In-house CFD Code Research Articles

A computational investigation of olive oil residues combustion in a vertical furnace

The paper presents a numerical investigation of combustion of pulverised olive waste and its impregnated samples. Olive oil by-products, mainly olive solid waste (OSW) and olive mill wastewater (OMWW) mixtures that are called impregnated solid waste (ISW) are investigated for different mixing ratios. The investigation is carried out by an in-house two-dimensional CFD code based on the finite volume method, developed for this purpose. The model is first validated by comparisons to the experimental data and predictions of other authors. Subsequently, it is applied to the cases of present interest, predicting the efficiency of the thermal process, species and temperature distributions. The combustion behaviour of ISW is compared with that of the original OSW under the same conditions. The findings of the present study shows that a beneficial effect of using ISW is observed, as the addition of OMWW leads to a decrease of carbon oxides production. Furthermore, the OMWW addition does not have a negative effect on their firing quality since the different samples exhibit quite close temperature profiles. Obtained results are important for the investigation the possibility of using olive residues as an alternative energy source in olive producing countries.

Computational fluid dynamics simulations of unsteady mixing in spacer-filled direct contact membrane distillation channels

Direct contact membrane distillation (DCMD) is a promising means of concentrating brines to their saturation limit. During that process, membrane spacers play a key role in temperature polarization, concentration polarization, and mineral scaling. These interactions are not well understood, because they are difficult to study experimentally and numerically, and the flow regimes are not fully charted. We consequently develop a tailored in-house CFD code that simulates unsteady two-dimensional heat and mass transport in plate-and-frame DCMD systems with cylindrical spacers. The code uses a combination of finite-volume methods in space, projection methods in time, and recent advances in immersed boundary methods for the spacer surfaces. Using the code, we explore how the transition to unsteady laminar vortex shedding affects polarization and permeate production of DCMD systems. We show that the impact of spacers can be explained by examining the various steady and unsteady vortical flow structures generated in the bulk and near the membranes. Overall, we show that though unsteady vortex structures tend to mix temperature polarization layers with the bulk, they are not similarly able to mix the concentration layers. Rather, vortical structures tend to create regions of preferential salt accumulation. In the vortex shedding regime, the net result is that spacers often increase vapor production at the expense of increasing the risk of mineral scaling.

Enhancing the in Situ Visualization of Performance Data in Parallel CFD Applications

This paper continues the work initiated by the authors on the feasibility of using ParaView as visualization software for the analysis of parallel CFD codes’ performance. Current performance tools are unable to show their data on top of complex simulation geometries (e.g. an aircraft engine). In our previous paper, a plugin for the open-source performance tool Score-P has been introduced, which intercepts an arbitrary number of manually selected code regions (mostly functions) and send their respective measurements – amount of executions and cumulative time spent – to ParaView (through its in situ library, Catalyst), as if they are any other flow-related variable. This paper adds to such plugin the capacity to also show communication data (messages sent between MPI ranks) on top of the CFD mesh. Testing is done again with Rolls-Royce’s in-house CFD code, Hydra. The plugin’s original feature (regions’ measurements) is here revisited, in a bigger test-case, which is also used to illustrate the new feature (communication data). The benefits and overhead of the tool are discussed.

Numerical solution of the conjugate heat transfer problem for turbulent liquid flow in a tube using the large eddy simulation method

This article presents the results of turbulent flow and temperature fluctuations simulation in the liquid and the tube wall in a conjugate formulation using the hybrid Reynolds-averaged Navier-Stokes / large eddy simulation (RANS/LES) method. Calculations are performed for water, air and mercury. The results show that the “conjugate” LES model implemented in in-house CFD code ANES provides a good reproduction of the available experimental results on temperature fluctuations near the wall for cases of different wall thicknesses and thermal activity of the wall material and the fluid. An approximate model for the wall is proposed and can be used as a boundary condition for calculations by direct numerical simulation (DNS) and LES methods in the non-conjugate formulations.

