Transition SST Model Research Articles

Study of intensification of the heat transfer in helically coiled tube heat exchangers via coiled wire inserts

In the current study, a numerical method is used to investigate the thermal energy transfer and pressure drop augmentation in helically coiled tube heat exchanger with a coiled wire insert made from copper. The impact of geometrical parameters of the inserts like diameter and cross sectional form on the intensification of the Nusselt and the friction factor number is studied. The Transition SST model is used to simulate the impact of turbulence. The model validation is performed by comparing the results with the empirical equations of prior experimental works. Results indicate that, using a circular cross section coiled wire inserts will enhance the Nusselt number and friction factor by 340.9%, 536.1%, respectively. Furthermore, using inserts with concentric circular cross section with diameter of 0.008 m and two rectangular cross sections are recommended for the intensification of heat transfer at the inlet mass flow rate 0.05 kg/s while, all inserts are suggested at the inlet mass flow rate of 0.075 kg/s. As a part of the study, a correlation is proposed for estimating the Nusselt number of these heat exchangers.

Super-hydrophobicity effects on performance of a dynamic wind turbine blade element under yaw loads

Flow characteristics over a dynamic section of wind turbine blade in the presence of super-hydrophobic coatings were studied numerically using computational fluid dynamics method. Assuming a super-hydrophobic coating, blade icing can potentially be prevented. Fluids on super-hydrophobic surfaces are moving; so slip velocity exists on their walls. The dynamic motion was in sinusoidal mode that caused deep dynamic stall (DS) phenomenon. The occurrence of DS results in enlarged vortices with high strength of vorticity. These vortices cause overshoot in aerodynamic loads. By applying a super-hydrophobic surface on different locations of the airfoil, the DS vortex generation can be definitely affected. Investigation of these effects, is the aim of this paper. There is no study in the literature that analyzed DS in the presence of slip velocities at different locations. Thus, an SD7037 airfoil was simulated at Re≈4×104 using Transition-SST model. Results demonstrated that as the coating covered the leading edge, DS and vortex formation were postponed and the lift peak reduced about 10.6%; while for airfoil entirely covered by coating, the maximum lift value was augmented 14.6% and DS delayed. Applying slip boundary conditions at trailing edge as well as the pressure side had not considerable changes in cyclic loads.

Numerical Investigation of Added Mass and Hydrodynamic Damping on a Blunt Trailing Edge Hydrofoil

Added mass and hydrodynamic damping play significant roles in fluid-structure interaction (FSI) in hydraulic turbines. Added mass can reduce natural frequencies, while hydrodynamic damping could result in a higher amplitude decay speed of the vibration. In order to quantify the added mass and hydrodynamic damping of a three-dimensional (3D) NACA 0009 hydrofoil with a blunt trailing edge, a two-way FSI simulation method was employed. The effects of grid scale, time-step, turbulence model, exciting force, and numerical damping on the calculation accuracy of the two-way FSI numerical simulation were analyzed in great detail through comparison with the previously published experimental data. Hydraulic force was obtained by using a transitional shear stress transport model at the flow region of the Reynolds number ReL = 0.2 × 106–2 × 106. The vortex shedding frequency, the natural frequency of the first-order bending mode in water, and the hydrodynamic damping ratio obtained from the numerical simulations agree well with the experimental data, with maximum deviations in 6.12%, 4.53%, and 8.82%, respectively. As the flow velocity increases, the natural frequency may not significantly change, while the added mass coefficient gradually increases, considering the effect of added stiffness. Above the first-order bending mode lock-in region, the results indicate that the first-order bending mode hydrodynamic damping ratio increases linearly with velocity. The present numerical achievements offer a higher level of accuracy for predicting the added mass and hydrodynamic damping characteristics of a hydrofoil.

