Standard K-epsilon Research Articles

Flow field optimizing study of low temperature economizer in a coal-fired boiler

The complex structure of double inlet gas flue has a significant influence on gas flow field distribution in a 1000MW coal-fired boiler’s low temperature economizer. In order to optimize gas flow field of the low temperature economizer with double inlet gas flues and reduce its failure rate, this paper presents a flow field simulation of the low temperature economizer based on the computational fluid dynamics (CFD). This numerical simulation was operated by using porous media model instead of the complex structure inside heat exchanger and the standard k-epsilon model. Velocity contours of a same vertical cross-section inside of inlet gas flue of the heat exchange zone in different numerical simulations were evaluated by the evaluation standard of RSM. The results of numerical simulation show that the main reasons for uneven distribution of flow field in economizer and its inlet gas flues are unequal diameter of flue elbow and straight flue, unreasonable setting of guide plate and diffusion flue elbow. After making structural optimization of the low temperature economizer such as equalizing elbow to the straight flue diameter and setting the guide plate reasonably, the flow field in the low temperature economizer and its flues are obviously improved.

Flow field simulation and structural parameter optimisation of vacuum adsorption system for textile fabrics

Vacuum adsorption and grabbing for textile fabrics is one key technology for intelligent garment processing. Owing to the softness and air permeability of textile fabrics, the design of vacuum adsorption grab device has lacked an effective method. In this paper the standard k-epsilon turbulence model is used to analyse the flow field in the suction cavity of vacuum adsorption device for textile fabrics, the optimisation design model of structural parameters is established and solved by the Fluent software, and the verification experiment is carried out. The experimental results show that the suction mechanism with optimised parameters can effectively absorb and grasp the fabric, and the negative pressure required is minimum. The research results show that the structure parameters such as the thickness of the suction cup cavity, the diameter of the suction hole, and the depth of the suction hole affect the adsorption performance of the vacuum adsorption device.

Effect of different turbulence model on prediction of lean blowout limit for micro gas turbine combustor

ABSTRACT This paper aims to present turbulence modelling results of methane/air combustion in a micro gas turbine combustor. Modelling studies were carried out with different turbulence models to evaluate Lean Blowout (LBO) performance on the micro gas turbine combustion chamber by using a commercially available computational fluid dynamics code. Turbulence models implemented in this study are k-epsilon standard, k-epsilon RNG, K-epsilon realisable, k-omega standard, and k-omega SST. A two-dimensional axisymmetric 3 kW swirl stabilised micro gas turbine combustor model was built to investigate the impact of various turbulence models on the prediction of LBO behaviour of the combustor fuelled with methane. Performance evaluation of these models was executed by calculating the average exit gas temperature (AEGT) that have been used for predicting the blowout conditions. Results obtained from this study were validated with experimental results. Numerical results showed that two-equation turbulence models give consistent simulation results with experimental results employing trend and value. Standard k-epsilon was found to give consistent simulation results with experimental data.

Investigating the Effect of Different Parameters on CHTC Using Wind-Tunnel Measurement and Computational Fluid Dynamics (CFD) to Develop CHTC Correlations for Mixed CHTCS

ABSTRACTConvective Heat Transfer Coefficient (CHTC) is a determining factor in building energy simulation (BES) tools for building thermal calculations. The accuracy of CHTC calculation has a direct effect on building energy analysis.This study aims to assess the impact of multiple parameters, namely temperature difference, wind speed, and wind direction on CHTC of building exterior surfaces. Then the overall high accuracy correlation based on these parameters for CHTC is provided. According to the specified values for temperature and velocity, Richardson’s number range from 0.1 to 10, representing a mixed heat transfer. The simulated results are compared with a wind tunnel experiment for validation. The standard k-epsilon model is used for turbulence simulation. Several cases are numerically simulated, considering various velocities, wind directions, and temperature differences. Results indicate that the studied parameters could be ranked as velocity, building orientation, and temperature difference in the order of effectiveness. All of the correlations used in EnergyPlus software for the exterior surface of the building are compared with the presented correlation and simulated data. The comparison shows that the proposed expression could predict CHTC for various angles, velocities, and temperature differences with an error of below 3%.

