Anupam Dewan

Review of passive heat transfer augmentation techniques

Abstract Heat transfer augmentation techniques (passive, active or a combination of passive and active methods) are commonly used in areas such as process industries, heating and cooling in evaporators, thermal power plants, air-conditioning equipment, refrigerators, radiators for space vehicles, automobiles, etc. Passive techniques, where inserts are used in the flow passage to augment the heat transfer rate, are advantageous compared with active techniques, because the insert manufacturing process is simple and these techniques can be easily employed in an existing heat exchanger. In design of compact heat exchangers, passive techniques of heat transfer augmentation can play an important role if a proper passive insert configuration can be selected according to the heat exchanger working condition (both flow and heat transfer conditions). In the past decade, several studies on the passive techniques of heat transfer augmentation have been reported. The present paper is a review on progress with the passive augmentation techniques in the recent past and will be useful to designers implementing passive augmentation techniques in heat exchange. Twisted tapes, wire coils, ribs, fins, dimples, etc., are the most commonly used passive heat transfer augmentation tools. In the present paper, emphasis is given to works dealing with twisted tapes and wire coils because, according to recent studies, these are known to be economic heat transfer augmentation tools. The former insert is found to be suitable in a laminar flow regime and the latter is suitable for turbulent flow. The thermohydraulic behaviour of an insert mainly depends on the flow conditions (laminar or turbulent) apart from the insert configurations. The present review is organized in five different sections: twisted tape in laminar flow; twisted tape in turbulent flow; wire coil in laminar flow; wire coil in turbulent flow; other inserts such as ribs, fins, dimples, etc.

Recent Trends in Computation of Turbulent Jet Impingement Heat Transfer

A review of the current status of computation of turbulent impinging jet heat transfer is presented. It starts with a brief introduction to flow and heat transfer characteristics of jet impinging flows considering the simplest jet impinging geometry: normal impingement of a single jet into a flat surface. Subsequently, a review of recent computational studies related to the same geometry is presented. The effects of different subgrid scale models, boundary conditions, numerical schemes, grid distribution, and size of the computational domain adopted in various large eddy simulations of this flow configuration are reviewed in detail. A review of direct numerical simulation of the same geometry is also presented. Further, some recent attempts in Reynolds-averaged Navier–Stokes modeling of impinging flows are also reviewed. A review of computation of other complex impinging flows is also presented. The review concludes with a listing of some important findings and future directions in the computation of impinging flows.

Tackling Turbulent Flows in Engineering

In this chapter we present basics of fluid mechanics which include concept of fluid, important fluid properties, different methods of flow visualization, difference between Eulerian and lagrangian approaches, between integral and differential treatments. We present differential equations for the conservation of mass, momentum and energy and the corresponding boundary conditions. We also introduce the reader to convective heat transfer. The chapter concludes with two examples of flow normal to an infinite circular cylinder and convective heat transfer from a heated tube surface to a cold fluid flowing axially within the tube. 1.1 Fluid Properties A fluid deforms continuously when a tangential stress is applied irrespective of the amount of stress. A solid can be distinguished from liquid based on its response to an applied stress. A solid can resist a shear stress by a static deformation, but a fluid cannot resist a shear stress. Both liquids and gases are treated as fluid. A fluid can be treated as continuum in most situations, except at extremely low pressure where the molecular spacing and mean free paths are comparable to the physical dimensions of the flow. Fluid mechanics is a broad subject. In the subsequent sections we will consider important physical properties of fluids that have an influence on their fluid and thermal behaviours.

Analysis of Non-Darcy Models for Mixed Convection in a Porous Cavity Using a Multigrid Approach

This article investigates the performance of two models; namely the Brinkman-Forchheimer Darcy model (BFDM) and the Brinkman-extended Darcy model (BDM), in a problem involving mixed convection in a square cavity filled with a porous medium using the multigrid method. The left and right walls, moving in opposite directions, are maintained at different constant temperatures, while the top and bottom walls are thermally insulated. The transport equations were solved numerically by the finite-volume method on a colocated grid arrangement using a quadratic upwind interpolation for convective kinematics (QUICK) scheme. The influence of the key parameters, namely the Darcy number (Da) and Grashof number (Gr) on the flow and heat transfer pattern is examined. Further, the issue of reliability of the results is addressed. The results demonstrate that BDM over-predicts the momentum and heat transfer rates compared with BFDM, which is in conformity with the fact that the additional term present in the BFDM hinders convective effects. The full approximation storage (FAS) multigrid method achieves considerable acceleration of convergence for the present relatively unexplored problem.

Prediction of turbulent plane jet in crossflow

In this article numerical predictions of turbulent plane jets discharged normal to a weak or moderate cross stream are presented. The Reynolds-averaged Navier-Stokes equations with the standard k- k turbulence model have been used to formulate the flow problem. The governing equations that are elliptic in nature are solved using the finite volume method. The predictions are presented to illustrate the flow pattern involved and to assess the performance of the standard k- k model by comparison with available experimental data for three different jet to cross stream velocity ratios (six, nine, and ten) and the agreement is found to be satisfactory.

