Flow In Ribbed Channel Research Articles

Large eddy simulation of turbulent transport and thermal enhancement in a steam-cooled ribbed channel

Pulsating flow, as an active heat transfer enhancement technique, is expected to further improve thermal performance. To demonstrate the potential value of pulsating flow in blade cooling, a numerical investigation is conducted on the flow and heat transfer in a steam-cooled ribbed channel under steady and pulsating inflow at Re = 12000. LES, based on OpenFOAM-8, is employed and precursor simulation method is used to generate fully developed turbulent inflow. Three operating conditions, steady inflow (StF = 0), low frequency pulsation (StF = 0.02) and medium frequency pulsation (StF = 0.08), are studied. The features under steady inflow condition (StF = 0) are firstly discussed, indicating that, due to the entry effect, the ribbed channel can be divided into two regions with different vortex motion behaviors. The ejections induced by vortex evolution enhance turbulent and energy transport between the sidewall and mainstream. Two cases of pulsating inflow are then carried out, finding that stronger ejections appear and low heat transfer area is significantly improved. Corresponding to low frequency pulsation (StF = 0.02) and medium frequency pulsation (StF = 0.08), the Nusselt number has an increase of 3.511 % and 14.242 %, respectively, and friction factor has a rise of 3.934 % and 8.253 %, compared with steady inflow. The thermal performance factor is increased by 11.262 % at StF = 0.08, indicating better cooling performance than the rise of 2.188 % at StF = 0.02. It is suggested that pulsating flow with medium frequency pulsation has positive performance in a steam-cooled ribbed channel.

Numerical analysis and optimization on flow and heat transfer performance of a steam-cooled ribbed channel

It aims to enhance the heat transfer performance and reduce the pressure loss of the steam-cooled ribbed channels in the gas turbine blades. Numerical calculations were performed to analyze the flow and heat transfer characteristics in the steam-cooled ribbed channel for different inlet Reynolds number (10,000 ≦ Re ≦ 90,000), rib angle (30° ≦ α ≦ 90°), rib height ratio (0.05 ≦ e/D ≦ 0.15) and rib pitch ratio (0.5 ≦ P/D ≦ 1.5). The flow and heat transfer analysis, response surface analysis, significance analysis and parameter optimization were performed by using the Response Surface Methodology. The results show that the variations of Re, α, e/D and P/D change the size, shape and swirling strength of the secondary flows in different extent, leading to the change of flow and heat transfer characteristics in the steam-cooled ribbed channel. Increasing Re and e/D, and decreasing P/D at the low-medium level of e/D obviously enhance the Nu/Nu0; Decreasing Re and e/D, and increasing P/D result in a reduction in the f/f0. An increment in α first increases and then decreases both the Nu/Nu0 and f/f0. Among the four influence parameters Re, α, e/D and P/D, Re is the most significant parameter affecting the Nu/Nu0, followed by e/D, α and P/D; e/D has the most significant influence on the f/f0, followed by α, Re and P/D. The multi-objective optimization for lower pressure drop and higher heat transfer is achieved at Re of 90,000, α of 41.5152, e/D of 0.1157 and P/D of 0.9747. Compared with the reference channel, the optimized channel has a 86.19% enhancement in heat transfer coefficient and a 38.32% improvement in comprehensive thermal coefficient, at the same Reynolds number (Re = 90,000).

A pressurized ammonia-fed planar anode-supported solid oxide fuel cell at 1–5 atm and 750–850°C and its loaded short stability test

