A. N. Pavlenko

Dynamics of transient processes at liquid boiling-up in the conditions of free convection and forced flow in a channel under nonstationary heat release

Results on experimental investigation of the dynamics of boiling-up at stepwise heat release on a horizontally oriented cylindrical surface in a large volume of freon-21 are presented. Experimental data on the propagation velocity, structure, and other local characteristics of development of self-sustained evaporation fronts at different temperature differences of boiling-up in saturated liquid were obtained. New experimental results on the dynamics of vapor phase incipience and evolution on the surface of a vertical heat releasing tube and on the dynamics of changing the heater temperature and pressure in a flow of liquid (water, ethanol) subcooled to saturation temperature in the channel under nonstationary heat release conditions are represented. It was revealed that the dependence of the expectation time of intense bubble growth on the water motion velocity is nonmonotonic.

Transient processes in falling films of liquid under conditions of unsteady-state heat release

Results are given of experimental investigation of the dynamics of development of heat transfer and critical phenomena in falling wavy films of nitrogen under conditions of unsteady-state heat release from the surface of a thin-walled heater. The experiments involve the investigation of transient modes with formation of dry spots and development of dryout in the case of stepwise and quasi-steady laws of heat release. It is demonstrated that the disintegration of laminar-wavy film of liquid in the case of low heat flux in critical modes under conditions of stepwise increase involves the emergence of metastable regular structures with jets of liquid and large-scale non-wetted zones between them. The generalization of experimental data is used to demonstrate that the value of characteristic transverse dimension between jets of liquid is more than twice the respective values obtained under conditions of steady-state heat release with boundary condition Theater ∼ const. In the case of high heat flux, the vigorous boiling of liquid leads to a rapid ejection of liquid in the form of droplets and to complete drying (almost simultaneously) of the entire heat-emitting surface. Experimental data are obtained on the times of expectation of boiling of liquid, of development of regular structures, and of drying of the heat-emitting surface as functions of heat flux in the range of variation of Reynolds number Rein = 90–1690. It follows from analysis of experimental data that, in calculating the time of expectation of boiling of liquid in the case of stepwise heat release in the investigated range of variation of heat flux, one must take into account the development of intense evaporation from the free surface of laminar-wavy film.

Investigation of transient processes at liquid boiling under nonstationary heat generation conditions

The paper presents results on experimentally investigated dynamics of boiling development and formation of film boiling zones under stepwise heat generation on a horizontally and vertically oriented cylindrical surface in a large volume of Freon-21. Experimental data on the expectation time and boiling temperature, the propagation velocity and structure of evaporation and boiling fronts for different heat flux density both in saturated liquid and in subcooling conditions are obtained. Results of experiments on investigating the nucleation forms under development of nonstationary heat release crisis caused by heat loading on the vertical heater immersed into the volume of liquid (water, ethanol) subcooled to saturation temperature are presented. A calculation ratio for determining the expectation time to the beginning of intense vaporization in water is proposed and compared with experimental data obtained on surfaces with different-size roughness. Peculiarities of evolution of evaporation fronts from incipient bubbles are investigated in the experiments with ethanol. Data on the evaporation front velocity as a function of wall overheating are obtained. The obtained experimental data on the propagation velocity of self-sustained evaporation fronts are compared with the known calculated data.

Experimental study of the effect of maldistribution at the structured packing inlet on the freon mixture separation efficiency

The results of an experimental study of the effect of the initial maldistribution over a structured packing on the separation efficiency of a binary freon mixture are considered. The study was performed in a distillation column 0.9 m in diameter on a mixture of R114 and R21 freons. The results on the efficiency of freon mixture separation, the pressure drop on the packing, and the distribution of the local liquid flow rate under the packing are presented. The experimental results showed that the rotation angle of structured packed layers have a significant effect on the efficiency of mixture separation in the case of maldistribution.

Disintegration of flows of superheated liquid films and jets

This paper represents results on investigating the dynamics of boiling and disintegration of superheated liquid films and jets. The first part deals with experimental study of boiling liquid outflow through short cylindrical and slit channels. Evolution of disintegration of a hot water jet flow is observed both at low and moderate superheating and at high and limit superheating, and also for vaporization mechanisms corresponding to these superheatings. Peculiarities of disintegration of jets through slit and cylindrical channels are noticed. Results on measuring the reactive thrust of the jet through a slit channel under different geometrical conditions behind the channel outlet are represented. The 1/f fluctuations in transient regimes of superheated liquid boiling and in transient regimes of behavior of the jet shape are found. The second part of this article represents results on experimental investigation of nonsteady heat transfer and dynamics of the development of crisis phenomena at boiling of a falling subcooled liquid film in the conditions of stepwise heat release. The experimental data were obtained using synchronized high-speed infrared thermography and video. It is shown that with growth and condensation of vapor bubbles, on the liquid film interface appear large-amplitude waves that lead to considerably increasing local intensity of heat transfer. New data on the boiling incipience temperature in a subcooled liquid film, depending on the heat flux density, are obtained. It is found that the development of boiling crisis is a result of appearance of local dry patches and their subsequent growth by the mechanism of longitudinal thermal conductivity in the heat transfer wall as the equilibrium heat flux density is exceeded.

