Rib-roughened Channel Research Articles

Optimization of rib-roughened annular gas-coolant channels

The present study deals with the effects of wall geometry on the fluid flow and heat transfer in an annulus. Rectangular-rib structural wall roughness on the inner wall is considered. This roughness is characterized by rib height, width and pitch. A detailed parametric investigation of the effect of rib width-to-height ratio on the friction coefficient is performed for different rib pitches. This ratio varies from 0.125 to 6.0. Various rib pitches are considered, from those corresponding to a smooth wall, and up to the pitch-to-rib-height ratio of 16. Examples of various rib heights are also shown. The hydraulic diameter and the flow Reynolds number serve as additional parameters. Numerical simulations are performed using a modified k– ɛ turbulence model, which makes possible the prediction of flow separation, reattachment and adverse pressure gradients. The model is validated by comparing its results with experimental and numerical results for overall and local flow parameters reported in the literature. The results of the present study indicate that for the given hydraulic diameter of the annulus and flow Reynolds number, the pitch that corresponds to maximal flow resistance depends on the rib width-to-height ratio. It appears that when the width-to-height ratio decreases, this optimal pitch approaches an asymptotic value. On the other hand, the optimum rib distance-to-height ratio remains almost constant. These findings are discussed in context of the available literature. Generalization of the results is attempted through the use of dimensionless groups based on the flow and geometrical parameters of the systems. A correlation for the friction factor is suggested.

Numerical investigation of a water flow in a rib-roughened channel by using Reynolds stress model

In this paper, water flow in a rib-roughened channel is investigated numerically by using Reynolds stress turbulence models (RSM) on a three-dimensional (3-D) domain. Computational results for mean streamwise velocity component and turbulent kinetic energy show good agreements with available experimental data. Five rib pitch-to-height ratios (p/h) of 1, 5, 10, 15 and 20 are analysed for six different Reynolds numbers (Re) of 3000, 7000, 12,000, 20,000, 40,000 and 65,000. Velocity vectors, streamlines and Reynolds stresses are showed for these ratios and Re numbers. Streamlines revealed that Reynolds numbers do not affect flowfield but play an important role in the Reynolds stresses.

Effect of Coriolis and centrifugal forces on turbulence and transport at high rotation and density ratios in a rib-roughened channel

Prediction of three-dimensional flow field and heat transfer in a two pass rib-roughened square internal cooling channel of turbine blades with rounded staggered ribs rotating at high rotation and density ratios is the main focus of this study. Rotation, buoyancy, ribs, and geometry affect the flow within these channels. The full two-pass channel with bend and with rounded staggered ribs with fillets ( e / D h = 0.1 and P / e = 10 ) as tested by Wagner et al. [J.H. Wagner, B.V. Johnson, R.A. Graziani, F.C. Yeh, Heat transfer in rotating serpentine passages with trips normal to the flow, ASME J. Turbomach. 114 (1992) 847–857] is investigated. Reynolds stress model (RSM) turbulence model is used for this study. To resolve the near wall viscosity-affected region, enhanced wall treatment approach is employed. RSM model was validated against available experimental data (which are primarily at low rotation and buoyancy numbers). The model was then used for cases with higher rotational numbers (0.24, 0.475, 0.74 and 1) and higher density ratios (0.13, 0.23, and 0.3). Particular attention is given to how secondary flow, Reynolds stresses, turbulence intensity, and heat transfer are affected by Coriolis and buoyancy/centrifugal forces, caused by high levels of rotation and density ratios. A linear correlation for 4-side-average Nusselt number as a function of rotation number is derived.

Two-Dimensional Velocity Measurement of Turbulent Flow in Smooth and Rib-Roughened Square Two-Pass Channels by Using Particle Image Velocimetry

This study experimentally deals with turbulent flow inside smooth and transverse-rib-roughened square two-pass channels in stationary condition. Details of the two-dimensional flow field were quantitatively measured by using a particle image velocimetry which has high spatial- and time-resolution. In the transverse rib case, reattachment and recirculation behind the rib, and small separation bubble in front of the rib were observed. High turbulence intensity in front of the rib as a result of highly fluctuating unsteady fluid motion was clearly captured. Furthermore vortices caused by the sharp turn were observed in the secondary flow.