Coarse-grid simulations of full-loop gas-solid flows using a hybrid drag model: Investigations on turbulence models

The effect of fluid turbulence models on the coarse-grid simulation for the mesoscale gas-solid flow system is not as clear as that for gas-liquid/single flows. In this study, the effect of different turbulence models on predictions of riser-only, turbulent and bubbling flow hydrodynamics is quantified. Then the selected turbulence models are examined by coarse-grid simulations in a full-loop circulating fluidized bed (CFB) with a proposed hybrid drag model which integrates two SGMs and Hulin-Gidaspow model. Results show that the standard k-epsilon and k-omega models fail to capture a correct fluidization pattern in riser. Meanwhile, the RNG and realizable k-epsilon models and SST k-omega model enable satisfactory predictions which are highly comparable to discrete model predictions and experiments over different flow regimes. However, the realizable k-epsilon model produces more pronounced flow fluctuations causing computational instability. Using superior turbulence closures, full-loop simulations with the proposed hybrid model can predict desirable hydrodynamics.

COMPARISON OF STANDARD k-epsilon AND SST k-omega TURBULENCE MODEL FOR BREASTSHOT WATERWHEEL SIMULATION

Currently, Computational Fluid Dynamics (CFD) was utilized to predict the performance, geometry optimization or physical phenomena of a breastshot waterwheel. The CFD method requires the turbulent model to predict the turbulent flow. However, until now there is special attention on the effective turbulent model used in the analysis of breastshot waterwheel. This study is to identify the suitable turbulence model for a breatshot waterwheel. The two turbulence models investigated are: standard k-epsilon model and shear stress transport (SST) k-omega. Pressure based and one degrees of freedom (one-DoF) feature was used in this case with 75 Nm, 150 Nm, 225 Nm and 300 Nm as preloads. Based on the results, the standard k-epsilon model gave similar result with the SST k-omega model. Therefore, the simulation for breastshot waterwheel will be efficient if using the standard k-epsilon model because it requires lower computational power than the SST k-omega model. However, to study about physical phenomenon, the SST k-omega model is recommend.

A study effects of injection pressure and wall temperature on the mixing process of NOx and NH3 in Selective Catalytic Reduction system

Diesel engines are commonly used for public transportation on-road and off-road applications. Growth production of the diesel engine is very significant from year to year. Nitride Oxide (NOx) from diesel engine was one of the major sources of air pollution. Selective Catalytic Reduction (SCR) has been successfully used to reduce NOx from a diesel engine with a chemical reaction from ammonia (NH3). The mixing reaction between NOx and NH3 reaction can produce steam (H2O) and Nitrogen (N2). However, ammonia uniformity pattern usually not homogenization and the ammonia was difficult to mix with NOx. The constant air flows incomplete to assist the spray injector to spread NH3 to all corners of SCR. The impact study of turbulent phenomena and standard k-epsilon Low-Reynolds Number model to the mixing process in the SCR system using STARCCM+. The simulation studies are conducted under different pressure (4 to 6 bars), the injection rate (0.04 g/s) and temperature (338 K – 553 K) and the high pressure and high velocity magnitude creating turbulent swirl flow. The ammonia decomposition process and mixing process with NOx were investigated using a box with optical access. The simulation and numerical study results validated using back pressure value and the distribution of NOx concentration value from the catalyst outlet. The wall temperature will increase the urea evaporation to generate ammonia and gas pressure will increase the mixing process and chemical process in the SCR system. These reactions enable to optimize the SCR system technology which eventually able to reduce the NOx quantity from a diesel engine.

