Jacek Smolka

Shape Optimization of Coils and Cooling Ducts in Dry-Type Transformers Using Computational Fluid Dynamics and Genetic Algorithm

In this paper, shapes of cooling ducts in dry-type transformers are optimized using computational fluid dynamics (CFD) and the Genetic Algorithm (GA). The GA is used to optimize diameters of both ducts and coils. Constraints in the optimization process are the minimum distance between the high-voltage (HV) and the low-voltage (LV) windings and the outer diameter of coils. Since the investigated transformer is a special unit, two objective functions (OF) were applied to minimize the average and the maximum temperature of the windings, and thus the coil power losses. The OF value is determined using a CFD model that accounts for all three heat transfer modes. The local total heat fluxes are specified on the model external boundaries. The thermal properties of the coils and core are treated as anisotropic and temperature-dependent quantities, while the power losses are treated as heat sources and are computed based on the coupled CFD-EMAG model. Both coil properties and losses vary with each generated coil configuration. The results show that the nonuniform positioning of the wires and air ducts can significantly improve the heat dissipation. Consequently, the coil losses are substantially reduced.

Application of Bezier surfaces to the 3-D inverse geometry problem in continuous casting

In this article, a three-dimensional (3-D) numerical solution of the inverse geometry problem for a continuous casting process of an aluminium alloy is presented. In particular, an accurate determination of the interface location between the liquid and solid phases based on temperature measurements at several internal points of the body is discussed. This problem is crucial for the design and control of the casting process. To solve it, an algorithm was proposed that is an extension into 3-D of the solution procedure developed for the two-dimensional (2-D) geometrical inverse problem. Namely, in the solution algorithm, the Bezier splines for the 2-D procedures were replaced with the Bezier surfaces for the 3-D phase change boundary approximation. In the inverse procedure, a sensitivity analysis was used to estimate the location of the Bezier surface control points. While the measured temperatures required to solve the problem always contain measurement errors, a comparison of the numerically measured and retrieved values showed a very high computational accuracy. Consequently, the average percentage error of the control point locations is very satisfactory compared with the reference phase change solution. In addition, the computationally effective method was independent of the maximum error of measurements used for calculations.

A comparison of heterogenous and homogenous models of two-phase transonic compressible CO2 flow through a heat pump ejector

This paper presents mathematical model of a two-phase transonic flow occurring in a CO2 ejector which replaces a throttling valve typically used in heat pump systems. It combines functions of the expander and compressor and it recovers the expansion energy lost by a throttling valve in the classical heat pump cycle. Two modelling approaches were applied for this problem, namely a heterogenous and homogenous. In the heterogenous model an additional differential transport equation for the mass fraction of the gas phase is solved. The evaporation and condensation process in this model is described with use of the Rayleigh-Plesset equation. In the homogenous model, phases are traced based on the thermodynamic parameters. Hence the heterogenous model is capable to predict non-equilibrium conditions. Results obtained with both models were compared with the experimental measurements.

3-D periodic CFD model of the heating system in a coke oven battery

Purpose – The purpose of this paper is to develop a 3-D fully transient numerical model of the heat and fluid flow associated with the chemical reactions that occur in the heating system of the coke oven battery. As a result, the model can be used to provide data for the control system of the battery to reduce energy consumption and emissions and to obtain a product of the desired quality. Design/methodology/approach – In the proposed model, an accurate representation of the heating flue geometry, the volumetric heat sources as a result of the coke oven gas combustion, the temperature- and mole fraction-dependent properties of the gases were taken into account. The most important part of the model was the unsteady boundary condition definition that allowed the modeling of the periodic heat delivery to the two oven heating walls, both in the coking and the reversion cycles. Findings – The temperatures obtained using the computational fluid dynamics (CFD) model showed the same pattern of temperature variati...

A validated numerical model of heat and mass transfer in a PM BLDC electric motor

In the paper, a validated numerical model describing heat transfer and air flow phenomena in a PM BLDC motor is presented. Validation was performed using the results of multiple velocity and temperature measurements. The air velocity was measured by constant temperature anemometers (CTA) and using Laser Doppler Anemometry (LDA) outside and inside the motor casing, respectively. The temperature measurements were conducted involving over 40 thermocouples located on the stator winding, internal and external casing walls and in the surrounding air. The numerical and experimental results were in a satisfactory agreement.

