CFD Techniques Research Articles

Development of A Process for Predicting the Overall Heat Transfer Coefficient for Transient Heat Transfer in An Exhaust System using CFD

The analysis of heat transfer of automotive exhaust system is most important since their prominence in the design and also in the optimization phase of exhaust after treatment system.This paperdeals with the process which can be useful to predict the overall heat transfer coefficient for the transient flow of pipe in the after treatment system. This considers the convection of heat along gas flow, the convection between gas and wall, conduction through wall, radiation and of course convection to the ambient. Governing equations are obtained for the transient flow in a pipe for calculating gas temperature and wall temperature at distance x and time t. Analytical solution will be computed using CFD techniques for these governing equations. From the obtained analytical solution to the transient flow in pipe an excel tool will be developed which can be able to give the outlet temperature of the pipe in transient flow at length x and time t, total heat loss from pipe to the ambient, overall heat transfer coefficient for the pipe.

Parametric optimization of packed bed for activated coal fly ash waste heat recovery using CFD techniques

Abstract Coal fly ash is an industrial solid waste generated from coal preparation during the processing and cleaning of coal for electric power generation. Comprehensive investigation on the reutilization of waste heat of activated coal fly ash is of great economic significance. The method of recovering the waste heat, proposed in this study, is the transfer of heat from activated coal fly ash to gas with the movement of air using the packed bed, providing valuable energy sources for preheating the raw coal fly ash to reduce the overall energy consumption. The investigation is carried on the heat transfer characteristics of gas–solid (activated coal fly ash) phases and air temperature fields of the packed bed under some key conditions via computational fluid dynamics. A two dimensional geometry is utilized to represent key parts of packed bed. The distribution mechanism of the temperature field for gas phase is analyzed based on the transient temperature contours at different times. The results show that the obtained rule of gas–solid heat transfer can effectively evaluate the influences of operating parameters on the air temperature in the packed bed. Simultaneously, it is found that no temperature differences exist in the hot air at the outlet of the packed bed. The investigation provides guidance for the design and optimization of other similar energy recovery apparatuses in industries.

Numerical and experimental study of the impact of conical chimney angle on the thermodynamic characteristics of a solar chimney power plant

The goal of this paper is to study and optimize the conical chimney angle (α) of a divergent solar chimney power plant (DSCPP) by using CFD technique. The local airflow characteristics were analyzed in four configurations with different conical angles α = 0°, α = 3°, α = 6° and α = 9°. The first design is validated experimentally by using a pilot prototype build at the National School of Engineers of Sfax, Tunisia. In addition, some experimental results of the temperature, the velocity and the power output were presented during a typical day. A novel mathematical correlation was developed to prove the effect of the conical angle and the DSCPP scale on the power output. In fact, the relationship between the optimum conical angle and the system scale was performed based on both quadratic and cubic regressions. The computational results ensure that the conical chimney angle presents a parabolic tendency with the turbulence airflow characteristics and the power output. The performance of the DSCCP was degraded since the conical angle is greater than α = 3°. Furthermore, the optimum angle decreases with an increasing system scale. A commercial solar chimney with a conical angle around α = 1° presents an efficient system.

Low Speed Wind Tunnel Design and Optimization Using Computational Techniques and Experimental Validation

Generally, the experimental aerodynamics is related to wind tunnel experiments. The wind tunnel design topic is very old but the development in computational fluid dynamics led to improvement in the wind tunnel design. This paper describes the design and optimization of low speed wind tunnel using CFD techniques. The new optimum wind tunnel will replace the old one featuring poor air quality and small area with lower wind speed at the test section. A computational domain was generated and adopted using ANSYS mesh generator and the solution domain was analysed by simulation technique using FLUENT CFD code in ANSYS Workbench package. The pressure drop calculations comparison between analytical, computational and experimental is included for different sections in the wind tunnel. The contraction cone was optimized using the response surface technique. The results identified that the pressure drop and turbulence level are modified as compared to the old wind tunnel.