Influence of wall roughness on boundary layer transition position of the sonic nozzles

The discharge coefficient of sonic nozzle is mainly dependent on the viscous boundary layer which is affected by the Reynolds number, wall roughness and turbulence intensity. Six sonic nozzles with various throat diameter of (1.921–9.086) mm and the average roughness of 0.6 μm in NIM were designed and calibrated to experimentally analyze the influences of wall roughness on the discharge coefficient and boundary layer transition by two pVTt gas flow standard facilities. Furthermore, a series of numerical simulations through the transition SST model for the axisymmetric nozzles were established to research the discharge coefficients of the sonic nozzles with the throat diameter of 3.808 mm, 5.350 mm and 7.453 mm when the Reynolds numbers ranges from 2.5 × 104 to 4.4 × 107 and the relative equivalent sand roughness Ks/d from 1.3 × 10−5 to 1.3 × 10−3. The calculating results of the simulation model were validated by the ISO 9300 empirical equation and the experimental data of NIM, PTB and UK. Finally, the relationships between the displacement thickness of boundary layer, namely the discharge coefficient and the relative roughness of the wall were obtained in detail. The starting positions of boundary layer transition with different Reynolds number and relative roughness were also analyzed.

Computational investigation of the flow inside a Tesla turbine rotor

Tesla expander is a bladeless turbine suited to low power range applications. In this article, a comparison between the performance prediction, as well as the assessment of the main flow characteristics, of a Tesla turbine working with organic fluids obtained through an in-house 2D code developed in EES environment and a simulation run with a computational fluid dynamics commercial software was done.Three working fluids (R404a, R134a and R245fa) were analysed in order to determine the related performance parameters. Various computations were carried out at several speeds of revolution, both with the laminar model and the Langtry-Menter transitional shear stress transport model for turbulence processing. High rotor efficiency was predicted for a small-scale prototype working with all analysed fluids (69% at 3000 rpm). The results obtained by the CFD simulations and by the in-house code showed an excellent matching.Finally, absolute and relative flow path lines were computed in order to determine fluid dynamics inside the channel and to analyse the fundamental flow phenomena.

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

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

Numerical Investigation For Convective Heat Transfer and Friction Factor Under Pulsatile Flow Conditions

Under pulsatile flow conditions, the characteristics of convective heat transfer and friction factor for periodic corrugated channel are investigated numerically. The Finite Volume Method (FVM) is used in the numerical study. Three different Reynolds Averaged Numerical Simulation (RANS) based turbulence models, namely the k-ω, the Shear Stress Transport (SST) and the transition SST model are employed and compared with each other. The results are also compared with the previous measurements performed for non-pulsating conditions. Investigations are performed for air flowing through corrugated channel which has sharp corrugation peak with an inclination angle of 30° and a 5mm minimum channel height. Reynolds number is varied within the range 6294 to 7380, while keeping the Prandtl number constant at 0.70. Four different sinusoidal pulsatile flow conditions, which are combination of two different frequencies and two different amplitudes, are used. Variations of the Nusselt number and friction factor with the Reynolds number are studied. Effects of the amplitude and period of sinusoidal pulsatile flow conditions are discussed.

Investigation of Inclined Turbulators for Heat Transfer Enhancement in a Solar Air Heater

ABSTRACTIn the present study, the effect of forward inclined turbulators on the heat transfer enhancement in a duct is investigated, for forced convection. Turbulator configurations with three different pitch ratios and three different inclination angles are investigated for seven Reynolds numbers within the range 500–50,000. Investigations are performed experimentally as well as computationally, within a computational fluid dynamics framework. A distinguishing feature of the latter has been the employment of a turbulence model, the transitional shear stress transport model that is applicable throughout the presently considered range of Reynolds numbers containing laminar, transitional, and turbulent regions. At the beginning of the study, measurements and predictions are validated against analytical and empirical expressions known for a plain duct. The results obtained for turbulators configurations indicate that Nusselt number increases with the inclination angle but decreases with the pitch ratio. The influence of the inclination angle on the Nusselt number and thermal enhancement factor is found to be stronger than that of the pitch ratio. For all Reynolds numbers and for all configurations, the thermohydraulic performance is observed to increase, leading to thermal enhancement factors within the range 2–5. In all cases, a quite good agreement of the predictions and experiments is observed, which increases the confidence in the accuracy of both approaches.

