Two-layer Zonal Model Research Articles

The effect of internal plate vibration on the rate of heat transfer in a glass recycling process dryer

This study concerns with the effect of transverse vibration of an internal plate on mixed convection heat transfer inside a square enclosure of a glass recycling process dryer. The plate can oscillate in horizontal and vertical directions. The Reynolds-averaged Navier-Stokes equations are used to describe the unsteady turbulent flow field. The governing equations are solved numerically applying a finite-volume approach. OpenMP Parallel SIMPLE algorithm is employed with power-low scheme for the convective terms. Also, the turbulent flow properties are estimated by a two-layer zonal model based on k−εmodel and Wolfstein near wall treatment. Dimensionless effective parameters including Reynolds number (Re), Grashof number (Gr) and vibration angular frequency (ω) are varied to investigate inherent flow structures and the heat transfer rate. It was deduced from the results that at high Gr and low Re numbers, the effect of internal plate vibrations is negligible and the fluid flow is induced only by the buoyancy force. As Re and ω are increased, the effect of internal plate vibration becomes dominant. At high Re and ω values while Gr kept low, benchmark cavity heat transfer is increased up to 90 and 80 times due to the vibration of internal horizontal and vertical plates, respectively. The obtained results can be beneficial for the dryers of glass recycling industries in order to heat and smash the glass particles without a significant heat loss. Desired condition occurs at high Gr and ω when the flow Re number is comparatively low.

The effects of an internal isolated plate on natural convective heat transfer inside an industrial oven

This paper concerns with the problem of natural convection heat transfer inside an industrial oven with an internal plate. The model is simulated numerically as a square cavity with an isolated plate laid horizontally or vertically inside it. Different ranges of Rayleigh number from laminar to turbulent flow regimes were investigated numerically and inherent flow structures and the amount of transferred heat were obtained. The governing Reynolds-averaged Navier-Stokes and energy equations are discretized and solved applying finite-volume method. Moreover, a two-layer zonal model is applied for near-wall turbulent properties. Comparing the present results with similar previous studies and experimental data shows that the employed zonal model is more accurate than other existing turbulent models. Our results also demonstrate that the total rate of heat transferred by a cavity with internal isolated plate is always lower than that of a bare cavity. As the distance of internal plate from the cavity wall decreases, its effect on the reduction of overall heat transfer increases accordingly. We also determined a distance between the plate and the cavity wall with less than 2% change in relative Nusselt number and deduced that the maximum distance of vertical plate is always greater than that of the horizontal plate at each Rayleigh number. Furthermore, a dead zone is created between the hot wall and the plate with low velocities, the temperatures close to the hot wall temperature, and very weak circulation, which can significantly reduce the overall heat transfer in the cavity. The results of this work can be used in glass industries and reduction of heat loss in industrial ovens by placing the glass sheets inside them in appropriate position.

Numerical Simulations of the Near-Field Region of Film Cooling Jets Under High Free Stream Turbulence: Application of RANS and Hybrid URANS/Large Eddy Simulation Models

This paper investigates the flow field and thermal characteristics in the near-field region of film cooling jets through numerical simulations using Reynolds-averaged Navier–Stokes (RANS) and hybrid unsteady RANS (URANS)/large eddy simulation (LES) models. Detailed simulations of flow and thermal fields of a single-row of film cooling cylindrical holes with 30 deg inline injection on a flat plate are obtained for low (M = 0.5) and high (M = 1.5) blowing ratios under high free stream turbulence (FST) (10%). The realizable k‐ε model is used within the RANS framework and a realizable k‐ε-based detached eddy simulation (DES) is used as a hybrid URANS/LES model. Both models are used together with the two-layer zonal model for near-wall simulations. Steady and time-averaged unsteady film cooling effectiveness obtained using these models are compared with available experimental data. It is shown that hybrid URANS/LES models (DES in the present paper) predict more mixing both in the wall-normal and spanwise directions compared to RANS models, while unsteady asymmetric vortical structures of the flow can also be captured. The turbulent heat flux components predicted by the DES model are higher than those obtained by the RANS simulations, resulting in enhanced turbulent heat transfer between the jet and mainstream, and consequently better predictions of the effectiveness. Nevertheless, there still exist some discrepancies between numerical results and experimental data. Furthermore, the unsteady physics of jet and crossflow interactions and the jet lift-off under high FST is studied using the present DES results.