Dynamics of transition processes and structure formation in critical heat-mass transfer regimes during liquid boiling and cavitation

The results of experimental studies concerning the development of critical phenomena and structure formation in the process of boiling in falling films and during liquid cavitation are given. In conditions of stepwise and periodic pulsed surges of a thermal load, the parameters of formed metastable regular structures and critical parameters of heat-releasing surface drying are shown to be determined by the dynamics of moving wetting boundaries in the process of system self-organization. In the case of high-intensity heat fluxes, decomposition of a falling film is determined by propagation regimes of self-maintaining boiling fronts with a complex shape of intermediate structures. The study of ultrasonic cavitation of water, glycerin, and vacuum oil shows that structures of interacting gas-vapor bubbles (having the form of fractal clusters) are formed near the emitter surface. Spatial structures are characterized by a low-frequency divergence of the power spectra and a scale-invariant function of the fluctuation distributions. The experimental results are in good qualitative agreement with the numerical simulations performed within the theory of 1/f fluctuations in the case of nonequilibrium phase transitions in a spatially distributed system.

Heat transfer and crisis phenomena with intense boiling in the falling wave liquid films

Experimental data on heat transfer with intense evaporation in the falling films of liquid nitrogen were analysed. According to data generalization, heat transfer at evaporation becomes more intense under the precrisis modes at high heat fluxes for two studied boundary conditions on the heat-releasing surface: Tw ≈ const and qw ≈ const. The relative contributions of conductive and convective components of heat transfer for different heat fluxes were estimated due to statistical treatment of the wave characteristics carried out by the capacitance probes for measurement of the local liquid film thickness. It was found out that heat transfer intensification is mainly caused by a drastic decrease in thermal resistance of the local zones with intensely evaporating residual layer between large waves. At that, the convective component of heat transfer related to wave perturbations on a free surface of a liquid film decreases significantly with a rise of heat fluxes. New data on pulsations of the local temperature of the heat-releasing surface were obtained at different points along the flow with the modes of “dry spot” formation.

Effect of dynamically controlled irrigation of a structured packing on mixture separation efficiency

The paper presents experimental results on the separation efficiency and distribution of flow parameters over the cross section for uniform and nonuniform modes of irrigation of aMellapak 350.Y structured packing in a distillation column 0.9 m in diameter using a mixture of Freon R114 and R21. Empirical data are obtained for the dynamic characteristics of forming a large-scale temperaturemaldistribution over the packing cross section as the column operation becomes steady. The dynamics of changes in the structure of large-scale temperatures maldistribution over the packing cross section is studied under various disturbing effects on the parameters of jet irrigation. The gained results form a methodological and comparative base for further investigations aimed at searching for new methods of mass transfer intensification in mixture separation, which are based on dynamically controlled irrigation of the regular packing by a liquid.

Separation of mixtures and distribution of a liquid on a structured packing in a large-scale model of a distillation column

A large-scale experimental setup is designed to study the hydrodynamic parameters and the mass-transfer efficiency in a countercurrent flow of mixtures of Freon R21 and Freon R114 on structured packings in a column 0.9 m in diameter. The study deals with the local and integral parameters of separation of the mixture, the degrees of nonuniformity of the flow rates and concentration distributions of the liquid and vapor phases over a cross section and on the column wall, and the pressure difference while varying the number of packing layers and their rotation angle. The mass-transfer processes while independently varying the liquid and vapor flow rates over wide ranges are also investigated. Experimental data are presented to illustrate the effect of the operating parameters on the height of an equivalent theoretical plate, the relative pressure difference, and the distributions of the local parameters of the flows.

Effects of layers and holes on performance of wire mesh packing

To investigate the effect of layer number and hole on the performance of structured packing, pressure drop, flooding velocity, and mass transfer efficiency of the BXH- and DM I- type structured packing are measured. The experimental results show that, compared with the BX-type structured packing, the DM I-type packing has a higher pressure drop and mass transfer efficiency with lower flooding velocity, while BXH type packing has a lower pressure drop and mass transfer efficiencywith higher flooding velocity. This indicates that the increase of the mesh layer can improve mass transfer efficiency of structured packing but reduce its capacity. On the contrary, the surface hole of the structured packing can improve the capacity but reduce the mass transfer efficiency. Experimental data on the influence of holes and microtexture on the liquid spreading over the surface of single cells of the structured aluminum packing are analyzed.