CFD Techniques Research Articles

Stability analysis of stratified Rayleigh–Bénard–Poiseuille convection: Influence of the shear flow

The presence of horizontal flow in stratified systems where a vertical temperature gradient leads to heat transfer through natural convection can be observed in several technological and natural phenomena. In the study reported herein, a linear stability analysis using normal modes and the direct simulation of the governing equations using CFD techniques are applied to investigate the influence of the horizontal flow intensity on the onset of natural convection in a double-layer system heated from below. The results obtained with the two methodologies are in good agreement and complement each other, since, while linear analysis is suitable for defining the critical Rayleigh values, direct simulations allow a detailed analysis of the flow field when the convective motion is fully developed. Due to the large number of phenomena governing the system stability, this study focuses on a part of the spectrum of parameters selected to allow the determination of the minimum values that the Rayleigh number must achieve in order to make natural convection possible. The aim of the study is to gain a better understanding of some basic characteristics of the flow, such as the influence of the boundary conditions and the most common ways in which the convective cells can develop. The results are consistent with previously published data for double-layer Rayleigh–Bénard and single-layer Rayleigh–Bénard–Poiseuille convection. However, the existence of different modes that can make the system unstable creates a more complex scenario for some intervals of the governing parameters.

Effectiveness of meta-models for multi-objective optimization of centrifugal impeller

The major issue of multiple fidelity based analysis and optimization of fluid machinery system depends upon the proper construction of low fidelity model or meta-model. A low fidelity model uses responses obtained from a high fidelity model, and the meta-model is then used to produce population of solutions required for evolutionary algorithm for multi-objective optimization. The Pareto-optimal front which shows functional relationships among the multiple objectives can produce erroneous results if the low fidelity models are not well-constructed. In the present research, response surface approximation and Kriging meta-models were evaluated for their effectiveness for the application in the turbomachinery design and optimization. A high fidelity model such as CFD technique along with the meta-models was used to obtain Pareto-optimal front via multi-objective genetic algorithm. A centrifugal impeller has been considered as case study to find relationship between two conflicting objectives, viz., hydraulic efficiency and head. Design variables from the impeller geometry have been chosen and the responses of the objective functions were evaluated through CFD analysis. The fidelity of each meta-model has been discussed in context of their predictions in entire design space in general and near optimal region in particular. Exploitation of the multiple meta-models enhances the quality of multi-objective optimization and provides the information pertaining to fidelity of optimization model. It was observed that the Kriging meta-model was better suited for this type of problem as it involved less approximation error in the Pareto-optimal front.

Multiple surrogate based optimization of a bidirectional impulse turbine for wave energy conversion

Oscillating water column based wave energy extracting system has a low efficiency due to the poor performance of its principal power extracting component, the bidirectional turbine. In the present work, flow over a bidirectional impulse turbine was simulated using CFD technique and optimized using multiple surrogates approach. The surrogates being problem dependent may produce unreliable results, if a wrong surrogate is selected. Hence, multiple surrogates such as response surface approximation, radial basis function, Kriging and weighted average surrogates were incorporated in this problem. Same design points were used to generate multiple optima via multiple surrogates to enhance the robustness of the optimization process. Numbers of guide vanes and rotor blades were chosen as the design variables, and the objective was to maximize the blade efficiency. Reynolds-averaged Navier–Stokes equations were solved for analyzing the flow physics. The computed results were used to train the surrogates and find the optimal points via hybrid genetic algorithm. The surrogates were further applied to find the optimal flow parameters by changing flow velocity and turbine speed. The relative efficiency enhancement through our present approach was about 16%. Detailed methodologies, analysis of the results and surrogate applicability have been presented in this paper.

Numerical predictions of flow boiling characteristics: Current status, model setup and CFD modeling for different non-uniform heating profiles

A detailed analysis of two-phase flow boiling characteristics inside high pressure systems is presented focusing on non-uniform axial heating profiles. For this purpose, a detailed numerical model has been developed after presenting the current status of the use of CFD techniques in flow boiling predictions. User defined functions written in C++ were compiled and hooked to the software in order to account for mass interaction between phases using the Eulerian multiphase flow model. The modeled domain is a 2 m long stainless steel pipe with inside and outside diameters of 15.4 mm, 25.4 mm, respectively. The base imposed uniform heat flux was 345.6 kW/m2, for mass flow rate of water of 0.161 kg/s and at a temperature of 200 °C. The model was validated against range of experimental data and the results are very promising for the use of CFD in flow boiling characterization. Effects of increasing uniform heat flux were considered for different increments of 30, 50 and 75% as reference to the basic applied heat flux. The influences of heat flux profile in the axial direction were investigated while maintaining the same total power. Different heat flux profiles of linearly increasing, linearly decreasing, sine, and cosine shapes were considered.

