Defect and Diffusion Forum Vol. 362

Abstract: The thermal aerodynamic analysis of the processes in thermal turbomachinery is of great importance when it comes their design and operation in order to achieve reliable and trouble-free operation in the required turbo-power range. The distribution of kinematic and thermodynamic parameters of the working medium around heavy loaded working disks and labyrinth seals has a significant influence on heat-mass exchange and energy transformation processes. Object of this work is thermo-aerodynamic research of mass exchange processes associated with the movement of the working medium in typical complex clearances between the rotor and stator of the steam turbines of disk type and determining axial forces in the rotor. Results based on one-dimensional and two-dimensional formulations of the problem are analyzed and compared with the results of field experiment of turbine P12-90 / 18, which before the reconstruction had problems with unstable axial loading during operation in wide power range. After proper reconstruction the turbine is in a sustainable balance throughout the whole range of operation modes. There is consistency in the results in quantitative and qualitative terms regarding the extreme conditions of axial loading. There is a three-dimensional approach to solving the problem of distribution of axial loading on the structural elements of the rotor, whose advantage is the obtaining of a detailed picture of the passing fluid in the clearances between the rotor and stator of the aggregate and diaphragm-disk spaces, and a detailed presentation of the uneven distribution of the axial forces on the front surfaces. The applied thermal aerodynamic approach allows to predict the main characteristics of steam turbines at different axial and radial clearances, changing during the operation in case of wear of the the crest of the labyrinth seals. This approach can serve as a thermo-aerodynamic diagnosis of the condition of the flow part of different thermal turbomachinery and in variable working modes.

Abstract: The aim of considerations is the numerical modeling of thermal processes proceeding in the system external heat source - protective clothing - air gap - human skin and next the application of sensitivity analysis methods to study the impact of clothing parameters on the transient temperature field in domain of skin tissue. From the mathematical point of view the problem is described by the system of Fourier and Pennes equations determining the transient temperature field in the successive sub-domains. This system is supplemented by the appropriate boundary and initial conditions. Taking into account the geometrical properties of the domain considered, the 1D solution seems to be sufficiently exact (e.g. chest or back). The sensitivity model is constructed on the basis of the so-called direct approach. The equations creating the mathematical model of the process considered are differentiated with respect to selected parameter. At the stage of numerical modeling the 1^st scheme of the boundary element method for parabolic equations has been used.

Abstract: In this work, the coarse-grained molecular dynamics simulation is employed to study lubricant evolution and depletion when subjected to a moving laser heat source. A layered film structure is formed in the equilibrium lubricant system due to the polar interactions of the lubricant functional end groups with the disk substrate. The lubricant surface morphology and depletion profiles during laser heating are studied. It is shown that the lubricant undergoes severe depletion increasing as the laser heats up with time, resulting in aggravated lubricant diffusion and evaporation. Moreover, the surface temperature profile is examined under a moving laser heat source and it reveals that the increased temperature is centered around the laser beam and quickly decays away from the laser beam. The non-uniform temperature is formed due to heat transfer between heated beads and surrounding beads, which leads to non-uniformity of surface tension and thermocapillary stress, thereby depleting the lubricant away from the scanning laser beam on the disk surface.

Abstract: Mechanisms related to diffusion flame stabilization have been the subject of several studies within the last decades due the industrial and scientific interests. Information on flame stability is of fundamental importance in energy efficiency and safety regarding industrial applications. Thus, an experimental study was performed in order to examine the flame characteristics and regions of stability limits. In this study, a representative burner of industrial applications was employed, which allows the stabilization of several combustion regimes. The lift-off and blow-out flame regimes were investigated for different proportions of carbon dioxide in natural gas. In this way, an analysis of the calorific fuel influence on the flame stability was performed based on the measurements and a comparison with classical literature models was done. The fuel dilution by adding carbon dioxide was found to decrease the soot production, leading to lower flame heights and also, lower lift-off and blow-out limits. Results obtained from this study encourage future works which consider flames in large scale, in order to equate to industrial applications.