Optimizing Cooling Fan Power Consumption for Improving Diesel Engine Fuel Efficiency Using CFD Technique

Optimizing Cooling Fan Power Consumption for Improving Diesel Engine Fuel Efficiency Using CFD Technique

CFD modeling of liquid film reversal of two-phase flow in vertical pipes

The phenomenon of two-phase flows is characterized by its wide range of presence in nature and industrial applications. In gas and oil production, the simultaneous two-phase flow of gases and liquids often occurs in gas wells. The produced gases flow rate decreases over the years until reaching a critical flow rate when they lose their ability to lift the associated liquids upward and the liquid film reverses direction, which triggers liquid loading. Liquid loading causes the produced liquids to accumulate in the bottom of the wellbore, which causes a high back pressure that reduces the well production rate till production is ceased eventually. The critical gas velocity exists in the churn flow regime which is mainly characterized by the oscillatory behavior of the liquids flow field. This study employs CFD techniques to model the churn flow in a 3-inch-diameter vertical pipe near the critical gas flow rate for different liquid flow rates. This work utilizes the two-fluid Eulerian model along with the RNG (Re-Normalization Group) k-ε turbulence model to investigate the behavior of the flow field in a two-dimensional axisymmetric computational domain. Simulations were carried out using the commercial software ANSYS Fluent 18.2 with air and water as the two working fluids. The model results showed a good agreement with the experimental data and proved the mesh and time independence of the model. Oscillatory behaviors of the liquid film flow rate, shear stress, and pressure were observed along with the formation of interfacial waves. Detailed information about the velocity, shear stress, and pressure behaviors is presented. Accordingly, recommendations are suggested for future considerations.

CFD Analysis of a Buffer Tank Redesigned with a Thermosyphon Concentrator Tube

This study analyzes a buffer tank simulated in both continuous operation mode and heating mode using CFD techniques. The analysis is focused in the thermal behavior of the tank, especially in parameters such as heat exchanged, heating time, and temperature distributions into the tank, in order to propose a better design. The results of the different simulations show that the tank heats water extremely slowly and extremely evenly when producing domestic hot water (DHW), which negatively affects the thermal stratification that is critical for rapidly reaching the DHW temperature. Therefore, the main problem of the tank is an inefficient heat exchange and a poor distribution of temperature. In order to overcome these operational limitations, a new design is proposed by installing a tube inside the tank that encloses the heating coil and sends hot water directly to the tank top region such that high-temperature DHW is rapidly provided, and thermal stratification is improved. Several simulations are performed with different open and closed configurations for the outlets of the inner tube. The different results show that the heating times significantly improve, and the time needed to reach the 45 °C set point temperature is reduced from 44 to 15 min. In addition, the simulations in which the opening and closing of the water outlets are regulated, the outlet DHW temperature is kept within 45–60 °C, which prevents overheating to unsafe use temperatures. Furthermore, the results of the simulation in continuous operation mode show a clear improvement of thermal stratification and an increase in the heat transmitted to the inside of the tank.

Turbulent forced convection in a shell and tube heat exchanger equipped with novel design of wing baffles

PurposeThis work presents a numerical study of the dynamic and thermal behavior of a turbulent flow in a shell and tube heat exchanger equipped with a new design of baffle type wing. The implementation of this type of baffle makes it possible to lengthen the path of the fluid in the shell, to increase the heat flux exchanged on the one hand and is to capture the weakness of the shell and tube heat exchanger with segmental baffles on the other hand.Design/methodology/approachThis paper aims to analyze numerically the thermo-convective behavior of water using CFD technique by solving the conservation equations of mass, momentum and energy by the finite volume method based on the SIMPLE algorithm for coupling velocity-pressure. To describe the turbulence phenomenon, the Realizable k–ε model is employed. The analysis is done for different mass flow rates. The parameters studied are: the fluid outlet temperature, the average heat transfer coefficient, the pressure drop, the total heat transfer rate, the effect of the geometric shape of the baffle on the thermal behavior. The purpose of this study is to propose a new design of a shell and tube heat exchanger with a high heat transfer coefficient and a lower pressure drop compared to a shell and tube heat exchanger with transverse and segmental baffles.FindingsThe results showed that the use of the wing baffles enhanced the heat transfer coefficient significantly and reduced the friction coefficient. Compared with segmental baffles, the wing baffles are 11.67, 18.53 and 11.5 per cent lower in the pressure drop and 1.79, 1.9 and 2.39 per cent higher in the Nusselt number for the three mass flow rates 0.5, 1 and 2 kg/s, respectively.Originality/valueThe originality of this work lies in proposing a three-dimensional analysis for a novel heat exchanger.

