Browsing by Author "Penkova N."
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Item A conjugate heat transfer in a fixed-bed ceramic regenerator for air conditioning applications(2020-01-01) Penev A.; Tsokov L.; Penkova N.A mathematical model of the conjugate heat transfer in ceramic regenerative heat exchangers aiming a heat recovery in air conditioning systems is elaborated. The transient thermal and fluid flow fields in such a regenerator are obtained via numerical solutions by the finite volume method. The models are validated by comparing the computed and measured outlet temperatures and the heat transfer coefficients obtained by the simulations and the theory of similarity. The results related to the heat transfer and the thermal efficiency of the regenerator are discussed.Item Analysis and optimization of temperature stratification in a thermal energy storage tank(2014-01-01) Penkova N.; Harryzanov N.A mathematical model of the transient temperature and fluid flow fields in the multiphase domain at a water thermal energy storage tank is composed. The model is applied for an analysis of the transport phenomena at the heat storage accumulator, charged by waste heat flows of a cogeneration unit and discharged by thermal energy supply to the heating installation of a conservatory. Numerical investigations of the transient 3D temperature and fluid flow fields in the accumulator at different charging and discharging regimes have been performed in order to estimate the temperature variations of the outflows, situated at different levels. Recommendations for the improvements of the design and the operation of the accumulator are formulated to ensure higher temperature stratification in the water domain and subsequent continuous and smooth work of the CHP and heating installations.Item Analysis of the efficiency of an embedded floor heating for snow melting and freeze protection(2020-11-18) Penkova N.; Zlateva M.; Mladenov B.An algorithm for modelling and numerical simulation of the three dimensional conjugate heat transfer in slab-embedded snow melting and freeze protecting systems is developed. The influence of the climate conditions on the heat transfer in the constructions is modelled according to the correspondent design method of ASHRAE. The models are verified and solved numerically via finite volume method for a road embedded hydronic installation at different piping configurations. The fluid flow parameters and temperature fields in the construction are obtained at steady state climate conditions, hardest for 95 % of the wintertime for the region. An analysis of the efficiency of the modelled configurations is implemented based on the computed snow-free area ratio of the road surfaces.Item Cost analysis at different energy sources for heating of an air-supported dome for indoor tennis(2021-07-12) Gavrilov V.; Penkova N.; Zlateva P.; Mladenov B.Numerical analyses of the energy supply for heating of air supported structure for indoor tennis in Sofia via district heating, gas and liquid fuels are performed. The computational procedures are implemented through thermal balances of the structures, calibrated on the base of in situ measurements of the indoor temperature and the amount of the fuel, consumed at heating of an air supported balloon for tennis in University of Chemical technology and Metallurgy. The profitability of the different heating variants is estimated using the price of the energy and the systems.Item Efficiency of transparent structures in Trombe walls(2018-01-01) Kaloyanov N.; Stankov B.; Tomov G.; Penkova N.The Trombe wall is a passive solar design solution suitable for buildings in temperate and warm climates. The transparent units, which constitute the external boundary of the structure, are among the most important factors for its energy performance. This paper presents an analysis of the energy performance of unvented Trombe walls using two of the most commonly encountered types of glazing - single glass and double insulting glass units (IGU). The analyses are based on physical measurements obtained at a test site and numerical simulations of the heat transfer processes in the system. In addition to the energy performance assessment, a thermal load analysis is performed for the more efficient solution, i.e., the double glazed unit, to predict the influence of the occurring heat transfer processes on the mechanical behavior and reliability of the construction.Item IMPROVEMENT OF FIRING CURVE OF CERAMIC WARE VIA MODELLING AND NUMERICAL SIMULATION OF THE COUPLED THERMO-MECHANICAL PROCESSES IN THE MATERIAL(2021-01-01) Penkova N.; Krumov K.; Zlateva P.A mathematical model of coupled heat transfer and mechanical processes in ceramic ware at firing is developed. It is based on information about the thermo-mechanical behaviour of the material at sintering, obtained experimentally. The model is solved numerically for massive bodies of technical ceramics to estimate and prevent the reasons for cracks appearance at their firing in chamber kilns. The transient thermo-mechanical processes in the fired mass are simulated varying with the heating rate of the firing curves. The investigated thermal regimes are analysed on the base of the results about the temperature and stresses fields in the articles. Approaches for improvements of the fired curve are discussed in order to prevent defects of the products.Item Investigation of the possibilities for energy economy in continuous dryers through reduction of the drying time(2020-11-18) Penkova N.; Mladenov B.The possibility for a reduction of the time duration of an industrial drying of ceramic bricks are analysed based on numerical simulation of the transient coupled mass transfer and mechanical processes in the material. The differences between the local mechanical stresses and strength in the dried bodies are computed and used as a criterion for this possibility. Ways for reorganization of the existing process parameters in the dryers to achieve the corrected drying kinetic curve are discussed. The expected effects on the efficiency of the drying installation are