Experimental Investigation of Heat Transfer Enhancement in a Two-Pass Square Duct by Permeable Ribs

This investigation presents the heat transfer enhancement results of flow past repeated permeable ribs mounted on the bottom surface of a two-pass square channel. Spatially periodic flow interruption generated by rib arrays mounted on the walls is extensively used for augmentation of heat transfer in turbine blade cooling passages. To remove the local heat transfer deterioration in the vicinity region of the solid ribs, permeable ribs have been proposed in the literature for single pass coolant passages. This study intends to investigate the performance of permeable ribs array placed on the lower wall of a two-pass channel compared to that of the solid rib array. The heat transfer and friction characteristics are investigated for smooth, solid-ribbed, slit-ribbed, and split-slit-ribbed square channels for the Reynolds numbers of 5,500, 12,800, and 16,400. An array of ribs (consisting of fifteen ribs) has been mounted over the total test section, with seven each in the first and second passes, and one in the bend. The performance analysis has been carried out using thermal performance on the basis of constant pumping power and entropy generation principle.     

Influence of rib height on the local heat transfer distribution and pressure drop in a square channel with 90° continuous and 60° V-broken ribs

Internal channel cooling is employed in advanced gas turbines blade to allow high inlet temperatures so as to achieve high thrust/weight ratios and low specific fuel consumption. The objective of the present work is to study the effect of rib height to the hydraulic diameter ratio on the local heat transfer distributions in a double wall ribbed square channel with 90° continuous attached and 60° V-broken ribs. The effect of detachment of the rib in case of broken ribs on the heat transfer characteristics is also presented. Reynolds number based on duct hydraulic diameter is ranging from 10,000 to 30,000. A thin stainless steel foil of 0.05 mm thickness is used as heater and infrared thermography technique is used to obtain the local temperature distribution on the surface. The images are captured in the periodically fully developed region of the channel. It is observed that the heat transfer augmentations in the channel with 90° continuous attached ribs increase with increase in the rib height to hydraulic diameter ratio (e/D) but only at the cost of the pressure drop across the test section. The enhancements caused by 60° V-broken ribs are higher than those of 90° continuous attached ribs and also result in lower pressure drops. But, with an increase in the rib height, the enhancements are found to decrease in channel with broken ribs. The effect of detachment incase of broken ribs is not distinctly observed. The heat transfer characteristics degraded with increase in the rib height in both attached and detached broken ribbed cases.     

An experimental investigation of heat transfer characteristics for steam cooling in a rectangular channel with parallel ribs

An experimental study of heat transfer characteristics in superheated steam cooled rectangular channels with parallel ribs was conducted.The distribution of the heat transfer coefficient on the rib-roughed channel was measured by IR camera.The blockage ratio(e/Dh) of the tested channel is 0.078 and the aspect ratio(W/H) is fixed at3.0.Influences of the rib pitch-to-height ratio(P/e) and the rib angle on heat transfer for steam cooling were investigated.In this paper,the Reynolds number(Re) for steam ranges from 3070 to 14800,the rib pitch-to-height ratios were 8,10 and 12,and rib angles were 90°,75°,60°,and 45°.Based on results above,we have concluded that:In case of channels with 90° tranverse ribs,for larger rib pitch models(the rib pitch-to-height ratio=10 and12),areas with low heat transfer coefficient in front of rib is larger and its minimum is lower,while the position of the region with high heat transfer coefficient nearly remains the same,but its maximun of heat transfer coefficient becomes higher.In case of channels with inclined ribs,heat transfer coefficients on the surface decrease along the direction of each rib and show an apparent nonuniformity,consequently the regions with low Nusselt number values closely following each rib expand along the aforementioned direction and that of relative high Nusselt number values vary inversely.For a square channel with 90° ribs at Re= 14800,wider spacing rib configurations(the rib pitch-to-height ratio=10 and 12) give an area-averaged heat transfer on the rib-roughened surface about8.4%and 11.4%more than P/e=8 model,respectively;for inclined parallel ribs with different rib angles at Re=14800,the area-averaged heat transfer coefficients of 75°,60° and 45° ribbed surfaces increase by 20.1%,42.0%and 44.4%in comparison with 90° rib angle model.45° angle rib-roughened channel leads to a maximal augmentation of the area-averaged heat transfer coefficient in all research objects in this paper.     