Ammonia is a useful energy carrier for solid oxide fuel cell (SOFC) with advantages over hydrogen. Understanding of the performance and stability of ammonia-fed SOFC operated at elevated pressure (p) is an important step towards the development of high-efficiency hybrid SOFC power system with micro gas turbine (MGT). This paper reports cell performance, electrochemical impedance spectra (EIS), and stability measurements of a pressurized ammonia-fed anode-supported SOFC at p = 1–5 atm and T = 750–850°C using a planar (50 × 50 mm2) single-full-cell (400 μm Ni-YSZ anode/3 μm YSZ electrolyte/12 μm LSC-GDC cathode). The full cell together with metallic frames and current collectors are sandwiched by a pair of rib-channel flow distributors (interconnectors) in a high-pressure testing facility. Results show that pressurization and increasing temperature enhance the ammonia-fed SOFC performance significantly having almost the same power densities as those of hydrogen/nitrogen-fed SOFC, as substantiated and explained by EIS data and an equivalence circuit model where the effects of p and T on ohmic, gas diffusion, and gas conversion impedances are shown. Moreover, the loaded short (10 h) stability tests at 700°C and 0.8 V for 1 atm/3 atm cases reveal no/little power degradation, where the microstructures without any crack within the scale of SEM observation and nearly the same element atomic percentages of Ni-YSZ anode surfaces from EDX spectra are found. These results suggest that the pressurized ammonia-fed SOFC is a promising candidate for the hybrid SOFC-MGT power generation.

Experimental and Numerical Studies of Two-Phase Flow in Ribbed Periodic Vertical Channel

Pressure drop in a ribbed vertical channel was studied for a two-phase (air and water) flow, different shapes of ribs (triangle, semi-circular, and rectangle) were used in this investigation. An experimental rig was constructed with a test channel having the dimensions (10×3×70 cm) that enabled the flow patterns visual observation. An experimental setup of a two-phase was done for the present investigation, as well as a sum of (150) two-phase data with various flow patterns was determined. The used water superficial inlet velocities were in the range (0.0333 - 0.0888 m/s), and the air superficial inlet velocities were in the range (0.0555 - 0.1666 m/s). The experimental pressure drop according to a smooth plate was (41%, 38%, and 35%) for a channel fitted with semi-circle, rectangular and triangular ribs, respectively. The less pressure drop recorded was in the triangular ribbed channel, which indicates that the triangular shaped ribbed channel is the finest shape among the studied ones to lower the pressure drop. As future work, the numbers and dimensions of ribs can be changed to investigate its effect on the two-phase flow and the pressure drop, another suggestion for the future investigations is to include other fluid to have a three-phase flow, also the inclination angle of the duct can be changed to study its effect.

Improved Eddy-Viscosity Modelling of Turbulent Flow around Porous–Fluid Interface Regions

The RANS modelling of turbulence across fluid-porous interface regions within ribbed channels has been investigated by applying double (both volume and Reynolds) averaging to the Navier–Stokes equations. In this study, turbulence is represented by using the Launder and Sharma (Lett Heat Mass Transf 1:131–137, 1974) low-Reynolds-number $$k-\varepsilon $$ turbulence model, modified via proposals by either Nakayama and Kuwahara (J Fluids Eng 130:101205, 2008) or Pedras and de Lemos (Int Commun Heat Mass Transf 27:211–220, 2000), for extra source terms in turbulent transport equations to account for the porous structure. One important region of the flow, for modelling purposes, is the interface region between the porous medium and clear fluid regions. Here, corrections have been proposed to the above porous drag/source terms in the k and $$\varepsilon $$ transport equations that are designed to account for the effective increase in porosity across a thin near-interface region of the porous medium, and which bring about significant improvements in predictive accuracy. These terms are based on proposals put forward by Kuwata and Suga (Int J Heat Fluid Flow 43:35–51, 2013), for second-moment closures. Two types of porous channel flows have been considered. The first case is a fully developed turbulent porous channel flow, where the results are compared with DNS predictions obtained by Breugem et al. (J Fluid Mech 562:35–72, 2006) and experimental data produced by Suga et al. (Int J Heat Fluid Flow 31:974–984, 2010). The second case is a turbulent solid/porous rib channel flow to examine the behaviour of flow through and around the solid/porous rib, which is validated against experimental work carried out by Suga et al. (Flow Turbul Combust 91:19–40, 2013). Cases are simulated covering a range of porous properties, such as permeability and porosity. Through the comparisons with the available data, it is demonstrated that the extended model proposed here shows generally satisfactory accuracy, except for some predictive weaknesses in regions of either impingement or adverse pressure gradients, associated with the underlying eddy-viscosity turbulence model formulation.