Evaluation of correlations for prediction of onset of heat transfer deterioration for vertically upward flow of supercritical water in pipe

Supercritical water has great potential as a coolant for nuclear reactor. Its use will lead to higher thermal efficiency of Rankine cycle. However, in certain conditions heat transfer may get deteriorated which may lead to undesirable high clad surface temperature. It is necessary to estimate the operating conditions in which heat transfer deterioration (HTD) will take place, so as to establish thermal margins for safe reactor operation. In the present work, the heat flux corresponding to onset of HTD for vertically upward flow of supercritical water in a pipe is obtained over a wide range of system parameters, namely pressure, mass flux, and pipe diameter. This is done by performing large number of simulations using an in-house CFD code, which is especially developed and validated for this purpose. The identification of HTD is based on observance of one or more peak/s in the computed wall temperature profile. The existing correlations for predicting the onset of HTD are compared against the results obtained by present simulations as well as available sets of experimental data. It is found that the prediction accuracy of the correlation proposed by Dongliang et al. is best among the existing correlations.

Magneto-thermal convection in lid-driven cavity

The fluid flow and convective heat transfer occurring in a lid-driven cavity which is filled with electricity conductive fluid and subjected to external magnetic field, is analyzed comprehensively. The variations in inclination angle of magnetic field and its strength are addressed during the investigation. The cavity is heated by a linearly varying heat source applied on the left wall. Different regimes of heat transfer are considered by varying pertinent flow-parameters namely Reynolds number (Re), Richardson number (Ri) and Hartmann number (Ha). The effects of magnetic field and the wall motion are studied extensively. The study is carried out using a well-validated in-house CFD code based on finite volume method and SIMPLE algorithm. The obtained results reveal strong influence of Ri and Ha on the heat transfer characterization.

Identification of upper branch for vortex-induced vibration of a circular cylinder at [formula omitted

In this study flow around a circular cylinder undergoing transverse-only vortex-induced vibrations is investigated using a two-dimensional in-house CFD code. The Navier–Stokes equations, the continuity and pressure Poisson equations written for an incompressible constant property Newtonian fluid coupled with the structural equation are solved using the finite difference method. Systematic computations are carried out for different damping ratio values between ζ=0% and 5%. Reduced velocity is varied in U∗=2.5−7.5, while Reynolds number and mass ratio are fixed at Re=300 and m∗=10, respectively. Up until now, researchers have reported an upper branch only at high Reynolds numbers and low m∗ζ values. However, in this study we have observed a three-branch behavior (initial, upper and lower branches) at Re=300 for ζ≤1%. The upper branch is bounded by two gradual phase changes: at the boundary adjacent to the initial branch, the time-averaged phase difference of vortex force, and at the boundary to the lower branch, the time-averaged phase difference of transverse fluid force relative to the cylinder displacement changes between 0° and 180°. Unbounded variations and phase slips are observed in the time-dependent phase differences, which explains the gradual changes in their time-mean values. In some ranges of the upper branch the second harmonic frequency component plays an important role in the spectra of transverse fluid force, which is closely related to the observed asymmetrical vortex structure. Increasing the structural damping above ζ=1%, only initial and lower branches are found. The comparison of peak oscillation amplitude against an empirical relationship provided in the literature shows very good agreement for ζ≥1%. A Griffin plot is also used for the comparison of peak response data where the currently obtained results and those in the literature collapse onto almost a single curve.

Thermal magneto-hydrodynamics in a ventilated porous enclosure

The present study examines mixed convection in a ventilated cavity saturated with porous substance and heated at the right-bottom and top-left corners under the influence of uniform magnetic field applied externally. The analysis is carried out numerically using in-house CFD code. Five different heating configurations are analyzed. The effects of pertinent parameters such as Reynolds number (Re = 1–1000), Richardson number (Ri = 0.1–100), Darcy number (Da = 10−7–10−3), Hartmann number (Ha = 1–100), angle of magnetic field ( $$ \gamma $$ = 0–180°) and porosity (e = 0.1–1.0), on the flow and temperature fields are examined for the search of the appropriate configuration yielding optimum heat transfer rate. Furthermore, heat transfer characterization at different heating configurations and parametric combinations is assessed.

Population balance modelling and laser diagnostic validation of soot particle evolution in laminar ethylene diffusion flames

