Liquid Crystal Thermography Technique Research Articles

Experimental Study for the Enhancement of Thermal Efficiency and Development of Nusselt Number Correlation for the Roughened Collector of Solar Air Heater

Abstract The solar air heater’s thermal efficiency is relatively poor owing to the flat collector surface. This article’s primary objective is to increase the collectors’ thermal efficiency of rectangular ducts of solar air heater by adopting a novel V-shaped twisted rib element with staggering orientation. Experimentations are performed for various flow Reynolds numbers ranging from 3 k to 21 k, roughness pitch-to-rib height ratio ranging from 7 to 11, and staggering distance-to-rib height ratio between 2 and 6. Dispersion of Nusselt number over the collector surface is achieved through the liquid crystal thermography technique. Among the varied rib and flow constraints, it is observed that a maximum thermal performance enhancement index of 2.69 is observed, with the optimum value of the roughness parameter at a rib pitch-to-height ratio of 9 and a staggering distance-to-height ratio of 4. Mathematical correlation has also been developed using a regression model to estimate the Nusselt number in terms of non-dimensional roughness parameters. The percentage deviation between the Nusselt number attained from established relationships and the investigational results is found to be giving very satisfactory outcomes. The thermal efficiency of the smooth surface is recognized at 41.64% which increases for the roughened surface of twisted V-ribs to 73.63%. Hence employing twisted V-ribs as an artificial roughness element no doubt increases the Nusselt number, thermohydraulic performance enhancement index, and thermal efficiency, but it also exerts less frictional power of solar air heater.

Nusselt number and friction characteristics of solar air heater roughened with novel twisted V-shaped staggered ribs using liquid crystal thermography

The objective of this study is to investigate the Nusselt number and friction characteristics of rectangular channel solar air heater (SAH) ducts roughened with staggered V-shaped ribs using the liquid crystal thermography technique. Experimentation is performed for Reynolds numbers ranges from 3000 to 21000, relative roughness pitch (P/e) of 7–11, relative roughness length (S/e) of 6.15–10.26, relative staggered distance (d/e) between 2 and 6, and fixed angle of attack (α) and relative twist length (y/e). The change in temperature of the roughened absorber surface that produced the RGB (red, green, blue) colour pattern is acquired by a charge-coupled device and converted into (Hue, Saturation, and Intensity) HSI using MATLAB. For all the varied parameters, results are discussed in terms of Nusselt number, friction factor, thermal enhancement factor, and visualization of fluid flows. Results show that the maximum augmentation ratio in Nusselt number and friction factor is 3.43 and 2.57 times more than that of a smooth duct, respectively. Among the varied roughness parameter, a maximum TEP value of 2.69 is noticed with P/e of 9, d/e of 4, and S/e of 6.15. It is concluded that inclusion of staggered twisted V-shaped ribs helps in enhancing heat transfer with the least amount of friction penalty.

Heat Transfer Augmentation of a Solar Air Heater Using a Twisted V-Shaped Staggered Rib Over the Absorber Plate

Abstract An effort is made to study the performance of a solar air heater having the absorber surface roughened by providing staggered, twisted, and V-shaped ribs roughness. The liquid crystal thermography technique is applied to get the Nu distribution over the surface. Experimentation is done for Reynolds number ranges from 3000 to 21,000, relative roughness to pitch ratio (P/e) from 7–11 and relative staggered distance (d/e) of 2–6, for the fixed angle of attack (α), relative twist length (y/e), and relative roughness length (S/e). It is found that the maximum Nusselt number and friction factor augmentation are 3.43 and 2.57 times than that of the smooth duct. The maximum thermo-hydraulic performance (THP) value obtained is 2.69 for the P/e of 9 and d/e of 4. It can be inferred that staggered twisted V-shaped rib roughness helps to get the enhanced Nu with minimum friction penalty.