evaluated via material and thermal balance.Item Mathematical modeling and numerical simulation of coupled mass transfer and mechanical processes in ceramic ware at industrial drying(2019-07-01) Penkova N.; Chervenliev P.; Mladenov B.; Krumov K.An algorithm for numerical simulation of transient moisture content fields and mechanical processes in ceramic ware at drying in industrial aggregates is developed. It is based on mathematical models of the mass transfer and mechanical behavior in the ceramic bodies, data for the drying regime and physical properties of the material as function of water content. The models allow variations of the drying conditions in order to choice the most efficient regime at existing or design dryers. The algorithm is applied for a direct coupled finite element analysis of wet bricks behavior in continuous working drying installation. The shrinkage mode, modulus of elasticity, Poisson ratio, modulus of rupture, effective mass transfer coefficient and critical moisture content are determined by experimental tests of the material. They are used to simulate numerically three-dimensional moisture, stress and strain fields in ceramic bodies at the existing drying regime. Ways for improvement of the models and their application for estimation of the potential for energy savings in industrial dryers are discussed.Item Modelling and numerical simulation of fire in a coffee storage hall(2021-11-22) Krumov K.; Penkova N.; Mladenov B.; Stoyanov Y.An approach for modelling of fire in halls at solid materials burning, based on gas phase combustion models in ANSYS/CFX, is proposed. The models are suitable for quick prediction of the flue gases distribution and thermal loads on the building envelopes at a design stage. They was applied for numerical simulation of such processes in a coffee storage hall at different fire positions and forced anti-smoke ventilation. The thermal, fluid flow and concentration field were obtained at the modelled scenarios. Maximal thermal loads on the building envelopes are established at the case of fire near their non-streamlined parts.Item Modelling and numerical simulation of the heat transfer and natural ventilation in storage halls(2020-11-18) Penkova N.; Krumov K.; Mladenov B.; Stoyanov Y.An algorithm for modelling of the heat transfer in storage halls at different combinations of open doors and smoke vents is developed. The models allow numerical simulations of velocity, pressure and temperature fields in the halls in cases of transient and steady state conditions, taking into account the indoor buoyancy, heat sources, heat transfer through the building envelopes and outdoor wind pressures. The models are solved numerically via finite volume method in order to estimate the necessity of additional heating at a design stage of a courier logistic storage hall, situated in the temperate climate zone, at the hardest winter conditions of the region. Recommendations about the organization of the heating and the cargo input are done based on the analyses of the fluid flow and thermal fields at the modelled scenarios.Item Modelling and numerical simulation of the transport phenomena in water thermal energy storage tanks(2021-11-22) Kaloyanov N.; Tsecov R.; Penkova N.Mathematical model for numerical simulation of the transient heat transfer and fluid flows in water thermal energy storage tanks is developed. The model allows analysis of the thermal fields in the accumulators at different schemes and modes of charging and discharging. It was verified and validated based on experimentally obtained information about the temperature stratification at charging of a thermal accumulator at a laboratory solar system. The proposed approach for numerical study of the thermal energy storage is convenient for parametrical estimation and improvement of the efficiency of the thermal systems.Item Numerical analysis of the transient heat transfer in high temperature chamber furnaces(2019-09-20) Krumov K.; Penkova N.An algorithm for numerical analysis of the transient thermal processes in high temperature gas chamber furnaces for ceramic firing is developed. It is based on mathematical models of the conjugate heat transfer, including combustion, turbulent flows of the torch and gas mixtures, heat transfer through radiation, conduction and convection. The algorithm is used for analysis and improvement of the efficiency of a furnace for firing of technical ceramics.Item Performance of diffusion absorption refrigerators at variable power input(2024-01-01) Stoyanov Y.; Penkova N.; Krumov K.Absorption refrigerators have been used in recent decades for utilization of waste or solar energy to produce heat or cold for industrial purposes or to condition the microclimate in buildings. Although these units are designed at nominal power input in the generator, they can also operate at variable parameters of the heat source, which is possible in the real situations. An experimental determination of the operating range and the performance of a diffusion absorption refrigerator at heat pump and refrigeration mode were carried out in the present study. The results are useful for predicting the efficiency and the effectiveness of such systems at variable heat source capacities.Item Stress analysis of insulating glass units under transient thermal loadings(2018-01-01) Ivanov I.; Velchev D.; Penkova N.; Krumov K.; Iliev V.Finite element stress analysis of glass panels in insulating glass units (IGU) is carried out based on the temperature field determined by serial numerical simulations of the conjugate heat transfer varying with the ambient temperature and solar radiation during a year. The hermetically sealed gas cavity between glass panels is modelled by finite elements and the stress is analyzed when the gauge pressure is maximum positive or negative due to gas temperature change at different seasons. The influence of the primary and secondary sealant of IGU is discussed. The combination of loadings due to thermal expansion and to cavity pressure is analyzed. Important conclusions about IGU structure and its durability are drawn.