Numerical Study of Interrupted Impinging Jets for Cooling of Electronics

The objective of this paper is to present the results of a numerical investigation of the effect of flow pulsations on local, time-averaged Nusselt number of an impinging air jet. The problem was considered to provide inputs to augmenting heat transfer from electronic components. The solution is sought through the FLUENT (Version 6.0) platform. The standard k-epsiv model for turbulence equations and two-layer zonal model in wall function are used in the problem. Pressure-velocity coupling is handled using the SIMPLEC algorithm. The model is first validated against some experimental results available in the literature. A parametric study is carried out to quantify the effect of the pulsating jets. The parameters considered are (1) average jet Reynolds number (5130 < Re < 8560), (2) sine and square wave pulsations, (3) frequencies of pulsations (25 < / <400 Hz), and (4) height of impingement to jet diameter ratios (5 < H/d < 9). In the case of sine wave pulsations, the ratio of root mean square value of the amplitude to the average value (AN) was varied from 18% to 53%. The studies are restricted to a constant wall heat flux condition. Parametric conditions for which enhancement in the time-averaged heat transfer from the surface can be expected are identified.

Evaluation of Near-Wall Turbulence Models for Deliberately Roughened Liquid Annular Seals

An extensive numerical evaluation of the performance of two near-wall treatments and two core-flow turbulence models has been done while simulating experimental tests of water flowing between flat plates with deliberately roughened surfaces. These flat-plate tests replicated the results of several annular seal experiments in which friction factor increased as the seal clearance increased. Annular seals are extensively used in a wide range of turbomachinery, and their design influences efficiency and rotordynamic stability. A commercial code, FLUENT, was used to solve the Reynolds-averaged Navier‐Stokes equations with the flat-plate tests as a reference. The performance of the standard wall functions, based on the law of the wall, as near-wall treatment of turbulence was evaluated and compared to the two-layer-zonal approach. Similarly, the solutions obtained with the standard and “renormalization group” method versions of the first-order closure k‐e model were contrasted to those obtained with a second-order closure, the Reynolds-stress model. Although the main features of the friction factor behavior observed in the experiments under study were reproduced with a simple two-dimensional approach, the present work concentrates on the numerical detail and lessons learned while obtaining such results. It was found that wall function solutions are extremely sensitive to the location of the first grid point near the wall, even if it is located within the overlap region. In addition, the low-Reynolds nature of the flow requires coarse meshing making any k‐e model solution grid dependent. On the other hand, the two-layer zonal model is more consistent, and it is not sensitive to the location of the first grid point near the wall, provided it is located at a y + distance no larger than 10. Solutions obtained with the latter approach and the Reynolds-stress model are found to be grid independent. In summary, a numerical solution is truly grid independent when it is consistently replicated upon variation of the discretization scheme, the pressure-to-velocity linking method, the algebraic equation solving algorithm, and the mesh size and type. In low-Reynolds-number turbulent flow, the two-layer zonal approach fulfills such statement as near-wall treatment of turbulence.

Numerical investigation of bend and torus flows, part I : effect of swirl motion on flow structure in U-bend

As a preliminary study to flow modeling in torus reactors, simulations are carried out in well-known standard geometries, namely 90° and 180° bends. For the latter, two flow configurations are considered, with and without initial swirl motion so as to approach torus reactor conditions. Efficiency of the commercial CFD code FLUENT is investigated by comparing predicted results with experimental measurements available in the literature for both bended configurations. Different turbulent models and near-wall considerations are considered, including k– ε and high Reynolds-stresses models, with the standard wall-function approach as well as the two-layer zonal model and low- Re k– ε models. After validation of the numerical strategy, a parametric study is made to better understand the interactions between Dean vortices, involved by the bend curvature, and the main rotating motion generated by the swirl motion. Simulations are achieved for various values of the initial swirl intensity applied at the bend entry. Numerical simulations show different flow structures, resulting from the progressive Dean vortices perturbations with the increase of swirl intensity.

