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- ItemDetermination of transient heat transfer by cooling channel in high-pressure die casting using inverse method(IOP Publishing, 2024-05-01) Boháček, Jan; Mráz, Kryštof; Hvožďa, Jiří; Lang, Filip; Raudenský, Miroslav; Vakhrushev, Alexander; Karimi-Sibaki, Ebrahim; Kharicha, AbdellahComplex shapes of aluminum castings are typically manufactured during the short cycle process known as the high-pressure die casting (HPDC). High productivity is ensured by introducing die cooling through a system of channels, die inserts or jet coolers. Die cooling can also effectively help in reducing internal porosity in cast components. Accurate simulations based on sophisticated numerical models require accurate input data such as material properties, initial and boundary conditions. Although the heat is dominantly dissipated through die cooling, indicating the importance of knowing precise thermal boundary conditions, open literature lacks a detailed information about the spatial distribution of heat transfer coefficient. This study presents an inverse method to determine accurate heat transfer coefficients of a die insert based on temperature measurements in multiple points by 0.5 mm K-type thermocouples and a subsequent solution of the two-dimensional inverse heat conduction problem. The solver was built in the open-source CFD code OpenFOAM and the free library for nonlinear optimization NLopt. The results are presented for the commonly used 10 mm die insert with a hemispherical tip and coolant flow rates ranging from 100 l/h to 200 l/h. Heat transfer coefficients reach values well above 50 kW/m2K in the hemispherical tip, which is followed by a secondary peak and then a gradual drop to values around 1 kW/m2K further downstream.
- ItemEffects of the asymmetric and oscillating turbulent melt flow on the heat transfer and solidification inside the thin slab continuous casting (TSC) mold under the applied electromagnetic brake (EMBr)(IOP Publishing, 2024-05-01) Vakhrushev, Alexander; Karimi-Sibaki, Ebrahim; Wu, Menghuai; Tang, Yong; Hackl, Gernot; Watzinger, Josef; Boháček, Jan; Kharicha, AbdellahThe thin slab casting (TSC) is a breakthrough near-net-shape technique for flat products accompanied by rapid casting and solidification rates. The TSC quality hinges on the turbulence, super-heat flow and growth of the solidified shell. The electromagnetic brake (EMBr) is commonly applied to control the fresh melt flow after feeding through a submerged entry nozzle (SEN). Numerical modelling is a perfect tool to investigate the multiphase phenomena in the continuous casting (CC). The presented study considers the heat transfer through the solid shell and water-cooled copper mold including the averaged thermal resistance of the slag skin and the air gap coupled with the turbulent flow and magnetohydrodynamics (MHD) model using an in-house code developed inside the open-source computational fluid dynamics (CFD) package OpenFOAM®. The model is applied to investigate different undesired asymmetric melt flow issues: (i) with the misaligned or (ii) partially blocked SEN; (iii) caused by the mean flow fluctuations with the natural frequencies; (iv) related to the oscillations of the fresh melt jets for the specific SEN designs and casting regimes. The variation of the flow pattern and superheat distribution is studied and presented for different scenarios both with and without applied EMBr.
- ItemCase Study on Thermal Management of Planar Elements with Various Polymeric Heat Exchangers: Experiment and Simulation(Springer Nature, 2024-05-02) Hvožďa, Jiří; Mráz, Kryštof; Raudenský, Miroslav; Vakhrushev, Alexander; Karimi-Sibaki, Ebrahim; Boháček, JanA reliable battery thermal management system (BTMS) is essential to ensure proper performance, a long life span, and high electric vehicle safety. The primary objective of BTMS is to maintain the cells’ temperature in the range of 15–35 °C while limiting the temperature spread between cells to below 5 °C. Active thermal management with polymeric hollow fibers (PHFs) has been reported in a few articles, but its tremendous flexibility is mainly advantageous for cylindrical cells. Extruded polymeric cold plate heat exchangers with rounded rectangle channels (RRCs) are proposed as a more elegant solution for planar batteries. Heat exchangers using PHFs and RRCs were experimentally compared, with a strong focus on minimizing the maximum temperature and temperature spread of the experimental setup while simultaneously achieving minimal pressure drops. The system behaviour with different parameters, including materials, geometry, and thermophysical properties, was further studied using properly validated CFD models.
- ItemCrossflow polymeric hollow fiber heat exchanger: fiber tension effects on heat transfer and airside pressure drop(Springer Nature, 2024-03-01) Kroulíková, Tereza; Mráz, Kryštof; Hvožďa, Jiří; Boháček, JanIn various applications, a polymeric hollow fber heat exchanger (PHFHE) is a competitive alternative to a conventional heat exchanger (HE). Standard empirical models for predicting the crossfow tube HE characteristics are defned for devices with rigid tubes with relatively large diameters compared to the polymeric hollow fbers with an outer diameter of around 1 mm. This study examines the impact of tension force on airside heat transfer rate and pressure drop in a crossfow PHFHE. The tension force infuences the stifness of the fexible polymeric fbers and their response to applied airfow. Two liquid– gas PHFHEs were designed and manufactured to ensure uniformity of the fbers' arrangement (inline and staggered). An experimental stand enabling the application of defned tension force in the range of 0–9000 N was designed, manufactured and placed into the calorimetric tunnel, where heat transfer rate and pressure drop measurement were performed with varying air velocity between 2 and 8 ms1 (corresponding to Reynolds number of 240–970). Among our key fndings was that the elongation of the fbers due to thermal expansion or stress relaxation has a considerable impact on the fbers' arrangement and resulting fuid fow. Moreover, the application of tension force yielded no substantial change in air pressure drop; however, it led to a notable enhancement in heat transfer rate. Specifcally, under a maximal tension force of 9000 N, the heat transfer rate increased by around 11% compared to the unloaded state.
- ItemTransient Simulation of Diffusion-Limited Electrodeposition Using Volume of Fluid (VOF) Method(ELECTROCHEMICAL SOC INC, 2023-07-04) Karimi-Sibaki, Ebrahim; Vakhrushev, Alexander; Wu, Menghuai; Ludwig, Andreas; Boháček, Jan; Kharicha, AbdellahA numerical model utilizing the volume of fluid (VOF) method is proposed to simulate the transient shape changes of the deposit front, considering the diffusion-limited electrodeposition process. Modeling equations are proposed to accurately handle transport phenomena in both electrolyte (fluid) and deposit (solid). Transient evolutions of field structures, including flow, concentration, electric current density, and electric potential, are computed considering electrodeposited copper bumps. Two cases, including single cavity and multiple cavities, are studied. Based on the modeling results, the maximum height of the hump and the thickness of the deposited layer in each consecutive cavity decreases going from upstream to downstream. Conversely, the location of the maximum height of the hump remains unchanged in all cavities. Results are validated against available experiments.