Yu. P. Ivochkin

An experimental investigation of the characteristics of explosive boiling of subcooled water on a hot surface under conditions of change of boiling modes

A procedure of estimating the area of contact between cold water and a hot hemispherical surface is developed and described. Synchronous measurements of pressure pulses in liquid, the temperature of a body, and characteristics of contact between a heated body and a coolant are performed under conditions of abrupt change of boiling modes (from film to nucleate boiling). Characteristic features of the investigated processes are determined, which are largely defined by the temperature of the hemisphere, by its thermophysical properties, and by the presence of surface oxide films. It is found that the maximal value of the amplitude of pressure pulses, which may be as high as ∼1 MPa in the experiments, is observed in the region of temperatures of the hot body which are close to the temperature of limiting superheat of water. The dependences are obtained of the rate of spreading of liquid and of the time of delay of its explosive boiling from the instant of boiling on the temperature of the hemisphere.

A Study into Development of Instability and Collapse of Vapor Layer on a Heated Solid Hemispherical Surface

A study is performed into the processes of development of instability and collapse of a vapor film on a solid hemispherical surface. Optical-fiber sensors of pressure and vapor-film thickness are used in measurements; the process is videofilmed with subsequent computer processing of images. A physical model of the phenomenon is developed, which includes superheating and explosive boiling of water in recesses of wavy vapor film. The predictions are in qualitative agreement with the experimental results.

Temperature fluctuations in a liquid metal MHD-flow in a horizontal inhomogeneously heated tube

The investigation of heat transfer characteristics during liquid metal flow in a horizontal tube with one-sided heating of the lower half of the tube is carried out. Experimental investigations are conducted using the mercury MHD complex of the Moscow Power Engineering Institute and the Joint Institute for High Temperatures, Russian Academy of Science, in the context of a long-term collaboration program. The characteristics of the averaged and fluctuation temperatures are considered in different sections over the length of the heating zone in the area of a homogeneous transverse magnetic field. The laminar liquid metal flow revealed the occurrence and development of a low-frequency quasi-harmonic temperature fluctuation with great intensity.

An experimental investigation into the heat transfer developed along a uniformly heated tube carrying a flow of liquid metal and placed in transverse magnetic field

We present results from an experimental investigation into heat transfer for liquid-metal coolant flowing through a horizontal heated tube (at qw = constant on its wall) placed in transverse magnetic field. Three-dimensional fields of averaged temperature are measured, and heat-transfer coefficients along the tube length are calculated in a wide range of Reynolds numbers and Hartman numbers for different values of heat flux density.

Estimation of the influence of temperature pulses on piezoelectric pressure gauge readings

AbsractThe results of the pressure pulsations measurements connected with the process of the vapor films explosive destruction near heated bodies were analyzed. It was stated that under such conditions false pressure pulses are generated. It was experimentally shown and approved by numerical calculations that such interference may be caused by pressure pulsations due to thermal interaction of the piezotransducers used in the experiments with the high-temperature fragments of vapor. The mechanism of the false pressure pulses generation process caused by the thermal deformation of the membrane that affects the pressure sensor (piezoelectric crystal) was described.

Explosive regime of the development of instability leading to vapor-film collapse on a heated solid hemispherical surface

We investigated the local processes proceeding in the case of changing the boiling regime on a heated cylindrical rod with a hemispherical end immersed in water ( T = 285 K) at a depth equal to the radius of the hemisphere. The rod was heated in ambient air up to temperature T ≤ 1000 K. Vapor-film escape was observed visually using a microscope and two video cameras installed below and aside from the hemisphere immersed in the liquid. The pressure arising in the liquid and the vapor-film thickness were measured using fiber-optical sensors [1]. In our experiments, we used metallic rods made of steel, pure copper, and copper covered by PSr62 silver solder. After immersing the hemisphere in a liquid, freeconvection flow formed near the heated surface. Then, wave formations appeared on the vapor‐fluid interface. The amplitude and characteristic length of the surface waves were on the order of magnitude of a vapor-film thickness. The process of vapor-film escape and passing to bubble boiling proceeded following two different scenarios. According to one of them, the wave perturbances having the form of isolated solitons or wave trains were enhanced with time and enveloped the entire hemisphere immersed in the water. Afterwards, a vapor-film explosion occurred and the nucleate-boiling regime was established. In a number of cases, the explosive boiling of the film was accompanied by the formation of a jet flow directed from the lower end of the hemisphere into the fluid. The explosive escape of the vapor film accompanied by the formation of the jet flow can be repeated (up to 30 times) in intervals of 0.3 to 1 s. In the other regime, the wave perturbances, once arisen, attenuated gradually and passed relatively smoothly to the nucleate-boiling regime. Smooth passage to nucleate boiling was observed only at the first immersion in water for a new hemisphere or for that freshly cleaned from oxides. In repeated experiments with surfaces having the oxide film, the vapor escaped explosively. In Figs. 1‐3, we show photographs of a vapor film before explosion and with various types of its escape. The experiments were carried out under normal pressure, and the temperatures of the cooling water and heated surface were 293 and 795 K, respectively. The characteristic values of the vapor‐fluid jet velocity (Fig. 3) attained 0.3 m/s. Vibrations of the vapor-film surface were detected both at the second boiling crisis with quiet vapor-film escape and with vapor explosion. The characteristic longitudinal dimension of the wave structures attained

Mechanisms Governing Fine Fragmentation of Hot Melt Immersed in Cold Water

Hypotheses about the mechanisms governing fragmentation of superheated liquid metal droplets falling into cold water are analyzed. It is shown that a physical model based on the cavitation–acoustic mechanism governing fine fragmentation of melt under steam explosion conditions is likely the most suitable one for consistently describing the fragmentation of both low-melting and refractory metals. For checking this conjecture, special experiments for studying the processes triggered when cold (20°С) water comes into contact with a heated surface and for measuring the pressure impulses (arising both in coolant and in the hot body) accompanying the coolant flashing were carried out using liquid metal (tin and steel) droplets and superheated solid steel bodies. The working substance temperatures were varied in the range from 200 to 1600°С. The results obtained from the performed experiments are not in contradiction with the melt fine fragmentation process represented by the cavitation–acoustic model. It is shown that the acoustic waves generated during explosive growth of bubbles on a hot surface propagate in the solid body and are alternating in nature. Their intensity (including that at negative pressure values) differs only slightly in the modulus from the pressure impulses measured in the coolant and is sufficient for finely fragmenting the droplets. It is experimentally found—with the use of a conductance measuring technique—that the transition from the coolant film to bubble boiling mode is preceded by a short-term (lasting a few milliseconds) process involving intense interaction of waves at the steam–liquid boundary with the heated surface. The signal from the conductance measuring transducer was subjected to a wavelet analysis for different values of the heated surface temperature. The study results testify that high-frequency (several tens of kilohertz) pulsations of electric current are generated in the preburnout region with their characteristics varying (toward increasing the amplitude and intensity) with time as the heating and heated media come closer into contact with each other. A probabilistic process development scenario is suggested.

About lack of fragmentation for hot droplets at low subcooling of coolant

It was confirmed in experiments that during contact between cool and hot liquids, the lower sub-cooling of the cool liquid below the saturation temperature changes the characteristics of a vapor layer covering the fragments of hot liquid. This factor also decreases the probability of spontaneous direct contact between two kinds of liquid, explosive incipience of the cool liquid, and pressure pulse generation (the latter triggers fine fragmentation of hot coolant and vapor explosion). The mechanism that describes this trend in vapor layer behavior has been described.