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.     

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.     

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.     

A Possible Strategy for the Performance Enhancement of Turbine Blade Internal Cooling With Inclined Ribs

Ribbing the internal passages of turbine blades with 45 deg inclined ribs is a common practice to achieve a good compromise between high heat transfer coefficients and not too large pressure drop penalties. Literature studies demonstrated that, for channels having a large aspect ratio, the effect of the secondary vortices induced by angled ribs is reduced and the heat transfer performance is degraded. In order to enhance the performance, a possible strategy consists in introducing one or more longitudinal ribs (intersecting ribs) aligned to the main direction of flow. The intersecting ribs cut the ribbed channel into separate sub-channels and markedly affect the secondary flows with consequent increases in heat transfer performance. Experiments were performed for a rectangular channel with a large aspect ratio (equal to five) and 45 deg inclined ribs, regularly spaced on one of the principal walls of the channel. The effect of one and two intersecting ribs on friction and heat transfer characteristics has been investigated. The ribbed surface of the channel has been electrically heated to provide a uniform heat flux condition over each inter-rib region. The convective fluid was air. Heat transfer experiments have been conducted by using the liquid crystal thermography. Results obtained for the ribbed channel without intersecting rib and with one/two intersecting ribs are compared in terms of dimensionless groups.     

Numerical investigation on synthetical performances of fluid flow and heat transfer of semiattached rib-channels

Although the conventional internal ribs or turbulators can significantly improve the performances of the convective heat transfer within a channel, the added ribs can also cause two demerits, a larger friction factor and some lower heat transfer areas (LHTA) than that in the corresponding smooth channel, especially behind fully attached (solid-type) ribs and at the corners formed by bottom and side walls. This paper presents a novel design of the ribbed channel, which is here called semiattached rib-design. The ribs are perforated at the rib corners to form two rectangular holes, so a portion of the fluid can pass through the holes. The characteristics of the semiattached rib-design are numerically investigated by the commercial software Fluent 6.3 in a Reynolds number range from 10⁴ to 2.5 × 10⁴. Five different structures of the rib (width ratios of channel to hole) and two positions (transverse rib and 45° angled ribs) are analyzed. The numerical results show that the semiattached rib-design can significantly improve local heat transfer and fluid flow performances; the semiattached ribs with 45° angle of attack can even achieve a higher efficiency of synthetical heat transfer than that of the fully attached and detached rib-channels, at the same time eliminate the LHTA; although the average Nusselt number over a pitch in the transverse ribbed channel is lower than that of fully attached and detached rib-channels, this semiattached ribs can also fully eliminate the LHTA.     

Detailed measurement of heat/mass transfer with continuous and multiple V-shaped ribs in rectangular channel

Effects of aspect ratio on heat/mass transfer were investigated in rectangular channels with two different V-shaped rib configurations, which are continuous V-shaped rib configuration with a 60° attack angle, and multiple (staggered) V-shaped rib configuration with a 45° attack angle. The square ribs were attached on the test section in a parallel manner. A naphthalene sublimation method was used to measure the local heat/mass transfer coefficients. For the continuous V-shaped rib configuration, two pairs of counter-rotating vortices were generated in the channel, and high transfer region was formed at the center of the ribbed walls. However, for the multiple V-shaped rib configuration with 45° attack angle, asymmetric secondary flow patterns were generated due to its geometric features, resulting in uniform heat/mass transfer distributions. The effect of channel aspect ratio was more significant for the continuous 60° V-shaped rib than for the multiple 45° V-shaped rib configuration.     

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.     

Thermal performance assessment of turbulent channel flows over different shaped ribs

Experiments are conducted to assess turbulent forced convection heat transfer and friction loss behaviors for air flow through a constant heat flux channel fitted with different shaped ribs. The rib cross-sections used in the present study are triangular (isosceles), wedge (right-triangular) and rectangular shapes. Two rib arrangements, namely, in-line and staggered arrays, are introduced. Measurements are carried out for a rectangular channel of aspect ratio, AR = 15 and height, H = 20 mm with single rib height, e = 6 mm and rib pitch, P = 40 mm. The flow rate is in terms of Reynolds numbers based on the inlet hydraulic diameter of the channel in a range of 4000 to 16,000. The experimental results show a significant effect of the presence of the ribs on the heat transfer rate and friction loss over the smooth wall channel. The in-line rib arrangement provides higher heat transfer and friction loss than the staggered one for a similar mass flow rate. In comparison, the wedge rib pointing downstream yields the highest increase in both the Nusselt number and the friction factor but the triangular rib with staggered array shows better thermal performance over the others.     