Research on mixed gas filling process in laser transmission pipeline

Aiming at the problem of dynamic displacement of mixed gas in laser transmission pipeline, by selecting a typical section of the pipeline, the problem can be simplified to study the filling of binary mixed gas in a relatively closed cavity with the single entrance and exit. The finite volume method is used to simulate the flow field of this section of pipeline, and the velocity distribution of the binary mixed gas filling process is simulated by using the incompressible N-S equation and the standard k-epsilon turbulence model; the molar concentration distribution of the binary mixed gas over time is simulated. Furthermore, the distribution of flow field and the change of gas composition inside the pipeline were calculated under different inlet velocity conditions. Through the above simulations, the change of flow field distribution in the pipeline with time is described; the formation, development and extinction process of vortex at different time or at different positions in the pipeline are simulated. Based on the works above, the basic research idea and theoretical basis of the study on the mixed gas filling progress of large scale laser transmission pipeline are provided. By comparing and analyzing the characteristics of flow field distribution at different inlet velocity or at different time periods of the same inlet velocity, it will also have certain reference value to guide the implementation and optimization of the gas filling project of laser transmission pipeline.

Numerical parametric study on three-dimensional rectangular counter-flow thrust vectoring control

Recently, fluidic thrust vectoring control is popular for micro space launcher propulsion systems due to its several advantages, such as fast dynamic responsiveness, better control effectiveness, and no moving mechanical equipment. Counter-flow thrust vectoring control is an especially effective technique by utilizing less suction flow to control the mainstream deflection flexibly. In the current work, theoretical and numerical analyses are performed together to elaborate on the performance of the three-dimensional rectangular counter-flow thrust vectoring control system. A new propulsion nozzle of Mach 2.5 is devised by method of characteristics. To testify the feasibility and accuracy of the present research methodology, numerical results were validated against experimental data from the open literature. The computational result using the standard k-epsilon turbulence model reveals a good match with experimentally measured static pressure values along the centerline of the upper suction collar. The influence of several key parameters on vectoring performance is investigated herein, including the mainstream temperature, collar radius, horizontal collar length, and gap height. Critical parameters have been quantitatively analyzed, such as static pressure distribution along the centerline of the upper suction collar, pitching angle, suction mass flow ratio, and thrust coefficient. Furthermore, the flow-field features are qualitatively expounded based on the static pressure contour, streamline, iso-turbulent kinetic energy contour, and iso-Mach number contour. Some important conclusions are offered for further studies. The mainstream temperature mainly affects different dynamic characteristics of the mixing shear layer, including the convective Mach number of the shear layer, density ratio, and flow velocity ratio. The collar radius influences the pressure gradient near the suction collar surface significantly. The pitching angle increases rapidly with the increasing collar radius. As the horizontal collar length increases, the systematic deflection angle initially increases then decreases. The highest pitching angle is obtained for L/ H = 3.53. With regard to the gap height, a larger gap height can achieve a higher pitching angle.

Effect of Flow Separation Control with Suction Velocity Variation: Study of Flow Characteristics, Pressure Coefficient, and Drag Coefficient

Flow separation is expected to have the effect of increasing aerodynamic drag due to decreased pressure distribution at the rear of the vehicle. The faster the flow separation occurs, the lower the pressure distribution is in the area, thereby reducing vehicle performance. Therefore, flow modification is needed with expected effects on the separation delay and the reduction in wake and vortex formation. This modification can be done through the application of suction active control in the separation area. The research is intended to analyze the effect of suction active control on flow characteristics, pressure distribution and aerodynamic drag on vehicle models with suction velocity variations. The test model used is an Ahmed model modified by changing the orientation of the flow. The study used a numerical computational approach with a standard k-epsilon turbulence model at 19.4 m/s upstream velocity. Results revealed that the use of flow active control was able to reduce wake and vortex formation through separation delay and to increase the minimum pressure coefficient by 73% on the model with Usc₂ suction velocity of 0.5 m/s, gaining the highest drag coefficient reduction of 10.897% in the same model.