Mean-line modeling and CFD analysis of a miniature radial turbine for distributed power generation systems

Distributed power generation (DPG) based on organic Rankine cycle offers potential in the effective use of energy from low grade heat sources up to 2008C. In this regard, developing an effective expander plays a major role in determining the overall cycle efficiency. In this work mean-line modeling and CFD techniques are employed to develop a small-scale radial turbine for DPG systems with a power output of 5 kWe. A parametric study is carried out using the mean-line approach to investigate the effects of key input parameters such as operating conditions, velocity ratio, rotational speed and rotor flow angles on the turbine rotor inlet diameter and overall performance. Results from the mean-line approach show that in order to achieve high power output, inlet total temperature, mass flow rate and pressure ratio should be increased. However, for reducing the rotor inlet diameter the velocity ratio should be decreased. CFD technique is then used to assess the flow field and to improve the blade loading by modification of blade angle distribution. CFD is also used to determine the minimum number of rotor blades and the results show that the value suggested by mean-line modeling overestimates this parameter. By using these two approaches a wide range of design configurations are explored and the most effective design is identified to be with specific diameter of 4.83 (rotor inlet diameter of 0.0787 m), specific speed of 0.433 (rotational speed of 55 000 rpm), 10 blades and output power of 4.662 kW.

A COMPARATIVE STUDY OF OMEGA RSM AND RNG K-ε MODEL FOR THE NUMERICAL SIMULATION OF A HYDROCYCLONE

The design and optimization of hydrocyclones using CFD techniques are gaining popularity and the key to a successful simulation lies with the accurate description of the high turbulent swirling behavior of the flow. This paper presents a detailed comparison between the Omega RSM and the RNG k–e turbulence model, which are both derived specially for modeling swirling or rotational flow, in the simulation of a hydrocyclone. The predictions of velocity field, volume of vortexes, mass split and turbulent viscosity were obtained and compared. It is showed that in general both models gave similar predictions of the flow field under different inlet velocities, while the predictions of turbulent viscosity and in the core region of hydrocyclone were found more closely aligned with the reality using Omega RSM.

Computational Analysis of Flow Dynamics in a Four-Stroke Internal Combustion Engine

This paper presents a predictive investigation using CFD techniques focusing on the tradeoff between steady-state airflow considerations and unsteady transient pressure wave tuning of a single cylinder IC engine. An axisymmetric CFD model has been developed to initially examine the steady-state flow characteristics generated by different inlet entry radii. This has then been further developed to incorporate the unsteady, transient pressure wave effects caused by the motion of inlet valve and piston. Detailed simulation results, which are validated against experimental data provided by Group Lotus plc, are reported here and quantitative conclusions are drawn for maximising airflow in future inlet manifold design.

Modelling and simulation of carbon dioxide absorption by aqueous MEA solution in hollow fibre membrane contactors

AbstractIn this article, a comprehensive model has been developed for analyzing the transport phenomena in hollow fibre membrane contactors operated under non‐wetted or partially wetted conditions. In this regard, the dynamic behaviour of membrane contactors, considering the entrance regions of momentum, thermal energy, and mass transfers, was investigated carefully. Moreover, effects of temperature distribution on the contactor efficiency were examined by taking into account the influences of heat of solution, heat of reaction, and the viscous dissipation. The chemical system studied was a gas sweetening process, including methane and carbon dioxide as the gaseous mixture, and MEA aqueous solution as the solvent. CFD techniques have been used to solve the nonlinear governing equations simultaneously, and the model predictions were validated against reported experimental data in the literature, and excellent agreement was found. Comparing the model predictions with those of previous models shows relatively better accuracy of the model predictions. Furthermore, distributions of velocity, temperature, and species concentrations in the tube side, shell side, and the membrane were obtained and the effects of various operating and design parameters such as wetting fraction, gas and liquid inlet velocities, inlet temperature of the solvent, MEA concentration, and CO2 volume fraction of the feed on these distributions were carefully studied. Finally, the dynamic response of the system was investigated by analyzing the contactor's response to different step and pulse changes.