Pressurized Ammonia and Syngas Planar Anode-Supported Solid Oxide Fuel Cells and Their Performance Stability Test

This paper reports a stability test comparison between ammonia- and syngas-fueled anode-supported cells (ASC). The planar (50 x 50 mm2) ASC is consisted of 400-mm-Ni-YSZ anode, 3-mm-YSZ electrolyte, and 12-mm-LSC-GDC cathode together with metallic frames and current collectors which are sandwiched by a pair of rib-channel flow distributors (interconnectors) with guide vanes on both anode and cathode sides to form a single-full-cell stack in a dual-chamber pressure/temperature-controlled facility. Pressurization enhances the power density noticeably for both ammonia- and syngas-fueled ASCs. Little degradation is observed for the pressurized ammonia-fueled ASC at 0.8 V at 700°C (power density and stability increase with increasing temperature). However, the syngas-fueled ASC can only remain stable at 1 atm and at higher temperature (≥ 800°C); after five hours stable operation, a rapid degradation is observed at 3 atm due to severe carbon deposition even at 850°C.

ENHANCEMENT THE HEAT TRANSFER FOR TWO-PHASE FLOW THROUGH A RECTANGULAR RIBBED VERTICAL CHANNEL

The heat transfer coefficient and temperature distribution of two phase flow (water, air)in rectangular ribbed vertical channel was investigated experimentally and numerically inthis work for different values of water and air superficial velocities (0.0421, 0.0842, 0.1158,0.1474 and 0.1684 m/s) and (1.0964, 1.425, 1.644, 1.864 and 2.193 m/s), respectively, atconstant heat flux (120 W). The distribution of temperature along the channel wasphotographed using thermal camera and compared with images for the correspondingcontours which found numerically. The experimental results of heat transfer coefficientcompared with computational fluid dynamics model simulated by Ansys fluent 15.0. Agood agreement has been found between the experimental and numerical data, where thepercentage deviation between the experimental and the numerical results is (1% - 6% ). Theresults showed that, the local heat transfer coefficient increased by adding ribs, it alsoincreased as the velocity of the flow increased.

ENHANCEMENT THE HEAT TRANSFER FOR TWO-PHASE FLOW THROUGH A RECTANGULAR RIBBED VERTICAL CHANNEL

The heat transfer coefficient and temperature distribution of two phase flow (water, air)in rectangular ribbed vertical channel was investigated experimentally and numerically inthis work for different values of water and air superficial velocities (0.0421, 0.0842, 0.1158,0.1474 and 0.1684 m/s) and (1.0964, 1.425, 1.644, 1.864 and 2.193 m/s), respectively, atconstant heat flux (120 W). The distribution of temperature along the channel wasphotographed using thermal camera and compared with images for the correspondingcontours which found numerically. The experimental results of heat transfer coefficientcompared with computational fluid dynamics model simulated by Ansys fluent 15.0. Agood agreement has been found between the experimental and numerical data, where thepercentage deviation between the experimental and the numerical results is (1% - 6% ). Theresults showed that, the local heat transfer coefficient increased by adding ribs, it alsoincreased as the velocity of the flow increased.

Effects of rib angle and rib orientation on flow and heat transfer in two-pass ribbed channels

The secondary flows generated by the rib and the U-shaped bend play a vital role in heat transfer enhancement in two-pass ribbed channels. This paper focuses on the combined effects of rib orientations (N-type and P-type) and rib angles (from 30o to 75o) on flow and heat transfer in two-pass ribbed channels. The 3D steady Reynolds-averaged Navier–Stokes equations coupled with SST k-ω model are solved by using commercial software ANSYS CFX. The CFD model has been validated by experimental data with a good agreement. In addition, the comprehensive evolution of secondary flows and its effect on heat transfer are analyzed in detail by using vortex core technology. The results indicate that the key factor for local heat transfer enhancement is to transport more secondary flows into downstream passage without weakening local secondary flow and energy dissipation of upstream secondary flow. Furthermore, the rib orientation in passage 2 should consider the influence of rib angle so that the local main flow secondary flow has the same rotational direction with the most powerful upstream secondary flow to reduce the energy loss and speed up the growth of local main flow secondary flow as much as possible.