CFD analyses of the wind drags on Khaya Senegalensis and Eugenia Grandis

The drag coefficients of one mature Khaya Senegalensis tree and one Eugenia Grandis tree in Singapore were analysed using computational fluid dynamics (CFD) modelling and validated with field measurements. For the numerical method, an innovative laser scanning approach was used to generate the tree geometries and to calculate the three-dimensional (3D) leaf area density distribution within the canopies. The canopies were represented by multiple porous domains and the turbulent effect of leaves was simulated by source and sink terms as a function of the calculated leaf area density. Computational fluid dynamics analyses using ANSYS 17.2 software were carried out on both leafless and leafed tree models. Three turbulent models, the Realizable k-ε model, Transition Shear Stress Transport model and Reynolds Stress Model were compared, and it was found that the differences in the drag forces among the turbulent models were negligible when they were meshed appropriately based on the grid independence study. The computed drag coefficient of the Khaya Senegalensis tree and the Eugenia Grandis tree from CFD modelling were similar within the range between 0.6 and 0.7. From field monitoring including the wind velocity and stem strains, the drag coefficients of both trees were calculated to be 0.51 and 0.40. Possible reasons causing the difference and limitations of the numerical method are discussed.

Numerical investigation of jet impingement cooling of a low thermal conductivity plate by supercritical pressure carbon dioxide

Jet impingement cooling is widely used due to its high heat transfer rates, especially within the stagnation region. Jet impingement cooling by supercritical pressure fluids has even higher heat transfer coefficients with the proper working conditions than regular fluids with no burnout. This paper presents a numerical investigation of supercritical pressure carbon dioxide jet impingement cooling of a low thermal conductivity plate. Predictions of 11 RANS turbulence models were first validated against the published experimental data done by our research group. The effects of the inlet temperatures and pressures on the heat transfer were studied numerically for a wide range of heat fluxes with the flow phenomena observed in the experiment further explained by the numerical results. The SST k-ω and Transition SST models gave more accurate predictions of the average heat transfer coefficient with differences of less than 15% for the validated case. The maximum average heat transfer coefficient occurred when the inlet temperature was lower than and the surface temperature was slightly higher than the pseudocritical temperature. The maximum average heat transfer coefficient increased with increasing inlet temperature when the inlet temperature was lower than the pseudocritical temperature and decreased with increasing pressure.

Numerical investigation of three turbulence simulation models for S809 wind turbine airfoil

The contributions in terms of energy production and reliability benefits of horizontal axis wind turbines are growing on a yearly basis throughout the world, which has made them the predominant technology for wind energy harvesting. When numerically simulating wind turbines, the selection of turbulence modelling approach can be of pivotal importance since it defines the numerical costs and gains of the simulation. In this paper, 2D computational fluid analysis of S809 Don Somners airfoil is targeted to evaluate the merits, drawbacks, and capabilities of three different common turbulence-modeling approaches, namely Spalart-Allmaras, k-w SST, and Transition SST models. The results are validated through comparison against experimental data on pressure coefficient distribution; a significant agreement between experimental data and Transition SST model is observed, which makes this model the reference model in this study. The other two approaches are compared with the results of the aforementioned model in terms of pressure distribution and flow separation predictions. The results indicate that k-w and Spalart-Allmaras models are as reliable as Transition SST when the Reynolds number and angle of attack are high enough (i.e. higher than 10 to 12°). At low Reynolds numbers, which means under 5 to 6°, the reference model is observed to be the most reliable among the three. The results of this study are of great importance when full rotor simulation is the task in hand, since they define the most appropriate approach to use with each flow condition to achieve an optimized numerical simulation procedure.

