Heat transfer augmentation in a wedge-ribbed channel using winglet vortex generators

Experimental investigations have been carried out to study the effect of combined wedge ribs and winglet type vortex generators (WVGs) on heat transfer and friction loss behaviors for turbulent airflow through a constant heat flux channel. To create a reverse flow in the channel, two types of wedge (right-triangle) ribs are introduced: wedge ribs pointing downstream and pointing upstream. The arrangements of both rib types placed inside the opposite channel walls are in-line and staggered arrays. To generate longitudinal vortex flows through the tested section, two pairs of the WVGs with the attack angle of 60° are mounted on the test channel entrance. The test channel has an aspect ratio, AR = 10 and height, H = 30 mm with a rib height, e/H = 0.2 and rib pitch, P/H = 1.33. The flow rate in terms of Reynolds numbers is based on the inlet hydraulic diameter of the channel ranging from 5000 to 22,000. The presence of the combined ribs and the WVGs shows the significant increase in heat transfer rate and friction loss over the smooth channel. The Nusselt number and friction factor values obtained from combined the ribs and the WVGs are found to be much higher than those from the ribs/WVGs alone. In conjunction with the WVGs, the in-line wedge pointing downstream provides the highest increase in both the heat transfer rate and the friction factor while the staggered wedge pointing upstream yields the best thermal performance.     

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.     

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.     

Thermal behavior in solar air heater channel fitted with combined rib and delta-winglet

Effects of combined ribs and delta-winglet type vortex generators (DWs) on forced convection heat transfer and friction loss behaviors for turbulent airflow through a solar air heater channel are experimentally investigated in the present work. Measurements are carried out in the rectangular channel of aspect ratio, AR = 10 and height, H = 30 mm. The flow rate is presented in the form of Reynolds numbers based on the inlet hydraulic diameter of the channel ranging from 5000 to 22,000. The cross-section shape of the rib placed on the absorber plate to create a reverse-flow is an isosceles triangle with a single rib height, e/H = 0.2 and rib pitch, P_l/H = 1.33. Ten pairs of the DW with its height, b/H = 0.4; transverse pitch, P_t/H = 1 and three attack angles (α) of 60°, 45° and 30° are introduced and mounted on the lower plate entrance of the tested channel to generate longitudinal vortex flows. The experimental results show that the Nusselt number and friction factor values for combined rib and DW are found to be much higher than those for the rib/DW alone. The larger attack angle of the DW leads to higher heat transfer and friction loss than the lower one. In common with the rib, the DW pointing upstream (PU-DW) is found to give higher heat transfer rate and friction loss than the DW pointing downstream (PD-DW) at a similar operating condition. In comparison, the largest attack angle (α = 60°) of the PU-DW yields the highest increase in both the Nusselt number and friction factor while the lowest attack angle of the PD-DW provides the best thermal performance.     

Periodic laminar flow and heat transfer in a channel with 45° staggered V-baffles

A numerical investigation has been carried out to examine periodic laminar flow and heat transfer characteristics in a three-dimensional isothermal wall channel of aspect ratio, AR = 2 with 45° staggered V-baffles. The computations are based on the finite volume method, and the SIMPLE algorithm has been implemented. The fluid flow and heat transfer characteristics are presented for Reynolds numbers based on the hydraulic diameter of the channel ranging from 100 to 1200. To generate two pair of main streamwise vortex flows through the tested section, V-baffles with an attack angle of 45° are mounted in tandem and staggered arrangement on the lower and upper walls of the channel. Effects of different baffle heights on heat transfer and pressure drop in the channel are studied and the results of the V-baffle pointing upstream are also compared with those of the V-baffle pointing downstream. It is apparent that in each of the main vortex flows, a pair of streamwise twisted vortex (P-vortex) flows can induce impinging flows on a sidewall and a wall of the interbaffle cavity leading to drastic increase in heat transfer rate over the channel. In addition, the rise in the V-baffle height results in the increase in the Nusselt number and friction factor values. The computational results reveal that the optimum thermal enhancement factor is around 2.6 at baffle height of 0.15 times of the channel height for the V-baffle pointing upstream while is about 2.75 at baffle height of 0.2 times for the V-baffle pointing downstream.     

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.     