Effects of jet-to-target spacing on primary and secondary peak heat transfer for an obliquely impinging jet

This paper presents an experimental investigation on the primary and secondary peak heat transfer characteristics of an obliquely (45°) impinging jet on a flat and smooth target surface. Local convective heat transfer coefficients have been calculated through a transient liquid crystal thermography technique employing one-dimensional semi-infinite conduction model for the target surface. The effects of jet Reynolds number (Re) and jet-to-target spacings (z/d) have been investigated where Re was varied from 9,000 to 18,000 and z/d was varied from 2.5 to 6. Local heat transfer measurements reveal several important features of oblique jet impingement, e.g., the location of the primary and secondary peaks with respect to the geometric center (GC), its dependence on Re and z/d, and the shape of enhanced heat transfer zone with stagnation, separation, and reattachment features. The two-dimensional heat transfer map is presented in a normalized form Nu/Numax, where Numax is the primary peak heat transfer. It has been found that for a certain z/d, the primary peak occurs at a fixed distance from the geometric center for the range of Re investigated, while the secondary peak location varies with Re. For the smallest z/d, the primary peak heat transfer location was offset by one jet diameter from the GC in the uphill direction, whereas for the moderate z/d = 4, the primary peak occurred near the GC. For the highest z/d, the primary peak was again in the vicinity of GC, however, slight offset on the downhill side. The spent flow reattachment (post the first impingement) which also determined the secondary peak location and heat transfer level was strongly dependent upon the Re. The oblique jet impingement radial variation is also compared against 90° (orthogonal) jet impingement at z/d = 2.5 and 6 at Re = 18,000. It has been observed that the primary peak for above two z/d configurations were sharper for the 45° case when compared to 90°, and the secondary peak was further offset in the downhill direction with higher heat transfer levels for the 45° impingement case compared to 90°.

Application of liquid crystal thermography for temperature measurement of the absorber plate of solar air heater

Thermochromic liquid crystals (TLC) have indeed been utilized effectively in quasi heat transport and fluid dynamic research in recent years. Researchers in the areas of heat transfer and flow circulation have long used TLCs to map surface and spatial temperature changes. Before TLCs can be utilized for quantitative temperature measurements, they must first be calibrated to determine their color–temperature relationship. The goal of this article is to provide new and intermediate TLC users with an overview of the fundamentals and difficulties of TLC calibrations, with a focus on surface thermography. The first part of the article provides a comprehensive overview of thermochromic liquid crystals and the liquid crystal thermography technique. The final part of this paper concludes with a description of how liquid crystal thermography, both steady and transient, may be used to study heat transfer in turbulent flows. In the present article, Surface temperature TTLC is evaluated with the color response of TLC sheets and thermodynamic properties can be further calculated with the thermal relationship provided. The Imaging, colorimetry, lighting, and expected measurement error are all important elements of TLC calibration are discussed in this article.

Heat Transfer Measurement in a Rectangular Channel of Solar Air Heater With Winglet-Type Ribs Using Liquid Crystal Thermography

Abstract This paper deals with the study of heat transfer in solar air heater consisting of winglet-shaped roughness on the absorber plate using liquid crystal thermography technique. The winglet-type roughness element was placed on the absorber surface of a rectangular channel solar air heater having an aspect ratio of 4. The absorber surface was heated uniformly by a constant heat flux of 800 W/m2. The non-dimensional roughness parameter considered was relative roughness pitch, i.e., P/e, and its values range between 5 and 12 with Reynolds number (Re) ranges between 6500 and 22,000. The value of angle of attack, i.e., alpha and relative roughness width (W/w) was kept constant, and the relative roughness pitch was varied to measure the heat transfer coefficient. The enhancement in heat transfer has been compared, and it is observed that at P/e of 8 for angle of attack (α) of 60 degrees resulting its optimum value. The enhancement of heat transfer with the increase in Reynolds number is also noted.