PLATE HEAT EXCHANGERS FOR OCEAN THERMAL ENERGY CONVERSION

Ocean Thermal Energy Conversion (OTEC) utilizes the temperature difference between the warm surface seawater and cold deep ocean water to generate electricity. The heat exchanger in an OTEC power plant is significant as it exchanges a large quantity (nearly 40-50 times of the gross power) of heat energy due to the low thermal efficiency of the power cycle. Nearly 30-40% of the total cost of the plant should be allotted to this single component. For the present, 1-MW OTEC plant plate heat exchangers are used both as an evaporator and a condenser. This paper reports a Computational Fluid Dynamics (CFD) investigation for a dimple plate heat exchanger. The analysis uses the Log Mean Temperature Difference Method (LMTD) in all its calculations. Whilst the shell side flow highly resembles the flow over a rough or wavy plate, the tube side passage in these represents the flow over short hexagonal tube banks with the flow across the sectional areas between the hexagons having the shape of a benzene ring. The CFD code employed is Fluent 5 using the Re-Normalization Group (RNG) turbulence model and the two-layer zonal model for near wall areas on the shell side. For tube side flow, the normal k-ε turbulence model is used with a turbulent Prandtl number of 0.63 suggested for flow in tube banks by the literature.

Modeling of Turbulent Fluid Flow Over a Rough Wall With or Without Suction

The turbulent flow close to a wall with two-dimensional roughness is computed with a two-layer zonal model. For an impermeable wall, the classical logarithmic law compares well with the numerical results if the location of the fictitious wall modeling the surface is considered at the top of the rough boundary. The model developed by Wilcox for smooth walls is modified to account for the surface roughness and gives satisfactory results, especially for the friction coefficient, for the case of boundary layer suction.

Numerical modelling of flow and heat transfer in the rotating disc cavities of a turboprop engine.

A numerical analysis of the flow and heat transfer in the cavity between two co-rotating discs with axial inlet and radial outflow of fluid, a configuration common in gas turbine engines, is described. The results are compared with the experimental data of Northrop and Owen. The effectiveness of the k-epsilon turbulence model with the two-layer zonal model for near-wall treatment of Chen and Patel is tested for this type of flow. Using three-dimensional models it is shown that modelling discrete holes at the outlet as opposed to a continuous slot, which is the approximation inherent in the two-dimensional axisymmetric model, has little effect on the predicted Nusselt number distribution along the disc surface. Results of a conjugate heat transfer analysis of a spacer in the turbine section of a turboprop engine are then presented.

Numerical investigation of developing turbulent flow in a helical square duct with large curvature

A fully elliptic numerical study has been carried out to investigate the three-dimensional turbulent developing flow in a helical square duct with large curvature. A two-layer zonal model is proposed and used, in which the whole region is divided into a viscosity-affected near wall layer and a fully turbulent region. A DSM closure is applied in the former, and a one-equation model is solved in the latter. The results presented in this paper cover a Reynolds number range of (1∼10)×104. The development of flow is found to be dominated by radial pressure gradient and Dean-type secondary motion. The distribution of Reynolds stresses in fully developed flow exhibit a complex pattern of turbulence anisotropy. The development of peripherally averaged friction factor and the distribution of local friction factor in fully developed flow are given and discussed.

Numerical simulation of flow in an array of in-line blunt boards: mass transfer and flow patterns

The airflow patterns around an array of in-line truncated boards and the sublimation process of thin naphthalene layers over the in-line boards have been simulated numerically. The simulation used the renormalization group k– ε turbulence model and the two-layer zonal model for the near-wall treatments. When only one of the in-line boards is coated with naphthalene, as in the experiments by previous authors, the predicted local mass-transfer coefficients over that board show a substantial increase at the leading edge of the board and are in good agreement with the measurement data. However, when all the in-line boards are naphthalene coated, the calculated mass-transfer coefficients decrease in an approximately asymptotic manner from a maximum value near the leading edge of the first board. These results indicate that the small gaps between in-line boards have an insignificant effect on the mass-transfer coefficients. Vortex shedding from in-line boards separated by small gaps has also been investigated numerically. It is shown that in a wind tunnel with a single layer of boards, all the recirculating flow eddies at different locations around the in-line boards are characterized by vortex shedding, in agreement with the experimental observations by previous authors, while in a kiln stack with multiple layers of in-line boards, only the eddies in the near wake of the stack are formed periodically.