Heat transfer and friction characteristics of rectangular solar air heater duct using rib-grooved artificial roughness

Experimental investigation on the heat transfer and friction characteristics of rib-grooved artificial roughness on one broad heated wall of a large aspect ratio duct shows that Nusselt number can be further enhanced beyond that of ribbed duct while keeping the friction factor enhancement low. The experimental investigation encompassed the Reynolds number range from 3000 to 21,000; relative roughness height 0.0181–0.0363; relative roughness pitch 4.5–10.0, and groove position to pitch ratio 0.3–0.7. The effect of important parameters on the heat transfer coefficient and friction factor has been discussed and the results are compared with the results of ribbed and smooth duct under similar flow conditions. The present investigation clearly demonstrates that the heat transfer coefficient for rib-grooved arrangement is higher than that for the transverse ribs, whereas the friction factor is slightly higher for rib-grooved arrangement as compared to that of rectangular transverse ribs of similar rib height and rib spacing. The conditions for best performance have been determined. Correlations for Nusselt number and friction factor have been developed that predict the values within reasonable limits.     

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.     

Experimental Investigation of Heat Transfer Enhancement in Rectangular Duct with Pentagonal Ribs

Rib turbulators are extensively used in augmentation of convective heat transfer in several applications related to heat exchange and cooling in thermal energy systems. Present experimental investigation examines the local heat transfer and friction factor characteristics of pentagonal ribs mounted on bottom heated wall of a rectangular channel. The emphasis is towards assessing and analysing the potential impact of varying chamfering angle (0 to 20°) and rib pitch to height ratio (6 to 12) on the overall heat transfer enhancement and its distribution on the surface. Experiments are performed at different Reynolds numbers ranging from 9400 to 58850. Liquid crystal thermography is applied to measure surface temperature distribution and finally to demonstrate the local heat transfer coefficient over the ribbed surface. The results depict that the local augmentation Nusselt number distribution is axisymmetric and shows 2-dimensionalty in heat transfer distribution. Pentagonal ribs show a significant improvement for the low heat transfer zones in leeward vicinity of the square rib, specially prominent at higher Reynolds number, and therefore seen as the potential benefit in terms of obviating the hotspots. It is observed that the pentagonal ribs lead to superior heat transfer enhancement in conjunction with significant reduction in pressure penalty as compared to square ribs and thus ensures an enhanced thermo-hydraulic performance.     

Secondary flows and extra heat transfer enhancement of ribbed surfaces with jet impingement

Previous experiments recognize that substantial heat transfer augmentation is achieved by adding ribbed turbulators after jet impingement with cross flow present. To address fundamental working mechanisms, conjugate CFD simulations are employed for ribs, jet impingement, and their combinations. Flow characteristics and drawbacks for the individual and combined enhancement techniques are highlighted. New analysis on the coupled design arrangement reveals that the counter-rotating vortices generated by the jet flow can energize inter-rib recirculating vortices and promote span-wise convection. With an optimal design combination arrangement, extra heat transfer benefit is achieved beyond that associated with simple superposition of rib and jet impingement techniques.     

Influence of channel-height on heat transfer in rectangular channels with skewed ribs at different bleed conditions

An infrared imaging system is used to measure detailed distributions of local heat transfer coefficient from rectangular channels with two opposite wide walls roughened by 45° staggered ribs of twenty test cases. Nusselt number (Nu) contours over the ribbed surfaces are presented for five sets of channels with an identical rib-floor configuration, but different channel-heights where the complex Nu distributions without bleed and then with bleeds from the scanned rib-floor or from the ribbed wall opposite to the scanned rib-floor or from the smooth sidewall are analyzed to examine the synergetic effects of variable channel-heights and different bleed conditions on heat transfer. Spatially averaged heat transfer results for the entire set of test channels are generated with the thermal performance factors compared to establish the heat transfer correlations with applications to the design of coolant channels in a gas turbine blade.     