Some Case Studies

This chapter presents some examples of turbulent flows encountered in industry. We see how they have been successfully modeled and computed using some of the techniques presented earlier in this book. Turbulent flow fields in these examples have been treated using RANS based models of different complexities as well as more advanced techniques such as DNS and LES. The examples cover a wide spectrum involving heat exchangers with turbulent convective heat transfer, stirred vessel with complex three-dimensional, turbulent and rotating flow, turbulent flow in tundish used in steel industry, ventilation in buildings, film cooling effectiveness. A reader may like to read the corresponding papers for further details of the cases presented.

Computation of the turbulent plane plume using the k–ϵ–t′2–γ model

Abstract The k–ϵ– t ′2 –γ turbulence model is used to predict the self-similar plane plume in a quiescent environment. This model has been recently used to predict the turbulent axisymmetric plume. Modelled transport equations for the turbulent kinetic energy (k), its dissipation (ϵ), mean square temperature fluctuations ( t ′2 ) and intermittency factor (γ) have been solved numerically along with the equations for the mean quantities. A small change in one of the model constants, incorporation of the dissipation term in the intermittency transport equation and withdrawal of the intermittency interaction invariant term from the dissipation equation yield predictions of mean and turbulent quantities including intermittency that are in good agreement with the experimental data.

LES of a Turbulent Slot Impinging Jet to Predict Fluid Flow and Heat Transfer

In this article, large eddy simulation (LES) is performed for a turbulent slot jet impingement heat transfer at a Reynolds number of 13,500 and a nozzle to plate spacing of 10. Various aspects of predicting a turbulent jet impinging flow in an optimum domain size and grid resolution for LES have been assessed. Two inflow conditions, one without any fluctuations and the other with fluctuations generated by the spectral synthesizer, were tested and comparisons of various mean flow, turbulence, and heat transfer data showed that LES without any inflow fluctuations provides good agreement with the corresponding experimental and numerical results reported in the literature. Further, various important dynamical flow structures have been visualized from the instantaneous computed data. Finally, mean flow and turbulence statistics have been presented in the wall jet region close to the stagnation point, which could be useful as data for validation of RANS-based turbulence models.

Large Eddy Simulation of Turbulent Slot Jet Impingement Heat Transfer at Small Nozzle-to-Plate Spacing

We present fluid flow and heat transfer of a slot jet impingement heat transfer at a small value of the nozzle-to-plate spacing at which a secondary peak in the Nusselt number is observed. Large eddy simulation has been performed with a finite-volume-based computational fluid dynamics code and using a dynamic Smagorinsky model. The optimum domain size and grid for large eddy simulation (LES) have been produced based on LES computations on a coarse mesh and Reynolds-averaged Navier–Stokes-based computations. Two inflow conditions, namely, using the vortex method and no perturbations, were compared. The present LES results, using the vortex method, capture the secondary peak in the Nusselt number better as compared to the case with no perturbations. Results show that mean velocity profile in the stagnation region deviates from the standard law of the wall. Further, large-scale vortical structures were observed near the location of the secondary Nusselt number peak. Increases in both the streamwise and wall normal turbulence fluctuations are observed near the secondary peak in the Nusselt number. The secondary peak in Nusselt number is found to be associated with the combined effect of flow acceleration and an increase in the turbulence kinetic energy.

Computational Models for Turbulent Thermal Plumes: Recent Advances and Challenges

Thermal plumes have been the subject of research due to their technological and environmental importance in many physical processes, such as spread of smoke and toxic gases from fires, and release of gases from volcanic eruptions and industrial stacks. In this paper, history and current trends in the computation of turbulent buoyant plume have been reviewed. First an introduction to the turbulent buoyant plume is presented, which includes its importance, the flow physics, and heat transfer characteristics. Subsequently, a brief review of the experimental works reported in the literature is presented, followed by a review of the computational methods. Several subgrid-scale models used in large eddy simulation performed in the literature to simulate buoyant plume are discussed. Further, the boundary conditions, computational schemes, and computational grid sizes are discussed. The efficacy of various Reynolds-averaged Navier–Stokes (RANS)-based modeling techniques to simulate turbulent buoyant plumes has been reviewed. It has been concluded that the dynamic subgrid-scale models perform fairly well in predicting the statistics of turbulent buoyant plume. However, no subgrid-scale model has been able to capture the inverse energy cascade, called backscatter, which is an important phenomenon in the evolution of a plume. In case of RANS modeling of plumes, models based on the generalized gradient diffusion hypothesis capture the flow accurately as compared with the models based on the simple gradient diffusion hypothesis.