Experimental and numerical investigation of turbulent flow heat transfer in a serpentine channel with multiple short ribbed passes and turning vanes

Detailed experimental and numerical investigations are conducted on the heat transfer, pressure loss and flow structure characteristics of a serpentine channel with multiple short ribbed passes and turning vanes, and the channel Reynolds numbers range from 10,000 to 60,000. The results are compared with the counterparts of a smooth three-pass serpentine channel and another one without turning vanes in the bend regions. The globally averaged heat transfer and the pressure loss characteristics of the three-pass serpentine channels were obtained by steady-state heat transfer experiments. The local heat transfer distributions of each ribbed pass were obtained by using transient liquid crystal thermography technique. Furthermore, the SST k-ω turbulence model was adopted for the steady-state three-dimensional numerical simulations to examine the detailed flow characteristics in the three-pass channels. The experimental results show that within the studied Reynolds number range the ribbed channel with the turning vanes shows averaged Nusselt number ratios of 1.56–1.72 in the first pass, 2.05–2.18 in the second pass and 2.15–2.23 in the third pass respectively, which are correspondingly about 10% lower than those of the counterpart without the turning vanes. The total pressure loss of the serpentine channel with the turning vanes is reduced by 13.4%–48.4% compared to the case without the turning vanes. The experiments by the transient liquid crystal thermography indicate that the turning vanes in the bend regions appreciably improve the heat transfer uniformity in the second and third passes. The numerical simulations show similar heat transfer characteristics as the experiments for the ribbed serpentine channels, and further show that the turning vanes improve the flow structure in the bend regions, and reduce the flow separation in the second and third passes, which contributes a more uniform heat transfer in the three-pass serpentine channel.

Experimental and Numerical Study of Heat Transfer and Turbulent Flow Characteristics in Three-Short-Pass Serpentine Cooling Channels With Miniature W-Ribs

Abstract Detailed experimental and numerical studies have been conducted on the heat transfer, pressure loss, and turbulent flow structure of a three-short-pass serpentine cooling channel with miniature W-shaped ribs on the wall under the Reynolds numbers from 8500 to 60,000. Steady-state heat transfer experiments were done to obtain the globally averaged and total heat transfer performance of each ribbed pass of the serpentine channel, and the streamwise pressure loss characteristics of the serpentine-channel flow were also obtained by multipoint pressure measurements. Additionally, the transient liquid crystal thermography technique was also used to obtain the local heat transfer distributions on the miniature W-ribbed surface of each pass. Furthermore, numerical simulations were done by using the AKN k–ε turbulence model to reveal the detailed turbulent flow and heat transfer characteristics in the serpentine channel. The experiments indicate that the miniature W-ribbed short pass has significantly enhanced total heat transfer by a factor of up to 4.0. The total heat transfer enhancement shows appreciably different values in different passes of the serpentine channel, and the second pass shows about 15% higher heat transfer enhancement than the first pass, and the third pass shows the highest heat transfer enhancement, which is about 15% higher than the second pass. The pressure loss measurements indicate that the two flow turnings contribute more than 90% of the total pressure loss in the serpentine channel with one ribbed pass with the miniature W ribs. The numerical simulations indicate that the flow turnings significantly increase the turbulent mixing in the flow of the downstream pass, and the miniature W-ribs on the wall appreciably improve the near-wall vortex mixing, which contributes the heat transfer enhancement.

Heat transfer enhancement due to a staggered element using liquid crystal thermography in an inclined discrete rib roughened solar air heater

In the present work, flow topology and heat transfer characteristics have been studied experimentally to visualize the Nusselt number distribution of inclined discrete rib with staggered elements inside a rectangular duct using liquid crystal thermography technique. Thermo-chromic liquid crystals act as a thermal imaging tool and used for visualizing the surface temperature distributions throughout the specified zone of a roughened surface. The experiment has been carried out with an applicable range of waft parameters, Reynolds number varied from 4000 to 12,500. The roughness pitch to rib height ratio assorted from 8 to 14, the ratio of length of staggered element to gap width as 2, relative staggered position as 0.6 and ratio of width of gap to roughness height as 1. The comparison of Nusselt number, flow friction, and thermal performance of the rectangular channel having inclined rib staggered arrangement with inclined rib without a staggered element has done. The staggered element helps to strengthen the flow by increasing the turbulence intensity in its nearby region and resulting in more heat transfer. The maximum increment in the Nusselt number and friction factor is 3.29 & 4.04, respectively, at a pitch of 10. The maximum thermo-hydraulic performance obtained as 2.06 at Reynolds number of 12,438.