Hydrodynamic Behavior in Solar Oil Heat Exchanger Ducts Fitted with Staggered Baffles and Fins

The attachment of turbulators, such as baffles, fins, ribs, bars, and blocks, inside the thermal solar receiver ducts, is among the most effective mechanisms for important thermal exchange by creating the turbulence, extending the trajectory of the flow, increasing the surface of heat exchange, forcing recycling cells, and hence a high thermal exchange. The solar finned and baffled heat exchangers are employed in a wide application interval, and it is important to examine the design of a duct for this configuration of the flow field and its effect on the heat transport phenomenon. In This study, dynamic field simulations are reported in horizontal rectangular form ducts, using three obstacles with oil HTF (heat transfer fluid). Two various finned and baffled duct configurations are treated, i.e., case (A) with one fin and two baffles, and case (B) with two fins and one baffle. Different hydrodynamic fields, i.e., X-velocity and Y-speed, as well as various X-velocity profiles in many flow stations, related to Re value, are analyzed. A computational approach is applied in order to simulate the oil flow, using finite volume (FV) integration method, SIMPLE discretization algorithm, QUICK interpolation scheme, Standard k-epsilon turbulence model, and ANSYS FLUENT 12.0 software. Simulation results reported an unstable flow structure, with powerful recycling cells, on the backsides of each fin and baffle, as a result of fluid detachment at their upper front sharp edges, in both studied models (A and B). As expected, the first duct model, i.e., Case A, has better X- and Y-velocity values, due to its large recirculation regions. In This paper, many physical phenomena, such as the turbulence, instability, flow separation, and the appearance of reverse secondary currents, are reported. As its data confirmed by many previous numerical and experimental results, the suggested new models of finned and baffled heat exchangers filled with high thermal conductivity oil, allow an improvement in the dynamic thermal-energy behavior of many thermal devices such as flat plate solar collectors.

Experimental and computational fluid dynamics modeling of a novel tray humidifier column in humidification dehumidification desalination; evaluation of hydrodynamic and heat transfer characteristics

In this article, a novel tray humidifier column for humidification dehumidification desalination was proposed. The performance of the humidifier column has been investigated with experimental and computational fluid dynamics simulations. The hydrodynamics and heat transfer characteristics of this tray humidifier has been studied. A stainless steel sieve tray with a rectangular cross section with a dimension of 20 × 50 cm was used in the experimental study. In computational fluid dynamics modeling, a transient three-dimensional model has been developed based on the volume of fluid framework by using standard k-epsilon model. The effect of air and seawater flow rate and inlet seawater temperature on the exit air temperature has been investigated. The results show that the humidifier effectiveness of the tray humidifier column varies between 0.67 and 0.87 depending on operating conditions. Then, tray column can be used in humidification dehumidification desalination systems with advantages such as compact equipment, low-pressure drop, and handling solids or other sources of fouling.

Hydrodynamic and thermal analysis of water, ethylene glycol and water-ethylene glycol as base fluids dispersed by aluminum oxide nano-sized solid particles