Analytical Modeling of Two-Phase Water Transport in a PEMFC

While significant progress has been achieved in PEMFC modeling, simulating liquid water formation and transportation in a PEMFC remains a challenge. Under wet operating conditions, liquid water may condense in the channel, gas diffusion layer, or electrode, which introduces significant complexities in simulating fuel cell performance. There have been several approaches to model two-phase water transport using both macroscopic and microscopic methods. Two-phase macroscopic models are mostly based on volumetric averaged method and solve the governing equation using CFD techniques, which require intensive computational resources. In recent years, microscopic approaches based on Lattice Boltzmann Method (LBM) and pore-network approach have been applied to detailed morphological geometric domain of gas diffusion layer. While microscopic approach provide useful information, they cannot be coupled with a full fuel cell model. Therefore, constructing an effective analytical two-phase model is crucial in developing materials and design to optimizing fuel cell performance. In this work, we present an empirical-based, steady-state, 1-D, non-isothermal, and two-phase PEMFC model that includes full cell geometry and water balance. Limiting current experiments based on Caulk and Baker [1] is employed to study both dry and wet oxygen transport phenomena with wet-proofed Toray-H-060 carbon fiber paper with microporous layer. Specifically, the ratio of tortuosity and porosity under wet and dry conditions are characterized by the limiting current method and incorporated into the model. The two-phase model uses tendril length to represent the liquid water saturation and penetration in the diffusion layer. Figure 1 shows that the newly constructed two-phase model can capture the total transport resistance as a function of limiting current density in the dry, transition, and wet regions well for operating conditions at 300 kPa, 70°C, 80% and 90% relative humidity. This two-phase model also includes a two-phase electrode model, which correlates electrode relative humidity to available ECSA for fuel cell performance simulation. The details of the model and prediction results will be presented at the conference. Referece: 1. Caulk, D.A. and Baker, D.R., 2010. Heat and water transport in hydrophobic diffusion media of PEM fuel cells. Journal of The Electrochemical Society, 157(8), pp.B1237-B1244. Figure 1

Multiple speed integrated optimization design for a SWATH using SBD technique

Hull geometry modification and reconstruction, optimization algorithms and CFD technique are combined together into what is known as simulation-based design (SBD) techniques, and its essence is the hydrodynamic configuration design driven by global flow optimization. The purpose of this paper is to show how the improvement in the hydrodynamics performance of a Small Waterplane Area Twin Hull (SWATH) can be obtained by solving a shape optimization design problem at different speeds using the SBD technique. In this paper, an example of the technique application for the SWATH hull-form optimization at different speeds is demonstrated. In the procedure, the free-form deformation method is chosen to automatically modify the geometry of submerged pontoon, and the multi-objective particle swarm optimization (MOPSO) algorithm is adopted for exploring the design space. The two objectives functions, the total resistance at two different speeds (11 kn and 15 kn), are assessed by RANS solvers. The optimization results show that the decrease in total resistance is significant for the optimization case at two different speeds (11 kn and 15 kn), with a reduction of about 5.1% and 6.3%, respectively. Meanwhile, the displacement increases 3.7% and the transverse section of submerged pontoon becomes “flower vase” type from ellipse. Finally, dedicated experimental campaigns for optimized model have been carried out to validate the computations and establish the effects of the optimization processes. It shows that the computations of optimization scheme mainly fit the results of model test. The given, practical examples demonstrate the practicability and superiority of the proposed SBD technique for the SWATH multiple speed integrated optimization problem. In addition, the technique can also be applied to other fields of shape optimization design.

Numerical study on dynamic thermal characteristics and optimum configuration of internal walls for intermittently heated rooms with different heating durations

Numerical simulations are carried out to investigate the heating load of intermittently heated rooms and dynamic thermal characteristics of the internal walls in residential buildings by employing CFD techniques. The buildings are located in the lower reaches of the Yangtze River region of China. Moreover, thorough experiments on temperature distributions of thermal mass and indoor air were conducted to validate the simulation model. The results show that the heating load caused by the internal walls accounts for more than 65% of the room heating load. Furthermore, for the intermittently heated room with heating duration shorter than 2 h, the heating load of the internal walls increases linearly with the increase of the surface thermal absorption coefficient, which can be reduced by placing thermal insulation on the surface of the internal walls. When the heating duration is longer than 2 h, the heating load of the internal walls grows linearly with the inside thermal storage coefficient, which can be reduced by increasing the thermal resistance of the thermal insulation or decreasing heat capacity of the internal walls.

Numerical Study on the Performance Sensitivity of a Pulse Tube Cryocooler with Active Phase Shifter

Pulse tube cryocoolers have been widely used for cooling infrared sensors, superconducting and gas liquefaction. An active phase shifter allows the phase angle between the pressure and the mass flow rate to be varied, thus high efficiency and better performance can be achieved. In this work, a CFD technique using dynamic meshing was adopted to investigate the oscillating flow and performance sensitivity of an in-line pulse tube cryocooler with an active phase shifter. A high performance computer (HPC) was used for the 2D simulation. The effect of phase angle, operating frequency, pressure drop, and fill pressure were studied. The pulse tube cryocooler appears to have an optimal operating frequency (55 Hz) and displacement phase angle (40 deg) between compressor and displacer. A fill pressure of 28 bar leads to better performance than 24 bar and 20 bar. The CFD results are validated by the experimental results of a prototype in-line pulse tube cryocooler.

