scholarly journals Turbulence Modeling in Side-Entry Stirred Tank Mixing Time Determination

2021 ◽  
Vol 333 ◽  
pp. 02003
Author(s):  
Suci Madhania ◽  
Ni’am Nisbatul Fathonah ◽  
Kusdianto ◽  
Tantular Nurtono ◽  
Sugeng Winardi
Keyword(s):  
Turbulence Model ◽  
Turbulence Modeling ◽  
Mixing Time ◽  
Paper Industry ◽  
Turbulence Models ◽  
Computational Prediction ◽  
Operating Conditions ◽  
Stirred Tank ◽  
Critical Parameters ◽  
Working Fluid

Mixing is one of the critical processes in the industry. The stirred tank is one of the operating units commonly used in the mixing process. Several factors greatly influence the efficiency of the stirred tank, including the stirred-tank design, operating conditions, and working fluid properties. The side-entry stirred tank is widely applied in industry, among others; the processing of crude oil in the refinery industry, water-molasses mixing in the bioethanol industry, pulp stock chest in the pulp and paper industry, and anaerobic digester for biogas reactors. Mixing time is one of the critical parameters used in the design of the stirred tank. This research will model mixing time in a flat bottomed-cylindrical side-entry stirred tank with dimensions D = 40 cm and T = 40 cm using CFD ANSYS 18.2 by applying the Standard κ − ε (SKE) and Realizable κ − ε (RKE) turbulence models. The stirrer used is a three-blade marine propeller d = 4 cm which is an axial type impeller. The phenomenon of mixing in the side-entry stirred tank, qualitatively described through computational prediction results in the form of flow profiles and tracer density change contours locally. Moreover, quantitatively indicated by mixing time validated using experimental data carried out by the conductometry method. The computational prediction shows that the mixing time modeled using the SKE turbulence model shows a similarity level of 68.16%, while the RKE turbulence model shows 31.94%.

A Critical Evaluation of Turbulence Modeling in a Model Combustor

2012 ◽  
Author(s):  
Leiyong Jiang
Keyword(s):  
Turbulence Modeling ◽  
Critical Evaluation ◽  
Turbulence Models ◽  
Reynolds Stress Model ◽  
Operating Conditions ◽  
Moment Closure ◽  
Mean Velocity ◽  
Second Moment Closure ◽  
Heat Transfers ◽  
Air Diffusion

Based on the previous benchmark studies on combustion, scalar transfer and radiation models, a critical evaluation of turbulence models in a propane-air diffusion flame combustor with interior and exterior conjugate heat transfers has been performed. Results obtained from six turbulence models are presented and compared in detail with a comprehensive database obtained from a series of experimental measurements. It is found that the Reynolds stress model (RSM), a second moment closure, is superior over the five popular eddy-viscosity two-equation models. Although the main flow patterns are captured by all six turbulence models, only the RSM is able to successfully predict the lengths of both recirculation zones and give fairly accurate predictions for mean velocity, temperature, CO2 and CO mole fractions, as well as turbulence kinetic energy in the combustor chamber. In addition, the realizable k-ε (Rk-ε) model illustrates better performance than four other two-equation models and can provide comparable results to those from the RSM for the configuration and operating conditions considered in the present study.

CFD Simulation of a Supersonic Steam Ejector for Refrigeration Application

2016 ◽  
Author(s):  
Liju Su ◽  
Ramesh K. Agarwal
Keyword(s):  
Flow Field ◽  
Cfd Simulation ◽  
Fluid Pressure ◽  
Turbulence Models ◽  
Industrial Applications ◽  
Operating Conditions ◽  
Working Fluid ◽  
Entrainment Ratio ◽  
Ansys Fluent ◽  
Entrainment Velocity

Supersonic steam ejectors are widely used in many industrial applications, for example for refrigeration and desalination. The experimental evaluation of the flow field inside the ejector is relatively difficult and costly due to the occurrence of shock after the velocity of the steam reaches over the sonic level in the ejector. In this paper, numerical simulations are conducted to investigate the detailed flow field inside a supersonic steam (water vapor being the working fluid) ejector. The commercial computational fluid dynamics (CFD) flow solver ANSYS-Fluent and the mesh generation software ANSYS-ICEM are used to predict the steam performance during the mixing inside the ejector by employing two turbulence models, the k-ω SST and the k-ε realizable models. The computed results are validated against the experimental data. The effects of operating conditions on the efficiency of the ejector such as the primary fluid pressure and condenser pressure are studied to obtain a better understanding of the mixing process and entrainment. Velocity contours, pressure plots and shock region analyses provide a good understanding for optimization of the ejector performance, in particular how to increase the entrainment ratio.