Numerical investigation for convective heat transfer and friction factor under pulsating flow conditions

For pulsating flow, the behaviours of the convective heat transfer and friction factor for a periodic corrugated channel are investigated numerically. The finite volume method is used in the numerical study. Three different Reynolds Averaged Numerical Simulation based turbulence models, namely the k-ω model, the Shear Stress Transport (SST) model and the transition SST model are used and compared. The results are also compared with the previous experiments for non-pulsating flow. Analyses are conducted for air flow through a corrugated channel which has sharp corrugation peaks with an inclination angle of 30° and a 5mm minimum channel height. Reynolds number is changed in the range 6294 to 7380, while keeping the Prandtl number constant at 0.70. A sinusoidal pulsatile flow condition which is F=400 and uA*=0.5 is used. Variations of the Nusselt number and the friction factor with the Reynolds number are studied. Non-pulsating flow results and pulsating flow results are compared with each other.

Computational investigation of the velocity and temperature fields in corrugated heat exchanger channels using RANS based turbulence models with experimental validation

The characteristics of convective heat transfer and friction factor for periodic corrugated channels are investigated numerically. In the numerical study, the finite volume method (FVM) is used. Three different Reynolds averaged numerical simulation (RANS) based turbulent models, namely the k-ω, the shear stress transport (SST) model and the transition SST model are employed and compared with each other. Experimental results obtained from a previous study are used for the assessment of the numerical results. Investigations are performed for air flowing through corrugated channels with an inclination angle of 30°. The Reynolds number is varied within the range 2,000 to 11,000, while keeping the Prandtl number constant at 0.70. Variations of the Nusselt number, Colburn factor, friction factor and goodness factor with the Reynolds number are studied. Effects of the corrugation geometry and channel height are discussed. The overall performances of the considered turbulence model are observed to be quite similar. The SST model is observed to show a slightly better overall performance.

Prediction and measurement of the aerodynamic performance of a wind turbine

Aerodynamic behavior of a small wind turbine is analyzed, both experimentally and numerically. Mainly, an unsteady three-dimensional formulation is adopted, where the flow turbulence is modelled by an Improved Delayed Detached Eddy Simulation framework, using the four-equation transitional Shear Stress Transport model, as the turbulence model. A quite good agreement between the measurements and calculations is observed.

Heat transfer and flow field in a circular twisted channel

In the present paper, along with experimental study, computational fluid dynamics analysis is performed, using the transition SST model which can predict the change of flow regime from laminar through transition to turbulent. The differential governing equations are discretized by the finite volume method. The investigations are conducted for Reynolds numbers ranging from 100 to 50,000 covering laminar, transitional and turbulent regimes, and for three length and three pitch ratios. The predictions are observed to show a good agreement with the measurements and published correlations of other authors. The analysis indicates that the large length ratio and small pitch ratio yields a higher heat transfer rate with relatively low performance penalty. The transition from laminar to turbulent regime is observed between Reynolds numbers of 2,500 to 3,500 for all cases. For almost all investigated cases the performance factors are greater than unity.

Numerical simulations of the aerodynamic response of circular segments with different corner angles by means of 2D URANS. Impact of turbulence modeling approaches

ABSTRACTThis paper presents the results of numerical and experimental investigations on the force coefficients and Strouhal numbers of circular segments considering different corner angles or chord to sagitta ratios. The research is motivated because these geometries are becoming increasingly popular in several engineering disciplines. The so-called D-section (semi-circular cylinder with a corner angle of ) has been experimentally studied in the past, since it is a galloping prone geometry. However, there is a lack of research for cases with different corner angles, and the numerical investigations related to this topic are particularly scarce. In this work, a 2D Unsteady Reynolds Averaged Navier–Stokes approach has been adopted aiming to study the circular segments at the sub-critical regime, considering corner angles from to , and the flow parallel to the rectilinear side. These sections were found to be particularly challenging since they present massive flow separation on the rectilinear side, alongside the inherent difficulties related to modeling the flow along curved surfaces at high Reynolds numbers. The impact of introducing low-Reynolds-number and curvature corrections in the SST turbulence model and the performance of the Transition SST model have been extensively studied.