Laminar diffusion flames present an elementary configuration for investigating soot formation and validating kinetic models before these are transferred to turbulent combustors. In the present article, we present a joint experimental and modelling investigation of soot formation in a laminar co-flow burner. The diffusion flames are analysed with the aid of laser diagnostic techniques, including elastic light scattering (ELS), planar laser-induced fluorescence of OH (OH-PLIF) and line-of-sight attenuation (LOSA), to measure the spatial distribution of soot, gas phase species and the line-of-sight integrated soot volume fraction (ISVF), respectively. The experimental dataset is supplemented by location-specific TEM images of thermophoretically sampled soot particles. The simulation of the sooting flames is carried out with a recently developed discretisation method for the population balance equation (Liu and Rigopoulos, 2019, Combust. Flame 205, 506-521) that accomplishes an accurate prediction of the particle size distribution, coupled with an in-house CFD code. By minimising numerical errors, we ensure that the discrepancies on the modelling side are mainly due to kinetics and are able to carry out an investigation of alternative models. We include a complete set of soot kinetics for PAH-based nucleation and condensation, HACA-based surface growth and oxidation as well as size-dependent aggregation, and consider three different gas phase reaction mechanisms (ABF, BBP and KM2). Based on predictions of the gas phase composition and particle size distribution of soot, modelled counterparts of the laser diagnostic signals are computed and compared with the experimental measurements. The approach of directly predicting signals circumvents the difficulties of explicitly representing the OH concentration in terms of the measured OH-PLIF data and avoids using ‘hybrid’ modelled and measured values to approximate the OH concentration. Moreover, the LOSA signal is directly converted to the line-of-sight ISVF instead of a measure of local soot volume fraction to avoid tomographic inversion errors. Lastly, the predicted ELS signal is computed in terms of the particle size distribution resolved by the population balance model, thus circumventing the approximation of an integral soot property using a presumed size distribution. While we cannot obtain quantitative agreement between experiments and simulations, the accuracy of the numerical approach and the direct prediction of experimental signals allow us to conduct sensitivity analyses of key empirical parameters and investigate the importance of the PAH chemistry and its influence on the competition between nucleation, condensation and surface growth.

Preconditioning methods for compressible flow CFD codes: Revisited

The present article aims to implement and investigate a different preconditioning method based in a three-dimensional in-house compressible CFD code that ensures the robustness and numerical stability to determine the flowfield considering low Mach number flow. The present preconditioning method involve two different methodologies developed by references [8, 32]. The CFD solver was developed to calculate the Euler and Navier-Stokes equations, numerically, for steady-state regime based on the cell-centered finite volume method (FVM) using Reynolds Averaged Navier-Stokes equations (RANS). The centered second-order scheme was used for the discretization of convective terms from momentum equations. The explicit second-order five-step Runge-Kutta scheme was employed for the time-marching procedure, using an implicit residual smoothing technique to enhance the numerical stability. A local preconditioning method was implemented due to its robustness in predicting low Mach number flows in a compressible CFD code environment. However, for low Mach number flows, near stagnation points, numerical perturbations were amplified generating a stiffness in the convergence rate, which provided an inaccurate solution and numerical stability degradation. Aiming to improve the preconditioning robustness, a flux function and a new limiter were applied to operate with the preconditioning technique based on a pressure sensor. Those corrections re-scale the eigenvectors and ensure the locality of the algorithm, which improves the numerical stability and guarantees the convergence for low-speed flows. The inviscid flow over a NACA 0012 airfoil for compressible and incompressible cases shown accurate and robust solutions. For a viscous flow over a flat plate case in the compressible and incompressible cases, the preconditioning technique purposed in this work supplied good numerical solution in agreement with the analytical solution.

Natural Convection and Surface Radiation Heat Transfer in a Square Cavity with an Inner Wavy Body

An in-house CFD code has been developed to address the coupling between natural convection and surface radiation in irregular geometries encountered in cooling electronic components. In the current numerical study, the discrete ordinate methods with the concept of blocked-off region are used in order to investigate the conjugate natural convection and surface radiation in an enclosure containing a wavy inner circular cylinder. The validity of the numerical code is ascertained by comparing our results with previously published results dealing with the limiting case of a smooth circular cylinder enclosed in a cavity. Extensive computations have been performed to study in detail the effect of the Rayleigh number ( $$10^{3} \le {\text{Ra}} \le 10^{7}$$ ), the inner and outer surface emissivities ( $$0 \le \upvarepsilon_{{{\text{E}}\,{\text{or I}}}} \le 1$$ ), the undulations number ( $$4 \le {\text{N}} \le 1 2$$ , their amplitude ( $$0 \le {\upalpha} \le 1$$ ) and the average radius of the wavy circular cylinder ( $$0.05 \le {\text{R}}_{ 0} \le 0.25$$ ) on the flow structure as well as on the convective and radiative heat transfer characteristics. The obtained results showed that the amplitude and the average radius have more significant effect on the total heat transfer rate than the undulations number of the wavy wall.