Computational Fluid Dynamics Investigation of Air Cooled Heat Exchangers

Cooling of process streams is a standard operation in many industries, and where possible water cooling is the most cost effective solution. However in areas where water supply is limited Air Cooled Heat Exchangers (ACHE’s) are often the only alternative. The overall coefficient maybe limited by the air side heat transfer. To improve this coefficient the air flow should be evenly distributed and at as high velocity as possible over the whole bundle. The limits are set by the fan power consumption and the generated noise level. The required fan power forms a significant part of running costs. In order to keep these costs as low as possible knowledge concerning the flow distributions within the bundle can be important.Fluid bypass within the tube-side can be of similar importance. In design calculations an equal fluid flow velocity per tube is typically assumed from the total inlet mass flow. However if some of the fluid bypasses the tube bundle due to poor header design lower than expected fluid velocities in the tubes will be found, leading to underperforming ACHE’s, since heat transfer and fouling behaviour are influenced by the fluid velocity.Computational Fluid Dynamics (CFD) has been found to be a powerful tool to investigate how fluid flows through a defined geometry. In this paper CFD techniques will be used to investigate the tube side and air side flow of ACHE’s. The main focus will be on finding areas of fluid maldistribution within the air side and the effect of bypass on the tube-side of ACHE’s.As a benchmark the pressure drop results of CFD simulations were compared to correlations available in the public domain and agreed well, giving confidence in the fluid flow patterns produced by the CFD simulations.The effect of the tube-side bypass was found to reduce the duty of the ACHE by 27 % in the worst case scenario that was tested. Maldistribution of the air-side flow was found in the ACHE, with the number of fans and depth of the plenum influencing the amount seen.

Optimization of finned-tube heat exchangers for diesel exhaust waste heat recovery using CFD and CCD techniques

In this paper, response surface methodology (RSM) based on central composite design (CCD) is applied to obtain an optimization design of finned type heat exchangers (HEX) to recover waste heat from the exhaust of a diesel engine. The design is performed for a single point operation (1600rpm and 60Nm) of an OM314 diesel engine obtained from experimental measurements. Based on the CCD principle, fifteen HEX cases with different fins height, thickness and number are modeled numerically and the optimization is done to have the maximum heat recovery amount and minimum of pressure drop along the heat exchanger.

Feasibility analysis on natural ventilation scheme for large underground spaces based on the top cover design

A natural ventilation scheme was proposed for large underground spaces based on the top cover design. This is regarded as a design towards ‘green’ or ‘sustainable’ building. The main factors influencing the natural smoke extraction efficiency were studied by using orthogonal design method and CFD technique. A new model was built to estimate the feasibility of the natural ventilation scheme in case of fire accidents. Results show that when fire occurs in underground part of the building, smoke can be exhausted to the interlayer, which is added between the underground part and its top cover, and then the smoke spread along the ceiling of the interlayer to its open boundaries. When working together with smoke shafts, smoke screens can limit the spread of smoke and improve the smoke exhausting efficiency of shafts. In addition, whether the smoke can be vented above specified safety height both in underground part and in the interlayer can be theoretically estimated by the judging model constructed. In order to ensure safety smoke layer height, the size of the underground part is limited when the fire power is determinate. And it showed linear relationship between critical underground part size and critical fire power. The slop of the line decreases when smoke screens are added.