Heat transfer in rectangular channels with transverse and V-shaped broken ribs

Repeated ribs are used on heat exchange surfaces to promote turbulence and enhance convective heat transfer. Applications include fuel rods of gas-cooled nuclear reactors, inside cavities of turbine blades, and internal surfaces of pipes used in heat exchangers. Despite the great number of literature papers, only few experimental data concern detailed distribution of the heat transfer coefficient in channels with rib turbulators. This issue was tackled by means of the steady-state liquid crystal thermography: a pre-packaged liquid crystal film was glued onto the heated surface, and the colour map was taken by a video camera at the steady state of a given experiment. After calibration tests to assess the colour-temperature relationship had been performed, local heat transfer coefficients were obtained by applying custom-made software to process the digitised colour images. Liquid crystal thermography was applied to the study of heat transfer from a rectangular channel (width-to-height ratio equal to five) having one surface heated at uniform heat flux and roughened by repeated ribs. The ribs, having rectangular or square sections, were deployed transverse to the main direction of flow or V-shaped with an angle of 45 or 60 deg relative to flow direction. The effect of continuous and broken ribs was also considered. Local heat transfer coefficients were obtained at various Reynolds numbers, within the turbulent flow regime. Area-averaged data were calculated in order to compare the overall performance of the tested ribbed surfaces and to evaluate the degree of heat transfer enhancement induced by the ribs with respect to the smooth channel.     

Heat Transfer Measurements inside Narrow Channels with Ribs and Trenches

Detailed heat transfer coefficient distributions are obtained for high aspect ratio (width/height = 12.5) duct with rib and trench enhancement features oriented normal to the coolant flow direction. A transient thermochromic liquid crystal technique has been used to experimentally measure heat transfer coefficients from which Nusselt numbers are calculated on the duct surface featuring heat transfer enhancement features. Reynolds number (calculated based on duct hydraulic diameter) ranging from 7100 to 22400 were experimentally investigated. Detailed measurements of heat transfer provided insight into the role of protruding ribs and trenches on the fluid dynamics in the duct. Experimentally obtained Nusselt numbers are normalized by Dittus-Boelter correlation for developed turbulent flow in circular duct. The triangular trenches provide heat transfer enhancement ratios up to 1.9 for low Reynolds numbers. The in-line rib configuration shows similar levels to the trench whereas staggered rib configuration provides heat transfer enhancement ratios up to 2.2 for a low Reynolds number of 7100.     

On the Improvement of the Poor Heat Transfer Lee-Side Regions of Square Cross-Section Ribbed Channels

Heat transfer and flow characteristics of six ribbed channels of square cross section having different rib structure are computed with the objective of improving heat transfer in the lee-side of the ribs. Six ribs are installed on the bottom walls of each channel. The rib pitch-to-height ratio (P/e) is 10. Details of the turbulent flow structure, temperature fields, local heat transfer coefficients, flow friction coefficients, normalized heat transfer rates, and normalized friction factors are reported. The simulations use the v²f turbulence model and inlet Reynolds number range of 8,000 to 24,000. A uniform heat flux is appropriately applied on all surfaces. The heat transfer performances features of the ribbed channels of various designs are evaluated and compared. A case with an inclined lee-side structure having an inclination angle of 160° yields the highest Nusselt number and friction factor, about 4.6%–6.4% higher than those with rectangular ribs, and 7.1%–9.0% higher heat transfer when the heated-surface area is considered. Increased pressure drop is kept within certain limits when considering the balance between cooling effectiveness and pressure loss for the comparisons. Though having the best heat transfer, the case with the inclined back-wall geometry of the ribs does not present the better overall thermal performance due to the higher friction. The heat transfer enhancement is more prominent when improvements of the poor heat transfer regions downstream of the rib are computed with the surface area change excluded. A conclusion to be drawn is that lee-side improvement of heat transfer can be effected with suitable design of the rib downstream side. This finding can be applied to improvement of turbine airfoil cooling.     