PurposeThe purpose of this paper is to carry out a hydrodynamic and thermal analysis of turbulent forced-convection flows of pure water, pure ethylene glycol and water-ethylene glycol mixture, as base fluids dispersed by Al2O3 nano-sized solid particles, through a constant temperature-surfaced rectangular cross-section channel with detached and attached obstacles, using a computational fluid dynamics (CFD) technique. Effects of various base fluids and different Al2O3 nano-sized solid particle solid volume fractions with Reynolds numbers ranging from 5,000 to 50,000 were analyzed. The contour plots of dynamic pressure, stream-function, velocity-magnitude, axial velocity, transverse velocity, turbulent intensity, turbulent kinetic energy, turbulent viscosity and temperature fields, the axial velocity profiles, the local and average Nusselt numbers, as well as the local and average coefficients of skin friction, were obtained and investigated numerically.Design/methodology/approachThe fluid flow and temperature fields were simulated using the Commercial CFD Software FLUENT. The same package included a preprocessor GAMBIT which was used to create the mesh needed for the solver. The RANS equations, along with the standard k-epsilon turbulence model and the energy equation were used to control the channel flow model. All the equations were discretized by the finite volume method using a two-dimensional formulation, using the semi-implicit method for pressure-linked equations pressure-velocity coupling algorithm. With regard to the flow characteristics, the interpolation QUICK scheme was applied, and a second-order upwind scheme was used for the pressure terms. The under-relaxation was changed between the values 0.3 and 1.0 to control the update of the computed variables at each iteration. Moreover, various grid systems were tested to analyze the effect of the grid size on the numerical solution. Then, the solutions are said to be converging when the normalized residuals are smaller than 10-12 and 10-9 for the energy equation and the other variables, respectively. The equations were iterated by the solver till it reached the needed residuals or when it stabilized at a fixed value.FindingsThe result analysis showed that the pure ethylene glycol with Al2O3 nanoparticles showed a significant heat transfer enhancement, in terms of local and average Nusselt numbers, compared with other pure or mixed fluid-based nanofluids, with low-pressure losses in terms of local and average skin friction coefficients.Originality/valueThe present research ended up at interesting results which constitute a valuable contribution to the improvement of the knowledge basis of professional work through research related to turbulent flow forced-convection within channels supplied with obstacles, and especially inside heat exchangers and solar flat plate collectors.

FEM/CFD analysis of wings at different angle of attack

As we are moving towards future, our natural energy resources are exploited day by day so we have to limit our usage for future generation but simultaneously our need is also increasing drastically so we cannot limit our self by using them, but we can modify/alter the design of object so that we can increase the fuel efficiency. As wings are made of Airfoil section, so we can increase the lift force corresponding to drag force generated by the flow of air over airfoil section.The main motive of this study is to compare coefficient of lift and coefficient of drag at different angle of attack for constant air velocity at different airfoil sections provided by The National Advisory Committee for Aeronautics (NACA). We selected NACA 2412, NACA 2414 and NACA 2415 using Computational Fluid Dynamics.To obtain the solution we have to solve steady state governing equations, momentum equations with standard k-epsilon viscous model. The geometry is first imported in design modular of ANSYS, and then import the file into Ansys15 software using Fluent as a solver and CFD – Post as post processor.

Design of coiled tubing torque reducing device

During the operation of coiled tubing drilling, the power motor will produce reactive torque when it works, which will affect the drilling process. A torque reducing device is designed to counteract the torque by injecting drilling fluid. The device is designed as a short section, connected with cardan shaft and drive shaft, the nozzle of which can be automatically adjusted according to the pump pressure. The flow model of the device is established by SolidWorks software, and then leaded in FLUENT software for simulation. The standard k-epsilon model is used to simulate the operating conditions of 5 nozzle diameters and 2 outlet diameters respectively, and the parameters such as pressure drop, torque and flow rate are obtained. The designed torque reducing device can offset about 30% of the torque generated by the screw drill, and the reactive force of single jet is about 152N. If the jet direction is perpendicular to the axis, it can produce the drilling pressure about 3.4kN. If the direction of the nozzle is aligned with the axis, the effect of pressurization and torsion reduction can be produced simultaneously.

CFD analysis on Ranque–Hilsch vortex tube with different cold orifice diameter and cold mass fraction

The vortex tube or Ranque–Hilsch vortex tube is a device that allows hot and cold air to be separated by the tangential flow of compressed air into the vortex chamber through the inlet nozzles and energy separation effects in a vortex tube have been investigated using a computational fluid dynamics model. The governing equations have been solved in an ANSYS Fluent code in a 3D compressible and turbulent model when Standard k-epsilon was used. The effects of geometrical parameters have been investigated. The results showed that the cold end diameter to tube diameter ratio (dc/D) affects the performance of the tube, and this effect changes when operating the tube at different cold mass fraction. The results showed that computational data were in good agreement with the experimental values.

