Energetický ústav

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Impact of wind-induced resuspension on urban air quality: a CFD study with air quality data comparison
(NATURE PORTFOLIO, 2025-02-25) Linda, Jakub; Hasečič, Amra; Pospíšil, Jiří; Kudela, Libor; Brzezina, Jáchym
Wind-induced particulate matter (PM) resuspension is an increasingly recognized contributor to urban air pollution. A CFD model of 2D street canyon geometry was developed that can replicate the process of resuspension. Model created the wind speed vs concentration increase due to resuspension relationship and its key properties: threshold wind speed causing resuspension (TWSR) and concentration increase. At least 8.75 m/s in 10 m height inlet wind speed before street canyons was needed to start the resuspension, leading to PM10 concentrations often exceeding 1 mu g/m(3), with peak values reaching as high as 3 mu g/m(3). The model can predict the behavior of resuspension well, but it cannot capture all the factors acting in the real environment to match precisely the air quality data. Silt load remains the greatest unknown factor in determining the overall magnitude of resuspension, with observed increases in PM10 concentrations up to 6 mu g/m(3) in air quality data.
Estimation of added effects and their frequency dependence in various fluid-structure interaction problems
(Springer, 2024-10-18) Urban, Ondřej; Pochylý, František; Habán, Vladimír
This paper focuses on a fluid-structure interaction topic-the determination of added effects caused by fluid forces acting on a body, considering the standard linear equation of motion. We present various problems that assume small-displacement oscillations of single and multiple bodies in inviscid irrotational (potential) flow or viscous incompressible flow in both closed domain and external flow. For inviscid flow, effects of geometric parameters on the added effects were studied. The presented results extend results known from the literature. For viscous flow, frequency dependence of the added effects was studied for a wide range of frequency. The added effects were computed from data from numerical simulations of fluid flow, where the body oscillations were modeled using the dynamic mesh approach. Effects of the phase shift caused by the dynamic mesh were addressed. The added mass was compared with the corresponding value determined for inviscid flow where applicable. The results show strong dependence of the added effects on many parameters, making their proper computation challenging even for simplified cases.
Performance of vacuum-insulated central pipes for deep borehole heat exchangers in geothermal systems
(Oxford University Press, 2024-09-06) Kubačka, Ján; Karayiannis, Tassos G.
Geothermal energy is considered a promising future energy prospect, with the geothermal well outlet temperature being one of the important parameters affecting possible utilization options. For ground source heat pump applications or direct district heating, using lower temperatures can be acceptable. However, efficient electricity production requires a higher enthalpy gradient, which cannot be achieved without high temperature at the wellhead. The selection of the dry co-axial close-loop deep borehole systems (DBHE) may be, in some cases, very beneficial. The operating performance of co-axial DBHE can be optimized if the undesired heat transfer between the central pipe and annular fluid zones is minimized. Therefore, the operational performance of such a system depends strongly on the high thermal resistance of the central pipe. The most common option would be a low thermal conductivity material, such as high-density polyethylene (HDPE). In addition, vacuum-insulated tubing (VIT) used as the central pipe could be considered. The article presents results from the study aimed at the comparison of the homogeneous central pipe made of HDPE material and the gap-insulated central pipe. In the study, various air pressure levels as well as variations of surface emissivity were examined to reveal the effect on the heat transfer between the fluid channels. The simulation has been performed using a new purposely developed WellTH simulation software. A coaxial heat exchanger system using a VIT outperforms significantly the heat exchanger with an HDPE for deep geothermal wells. However, this advantage diminishes for shallow wells and therefore this tendency should be considered in the design stage.
Liquid dispersion in inner cavity of rotating packed bed
(EDP Sciences, 2024-07-04) Hájek, Jiří; Malý, Milan; Hájek, Ondřej; Cejpek, Ondřej; Rapta, Patrik; Jícha, Miroslav
Majority of power producing, and industrial processes generate a significant amount of carbon dioxide (CO2). To reduce their CO2 emissions, CCS (carbon capture and storage) can be used. One of the ways for CO2 capture is rotating packed bed (RPB), which operates on a similar principle as an absorption tower. However, in the RPB, mass transfer rate is driven by a centrifugal force, hence the RPB could be much smaller than the absorption tower, which relay on gravitational force. Also, the internal design of the RPB could affect efficiency of CO2 capture. There are several types of packing design, such as raised mesh, Zigzag, metal foam, or wire mesh. This study is focused on testing and construction of a transparent wire mesh packing, which could be used for analyse of fluid behaviour inside the packing e.g., a flow character, a liquid hold up and a liquid-gas interfacial. The operating packing speeds in the experimental part were 300 rpm, 600 rpm, 1200 rpm and 1800 rpm. The operating liquid (water) was supplied by six plain orifice nozzles with 1.44 mm diameter. Water was supplied to the system in a range of liquid flow rates from 44 to 176 kg/h. This corresponds to the jet velocity of 1.25 – 5.00 m/s. The observed area was the entry of the water jet into the wire mesh, where the atomization is the most intense. For the measurement, a high-speed camera was used. It is evident from results that with the higher jet velocity, the penetrating distance is larger, and the atomization is more intense.
Comparison of CFD simulations and measurements of flow affected by coanda effect
(EDP Sciences, 2012-04-16) Fišer, Jan; Jedelský, Jan; Vach, Tomáš; Forman, Matěj; Jícha, Miroslav
The article deals with experimental research and numerical simulations of specific phenomena in fluid flows called Coanda effect (CE), which has numerous important engineering applications. Although many researchers have concerned with wall jets, the physics of this flow still remains not well understood. This study is focused on analysis of behaviour of jet flow close to the wall and influence of its inclination. The flow has been visualized using smoke and velocity was measured by means of Hot Wire Anemometry (HWA). CFD simulations have been performed on the same geometry and compared with experiments in order to find a tool for correct prediction of the CE.

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