Full Computational Fluid Dynamics Model Research Articles

A Reduced Radiator Model for Simplification of ONAN Transformer CFD Simulation

In this paper, a reduced computational fluid dynamics (CFD) model is proposed for determining the total heat dissipation and the oil temperature distribution of transformer radiators in oil natural air natural (ONAN) cooling mode. In the reduced radiator CFD model, the air flow simulation is replaced with an optimized air heat transfer coefficient ( <bold xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>h</i><i>_air</i></b> ) equation. An experimentally verified full radiator CFD model, which includes the air flow simulation, has been introduced as a benchmark model for evaluating the validity of the reduced model. It was found that one <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${{\boldsymbol{h}}_{{\boldsymbol{air}}}}$</tex-math></inline-formula> can provide sufficient accuracy in simulating the air convection effect of transformer radiators. An <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${{\boldsymbol{h}}_{{\boldsymbol{air}}}}$</tex-math></inline-formula> equation was derived through a parametric study using the full radiator CFD model. The performance of the proposed <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${{\boldsymbol{h}}_{{\boldsymbol{air}}}}$</tex-math></inline-formula> equation was compared with the existing empirical equations, and its validities for different insulating liquids and different ambient temperatures were proven by additional CFD modelling. This <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${{\boldsymbol{h}}_{{\boldsymbol{air}}}}$</tex-math></inline-formula> equation can be transformed into a dimensionless form to help the radiator design and facilitate the complete-cooling-loop (CCL) based transformer CFD modelling.

A Compact Model for Lyophilizer Equipment Capability Estimation.

This work presents a compact model for the equipment capability limit of a common configuration of pharmaceutical lyophilizers, a product chamber separated from the condenser by a duct and isolation valve, at a wide range of design parameters. The equipment capability limit is one of the most important characteristics determining the lyophilization design space for a particular product, container, and equipment combination. Experimental measurements of equipment capability are time-consuming and expensive, especially at the production scale. Numerical modeling using computational fluid dynamics may reduce the number of experiments and provide insights into the physics of the process with high resolution. The computational fluid dynamics (CFD) modeling has been used in this work to develop a compact model for lyophilizer equipment capability. This eliminates the need for end users to create a full CFD model of the equipment and process. Full CFD and compact model simulations for laboratory and pilot-scale lyophilizers have been compared with tunable diode laser absorption spectroscopy measurements of the water vapor mass flow during ice slab tests. The compact model results average deviation from the experimental data is within 10%, whereas the full CFD simulations are within 5%. The compact model is based on several key parameters which are the main characteristics of a lyophilizer affecting the equipment capability curve. These parameters are discussed, and their effect on the modeling results is shown.

On the Hierarchy of Models for Pipe Transients: From Quasi-Two-Dimensional Water Hammer Models to Full Three-Dimensional Computational Fluid Dynamics Models

Abstract A hierarchy of models exists in the literature for the simulation of pipe transients. One-dimensional (1D) water hammer models provide a cost-effective tool for the analysis of such transients. Traditional 1D models implement a quasi-steady approximation of the frictional term, which results in poor modeling of the attenuation of the transient. To improve the modeling of the attenuation phenomenon, alternative unsteady friction models were developed for the 1D water hammer formulation. Moreover, quasi-two-dimensional (quasi-2D) water hammer models were introduced, which allow the computation of the unsteady velocity profile and hence provide improved modeling of the attenuation phenomenon. Recently, interest has developed in the use of computational fluid dynamics (CFD) models based on the Navier–Stokes equations in the simulation of fluid transients. Both axisymmetric and full three-dimensional (3D) CFD models are used in this regard. The aim of the current paper is to carry out a comparative study between the performance of quasi-2D water hammer models, axisymmetric CFD models, and full 3D CFD models. Numerical computations using the three models are performed for both laminar and turbulent flow cases. Present results show that the quasi-2D water hammer model and the axisymmetric CFD model provide near identical results in terms of computing the magnitude, phase, and attenuation of the transient. Reported results also demonstrate the computational efficiency of the quasi-2D model, which provides results that agree reasonably well with the full 3D CFD model results while using a grid density, which is an order of magnitude lower than the grid requirements for the full 3D CFD model.