Turbulent Scalar Flux Models for the Mixing of Hot Gases in the Swirling Flow of High-Temperature Reactor Lower Plenums

2008 ◽  
Author(s):  
Dominik von Lavante ◽  
Eckart Laurien
Keyword(s):  
High Temperature ◽  
Reynolds Stress ◽  
Turbulence Modeling ◽  
Swirling Flow ◽  
Turbulence Models ◽  
Operating Conditions ◽  
Vortical Flow ◽  
Mixing Efficiency ◽  
Scalar Flux ◽  
Flux Transport

With recent progress in high-temperature pebble-bed reactor programs research focus has started to include more ancillary engineering issues. One very important aspect for the realisability is the mixing of hot and colder helium in the reactor lower plenum. Under nominal operating conditions, depending on core design, the temperature of hot gas leaving the core can locally differ up to 210° C. Due to material limitations, these temperature differences have to be reduced to at least ±15° C. Several reduced-size air experiments have been performed on this problem, but their applicability to modern commercially sized reactors is not certain. With the rise in computing power CFD simulations can be performed in addition, but advanced turbulence modeling is necessary due to the highly swirling and turbulent nature of this flow. The presented work uses the geometry of the German HTR-Modul which consists of an annular mixing channel and radially arranged ribs. Using the commercial CFD code ANSYS CFX, we have made detailed analyses of the complex 3D vortical flow phenomena within this geometry. Several momentum transport turbulence models, e.g. the classical k-e model, advanced two-equation models and Reynolds-Stress Models were compared with respect to their accuracy for this particular flow. In addition, the full set of turbulent scalar flux transport equations was implemented for modeling the three components of turbulent transport of enthalpy seperately and were compared with the standard turbulent Prandtl number approach. As expected from previous work in related fields of turbulence modeling, the differences in predicting the mixing performance between models were significant. Only the full Reynolds-Stress model coupled with the scalar flux equations was able to reproduce the experimentally observed reduction of mixing efficiency with increasing Reynolds number. The correct scaling of mixing efficiencies demonstrates that the utilized turbulence models are able to reproduce the physics of the underlying flow. Hence they could be employed for the scaling and optimization of the lower plenum geometry. The results also showed that the original geometry used for the HTR-Modul is insufficient to provide adequate mixing, and that hence a not sufficiently mixed coolant for future reactor designs might be an issue. Based on this work, an optimization for future lower plenum geometries has become feasible.

scholarly journals Application of different turbulence models for improving construction of small-scale boiler fired by solid fuel

2017 ◽  
Vol 21 (suppl. 3) ◽  
pp. 809-823
Author(s):  
Nebojsa Manic ◽  
Vladimir Jovanovic ◽  
Dragoslava Stojiljkovic ◽  
Zagorka Brat
Keyword(s):  
Turbulence Model ◽  
Solid Fuel ◽  
Turbulence Modeling ◽  
Turbulence Models ◽  
Experimental Tests ◽  
Computer Hardware ◽  
Small Scale ◽  
Heat Output ◽  
Ansys Fluent ◽  
Model Fluid

Due to the rapid progress in computer hardware and software, CFD became a powerful and effective tool for implementation turbulence modeling in defined combustion mathematical models in the complex boiler geometries. In this paper the commercial CFD package, ANSYS FLUENT was used to model fluid flow through the boiler, in order to define velocity field and predict pressure drop. Mathematical modeling was carried out with application of Standard, RNG, and Realizable k-? turbulence model using the constants presented in literature. Three boilers geometry were examined with application of three different turbulence models with variants, which means consideration of 7 turbulence model arrangements in FLUENT. The obtained model results are presented and compared with data collected from experimental tests. All experimental tests were performed according to procedures defined in the standard SRPS EN 303-5 and obtained results are presented in this paper for all three examined geometries. This approach was used for improving construction of boiler fired by solid fuel with heat output up to 35 kW and for selection of the most convenient construction.

TURBULENCE MODELS FOR ACCURATE AEROTHERMAL PREDICTION IN HYPERSONIC FLOWS

2010 ◽  
Vol 24 (13) ◽  
pp. 1345-1348 ◽  
Author(s):  
XIANG-HONG ZHANG ◽  
YI-ZAO WU ◽  
JIANG-FENG WANG
Keyword(s):  
High Performance ◽  
Turbulence Modeling ◽  
Thermal Protection ◽  
Transport Model ◽  
Integrated Design ◽  
Turbulence Models ◽  
Computational Prediction ◽  
Hypersonic Flows ◽  
Design And Optimization ◽  
Sst Model