Experimental study and numerical analysis of heat transfer enhancement and turbulent flow over shallowly dimpled channel surfaces

Experiments and numerical simulations have been done to investigate the heat transfer, pressure loss and turbulent flow characteristics over the surfaces with arrays of shallow dimples of different dimple depths within channels. Three different dimple depth to diameter ratios of 0.067, 0.1 and 0.2 are investigated with the Reynolds number (Re) ranging from 10,000 to 60,000. Heat transfer measurements were done by using the transient liquid crystal thermography technique to obtain the local heat transfer enhancement characteristics. Additional steady-state experiments were also done to obtain the overall heat transfer enhancement and the pressure loss of the turbulent flow over the dimpled surfaces. The experimental results indicated that the dimple depth has appreciable effects on heat transfer enhancement and pressure loss. The Reynolds number also shows appreciable effects on the local heat transfer enhancement distribution on the surface with shallower dimples. The overall heat transfer enhancement of all the dimpled surfaces approximately keep constant with the Reynolds numbers. The experiments still indicate that the shallow dimples with the depth to diameter ratio of 0.067 obtained almost constant overall heat transfer enhancement up to 1.4 with negligible pressure loss penalty, which indicates a high and constant Reynolds analogy factor of about 1.33 over the studied Reynolds number range. However, as the dimple depth ratio increases from 0.067 to 0.2, the pressure loss increases rapidly and surpasses the heat transfer enhancement, which leads to lower Reynolds analogy factor than 1.0. Furthermore, the numerical simulations show detailed turbulent flow characteristics over the dimpled surfaces. Different from the dimples with relatively deep depth ratios of 0.1 and 0.2, the shallower dimples with the depth ratio of 0.067 generate an obvious low-speed swirling horseshoe vortex zone near the upstream rim in the dimples at the relatively high Reynolds number of 50,500. The horseshoe vortex shed from spanwise edges of the shallow dimples produces lower but more uniform turbulent mixing intensity in the near-wall region, which is responsible for the higher thermal performance.

Nusselt number and friction characteristics of a solar air heater that has a winglet type vortex generator in the absorber surface

The present investigation is based on the experimental analysis of heat transfer distribution and friction features of a solar air heater that has a rough absorbent surface with winglet type vortex generator has been made using liquid crystal thermography technique. The RrGgBb (red, green and blue) colour pattern obtained because of temperature variation which can be visualized by the captured image through a CCD camera. The captured colour pattern can be digitized and decoded it into HIS (Hue, Intensity, and Saturation). For the given winglet rib the roughnesss parameters considered as relative roughnesss pitch (P/e) values of 5 to 12, attack angle (α) of 30° to 75°, relative roughnesss width (W/w) values of 3 to 7 and flowing Reynolds number (Re) varies from 3000 to 22000. The effects of change in roughnesss parameters on heat transfer and friction factor are investigated. The optimum enhancement of heat transfer observed at P/e of 8 while the required friction factor decreases with the increases in relative roughnesss pitch. The presence of a winglet type vortex generator creates additional mixing, as well as a small hole at the winglet tip, produces air jet on the downstream side and enhances the heat transfer coefficient. The observation made for enhancement value of Nusselt number achieves its optimal at W/w of 5 while the friction factor observed to be maximum at W/w of 3 for the considered geometric parameters correspondings to an attack angle (α) of 60°.

