This paper investigates the injection of fuel into a constant volume spray chamber at high pressures by utilizing macroscopic fuel spray characteristics of mineral diesel and rapeseed oil biodiesel. In the case of lower spray chamber pressure the shapes of fuel sprays tend to be more inclined and indicate higher cavitation inside the nozzle hole. On this basis and from experimentally obtained spray tip penetration and spray angle, a modified mathematical model for spray tip penetration and spray angle is developed. This new model includes two-zone theory and the obtained results agree very well with experiments through the whole interval of spray development.
COBISS.SI-ID: 21277718
The work presents a computational approach using Computational Fluid Dynamics (CFD) for the purpose of modeling the combustion/gasification process inside the Waste -to-Energy (WtE) plant. The conversion of solid fuel in fuel bed on the grate is described with own developed empirical 1D model. Numerical results and comparison with experimental measurements showed that the used empirical model is appropriate and sufficiently reliable which provides the appropriate inlet boundary conditions for the freeboard CFD simulation.
COBISS.SI-ID: 19360022
In this paper we have developed a hybrid LES/URANS turbulent model for a BEM based turbulent fluid flow solver. We employed the unified LES/URANS approach, where the interface between the LES and URANS regions is defined using a physical quantity, which dynamically changes during numerical simulation. The main characteristic of the unified hybrid model is that only one set of governing equations is used for fluid flow simulation in both the LES and URANS regions. Regions where turbulent kinetic energy is calculated by LES and URANS models are determined using a switching criterion. We used the Reynolds number based on turbulent kinetic energy and the Reynolds number based on total turbulent kinetic energy to establish the LES/URANS interface switching criterion. Depending on flow characteristics and with the use of switching criterion, we chose between sub-grid scale viscosity (SGS) and URANS effective viscosity. The SGS or URANS effective viscosity is used in the transport equation for turbulent kinetic energy and in governing equations for fluid flow. The developed numerical algorithm was tested by simulating turbulent natural convection within a square cavity. The hybrid turbulent model was implemented within a numerical algorithm based on the boundary element method, where single domain and sub-domain approaches are used. The governing equations are written in velocity–vorticity formulation. We used the false transient time scheme for the kinematics equation.
COBISS.SI-ID: 18814742
Renewable energy production, storage and utilization has been studied in connection with fluctuating renewable power production and demand. The coupling has been considered of surplus renewable power with sewage sludge gasification and renewable syngas storage. High quality syngas produced with power support can be introduced into a gas distribution network or stored and utilized on site. Both systems enable gas consumption at power and heat demand periods. The efficiency of power conversion to syngas produced from high moisture sewage sludge has been studied using a developed equilibrium thermodynamic model for the gasification process providing the calculated prediction of the syngas amount and composition. Furthermore, the total energy efficiency has been studied of the syngas production and utilization system. The results show that the optimum scenario to get syngas suitable to introduce into a natural gas distribution network would be to gasify sewage sludge with 35–40 wt.% moisture, while the optimal parameter for syngas to be valorized onsite is, in terms of efficiency, with SS dried to 20 wt.% of moisture content. In the first scenario, a little more power is produced from stored gas in a natural gas distribution network than consumed, and almost twice as much is produced with on-site production, storage and utilization. The study has showed a technically feasible management option for energy production, storage and consumption with available technologies and infrastructure.
COBISS.SI-ID: 20335894
The paper reports on development of Boundary Element Method (BEM) based numerical algorithm for the numerical simulation of the freeze drying process in a vial. In the paper the problems of freeze-drying modeling are covered in detail. The BEM based algorithm is developed for the axisymmetrical geometry case using the Subdomain BEM approach. A special feature of the algorithm is an implicit representation of the interface conditions at the sublimation front, which is a great advantage of the proposed numerical scheme. As a test case the freeze drying of skim milk in a vial is selected. The numerical results show a good agreement with reference data proving that the developed numerical model is appropriate, accurate and fast in simulating the primary and secondary drying stage. The numerical analysis also shows that the time step during the secondary drying stage can be increased by a factor 100, which reduces the computational time drastically.
