Arul K Prakash

Numerical studies on fluid flow and heat transfer through a two-dimensional 180-degree sharp bend are performed using an in-house code based on streamline upwind Petrov-Galerkin finite element method. A new geometric parameter called inlet height to outlet height ratio (IOR) is defined to assess the heat transfer performance. Parametric analyses are carried out by varying the Reynolds number (Re) from 100 to 900 for five IORs (1:2, 1:1.5, 1:1, 1.5:1, 2:1). The fluid flow changes from steady to unsteady for all IORs with increasing Re. The critical Reynolds number range is identified for each IOR and is found to be lower for IOR < 1 or IOR > 1 when compared with IOR 1:1, thus enhancing the heat transfer due to unsteadiness. IOR also influences the rate of generation of the vortices and the vortex interactions, which emphasize the enhancement in heat transfer. Nusselt number (Nu) and thermal performance factor (TPF) for the domain are calculated and it is observed that though there is an increase of 20%–40% in Nu for IOR 2:1 configuration with respect to IOR 1:1, the TPF predicts that IOR 1:1.5 is an overall good domain for heat transfer and pressure drop considerations.

Forced convection and turbulent scalar transport over a structured superhydrophobic walls composed of periodic arrays of square posts and ridges textures are numerically investigated in a periodic channel. The flow physics and thermal transport within the fluid are studied using Direct numerical simulation (DNS), assuming the plastrons are flat. Slip velocities, Nusselt number, turbulent Prandtl number and turbulent heat fluxes are determined for both square posts and streamwise ridge shaped feature geometry configurations at a friction Reynolds number of Reτ=180. This article provides an insight that superhydrophobic surfaces enhance turbulent heat fluxes and thermal fluctuations in the laminar-sublayer region. With increasing feature wavelength, the turbulent heat fluxes in the buffer layer and outer layer drop for both square posts and streamwise ridges. When compared with a smooth no-slip channel, the diffusion term in the budget of thermal fluxes pointed to the role of organized lateral flow motions in enhancing wall-normal heat-flux, and a reduction in large scale transport of streamwise heat fluxes by near-wall coherent structures.

This article presents the numerical study of laminar forced convective heat transfer from elliptic cylinders of various axis ratios (AR=0.1, 0.4, 0.6, 0.8, and 1.0), angles of attack (AOA=30°, 45° , 60° , and 90°), and Reynolds numbers (Re=50, 100, 150, and 200). Simulations are carried out for both isothermal and isoflux wall boundary conditions. A detailed study of flow field reveals distinct instantaneous and time-averaged flow patterns behind the elliptic cylinder. The effect of flow patterns on isotherms and, thus, on heat transfer, is analyzed in detail. Local and surface averaged Nusselt number (Nu and Nuavg) is computed and their variation due to change in AR, AOA, and Re is studied. It is observed that increasing AR and Re increases Nu avg monotonically, while increasing AOA decreases Nuavg. Finally, correlations are proposed for Nuavg with respect to AR, AOA, and Re with minimum rms error. Copyright © Taylor & Francis Group, LLC.

Fluid flow and heat transfer characteristics past four elliptic cylinders arranged in square configuration are numerically investigated in this study. An in-house finite element method code based on streamline upwind Petrov - Galerkin algorithm has been used for simulations of two-dimensional, incompressible and laminar flow of air. Analyses are carried out by changing geometric parameters such as spacing ratio, axis ratio and incident angle. Results in terms of vorticity and temperature contours, Strouhal number, drag and lift coefficients and Nusselt number are studied for all cases. These results show that fluid flow patterns are strongly influenced by geometric parameters, based on which flow can be categorized into different types. A critical spacing ratio is found for some of the configurations, where a steep increase in drag coefficient is encountered. From this study, it is observed that incident angle 22.5 degree gives a higher heat transfer rate compared to the other two configurations (0 and 45 degree). Also, elliptic cylinders (axis ratio of 0.8 and 0.5) have similar Nusselt number value with less drag force compared to circular cylinders (axis ratio of 1.0) which justifies the utilization of elliptic cylinders as an alternative for circular ones.

Flow past any geometry contains structures of diverse sizes, albeit the shedding structure is the most prevalent. There are situations where the flow also has large-scale secondary dominant structures. This study attempts to provide the sources of such low frequency secondary structures in a laminar regime. To this end, we analyze the velocity field through fast Fourier transform in the near and far wake of an elliptic cylinder of axis ratio 0.4 whose major axis is kept perpendicular to the incoming flow. Numerical simulation is carried out for Reynolds number 130, where the flow is reported to be two-dimensional. This study reveals the presence of low-frequency structures besides the primary shedding structures in linear, transition, and saturation regions of temporal wake development. We show that the temporal source of the primary frequency is the saturated state of the wake development, while its physical source is the periodic nature of the saturation region, which inhibits the transmutation of the wavelength of flow structures. On the other hand, the secondary low frequency is embedded in the transitional developing stage of the wake and its physical source is the irregular behavior of the transition process, which aids transmutation of the wavelength of the structures.