A New Statistical-Based Correlation for the Rib Fin Effects on the Overall Heat Transfer Coefficient in a Rib-Roughened Cooling Channel

Heat transfer coefficients in the cooling cavities of turbine airfoils are greatly enhanced by the presence of discrete ribs on the cavity walls. These ribs introduce two heat transfer enhancing features: a significant increase in heat transfer coefficient by promoting turbulence and mixing, and an increase in heat transfer area. Considerable amount of data are reported in open literature for the heat transfer coefficients both on the rib surface and on the floor area between the ribs. Many airfoil cooling design software tools, however, require an overall average heat transfer coefficient on a rib-roughened wall. Dealing with a complex flow circuit in conjunction with180∘bends, numerous film holes, trailing-edge slots, tip bleeds, crossover impingement, and a conjugate heat transfer problem; these tools are not often able to handle the geometric details of the rib-roughened surfaces or local variations in heat transfer coefficient on a rib-roughened wall. On the other hand, assigning an overall area-weighted average heat transfer coefficient based on the rib and floor area and their corresponding heat transfer coefficients will have the inherent error of assuming a 100% fin efficiency for the ribs, that is, assuming that rib surface temperature is the same as the rib base temperature. Depending on the rib geometry, this error could produce an overestimation of up to 10% in the evaluated rib-roughened wall heat transfer coefficient. In this paper, a correction factor is developed that can be applied to the overall area-weighted average heat transfer coefficient that, when applied to the projected rib-roughened cooling cavity walls, the net heat removal from the airfoil is the same as that of the rib-roughened wall. To develop this correction factor, the experimental results of heat transfer coefficients on the rib and on the surface area between the ribs are combined with about 400 numerical conduction models to determine an overall equivalent heat transfer coefficient that can be used in airfoil cooling design software. A well-known group method of data handling (GMDH) scheme was then utilized to develop a correlation that encompasses most pertinent parameters including the rib geometry, rib fin efficiency, and the rib and floor heat transfer coefficients.

Effects of Bleed Flow on Heat/Mass Transfer in a Rotating Rib-Roughened Channel

The present study investigates the effects of bleed flow on heat/mass transfer and pressure drop in a rotating channel with transverse rib turbulators. The hydraulic diameter (Dh) of the square channel is 40.0mm. 20 bleed holes are midway between the rib turburators on the leading surface and the hole diameter (d) is 4.5mm. The square rib turbulators are installed on both leading and trailing surfaces. The rib-to-rib pitch (p) is 10.0 times of the rib height (e) and the rib height-to-hydraulic diameter ratio (e∕Dh) is 0.055. The tests were conducted at various rotation numbers (0, 0.2, 0.4), while the Reynolds number and the rate of bleed flow to main flow were fixed at 10,000 and 10%, respectively. A naphthalene sublimation method was employed to determine the detailed local heat transfer coefficients using the heat/mass transfer analogy. The results suggest that for a rotating ribbed passage with the bleed flow of BR=0.1, the heat/mass transfer on the leading surface is dominantly affected by rib turbulators and the secondary flow induced by rotation rather than bleed flow. The heat/mass transfer on the trailing surface decreases due to the diminution of main flow. The results also show that the friction factor decreases with bleed flow.

Heat Transfer in a Radially Rotating Square-Sectioned Duct With Two Opposite Walls Roughened by 45Deg Staggered Ribs at High Rotation Numbers

This paper describes an experimental study of heat transfer in a radially rotating square duct with two opposite walls roughened by 45deg staggered ribs. Air coolant flows radially outward in the test channel with experiments to be undertaken that match the actual engine conditions. Laboratory-scale heat transfer measurements along centerlines of two rib-roughened surfaces are performed with Reynolds number (Re), rotation number (Ro), and density ratio (Δρ∕ρ) in the ranges of 7500–15,000, 0–1.8, and 0.076–0.294. The experimental rig permits the heat transfer study with the rotation number considerably higher than those studied in other researches to date. The rotational influences on cooling performance of the rib-roughened channel due to Coriolis forces and rotating buoyancy are studied. A selection of experimental data illustrates the individual and interactive impacts of Re, Ro, and buoyancy number on local heat transfer. A number of experimental-based observations reveal that the Coriolis force and rotating buoyancy interact to modify heat transfer even if the rib induced secondary flows persist in the rotating channel. Local heat transfer ratios between rotating and static channels along the centerlines of stable and unstable rib-roughened surfaces with Ro varying from 0.1 to 1.8 are in the ranges of 0.6–1.6 and 1–2.2, respectively. Empirical correlations for periodic flow regions are developed to permit the evaluation of interactive and individual effects of ribflows, convective inertial force, Coriolis force, and rotating buoyancy on heat transfer.

