V. P. Koverda

1/f noise and self-organized criticality in crisis regimes of heat and mass transfer

Abstract Thermal fluctuations under the transition from nucleate to film boiling of water on a wire heater and fluctuations of jet form during an outflow of superheated liquid from a pressure vessel were experimentally investigated. It has been found that power spectrum of the fluctuations has got a low frequency component corresponding to a 1/f law (flicker noise). The effect given is connected with the occurrence of nonequilibrium phase transitions in the systems: a crisis of heat transfer under the transition from nucleate to film boiling and a crisis of a flow in boiling-up of superheated liquid jet.

Self-organized criticality and 1/f-noise at interacting nonequilibrium phase transitions

Oscillatory combustion regimes were experimentally investigated under the conditions of ignitable liquid boiling-up before the reaction front. Fluctuations with a power spectral density inversely proportional to frequency (1/f-noise) were observed and the power-law character of distribution functions was found. The phenomena are connected with the intersection and interaction of nonequilibrium phase transitions. A mathematical model describing the initiation of 1/f fluctuations at the interaction of a subcritical phase transition and supercritical one in a space-distributed system was suggested.

Self-organization of a critical state and 1/fα-fluctuations at film boiling

Abstract 1/ f and 1/ f 2 -fluctuations have been first revealed by experiment at film boiling on a vertically oriented platinum heater. Fluctuations of 1/ f α -spectrum have been observed over both a wide range of governing parameters and a band of five decimal orders. The experimental results testify to a possible self-organized critical behaviors of a dynamical system at nonequilibrium phase transitions.

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.

Nonequilibrium phase transitions in a jet of highly superheated water

Results are given of an experimental study into the dependence of integral and local characteristics of hot water flow on the value of superheating and on the kinetics of boiling in this water. Abrupt variations are revealed of parameters of a jet of boiling water such as the reactive force, the jet cone angle, and the rate of nucleation of vapor bubbles in the jet. Abrupt variations of modes of jet flow are treated as nonequilibrium phase transitions. This enables one to invoke model concepts of generation of 1/f fluctuations under conditions of nonequilibrium phase transitions in open systems. The inversely proportional dependence of power spectra of oscillation on frequency (1/f fluctuations) in a jet of highly superheated water is found experimentally both under laboratory conditions and in a commercial-scale steam generator. Characteristic scales are discussed for reducing the experimental results for reactive force to dimensionless form.

Self-organized criticality and 1/f fluctuation under conditions of nonequilibrium phase transitions

The results are given of an experimental investigation of fluctuation phenomena under conditions of electric arc discharge. Fluctuations are observed whose spectral density is inversely proportional to frequency (1/f noise). Power dependences are revealed of the fluctuation distribution functions. The behavior of spectral density and of distribution functions is associated with the simultaneous occurrence of various nonequilibrium phase transitions. Within the framework of the mean field theory, a mathematical model is suggested of interacting nonequilibrium phase transitions in a distributed system, which predicts the self-organization of the critical state and the generation of fluctuations with diverging spectral characteristics. An adequate agreement is observed between the suggested model and experimental data.

Peculiarities of superheated liquid discharging under strong and weak nonequilibrium conditions

Results obtained in experimental investigating the characteristics of superheated liquid jet discharging under different nonequilibrium conditions are presented. The cases of liquid discharge from a high-pressure chamber and under gravitational jet formation conditions are considered. Different mechanisms of liquid flash boiling with heating growing up to limit superheating are identified in the case of liquid discharging from the high-pressure chamber. Dependences of the reactive force and the jet shape on the value of liquid overheating in a flow are presented. It is shown that the rapidly decreasing value of the reactive force of the jet is connected with its complete disintegration caused by homogeneous nucleation and by the presence of a normal wall behind the outlet from the channel. The parameters of two-phase jet stability in the conditions of gravitational falling, the flow rates, and the size of droplet dispersion zones depend substantially on the shape of holes and the heat flux density. Pronounced effects of partial or full blocking of the holes as a result of soluble impurities deposition in the regimes of cryogenic liquid evaporation or boiling when vapor cavities are formed in the channel are observed in the conditions of long-term discharging under nonadiabatic conditions. This leads to complete disintegration of the jet and uncontrollable reduction of the flow rate through the hole up to full blocking of the latter.

Noise-induced transition and \(\frac{1}{f}\) fluctuations while intersecting nonequilibrium phase transitions

non in nature [1]. It is observed in radio engineering devices, in solid-state physics, in fracture mechanics, in chemical reactions, etc. An intrinsic feature of systems with noise lies in the fact that the power spectrum in this case is inversely proportional to the frequency f. Such a dependence suggests that a considerable part of the fluctuation energy is associated with slow processes and, moreover, implies a possibility of huge catastrophic surges in the system.

Scaling of 1/f noise in nonequilibrium phase transitions

Investigation of the dynamics of fluctuations of heat and mass transfer reveals that its crisis and transient modes exhibit high-energy pulsations with a power spectrum that is inversely proportional to frequency (flicker or 1/f fluctuations). Such a spectrum suggests energy transfer from high-to low-frequency modes and the possibility of large-scale catastrophic outbursts in the system being considered. The theory shows that such fluctuations arise in the system owing to the simultaneous occurrence of interacting phase transitions in the presence of white noise having a sufficiently high intensity. The distribution of fluctuations for scale transformations of the set of stochastic equations that describe the generation of 1/f noise is investigated. It is shown that, under a scale transformation, the Gaussian distribution of a random process having a 1/f spectrum passes to an exponential distribution, which is characteristic of the statistics of extreme outbursts. The probability of such outbursts must be taken into account in predicting the stability of various heat-transfer modes.