Experiments and Stress-Blended Eddy Simulations of turbulent flow over edge-rounded dimples

Experiments and numerical simulations have been conducted to reveal the detailed turbulent flow and heat transfer characteristics over the surfaces with spherical dimples with different edge schemes, including the sharp edge, the fully rounded edge and the upstream rounded edge. The dimples have the same depth-to-diameter ratio of hd/d = 0.2. Steady-state heat transfer experiments were carried out for the turbulent flow over the three dimpled surfaces within the Reynolds number range of 10,000 ≤ ReDh ≤ 60,000, and the globally averaged heat transfer enhancements and friction factor ratios were obtained. Furthermore, transient liquid crystal thermography technique was used to obtain the local heat transfer characteristics on the dimpled surfaces. In addition, numerical simulations were carried out with a newly developed hybrid RANS-LES method of Stress-Blended Eddy Simulation (SBES) and the local heat transfer enhancement, detailed flow structure and turbulence statistics data were obtained and analyzed. The experimental results show that the best thermal performance is achieved by the dimples with upstream rounded edge, indicating an increase of about 11.4% in total heat transfer enhancement and an increase of about 5.2% in pressure loss when compared to the conventional dimples with sharp edge. The numerical results show that the flow over the dimpled surfaces can be divided into five zones: the mainstream zone, the recirculation zone, the Kelvin–Helmholtz vortex zone, the upwash zone and the rear separation zone, and each zone has its unique flow features. It is also found that the lowest pressure is achieved by the dimple with fully rounded edge, which is due to the absence of flow separation behind the rear edge of the dimple. The upstream-edge-rounded dimples show the reduced recirculation flow in the recirculation zone, the enhanced impingement and upwash flow in the upwash zone near the rear edge. These cause the stronger production of turbulent kinetic energy behind the rear edge, which are responsible for the higher thermal performance.

Nusselt number and fluid flow analysis of solar air heater having transverse circular rib roughness on absorber plate using LCT and computational technique

The present study compares the heat transfer distribution over the absorber surface of a solar air heater using liquid crystal thermography (LCT) technique along with a CFD technique. The heat transfer distribution over the absorber surface is measured using the LCT technique. Computational analysis (CFD) is performed using ANSYS FLUENT 16.2 code with a general algorithm for the air flow to visualize the characteristics and distribution of the flowing air through the duct. The LCT measurement shows a two-dimensional temperature distribution as a color dispersal over the absorber surface. Comparison of the results for two different parameters having relative roughness pitch (P/e) and relative roughness height (e/D) are represented in graphical form as the effect of Nusselt number and friction factor obtained using LCT with computational technique over Reynolds number (Re) ranges of 8,551–11,149. The image captured by the CCD Camera of the LCT techniques to give a temperature distribution over the surface and the results are scrutinized for optimum values for the considered range of parameter studied resulting for maximum values of heat transfer rate at roughness pitch ratio of 10 with a minimum penalty of frictional losses. This shows approximately similar values obtained from both the approaches.

Experimental Study of Heat Transfer and Pressure Loss in Channels with Miniature V Rib-Dimple Hybrid Structure

In order to increase the thermal efficiency, the gas turbines are designed to operate at higher temperature, which requires highly efficient cooling structures for turbine blades. The dimples and ribs are effective surface structures to enhance the convective heat transfer in the gas turbine blade internal cooling. In the present study, a novel hybrid cooling structure with miniature V-shaped ribs and dimples is presented, and the heat transfer and pressure loss characteristics are obtained experimentally. The heat transfer performance of the rib–dimple structures, which include three different rib height-to-hydraulic diameter ratios of 0.017, 0.029 and 0.044 and one dimple configuration with the dimple depth-to-diameter ratio of 0.2, are studied by using the transient liquid crystal thermography technique for turbulent flow in rectangular channels within the Reynolds number range from 10,000 to 60,000. It is found that the miniature V-shaped ribs arranged upstream the dimples can significantly improve the heat transfer performance of the dimples, resulting in a more uniform heat transfer distribution on the surface. The V rib-dimple hybrid structure in the channel shows much higher heat transfer enhancement than the counterparts with only the dimples in the channels.

