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Crossflow 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, Jan
In 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.
Experimental and Numerical Investigations into Heat Transfer Using a Jet Cooler in High-Pressure Die Casting
(MDPI, 2023-11-28) Boháček, Jan; Mráz, Kryštof; Krutiš, Vladimír; Kaňa, Václav; Vakhrushev, Alexander; Karimi-Sibaki, Ebrahim; Kharicha, Abdellah
During high-pressure die casting, a significant amount of heat is dissipated via the liquid-cooled channels in the die. The jet cooler, also known as the die insert or bubbler, is one of the most commonly used cooling methods. Nowadays, foundries casting engineered products rely on numerical simulations using commercial software to determine cooling efficiency, which requires precise input data. However, the literature lacks sufficient investigations to describe the spatial distribution of the heat transfer coefficient in the jet cooler. In this study, we propose a solver using the open-source CFD package OpenFOAM and free library for nonlinear optimization NLopt for the inverse heat conduction problem that returns the desired distribution of the heat transfer coefficient. The experimental temperature measurements using multiple thermocouples are considered the input data. The robustness, efficiency, and accuracy of the model are rigorously tested and confirmed. Additionally, temperature measurements of the real jet cooler are presented.
Transient 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, Abdellah
A 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.
The Efficient Way to Design Cooling Sections for Heat Treatment of Long Steel Products
(MDPI, 2023-05-26) Kotrbáček, Petr; Chabičovský, Martin; Resl, Ondřej; Komínek, Jan; Luks, Tomáš
To achieve the required mechanical properties in the heat treatment of steel, it is necessary to have an adequate cooling rate and to achieve the desired final temperature of the product. This should be achieved with one cooling unit for different product sizes. In order to provide the high variability of the cooling system, different types of nozzles are used in modern cooling systems. Designers often use simplified, inaccurate correlations to predict the heat transfer coefficient, resulting in the oversizing of the designed cooling system or failure to provide the required cooling regime. This typically results in longer commissioning times and higher manufacturing costs of the new cooling system. Accurate information about the required cooling regime and the heat transfer coefficient of the designed cooling is critical. This paper presents a design approach based on laboratory measurements. Firstly, the way to find or validate the required cooling regime is presented. The paper then focuses on nozzle selection and presents laboratory measurements that provide accurate heat transfer coefficients as a function of position and surface temperature for different cooling configurations. Numerical simulations using the measured heat transfer coefficients allow the optimum design to be found for different product sizes.
Impact of Submerged Entry Nozzle (SEN) Immersion Depth on Meniscus Flow in Continuous Casting Mold under Electromagnetic Brake (EMBr)
(MDPI, 2023-02-21) Vakhrushev, Alexander; Karimi-Sibaki, Ebrahim; Boháček, Jan; Wu, Menghuai; Ludwig, Andreas; Tang, Yong; Hackl, Gernot; Nitzl, Gerald; Watzinger, Josef; Kharicha, Abdellah
Complex multi-phase phenomena, including turbulent flow, solidification, and magnetohydrodynamics (MHD) forces, occur during the continuous casting (CC) under the applied electromagnetic brake (EMBr). The results of the small-scale experiment of the liquid metal model for continuous casting (mini-LIMMCAST) at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), investigating MHD flow with a deep immersion depth of 100 mm, are supplemented by newly presented numerical studies with the shallow position of the submerged entry nozzle (SEN) at 50 mm below the meniscus. Herein, the focus is on the MHD effects at the meniscus level considering (i) a fully insulating domain boundary, (ii) a perfectly conductive mold, or (iii) the presence of the solid shell. The volume-of-fluid (VOF) approach is utilized to model a Galinstan flow, including free surface behavior. A multiphase solver is developed using conservative MHD formulations in the framework of the open-source computational fluid dynamics (CFD) package OpenFOAM®. The wall-adapting local eddy-viscosity (WALE) subgrid-scale (SGS) model is employed to model the turbulent effects on the free surface flow. We found that, for the deep immersion depth, the meniscus remains calm under the EMBr for the conductive and semi-conductive domain. For the insulated mold disregarding the SEN position, the self-inducing MHD vortices, aligned with the magnetic field, cause strong waving of the meniscus and air bubble entrapment for shallow immersion depth. Secondary MHD structures can form close to the meniscus under specific conditions. The influence of the EMBr and immersion depth on the flow energy characteristics is analyzed using power spectral density (PSD).

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