Mohammad Nikjoo

Bio: Mohammad Nikjoo is an academic researcher. The author has contributed to research in topics: Vortex & Reynolds stress. The author has an hindex of 2, co-authored 2 publications receiving 13 citations.

Study of non-linear k-epsilon model for turbulent swirling flows

Abstract: Computations on the axisymmetric turbulent flows were conducted to investigate the applicability of the non-linear models. Three kinds of the turbulence models of the standard k-s model, quadratic Reyno Ids stress-strain model, and cubic Reynolds stress-strain model were applied and computations were compared with data. The effects of inlet dissipation rate on predicted mean and turbulence quantities are investigated. The results show that, the cubic model can improve the prediction of the axial velocity component as compared with the standard k-e and quadratic models. Furthermore, the conventional k-e model and quadratic model fail to predict the parabolic profiles of the tangential velocity component. On the other hand, the cubic model can predict the experimental tendency of the parabolic profiles of tangential velocity component. Introduction Swirling flows are commonly encountered in gas turbine combustors to aid stabilizing the flame and create a region of strong shear to enhance mixing process. In these flow fields, the interaction between turbulence and the centrifugal force induced by the swirl has influence on the characteristics of the turbulent transfer of momentum, mass and energy. Numerous studies have been conducted to obtain a better understanding of these phenomena and to establish the prediction procedures for the swirling flows [1-3]. So far, the k-e model has been extensively used in engineering calculations, hi direct comparison of the k-e model predictions with experimental data, the model is successful in predicting the basic features of turbulent flows. When significant streamline curvatures are introduced into the flow field, the model does not adequately account for the enhanced turbulence diffusion caused by the extra strain rate associated with streamline curvature. Copyright © 1998 by authors. Published by AIAA Inc. with permission. The experimental data on the turbulent swirling flows in pipes indicate that there are two distinct rotational motions [4-5]. The one hi the inner region, near the centerline, where the swirl velocity is close to solidbody rotation, and the other is at the outer region and is dominated by the free-vortex motion. Forced vortex has a stabilizing effect, which provokes a reduction in the stresses and promotes the retardation of mixing and combustion in swirling flames [6]. On the contrary, the swirl of the free vortex results in destabilizing the flow and enhances the turbulent exchange of momentum, mass and energy. These phenomena seem to have concern with the increase of the wall friction and heat transfer rate in the swirling flow [7]. The two-equation k-e type model has failed to predict the observed combined forced-free vortex motion [2]. The standard k-e model has been subjected to a number of modifications in an effort to enhance its responsiveness to the extra strain rates imposed by rotation and streamline curvature [8-9]. The manner in which these corrections are implemented depends on the suppression or augmentation of turbulence occurs as a result of swirl motion. These approaches have been designed to ultimately increase or decrease the effective viscosity. Although the source terms of the ke model may require optimization with respect to strongly swirling flows, adjustment of the eddy viscosity level through stabilization or destabilization procedure is not sufficient. Perhaps the biggest limitation of the applicability of the k-e model is due to the assumption of isotropic eddy viscosity. One obvious choice of method for overcoming some of the limitations inherent in the k-e model is provided by Reynolds stress models. These models have been applied with considerable success to many complex swirling flows [10-11]. However, they are computationally complex and expensive and have not reached the state of practical application. Therefore, there is a real engineering need to develop a turbulent closure model for the flow equations that would give realistic results and yet is fast and reliable computationally. 1 American Institute of aeronautics and Astronautics

Evaluation of one‐ and two‐equation low‐Re turbulence models. Part I—Axisymmetric separating and swirling flows

Abstract: This first segment of the two-part paper systematically examines several turbulence models in the context of three flows, namely a simple flat-plate turbulent boundary layer, an axisymmetric separating flow, and a swirling flow. The test cases are chosen on the basis of availability of high-quality and detailed experimental data. The tested turbulence models are integrated to solid surfaces and consist of: Rodi's two-layer k–e model, Chien's low-Reynolds number k–e model, Wilcox's k–ω model, Menter's two-equation shear-stress-transport model, and the one-equation model of Spalart and Allmaras. The objective of the study is to establish the prediction accuracy of these turbulence models with respect to axisymmetric separating flows, and flows of high streamline curvature. At the same time, the study establishes the minimum spatial resolution requirements for each of these turbulence closures, and identifies the proper low-Mach-number preconditioning and artificial diffusion settings of a Reynolds-averaged Navier–Stokes algorithm for optimum rate of convergence and minimum adverse impact on prediction accuracy. Copyright © 2003 John Wiley & Sons, Ltd.

Perspective of Combustion Modeling for Gas Turbine Combustors

Abstract: • Empirical/analytical design tools A status summary report is given on the development of comprehensive computational combustor design (CCD) tool activities of GEAE combustion group conducted for the last three years. A “no-fudge factor” pretest prediction approach is being pursued for the combustors that use rich or lean-dome swirl cups (Single Annular and Dual Annular combustors, SAC, DAC); Twin Annular Premixing Swirler, TAPS (SAC and DAC), and Dry Low Emissions’ DACRS mixers for triple annular combustor domes. The pretest prediction capability is being gauged with existing data including emissions, exit temperature quality and wall temperatures from nine different combustors selected from a group of production and technology combustors. The agreement between predictions and data is good considering this is the 1 st

3D RANS Simulation of Turbulent Flow and Combustion in a 5 MW Reverse-Flow Type Gas Turbine Combustor

Abstract: This study is concerned with 3D RANS simulation of turbulent flow and combustion in a 5 MW commercial gas turbine combustor The combustor under consideration is a reverse flow, dry low NO x type, in which methane and air are partially mixed inside swirl vanes We evaluated different turbulent combustion models to provide insights into mixing, temperature distribution, and emission in the combustor Validation is performedfor the models in STAR-CCM+ against the measurement data for a simple swirl flame (http:// publiccasandiagov/TNF/swirlflameshtml) The standard k-s model with enhanced wall treatment is employed to model turbulent swirl flow, whereas eddy break-up (EBU), presumed probability density function laminar flamelet model, and partially premixed coherent flame model (PCFM) are tried for reacting flow in the combustor Independent simulations are carried out for the main and pilot nozzles to avoid flashback and to provide realistic inflow boundary conditions for the combustor Geometrical details such as air swirlers, vane passages, and liner holes are all taken into account Tested combustion models show similar downstream distributions of the mean flow and temperature, while EBU and PCFM show a lifted flame with stronger effects of swirl due to limited increase in axial momentum by expansion