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Abstract: Subcooled flow boiling is a crucial heat transfer mechanism in which the fluid temperature exceeds its saturation point, leading to phase change. Computational Fluid Dynamics (CFD) is a powerful tool for investigating such phenomena. However, commonly used CFD wall boiling models, such as the Rensselaer Polytechnic Institute (RPI) model, may under-predict convective heat transfer in flow boiling conditions, do not account for conjugate heat transfer (CHT), and often fail to accurately capture vapour flow behaviour after vapour detachment. This study presents a new model, named the Combined Boiling Model (CBM), which integrates CHT, wall boiling, and flow evaporation/condensation to improve predictive accuracy. Simulation results were compared against experimental data, demonstrating that the CBM accurately predicts heat transfer in channel flows with inlet velocities up to 1 m/s and heat fluxes up to 1.7 × 10⁶ W/m².
Keywords: boiling, evaporation, condensation, heat transfer, CHT, CFD
Published in DKUM: 28.11.2025; Views: 0; Downloads: 7
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Abstract: The aim of this paper is to present a novel and comprehensive methodology for the dynamic modeling and experimental validation of a photovoltaic/thermal system. The dynamic model is divided into thermal and electrical subsystems, encompassing the photovoltaic/ thermal module and the thermal energy storage. The thermal subsystem of both the photovoltaic/thermal module and the thermal energy storage is described by a one-dimensional dynamic model of heat transfer mechanisms and optical losses, while the electrical subsystem is presented as an electrical equivalent circuit of double diode solar cell. Model validation was conducted on a modern experimental photovoltaic/thermal system over an extended operational period at a five-minute resolution, with validation days classified as sunny, cloudy, or overcast based on weather conditions, thereby demonstrating an applied approach. The results demonstrate the lowest deviation values reported to date, confirmed using six quantitative indicators. The added value of the proposed methodology, not previously addressed in the literature, lies in the following contributions: (i) comprehensive modeling of the entire photovoltaic/thermal system, (ii) accurate consideration of optical losses in the photovoltaic/thermal module, and (iii) long-term experimental validation. Overall, the proposed methodology provides a reliable and efficient framework for PV/T system design, optimization, and long-term performance assessment.
Keywords: photovoltaic/thermal system, thermal energy storage, dynamic modeling, experimental validation, heat transfer mechanism, temperature, electrical power
Published in DKUM: 10.11.2025; Views: 0; Downloads: 6
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Abstract: With computational modeling of lyophilization in vials, the pressure coupling between the sublimation front and the drying chamber has traditionally been calculated using a simplified mass transfer resistance model in the form of a model, which takes into account the headspace and the stopper in a simplified way. In developing a 3D CFD-based digital twin of lyophilization in vials, a need arises for a mass flow rate-dependent vial headspace/stopper model, as it enables a more accurate calculation of the pressure conditions above the shelf as well as pressure conditions directly at the sublimation front, the latter directly affecting the sublimation mass transfer rate as well as the temperature inside the product, which is crucial for determining the risk of product collapse. The local pressure variations at a shelf level affect the heat transfer conditions due to heat conduction in the low pressure environment of the drying chamber. In the present work the development of a coupled multilevel vial lyophilization model for the freeze-drying of vials is reported, with the time-dependent 1D heat and mass transfer model at the vial level coupled with the time-dependent 3D low-pressure CFD model of the flow of the water vapor–air mixture in the drying chamber heated by the shelves. A direct pressure coupling between the sublimation front and the drying chamber space in form of vial type specific headspace/stopper resistance model is implemented. The developed multilevel lyophilization model is used to study the pressure build-up above the shelf and the headspace of the vial and its influence on the product temperature at the bottom of the vial using simulations carried out for different chamber pressures (6 Pa and 22 Pa), shelf temperatures (−20 oC and +10 oC) and vial types (10R and 15R). By implementing previously developed vial headspace/stopper pressure resistance models, the computational results show that the pressure build-up above the shelf and vial headspace significantly affect the product temperature at the bottom of the vial, especially at low chamber pressures ( Pa) and small gap sizes between the rubber stopper and the shelf above it. The increased pressure outside the vial leads also to higher heat transfer by conduction, which is particularly pronounced at the central shelf positions and within smaller shelf gaps. These results underline the importance of using a coupled multilevel model when analyzing the relationship between the local pressure variations above the shelf and their direct influence on product drying conditions, further improving the predictive capabilities of CFD based multilevel lyophilization models, especially with respect to detecting the product collapse temperature.
Keywords: freeze-drying, conjugate heat and mass transfer, computational fluid dynamics, multi-scale modeling
Published in DKUM: 17.06.2025; Views: 0; Downloads: 14
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Abstract: In this paper, a quadratic time interpolation for temperature and a linear time interpolation for fluxes are implemented for the parabolic (time-dependent) fundamental solution-based scheme for solving transient heat transfer problems with sources using the subdomain BEM (boundary element method), which is the main innovation of this paper. The approach described in this work to incorporate the quadratic time variation does not require doubling the number of equations, which is otherwise required in the BEM literature, for the discretized problem to be well-conditioned. Moreover, the numerical accuracy, compared over an unprecedented range of the Fourier number (Fo) and source strength values, can help in selecting the appropriate scheme for a given application, depending on the rate of the heat transfer process and the included source term. The newly implemented scheme based on the parabolic fundamental solution is compared with the well-established elliptic (Laplace) scheme, where the time derivative of the temperature is approximated with the second-order finite difference scheme, on two examples.
Keywords: quadratic time elements, time-dependent fundamental solution, heat transfer modeling, boundary element method
Published in DKUM: 07.05.2025; Views: 0; Downloads: 5
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Abstract: Much remains to be done to understand better and thus achieve better control over magnetic materials to maximize their magnetocaloric properties and performance, specifically for gadolinium. A clear path forward is highlighted by thorough experiments coupled with theory, in which the latter is tested and re%ned against the former, thus resulting in discoveries of new and improved materials and bringing near-room-temperature magnetic refrigeration technology to fruition in the near future.
Keywords: Gadolinium, heat transfer, magneto-calorific, magnetic field
Published in DKUM: 01.12.2023; Views: 425; Downloads: 321
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