Rib Turbulator Heat Transfer Enhancements at Very High Reynolds Numbers

High-pressure stage gas turbine blades feature serpentine passages where rib turbulators are installed to enhance heat transfer between the relatively colder air bled off from the compressor and the hot internal walls. Most of the prior studies have been restricted to Reynolds number of 90,000 and several studies have been carried out to determine geometrically optimized parameters for achieving high levels of heat transfer in this range of Reynolds number. However, for land-based power generation gas turbines, the Reynolds numbers are significantly high and vary between 105 and 106. The present study is targeted toward these high Reynolds numbers where traditional rib turbulator shapes and prescribed optimum geometrical parameters have been investigated experimentally. A steady-state liquid crystal thermography technique is employed for measurement of detailed heat transfer coefficient. Five different rib configurations, viz., 45 deg, V-shaped, inverse V-shaped, W-shaped, and M-shaped have been investigated for Reynolds numbers ranging from 150,000 to 400,000. The ribs were installed on two opposite walls of a straight duct with an aspect ratio of unity. For very high Reynolds numbers, the heat transfer enhancement levels for different rib shapes varied between 1.4 and 1.7 and the thermal hydraulic performance was found to be less than unity.

Nusselt number-friction characteristic for a twisted rib roughened rectangular duct using liquid crystal thermography

Liquid crystal thermography technique is applied over a wide wall of a rectangular channel having twisted rib roughness to evaluate heat transfer distribution. The temperature distribution can be easily visualized easily as color pattern obtained by a CCD camera. The captured color pattern is digitized and converted to HIS (Hue, Saturation and Intensity) from RGB (red, Green and Blue). In this study the effect of different roughness parameters such as relative roughness pitch, twist ratio and rib inclination angle on heat transfer coefficient and friction factor are investigated. The range of Reynolds number and aspect ratio of the duct is selected those are most suitable for solar air heater. It is observed that relative roughness pitch of 8 shows the maximum heat transfer while friction factor decreases with increase in relative roughness pitch. The effect of twist ratio is to create the jet formation on the downstream side of the ribs, which creates vigorous mixing to increase in heat transfer. The increase in twist ratio decreases the number of jets formed and the Nusselt number, while friction factor increases. The rib inclination to the main flow generates secondary flow, increases turbulence level as well as Nusselt number. The secondary flow increases with increase in inclination angle to create maxima in Nusselt number at an angle of 60°. The friction factor increases with increase in rib inclination angle.

Jet Impingement Heat Transfer in Narrow Channels With Different Pin Fin Configurations on Target Surfaces

A comparative experimental and numerical study has been done on multiple-jet impingement heat transfer in narrow channels with different pin fin configurations on the target surfaces. Three different target plates including a flat plate, a plate with full-height pin fins, and another plate with miniature pin fins are investigated in the jet impingement cooling systems comparatively. The experiments were done under maximum cross flow scheme for the jet Reynolds numbers from 15,000 to 30,000. Narrow jet impingement spacing is kept the same as 1.5 times jet diameter for all the target plates. In the experiments, detailed jet impingement heat transfer characteristics on the flat plate and the full-height pin-fin plate were obtained by using the transient liquid crystal thermography technique, and additionally steady experiments were done to obtain the overall heat transfer performance of the jet impingement systems with all the three different target plates, which accounts for the heat transfer contribution from the pin fins' surface. Significant overall jet impingement heat transfer enhancement can be obtained with full-height pin-fin roughened surfaces with appreciable pressure loss; however, with miniature pin fins on the target plate, the jet impingement overall heat transfer performance can be remarkably improved with negligible pressure loss penalty. Furthermore, three-dimensional (3D) computational fluid dynamics (CFD) analysis was done to analyze the detailed flow structure and heat transfer characteristics in the jet impingement systems with different pin fin configurations.