Accurate description of the aerodynamic and aerothermal environment is crucial to the integrated design and optimization for high performance hypersonic vehicles. In the simulation of aerothermal environment, the effect of viscosity is crucial. The turbulence modeling remains a major source of uncertainty in the computational prediction of aerodynamic forces and heating. In this paper, three turbulent models were studied: the one-equation eddy viscosity transport model of Spalart-Allmaras, the Wilcox k -ω model and the Menter SST model. For the k -ω model and SST model, the compressibility correction, press dilatation and low Reynolds number correction were considered. The influence of these corrections for flow properties were discussed by comparing with the results without corrections. In this paper the emphasis is on the assessment and evaluation of the turbulence models in prediction of heat transfer as applied to a range of hypersonic flows with comparison to experimental data. This will enable establishing factor of safety for the design of thermal protection systems of hypersonic vehicle.

scholarly journals THREE-DIMENSIONAL COMPUTATIONAL PREDICTION OF VORTEX SEPARATION PHENOMENON INSIDE THE RANQUE-HILSCH VORTEX TUBE

Aviation ◽  
2016 ◽  
Vol 20 (1) ◽  
pp. 21-31 ◽  
Author(s):  
Seyed Ehsan RAFIEE ◽  
Mohammad Bagher Mohammad SADEGHIAZAD
Keyword(s):  
Turbulence Model ◽  
Fluid Dynamic ◽  
Three Dimensional ◽  
Turbulence Models ◽  
Computational Prediction ◽  
Separation Process ◽  
Counter Flow ◽  
Vortex Separation ◽  
The Impact ◽  
Air Separator

The air separators are used to provide safe, clean and appropriate air to the helicopter’s engine. In this operational study, the separation process inside a Ranque-Hilsch air separator cleaning system has been investigated to analyze the impact of choosing the appropriate turbulence model for predicting the separation process inside the air separator. This research is directed towards presenting a computational fluid dynamic explanation performed on a counter-flow air separator using air at different magnitudes of air flow fraction and applying different turbulence models. In a numerical investigation of counter-flow air separator, air has been chosen and its vortex separation phe- nomenon has been analyzed as a function of flow fraction. Furthermore, a numerical analysis to compare the outputs of a seven equation RSM turbulence model applied for the study of vortex separation of a counter-flow air separator with some two-equation turbulence methods, namely, k-ε and k-ω model as well as LES has been presented. All of the turbulence numerical methods are seen to present and predict the same flow pattern inside an air separator, but, with various details. The results show that among the tested methods the RSM creates the most accurate separation pattern. The numerical results are validated by some available experimental data with good agreement.

scholarly journals Parametric Analysis of a Polygeneration System with CO2 Working Fluid

2021 ◽  
Vol 11 (7) ◽  
pp. 3215
Author(s):  
Evangelos Bellos ◽  
Christos Tzivanidis
Keyword(s):  
Heating Temperature ◽  
Operating Conditions ◽  
Critical Parameters ◽  
Working Fluid ◽  
Inlet Temperature ◽  
Present System ◽  
High Heating ◽  
Pressure Medium ◽  
Polygeneration System ◽  
Temperature Levels

The objective of the present work is the investigation of a novel polygeneration system for power, refrigeration and heating production at two temperature levels. The present system uses CO2 as the working fluid, which is an environmentally friendly fluid. The total configuration is a combination of a transcritical refrigeration cycle coupled to a Brayton cycle with recompression, which is fed by a biomass boiler. The examined system, at nominal operating conditions, produces refrigeration at 5 °C, and heating at 45 °C and 80 °C. Additionally, the system can be converted into a trigeneration system where the two heating outputs are produced at the same temperature level. The system was studied parametrically by changing the following seven critical parameters: turbine inlet temperature, high pressure, medium pressure, heat exchanger effectiveness, refrigeration temperature, heat rejection temperature and high heating temperature. In nominal operating conditions, the system energy and exergy efficiencies were 78.07% and 26.29%, respectively. For a heat input of 100 kW, the net power production was 24.50 kW, the refrigeration production was 30.73 kW, while the low and high heating production was 9.24 kW and 13.60 kW, respectively. The analysis was conducted with a developed model in Engineering Equation Solver.

scholarly journals Effects of turbulence models and grid densities on computational accuracy of flows over a vertical axis wind turbine

2018 ◽  
Vol 7 (3) ◽  
pp. 213-222
Author(s):  
Jaruwan Chaiyanupong ◽  
Tawit Chitsomboon
Keyword(s):  
Shear Stress ◽  
Wind Turbine ◽  
Turbulence Model ◽  
Turbulence Models ◽  
Vertical Axis ◽  
Operating Conditions ◽  
Speed Ratio ◽  
Vertical Axis Wind Turbine ◽  
Computational Accuracy ◽  
Shear Stress Transport