Numerical simulations on flow and heat transfer in ribbed two-pass square channels under rotational effects

The main objective of this research is to study the flow and heat transfer characteristics in a rotating two-pass square channel with ribbed walls. In this study, the channel length-to-hydraulic diameter ratio of the rotating two-pass square channel (L/Dh), the rib height-to-hydraulic diameter ratio (e/Dh), rib angle of attack (α) and the rib pitch-to-height (p/e) ratio are fixed at 11.33, 0.13, 60° and 10, respectively. The test fluid is air having the flow rate in terms of constant Reynolds number (Re) of 10,000. The rotation numbers (Ro) are varied from 0.1 to 0.4. The details of the local heat transfer distribution and the flow field of the rotating two-pass square channel are numerically studied by using commercial software ANSYS Fluent (ver.15.0). The results show that the ribbed walls enhance the heat transfer rate significantly. Under rotation, the average Nu in the first pass with radial outward flow is increased while that in the second pass is decreased, and also found that maximum heat transfer rate is observed for rotation number of 0.4 which is higher about 10-20% when compared with the other rotation number cases.

A dynamic delayed detached-eddy simulation model for turbulent flows

Abstract The current study developed a new dynamic delayed detached-eddy simulation (dynamic DDES) model based on the k-ω SST model and the well-established dynamic k -equation subgrid-scale model. Instead of using a constant model coefficient C DES in traditional DES formulations, the present model employs two coefficients C k and C e , which are computed dynamically by taking into account the spatial and temporal variations of the flow field at the grid and test filter levels. A modification on shielding function f d is proposed, with a spatial uniformization operator imposed on the velocity gradient to obtain a smooth and monotonous hybrid interface. A damping function φ d is introduced based on the local grid resolution and flow condition to damp the Reynolds-averaged Navier–Stokes (RANS) region and achieve wall-modeled LES (WMLES) mode dynamically. The test of the model in developed channel flow shows the log-layer mismatch (LLM) problem is significantly improved with respect to the dynamic LES model and original DDES model. The use of the spatial uniformization operator and the damping function convincingly demonstrates the improvement in prediction of separated flows, with the model coefficients dynamically computed. The LES region is maximized at the limit of grid resolution and more turbulent vortical structures are resolved. The test in the ribbed channel flow shows the present model has considerably better performance in prediction of the mean and turbulence velocity in the strong shear layer and the recirculation bubble. In addition, the simulation of impinging jet shows the model exhibits rapid switching from the RANS to LES under the flow instabilities when the inflow does not include turbulence content.

Numerical investigation of turbulent flow and heat transfer in two-pass ribbed channels

In this study, the heat transfer and friction characteristics of four different rib geometries- 45° angled, V-shaped, W-shaped and M-shaped ribs in a two-pass stationary channel have been numerically investigated. The aspect ratio (Height to Width) of the cooling channel was 1:1 (square). The rib pitch-to-rib height ratio (p/e) and the rib-height-to-channel hydraulic diameter ratio (e/Dh) were 16 and 0.125 respectively. The Reynolds number was varied from 20,000 to 70,000. For the computations, the Reynolds averaged Navier–Stokes (RANS) equations were solved with the commercial software ANSYS Fluent using the realizable version of k-ε (RKE) model. The heat transfer results were benchmarked with experiments on a test rig with similar geometries and flow conditions. Detailed analysis of the flow characteristics in the two-pass channel was carried out so as to understand the interaction of the rib-induced secondary flows and the bend-induced secondary flows and their contribution to heat transfer enhancement. The heat transfer enhancement provided by V-shaped ribs was 7% higher than 45° ribs, 28% higher than W-shaped ribs and 35% higher than M-shaped ribs. However, the pressure penalty for V-shaped ribs was 19% higher than 45° ribs, 24% higher than W-shaped ribs and 28% higher than M-shaped ribs. On comparing the overall thermal hydraulic performance, V-shaped and 45° ribs were observed to perform significantly better than W-shaped and M-shaped ribs.