Comparison between EVM and RSM turbulence models in predicting flow and heat transfer in rib-roughened channels

A 3D analysis of two-equation eddy viscosity (EVMs) and Reynolds stress (RSM) turbulence models and their application to solve flow and heat transfer in rotating rib-roughened internal cooling channels is the main focus of this study. The flow in these channels is affected by ribs, rotation, buoyancy, bends and boundary conditions. The EVMs considered are the standard k–ϵ model of Launder and Spalding [1], the renormalization group k–ϵ model of Yakhot and Orszag [2], the realizable k–ϵ model of Shih et al. [3], the standard k–ω model of Wilcox [4] and the shear–stress transport (SST) k–ω model of Menter [5]. The viscosity-affected near-wall region is resolved by enhanced near-wall treatment using combined two-layer model with enhanced wall functions. The results for both stationary and rotating channels showed the advantages of Reynolds stress model (RSM), Gibson and Launder [6], Launder [7] and Launder et al. [8] in predicting the flow field and heat transfer compared to two-equation EVMs that need corrections to account for streamline curvature, buoyancy and rotation.

Comparative measurements of natural convection heat transfer in channels by holographic interferometry and schlieren

In this work, natural convection heat transfer in vertical channels is experimentally investigated by applying different optical techniques, namely holographic interferometry and schlieren. Both these techniques are based on the temperature dependence of the air refractive index but they detect different optical quantities and their use involves different instrumentation and optical components. Optical methods, non-intrusive in nature, are particularly suitable for the visualization of flow and thermal fields as witnessed by their increasing use in a range of scientific and engineering disciplines; for this reason, the introduction of these experimental tools into a laboratory course can be of high value. Physics and engineering students can get familiarized with optical techniques, grasp the basics of thermal phenomena, usually elusive, which can be more easily understood if they are made visible, and begin to master digital image analysis, a key skill in laboratory activities. A didactic description of holographic interferometry and schlieren is provided and experimental results obtained for vertical, smooth and rib-roughened channels with asymmetrical heating are presented. A comparison between distributions of the local heat transfer coefficient (or its dimensionless counterpart, the Nusselt number) revealed good agreement between the results separately obtained by the two techniques, thus proving their suitability for investigating free convection heat transfer in channels.

Experimental Investigation of the Aerothermal Performance of a High Blockage Rib-Roughened Cooling Channel

The present study deals with a detailed experimental investigation of the turbulent flow inside a rib-roughened turbine blade cooling channel. The measurements are carried out in a stationary straight channel with high blockage ribs installed on one wall. The main objective is to enhance the understanding and deepen the analysis of this complex flow field with the help of highly resolved particle image velocimetry measurements. A quasi-three-dimensional view of the flow field is achieved, allowing the identification of the main time-averaged coherent structures. The combined analysis of the present aerodynamic results with available heat transfer data emphasizes the role of the mean and fluctuating flow features in the heat transfer process. In particular, the stream wise/normal to the wall component of the Reynolds stress tensor is shown to be strictly related to the heat transfer rate on the channel surfaces. A correlation to estimate the heat transfer field from the aerodynamic data is presented for the high blockage rib roughened channel flow.

Design optimization of rib-roughened channel to enhance turbulent heat transfer

This work presents a numerical procedure to optimize the shape of two-dimensional channel with periodic ribs mounted on both of the walls to enhance turbulent heat transfer. The response surface method is used as an optimization technique with Reynolds-averaged Navier–Stokes analysis of flow and heat transfer. Standard k– ε turbulence model is used as a turbulence closure. Computational results for overall heat transfer rate show good agreements with experimental data. The width-to-height ratio of the rib, rib height-to-channel height ratio, pitch-to-rib height ratio and distance between opposite ribs to rib pitch ratio are chosen as design variables. The objective function is defined as a linear combination of heat-transfer and friction-loss related terms with weighting factor. D-optimal design is used to reduce the data points, and, with only 36 points, reliable response surface is obtained. Optimum shapes of the channel have been obtained in the range from 0.0 to 0.1 of weighting factor. In the weighting factor range where designer’s goal is shifted to reduction of pressure loss, both of pitch-to-rib height ratio and distance between opposite ribs to rib pitch ratio reach almost constant values.

