Rib-roughened Walls Research Articles

Direct numerical simulations of turbulent channel flow with a rib-roughened porous wall

To describe the effects of porous roughness on turbulence, we have carried out direct numerical simulations using the lattice Boltzmann method. The simulated flows are fully developed turbulent flows in channels consisting of a solid smooth top wall and a porous bottom wall with transverse porous ribs whose heights are 10 % of the channel height. The considered ratios of the rib spacing to the rib height are $w/k\simeq 1$ and 9. The Kelvin-cell structure is applied to construct faithfully the porous media whose porosities are $\varphi \ge 0.79$ . Three kinds of porous media having different permeabilities are considered. The most permeable one has an approximately one order higher permeability than that of the least permeable one. The higher permeability case is designed to have a pore scale that is the same as the rib height so that it is the most permeable case for the rib roughness with the designed porosity. In the simulations, the bulk Reynolds number is set to $Re_b=5500$ , and the corresponding permeability Reynolds numbers are $Re_K=2.2\unicode{x2013}7.5$ . The simulated field data and the drag coefficient, which includes both the pressure drag by the ribs and the frictional drag over the porous wall, are analysed to understand the characteristics of the permeable roughness in terms of permeability. The decomposition of the drag coefficient into the integrated laminar, rib-drag, dispersion and turbulence parts elucidates the transition mechanism between the typical d-type to k-type roughness depending on $Re_K$ . By the double (time and space) averaged budget equations for the dispersion and Reynolds stresses, we explain how the energy generated by the roughness transfers to turbulence through dispersion resulting in the k-type characteristics. The nominal roughness sublayer thickness and the characteristic roughness height are introduced with the parameters obtained by fitting the velocity data to Best's and Nikuradse's logarithmic velocity formulae. Along with data in the literature, it is suggested that the ratio of the characteristic roughness height to the nominal roughness sublayer thickness becomes constant irrespective of the rib spacing in the full permeable-wall turbulence at $Re_K> 7$ .

Effect of rotation number on flow and heat transfer characteristics in serpentine passage with ribbed walls

We studied the flow and heat transfer characteristics in a rotating serpentine passage with ribbed walls. The channel length-tohydraulic diameter ratio of the rotating serpentine passage (L/Dh), the rib height-to-hydraulic diameter ratio (e/Dh), rib angle of attack (a), the rib pitch-to-height (p/e) ratio and aspect ratio (AR) were fixed at 11.33, 0.13, 90°, 10 and 1, respectively. Numerical simulations were performed at a constant Reynolds number, Re = 10000. The rotation number (Ro) levels were 0.0, 0.1, 0.2 and 0.3. The distribution of local heat transfer coefficients and the flow field in a rotating serpentine channel were studied by numerical simulations using the commercial software ANSYS ver.15.0 (Fluent). The results show that the local heat transfer at the first-pass trailing and the second-pass leading surfaces has significantly higher rates than at the first-pass leading and second-pass trailing surfaces, due to rotational effects with Coriolis forces that are high in the middle region between two ribs, and very low immediately before and after the ribs. In addition, the overall thermal hydraulic performance with rib-roughened walls was best at the highest rotation number tested (Ro = 0.3).

The three-dimensional flow field and heat transfer in a rib-roughened channel at large rotation numbers

The turbulent velocity field in a rotating rib-roughened channel is studied by means of incompressible Large Eddy Simulations (LES). The computations are validated against Particle Image Velocimetry (PIV) measurements performed in the symmetry plane of an experimental model of the same geometry. The present simulations consider the effect of the Coriolis force on a periodic section of low aspect ratio (AR = 0.9) and one rib-roughened wall. The Reynolds number based on the bulk velocity and the hydraulic diameter is fixed to 15,000, whereas the rotation number is set to 0, 0.31 and 0.77. Beyond the analysis of the Coriolis force influence on the shear layer stability, the present simulations allow to characterize the stream-wise secondary flows that redistribute the momentum through the cross-section at the different rotation numbers, the temperature distribution, and the resulting heat transfer on the wall. The flow structure is similar at rotation numbers equal to 0.31 and 0.77 when the channel rotates in the clockwise direction, with reduced turbulence and heat transfer on the ribbed wall, which acts as leading side. Only minor differences in the secondary flows and mean velocity profiles are observed due to the different magnitude of the Coriolis force. On the other hand, it has been observed that the secondary flow structure differs significantly when the rotation number is increased from 0.30 to 0.77 under counter-clockwise rotation. In particular, Taylor-Görtler vortices are observed together with the Coriolis-induced secondary flows at the maximum rotation rate, leading to a redistribution of the mean and turbulent velocity fields, as well as a significant change in the heat transfer distribution.