Mass transfer from a Taylor bubble to the surrounding flowing liquid at the micro-scale: a numerical approach

Gas–liquid slug flow is characterized by complex and intermittent hydrodynamic features that offer an efficient alternative to promote biofilm control. In the present work, the mechanism of transferring a gaseous solute into a co-current liquid in a micro-scale slug flow system was inspected in detail. Specifically, the gas–liquid mass transfer from an individual Taylor bubble filled with oxygen was numerically studied using CFD techniques. To accurately describe the referred phenomenon, the hydrodynamic and concentration fields were simultaneously solved. Furthermore, the interface capturing based on the VOF methodology was also coupled to this solution approach. Three sub-categories within slug flow pattern were identified based on the flow behavior in the liquid phase: no liquid in recirculation (Case A); closed wake below the bubble tail (Case B); and recirculation ahead and below bubble (Case C). Regarding the solute distribution, in Case A the solute is dispersed only backwards, it accumulates in the closed wake structure in Case B, and it reaches the wall within the film region in Case C. Local and average mass transfer coefficients were also estimated for the different cases. The influence of the two most relevant dimensionless groups (Reynolds and Capillary numbers) was also briefly analyzed. Global mass transfer coefficients results confirmed that the penetration theory can provide reasonable estimations for systems like Case C.

Heat Transfer in Air Flow Past a Bottom Channel Wall-Attached Diamond-Shaped Baffle – Using a CFD Technique

A computational analysis has been conducted to investigate turbulent flow and convective thermal transfer characteristics in a two-dimensional horizontal rectangular section channel with a hot lower wall-mounted diamond-shaped baffle. The calculations are based on the finite volume method, by means of Commercial Computational Fluid Dynamics software FLUENT, standard k-epsilon turbulence model with QUICK numerical scheme, and the SIMPLE discretization algorithm has been applied. The fluid flow and heat transfer characteristics, i.e., dynamic pressure coefficient, stream function, mean, axial, and transverse velocities, turbulent viscosity, temperature field, skin friction coefficients, local and average Nusselt numbers, and thermal enhancement factor are presented for flow Reynolds numbers based on the aeraulic diameter of the computational domain ranging from 12,000 to 32,000 at constant surface temperature condition along the upper and lower walls. Effect of the diamond configuration of the insulated baffle is studied numerically and the data obtained from this same baffle model are also compared with that of the simple flat rectangular baffle under similar operating conditions. Over the range under investigation, the improvements are found to be around 3.962 and 29.820 times higher than the smooth air channel with no baffle for heat thermal transfer and skin friction factor, respectively. The maximum TEF is around 1.292 at the highest Reynolds number value, Re = 32,000.

Effects of parallel magnet bars and partially filled porous media on magneto-thermo-hydro-dynamic characteristics of pipe ferroconvection

In this experimental-numerical study, effects of transverse magnetic field of parallel magnet bars perpendicular to the fluid flow direction and partially filled porous media are investigated. Moreover, constant heat flux is applied on the outer surface of a circular straight horizontal pipe and concentric porous media is embedded inside it. Magnet bars are located right at the entrance of pipe and a thick layer of insulation is wrapped the pipe to prevent heat loss. In addition, a 3D numerical simulation is performed using CFD techniques to capture the influence of these two factors on the thermohydrodynamic behavior of ferrofluid flow. Hybrid scheme of finite volume method and SIMPLE algorithm are employed to discretize and solve governing equations as well as TDMA for solving the linear algebraic matrix equation. Mineral oil is used as a base fluid and ferrofluid with three different volume fractions 0.5, 1 and 2% are prepared for the experiment. Finally prepared experimental setup configuration in this investigation leads to this conclusion that in comparison with magnetic field, although porous media induce more unfavorable pressure loss, it will bring more heat transfer enhancement as well.