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.     

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.     

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.     

Two-dimensional-numerical investigation on the influence of first rib distance from the entrance of channel on Nusselt number and pressure drop

Enhancing the heat transfer coefficient in heat sinks can be achieved by surface modification techniques. Although the addition of ribs increases heat transfer capacity, it also increases pressure drop, lowering the channel's thermohydraulic performance (THP) factor. Rib research began a decade ago, with the majority of studies focusing on new rib designs or factors such as relative roughness pitch, relative roughness height, channel width, and channel height (geometric optimization) to improve THP. The goal of this study is to investigate the influence of the positioning of the first rib from the channel entrance on the Nusselt number, pressure drop, and THP factor with a simple design that could be manufactured easily. Three distinct rib designs are evaluated with rib positioning from the channel entrance, rib thickness, pitch, and Reynolds number as the parameters. It was found that the fluid starts settling up at the ribs as the ribs are moved closer to the channel entry point, thus increasing the pressure drop and reducing the fluid velocity. For the proposed design and dimensions, the Nusselt number increases by 3%–5%, and the pressure drop lowers by 4%–14% when the first rib is placed away from the channel entrance.     

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.     

Thermal visualization on surface with transverse perforated ribs

This paper presents the heat transfer and flow characteristics in a channel with different types of transverse perforated ribs. The effects of perforation/hole inclination angle (θ = 0°, 15° and 30°) and a location of hole on the rib (h = 0.2H, 0.5H and 0.8H), have been examined. The investigation was performed at constant Reynolds number (Re) of 60,000. The experimental heat transfer results via Thermochromic liquid crystal sheet are reported along with the numerical flow characteristics. The results reveal that due to jet-like flows impinging on the surface, the inclined perforated rib considerably improve the heat transfer immediately downstream from the ribs, compared to straight perforated and solid ones, resulting in superior overall heat transfer performance.     

An experimental investigation of the heat transfer in a ribbed triangular cooling channel

One of the most challenging aspects of gas turbine cooling is the cooling of the first stages of turbine blades. Here the highest external heat load is seen at the leading edge of the blade. The present study investigates the internal cooling in a triangular channel with a rounded edge as a model of a leading edge cooling channel for a gas turbine blade. A transient liquid crystal method is used to measure the heat transfer. Experimental results are reported for a number of new 3D rib configurations for Reynolds numbers between 50 000 and 200 000. From the experimental results it has been found that 60 deg. ribs provide in general higher heat transfer enhancements than 45 deg. ribs. However, this results in extremely high friction factors for the 60 deg. ribs. Taking the local and mean distributions of the heat transfer coefficients (as well as the increase in friction factors) into consideration, it was found that the most promising rib arrangement for leading edge cooling is a 3D rib with 45 deg. angle and double-sided fully overlapped ribs in the arc area. These ribs provide uniform heat transfer in the arc area as well as a high level of the heat transfer coefficients in the channel. The resulting friction factors are in an acceptable range for these ribs.     

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。     

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.     

Effect of rib spacing on heat transfer and friction in a rectangular channel with 45° angled rib turbulators on one/two walls

An experimental investigation of forced convection heat transfer in a rectangular channel (aspect ratio AR = 5) with angled rib turbulators, inclined at 45°, is presented. The angled ribs were deployed with parallel orientations on one or two surfaces of the channel. The convective fluid was air, and the Reynolds number varied from 9000 to 35,500. The ratio of rib height to hydraulic diameter (e/D) was 0.09, while four rib pitch-to-height ratios (p/e) were studied: 6.66, 10.0, 13.33, and 20.0. The aim of the work was to study the effect of rib spacing on the thermal performance of the ribbed channel. The maps of local heat transfer coefficient in the inter-rib regions have been reconstructed by liquid crystal thermography. The thermal performance of each ribbed channel is identified by the average Nusselt number and by the friction factor. Superior heat transfer performance was found at the optimal rib pitch-to-height ratio of 13.33 for the one-ribbed wall channel and at p/e = 6.66–10 for the two-ribbed wall channel.     