The nonaxismmetric endwall aerodynamic optimization design for a large turbine cascade with a midgap

The nonaxisymmetric endwall has been verified to be an effective method in reducing the endwall secondary flow loss. Some assembly features, such as the midgap in the hub of real aircraft engines, may have an influence on the endwall secondary flow. In the present work, a nonaxisymmetric endwall with midgap structure is designed for a large linear turbine cascade. A nonaxisymmetric endwall optimization design procedure is developed to minimize the total pressure loss coefficient at the passage exit. The profile of the endwall is designed using automatic numerical optimization with the Kriging surrogate method. The numerical simulation based on a transition shear stress transport model is used as the aerodynamic evaluation tool for the optimization system. When the midgap is considered in the design, mixing loss between midgap flows and main flow is significantly reduced. However, the loss relative to the passage vortex is increased to some extent.

Numerical investigation of hydrodynamic flow over an AUV moving in the water-surface vicinity considering the laminar-turbulent transition

Nowadays, Autonomous Underwater Vehicles (AUVs) are frequently used for exploring the oceans. The hydrodynamics of AUVs moving in the vicinity of the water surface are significantly different at higher depths. In this paper, the hydrodynamic coefficients of an AUV in non-dimensional depths of 0.75, 1, 1.5, 2, and 4D are obtained for movement close to the free-surface. Reynolds Averaged Navier Stokes Equations (RANS) are discretized using the finite volume approach and the water-surface effects modeled using the Volume of Fraction (VOF) method. As the operating speeds of AUVs are usually low, the boundary layer over them is not fully laminar or fully turbulent, so the effect of boundary layer transition from laminar to turbulent flow was considered in the simulations. Two different turbulence/transition models were used: 1) a full-turbulence model, the k-e model, and 2) a turbulence/transition model, Menter's Transition-SST model. The results show that the Menter’s Transition-SST model has a better consistency with experimental results. In addition, the wave-making effects of these bodies are studied at different immersion depths in the sea-surface vicinity or at finite depths. It is observed that the relevant pitch moments and lift coefficients are non-zero for these axi-symmetric bodies when they move close to the sea-surface. This is not expected for greater depths.

Numerical simulation of transport phenomena due to array of round jets impinging on hot moving surface

ABSTRACTAn attempt has been made to numerically evaluate the heat transfer from a moving surface due to impingement of array of round jets. This paper reports the effect of surface velocity on heat transfer. Transition SST model has been used for simulation to predict heat transfer under laminar, transition, and turbulent regimes. This model has been used as it bridges all flow conditions seamlessly. The computational domain considered in this study is a 3D model with symmetric planes on two sides and periodic interface on two sides, so as to represent an array of round jets. The range of Reynolds number adopted here is 100–5,000. Results were first validated with the correlation given by Martin[1] for array of round nozzles with stationary isothermal surface under turbulent conditions with an average error of 5.88%. It is observed that at higher surface velocities, the heat transfer from the moving surface is more than the case of heat transfer from a stationary surface. The value of surface velocity at which the heat transfers from moving surface is minimum decreases with increase in Reynolds number. An artificial neural network has been trained to accurately predict the Nusselt number for the given Reynolds number and surface velocity.

Numerical Models of Wind Effects on Temperature Loaded Object

Wind climate influencing wind loads on buildings and other structures, as well as the dispersion of pollutants from various surfaces is essentially determined by small-scale motions and processes occurring in the atmospheric boundary layer (ABL). The physical and thermal properties of the underlying surface, in conjunction with the dynamics and thermodynamics of the lower atmosphere influence the distribution of wind velocity in thermally stratified ABL. Atmospheric turbulence is characterized by a high degree of irregularity, three-dimensionality, diffusivity, dissipation, and a wide range of motion scales. This article describes a change of selected turbulent variables in the surroundings of flow around a thermally loaded object. The problem is solved numerically in Ansys Fluent 13.0 software using LES (Large eddy simulation) models as well as the Transition SST (Shear Stress Transport) model that is able to take into account the difference between high and low turbulence at the interface between the wake behind an obstacle and the free stream. The results are mutually compared and verified with experimental measurements in the wind tunnel.