Effects of pressurization and temperature on power generating characteristics and impedances of anode-supported and electrolyte-supported planar solid oxide fuel cells

Power generating characteristics of pressurized anode-supported cell (ASC) and electrolyte-supported cell (ESC) are measured using the same single-cell stack setup, a planar full cell sandwiched by a pair of rib-channel flow distributors. Both ASC and ESC apply the same flow rates (Qanode = 0.5 slpm H2 + 0.4 slpm N2 and Qcathode = 0.9 slpm air) measured at three operating temperatures (T = 750 °C, 800 °C, 850 °C), each T under five pressures (p = 1, 2, 3, 4, 5 atm), having a total of 30 data sets for comparison. It is found that under loaded conditions, ASC has much larger increments in power densities, about three folds higher, than ESC due to pressurization. As T increases from 750 °C to 850 °C at 0.7 V, power densities of ASC/ESC increase from 175/97 mW cm−2 to 309/193 mW cm−2 at p = 1 atm, while at p = 5 atm, the increases of power densities are 281/137 mW cm−2 to 476/250 mW cm−2, showing a stronger temperature dependence than pressurization. Corresponding electrochemical impedance spectra show that the better cell performance of ASC is attributed to both lower ohmic and polarization resistances found in ASC than that in ESC.

The Impact of Pressurization on Anode-Supported and Electrolyte-Supported Planar Solid Oxide Fuel Cells at 750oC ~ 850oC

How to reduce effectively the severe impact of various environmental and energy problems is one of the most important issues in our human society. This continues to drive the development of hybrid power generation systems aiming to achieve much higher efficiencies together with much lower emissions than conventional power systems. Among only a few available hybrid energy conversion systems, a pressurized solid oxide fuel cells (PSOFC) integrating with a gas turbine (GT) or micro gas turbine (MGT), the hybrid PSOFC-GT or -MGT power system, has the highest efficiency up to 70%. Moreover, the development of such hybrid PSOFC-GT or –MGT power system is not only beneficial for the environment, but also it is important to the consolidation of the fuel cell industry and the expansion of the gas turbine industry. Although several demos of the hybrid PSOFC-MGT power system have been established, many technical challenges existing in the design of such hybrid power system and its control still need to be solved in order to assure a stable operation among different components of the hybrid system. One of the most important technical challenges is the detail experimental information on electrochemical impedance spectra (EIS) of PSOFC, which is still rare in literatures. This motivates us to measure experimentally the effect of pressurization on EIS of PSOFC. In this work, we measure the pressurization influence on power generating characteristics and EIS for both anode-supported and electrolyte-supported cells (ASC and ESC) using a recently-established high-pressure SOFC facility. It is worth of noting that the present study has an important advantage due to the fact that both ASC and ESC use the same single-cell setup, a planar full cell having 40 × 40 mm2 effective reactive area sandwiched by a pair of rib-channel flow distributors (interconnectors) on both anode and cathode, under exactly the same experimental conditions. Specifically, both ASC and ESC apply exactly the same flow rates (Q anode = 0.5 slpm H2 + 0.4 slpm N2 and Q cathode = 0.9 slpm air) at three operating temperatures (T = 750 oC, 800 oC, 850oC), each T under five different pressures (p = 1, 2, 3, 4, 5 atm), having a total of 30 data sets for comparison. Hence, a direct comparison of experimental EIS data and power generating characteristics between ASC and ESC at elevated pressures can be clearly made. To the best knowledge of the authors, the present measured EIS comparison between ASC and ESC is new, and experimental EIS data of pressurized electrolyte-supported planar SOFC are still not available in literatures. These ASC and ESC measurements under the same experimental conditions suggest the following points: (1) At T = 850oC and 0.7 V, power densities of ASC/ESC increase from 309/193 mW cm-2 to 476/250 mW cm-2, as p increases from 1 atm to 5 atm. As T increases from 750oC to 850oC at 0.7 V, power densities of ASC/ESC increase from 175/97 mW cm-2 to 309/193 mW cm-2 at p = 1 atm, while at p = 5 atm, the corresponding increases of power densities are 281/137 mW cm-2 to 476/250 mW cm-2. It is found that ASC is more sensitive to pressurization than ESC, while the latter is more temperature-sensitive than the former. (2) The ohmic resistance is not varied with the loaded current density and it is independent of pressure for both ASC and ESC cases. Furthermore, ESC has higher values of the ohmic impedance, about 16 % larger, than that of ASC. The thicker the electrolyte thickness is, the larger the ohmic impedance is. (3) The polarization impedance decreases noticeably with increasing p for both ASC and ESC cases at either unloaded or loaded conditions. This explains why power densities increase with increasing pfor both ASC and ESC. (4) These measurements reveal that the better cell performance of ASC than that of ESC is not simply due to the larger ohmic overvoltage of ESC as previously predicted by numerical simulations. The variation of the polarization overvoltage between ASC and ESC also plays an important role. We hasten to note that it is not sure whether the present conclusions can be extended to all ASCs and ESCs due to different electrolytes and anodes used in the present ASC and ESC, but these results should be of help in the further development of PSOFC integrating with micro gas turbines for future stationary power generation. Figure 1