CFD simulations of transport phenomena during transcritical flow of real fluid (CO2) within ejector

Purpose – The purpose of this paper is to overview successful approaches to the computational simulation of real fluid (R744 – carbon dioxide (CO2)) flow within an ejector is presented. Important issues such as the ejector geometry and its optimisation, the adapted equations of state and the proposed models of the process, fluid parameters, etc., are examined and critically discussed. Whenever possible, the discussed models are experimentally validated. In the conclusion, some trends in future research are pointed out. Design/methodology/approach – Flow within CO2 ejector is generally transcritical and compressible. Models existing in the literature are shortly described and critically compared. Whenever possible, those models were validated against the experimental data. In a model validation process, the primary and secondary mass flow rates as well as the pressures at the selected points in the mixing section and diffuser were compared, showing a satisfactory agreement between experimental and computat...

Mathematical Modelling of Two‐Phase Transonic Compressible CO2 Flow Through Heat Pump Ejector

In this paper mathematical modelling of a two‐phase flow occurring in an ejector which replaces a throttling valve typically used in a heat pump cycle is numerically considered. The geometry of the model is three dimensional taking one cross‐sectional symmetry plane into account. In such model different discretizations were generated to study an influence of the rapidly changing quantities on the system operation. The density variations are defined using real gas model i.e. Redlich‐Kwong for CO2 and REFPROP databases were employed. Both models produced similar character of the results. In addition, it was observed that the effects of wall roughness value defined as in planned experiments in such high speed systems can be neglected.

IMPURITIES REMOVAL PROCESS FROM THE VACUUM INDUCTION FURNACE CHARGE: A NUMERICAL STUDY

The technology of mental melting in a vacuum induction furnace enables the efficient removal of impurities and provides an opportunity to melt refractory metals, such as titanium. These materials can be applied in cutting edge technologies, such as aviation (turbine blades) and biotechnology (prosthesis and implants). To control metallurgical heat and mass processes within an induction furnace, measurements and a numerical analysis can be conducted. In this paper, numerical approaches are discussed. Simulation requires the development of vacuum induction furnace coupling between fluid dynamics and electromagnetic fields. The proposed numerical domain was modelled as a threedimensional slice with a properly defined periodic boundary condition. To define the analysed electromagnetic problem, a set of Maxwell differential equations was specified. A fluid dynamics sub-model was composed of the mass and momentum conservation equations using the volume of fluid multiphase formulation, two-equation k- turbulence model and species transport to track the inclusion position within the melt. The main purpose of this study was an examination of the impurities removal process via the free surface of the melt within an induction furnace. The coupled computations were performed for five operating conditions, including different power inputs of the inductor. The results indicated a strong influence of the inductor power on the free surface area and therefore on the purification process intensity.

An experimental and numerical analysis of temperature and velocity field in PM BLDC motor

In this paper, a validated numerical model was introduced to determine the temperature and velocity fields outside the electric motor. The analysed object was a brushless permanent magnet motor (PM BLDC) having a rated power of 431 W with neodymium permanent magnets located on the rotor. The temperature and velocity measurements were conducted using thermocouples and constant-temperature anemometers. The numerical model covered the investigated motor and the same unit working as a generator and the air volume around them in order to improve the heat dissipation conditions. The numerical results show a satisfactory agreement with the values obtained during the measurements. Streszczenie. W pracy przedstawiono zwalidowany eksperymentalnie model numeryczny do wyznaczenia pola temperatury oraz predkości na obudowie oraz wokol bezszczotkowego silnika elektrycznego malej mocy. Pomiary temperatury i predkości przeprowadzono na stanowisku badawczym za pomocą termopar oraz anemometrow stalotemperaturowych i posluzyly one do walidacji modelu. Model numeryczny obejmowal silnik wraz z obciązającą go prądnicą oraz bryle powietrza wokol obu maszyn w celu dokladniejszego odwzorowania warunkow wymiany ciepla. Wyniki otrzymane z modelu numerycznego wykazaly satysfakcjonującą zgodnośc z wartościami otrzymanymi podczas pomiaru. Wyznaczania pola temperatury oraz predkości na obudowie oraz wokol bezszczotkowego silnika elektrycznego malej mocy

Numerical study of air staging in a coke oven heating system

The air staging to combustion approach introduced to a coke oven heating system as a primary method of nitrogen oxide (NO) formation reduction is considered in this paper. To numerically investigate the thermal and prompt NO formation, a heating flue model representing the most popular Polish coke oven battery was used. The model was developed and experimentally validated as a transient coupled model for the representative heating flue and the two coke ovens. Numerical simulations were performed to estimate the amount of NO passing into the atmosphere during the operation of such a heating system with and without the secondary air inlets. Various strategies for the secondary air distribution along the flue gas flow as well as the secondary air velocity were studied. The results of the numerical investigation demonstrated the substantial positive effect of the considered air staging on NO formation reduction.