Shape optimization of three-dimensional channel roughened by angled ribs with RANS analysis of turbulent heat transfer

A numerical procedure for optimizing the shape of three-dimensional channel with angled ribs extruded on both walls is presented to enhance turbulent heat transfer. The response surface based optimization is used as an optimization technique with Reynolds-averaged Navier–Stokes analysis of fluid flow and heat transfer. Shear stress transport (SST) turbulence model is used as a turbulence closure. Computational results for heat transfer rate show good agreements with experimental data. Four dimensionless variables such as, rib pitch-to-rib height ratio, rib height-to-channel height ratio, streamwise rib distance on opposite wall to rib height ratio, and the attack angle of the rib are chosen as design variables. The objective function is defined as a linear combination of heat transfer and friction loss related terms with a weighting factor. D-optimal method is used to determine the training points as a mean of design of experiment. Sensitivity of the objective function to each design variable has been evaluated. And, optimal values of the design variables have been obtained in a range of the weighting factor.     

Effects of macro and micro roughness in forced convective heat transfer

The article reports a comparative study of macro and micro type artificial roughness in convective heat transfer performance under laminar and low turbulent regime. Circular ribs with different rib height to channel height ratios (e/H = 0.05, 0.1, 0.15) fabricated on copper substrate are introduced as macro type roughness whereas copper (Cu) nano-porous layer (avg. thickness about 5 μm) is considered as micro roughness. Surface heat transfer and friction characteristics are investigated under different scale roughnesses on one principle wall of a rectangular channel having an aspect ratio (AR) of 7.5. Result shows that the average turbulence intensity between two ribs decreases with decreasing roughness height. On the other hand, nano-porous layer provides significant heat transfer efficiency (about maximum 42% more than bare copper plate) under laminar and low turbulent region without inducing significant turbulence into the channel. Nano-porous layer less than 5 μm is found to increase heat transfer surface area significantly that influences the dynamic behaviors of working fluids in the vicinity of heat transfer wall.     

Large eddy simulation of flow and heat transfer in a channel with a detached rib array

Ribbed channels are widely used to enhance heat transfer in various heat exchange equipment. However, the heat transfer is locally deteriorated immediately behind the rib due to the flow separation. To overcome this shortcoming, a detached rib array has been proposed recently. In the present study, large eddy simulations (LES) of turbulent flow and heat transfer in a channel with a detached rib array have been conducted. The no-slip and no-jump conditions on the rib surface are satisfied in the Cartesian coordinates by using an immersed boundary method. Experiments are conducted as well to validate the simulation. The velocity and temperature fields are obtained by a hot wire and a thermocouple, respectively. The surface heat transfer is measured using the thermochromic liquid crystal with a high spatial resolution. Compared with the attached rib, the detached one enhances the heat transfer underneath the rib, whereas the channel wall downstream of the rib shows lower heat transfer rate. By investigating the effect of the clearance between the rib and the wall, we have found an intermediate flow pattern, where the counter-rotating vortex pair and the separation bubble coexist. Including the new flow pattern, we discuss the flow characteristics such as the wake length and the locus of the saddle points, and the flow physics behind the local heat transfer distribution on the channel wall based on the instantaneous flow and thermal fields. The turbulence data near the solid surface are also presented, which have not been provided by previous experiments.     

Prediction of turbulent flow and heat transfer within rotating U‐shaped passages

Numerical predictions of three‐dimensional flow and heat transfer are presented for rotating serpentine passages with and without rib turbulators. The coolant air is pressurized and its operating conditions are selected closely to match actual turbine operating parameters. Two different arrangements of rib turbulators were studied: (1) transverse ribs on the leading and trailing walls and (2) transverse ribs on all four walls. The rib height‐to‐hydraulic diameter ratio (e/D_h) is 0.143; the rib pitch‐to‐height ratio (s/e) is 7. Results for the rib‐roughened serpentine passages were compared with those of smooth ones calculated in the literature. It was shown that a significant enhancement is achieved by means of rib turbulators in a serpentine passage at a stationary state as well as in a rotating state. In the radially‐outward flow passages, the effect of rotation on heat transfer is relatively prominent. The secondary flows induced by the Coriolis forces are most intensive in the channel with four ribbed surfaces. The heat transfer after a 180° sharp turn in the smooth channel is influenced more by the sharp‐turn‐induced flow than the rib‐roughened ones. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(6): 410–420, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20125