In-house CFD Code Research Articles

Calibration of a Near-Wall Differential Reynolds Stress Model Using the Updated Direct Numerical Simulation Data and Its Assessment

In the article, a differential Reynolds stress model is recalibrated using turbulent channel flow direct numerical simulation data in the range of friction Reynolds numbers 550–5200. The calibration aims to produce a RANS sublayer model for use within the hybrid RANS/LES framework. The model is designed to capture the average field of a thin near-wall part of a boundary layer as accurately as possible. An a posteriori procedure is employed in which one-dimensional channel flow calculations are performed for all variations of the model coefficients at each stage of the optimization procedure. The coefficients are initialized with their original values and then optimized by minimizing the appropriately chosen norm. An improved representation of the mean velocity profile and peak Reynolds stress values is demonstrated. Both models—baseline and recalibrated—are implemented in an in-house CFD code, and several simulations, including a channel flow, a flat plate boundary layer and a boundary layer separation from a rounded step, are performed. The latter benchmark flow is also simulated in hybrid RANS/LES mode. The updated model is compared to the original one, demonstrating improvements over the baseline model in the cases it was designed for.

A mass transfer cavitation model for the numerical flow simulation of binary alkane mixture segregation

Based on the Rayleigh bubble dynamics equation a mass transfer model for cavitation of binary alkane mixtures is presented. Raoult's and Dalton's law, simple mixing rules, and an accurate Equation of State are utilized. The model is implemented into an in-house CFD code. For solver validation pure species literature cases are taken. The method is applied to a lighter n-octane/n-heptane and a heavier n-dodecane/n-heptane mixture in a rarefaction tube and a hydrofoil test case. Segregation of the species is observed during cavitation due to their different mass transfer rates. While for the lighter mixture, mass transfer of both species only moderately deviates, a significantly higher mass transfer of n-heptane compared to n-dodecane is observed for the heavier mixture, where the saturation pressure differs two orders of magnitude between the mixture ingredients. The strong segregation of the heavier mixture is associated with a predominant amount of n-heptane in the vapor phase. As a consequence, vapor composition is strongly affected by the volatilities of mixture ingredients.

Validation of the CFD Tools against In-House Experiments for Predicting Condensing Steam Flows in Nozzles

The issues addressed in this work concern the condensing steam flows as a flow of a two-phase medium, i.e., consisting of a gaseous phase and a dispersed phase in the form of liquid droplets. The two-phase character and the necessity to treat steam as a real gas make the numerical modeling of the flow in the last steam turbine channels very difficult. There are many approaches known to solve this problem numerically, mainly based on the RANS method with the Eulerian approach. In this paper, the two Eulerian approaches were compared. In in-house CFD code, the flow governing equations were defined for a gas–liquid mixture, whereas in ANSYS CFX code, individual equations were defined for the gas and liquid phase (except momentum equations). In both codes, it was assumed that there was no velocity slip between phases. The main aim of this study was to show how the different numerical schemes and different governing equations can affect the modeling of wet steam flows and how difficult and sensitive this type of computation is. The numerical results of condensing steam flows were compared against in-house experimental data for nozzles determined at the Department of Power Engineering and Turbomachinery of the Silesian University of Technology. The presented experimental data can be used as a benchmark test for researchers to model wet steam flows. The geometries of two half nozzles and an International Wet Steam Experimental Project (IWSEP) nozzle were used for the comparisons. The static pressure measurements on the walls of the nozzles, the Schlieren technique, and the droplet size measurement were used to qualitatively identify the location of the condensation onset and its intensity. The CFD results obtained by means of both codes showed their good capabilities in terms of proper prediction of the condensation process; however, there were some visible differences in both codes in the flow field parameters. In ANSYS CFX, the condensation wave location in the half nozzles occurred much earlier compared to the experiments. However, the in-house code showed good agreement with the experiments in this region. In addition, the results of the in-house code for the mean droplet diameter in the IWSEP nozzle were closer to the experimental data.

