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- ItemImportance of Melt Flow Direction during Injection Molding on Polymer Heat Sinks’ Cooling Efficiency(MDPI, 2021-04-07) Guzej, Michal; Zachar, Martin; Komínek, Jan; Kotrbáček, Petr; Brachna, RóbertPolymers with highly conductive fillers could possibly replace standardly used materials, such as aluminum and copper alloys, for passive cooling purposes. The main problem of the composite polymer-based heat sinks is that their high thermal conductivity is uneven. The orientation of this anisotropy is set according to the position of the highly thermally conductive filler. Its orientation is influenced by the melt flow during the polymer heat sink molding process. This article shows that change of the melt flow inside the mold cavity can improve the overall cooling efficiency of a polymer heat sink, which leads to lower temperatures on the heat source used. Two polymer heat sinks of identical geometries were produced. Their high thermal conductivity was given by the use of graphite flakes as the filler. The only difference between the heat sinks was in the position of the fan gate during their production. Different temperatures of the heat source between the two heat sinks were observed for the same measurement conditions. The measurements were conducted at Heatlab, BUT.
- ItemAn Assessment on Average Pressure Drop and Dust-Holding Capacity of Hollow-Fiber Membranes in Air Filtration(MDPI, 2021-06-24) Bulejko, Pavel; Krištof, Ondřej; Dohnal, MiroslavIn this work, we tried to analyze dust loading behavior of polypropylene hollow fiber membranes using average pressure drop models. Hollow fiber membranes varying in fiber diameter were loaded with a standardized test dust to simulate particle-polluted air. We measured pressure drop development of the membranes at different flowrates and dust concentrations, and, after each experiment, the dust deposited on the membrane fibers was weighed to obtain dust holding capacity (DHC). The obtained experimental data was analyzed using various average pressure drop models and compared with average pressure drop obtained from pressure drop/dust load dependence using a curve fit. Exponential and polynomial fitting was used and compared. Pressure drop in relation to the dust load followed different trends depending on the experimental conditions and inner fiber diameter. At higher flowrate, the dependence was polynomial no matter what the fiber diameter. However, with higher fiber diameter at lower permeate velocities, the dependence was close to exponential curve and followed similar trends as observed in planar filter media. Dust-holding capacity of the membranes depended on the experimental conditions and was up to 21.4 g. However, higher dust holding capacity was impossible to reach no matter the experiment duration due to self-cleaning ability of the tested membranes.
- ItemFully Polymeric Distillation Unit Based on Polypropylene Hollow Fibers(MDPI, 2021-03-26) Kůdelová, Tereza; Bartuli, Erik; Strunga, Alan; Hvožďa, Jiří; Dohnal, MiroslavAccess to pure water is a very topical issue today. Desalination represents a promising way of obtaining drinking water in areas of shortage. Currently, efforts are being made to replace the metal components of existing desalination units due to the high corrosivity of sea water. Another requirement is easy transportation and assembly. The presented solution combines two types of polymeric hollow fibers that are used to create the distillation unit. Porous polypropylene hollow fiber membranes have been used as an active surface for mass transfer in the distillation unit, while non-porous thermal polypropylene hollow fibers have been employed in the condenser. The large active area to volume ratio of the hollow fiber module improves the efficiency of both units. Hot water is pumped inside the membranes in the distillation unit. Evaporation is first observed at a temperature gradient of 10 °C. The water vapor flows through the tunnel to the condenser where cold water runs inside the fibers. The temperature gradient causes condensation of the vapor, and the condensate is collected. The article presents data for hot water at temperatures of 55, 60, and 65 °C. Optimization of the membrane module is evaluated and presented.
- ItemAn analysis on energy demands in airborne particulate matter filtration using hollow-fiber membranes(Elsevier, 2021-05-12) Bulejko, PavelThis work analyzes energy requirements for drawing the air through a hollow-fiber membrane (HFM) during air filtration. Polypropylene HFMs varying in inner fiber diameter (230 and ) were used to separate the ASHRAE A2 fine test dust from a simulated particle laden environment. During the experiments, pressure drop, permeate velocity (air flowrate), fan frequency and power input were recorded. The obtained experimental data was analyzed using theoretical models relating pressure drop with power consumption necessary to overcome the membrane resistance. The results obtained for HFMs of different parameters (especially inner fiber diameter) were compared in terms of air flowrate and dust loading rate. Their influence on the energy consumption to operate the fan in the filtration process was then evaluated. The results have shown the smaller diameter HFMs have significantly higher energy consumption to operate the blower (about 180 Wh/m3) compared to larger diameter membrane (approx. 100 Wh/m3). This was despite higher surface area (0.95 m2) of lower diameter membrane compared to lower surface (0.43 m2) of HFMs with larger diameter. The general course of energy consumption during dust loading varied depending on the inner fiber diameter rather than on dust loading. This was obvious no matter of the dust loading rate (2, 4 and 6 g/hr), which seems to have negligible effect at the adopted experimental conditions a consumption of 113, 102 and 116 Wh/m3, respectively).
- ItemA Numerical Study on the Influence of an Axial Magnetic Field (AMF) on Vacuum Arc Remelting (VAR) Process(Springer, 2021-07-07) Karimi-Sibaki, Ebrahim; Kharicha, Abdellah; Abdi, Mehran; Vakhrushev, Alexander; Wu, Menghuai; Ludwig, Andreas; Boháček, JanA comprehensive numerical model is proposed to study the influence of an axial magnetic field (AMF) on the solidification behavior of a Titanium-based (Ti–6Al–4V) vacuum arc remelting (VAR) ingot. Both static and time-varying AMF are examined. The proposed 2D axisymmetric swirl model includes calculating electromagnetic and thermal fields in the entire system composed of the electrode, vacuum plasma, ingot, and mold. A combination of vector potential formulation and induction equation is proposed to model the electromagnetic field accurately. Calculations of the flow in the melt pool and solidification of the ingot are also carried out. All governing equations are presented in cylindrical coordinate. The presence of a weak AMF, such as the earth magnetic field, can dramatically influence the flow pattern in the melt pool. The “Electro-vortex flow” is predicted ignoring AMF or in the presence of a time-varying AMF. However, the flow pattern is “Ekman pumping” in the presence of a static AMF. The amount of side-arcing has no influence on the pool depth in the presence of an AMF. Modeling results are validated against experiments.