Orientation effect in helical coils with smooth and rib-roughened wall: Toward improved gas heaters for supercritical carbon dioxide Rankine cycles

Helical coils have gained increasingly interest in the field of supercritical carbon dioxide (SCCO2) Rankine cycles during the last decade due to the compact structure and high heat transfer rate. Past studies mainly focused on effects of operating conditions and the gravitational buoyancy, and are not sufficient to fully understand the behavior of SCCO2 helically coiled gas heaters. Influence of some other key factors, such as the coil orientations and internal wall roughness, on overall performance has been seldom reported and is still unclear to date. In this work we filled this gap with a solid-to-fluid conjugate heat transfer model where supercritical flow turbulence is solved by the Shear-Stress Transport k-ω equations. The orientation effect on performance of helical coils was revealed with the coil axis arranged in horizontal, vertically upward and downward directions, respectively. Results demonstrate that the criterion of Bo < 10−5 for negligible buoyancy effect was applicable to coils with dimensionless curvature δ = 0.01–0.1. The Orientation effect only emerged when the buoyancy effect became significant with Bo > 10−5. Deterioration and enhancement of heat transfer occurred alternatively in horizontal coils. The overall performance of horizontal coils was much worse than vertical coils, among which the downward orientation was slightly better than the upward one. The increase of δ barely improved the performance of horizontal coils. Instead, at larger δ the intensive fluctuation in heat transfer coefficients was more frequent due to the more drastic change of the flow direction. The location and extent of local impairment also varied with the inlet arrangement. Finally, the passive compound enhancement technique of helical coils in conjunction with internal helical-rib roughness was found to be very effective in mixed convection of SCCO2, especially in the horizontal coils.

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

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

Thermohydraulics of rib-roughened helium gas running cooling channels for first wall applications

Improved cooling channel designs of helium cooled first wall (FW) applications focus on efficient heat transfer enhancement and reduced material temperatures, even for high incident heat flux densities of more than 0.5MW/m2. To this end, thermohydraulics of turbulent flow in a one-sided heated, one-sided rib-roughened channel with squared, round-edged cross section were predicted by Detached Eddy Simulations (DES) at Reynolds numbers ranging from ReDh=1.0E4 to ReDh=1.5E5 and at heat up rates ranging from q+=1.1E−3 to q+=7.48E−4, encompassing the envisaged operation envelope of helium cooled FW cooling channels. The rib-roughened channel wall consists of centrally positioned, transversally oriented rib elements with a rib-pitch-to-rib-height-ratio of p/e=10, a rib-height-to-hydraulic-diameter-ratio of e/Dh=0.0638 and a rib-width-to-channel-width-ratio of l/W=0.6. Mean flow and heat transfer quantities, turbulent fluxes and flow structures were analyzed. Anisotropic, large-scale eddies originated in separated shear layers are shed vertically and laterally to the flow. Maximum heat transfer correlates with regions of maximum span- and crosswise turbulent fluctuations and is located at the rib leading edge. Minimum heat transfer occurs within the region of the counter-rotating vortex behind the rib.

Contrastive experimental study on heat transfer and friction characteristics in steam cooled and air cooled rectangular channels with rib turbulators

The present experiment compares the heat transfer and friction characteristics in steam cooled and air cooled rectangular channels (simulating a gas turbine blade cooling passage) with two opposite rib-roughened walls. The Reynolds number (Re) whose length scale is the hydraulic diameter of the passage is set within the range of 10000–60000. The channel length is 1000 mm. The pitch-to-rib height ratio, the channel aspect ratio and the channel blockage ratio is 10, 0.5 and 0.047, respectively. It is found that the average Nu, the average friction coefficient, and the heat transfer performance of both steam and air in the ribbed channels show almost the same change trend with the increase of Re. Under the same test conditions, the average Nu of steam is 30.2% higher than that of air, the average friction coefficient is 18.4% higher, and the heat transfer performances of steam on the ribbed and the smooth walls are 8.4% and 7.3% higher than those of air, respectively. In addition, semi-empirical correlations for the two test channels are developed, which can predict the Nu under the given test condition. The correlations can be used in the design of the internal cooling passage of new generation steamcooled gas turbine blade/vane.