Effects of biomass particles size and shape on combustion process in the swirl-stabilized burner reactor: CFD and machine learning approach

When planning the development of the energy sector, significant attention is given to the energy from the renewable sources, amongst which the biomass has an important role. Computational fluid mechanics and machine learning models are the powerful and efficient tools which allow the analysis of various heat and mass transfer phenomena in energy facilities. In this study, the in-house developed CFD code and machine learning models (Random Forest, Gradient Boosting and Artificial Neural Network) for predicting the biomass trajectories, particle mass burnout and residence time in a swirl burner reactor are presented. Pulverized biomass combustion cases (fine straw, pinewood and switch grass) with various mean diameters (ranging between 60 and 650 μm) and different shape factors (within the range 0–1) are considered. The results of numerical simulations revealed a noticeably nonlinear dependence between the input values (particle types, sizes and shapes) and the output values (particle trajectories, mass burnout and residence time), mostly due to the complex swirling flow in the reactor. For particles with the mean diameters within the ranges considered, the mass burnout of particles generally decreases as the biomass particle shape factor increases. The residence time of pulverized biomass in the reactor shows in most cases a decreasing trend as the particle shape factor increases. Artificial Neural Network showed the best predictions for both particle mass burnout (RMSE = 0.083 and R2 = 0.937) and particle residence time (RMSE = 1.145 s and R2 = 0.900), providing the reliable assessment of these important indicators in the combustion process.

Assessment of Computational Fluid Dynamic Modeling of Multi-Jet Impingement Cooling and Validation With the Experiments

Abstract The current study involves numerical and experimental investigations of circular in-line jets impinging on a heated flat plate. The generic configuration is characterized by nine jets, each with a diameter of D = 0.0152 m. The jets are influenced by a self-generating crossflow and are positioned at a nozzle-to-plate distance (H/D) of 5 and a jet pitch (p/D) of 5. The steady Reynolds-averaged Navier–Stokes (RANS) simulations are performed for turbulent jet Reynolds numbers with the in-house CFD code TRACE. The Menter k–ω shear stress transport (SST) model is applied for turbulence modeling and the turbulent scalar fluxes are modeled based on the Reynolds analogy for a constant turbulent Prandtl number. To gain a closer insight into the impingement jet physics, high-resolution near-wall velocity and thermal fields are obtained through large eddy simulations (LESs) and measurements from particle image velocimetry (PIV). Focus is laid on the comparison of RANS results with the LES data and the experimental data. The results exhibit a qualitative similarity between the simulations and the experiments. Furthermore, correlations of the Nusselt number from the literature are used to validate the simulation results.

Liquid Phase Identification in Wet Steam Transonic Flows by Means of Measurement Methods

The major goal of this study is comparing the results of heterogeneous and homogeneous condensation through a CD nozzle. The experimental technique for liquid phase assessment in wet steam transonic flow is tested under real conditions in an in-house steam tunnel test rig. The flow was modelled numerically using an in-house CFD code for performing the heterogeneous and homogeneous condensation models. For the comparison between CFD results and experimental data, the static pressure distribution throughout the nozzle bottom wall were chosen. One may observe a good agreement of the CFD results with experimental data. The presented approach can be used in the future for searching the reasons for the lack of accuracy between experimental and numerical static pressure distribution in the condensing steam flows.

CFD SIMULATIONS OF OIL IMMERSED AND DRY TYPE TRANSFORMERS

At Siemens the in-house CFD code UniFlow is employed to investigate fluid flow and heat transfer in oil immersed and dry type transformers as well as transformer components like windings, cores, tank walls, and radiators. We outline its physical models and numerical solution methods. As an oil transformer application of the method, the simulation of oil flow and heat transfer in 5 windings of a prototype transformer with ONAN/ONAF cooling mode is described. It corresponds to a heat run test with the total losses. Furthermore, we outline an application to an AFWF cast resin transformer prototype operated at ships in an enclosure. The ventilator driven air flow is cooled by sea water. In addition to the LV and HV windings the core is simulated. Here also the heat radiation makes a significant contribution to the heat transfer.