Numerical study of the effect of oxidation zone inlet air temperature variation on municipal solid waste pellet gasification process on downdraft type reactor characteristics

Gasification is an efficient technology to extract energy from MSW biomass into energy in the form of syngas. One of the most influential paramerers to improve syngas quality is temperature, so the development of the design and operation of the downdraft type reactor is done by heating the inlet air of the oxidation zone through an external heater. Research with numerical studies was carried out on the effect of the inlet air in the oxidation zone to determine the temperature distribution and syngas composition along the reactor. Based on experimental research, heating the inlet air of the oxidation zone with external heaters in this study was carried out with 5 temperature variations, namely 80 °C, 110 °C, 150 °C, 180 °C, and 200 °C. The modeling used is the standard model k-epsilon, Radiation P1, the transport species model with turbulence used is finite-rate / eddy-dissipation, and Discrete Phase Model (DPM). The results obtained from this study were that the highest air temperature was obtained at 200°C at 1004 °C in the oxidation zone. The composition of syngas CO, H2, and CH4 at temperature increases of 80-200 °C increased from 21.64%, 9.24%, and 2.86% to 22.65%, 10.49%, and 3.10%. Then increasing the LHV syngas from 4,757.20 kJ/m3 to 5,106.02 kJ/m3. Cold gas efficiency increased from 72.17% to 79.31%.

CFD Analysis of Scramjet Engine Combustion Chamber with Diamond-Shaped Strut Injector at Flight Mach 4.5

The present work address the CFD analysis for scramjet combustor with diamond-shaped strut injectors at supersonic Mach 4.5 and it’s based on species transport combustor which is standard k-epsilon turbulence model. It is implied for supersonic combustion that the ramjet engine is focused for operating high speed engines. The combustion chamber assists as an envelope to clutch the propellant for a satisfactory period to certify complete mixing as well as combustion. The diamond-shaped strut injector gives the maximum temperature and pressure of 3517 K and 1.487 MPa respectively whereas the combustion efficiency is found to be 87.2%. The shear layers losses are extremely minimize the performance of the engine, thus interpreting trade-off programmes in a very difficult way to achieve maximum combustion efficiency. Further firmness by machines is essential for achieving higher combustion efficiency.

Experimental research and the numerical simulation of the non-standard pump for the flow-circulation type reactor

The paper shows the results of the experimental measurements and the numerical simulation of the flow-circulation type reactor pump main characteristics. The mathematical model based on Navier-Stokes equations and standard k-epsilon turbulence model was used for the pump CFD simulation. The pump power consumption was calculated by the proposed CFD post-processing method. It is found that the simulation error of the proposed method is not more than 2.9%. The obtained results include the regression equations of the pump full pressure, the pump feed and the power consumption at the pump cross-points.

Assessment of air flow distribution and hazardous release dispersion around a single obstacle using Reynolds-averaged Navier-Stokes equations

The flow around a cubical building, with a pollution source at the central point of the top of the cube, is studied. The Reynolds-averaged Navier-Stokes and species concentration equations are solved for Reynolds number, Re = 40,000, is based on the height of the cube. The predictions obtained with the standard, the Kato-Launder, and the low-Reynolds number k-epsilon models are examined with various wall functions for the near wall treatment of the flow. Results are compared against Martinuzzi and Tropea measurements (J. of Fluids Eng., 115, 85–92, 1993) for the flow field and against Li and Meroney (J. of Wind Eng. and Industrial Aerodynamics, 81, 333–345, 1983) experiments and Gaussian models for the concentration distribution. It is found that the present unstructured mesh model performs similarly to the structured mesh models. Results from the Kato-Launder model are closer to the experimental data for the flow patterns and contaminant distribution on the cube's roof. However, the Kato-Launder model has an over-prediction for the recirculation zone and the contaminant distribution windward of the cube. The standard k-epsilon and the low-Reynolds number k-epsilon models predict similar flow patterns and are closer to the experimental data of the cube's windward and side face.