Aiding and opposing contributions of centrifugal buoyancy on turbulent heat transfer in a two-pass transverse- or angled-rib-roughened channel with sharp 180° turns

Centrifugal buoyancy effect on heat transfer in a rotating two-pass rib-roughened channel with 180° sharp turns was numerically investigated by using the large eddy simulation. The effect of the aiding and opposing buoyancy contributions was seen in the suppressed turbulent transport in the radially inward flow, and in larger buoyancy-induced variation of the heat transfer on the pressure surface than that on the suction surface. As the buoyancy increased, the friction factor dominated by the pressure loss of the sharp turn decreased, and Colburn's j factor stayed almost constant. As a result, the heat transfer efficiency index slightly increased by the buoyancy, and it showed almost the same values among the three tested rib arrangements.

Aiding and opposing contributions of centrifugal buoyancy on turbulent heat transfer in a two-pass transverse- or angled-rib-roughened channel with sharp 180$deg; turns

Centrifugal buoyancy effect on heat transfer in a rotating two-pass rib-roughened channel with 180° sharp turns was numerically investigated by using the large eddy simulation. The effect of the aiding and opposing buoyancy contributions was seen in the suppressed turbulent transport in the radially inward flow, and in larger buoyancy-induced variation of the heat transfer on the pressure surface than that on the suction surface. As the buoyancy increased, the friction factor dominated by the pressure loss of the sharp turn decreased, and Colburn's j factor stayed almost constant. As a result, the heat transfer efficiency index slightly increased by the buoyancy, and it showed almost the same values among the three tested rib arrangements.

Local heat transfer in rotating square-rib-roughened and smooth channels with jet impingement

An experimental investigation was carried out to examine the effects of jet impinging positions on heat transfer from rib-roughened (square and semi-circular) channels with rotational speeds of up to 600 rpm. The rib height ( e) was fixed at 1.5 mm and pitch-to-height ratio ( p/ e) kept at 6.67. The study covered jet Reynolds number in the range 5000–9000 based on jet hole diameter. Results were presented for rotational number, jet impinging positions, and roughened surfaces effects on local Nusselt numbers. Significant heat transfer enhancement was found for the present physical geometry and within the ranges of operating parameters considered in the study. Moreover, detailed heat transfer coefficients on target wall in a rotating square smooth channel using transient liquid crystal techniques for the rotational speeds of 30 and 60 rpm at Re j=7000 and 9000 were also examined.

Low-Reynolds Number Turbulence Models: An Approach for Reducing Mesh Sensitivity

This study presents a new near-wall treatment for low-Reynolds number (LRN) turbulence models that maintains accuracy in ‘coarse’ mesh predictions. The method is based on a thorough examination of approximations made when integrating the discretized equations in the near-wall region. A number of modifications are proposed that counteract errors introduced when an LRN-model is used on meshes for which the first interior node is located at y+≈5. Here the methodology is applied to the k−ω turbulence model by Bredberg et al., although similar corrections are relevant for all LRN models. The modified model gives asymptotically, in the sense of mesh refinement, identical results to the baseline model. For coarser meshes y+⩽10, the present method improves numerical stability with less mesh-dependency than the non-modified model. Results are included for fully developed channel flow, a backward-facing step flow and heat transfer in a periodic rib-roughened channel.

Experimental analysis of turbulent flow structure in a fully developed rib-roughened rectangular channel with PIV

An experimental study was carried out to investigate the turbulent water-flow structure over one-side micro-repeated ribs in a narrow two-dimensional rectangular channel by particle image velocimetry (PIV). Two rib pitch-to-height ratios p/k of 10 and 20 were investigated while the rib height was held constant at 4 mm. The rib height-to-channel equivalent diameter ratio k/De was 0.1. The streamwise mean velocity and turbulent kinetic energy fields in the fully developed flow region of the channel were calculated at three different positions of x=0, 3.8, and –6.2 mm, which corresponded to center, downstream, and upstream of the rib, respectively, and for two Reynolds numbers Re of 7,000 and 20,000. A large-scale turbulent eddy was generated by the rib promoter and then propagated into the mainstream flow, which led to the deformation of the velocity profile. Downstream of the rib, rotating and counter-rotating eddies were also generated by the rib promoter. The enhancement of the turbulent kinetic energy was not changed when the Reynolds number increased from 7,000 to 20,000 between p/k=20 and 10. The reattachment length LR was measured from velocity vector fields in the developing, fully developed, and exit regions of the flow over the range Re=1,400–50,000. The results showed that the ratio p/k and the Reynolds number had no significant effect on the reattachment length beyond a critical value of Re=15,000, where LR was found to be approximately 4 times the rib height.