CFD simulation of pressure loss in HVDC transformer winding

At Siemens the in-house CFD code UniFlow is used to analyse fluid flow and heat transfer in oilimmersed and dry-type transformers, as well as transformer components like windings, cores, tank walls, and radiators. It can be employed to perform steady state as well as transient analyses. This paper describes its physical models and numerical solution methods. Moreover, it presents an application to a valve winding of a HVDC transformer, cooled by mineral oil. This study is aimed at finding the flow induced pressure loss in the winding and the static ring assembly below and above the winding. The investigation includes isothermal runs with different inlet velocity and a conjugate heat transfer run with a conductor representation. In the isothermal simulations a steady state is established and the pressure loss is an almost linear function of the inlet velocity. In the run involving heat transfer, the high buoyancy forces hamper the development of a steady state and the possibility to calculate a flow induced pressure loss.

Assessment of RANS turbulence models on predicting supercritical heat transfer in highly buoyant horizontal flows

This paper rigorously validates the RANS models against the DNS on predicting turbulent flow and heat transfer of highly buoyant horizontal supercritical fluids. The low-Reynolds number turbulence models of RNG, AKN, V2F and ( k − ω ) SST that are recommended in literature are selected. Also, the LS model demonstrating good performance in some in-house CFD codes has been reimplemented via User-Defined Functions (UDFs) and examined for supercritical simulations, in particular with strong buoyancy effects. The results indicated that the AKN model works best among the tested models on the reproductions of some bulk parameters that are of interest (such as wall temperature, Nusselt number and skin friction coefficient) and the distortions of the turbulent velocity profile caused by the strong buoyancy, closely followed by the V2F model. The UDFs implemented LS model exhibits much better performances than the originally incorporated version in FLUENT, which is attributed to the more proper model implementation that leads to better treatments on the fluid flow and heat transfer, especially in the near-wall regions. Regarding the reproductions on the turbulence kinetic energy generations under the studied conditions, the RANS models are unable to give satisfactory results, the simulated suppression in the top half is more drastic. Similar to the vertical supercritical fluids, with quite high-strength buoyancy, the heat transfer recovers around the horizontal pipe top wall but the RANS models' response to the “recovery” is poor. The model performance on reproducing the turbulent statistics is closely related to the implementation of the vital structural parameters.

Simulation of the Formation and Growth of Soot Aerosol Particles in a Premixed Combustion Process Using a Soot Aerosol Dynamics Model

Recently, an aerosol dynamics model—the Soot Aggregate Moment Model (SAMM)—that can efficiently trace the size distribution and morphology of soot particles was developed. In order to examine the applicability of SAMM in association with open-source CFD and combustion chemistry solvers, the formation and growth of soot particles in a premixed ethylene/air combustion were simulated by connecting SAMM with OpenSMOKE++ in this study. The simulation results were compared with available measurements and with the results of a previous study conducted using SAMM connected with an in-house CFD code and the CHEMKIN combustion chemistry package. Both CHEMKIN and OpenSMOKE++ underestimated C2H2 concentration compared to previous measurements, with deviation from the measured data being smaller for OpenSMOKE++. The chemical mechanism adopted in the CHEMKIN package was found to underestimate pyrene concentration by a factor of several tens. OpenSMOKE++ predicted much higher soot precursor concentrations than CHEMKIN, leading to a higher nucleation rate and a faster surface growth in the latter part of the reactor. This resulted in a reasonable soot production rate without introducing an artificial condensation enhancement factor. The overestimation of low-molecular-weight polycyclic aromatic hydrocarbons in the latter part of the reactor and the neglect of sintering led to an overprediction of soot production and primary particle number. This result indicates that accounting only for obliteration without sintering in SAMM could not simulate the merging of primary particles sufficiently. This indication merits further investigation.

