Home – Home – Jornals – Advanced Search

The effect of wall treatment on the performance of κ-ε model in incompressible, turbulent, separated flows with and without heat transfer has been evaluated in this study. We have simulated two benchmark cases: (i) flow past a circular cylinder at Re = 3900, and (ii) flow past a heated square cylinder at Re = 21 400 using the open source CFD package: OpenFOAM. We have compared three variants of the κ-ε model namely, Launder-Sharma κ-ε model (Yap corrected) (LSKEY), Lam-Bremhorst κ-ε model (Yap corrected) (LBKEY) and two-layer κ-ε model (TLKE) along with the available experimental and direct numerical simulation (DNS) data. Comparisons are made in terms of the models' capability to predict the mean flow variables, surface integral quantities and heat transfer characteristics at different wake locations. On the basis of the presented study, we conclude that LSKEY performs better than the other models in predicting the wake and surface flow and heat transfer parameters. Further our comparisons show that, while LSKEY and LBKEY require comparable clock time per flow-through cycle, the computational time needed by TLKE is almost twice as compared to LBKEY or LSKEY. These results call for more attention from the CFD community onto the LSKEY model, in particular, so that, it can be incorporated in various other flow fields, especially the scale resolving methodologies like the partially-averaged Navier-Stokes (PANS), wherein a superior wall treatment along with a shorter computational time could be of immense advantage. In authors' opinion, these benefits of the LSKEY model have largely been overlooked, perhaps because of a biased preference to the TLKE model, which enjoys the default presence in popular commercial computational fluid dynamics (CFD) packages.

An experimental study of the process of air outflow from a container with a characteristic duration of 0.6 to 9 s through throttling tubes of various configurations was carried out. The equivalent area of the outlet orifice is determined depending on the ratio of the length of the throttling tube to its nominal diameter. It was found that during the outflow process, the temperature of the gas inside the container decreases by 10-15%, which value differs significantly from the theoretical estimate made under the assumption that the process is adiabatic (60 %). Based on the results of measuring the pressure and temperature of the gas in the container, a method is proposed for calculating the heat flow from the walls of the container to the outflowing gas.

A novel thermal lattice Boltzmann approach is proposed for the implementation of the thermal boundary conditions at the fluid-solid interface. The numerical code, developed on the basis of the new approach, was validated against reliable numerical data from the literature in the cases of both square and circular conducting blocks. Analytical and experimental validations were also performed in the case of a circular block. The numerical tests show that the adopted approach allows to handle interface problems with large thermal conductivity ratios. In the present study, this approach is validated first in the case of a square conducting block and used to simulate a conjugate convection-conduction problem in a square cavity enclosing a circular block. The novel developed TLBM approach reduces computational memory as well as numerical programming issues associated with the use of a hybrid method that combines the lattice Boltzmann method and classical methods.

With the aim to model the self-excited oscillations of a body, a hypothesis is proposed for the formation of periodic vortex structures in the bottom wake whose frequency coincides with the natural frequency of oscillations of the body, and the force effect of the oscillations on the body is mathematically described with a harmonic function of time. Analytical formulas for aerodynamic derivatives and equivalent aerodynamic derivatives are obtained. It is shown that the mathematical model satisfactorily describes the dependence of the pitch angle on time and the dependence of the equivalent aerodynamic derivatives on the amplitude of oscillations for two moments of inertia of the body. The mathematical model predicts the hyperbolic law of dependence of the amplitude of self-excited oscillations on the reduced frequency.

The morphology of carbon material formed in an arc discharge in a mixture of i-butane, n-butane and propane when spraying a graphite-nickel electrode was studied. The experiments were carried out with changing the gas medium pressure. Carbon globules, graphene structures and carbon nanotubes have been discovered. It was found that at pressures of 75 and 400 torr, carbon globules predominate in the resulting materials. At gas pressures of 200 torr, the material collected from the cold screen surface contains both graphene-like structures and significant amounts of carbon nanotubes. The physical reasons influencing the observed phenomena are discussed.

The paper presents the calculation estimates for efficiency of regenerative cooling for a model cylinder-shaped duct using a suspension of heat-conductive metal nanoparticles in n-decane as fuel/coolant. We adapted a standard mathematical model of conjugated heat transfer that accounts for thermophysical properties of the metal nanoparticle suspension and n-decane. The data are presented for heating the nanosuspension and the model duct walls for the cases of different content of metal nanoparticles in nanosupension. The range where the use of nanosuspension gives advantages in terms of heat removal relative to the n-decane has been shown.

Heat conductivity for a Novec 7100 fluid sample was measured with a method of coaxial cylinders. Experiments were performed in the temperature range 350 - 385 K and pressure range 0.12 - 0.21 MPa. The error for experimental data on heat conductivity is about 1.5 - 2.5 %, The error in measuring temperature and pressure was less than 0.05 K and 4 kPa, correspondingly. The general equation for calculating the heat conductivity as a function of pressure and temperature was formulated. The heat conductivity was defined for the ideal gas state. A previously developed approach was tested in application for a single-measurement prognosis of heat conductivity.

Numerical simulation of unsteady processes proceeding during the laser welding of plates made of porous and monolithic (non-porous) metals was carried out. The influence of the welding speed on the quality of the resultant joints and on the seam morphology was studied. The calculated characteristics of connections between porous and monolithic stainless steel plates are in qualitative agreement with the results of physical experiments.

Physicochemical and thermal processes occurring during methane conversion into synthesis gas under non-isothermal conditions in microstructural heat exchanger-reactors based on microchannels are considered in this paper. A method for synthesizing a rhodium-based composite thin-layer catalyst for steam reforming of methane and carbon monoxide is proposed, and the results of experimental and numerical studies of the features of steam reforming under controlled thermal conditions of a microchannel reactor are presented. The determining influence of thermal processes on the rate and sequence of multi-stage heterogeneous reactions was obtained; the methods for controlling the steam reforming process have been developed to achieve high completeness of chemical transformations.

The paper presents results of experimental study on the solubility (VLE-properties) for crystalline tricosane in supercritical carbon dioxide (in pure and modified form) in the temperature range 308.15 - 315.15 K and the pressure range 8.00 - 20.32 MPa using the dynamic method. Modification of carbon dioxide fluid was performed by adding several organic solvents, such as dimethylsulfoxide, ethanol, acetone, and chloroform. It was found that using the fluid co-solvents at the concentration of 5 wt. % improves the tricosane solubility by factor of two. The measurement results can be described by the Penga-Robinson equation of state.