An experimentally validated multifidelity hybrid model for analyzing the pressure variation of a relief system

In a relief system, the pressure variation in the system is often caused by the operations of the pressure control elements such as the pressure relief valve and ball valve, in addition to the pressure fluctuations in the pressure resources. This pressure variation couples with the acoustics or the motion of passive pressure control elements, which may result in system malfunction. To investigate the principles of the pressure variation in a relief system including a reservoir, a pipeline and a spring-loaded pressure relief valve, a multifidelity hybrid model combining the method of characteristics (MOC) and computational fluid dynamics (CFD) method is proposed. In the multifidelity hybrid model, the characteristics of the pressure resources are modeled using their performance curves, the compressible gas transmitted in the pipe is calculated by one-dimensional MOC, and the air flow in the valve as well as the valve motion is simulated by a two-dimensional CFD model. To validate the model, analyses are conducted with a 1:1 scale test rig and a full CFD model. The comparison of the results shows that both the multifidelity hybrid model and the full CFD model can accurately capture the pressure fluctuations in straight pipelines induced by valve closure and predict the forms of valve motion, but the calculation speed of the multifidelity hybrid model is four times faster than that of the full CFD model.

Development and validation of a CFD based optimization procedure for the design of torque converter cascade

ABSTRACTTraditional one-dimensional (1D) design theory no longer satisfies modern torque converter design requirements due to the lack of accuracy and long trial-and-error process. A computational fluid dynamics (CFD) based optimization approach was brought forward. This study is devoted to two key processes in bringing CFD into torque converter design – blade parameterization/evaluation and optimization. A six-parameter blade camber line design method was employed to represent the blade shape. The blade profile was evaluated based on manufacturability considerations. A low-fidelity CFD model was developed for parameter study and optimization, and a high-fidelity CFD model considering the transient and cavitation effects was employed to evaluate the optimization outcome. The parameter sensitivity study was performed using design of experiment (DOE) technique and the variables were categorized into primary design variable, effective design variable and insignificant design variable groups. Optimization on the primary variables was carried out using the genetic algorithm. Two optimum solutions were evaluated with the transient full 3D CFD model and then tested. The optimum torque converters’ test results indicated significant performance improvement. The proposed design procedure can improve the design efficiency & accuracy and shorten the design cycle by the application of different CFD models in conjunction with an optimization algorithm.

Incorporating convective feedback in wildfire simulations using pyrogenic potential

Modelling the dynamics of wildfires is very computationally challenging. Although three-dimensional computational fluid dynamics (CFD) models have been successfully applied to wildfires, the computational time required makes them currently impractical for operational usage. In this study, we develop a two-dimensional propagation model coupled to a ‘pyrogenic’ potential flow formulation representing the inflow of air generated by the fire. This model can accurately replicate features of fires previously unable to be simulated using current two-dimensional models, including development of a fire line into a parabolic shape, attraction between nearby fires and the observed closing behaviour of ‘V’ shaped fires. The model is compared to experimental results with good agreement. The pyrogenic potential model is orders of magnitude faster than a full CFD model, and could be used for improved operational wildfire prediction.

Development of a Methodology for Interface Boundary Selection in the Multiscale Road Tunnel Fire Simulations

The simulation of large complex dynamical systems such as a fire in road tunnels is necessary but costly. Therefore, there is a crucial need to design efficient models. Coupling of computational fluid dynamics (CFD) models and 1D network modeling simulations of a fire event, a multiscale method, can be a useful tool to increase the computational efficiency while the accuracy of simulations is maintained. The boundary between a CFD model (near field) and a 1D model (far field) plays a key role in the accuracy of simulations of large systems. The research presented in this paper develops a novel methodology to select the interface boundary between the 3D CFD model and a 1D model in the multiscale simulation of vehicle fire events in a tunnel. The development of the methodology is based on the physics of the fluid structure, turbulent kinetic energy of the dynamical system, and the vortex dynamics. The methodology was applied to a tunnel with 73.73 m2 cross section and 960 m in length. Three different vehicle fire scenarios were investigated based on two different heat reslease rates (10 MW and 30 MW) and two different inlet velocities (1.5 m/s and 5 m/s). all parameters upstream and downstream of the fire source in all scenarios were investigated at t = 900 s. The effect of changes in heat release rate (HRR) and air velocity on the selection of an interface boundary was investigated. The ratio between maximum longitudinal and transversal velocities was within a range of 10 to 20 in the quasi-1D region downstream of the fire source. The selected downstream interface boundary was 12Dh m downstream of the fire for the simulations. The upstream interface boundary was selected at 0.5 Dh m upstream the tip of the object when the velocity was greater than equal to the Vc. In the simulations with backlayering (V < Vc), the interface boundary was selected 10 m further from the tip of the backlayering (1.2 Dh). An indirect coupling strategy was utilized to couple CFD models to 1D models at the selected interface boundary; then, the coupled models results were compared to the full CFD model results. The calculated error between CFD and coupled models for mean temperature and velocity at different cross sections were calculated at less than 5%. The findings were used to recommend a modification to the selection of interface boundary in multiscale fire simulations in the road tunnels and more complex geometries such as mines.