Jet impingement on a rib-roughened wall inside semi-confined channel

Convective heat transfer on the rib-roughened wall impinged by a row of air jets inside semi-confined channel was experimentally investigated. Four rows of transverse ribs were arranged in the wall-jet zone downstream from the impinging jet stagnation to enhance heat transfer. Three typical rib configurations, including orthogonal ribs, V-shaped ribs and inverted V-shaped ribs, were considered under different non-dimensional jet-to-target distances ranging from 1 to 3 diameters and impinging jet Reynolds numbers ranging from 6000 to 30,000. The results show that the rib-roughened wall enhances the convective heat transfer up to 30% in the ribbed region by comparison with the smooth wall under the same jet Reynolds number. Among three rib configurations, the inverted V-shaped rib seems to be advantageous on the convective heat transfer enhancement, especially at lower jet-to-target spacing. The ribs on the impinging target do provide stronger convective heat transfer in the wall-jet region, but at greater expense of pressure drop inside the channel. At the jet-to-target spacing ratio of 1, the flow coefficient of the rib-roughened channel is decreased 5%–10% in related to the smooth channel.

Influence of Rib Height on Heat Transfer Augmentation – Application to Aircraft Turbines

This paper presents the influence of rib height on heat transfer and friction factor augmentation in convergent rectangular channel with bottom rib-roughened walls has been conducted in experimentally. In this experiment the square sized ribbs at three different height were (3, 6 and 9 mm) used. A specified constant heat-flux is applied to the bottom surface of the test section and the ribs are arranged in the form of a staggered array. Measurements were taken of a convergent rectangular channel with an inlet aspect ratio of 1.25 and an exit aspect ratio of 1.35. This yields a rib height (e) to mean hydraulic diameter of the duct (Dm) ratio of 0.0348, 0.0697 and 0.1046 respectively, with a fixed rib pitch (p) to test the section inlet height (hin) ratio of 0.6 along the sloped surface of the test section. The Reynolds number based on the mean hydraulic diameter (Dm) of the channel was kept in a range of 20,000 to 60,000. The results from the ribbed channel are compared with that of the smooth convergent rectangular channel, to estimate the heat transfer enhancements from the ribbed channel. The comparison shows that among the tested convergent smooth channel, 3, 6 and 9 mm ribbed channel; the heat transfer from the channel with 6 mm rib height is better and showed comparatively lower friction loss than the other ribbed channels.

Experimental investigation of impinging jet array heat transfer from a surface with V-shaped and convergent-divergent ribs

Detailed heat transfer measurements over a surface with V-shaped ribs (V-SR) and convergent-divergent shaped ribs (CD-SR) by a circular impinging jet array was investigated using thermal infrared camera. Both V-SR and CD-SR configurations with an angle of 45° are considered. In-line jet arrays with different exit flow orientations were also considered. The range of parameters for the analysis has been decided on the basis of practical considerations of the system and operating conditions. The effects of different rib heights on the impinging heat transfer along the wall are studied. Five different ribbed surfaces with different rib height and shapes were selected. The rib pitch (p) to rib height (e) ratio is 6. During the experiments, the Reynolds number was varied from 2000 to 10,000, the jet diameter-to-rib height ratio from 0.6 to 1.2, and jet-to-plate spacing from 2 to 12. The heat transfer results of the rib-roughened plate are compared with those of a smooth plate. The heat transfer from the rib-roughened wall may be enhanced or retarded. Best heat transfer performance was obtained with the V-SR arrangements. The presence of rib turbulators on the target plate produces higher heat transfer coefficients than the smooth plate. The average Nusselt number values for the V-SR plate showed an increase ranging from 4% to 26.6% over those for the smooth plate. Correlations have been developed for the average Nusselt number for the rib-roughened surfaces.

Heat transfer characteristics in steam-cooled rectangular channels with two opposite rib-roughened walls

An experimental study of heat transfer characteristics in steam-cooled rectangular channels with two opposite rib-roughened walls for Reynolds number (Re) in the range of 10,000–80,000 was conducted. To simulate the actual geometry and heat transfer structure of turbine blade/vane internal cooling passage, each of the test channels was made by welding four stainless steel plates. The pitch-to-rib height ratio p/e was kept at 10 and the channel length L was kept at 1000 mm. The channel aspect ratios (W/H) were 0.25, 0.5 and 1, respectively. The channel blockage ratios were 0.047 for W/H = 1, 0.5 and 0.078 for W/H = 0.25. We have found that the average Nusselt number (Nu) for the channel with α = 45° is about 15–25 percent higher than that for the channel with α = 60°. For the channels with W/H = 0.5 or 1, the average Nu decreases along the rib axes because of the rib-induced secondary flow that moves from the left-hand side to the right-hand side. In addition, based on the heat transfer results, we have developed the semi-empirical correlations for the two test channels with the highest and the lowest heat transfer. The correlations can be used in the design of the internal cooling passage of new-generation steam-cooled gas turbine blade/vane.