Analysis of heat transfer augmentation and flow characteristics due to rib roughness over absorber plate of a solar air heater

A computational analysis of heat transfer augmentation and flow characteristics due to artificial roughness in the form of ribs on a broad, heated wall of a rectangular duct for turbulent flow (Reynolds number range 3000–20,000, which is relevant in solar air heater) has been carried out. Shear stress transport k−ω turbulence model is selected by comparing the predictions of different turbulence models with experimental results available in the literature. A detailed analysis of heat transfer variation within inter rib region is done by using the selected turbulence model. The analysis shows that peak in local heat transfer coefficient occurs at the point of reattachment of the separated flow as observed experimentally. The results predict a significant enhancement of heat transfer in comparison to that for a smooth surface. There is a good matching between the predictions by SST k−ω and experimental results. In this work, nine different shapes of rib are examined using SST k−ω model and compared on the basis of heat transfer enhancement, friction characteristics and performance index considering heat transfer enhancement with the same pumping power.     

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

Abstract

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.     

Effect of laminar separation flow and nanofluids on heat transfer augmentation with passive techniques: A review

Heat transfer in flow channels can be improved by using passive techniques such as ribs on wall and change cross section area where these modifications have practical engineering application for thermal power plant, refrigerators, and radiators. Effects of separation flow and nanofluids on thermal performance for laminar range presented experimentally and numerically in this review. The augmentations of heat transfer with fluid and nanofluid flow through sudden expansion, over backward and forward facing step, and rib channels have been concerned. The experimental results showed good agreement with numerical results and indicated the effects of separation flow and nanoparticles on augmentation of heat transfer rate. The results showed increase in Nusselt number with increase of Reynolds number, step height, and number of ribs. It was detected that by increasing the nanoparticle volume concentrations of nanofluids, improves the heat transfer coefficient. Also different nanoparticles used in the literature investigations are based on thermal conductivity where enhancement of heat transfer rate was obtained significantly.     

The importance of rib shape effects on the local heat transfer and flow friction characteristics of square ducts with ribbed internal surfaces

Accurate prediction of ribbed duct flow and heat transfer is of importance to the gas turbine industry. In the present work, a computer code has been developed to study the turbulent heat transfer and friction in a square duct with various-shaped ribs mounted on one wall. The simulations were performed for four rib shapes, i.e., square, triangular, trapezoidal with decreasing height in the flow direction, and trapezoidal with increasing height in the flow direction. The prepared algorithm and the computer code are applied to demonstrate distribution of the heat transfer coefficient between a pair of ribs. The results show that features of the inter-rib distribution of the heat transfer coefficient are strongly affected by the rib shape and trapezoidal ribs with decreasing height in the flow direction provide higher heat transfer enhancement and pressure drop than other shapes.     

Computational analysis of heat transfer in turbulent flow past a horizontal surface with two-dimensional ribs

Heat transfer and flow behaviours past a horizontal surface with two-dimensional transverse ribs were numerically investigated using a CFD model. The present model was adopted for turbulent flow of air past a number of rib arrays with different rib pitch to height ratios heated with a uniform heat flux. The temperature profiles and the local heat transfer coefficients as well as the flow velocity and turbulence characteristics were predicted. The results showed that the presence of the transverse ribs yields a significant enhancement of the heat transfer compared with that for a flat plate and the predicted heat transfer coefficients showed good agreement with previous experimental results. Also, peaks in local heat transfer coefficients were predicted within the interrib regions and were found to coincide with the points of reattachment of the separated flows.     

Comparative Study of Convective Heat Transfer Performance of Steam and Air Flow in Rib Roughened Channels

A comparative experimental study of heat transfer characteristics of steam and air flow in rectangular channels roughened with parallel ribs was conducted by using an infrared camera. Effects of Reynolds numbers and rib angles on the steam and air convective heat transfer have been obtained and compared with each other for the Reynolds number from about 4,000 to 15,000. For all the ribbed channels the rib pitch to height ratio(p/e) is 10, and the rib height to the channel hydraulic diameter ratio is 0.078, while the rib angles are varied from 90° to 45°.Based on experimental results, it can be found that, even though the heat transfer distributions of steam and air flow in the ribbed channels are similar to each other, the steam flow can obtain higher convective heat transfer enhancement capability, and the heat transfer enhancement of both the steam and air becomes greater with the rib angle deceasing from 90° to 45°. At Reynolds number of about 12,000, the area-averaged Nusselt numbers of the steam flow is about 13.9%, 14.2%, 19.9% and 23.9% higher than those of the air flow for the rib angles of 90°,75°, 60° and 45° respectively. With the experimental results the correlations for Nusselt number in terms of Reynolds number and rib angle for the steam and air flow in the ribbed channels were developed respectively.     

