EÚ-odbor termomechaniky a techniky prostředí

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    Effects of secondary breakup, collision dynamics, gravity and evaporation on droplet size distribution in a pressure-swirl JET A-1 spray
    (Elsevier, 2024-03-01) Jedelský, Jan; Malý, Milan; Vankeswaram, Sai Krishna; Zaremba, Matouš; Kardos, Réka Anna; Csemany, David; Červenec, Adam; Józsa, Viktor
    Size and velocity of droplets in pressure-swirl sprays vary with distance downstream the nozzle. This study aims to explain the phenomena contributing to size-resolved spatial variation of the drop size distribution for Jet A-1 spray from a small pressure swirl atomizer injected into quiescent ambient air at atmospheric conditions. The effects of secondary breakup, droplet collisions, gravity, drag-driven spray dispersion, and evaporation are evaluated. Phase Doppler anemometer (PDA) was used to resolve the velocity and size of the droplets in radial profiles at several axial distances (Z) from the nozzle exit at injection pressures between 0.5 and 1.5 MPa. The droplet motion and collision dynamics were qualitatively characterised by high-speed imaging. The analysis focused on areas along 1) the spray axis and 2) the liquid sheet direction. The first area covers small droplets with marginal evolution, while the mean drop size in the second area significantly increases downstream. The local drop size distribution and its axial evolution results from a combined effect of ballistic filtering (drifting of small droplets from periphery to spray centre and large droplets vice versa), centrifugal and turbulent droplet dispersion, and droplet collisions (increasing drop size with distance). The relative droplet-gas velocity was found at investigated Z positions too small for drag-driven secondary droplet breakup to occur. Evaporation of Jet A-1 sprayed at room temperature and pressure into still atmosphere reduces the drop size marginally. Trajectory of all drop sizes is insignificantly altered by gravity. The smallest droplets are strongly drag-driven to the spray axis, while large ones disperse centrifugally and have sufficient momentum to neglect the gravity. Combining results from imaging and laser diagnostics, various collision outcomes were identified, with a conclusion that these depend on the size and velocity of colliding droplets. Coalescing collisions dominate and strongly increase the mean drop size downstream the spray at off-axis positions while insignificantly contribute along the spray axis. The drop size span factor reduces with Z as separative collisions between large droplets and mixed-type collisions between small and large droplets narrow the drop size distribution. The above effects can be most easily amplified for drop size reduction via modification of physical properties of sprayed liquid by its heating.
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    Schlieren analysis of non-MILD distributed combustion in a mixture temperature-controlled burner
    (Elsevier, 2023-06-15) Józsa, Viktor; Malý, Milan; Füzesi, Dániel; Rácz, Erika; Kardos, Réka Anna; Jedelský, Jan
    It was recently demonstrated that distributed combustion is accessible outside the MILD combustion regime without needing inner or outer flue gas recirculation. The Mixture-Temperature Controlled combustion concept, which made it possible, offers excellent flame stability besides ultra-low emission. This concept is investigated presently to reveal the qualitative characteristics of the cold discharging mixture jet from the burner and its ignition. The Schlieren technique with a high-speed camera is the most suitable approach for this purpose, revealing the line-of-sight density gradients. Nine cases were evaluated, utilizing natural gas and diesel fuel, various equivalence ratios, and atomizing pressures. V-shaped flames were used as a baseline for comparing distributed combustion to it via direct images and velocity field using the PIVlab Matlab application. The results confirm the previous hypothesis that distributed combustion features a cold fuel-air mixture at the burner discharge that ignites downstream. The excellent flame stability comes from the fishbone-tiled coherent structures with significant random features, resulting in no characteristic frequency related to the flame. All these results comply with the previous findings by chemiluminescence emission and acoustic signal of distributed combustion, which techniques cannot be used to investigate the flame structure, unlike Schlieren imaging.