The Impact of Pressurization on Anode-Supported and Electrolyte-Supported Planar Solid Oxide Fuel Cells at 750oC ~ 850oC

The impact of pressurization on anode and electrolyte-supported cells (ASC/ESC) is measured in a high-pressure SOFC test facility using the same single-cell stack setup, a planar fuel cell with rib-channel flow distributors, under the same experimental conditions (i.e. same flow rate, temperature, pressure). Results show that at T= 850°C and 0.7 V, power densities of ASC/ESC increase from 309/193 mW cm-2 to 476/250 mW cm-2, as p increases from 1 atm to 5 atm. As T increases from 750°C to 850°C at 0.7 V, power densities of ASC/ESC increase from 175/97 mW cm-2 to 309/193mW cm-2 at p = 1 atm, while at p = 5 atm, the corresponding increases of power densities are 281/137 mW cm-2 to 476/250 mWcm-2. It is found that ASC is more sensitive to pressurization than ESC, while the latter is more temperature-sensitive. Corresponding electrochemical impedance spectra explain these results.

Comparison of Electrochemical Impedance Measurements between Pressurized Anode-Supported and Electrolyte-Supported Planar Solid Oxide Fuel Cells

Pressurization influence on electrochemical impedance spectra (EIS) of both anode-supported and electrolyte-supported cells (ASC and ESC) are measured and compared using a high-pressure SOFC facility. In it both ASC and ESC apply the same single-cell setup, a planar full cell having 40 × 40 mm2 effective reactive area sandwiched by a pair of rib-channel flow distributors (interconnectors) on both anode and cathode, using the same flow rates (Qanode = 0.5 slpm H2 + 0.4 slpm N2 and Qcathode = 0.9 slpm air) at 850°C, with five different pressures (1 ∼ 5 atm). It is found that ASC is more sensitive to pressurization than ESC, of which power densities of ASC/ESC at 0.7 V increase from 309/193 mW cm−2 to 476/250 mW cm−2, as p increases from 1 atm to 5 atm, corresponding to a 54%/30% increase. This result is explained by EIS measured data, in which both ohmic and polarization resistances of ASC are smaller than that of ESC. Moreover, the ohmic resistance is independent of pressurization for both ASC and ESC, remaining constant regardless of either unloaded or loaded conditions, while the polarization resistance decreases noticeably with increasing pressure for both ASC and ESC.