Parallelization of a stochastic Euler-Lagrange model applied to large scale dense bubbly flows

A parallel and scalable stochastic Direct Simulation Monte Carlo (DSMC) method applied to large-scale dense bubbly flows is reported in this paper. The DSMC method is applied to speed up the bubble-bubble collision handling relative to the Discrete Bubble Model proposed by Darmana et al. (2006) [1]. The DSMC algorithm has been modified and extended to account for bubble-bubble interactions arising due to uncorrelated and correlated bubble velocities. The algorithm is fully coupled with an in-house CFD code and parallelized using the MPI framework. The model is verified and validated on multiple cores with different test cases, ranging from impinging particle streams to laboratory-scale bubble columns. The parallel performance is shown using two different large scale systems: with an uniform and a non-uniform distribution of bubbles. The hydrodynamics of a pilot-scale bubble column is analyzed and the effect of the column scale is reported via the comparison of bubble columns at three different scales.

A numerical and analytical coupling method for predicting the performance of intermediate-pressure steam turbines in operation

This study proposes a numerical and analytical coupling method for simulating unsteady steam flow through three-stage stator and rotor blade rows of an intermediate-pressure steam turbine operating in a power plant by considering both the secular-changed blades and the leakage flows in labyrinth seals. The flow simulation was based on the in-house CFD code developed by Tohoku University, whereas the leakage flows in labyrinth seals were predicted by an analytical method. The effects of secular-changed blades and leakage flows on unsteady steam flows are discussed on the basis of time-dependent solutions. Next, the total performances obtained under conditions using manufactured and secular-changed blades both with and without leakage flows were compared with the actual data measured in operation. The obtained results indicate that the increase in static temperature at the outlet of actual intermediate-pressure steam turbines during a long-time operation is obviously caused by the secular change of blades. This paper presents the first results based on a numerical approach to clarify the temperature increase in a working intermediate-pressure steam turbine.

Numerical study of the flow field and heat transfer in a channel equipped with phase change material blocks

In this paper, pressure drop and change in bulk temperature in a channel with two isothermal blocks have been investigated. Different geometrical parameters have been studied numerically to find the effects of blocks size and position on the flow field and heat transfer. The governing equations, including mass, momentum, and energy conservations, have been discretized using the finite-volume method, then have been solved by SIMPLE method, collocated grid, and Rhie-Chow interpolation using an in-house CFD code. Investigating the effects of geometrical parameters including blocks size and position shows that increase in the block size (dx = dX/H) and increase in the distance of the center of the blocks from the walls (y = Y/H) of the channel, the pressure drop and change in bulk temperature is increased. Also, the efficiency coefficient defined by dividing the bulk temperature change to pressure drop shows an optimum value of 15%–20% of channel height for distance from wall and 10%–12% of channel height for the block size in the studied range. The results of this study could be impressive in designing configurations such as PCM equipped air-conditioning systems to shift the peak times of energy consumption.

Coupled thermal radiation and natural convection heat transfer in a cavity with a discretely heated inner body

The natural convection coupled to surface radiation in an asymmetrically cooled square cavity including a partially heated inner circular cylinder is numerically simulated in this paper following the finite volume and the discrete ordinate methods. The flow inside the cavity is driven by the heated part of the circular cylinder maintained at high temperature 297K≤TH≤357K. The cavity is filled with the air (Pr=0.71) cooled by a single vertical wall maintained at a cold temperature TC=293K. The other portion of the inner circular cylinder and the remaining boundaries of the enclosure are insulated. The hydrodynamic and the thermal behavior of the fluid, the convective and the radiative heat transfer are investigated for Rayleigh number ranging from 103 to 107and the common emissivity 0≤ε≤1 of the inner and outer surfaces. The numerical work is carried out using an in-house CFD code. The obtained results show a considerable effect of the surface radiation on the flow structure and heat transfer characteristics in the cavity.

Natural convection and surface radiation heat transfer in a cavity with vertically oriented fins

In this paper, the finite volume method is combined with the discrete ordinate method to study the natural convection coupled to surface radiation in a cooled square finned cavity. The fin array under consideration is made of N thin fins arranged in a single horizontal row. For better heat transfer performances, all these fins are oriented vertically inside the cavity and heated isothermally at 299K≤TH≤341K. The cavity is filled with air (Pr=0.71) and cooled by the side vertical walls maintained at a cold temperature TC=293K. The study investigates the effect of the Rayleigh number ranging from 103 to 107, the fins number 1≤N≤15 and the emissivity 0≤ε≤1. The results are reported in terms of streamlines, isotherms and the local and the average convective and radiative Nusselt numbers. The numerical work is carried out using an in-house CFD code written in FORTRAN. The obtained results show a considerable effect of the surface radiation on the flow structure in the cavity and the maximum rate of heat transfer is reached when the fins number exceeds 9.