Heat Transfer in Rotating Rectangular Cooling Channels AR=4 With Angled Ribs

An investigation into determining the effect of rotation on heat transfer in a rib-roughened rectangular channel with aspect ratio of 4:1 is detailed in this paper. A broad range of flow parameters have been selected including Reynolds number (Re=5000–40000), rotation number (Ro=0.04–0.3) and coolant to wall density ratio at the inlet Δρ/ρi=0.122. The rib turbulators, attached to the leading and trailing surface, are oriented at an angle α=45deg to the direction of flow. The effect of channel orientations of β=90 deg and 135 deg with respect to the plane of rotation is also investigated. Results show that the narrow rectangular passage exhibits a much higher heat transfer enhancement for the ribbed surface than the square and 2:1 duct previously investigated. Also, duct orientation significantly affects the leading and side surfaces, yet does not have much affect on the trailing surfaces for both smooth and ribbed surfaces. Furthermore, spanwise heat transfer distributions exist across the leading and trailing surfaces and are accentuated by the use of angled ribs. The smooth and ribbed case trailing surfaces and smooth case side surfaces exhibited a strong dependence on rotation number.

An improved k− ω turbulence model applied to recirculating flows

In this paper an improved k− ω turbulence model is proposed, which brings the asymptotic boundary value for ω into accord with direct numerical simulation (DNS) data. In the new ω-equation both a turbulent and a viscous cross-diffusion term are included, justified by an analogy to the Yap-correction and the pressure-diffusion process, respectively. The importance of cross-diffusion terms in removing the freestream sensitivity with respect to ω for free shear flows is shown. The performance of the model is evaluated and compared with DNS and with other turbulence models in a channel flow, a backward-facing step flow and a rib-roughened channel flow with heat transfer. The model requires neither wall-function nor wall-distance information and is fully integrable over the near-wall region.

Forced convective heat transfer in square-ribbed coolant channels with helium gas for fusion power reactors

Square-ribbed coolant channels with helium gas for fusion power reactors were designed to enhance the turbulent heat transfer in comparison with smooth coolant channels. Heat transfer coefficients and friction factors in the square-ribbed annular channels were investigated quantitatively under high pressure. The turbulent heat transfer coefficients in the square-ribbed annulus, with a ratio of the rib pitch to a height of ten, were 200–300% higher than those in the smooth annulus and then the thermal–hydraulic empirical correlations were derived. The average heat fluxes from the heated wall were predicted using the present empirical correlations and the maximum heat flux at p/ h=10 reached ≈1.5 MW/m 2 at Re=65 000. From the results of the present study, it can be expected that the proposed rib-roughened coolant channels are available to remove surface heat fluxes at the first wall and blanket in the fusion power reactors.

Experimental Study on Heat Transfer Augmentation for High Heat Flux Removal in Rib-Roughened Narrow Channels

Frictional pressure drop and heat transfer performance in a very narrow rectangular channel having one-sided constant heat flux and repeated-ribs were studied experimentally for turbulent water flows. Their empirical correlations were derived for designing target cooling channels to remove high heat flux generated at target plates in a high-intensity proton accelerator system. The rib pitch-to-height ratios (p/k) were 10 and 20 while holding the rib height constant at 0.2mm the Reynolds number (Re) from 2,400 to 98,500 under different channel heights (H) of 1.2 mm and 3.2 mm, the rib height-to-channel equivalent diameter ratio (k/De ) of 0.088 and 0.036, respectively. The results show that the rib-roughened surface augments heat transfer by about 2–2.5 times compared with the smooth surface at the expense of around 2.5 times higher frictional pressure drop under a range of Re=8,000-to-30,000 at p/k=10, and H=1.2mm. Experimental results of channel height, H=1.2mm show slightly higher heat transfer and friction factor performance than that of the channel height, H=3.2 mm.