Flow field investigation in rotating rib-roughened channel by means of particle image velocimetry

The turbulent velocity field over the rib-roughened wall of an orthogonally rotating channel is investigated by means of two-dimensional particle image velocimetry (PIV). The flow direction is outward, with a bulk Reynolds number of 1.5 × 104 and a rotation number ranging from 0.3 to 0.38. The measurements are obtained along the wall-normal/streamwise plane at mid-span. The PIV system rotates with the channel, allowing to measure directly the relative flow velocity with high spatial resolution. Coriolis forces affect the stability of the boundary layer and free shear layer. Due to the different levels of shear layer entrainment, the reattachment point is moved downstream (upstream) under stabilizing (destabilizing) rotation, with respect to the stationary case. Further increase in rotation number pushes further the reattachment point in stabilizing rotation, but does not change the recirculation length in destabilizing rotation. Turbulent activity is inhibited along the leading wall, both in the boundary layer and in the separated shear layer; the opposite is true along the trailing wall. Coriolis forces affect indirectly the production of turbulent kinetic energy via the Reynolds shear stresses and the mean shear. Two-point correlation is used to characterize the coherent motion of the separated shear layer. Destabilizing rotation is found to promote large-scale coherent motions and accordingly leads to larger integral length scales; on the other hand, the spanwise vortices created in the separating shear layer downstream of the rib are less organized and tend to be disrupted by the three-dimensional turbulence promoted by the rotation. The latter observation is consistent with the distributions of span-wise vortices detected in instantaneous flow realizations.

Aerodynamic investigation of a rotating rib-roughened channel by time-resolved particle image velocimetry

Particle image velocimetry (PIV) is used to study the turbulent flow over the rib-roughened wall of a cooling channel model in rotation. The aspect ratio is 0.9, the blockage ratio is 0.1 and the rib pitch-to-height ratio is 10. The flow direction is outward, with a Reynolds number of 1.5 × 104 and a rotation number of 0.3 in both rotational directions. The PIV system rotates with the channel, allowing to directly measure the relative flow velocity with high spatial and temporal resolution. Coriolis forces affect the stability of the shear layers: cyclonic (anticyclonic) rotation inhibits (enhances) the turbulent motion, influencing velocity, vorticity, and turbulence intensity of the flow. The time-resolved measurements show the effect of rotation on the shear layer instability and on the consequent formation of spanwise vortices, as well as on the time trace of the reattachment point. Turbulent energy spectra suggest that all temporal scales are affected by rotation.

Experimental Investigation of Conjugate Heat Transfer in a Rib-Roughened Trailing Edge Channel With Crossing Jets

The present contribution is devoted to the experimental study of the conjugate heat transfer in a turbine blade cooling cavity located near the trailing edge. The cooling scheme is characterized by a trapezoidal cross-section, one rib-roughened wall, and slots along two opposite walls. The Reynolds number, defined at the inlet of the test section, is set at 67,500 for all the experiments. The values of all the important nondimensional parameters characterizing the experiment, including the solid-to-fluid conductivity ratio, are engine-representative. Uniform heat flux is imposed along the outer side of the rib-roughened wall. The measurements are performed using three different ribbed walls, with thermal conductivities ranging from 1 W m−1 K−1 to 18 W m−1 K−1. Highly resolved distributions of nondimensional temperature and Nusselt number over the rib-roughened wall are obtained by means of infrared thermography and finite element analysis. The impact of the conduction through the wall on the thermal performance is demonstrated by comparison with purely convective results, previously published by the authors on the same configuration.

Aerothermal Investigation of a Rib-Roughened Trailing Edge Channel With Crossing Jets—Part II: Heat Transfer Analysis

The present contribution addresses the aerothermal experimental and computational study of a trapezoidal cross-section model simulating a trailing edge cooling cavity with one rib-roughened wall and slots along two opposite walls. Highly resolved heat transfer distributions for the geometry with and without ribs are achieved using a steady state liquid crystals method in part II of this paper. The reference Reynolds number defined at the entrance of the test section is set at 67,500 for all the experiments. Comparisons are made with the flow field visualizations presented in part I of this paper. The results show the dramatic impact of the flow structures on the local and global heat transfer coefficients along the cavity walls. Of particular importance is the jet deflected by the rib-roughened wall and impinging on the opposite smooth wall. The experimental results are compared with the numerical predictions obtained using the finite volume, Reynolds-Averaged Navier–Stokes solver Calcul d'Écoulements Diphasiques Réactifs pour l'Énergétique (CEDRE).