In this analysis, forced convective heat transfer characteristics of Al2O3-water nanofluid through a porous channel with several combinations of heaters and coolers is investigated numerically. The two-dimensional equations governing nanofluid flow and heat transfer through porous media are discretized using in-house code with Streamline Upwind Petrov Galerkin-based Finite Element Method. Darcy–Brinkman–Forchheimer’s generalized porous media model is used in this study. The average Nusselt number of basefluid without porous media, nanofluid with and without porous media cases are compared for different Peclet numbers and the effect of Peclet number on stream lines and isotherms are studied for nanofluid with and without porous media cases. In addition to this the effect of Darcy number, porosity, and nanoparticle volume fraction on the performance of average Nusselt number is investigated. From these results, it is observed that the average Nusselt number increases with decrease in Darcy number. From this analysis, it can be concluded that addition of porous media results in enhancement of heat transfer and can be used as a potential technique for electronic cooling applications.

Numerical solutions are presented for fluid flow past a rotating elliptic cylinder placed in a uniform flow for different non-dimensional rotation rates (α). The two dimensional, incompressible Navier-Stokes equations are solved using immersed boundary method (IBM) on a non-body conforming Cartesian grid. Reynolds number based on the perimeter of elliptic cylinder is 100 and axis ratio of the cylinder is 0.1. The periodic flow, after the cylinder is impulsively started, is studied. Vortex shedding is observed in all the cases with significant differences in shedding pattern. For α ≥ 1, we observe an interesting phenomenon of formation of 'Hovering vortex’. The factors contributing to hovering vortex and associated changes in the aerodynamic forces are studied. Significant increase in lift coefficient is observed with increase in rotation rate. For some cases we observe thrust generation for major portion of the cycle.

In the present study, a multi-objective optimization procedure combining finite element modeling of impingement cooling system, Response Surface Approximation (RSA) of objective functions and optimization based on Multi-Objective Genetic Algorithm (MOGA), to achieve maximum heat transfer and minimum entropy generation is demonstrated. For the purpose, numerical simulations are performed for impingement cooling system with Al2O3/water nanofluid, to investigate the influence of Reynolds number (Re), non-dimensional channel height (H/L) and nanoparticle volume fraction (Pdbl) on fluid flow, heat transfer and entropy generation. The simulated results illustrate that, a secondary recirculation bubble observed on upper surfaces of fourth (at Re ≥ 500) and fifth (for Re ≥ 800) heat sources, lead to an accumulation of heat. The magnitude of local Nusselt number (Nu) is found to be maximum along stagnation region whereas in the regions of secondary recirculation a minimum value is observed. Further, an increase in overall surface averaged Nusselt number (Nuov) and global total entropy generation (Stot,Ω) is observed with increasing Re,Pdbl and decreasing H/L. Subsequently, Nuov and Stot,Ω are selected as objective functions and are modeled using RSA. Furthermore, MOGA has been implemented to obtain optimum configurations of impingement cooling system encapsulating in the functional space lying on the Pareto-optimal frontier where a trade-off between two performance parameters, Nuov and Stot,Ω are obtained.

In the present study, fluid flow, heat transfer and entropy generation in impingement cooling system with an array of air jets for different values of Reynolds number (Re), Velocity Ratio (VR) and Channel Height (H/L) are investigated. The magnitude of overall Nusselt number (Nuov‾) and global total entropy generation (Stot,Ω) is found to increase with increasing Re,VR and decreasing H/L. Further, spectral and proper orthogonal decomposition analyses are performed to analyze spatio-temporal dynamics of vortex structures for unsteady configurations of impingement cooling system. It is observed that, along the interface of jet (both primary and secondary) and ambient fluid, the destabilizing effect of shear forces overcome the stabilizing effect of momentum diffusion. This results in evolution of counter-rotating vortex rings along the interfaces of jet and ambient fluid due to shear layer instability. Finally, Multi-Objective Genetic Algorithm (MOGA) has been implemented to obtain optimum configurations of impingement cooling system where a trade-off between two performance parameters, Nuov‾ and Stot,Ωis obtained.

The severe reactor accident at Fukushima Daiichi Nuclear Power Plant (2011) has confirmed the need to understand the flow and transport processes of steam and combustible gases inside the containment and connected buildings. Over several years, Computational Fluid Dynamics (CFD) models, mostly based on proprietary solvers, have been developed to provide highly resolved insights; supporting the assessment of effectiveness of safety measures and possible combustion loads challenging the containment integrity. This paper summarizes the design and implementation of containmentFOAM, a tailored solver and model library based on OpenFOAM®. It is developed in support of Research & Development related to containment flows, mixing processes, pressurization, and assessment of passive safety systems. Based on preliminary separate-effect verification and validation results, an application oriented integral validation case is presented on the basis of an experiment on gas mixing and H2 mitigation by means of passive auto-catalytic recombiners in the THAI facility (Becker Technologies, Eschborn, Germany). The simulation results compare well with the experimental data and demonstrate the general applicability of containmentFOAM for technical scale analysis. Concluding the paper, the strategy for dissemination of the code and measures implemented to minimize potential user errors are outlined.