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    Hight Speed Temperature Measurement
    (EPJ Web of Conferences, 2022-07-11) Bouchal, Patrik; Hejčík, Jiří; Jícha, Miroslav
    In certain applications where temperature changes rapidly, e.g., compressor cylinder or explosion chamber, it is necessary to obtain temperature data with sample rate high enough to register the change, i.e., the temperature probe response time must be very low. Type E coaxial thermocouple with response time in the range of us can be an example of such temperature probe. This paper describes several approaches for high frequency temperature data obtaining using the coaxial thermocouples. Individual approaches are described together with their related issues. The possible method of data evaluation is discussed.
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    Experimental set up for the investigation of partial phase changes of phase change materials
    (EDP Sciences, 2022-07-11) Zálešák, Martin; Bouchal, Patrik; Ostrý, Milan; Hejčík, Jiří
    Thermal energy storage with phase change materials (PCMs) has attracted a lot of attention in the last several decades. Most PCMs do not change phase at a constant temperature but rather in a certain temperature range. It means that the PCM need to transit through its phase change temperature range to fully change phase from the solid state to the liquid state and vice-versa. The situation, in which the phase transition begins and/or ends within the phase change temperature range (in the mushy zone), is usually called a partial phase transition (or a partial phase change). The partial phase transitions occur quite often in real-life thermal energy storage systems with PCMs; especially when a PCM has a wide phase change temperature range. The behavior of PCMs during the partial phase transitions is poorly understood at the moment, because the experimental techniques used for the characterization of PCMs (such as the differential scanning calorimetry – DSC) are difficult to apply for the study of partial phase transitions. The lack of knowledge in this area influences the accuracy of phase change simulation models. The main goal of the experimental investigations, described in the paper, was to obtain data for the development of a simulation model for partial phase changes. The experimental set up for the investigation of partial phase changes of PCMs has been proposed, assembled, and the pilot measurements have been conducted. The experimental set up consists of two water storage tanks (that can be maintained at different water temperatures), a water-PCM concentric tube type heat exchanger and a data acquisition system. The water flows through the central tube of the heat exchanger while the PCM is located in the annular space of the exchanger. The water storage tanks, maintained at the temperatures within the phase change temperature range of a PCM, allow for the investigations of the heat storage cycles consisting of partial phase changes.
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    Searching for a Numerical Model for Prediction of Pressure-Swirl Atomizer Internal Flow
    (MDPI, 2022-06-22) Malý, Milan; Sláma, Jaroslav; Cejpek, Ondřej; Jedelský, Jan
    Numerical prediction of discharge parameters allows design of a pressure-swirl atomizer in a fast and cheap manner, yet it must provide reliable results for a wide range of geometries and operating regimes. Many authors have used different numerical setups for similar cases and often concluded opposite suggestions on numerical setup. This paper compares 2D (two-dimensional) axisymmetric, 3D (three-dimensional) periodic and full 3D numerical models used for estimation of the internal flow characteristics of a pressure-swirl atomizer. The computed results are compared with experimental data in terms of spray cone angle, discharge coefficient (C-D), internal air-core dimensions, and velocity profiles. The three-component velocity was experimentally measured using a Laser Doppler Anemometry in a scaled transparent model of the atomizer. The internal air-core was visualized by a high-speed camera with backlit illumination. Tested conditions covered a wide range of the Reynolds numbers within the inlet ports, Re = 1000, 2000, 4000. The flow was treated as both steady and transient flow. The numerical solver used laminar and several turbulence models, represented by k-epsilon and k-omega models, Reynolds Stress model (RSM) and Large Eddy Simulation (LES). The laminar solver was capable of closely predicting the C-D, air-core dimensions and velocity profiles compared with the experimental results in both 2D and 3D simulations. The LES performed similarly to the laminar solver for low Re and was slightly superior for Re = 4000. The two-equation models were sensitive to proper solving of the near wall flow and were not accurate for low Re. Surprisingly, the RSM produced the worst results.