Numerical Calculations on Flow and Heat Transfer in Smooth and Ribbed Two-Pass Square Channels under Rotational Effects

U-shaped channel, which is also called two-pass channel, commonly exists in gas turbine internal coolant passages. Ribbed walls are frequently adopted in internal passage to enhance the heat transfer. Considering the rotational condition of gas turbine blade on operation, the effect of rotation is also investigated for the coolant channel which is close to real operation condition. Thus, the objective of this study is to discuss the effect of rotation on fluid flow and heat transfer performance of U-shaped channel with ribbed walls under high rotational numbers. Investigated Reynolds number is Re=12500and the rotation numbers areRob=0.4and 0.6. In the results, the spatially heat transfer coefficient distributions are exhibited to discuss the effect of rotation and roughened walls. It is found that ribbed walls enhance the heat transfer rate significantly. Under the rotational condition, theNuin the first pass with outward flow is increased while that in the second pass is decreased. Finally, averageNuratio, friction ratio, and thermal performance are all presented to discuss the thermal characteristics.

Numerical Study of Turbulent Periodic Flow and Heat Transfer in a Square Channel with Different Ribs

A numerical investigation has been carried out to examine turbulent flow and heat transfer characteristics in a three-dimensional ribbed square channels. Fluent 6.3 CFD code has been used. The governing equations are discretized by the second order upwind differencing scheme, decoupling with the SIMPLE (semi-implicit method for pressure linked equations) algorithm and are solved using a finite volume approach. The fluid flow and heat transfer characteristics are presented for the Reynolds numbers based on the channel hydraulic diameter ranging from 104 to 4 ′ 104. The effects of rib shape and orientation on heat transfer and pressure drop in the channel are investigated for six different rib configurations. Rib arrays of 45° inclined and 45° V-shaped are mounted in inline and staggered arrangements on the lower and upper walls of the channel. In addition, the performance of these ribs is also compared with the 90° transverse ribs.

A dynamic global subgrid-scale model for large eddy simulation of scalar transport in complex turbulent flows

In the present study, a dynamic global subgrid-scale eddy diffusivity model for large eddy simulation of scalar transport in complex turbulent flows is proposed by generalizing the dynamic global subgrid-scale eddy viscosity model [Phys. Fluids, 18 125109 (2006) and Phys. Fluids, 22 075106 (2010)]. The global model coefficient in the subgrid-scale model is determined by a dynamic procedure based on the Germano identity such that the model coefficient is globally constant in space but varies only in time. Large eddy simulations of passive scalar transport in turbulent channel flow, turbulent boundary layer, and turbulent ribbed channel flow are conducted and the proposed model shows an excellent performance for all the flows considered. Since the proposed model requires no ad hoc clipping and/or averaging over a homogeneous direction, it can be readily applied to large eddy simulation of scalar transport in complex turbulent flows.

The impact of flow distributors on the performance of solid oxide fuel cell—Part II: Electrochemical impedance measurements

This paper presents newly measured electrochemical impedance spectra of two sets of nearly identical single-cell stacks except using different designs of flow distributors for planar solid oxide fuel cells (SOFCs). It is found that by using small guide vanes around the feed header of commonly used rib-channel flow distributors to improve significantly the degree of flow uniformity, values of ohmic and polarization resistances of the single-cell stack can be decreased by 32% and 60% as compared to that without using guide vanes under the same experimental conditions at 0.6 V and 850 °C. This finding explains why by improving flow uniformity in interconnects can result in more than 10% increase of the cell power density. Moreover, the higher resistances measured for the case without using guide vanes are attributed to the anodic re-oxidation behavior due to non-uniform flow distributions in interconnects, as also verified by SEM images of anodic microstructures. These results show that the improvement of flow uniformity in flow distributors is useful to increase the performance and the longevity of planar SOFCs.