Numerical analysis of turbulent flow in a rotating U-bend with roughened walls

A numerical analysis has been performed for a developing turbulent flow in a rotating U-bend of strong curvature with rib-roughened walls using an anisotropic turbulent model. In this calculation, an algebraic Reynolds stress model is used to precisely predict Reynolds stresses, and a boundary-fitted coordinate system is introduced as a method of coordinate transformation to set the exact boundary conditions along the complicated shape of U-bend with rib-roughened walls. Calculated results for mean velocity and Reynolds stresses are compared to the experimental data in order to validate the proposed numerical method and the algebraic Reynolds stress model. Although agreement is certainly not perfect in all details, the present method can predict characteristic velocity profiles and reproduce the separated flow generated near the outer wall, which is located just downstream of the curved duct. The Reynolds stresses predicted by the proposed turbulent model agree well with the experimental data, except in regions of flow separation.

Aerothermal Investigation of a Rib-Roughened Trailing Edge Channel With Crossing-Jets—Part I: Flow Field Analysis

The present contribution addresses the aerothermal, experimental, and computational studies of a trapezoidal cross-sectional model simulating a trailing edge cooling cavity with one rib-roughened wall. The flow is fed through tilted slots on one side wall and exits through straight slots on the opposite side wall. The flow field aerodynamics is investigated in Part I of the paper. The reference Reynolds number is defined at the entrance of the test section and set at 67,500 for all the experiments. A qualitative flow model is deduced from surface-streamline flow visualizations. Two-dimensional particle image velocimetry measurements are performed in several planes around midspan of the channel and recombined to visualize and quantify three-dimensional flow features. The crossing-jets issued from the tilted slots are characterized and the jet-rib interaction is analyzed. Attention is drawn to the motion of the flow deflected by the rib-roughened wall and impinging on the opposite smooth wall. The experimental results are compared with the numerical predictions obtained from the finite volume Reynolds-averaged Navier–Stokes solver, CEDRE.

Vertical diffusion of pollution from line source near a wall

In this paper, the vertical diffusion was measured from line source injector under neutral boundary condition over rib-roughened wall in a water channel. The variation in gap distance between the rib-roughened wall characterized skimming flow (ribs closely spaced), wake interference flow and isolated roughness flow (ribs widely spaced). The effect of change in local bed geometry was noticed with the increase in turbulence energy production, which was correlated with the vertical diffusion.

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.

Influence of duct aspect ratio on heat/mass transfer in coolant passages with rotation

The combined effects of duct aspect ratio and rotation on heat/mass transfer characteristics are investigated. Mass transfer experiments are conducted to obtain detailed local heat/mass transfer coefficients on the leading and trailing surfaces in rotating two-pass ducts with a pair of opposite rib-roughened walls. The ducts of three different aspect ratios ( W/ H = 0.5, 1.0, and 2.0) are employed with a fixed hydraulic diameter ( D h) of 26.7 mm. In all duct cases, the rib height-to-hydraulic diameter ratio ( e/ D h) is 0.056 and the rib pitch-to-rib height ( p/ e) is 10. The rotation number ranges from 0.0 to 0.20 while the Reynolds number is fixed at 10,000. To verify the heat/mass transfer augmentation, internal flow structures are calculated for a smooth two-pass square duct using numerical simulations. The results show that Sherwood number ratios are approximately 2.5 times higher than the fully developed value in a stationary smooth pipe due to the flow reattachment near ribbed surfaces. The overall heat/mass transfer coefficient increases as the duct aspect ratio increases. It is because the core flow is highly disturbed and accelerated in the midsections of the ribs, as the rib height-to-duct height ratio ( e/ H) increases. Dean vortices generated due to 180°-turn augment heat/mass transfer in the turning region and in the upstream region of the second pass. The rotation of duct produces heat/mass transfer discrepancy between leading and trailing surfaces, having higher Sherwood number on the trailing surface in the first pass and on the leading surface in the second pass. However, the effects of duct turning curvature and rotation on heat/mass transfer become less significant for the higher aspect ratio.