Simulation Study for Optimization Design on Vertical Axis Wind Turbine blades

In this article is conducted research to harness wind energy which is firstly generated by vehicle / truck that is runing on the public road highway. To take advantage of wind energy of the moving truck is designed, otherwise advisor had some ideas during the proposal defense change into fixed vertical axis wind turbine. The purpose of this evaluation study is to get optimization for the design blades of the vertical axis fixed wind turbine and finding the best blades installed and angle of attack will result in highest lift/drag ratio. While other test parameters such as air pressure, wind speed and others are held constant. In this evaluation study the angle of attack are used ranging begin from 45 and until 90 degrees. Evaluation result showed that the best blades install and angle of attack that gives the best lift/drag ratio is 5 blades at AoA ninety degree. Blades diameter of the designed wind turbine are 0.35 m and the number of blades which is the best in analytical of CFD techniques in the designed wind turbine are five pieces. The speed of the wind which is used to test the blades is 8 m/s on turbine rotation 80 rpm. The evaluation study has suceeded to do parametric optimization of the turbine blades. The optimised blades have been ready to re-designed assamble with another componens of the wind turbine to construct the prototype but there some problems / handicaps during the changes the prootype of turbine from movable to fixed wind turbine. The assambled vertical axial wind turbine postponed to further be tested in order to know its performance. CFD simulation has been performed with ten different VAWT designed models. Moving mesh and fluid flow simulation have been developed in CFD software FLUENT. The findings of these numerical simulations provided pressure contour, velocity contour, C D or C L

Analyzing Rotor Profile’s Effects to Performance of Roots Vacuum Pump Applied the CFD Method

The paper is mainly focused on analysis characteristics of lobe pump with three different tooth profiles: Circular, Epi-cycloidal and Circular–Involute–Circular (CIC) rotor profiles. Based on the geometric analysis and mathematical model, the CIC rotor profile combined Circular–Involute–Circular is generated. In this paper, the CFD techniques are used to estimate the effects of tooth profile and lobe number to the pump performance. The simulation results obtained from FLUENT software illustrated that the rotor profile effects remarkably to the pump performance. The outlet pressure of CIC rotor profile is higher than that of the circular and epi-cycloidal rotor profiles. The numerical outcomes also showed that higher rotor’s speed is resulted in higher outlet pressure. Besides, the lobe number of rotor has effect slightly to the pump performance.

Numerical Investigation into Flutter and Flutter-Buffet Phenomena for a Swept Wing and a Curved Planform Wing

The results of numerical studies carried out on high-aspect-ratio wings with different planforms are discussed: the transonic regime is analysed for a swept wing and a curved planform wing. The wings have similar aspect ratios and similar aerodynamic profiles. The analyses were carried out by CFD and FE techniques, and the reliability of the numerical aerodynamic results was proven by a sensitivity study. Analysing the performances of the two wings demonstrated that in transonic flight conditions, a noticeable drag reduction can be obtained by adopting a curved planform wing. In addition, for such a wing, the aeroelastic instability condition, consisting in a classical flutter, is postponed compared to a conventional swept wing, for which a flutter-buffet instability occurs. In a preliminary manner, the study shows that, for a curved planform wing, the high speed buffet is not an issue and at the same time notable fuel saving can be achieved.

Geometric optimization of shell and tube heat exchanger with interstitial twisted tapes outside the tubes applying CFD techniques

Steady state numerical simulations were carried out with twisted tapes on shell side of a shell and tube heat exchanger. Thermal hydraulic performance of a heat exchanger with various twisted tape widths and twist ratios was conducted. The effects of structural modifications were investigated on the heat exchanger performance. Limited lab-scale experiments, using water as working fluid, were carried out to validate the numerical simulations. The entire model meshing method was adopted to mesh the geometry with Gambit 2.4 software while k-εrealizable model was adopted. Combining effects of both the geometric and structural modifications, an improved design with optimum comprehensive performance was suggested.

Mass transfer from a soluble Taylor bubble to the surrounding flowing liquid in a vertical macro tube — A numerical approach

Gas–liquid mass transfer is a phenomenon with a wide practical relevance and, besides many other possible advantages, its fundamentals can be used to enhance or regulate chemical and biological processes. In this work, the specific case of the mass transfer of oxygen from a Taylor bubble to the surrounding flowing liquid was studied using CFD techniques. This study was performed for a range of flow conditions (different Reynolds numbers) and a liquid with a viscosity (and constant Morton number) typical of bio-fluids. The flow field, gas–liquid interface and concentration field were determined simultaneously by coupling the VOF methodology with the species mass balance equation and setting a constant solute concentration at the bubble surface. The mass transfer behavior on the different hydrodynamic regions (nose, film and tail) was inspected and quantified based on local coefficients. It was possible to verify that these values increase along the nose and reach a maximum at the fully developed film region, where it tends to stabilize regardless of the bubble length (for bubble lengths between 2D and 9D). An opposite tendency was found along the bubble tail, where a decrease was verified on the local mass transfer coefficient values. Average mass transfer coefficients based on the simulation data were also determined. The comparison of these results with theoretical predictions allowed to infer that the film theory is the most adequate to describe the gas–liquid mass transfer between a Taylor bubble and the surrounding liquid.