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Boiling two phase flow in a coil-shaped duct combined with heat conduction in solid body
Novák, Aleš ; Hrubý, Jan ; Kozel, Karel
Forced-convection boiling in a coil-shaped duct heated from the outer side has an advantage of enhancing the heat transfer and reducing the risk of boiling crisis by pushing the liquid against the outer wall due to centrifugal force. Because of the large heat fluxes, the heat conduction in the solid walls is taken into account. The model is based on differential equations of mass, momentum and energy conservation. In the resulting quasi-1D model, the liquid and vapor phases are allowed to flow at different velocities. The model simulates a laboratory steam generator built in Institute of Thermomechanics AS CR.
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An influence of amplitude modulation of the synthetic jet to the flow
Matějka, M. ; Popelka, Lukáš
The paper deals with an experimental research of an active boundary layer control achieved by using periodic excitation of a synthetic jet. An influence of amplitude modulation of the synthetic jet flow control to the flow field of the wake was studied. It summarizes the influence of the basic factors that affect the design of the synthetic jet actuator. The principle of a flow control with and without amplitude modulation was used. Flow visualization of the flow field was done.
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PASSIVE CONTROL OF BOUNDARY LAYER TRANSITION AND SEPARATION
Popelka, Lukáš ; Matějka, M. ; Šimurda, David ; Součková, Natálie
Particle Image Velocimetry, smoke-wire, tuft filaments and oil-flow visualization techniques were used for wind-tunnel and in-flight investigation of boundary layer separation, both stall and separation bubbles, related to the low-Reynolds number transition mechanism. Airfoils of three Czech-designed sailplanes and their wing-fuselage interaction were subject to study. Experimental data were coupled with numerical modeling and synthesis gained. Effect of passive flow control devices - vortex generators - was surveyed, counter-rotating vortex generators and Zig-zag type turbulators were applied. Separation suppression was reached and consequent drag coefficient reduction of test aircrafts measured in flight. Investigation was further extended by PIV Time-Resolved technique.
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PERTURBANCE ANALYSIS AND FISSION MODEL OF COLLAPSING BUBBLE
Petrík, P. ; Zima, Patrik ; Ridha, A.
A new criterion for the assessment of the reliability of the Rayleigh-Plesset equation is proposed. An analysis of the shape stability of the spherical surface of the bubble during the collapse is used to develop a physical model, which incorporates the loss of shape stability, surface energy dissipation and fission process. The model also estimates the number of bubble fragments as well as conditions for the fission to occur. Finally, some aspects of the numerical code, especially error estimation and propagation of rounding error, are discussed.
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COLLAPSE OF A CAVITATION BUBBLE GENERATED BY LOW VOLTAGE DISCHARGE IN WATER
Müller, M. ; Unger, J. ; Buďárek, T. ; Zima, Patrik
The article presents experimental results of the optical study of cavitation bubble collapse close to a solid boundary in water. The bubble was generated by discharge of two low-voltage capacitors. High-speed CCD camera was used to record the time evolution of the bubble size. High-power halogen lamp was used for illumination. The system was synchronized by pulse generator connected to an oscilloscope. The velocity of the re-entrant jet was estimated from the time resolved photography for three different maximum bubble sizes.
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BASIC CHARACTERISTICS OF ARRAY OF PULSATILE JET
Novotný, P. ; Vít, Tomáš ; Vestfálová, M.
The presented paper shows the results of measurement of basic characteristics of array of pulsatile jets which are used to enhance the efficiency of ejectors. Four pulsatile jets forms cross like structure where perpendicular couples are operating in two basic modes (a) in phase and (b) in antiphase. Paper presents phase averaged velocity profiles and velocity fields. All of the presented experiments are realized using hot wire anemometry method.
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Digital holografic interferometry used for identification of 2D temperature field
Lédl, V. ; Vít, Tomáš ; Doleček, R. ; Psota, P.
The presented paper shows the possibility of digital holographic interferometry application in measurement of unsteady 2D temperature field generated by pulsatile flow which is impinging heated surface. Special holographic setup with double sensitivity was developed instead of the commonly used Mach-Zehnder type of holographic interferometer in order to attain the parameters sufficient for the studied case. This setup is not light efficient like the Mach- Zehnder type but has double sensitivity. The results from the digital holographic interferometry experiments are perfected by measurement of the velocity field achieved by methods of hot-wire anemometry.
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Droplet and Bubble Nucleation Modeled by Density Gradient Theory - Cubic Equation of State versus SAFT Model
Vinš, Václav ; Hrubý, Jan ; Planková, Barbora
The study presents some preliminary results of the density gradient theory (GT) combined with two different equations of state (EoS): the classical cubic equation by van der Waals and a recent approach based on the statistical associating fluid theory (SAFT), namely its perturbed-chain (PC) modification. It has been shown that PC-SAFT is a promising tool for accurate modeling of nucleation using the GT. Besides the basic case of a planar phase interface, the spherical interface was analyzed to model a critical cluster occurring either for nucleation of droplets (condensation) or bubbles (boiling, cavitation).
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Nucleation rate in binary cavitating systems - A comparison of experimental data and classical nucleation theory predictions
Němec, Tomáš
A brief review of recent literature on proton exchange membrane fuel cell (PEMFC) modelling is presented and the governing equations of the transport processes in the proton exchange membrane are presented. Consequently, the model of water transport in the membrane electrode assembly of a fuel cell in commercial software FLUENT is investigated. The aim of this work is to test the possibilities of the PEMFC modelling using FLUENT. Special attention is given to the water management and the properties of the fuel cell membrane.of four binary mixtures, i.e. diethylether - nitrogen, propane - carbon dioxide, isobutane - carbon dioxide, and chlorodifluoromethane - carbon dioxide. The predictions of the classical nucleation theory show very good agreement with the measured nucleation rates.
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