Flows through a vertical axis wind turbine (VAWT) are very complex due to their inherent unsteadiness caused by large variations of the angle of attacks as the turbine is rotating and changing its azimuth angles simultaneously. In addition, a turbine must go through a wide range of operating conditions especially the change in blade speed ratio (BSR). Accurate prediction of flows over VAWT using Reynolds-Averaged Navier-Stokes (RANS) model needs a well-tested turbulence model as well as a careful grid control around the airfoil. This paper aimed to compare various turbulence models and seek the most accurate one. Furthermore, grid convergence was studied using the Roache method to determine the sufficient number of grid elements around the blade section. The three-dimensional grid was generated by extrution from the two-dimensional grid along with the appropriate y+ controlling. Comparisons were made among the three turbulence models that are widely used namely: the RNG model, the shear stress transport k-ω model (SST) and the Menter’s shear stress transport k-ω model (transition SST). Results obtained clearly showed that turbulence models significantly affected computational accuracy. The SST turbulence model showed best agreement with reported experimental data at BSR lower than 2.35, while the transition SST model showed better results when BSR is higher than 2.35. In addition, grid extruding technique with y+ control could reduce total grid requirement while maintaining acceptable prediction accuracy.Article History: Received April 15th 2018; Received in revised form June 16th 2018; Accepted September 17th 2018; Available onlineHow to Cite This Article: Chaiyanupong,J and Chitsomboon, T. (2018) Effects of Turbulence Models and Grid Densities on Computational Accuracy of Flows Over a Vertical Axis Wind Turbine. Int. Journal of Renewable Energy Development, 7(3), 213-222.http://dx.doi.org/10.14710/ijred.7.3.213-222

Turbulence Modeling of Forced Convection Heat Transfer in Two-Dimensional Ribbed Channels

2008 ◽  
Vol 130 (3) ◽  
Author(s):  
E. Elsaadawy ◽  
H. Mortazavi ◽  
M. S. Hamed
Keyword(s):  
Heat Transfer ◽  
Thermal Analysis ◽  
Turbulence Model ◽  
Turbulence Modeling ◽  
Convection Heat Transfer ◽  
Turbulence Models ◽  
Experimental Results ◽  
Electronic Systems ◽  
Sst Turbulence Model ◽  
Stress Models

Although the problem of 2D ribbed channels has been studied heavily in the literature as a benchmark or basic case for cooling of electronic packing, there is still a contradiction in the literature about the suitable turbulence model that should be used in such a problem. The accuracy of the computational predictions of heat transfer rates depends mostly on the choice of the proper turbulence model that is capable of capturing the physics of the problem, and on the corresponding wall treatment. The main objective of this work is to identify the proper turbulence model to be used in thermal analysis of electronic systems. A number of available turbulence models, namely, the standard k-ε, the renormalization group k-ε, the shear stress transport (SST), the k-ω, and the Reynolds stress models, have been investigated. The selection of the most appropriate turbulence model has been based upon comparisons with both direct numerical simulations (DNSs) and experimental results of other works. Based on such comparisons, the SST turbulence model has been found to produce results in very good agreement with the DNS and experimental results and hence it is recommended as an appropriate turbulence model for thermal analysis of electronic packaging.

scholarly journals Analyzing the Turbulent Flow Characteristics by Utilizing k-ϵ Turbulence Model.

2017 ◽  
Vol 2 (11) ◽  
pp. 28
Author(s):  
Md. Safayet Hossain ◽  
Md. Ishtiaque Hossain ◽  
Somit Pramanik ◽  
Dr. Jamal Uddin Ahamed
Keyword(s):  
Reynolds Number ◽  
Turbulent Flow ◽  
Turbulence Model ◽  
Flow Characteristics ◽  
Turbulence Models ◽  
Skin Friction Coefficient ◽  
Working Fluid ◽  
Turbulent Dissipation ◽  
Quadrilateral Elements ◽  
Fully Developed Turbulent Flow

This study attempts to illustrate the behavior of a fully developed turbulent flow by using k-ε turbulence model. A two dimensional smooth bend channel is adopted for this experiment and water was chosen as working fluid. The Reynolds number was gradually increased to predict the diversity in turbulent kinetic energy (TKE), turbulent dissipation rate, turbulent intensity and eddy viscosity. Primarily the flow has been solved by employing three distinct k-ϵ turbulence models namely, Standard, Renormalization-group (RNG) and Realizable model. After experimenting with ten different sample (from 74E03 to 298E03) of Reynolds numbers, each of these analyses explicitly showed that Standard k-ε model gives much higher value of any aforementioned turbulent properties with respect to other two equation turbulence models. Later it’s been discovered that TKE obtained from Standard k-ω model is almost same as Realizable k-ε model (for Re=298E03, the difference is about 1.8%). It has been observed that the skin friction coefficient at the bend region obtained from different two equation models (Standard, Realizable and RNG k-ϵ model and Standard k-ω model) are almost similar to each other for each sample of Reynolds number. Quadrilateral elements were taken into consideration for grid generation in this analysis. Also, to decrease cost and to achieve further accuracy as well as reduced time consumption mapped faced meshing was utilized.

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