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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.
Applicability of Human Thermophysiological Model for Prediction of Thermal Strain in PPE
(MDPI, 2023-06-15) Lunerová, Kamila; Řehák Kopečková, Barbora; Pokorný, Jan; Mašín, Michal; Kaiser, David; Fialová, Vladimíra; Fišer, Jan
The use of personal protective equipment (PPE) is essential to protect the human body in hazardous environments or where there is a risk of CBRN agents. However, PPE also poses a barrier to evaporative heat dissipation, therefore increasing heat accumulation in the body. In our research, we investigated the applicability of thermophysiological models for the prediction of thermal strain and the permissible working time in a contaminated environment when the usage of protective ensembles is required. We investigated the relationship between the thermal insulation characteristics of four types of PPE against CBRN agents and the induced thermal strain in a set of real physiological strain tests with human probands wearing the PPE in a climatic chamber. Based on the results, we compared the predictions using two thermophysiological models-Predicted Heat Strain Index (PHS) and FIALA-based model of thermal comfort (FMTK)-with the experimental data. In order to provide a user-friendly platform for the estimation of thermal stress in PPE, a user-friendly computational tool, Predictor of Thermal Stress (PTS), was developed. The PTS tool is based on an extensive database of simulated calculations using an FMTK model based on PPE characteristics, environmental conditions, individual parameters, and expected workload. The PTS tool was validated by means of the results from real tests in a climatic chamber. The PTS was shown to be an easy-to-use computational tool, which can be run on a regular PC, based on real data applicable for the estimation of the permissible work time limit with regard to thermal strain in PPE under various conditions.
Spray Structure of a Pressure-Swirl Atomizer for Combustion Applications
(EDP Sciences, 2011-11-21) Ďurdina, Lukáš; Jedelský, Jan; Jícha, Miroslav
In the present work, global as well as spatially resolved parameters of a spray produced by a pressure-swirl atomizer are obtained. Small pressure-swirl atomizer for aircraft combustion chambers was run on a newly designed test bench with Jet A-1 kerosene type aviation fuel. The atomizer was tested in four regimes based on typical operation conditions of the engine. Spray characteristics were studied using two optical measurement systems, Particle Image velocimetry (PIV) and Phase-Doppler Particle Analyzer (P/DPA). The results obtained with P/DPA include information about Sauter Mean Diameter of droplets and spray velocity profiles in one plane perpendicular to the spray axis. Velocity magnitudes of droplets in an axial section of the spray were obtained using PIV. The experimental outputs also show a good confirmation of velocity profiles obtained with both instruments in the test plane. These data together will elucidate impact of the spray quality on the whole combustion process, its efficiency and exhaust gas emissions.

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