Decoupled CFD-based optimization of efficiency and cavitation performance of a double-suction pump

In this study the impeller geometry of a double-suction pump ensuring the best performances in terms of hydraulic efficiency and reluctance of cavitation is determined using an optimization strategy, which was driven by means of the modeFRONTIER optimization platform. The different impeller shapes (designs) are modified according to the optimization parameters and tested with a computational fluid dynamics (CFD) software, namely ANSYS CFX. The simulations are performed using a decoupled approach, where only the impeller domain region is numerically investigated for computational convenience. The flow losses in the volute are estimated on the base of the velocity distribution at the impeller outlet. The best designs are then validated considering the computationally more expensive full geometry CFD model. The overall results show that the proposed approach is suitable for quick impeller shape optimization.

Analysis of the Leakage Behavior of Francis Turbines and Its Impact on the Hydraulic Efficiency—A Validation of an Analytical Model Based on Computational Fluid Dynamics Results

The present contribution addresses the analysis of the leakage behavior of a small hydro Francis turbine using an analytical approach, which was validated based on the results of computational fluid dynamics (CFD). For a custom-designed Francis turbine with a specific speed of nq = 41.9 rpm, the flow chambers resulting from the labyrinth geometry were added to a traditional full CFD model of the turbine and numerical simulations were performed for several operation points ranging from part load (Qmin = 0.5 × Qopt) to over load (Qmax = 1.3 × Qopt). Consequently, the single losses occurring in the runner seals on crown and band side as well as the pressure distribution within the runner side spaces could be evaluated and compared to the results gained with an analytical approach, which was originally developed to calculate the leakage flow of centrifugal pumps. The comparison of the pressure distribution achieved with the numerical simulation and the analytical calculation shows that both approaches match well if the angular velocity of the fluid ωFluid trapped in the runner side spaces is calculated in an appropriate way. Furthermore, the achieved results demonstrate that the use of the analytical model enables the calculation of the disk friction and leakage losses with sufficient accuracy. This paper contributes to the improvement of the performance prediction of Francis turbines based on combined numerical and analytical calculations.

A Simple Thermal Model for Mixed Convection From Protruding Heat Sources

This paper reports the results of numerical and experimental investigations of conjugate mixed convection from a vertical channel with sets of protruding heat sources. The goal of this study is to investigate the possibility of obtaining reasonably accurate results with a simpler or compact thermal model that replaces a set of protruding heat sources with a heat source of appropriate thickness that occupies the whole of the channel wall. Commercially available FLUENT 6.3 was used for the simulations. In order to validate the numerical results, a low-speed vertical wind tunnel has been employed. These are followed by three-dimensional simulations for various heat transfer coefficients on the back side of the printed circuit board (PCB). The differences between the computational fluid dynamics (CFD) predicted and experimentally measured temperatures were minimized using least squares and the optimum value of heat transfer coefficient was obtained for use in the simple model. The effect of Reynolds number on heat transfer has been analyzed for both the full CFD and simpler thermal models. The variation of error in the maximum temperature between the full CFD model and the simpler thermal model under different conditions, as the number of chips changes from three to eight, is studied.

An efficient model development and experimental study for the heat transfer in naturally ventilated inclined roofs

Roof ventilation is an efficient way to reduce the heat transmission into building interior in summer. In this work, a theoretical model is developed to predict the heat flux transferred through the naturally ventilated inclined roof in a fast and accurate manner. In particular, the thermal resistance due to the coupled radiation and convection in roof cavity is modeled using the circuit transformation theory. Moreover, based on the computational fluid dynamics (CFD) analysis, correlations are proposed for the convective resistances in the naturally ventilated inclined cavity. Laboratory experiments are further carried out to validate the CFD model, and a satisfactory agreement is found between the experimentally measured and numerically simulated airflow velocity and temperature in the cavity. In order to evaluate the accuracy of developed model, the heat flux transferred into building interior is predicted by both the developed model and a full CFD model. A good agreement is achieved between the predictions of the two models. Based on the developed model, parametric studies are conducted to investigate the influences of key roof parameters on the heat flux transferred into building interior. Ranked in order of significance, the influential parameters are the solar reflectance of exterior roof surface, infrared emittance of cavity surface, thermal resistance of lower roof slab, thermal resistance of upper roof slab, roof inclination and cavity spacing.