General prediction of the thermal hydraulic performance for plate-fin heat exchanger with offset strip fins

Abstract The heat transfer and flow friction coefficients for offset strip fins (OSF) used in plate-fin heat exchangers are numerically investigated with well validated 3D models. The effect of the geometrical three-dimensionality is analyzed in a distinctive way to understand the general behavior of the thermal hydraulic performance. Based on the analysis of a large amount of numerical data by CFD techniques for OSF fins, a couple of new correlations for general prediction of the j and f factors are developed, which excellently correlate a variety of geometrical parameters with blockage ratio ranging from 10% to 60%. Moreover, 252 experimental data-points for j and 290 experimental data-points for f covering 24 OSF fins are collected from literatures to examine the predicting abilities of the proposed correlations. The comparisons show that the proposed correlations can present the basic behavior of experimental data with nice accuracy. Further comparisons with the previous correlations from literatures have been done with regards to the same experimental data, which reflect that the proposed ones provide well-adapted predictions for OSF fins with different fin thickness covering a broad range of blockage ratio, while the previous correlations express growing prediction deviations as the blockage ratio increases. On the whole, the proposed correlation predicts 92.5% of the j data and 90.3% of the f data within 20%, with the RMS errors of less than 15%. The present work might be very helpful to guide the optimum design of plate-fin heat exchangers and also evaluate the performance of heat transfer enhancement in the OSF fin channels.

Integrated analysis on static/dynamic aeroelasticity of curved panels based on a modified local piston theory

A flow field modified local piston theory, which is applied to the integrated analysis on static/dynamic aeroelastic behaviors of curved panels, is proposed in this paper. The local flow field parameters used in the modification are obtained by CFD technique which has the advantage to simulate the steady flow field accurately. This flow field modified local piston theory for aerodynamic loading is applied to the analysis of static aeroelastic deformation and flutter stabilities of curved panels in hypersonic flow. In addition, comparisons are made between results obtained by using the present method and curvature modified method. It shows that when the curvature of the curved panel is relatively small, the static aeroelastic deformations and flutter stability boundaries obtained by these two methods have little difference, while for curved panels with larger curvatures, the static aeroelastic deformation obtained by the present method is larger and the flutter stability boundary is smaller compared with those obtained by the curvature modified method, and the discrepancy increases with the increasing of curvature of panels. Therefore, the existing curvature modified method is non-conservative compared to the proposed flow field modified method based on the consideration of hypersonic flight vehicle safety, and the proposed flow field modified local piston theory for curved panels enlarges the application range of piston theory.

Stability and Drag Analysis of Wheeled Amphibious Vehicle Using CFD and Model Testing Techniques

Amphibious design of combat vehicle has become a challenging task in the context of increase in Gross Vehicle weight (GVW) of present generation combat vehicles due to demand for high protection levels and higher capacity engine and transmission, incorporation of multiple weapon systems, increased ammunition storage and larger addition of electrical and electronic items. Development of combat vehicles is complex and very expensive, and normally limited with less number of prototypes. The scale modeling and CFD analysis offers a viable solution to accomplish the amphibian design of a combat vehicle with adequate confidence before manufacturing the actual prototype. In the present work, an approach involving experimental towing test using scaled model and CFD simulation has been used to carry out the amphibious design of an 8X8, wheeled, combat vehicle with GVW of 22 ton. In this work, a 1/5th scaled model of the vehicle was manufactured and tested in the towing tank at different test speeds for drag and stability analysis. CFD analysis was carried out on the full scale model to gain adequate details about the dynamics of vehicle in the water in addition to drag estimation. Good correlation has been found in drag values and the flow patterns obtained from towing tank tests and CFD simulations.

English

The purpose of this paper is to present a 2D study regarding the numerical simulation of flow within a transonic highly-loaded rotating cascade from an axial compressor. In order to describe an intricate flow pattern of a complex geometry and given specific conditions of cascade's loading and operation, an appropriate accurate flow model is a must. For such purpose, the Navier-Stokes equations system was used as flow model; from the computational point of view, the mathematical support is completed by a turbulence model. A numerical comparison has been performed for different turbulence models (e.g. KE, KO, Reynolds Stress and Spallart-Allmaras models). The convergence history was monitored in order to focus on the numerical accuracy. The force vector has been reported in order to express the aerodynamics of flow within the rotating cascade at the running regime, in terms of Lift and Drag. The numerical results, expressed by plots of the most relevant flow parameters, have been compared. It comes out that the selecting of complex flow models and appropriate turbulence models, in conjunction with CFD techniques, allows to obtain the best computational accuracy of the numerical results. This paper aims to carry on a 2D study and a prospective 3D will be intended for the same architecture.