Abstract: There is a still dangerous effect in the form of methane leakage from coal seams in areas where mining activity still continuing or took place in the past. The issue of mining gas flow is still a matter that is necessary to pay attention and try to find new ways to solve this set of phenomena, no matter if it is the mining corridors or the flow in the rock massif. The number of measures to gradually reduce risk has been taken to protect the population. Nevertheless, the current situation is generally rated as serious as those risks still remain valid. Leakage of mine gases depend on many natural and technical mining conditions. With the closure of mines and thus the end of the ventilation situation considerably worse. This paper deals with the flow of dangerous mine gas, methane, through the rock mass using numerical flow modeling using CFD program Fluent. Using CFD codes can gain insight on the phenomenon under review and the results to take appropriate measures in the form of active or passive intervention.

Abstract: Knowledge of fluid rheology and flow characteristics is important when studying nanofluid flow in porous media. In this study, an experimental investigation is presented to determine the nanofluid viscosity, the permeability and the inertial (non-Darcy) parameter of a porous cylinder made of several capillary tubes. The applicability of the Darcy-Forchheimer equation for power-law fluids to estimate pressure drop through the porous material is discussed. The occurrence of particle losses from the base fluid (deposition) is also verified.Experiments are completed in two steps. In the first step, physical properties of nanofluids consisting of deionized water and different volume concentrations of Al₂O₃ nanoparticles is measured. In the second step, Al₂O₃-deionized water nanofluids are pumped through a porous cylinder (porosity 0.249) to evaluate hydraulic and intrinsic permeabilities, and the inertial parameter. The effect of Al₂O₃ volume fraction on these flow properties is studied, and the void morphology changes within the porous cylinder via deposition of nanoparticles are analyzed.

Abstract: An efficient numerical approximation for two phase flow in 1D is presented. Mathematicalmodel is based on two Richard’s type equations using Van Genuchten-Mualem (vG-M) model for capillarypressure-saturation and hydraulic permeability versus saturation of wetting liquid. The wettingand non-wetting liquids are incompressible and immiscible. The method is suitable for determinationof soil parameters (as a tunning parameters in vG-M model) via solution of inverse problem. Wettingliquid (-water) is injected into the sample originally saturated with non-wetting liquid (-oil) by gravitation,or centrifuge driving forces. In a series of experiments we discuss noninvasive (easy-to-measure)measurement scenarios which are satisfactory in a solution of inverse problem.

Abstract: The investigation on mixed convection boundary layer of a viscoelastic fluid over a sphere which is embedded in porous medium under convective boundary condition is carried out in this paper. The boundary layer equations of viscoelastic fluid are an order higher than Newtonian (viscous) fluid and the adherence boundary conditions are insufficient to determine the solution of these equations completely. Hence, the augmentation on extra boundary conditions is needed in order to solve this problem. The governing partial differential equations are first transformed into non-dimensional forms and then solved numerically using the Keller-box method by augmenting extra boundary conditions at infinity. The numerical results obtained for limiting case are comparing with related outcomes in order to validate the present results. Results on the effects of the viscoelastic parameter in the presence of porosity and mixed convection on the skin friction and heat transfer as well as velocity and temperature profile have been discussed.

Abstract: This paper studies the effect of periodical gravity modulation, or g-jitter induced magnetohydrodynamics (MHD) mixed convection flow past an inclined stretching sheet. Using appropriate non-dimensional variables, the governing partial differential equations are first transformed into non-dimensional form. The obtained non-dimensional equations are then solved numerically using Keller-box method. The features of the flow with heat transfer characteristics for different values of frequency, amplitude and magnetic parameters on the velocity and temperature profiles are analyzed and discussed. The behaviour of physical quantities such as skin friction and heat transfer coefficients are also investigated. To validate the present numerical results, comparison with published results is done and found to be in a good agreement.