带错排V肋和反V肋矩形通道强化传热机理及规律研究

采用热色液晶瞬态测量技术研究了带45°V肋和45°反V肋的矩形通道端壁的传热特性,分析V肋诱导产生二次流强化传热机理及其传热系数分布规律。通道进口雷诺数变化范围是10 460~32 100,肋高与当量直径的比为0.13,肋间距与肋高的比为10。实验结果表明:带V肋和反V肋矩形通道传热系数随着雷诺数的增大而增大;正V肋诱导产生沿V肋从中间向两侧发展的二次流,反V肋片诱导气流沿肋方向产生从两边流向中间的二次流;斜置V肋诱导产生的二次流增强了通道的传热能力;带V肋通道的传热强于带反V肋通道。     

Analysis of fluid flow and heat transfer in a rib grit roughened surface solar air heater using CFD

This paper presents the study of fluid flow and heat transfer in a solar air heater by using Computational Fluid Dynamics (CFD) which reduces time and cost. Lower side of collector plate is made rough with metal ribs of circular, square and triangular cross-section, having 60° inclinations to the air flow. The grit rib elements are fixed on the surface in staggered manner to form defined grid. The system and operating parameters studied are: e/D_h = 0.044, p/e = 17.5 and l/s = 1.72, for the Reynolds number range 3600-17,000. To validate CFD results, experimental investigations were carried out in the laboratory. It is found that experimental and CFD analysis results give the good agreement. The optimization of rib geometry and its angle of attack is also done. The square cross-section ribs with 58° angle of attack give maximum heat transfer. The percentage enhancement in the heat transfer for square plate over smooth surface is 30%.     

Flow and impingement cooling heat transfer along triangular rib-roughened walls

Experiments are performed to study slot air jet impingement cooling flow and the heat transfer along triangular rib-roughened walls. Both flow visualization and local heat transfer measurements along the ribbed wall are made. The effect of different rib protrusions (heights) on the impinging flow and heat transfer along the wall is studied, which is achieved by using different sizes of nozzles. Two different ribbed walls with different rib pitches are selected which have a rib pitch-to-height ratio of 2 and 4, respectively. The widely opened cavity between neighboring ribs make more intense transport of momentum between the wall jet and cavity flow so that recirculation cell in the cavity is hardly observed. This leads to a higher heat transfer around the cavity wall than in the case with rectangular ribs. However, in the region of laminar wall jet, a number of air bubbles enclosing the cavities are formed which prevent penetration of the wall jet into the cavities. This leads to a significant reduction in the heat transfer. The geometric shape of the triangular ribs is more effective in rebounding the wall jet away from the wall than in the case with rectangular ribs. The rebound of the jet away from the wall causes a significant reduction in the heat transfer. A comparison and correlations of the stagnating point Nusselt number under different conditions are presented and discussed. During the experiments, the Reynolds number varies from 2500 to 11,000, the slot width-to-rib height ratio from 1.17 to 6.67, and nozzle-to-plate spacing from 2 to 16.     