Overview of Coănda Jet and Vortex Cell Circulation Control

Flow control for enhanced aerodynamic performance have been utilized for enhanced lift, reduced drag and delay of stall at higher angle of attack. To this end, techniques such as continuous, synthetic and pulsed jets, compliant surface, vortex-cell, Coănda jet and the like have been developed and applied to alter the behavior of airfoils, wings, and bodies and other aerodynamic components. Two such techniques are here assessed as potentially promising technique in the framework of airfoil surface blowing circulation enhancement, i.e. Coănda-jet circulation enhancement and vortex-cell. Their progress and development in influencing the effectiveness and configuration of airfoil surface blowing for circulation enhancement of aerodynamic surfaces are discussed, using fundamental principles and CFD technique.

Using CFD Technique to Investigate the Effect of Tire Roiling-Noise with Different Pattern Design

In general, when evaluating the effect of a sedan vehicle’s tire tread pattern design on the noise generated, the noise is measured by first carving the tires and then using expensive laboratory equipment to perform the measurements. The advantages of using this method are that the experimental results and the noise level produced from the tires (that go into mass production) feature minimal differences and that reliable tested data are produced. The disadvantages of this method are that molds must be created to produce slicks, and that substantial time is invested in the process of artificial carving and performing subsequent experiments, all of which increase the tire development time and cost. Therefore, the effect of a sedan vehicle’s tire tread pattern design on rolling noise was predicted in this study by employing the numerical calculation methods used in calculating fluid mechanics. The experimental results obtained by the Automotive Research & Testing Center (ARTC) were used to test the improved tread pattern designs to validate the reliability of the method proposed in this study and that the tire development time can be effectively shortened

Investigation of Temperature and Air Flow inside Para Rubber Greenhouse Solar Dryer Incline Roof Type by Using CFD Technique

Para rubber solar greenhouse dryer is a one method of drying and it can be responded to requirement of agriculturist on amount of drying product. Present, the dryer was being built many type for made drying agriculture product. The air circulation and temperature scatter havent been used for the dryer built process, which are cause of fungus on product surface and product dehydrate not all over. Therefore, the objective of this research is to study temperature distributes and air circulations inside Para rubber solar greenhouse dryer of incline roof type using computational fluid dynamics method. The result showed that temperature and velocity of air were good distributing all area and havent air cluster under the roof.

Investigations on the ability of di-isopropanol amine solution for removal of CO2 from natural gas in porous polymeric membranes

In this study, capture of CO2 and H2S from natural gas mixture using porous polymeric membranes has been investigated numerically to assess the capacity of a novel absorbent, di-isopropanol amine (DIPA), in CO2 removal. Diffusion of acid gases through porous polymeric membranes was simulated by employing CFD techniques and considering a gas feed stream, a porous membrane and a reaction medium. For solving conservation equations, finite element method was applied to calculate the rate of CO2 and H2S absorption in the membrane. The type of membrane in this work is a hollow-fiber module. According to the modeling results, a high H2S removal can be achieved by DIPA absorber. Moreover, CO2 was captured from natural gas in an efficient manner in low gas/liquid flow rates. POLYM. ENG. SCI., 55:598–603, 2015. © 2014 Society of Plastics Engineers

A mass-conserving axisymmetric multiphase lattice Boltzmann method and its application in simulation of bubble rising

In many lattice Boltzmann studies about bubble rising, mass conservation is not satisfactory and the terminal bubble rising shape or velocity is not so consistent with experimental data as those obtained through other CFD techniques. In this paper, based on the multiphase model (He et al., 1999 [1]), a mass-conserving axisymmetric multiphase lattice Boltzmann model is developed. In the model, a mass correction step and an effective surface tension formula are introduced into the model. We demonstrate how the macroscopic axisymmetric Cahn–Hilliard equation and Navier–Stokes equation are recovered from the lattice Boltzmann equations through Chapman–Enskog expansion. The developed model is applied to simulate the bubble rising in viscous fluid. The mass correction step in our scheme significantly improves the bubble mass conservation. The surface tension calculation successfully predicts the terminal bubble shapes and reproduces the effect of initial bubble shape. The terminal bubble rising velocities are very consistent with experimental and numerical data in the literature. Qualitatively, the wakes behind the bubbles also agree well with experimental data. This model is useful for predicting the axisymmetric two-phase flows.