Control of flow and heat transfer around a cylinder through the action of hydrodynamic cavitation
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CUCIUC, Tudor; BOLOGA, Mircea. Control of flow and heat transfer around a cylinder through the action of hydrodynamic cavitation. In: Materials Science and Condensed Matter Physics. Ediția a 9-a, 25-28 septembrie 2018, Chișinău. Chișinău, Republica Moldova: Institutul de Fizică Aplicată, 2018, p. 268.
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Materials Science and Condensed Matter Physics
Ediția a 9-a, 2018
Conferința "International Conference on Materials Science and Condensed Matter Physics"
Chișinău, Moldova, 25-28 septembrie 2018

Control of flow and heat transfer around a cylinder through the action of hydrodynamic cavitation


CZU: 532.5+536.24+669
Pag. 268-268

Cuciuc Tudor, Bologa Mircea
 
Institute of Applied Physics
 
Disponibil în IBN: 11 februarie 2019


Rezumat

The phenomenon of cavitation attracts a special interest [1-3] associated with the prospects to attain superior results of various existing or novel technologies due to the effects produced by this phenomenon (extremely high local pressures and temperatures, generation of instabilities with high oscillation amplitudes, novel flow regimes of fluids, etc.). The transition processes that appear in the case of initiation and development of cavitation phenomenon in cavitation generators used for intensification of the processes of heat and mass transfer (mixing, control, extraction, atomization) were studied experimentally. It was stated from the analysis of distribution of the local heat transfer over the cylinder surface that in the case of a turbulent flow regime at the initial phase of the cavitation development (Fig. 1, curves 2 and 4) the integral convection coefficient increases by 40% in comparison with the case of a laminar boundary layer (Fig. 1, curves 1 and 3). This increasing can be elucidated due to the fact that the transition mechanism changes from the zone with cavitation in the form of vortices to the zone of supercavitation. In the case of the turbulent flow regime in a boundary layer in the transition region immediately near the singular cylinder or after the first cylinder when a tandem of cylinders is used at the distance between the cylinders greater than the crytical one, the cavitation phenomenon is accompanied by a periodic separation of cavitation vortices with a reduced intensity (Fig. 2, secondary vortex) with the following high amplitude separation of partial supercavitation cavities (Fig. 2, primary vortex) developed along the cylinder surface in the direction of the flow. This transition regime is stable and can be controlled via modification of the flow regime in the boundary layer by application of active control methods, for example, a continuous or periodic injection of microbubbles. The zone of flow with the partial supercavitation (Fig. 2, partial cavitation) spreads over the cylinder surface with the increasing of the speed or decreasing of the cavitation number and detaches at the same time when the attachment disappears. In this case in the spectrum of the pressure pulses only one dominant oscillation frequency is observed, in the same way as in the case of the cavitation in the wake of a single cylinder with a laminar boundary layer. At the same time, a substantial heat transfer decreasing is observed in the downstream part of the cylider. This research has shown that the indirect utilization of the cavitation phenomenon via evaluation of the possibility to control the flows near transition surfaces can be effective for intensification of various technological processes without detriment to materials structure and environmental contamination that is virtually inevitable in the case of a direct utilization of this phenomenon (effect of destruction and erosion of materials due to the violent implosion of the cavitation microbubbles).