Assessment of Heat Transfer Enhancement Using Metallic Porous Foam Configurations in Laminar Slot Jet Impingement: An Experimental Study

An experimental study using the liquid crystal thermography technique is conducted to investigate the convective heat transfer performance in jet impingement cooling using various porous media configurations. Aluminum porous foams are used in the present study. Four impinging jet configurations are considered: jet impingement (1) without porous media, (2) over the porous heat sink, (3) with porous obstacle case, and (4) through porous passage. These configurations are evaluated on the basis of the convective heat transfer enhancement for two different Reynolds numbers of 400 and 700. Jet impingement with porous heat sink showed deterioration in the average Nusselt number by 9.95% and 18.04% compared to jet impingement without porous media configuration for Reynolds numbers of 400 and 700, respectively. Jet impingement with porous obstacles showed a very negligible enhancement in the average Nusselt number by 3.48% and 2.73% for Reynolds numbers of 400 and 700, respectively. However, jet impingement through porous passage configuration showed a maximum enhancement in the average Nusselt number by 52.71% and 74.68% and stagnation Nusselt numbers by 58.08% and 53.80% compared to the jet impingement without porous medium for Reynolds numbers of 400 and 700, respectively. Within the porous properties considered, it is observed that by decreasing the permeability and porosity the convective heat transfer performance tends to increase.

Crossover Jet Impingement in a Rib-Roughened Trailing-Edge Cooling Channel

Airfoil trailing-edge cooling is the main focus of this study. The test section was made up of two adjacent trapezoidal channels, simulating the trailing-edge cooling cavity of a gas turbine airfoil and its neighboring cavity. Eleven racetrack-shaped holes were drilled on the partition wall between the two channels to produce 11 cross-over jets that impinged on the rib-roughened wall of the trailing-edge channel. The jets, after impinging on their respective target surface, turned toward the trailing-edge channel exit. Smooth target wall, as a baseline case, as well as four rib angles with the flow of 0 deg, 45 deg, 90 deg, and 135 deg are investigated. Cross-over holes axes were on the trailing-edge channel center plane, i.e., no tilting of the cross-over jets. Steady-state liquid crystal thermography technique was used in this study for a range of jet Reynolds number of 10,000–35,000. The test results are compared with the numerical results obtained from the Reynolds-averaged Navier–Stokes and energy equation. Closure was attained by k–ω with shear stress transport (SST) turbulence model. The entire test rig (supply and trailing-edge channels) was meshed with variable density hexagonal meshes. The numerical work was performed for boundary conditions identical to those of the tests. In addition to the impingement heat transfer coefficients, the numerical results provided the mass flow rates through individual cross-over holes. This study concluded that: (a) the local Nusselt numbers correlate well with the local jet Reynolds numbers, (b) 90 deg rib arrangement, that is, when the cross-over jet axis was parallel to the rib longitudinal axis, produced higher heat transfer coefficients, compared to other rib angles, and (c) numerical heat transfer results were generally in good agreement with the test results. The overall difference between the computational fluid dynamics (CFD) and test results was about 10%.

Characterization of heat transfer enhancement and frictional losses in a two-pass square duct featuring unique combinations of rib turbulators and cylindrical dimples

Transient heat transfer experiments using liquid crystal thermography have been carried out on a two-pass square channel for testing several unique combinations of ribs and cylindrical dimples. Four different rib shapes, viz. 45° angled, V, W and M have been studied. In compound channels, the rib pitch accommodates cylindrical dimples arranged in the shape of the rib, hence following the direction of rib induced secondary flows. For each rib shape, three different configurations – rib alone, dimple alone and rib–dimpled compound cases were studied. The rib-height-to-channel hydraulic diameter ratio was 0.125 and rib-pitch-to-rib-height ratio was 16. The dimple-depth-to-print diameter ratio was 0.3. The experiments were carried out for a wide range of Reynolds number (19,500–69,000), covering a spectrum typically found in both land-based and air-breathing engines. A total of 52 experiments were carried out to measure detailed heat transfer coefficient on the bottom wall of the two-pass channel. A transient liquid crystal thermography technique was used for heat transfer measurement. Static pressure measurements were carried out to measure the overall pressure drop in the two pass channel. From globally averaged Nusselt number and overall pressure drop, thermal-hydraulic performance of the 13 configurations were determined, compared and analyzed. It has been observed that 45° angled and V compound configurations resulted in higher heat transfer augmentation as well as higher thermal hydraulic performance when compared with their corresponding ribs alone and dimples alone configurations.