Modelling micropollutant degradation in UV/H2O2 systems: Lagrangian versus Eulerian method

The UV/H2O2 system is an effective barrier against micropollutants. The degradation of micropollutants largely differs due to variations in target compounds, water matrix composition and UV system. Models can account for these varying hydraulic and photochemical conditions. Computational fluid dynamics (CFD) models are combined with UV radiation, transport and (photo)chemical models to compute the degradation of compounds in a UV system. In this paper several modeling approaches with different levels of complexity are discussed. Depending on the type of UV system, a full CFD model, that resolves the hydraulics, UV radiation and transport of compounds, or simplified approaches that assume analytical hydraulic profiles and/or take the average of the fluence rate distribution. For the transport of chemical compounds, two methods were evaluated: the Lagrangian method and the Eulerian method. The Eulerian method calculates compound’s concentrations at a fixed computational mesh and is most often used for compound degradation prediction in UV/H2O2 systems. The Lagrangian method calculates compound’s concentrations along the tracks of individual particles. For a collimated beam and a pilot-scale flow-through reactor, both methods resulted in similar degradation predictions. This research shows that both methods can be used for oxidation predictions in UV/H2O2 systems.

Hemodynamic Flow Modeling Through an Abdominal Aorta Aneurysm Using Data Mining Tools

Geometrical changes of blood vessels, called aneurysm, occur often in humans with possible catastrophic outcome. Then, the blood flow is enormously affected, as well as the blood hemodynamic interaction forces acting on the arterial wall. These forces are the cause of the wall rupture. A mechanical quantity characteristic for the blood-wall interaction is the wall shear stress, which also has direct physiological effects on the endothelial cell behavior. Therefore, it is very important to have an insight into the blood flow and shear stress distribution when an aneurysm is developed in order to help correlating the mechanical conditions with the pathogenesis of pathological changes on the blood vessels. This insight can further help in improving the prevention of cardiovascular diseases evolution. Computational fluid dynamics (CFD) has been used in general as a tool to generate results for the mechanical conditions within blood vessels with and without aneurysms. However, aneurysms are very patient specific and reliable results from CFD analyses can be obtained by a cumbersome and time-consuming process of the computational model generation followed by huge computations. In order to make the CFD analyses efficient and suitable for future everyday clinical practice, we have here employed data mining (DM) techniques. The focus was to combine the CFD and DM methods for the estimation of the wall shear stresses in an abdominal aorta aneurysm (AAA) underprescribed geometrical changes. Additionally, computing on the grid infrastructure was performed to improve efficiency, since thousands of CFD runs were needed for creating machine learning data. We used several DM techniques and found that our DM models provide good prediction of the shear stress at the AAA in comparison with full CFD model results on real patient data.

Combined microwaves and convection heating: A conjugate approach

Microwave treatment has been gaining increasing recognitions in the food industry and household frameworks alike. Better energy and finishing efficiencies can be obtained by adding an additional transport mechanism, such as forced air convection heating. In this work, transient distributions of temperature and moisture during the combined treatment is analyzed by a full computational fluid dynamics model, coupled with custom moisture diffusion and evaporation notations. Non-linear, interdependent transfer phenomena equations are discussed, and the multi-physics effects are emphasized. Realistic transfer exchanges are inherently considered by using a conjugate approach and no resort to empirical, averaged heat and mass transfer coefficients is made. A validation with the experimental results from the available literature has been brought forth, for potato disks drying by either pure convection, pure MW exposure or both mechanisms combined. The effect of process time and working air velocity on the local temperature distributions are finally presented.

Role of turbulent shear rate distribution in aggregation and breakage processes

AbstractThe effect of spatial flow heterogeneity on shear‐induced aggregation and breakage of fine particles in turbulent flow conditions is investigated using computational fluid dynamics (CFD) and population balance modeling. The quadrature method of moments (QMOM), particularly suitable for implementation in commercial CFD codes, has been used to solve the corresponding population balance equation. QMOM is first tested and compared with alternative numerical methods (sectional/fixed‐pivot methods) for a specific set of realistic operating conditions. Then QMOM is implemented in a CFD code and the effect of spatial heterogeneities on the cluster mass distribution in a Taylor–Couette vessel is investigated. Simplified models have been derived based on the separation of the timescales of mixing on one hand and of aggregation and breakage on the other and compared with the full CFD model. Guidelines for use and limitations of such models and the identification of the underlying aggregation and breakage kernels are discussed. © 2005 American Institute of Chemical Engineers AIChE J, 2006