Numerical investigation of turbulent heat transfer in channels with detached rib‐arrays

A numerical investigation is conducted to analyze the flow‐field and heat transfer characteristics in a rectangular passage of width‐to‐height ratio of 6:1 with detached ribs on one wall, where constant wall temperature condition is applied. The effect of detached‐rib geometry on heat transfer coefficient, friction factor, and thermal enhancement factor is investigated covering the range of the detached‐clearance ratios (c/a) of 0.1, 0.2, 0.3, and 0.4, the Reynolds number based on the channel hydraulic diameter ranges from 8000 to 24,000. The numerical results show that the flow‐field, temperature pattern, local Nusselt number distribution, average Nusselt number, and friction factor are strongly dependent on the detached‐clearance ratios. The thermal enhancement factor (TEF) under the same pumping power constraint is calculated in order to examine the overall effect of the detached‐clearance ratio. For the present range investigated, the maximum TEF of 1.22 is achieved by the use of the ribs with c/a of 0.1 at Reynolds number of 8000. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20357     

Numerical investigation of the flow field and heat transfer characteristics for upstream continuous and truncated ribs

To verify the applicability of upstream ribs in film cooling, the present numerical study examines heat transfer characteristics and flow field for ribs located upstream of the film hole. Five ribs including bilaterally truncated ribs, centrally truncated ribs, and continuous ribs are explored with the smooth case at two blowing ratios and fixed crossflow Reynolds number. The results show that the film cooling effectiveness of cases with ribs outperforms the case without rib at a low blowing ratio. Centrally truncated ribs and continuous ribs provide superior cooling effectiveness than bilaterally truncated ribs and smooth cases. The introduction of ribs makes the distribution of the heat transfer coefficient (HTC) uneven after the hole. Among these, centrally truncated ribs increased the HTC, while bilaterally truncated ribs reduce the HTC in the far hole area at a high blowing ratio. It is found that anti-kidney-shaped vortex pairs are generated between two adjacent jets for centrally truncated rib cases, while they are generated in front of the jets for bilaterally truncated rib cases. For continuous rib, the impingement of the mainstream gas on the jet leads to a reduction in strength of the kidney-shaped vortex, which allows the coolant to form a better coverage.     

Measurement of detailed heat transfer along rib-roughened surface under arrays of impinging elliptic jets

The objective of the present study is to examine the detailed heat transfer coefficient distributions over a ribbed-surface under impingement of elliptic jet arrays using a liquid crystal thermograph technique. Both continuous and broken V-shaped-rib configurations with different exit flow orientations were considered. To examine the angled rib effects, three angled ribs were discussed under jet-to-plate spacing Z = 3 for different Reynolds numbers. Measured results show that the local heat transfer rates over the ribbed-surface are characterized by obvious periodic-type variation of Nusselt number distributions. The downstream peaks are diminished for increasing crossflow effect. Compared to the results without ribs, the heat transfer over the ribbed-surface may be enhanced or retarded. Whereas, among the test angled-rib arrangements, the best heat transfer performance is obtained with a surface with 45° V-shape ribs. In addition, the surface with continuous ribs provides a better impingement heat transfer than that with broken ribs.     

Experimental study of impinging heat transfer along rib-roughened walls by using transient liquid crystal technique

The objective of this work is to examine the detailed heat transfer coefficient distributions over a ribbed surface under impingement of in-line and staggered jet arrays by using a liquid crystal thermograph technique. In-line and staggered jet arrays with different exit flow orientations were considered. Three jet-to-target spacing Z of 3, 6 and 9 with in-line and staggered jet arrays were considered at jet Reynolds numbers of Re = 1500, 3000 and 4500 with three different exit flow orientations. In addition, the effects of rib configuration on the heat transfer distributions were discussed in detail. Results show that the local heat transfer rates over the ribbed surface are characterized by obvious periodic-type variation of Nusselt number distributions. The downstream peaks are diminished for increasing cross flow effect. Compared to the results without ribs, the heat transfer over the ribbed surface may be enhanced or retarded. Whereas, among the test angled-rib arrangements, the best heat transfer performance is obtained with a surface with 45° angled ribs.     

Effect of various rib arrangements on heat transfer in a semicylinder channel with effusion flow

The flow and heat transfer characteristics of various rib configurations on a concave channel surface with effusion holes were investigated. A semicylindrical channel with three rows of effusion holes was used to simplify the blade leading edge and eight kinds of ribs were attached on the internal concave surface for comparison. Continuous and broken ribs were both applied at 90°, as were upstream-pointed V-shaped and downstream-pointed V-shaped ribs. The Reynolds-averaged Navier–Stokes equation was solved using commercial software. The result included the divided-area-averaged and local Nusselt number distribution; the overall average Nusselt number on the concave surface is also discussed.