A rational hybrid RANS-LES model for CFD predictions of microclimate and environmental quality in real urban structures

We report on the application and comparative assessment of two rational turbulence modelling approaches for the computer simulation of air flow and pollutant dispersion in real urban environment: a stand-alone unsteady Reynolds-averaged Navier-Stokes eddy-viscosity model (URANS), and its blend with Large-eddy simulations (LES) in a hybrid mode (HRL). The elliptic-relaxation (k−ε)ζ−feddy-viscosity model was applied in both methods. The models, verified earlier in a range of engineering flows including heat transfer, were here validated in two environmental benchmark cases, a single building and an idealized urban settlement, both subjected to a steady wind. The velocity field and pollutant concentration are better predicted by the HRL method in both benchmarks. The HRL is then applied to predictions of air flow and spreading of pollutant from road traffic in downtown of a real city (Sarajevo) containing 100 realistic buildings. The simulations were performed with the in-house open-source CFD code T-Flows, specifically optimized for a fast and efficient high-quality unstructured meshing of the terrain orography and building configurations. The benchmark validations demonstrated that, compared with the standard k−ε, in typical urban applications with complex urban shapes and arrangements the accuracy and credibility of the predictions can substantially be improved by applying a physically sounder and yet still relatively simple ζ−f eddy-viscosity model that specifically accounts for the elliptic inviscid wall-blocking effects and near-wall stress anisotropy. The benchmarking also showed that still further improvements are achieved by using the HRL scheme which ensures a rational balance in capturing the important physics and the computational economy.

Modelling of laminar diffusion flames with biodiesel blends and soot formation

Modelling of soot formation and oxidation of biodiesel fuels is a challenge, due to the complexity of the chemical reactions and soot formation pathways. In this paper, the discretised population balance approach and a PAH-HACA based detailed soot model have been coupled with an in-house CFD code to simulate a laminar diffusion flame with blends of different components of oxygenated biodiesel fuel, and employed to predict combustion performance and soot formation. The simulation aims to reproduce a target experiment which investigated the effects of dibutyl ether (DBE) addition to the biodiesel surrogate (methyl decanoate, MD) by increasing the mole fraction of DBE from 0 to 40%. A combined and reduced MD–DBE–PAH mechanism developed from three sub-mechanism branches has been employed in the simulation. The simulation results show that temperature rises as the DBE percentage increases to 40%. The swallow-tail shape of the soot occurrence zone and the quantity of soot production are correctly predicted. Regarding the effect of soot suppression, the model has basically captured the reducing tendency of soot formation in the measurements as the DBE addition increases from 0% to 40%. Concentrations of PAHs and C3 species contributing to the formation of aromatic rings are slightly reduced due to the addition of DBE, which is a leading cause of soot suppression. However, on the whole, the numerical solution featured much smaller differences than those observed in the experiment among laminar flames with different MD/DBE ratios, because the combined MD–DBE–PAH mechanism only present a slight difference of concentrations of soot precursor species.

Effects of new solid propellant configurations on the combustion characteristics of a ramjet

The solid-fuel ramjet (SFRJ) motor is the most favorable air-breathing propulsive system and a suitable option for extending the speed and range of some military applications as in ramjet-powered missiles. However, SFRJ encountered severe challenges associated with low regression rate, as this latter plays an important role on the performance of the solid-fuel ramjet motor. To improve the solid-fuel ramjet's regression rate, this paper proposes new configuration which employs two solid fuel tubular grains with keeping overall simplicity of classic SFRJ motor, then the new design is numerically investigated by a developed in-house CFD code. The developed code is used to estimate Reynolds-averaged Navier–Stokes equations of reacting, unsteady, turbulent, and swirling flow which is validated and verified for several test cases. The numerical results for proposed designs are compared with classic SFRJ. The results reveal significant increase in solid fuel regression rate and heat transferred into grain's interior surface. Moreover, the thrust, specific impulse, and characteristics velocity are increased as well.

