were respective 2 and 4, Dj is a jet diameter of 5 mm. The thermochromic liquid crystals (TLCs) technique was used to measure the heat transfer coefficient distributions on an impingement surface. The results show that heat transfer enhancement on a jet impingement surface depended on the effects of crossflow and Coriolis force. The local Nusselt number at X/Dj?20 on the leading side (LS) was higher than on the trailing side (TS) while heat transfer on the LS at 20?X/Dj?40 gained the lowest, compared to on the TS. The average Nusselt number ratios ( ) on the TS at Ro = 0.0049 gave higher than on the LS of around 2.17%. On the other hand, the on the TS at Ro = 0.0099 was less than the LS of about 0.08%.