COBISS.SI-ID: 20240406
The paper studies the properties and sedimentation characteristics of sludge flocs, as they appear in biological wastewater treatment (BWT) plants. The flocs are described as porous and permeable bodies, with their properties defined based on conducted experimental study. The derivation is based on established geometrical properties, high-speed camera data on settling velocities and non-linear numerical model, linking settling velocity with physical properties of porous flocs. The numerical model for derivation is based on generalized Stokes model, with permeability of the floc described by the Brinkman model. As a result, correlation for flocs porosity is obtained as a function of floc diameter. This data is used in establishing a CFD numerical model of sedimentation of flocs in test conditions, as recorded during experimental investigation. The CFD model is based on Euler-Lagrange formulation, where the Lagrange formulation is chosen for computation of flocs trajectories during sedimentation. The results of numerical simulations are compared with experimental results and very good agreement is observed.
COBISS.SI-ID: 14734358
This paper discusses the influence of biodiesel on the engine combustion characteristics. The considered fuel is neat biodiesel from rapeseed oil. The considered engine is a bus diesel engine with injection M system. The engine characteristics are obtained by experiments and numerical simulation. The results obtained with biodiesel are compared to those obtained with mineral diesel under various operating regimes. In this way, the influences of biodiesel usage on the injection pressure, injection timing, ignition delay, in-cylinder gas pressure and temperature, heat release rate, exhaust gas temperatures, harmful emissions, specific fuel consumption, and on engine power are analyzed. Furthermore, the relationships among fuel properties, injection and combustion characteristics, harmful emissions, and other engine performance are determined. Special attention is given to possible explanations of higher NOx emission in spite of lower in-cylinder gas temperature.
COBISS.SI-ID: 14779158
The one-dimensional steady Pennes (bioheat) equation was applied to analyze heat conduction inside a combined layer of human muscle and fat, under Fanger thermal comfort conditions. The bioheat equation was solved subject to two boundary conditions at the skin surface: a prescribed skin temperature satisfying the Fanger comfort criterion, and a prescribed heat flux obtained from the overall energy balance for the system. In addition to a fixed body core temperature, an adiabatic condition was imposed as an auxiliary condition at the core of the body, and a pair of equations were derived, relating the blood perfusion and the volumetric heat generation rate for a given activity level and environmental conditions. By solving the two equations, we determined the functional dependence of blood perfusion and metabolic heat generation on the human activity level. For convenience, we presented simple explicit expressions for the key relations, with the aid of asymptotic analyses. Additional results include the temperature distribution inside the muscle layer, and the effects of muscle and fat layer thickness on the heat transfer processes.
COBISS.SI-ID: 22074390
The present contribution is the second part of a two-part research work presenting a generic method to extend lift force models that were originally devised for single linear shear flow to arbitrary flow conditions. The method can be applied to the computation of lift forces exerted on prolate spheroidal particles (or fibres) in arbitrary non-uniform flows. The method proposed in the Part I calculates the lift force arising from the dominant streamwise flow shear. In Part II the influence of the non-streamwise flow shear on the lift force is also taken into account. The present method assumes that the particle slip velocity is parallel to the fluid velocity along the particle trajectory. The novelty in the presented method is the computation of the shear lift force model for prolate spheroidal particles taking into account also non-streamwise flow shear. The accuracy of the novel shear lift force model for prolate spheroidal particles is verified by comparing it with the lift force model proposed in Part I via simulating the axial migration of a prolate spheroidal particle in the Poiseuille flow. In order to validate the ability of the present method for capturing the lift component arising from non-streamwise flow shear, the lift force model is compared with established generalised Saffman-type lift models by simulating the motion of a particle in lid-driven cavity flow. The computational results demonstrate that the present lift force model for prolate spheroidal particles is applicable in flow cases with streamwise and non-streamwise flow shear, even if some (reasonably small) accuracy for the case of the streamwise-only shear is lost.
COBISS.SI-ID: 21969942
Spray drying of particles, which contain a significant amount of moisture inside the particle or in a form of crystalline bound moisture, present an important challenge in engineering as well as in development of modern computational transport phenomena simulation tools. Among those the Computational Fluid Dynamics can be applied based on a combination of two-phase models and additional drying kinetics models, describing transport phenomena on a particle level. While the choice of two-phase flow models, based on EulerLagrange approach, is the most obvious choice, a choice of a correct drying kinetics model is mostly limited to models considering surface bound moisture only. A combination of the multistage drying model and an Euler-Lagrange two-phase model, therefore presents an excellent starting point for improving the accuracy of computational models. The contribution reports on the most important properties of such a model as well as on computational results for the case of laboratory spray dryer of zeolite-water slurry.
COBISS.SI-ID: 20122390