Numerical simulation of fluidization for application in oxyfuel combustion

<p style='text-indent:20px;'>This paper is concerned with the simulation of multiphase flow hydrodynamics in an experimental oxyfuel fluidized bed combustor designed for biomass fuels. The aim is to perform cross-validation between several models and solvers that differ in the description of some phenomena in question. We focus on the influence of turbulence modeling, inter-phase drag force models, the presence of biomass in the mixture. Also the possibility to simplify the full 3D description to a quasi-1D model is tested. However, the results indicate that such simplification is not suitable for chaotic phenomena in considered scenarios. The models were developed using ANSYS Fluent and OpenFOAM CFD software packages as well as our in-house CFD code CFBSim. The quantities relevant for comparison (the densities of the dispersed solid phases and the phase velocities) are presented in the form of cross-section averaged vertical profiles.

Numerical Simulation of Steam Condensation in a Steam-Gas Mixture Flow in a Variable-Section Channel with a Bundle of Smooth Horizontal Tubes

In this paper, the results from calculations of heat and mass transfer in a variable-cross-section channel with a bundle of smooth horizontal tubes, on the surface of which steam from a moving steam-gas mixture (SGM) condenses, are presented. The decrease in the channel’s cross section and, accordingly, the number of tubes in the vertical rows along the SGM movement provides the mixture with approximately constant velocity as the steam condenses. The mathematical model used in this study is described in detail in our previous publications. The two-dimensional equations of single-phase hydrodynamics, energy, and diffusion are solved for the external SGM flow. The condensation process is modeled at the level of the boundary conditions on the tube surface, taking into account a moving laminar condensate film. The heat transfer through the tube wall from the film to the cooling water is described using a one-dimensional model of the wall. To account for the irrigation of the bundle’s lower tubes with condensate formed on the upper tubes (inundation effect), a simplified model is used. The data on the velocity fields and impurity concentration in the condenser and the heat transfer characteristics are presented. The calculation results of the heat transfer coefficients on the tubes of the first vertical row of the bundle and the heat transfer coefficients for individual sections of the simulated condenser containing several tube rows at a 0–8.5% volume fraction of air in the SGM at the inlet to the apparatus are compared with experimental data. A quite satisfactory agreement between the calculated and experimental data is obtained, which confirms the efficiency of the used model. The calculated data on the local velocity fields and the composition of the steam-air mixture indicate a significant heterogeneity of these characteristics. This complicates the development of relatively simple engineering methods for calculating the heat load of condensers at high air concentrations. The calculations were performed using in-house CFD-code ANES.

Influence of Air Humidity on Transonic Flows with Weak Shock Waves

The paper presents CFD results for the transonic flow of dry and moist air through a diffuser and a compressor rotor. In both test geometries, i.e. the Sajben transonic diffuser and the NASA Rotor 37, the air humidity impact on the structure of flows with weak shock waves was examined. The CFD simulations were performed by means of an in-house CFD code, which was the RANS-based modelling approach to compressible flow solutions. It is shown that at high values of relative humidity, above 70%, the modelling of the transonic flow field with weak shock waves by means of the dry air model may produce wrong results.

Numerical study of enhanced and deteriorated heat transfer phenomenon in supercritical pipe flow

The mechanism of heat transfer enhancement and heat transfer deterioration for supercritical water flowing in vertical pipe is numerically studied using in-house 2D axi-symmetric CFD code, named NAFA. The numerical results obtained by the NAFA code for Yamagamata’s experiments, agree with their experimental results, thus validating the code. Simulations are carried out for low as well as high mass fluxes keeping the same heat flux to mass flux ratio. The study of effect of flow direction is also performed. The major contribution of this work is explanation of mechanism of heat transfer enhancement and deterioration in various conditions and various factors assisting or resisting the two. This explanation is given on the basis of radial variation of thermo-physical properties of the working fluid and turbulence parameters, plotted at selected axial locations where heat transfer coefficient ratio is high or low indicating enhancement/deterioration.