Mathematical modeling of porous combustion under various working conditions

Porous burner offers attractive advantages including high combustion performance and power ranges, and approximately zero pollutant emission. In this paper, the thermal behavior of premixed flame of a CH4/air mixture in a porous burner was examined using an unsteady In-house CFD code with an appropriate single reaction mechanism. In this regard, to solve the transient term, an implicit scheme was used, and displacement terms were solved using the upwind methodology. Additionally, the discretized central difference was employed to solve diffusion terms. At first, the results were compared with available published data, and then the influences of various working parameters, including solid matrix conductivity and volumetric heat transfer coefficient, were investigated on the flame structure. Thermal performance of the burner is studied by varying relevant parameters within a practical range. The predicted gas temperature profile near the reaction zone is much broader than that in an adiabatic premixed flame. The effect of volumetric heat transfer coefficient on the temperature profile is investigated. The local gas temperature decreases in the reaction zone and the solid temperature increases in the preheat zone with an increase in convective heat transfer. Also, the corresponding local convective heat transfer rates for different values of volumetric heat transfer coefficient are calculated and compared. Simulation results also indicate that increasing the effective thermal conductivity of the solid will decrease the solid phase temperature downstream of the flame location. Due to high temperature gradients in the solid matrix, its effective thermal conductivity has a significant impact on the conductive heat transfer rates.

Heat Transfer Performance of Uni-Directional and Bi-Directional Lid-Driven Cavities Using Nanoparticle Enhanced Ionic Liquids (NEILS)

Two-dimensional numerical investigation has been carried out using an in-house CFD code to obtain the heat transfer performance of normal, uni-directional and bi-directional lid-driven cavity. The 2D in-house code is based on single phase approach for studying steady state laminar flows. Pure water (PW), ionic liquid (IL): 1-ethyl-3-methylimidazolium methanesulfonate; [C2mim][CH3SO3], ionic liquid mixtures (PW + IL) and alumina (Al2O3) nanoparticle enhanced ionic liquids (NEILS) are used in the present investigation. The thermophysical properties of all the heat transfer fluids used in the present numerical investigation were obtained from the experimentally measured data available in the literature. The obtained results showed that uni-directional lid-driven cavity exhibits 68.78 % higher Nuavg value at Re = 1900 in comparison with normal lid-driven cavity for pure water. The use of [C2mim][CH3SO3] ionic liquid also enhances the heat transfer performance by exhibiting 115.48 % higher Nuavg value in comparison with pure water at Re = 1900. Moreover, 15 % wt Al2O3 nanoparticles in 0.25PW + 0.75IL gives 21.2 % and 161.16 % higher heat transfer performance in comparison with ionic liquid and pure water respectively at Re = 1900. Furthermore, it is also observed that 10 % wt Al2O3 nanoparticles in 0.25PW + 0.75IL shows similar heat transfer performance like ionic liquid at all Re.

Pressure recovery during pressure reduction experiment with large-scale liquid hydrogen tank

The world's first Liquefied Hydrogen (LH2) Carrier which will transport LH2 from Australia to Japan has been built. At Japanese port, reducing the tank pressure will be required for the safe tank operation. However, pressure reduction will cause flashing, leading to an excess of the venting capacity or liquid leakage. Hence, the purpose of this research is to clarify the pressure reduction rate and liquid behavior through experimental and numerical approaches respectively. Pressure reduction experiment under high liquid level condition was conducted by using 30 m3 tank filled with saturated LH2. The experiment showed that the pressure recovery occurred at the beginning of depressurization, and then the pressure decreased based on the equilibrium state. From numerical analysis by VOF based in-house CFD code, it was found that the pressure recovery was caused by the boiling delay, and the tank pressure followed the saturation pressure after the liquid fully stirred.