Numerical heat transfer study of turbulent square-duct flow through inline V-shaped discrete ribs

A numerical work has been conducted to examine turbulent periodic flow and heat transfer characteristics in a three dimensional square-duct with inline 60° V-shaped discrete thin ribs placed on two opposite heated walls. The isothermal-flux condition is applied only to the upper and lower duct walls while the two sidewalls are insulated, similar to internal passage cooling of gas turbine blades. The computations are based on the finite volume method with the SIMPLE algorithm for handling the pressure–velocity coupling. Air is the working fluid with the flow rate in terms of Reynolds numbers ranging from 10,000 to 25,000. The numerical result is validated with available square-rib measured data and found to agree well with measurement. The computation reveals that the ribbed duct flow is fully developed periodic flow and heat transfer profiles at about x/D = 7–11 downstream of the inlet. Effects of different rib height to duct diameter ratios, BR, on thermal characteristics for a periodic ribbed duct flow are investigated. It is found that a pair of counter-rotating vortices (P-vortex) caused by the rib can induce impingement/attachment flows on the walls leading to greater increase in heat transfer over the test duct. In addition, the rise of BR values leads to the increase in heat transfer and friction loss. The maximum thermal performance is around 1.8 for the rib with BR = 0.0725 where the heat transfer rate is about 4.0 times above the smooth duct at lower Reynolds number.     

Performance of solar air heater ducts with different types of ribs on the absorber plate

Repeated ribs are considered an effective technique to enhance forced convection heat transfer in channels. In order to establish the performance of rib-roughened channels, both heat transfer and friction characteristics have to be accounted for. In the present paper, heat transfer coefficients and friction factors have been experimentally investigated for a rectangular channel having one wall roughened by repeated ribs and heated at uniform flux, while the remaining three walls were smooth and insulated. Angled continuous ribs, transverse continuous and broken ribs, and discrete V-shaped ribs were considered as rib configurations. Different performance evaluation criteria, based on energy balance or entropy generation analysis, were proposed to assess the relative merit of each rib configuration. All the rib-roughened channels performed better than the reference smooth channel in the medium-low range of the investigated Reynolds number values, which is that typically encountered in solar air heater applications.     

Heat/mass transfer in rotating impingement/effusion cooling with rib turbulators

A detailed measurement of the heat/mass transfer coefficients on the ribbed surfaces for the rotating impingement/effusion cooling system has been conducted. Three different jet orientations (front, leading, and trailing) were investigated at the same rotating speed and impinging jet Reynolds number of 3000. A naphthalene sublimation method was used to obtain local heat/mass transfer coefficients. Regardless of rib turbulators, the leading and trailing orientations lead to totally changed heat/mass transfer distributions due to the jet deflection, while the Sh distributions of the front orientation were similar to those of the stationary case. For leading and trailing orientations, the influence of crossflow, which deflected wall jets, decreased due to the blockage effect of the rib turbulators. Therefore, the wall jets spread more widely and the interaction between adjacent wall jets along spanwise direction became stronger, enhancing the heat/mass transfer compared to that on smooth surface.     

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 and friction behaviors in rectangular channels with varying number of ribbed walls

An experimental study of surface heat transfer and friction characteristics of a fully developed turbulent air flow in a square channel with transverse ribs on one, two, three, and four walls is reported. Tests were performed for Reynolds numbers ranging from 10,000 to 80,000. The pitch-to-rib height ratio, P/e, was kept at 8 and rib-height-to-channel hydraulic diameter ratio, e/D_h was kept at 0.0625. The channel length-to-hydraulic diameter ratio, L/D_h, was 20. The heat transfer coefficient and friction factor results were enhanced with the increase in the number of ribbed walls. The friction roughness function, R(e⁺), was almost constant over the entire range of tests performed and was within comparable limits of the previously published data. The heat transfer roughness function, G(e⁺), increased with roughness Reynolds number and compared well with previous work in this area. Both correlations could be used to predict the friction factor and heat transfer coefficient in a rectangular channel with varying number of ribbed walls. The results of this investigation could be used in various applications of turbulent internal channel flows involving different number of rib roughened walls.     

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。     

Numerical Evaluation of Heat Transfer Enhancement in Rocket Thrust Chambers by Wall Ribs

Heat transfer enhancement due to longitudinal wall ribs inserted in a rocket engine thrust chamber is analyzed by means of a Reynolds-Averaged Navier-Stokes equations solver. A ribbed wall experiment is numerically reproduced to assess the capability of the simplified approach to properly capture the heat transfer enhancement. Then, a parametric analysis on the role of the longitudinal rib height on heat transfer enhancement is made on a sample thrust chamber. Results show the expected heat increase related to the surface increase, and highlight the reduction of efficiency for increasing rib height due to the thermal stratification between ribs.