Unsteady development of Marangoni convection in a sidewall moving open cavity

In this study, simultaneous effect of free, forced and Marangoni convection on fluid flow and heat transfer in a sidewall moving open cavity is investigated. The cavity is filled with low Prandtl number fluid (Pr = 0.054); such a situation can be find in the application of liquid metal and semiconductor melts. Free convection is induced due to the heating of fluid from the bottom of the cavity. Forced convection is originated due to the translation of the sidewalls either in upward or downward direction. Whereas, the Marangoni convection is generated in the upper portion of the fluid due to imposed temperature gradient along the top free surface. The transient phenomena of the combined effects are analyzed numerically in dimensionless forms utilizing an in-house CFD code. The analyses are conducted under different pertinent parameters such as Marangoni number, Biot number, Reynolds number and Richardson number. The study is conducted to explore the flow-field structures and heat transfer characteristics in the presence of three convection modes with their relative strengths.

Preliminary study on heat flux measurement data of TT-0 flight test

With the explosive development of aerospace science, the design of the new generation airliner at higher speeds is attracting more attentions. To achieve this goal, it is necessary to achieve accurate prediction of the aerodynamic heating / force loads and successful reduction of drag and heat flux. As a remedy for the existing studies which are based upon the CFD and wind tunnel tests, this study presents a flight test for the drag and heat reduction spike technology. The principal goals of this flight test were to provide reference for verifying the accuracy of the prediction technology on ground and promote the development of the drag and heat reduction technology. By adopting the OS-X rocket, the TT-0 test vehicle designed by Shenyang Aircraft Design & Research Institute reached a maximum Mach number of 5.8 and a maximum altitude of 38 km. Hypersonic and supersonic pressure data by pressure scanning valves and heat fluxes by gauges at different locations were obtained successfully. Also, heat fluxes obtained by in-house CFD code are illustrated in comparison with the flight data. The results indicate that the numerical errors are large in most cases. More technologies, such as more CFD codes and more numerical procedures, should be adopted to conduct studies on this issue in the future.

Enhanced Pressure Based Coupled Algorithm to Combine with Pressure–Velocity-Enthalpy for all Mach Number Flow

In this paper, a pressure-based coupled computational fluid dynamics algorithm for numerical analysis of all Mach number region flow is developed. For this purpose, an enhanced pressure based coupled algorithm was developed through the pressure–velocity coupled algorithm and the pressure-enthalpy coupled algorithm were combined. In additional, the Kurganov–Tadmor flux splitting scheme, which is mainly used in density-based solvers, was applied to a developed pressure-based coupled solver. To confirm the analytical ability of developed solver, the variety of Mach number flow problems were performed using the developed solver. It was confirmed that the developed solver had the similar analytical ability with that of the other numerical codes through the analysis of the shock tube problems. In order to verify the analytical ability for the variety Mach number flow region of the developed solver, 2D bump and nozzle problems and 3D missile and wing problems were analyzed and compared with results of experiments and other numerical analysis codes. It is confirmed that the analytical ability of developed solver in the all speed flow region is somewhat improved than the commercial analysis package and is similar to the density based in-house CFD code.

Natural convection in a square cavity containing two heat-generating cylinders with different geometries

This work presents a numerical study of laminar natural convection in an air-filled square cavity (Pr = 0.71). Inside this cavity, there are two cylinders having the same surface and different geometric shapes (square, circular, and elliptical). These two cylinders generate uniform volume powers represented by two Rayleigh numbers. The right wall of the cavity is cooled uniformly with a constant temperature Tc, while the other walls are assumed to be perfectly insulated. Numerical simulations are performed with an in-house CFD code using the Finite Volume Method (FVM) based on the SIMPLE algorithm. The objective of this study is to determine the effects of the two studied Rayleigh numbers (ranging from 103 to 107) and the effect of the cylinders’ geometry on the temperature and flow fields, as well as on the local heat transfer. The results obtained show that Rayleigh numbers significantly affect the flow and heat transfer in the cavity.