Interconnection between dynamics of liquid boiling-up and heat transfer crisis for nonstationary heat release

This paper deals with investigation results on crisis phenomena for nonstationary heat release under the conditions of free convection and in falling liquid films. It is shown that the character of the crisis development and the critical heat flux for nonstationary heat release significantly depend on the characteristics determining the temperature head of liquid boiling-up. According to experimental data with an arbitrary temporal function of heating, the character of the dependence between the critical heat flux and the heat release increasing rate is significantly effected by ready nucleation sites. It is found that a change in the nonstationary critical heat flux in the range of high times between impulses for periodic heat release is connected with deactivation of ready nucleation sites on the heat-releasing surface. According to new experimental results, in the studied range of irrigation degree alteration (Re_in = 30–1660), parameters characterizing decay of the falling liquid film with stepped heat release (the distribution of the time of boiling-up expectation along the liquid film, the velocities of movable boundaries in the boiling-up and drying fronts, the intensity of liquid ejection from the heat-releasing surface) complexly depend on the Reynolds number, wave characteristics, and heat flux density. The text was submitted by the authors in English.     

Third heat transfer crisis at subcooling

The effect of liquid subcooling below the saturation temperature on the third heat transfer crisis was studied experimentally at pool boiling. Experimental data on the threshold values of superheating and heat fluxes, above which the evaporation front and third heat transfer crisis for acetone at subcooling from 0.3 to 10 K are formed, were shown. Formation of evaporation fronts is the necessary, but not sufficient condition for the third heat transfer crisis at subcooling. It was found that formation of a stable vapor film after propagation of condensation fronts over the heater surface is possible at heat fluxes considerably lower than the first critical one.     

Features of dry spot evolution at the film flow of cryogenic liquids at non-stationary heat release

Transitional processes with dry spot formation and drying crisis development were studied experimentally for the gradual and quasi-stationary laws of heat release on a thin-wall heater cooled by a falling film of cryogenic liquid. It is shown that for low densities of the heat flux, the laminar-wave liquid film decays with formation of a self-organizing system of metastable regular structures with boiling liquid jets and large dry zones between them. The numerical experiment modelling the process of repeated wetting of a superheated surface dried by impulse heat release was carried out. It was found for the first time that the local motion velocities of different zones of the 2D wetting front differ significantly. Reliability of results obtained by numerical methods was proved by direct comparison with experimental data. The work was financially supported by the Russian Foundation for Basic Research (Grant No. 5-08-18022-a) and Siberian Branch of RAS (Integration project of SB RAS together with ITP of UB RAS No. 2.5).     

Experimental Study of the Influence of Structured Capillary-Porous Coatings on the Dynamics of Development of Transient Processes and the Crisis Phenomena at Stepwise Heat Release

The effect of structured plasma-sprayed capillary-porous coatings on transient processes and the development of crisis phenomena at boiling under pulsed heat release was studied. The working fluid was liquid nitrogen on the saturation line at atmospheric pressure. It is shown that under unsteady heat release, there is a degeneration of the development of the boiling crisis on heaters with structured capillary-porous coatings at q < q_CHF (critical heat flux at steady heat release). Under unsteady pulsed heat release, no rapid transition to the film boiling regime (without passing through the nucleate boiling stage) is observed on heaters with such coatings until the thermal load is more than two times higher than the critical heat flux for steady heat release. This significantly increases the times of transition to post-critical heat transfer. Analysis of synchronized measurements of surface temperature of heaters and high-speed video recording of transient processes shows that the degeneration of the heat transfer crisis at q < q_CHF on samples with coatings occurs due to significantly lower liquid boiling temperature differences and specific features of the dynamics of propagation of self-sustaining evaporation fronts in comparison with a smooth heater.     

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.     

微液膜动力学特性与稳定性实验研究

气-液两相流设备的性能受限于临界热流密度,开展流动微液膜动力学特性及其稳定性的相关研究是深入理解沸腾危机及临界热流密度机理的关键。采用光学玻璃制成的矩形通道作为实验段,使用微流量齿轮泵驱动去离子水,使其在实验通道入口处与在其上部流动的压缩空气接触形成同向流动的分层流。利用共轭光学探测器对流动微液膜的厚度进行了测量,利用高速摄像机对气-液两相分层流波动特性进行了可视化观测。研究表明,在绝热情况下,当液速一定时,液膜的平均厚度随着气速增加而减小,当气速增加到某一阈值时会导致液膜破裂。     

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.     

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.     

Heat transfer and crisis phenomena at boiling in Freon mixture films falling down the structured tube

Investigation results on hydrodynamics, heat transfer, and crisis phenomena in laminar-wave liquid films falling down the surfaces of different geometry are presented in this study. Freon mixture R21/R114 with initial concentration of low-boiling component of 4?C16.6 % was used as the working liquid. The film Reynolds number at the inlet to the experimental section varied from 60 to 700. The heat flux density was changed within 0?C5 W/cm². The images of wave surface of the falling liquid film and formation of dry spots were visualized and recorded by the high-speed video camera. Results of investigation of the wave structure of the film surface, measurements of heat transfer coefficients under the conditions of boiling, and critical heat fluxes in the film flow over the smooth and structured surfaces are presented.     

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: T _w ≈ const and q _w ≈ 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. The work was financially supported by the Russian Foundation for Basic Research (grant No. 03-02-04027-NNIO-a) and DFG (Deutsche Forschungsgemeinschaft, project Nos. Re-463-37-1 and SFB-540).     

Thermographic investigation of the temperature field dynamics at the liquid-air interface in drops of water solutions drying on a glass substrate

The temperature field distribution at the liquid-air interface in drops of water and water solutions drying on a glass substrate is studied with thermal imaging means. It is shown that irrespective of the liquid composition, the circumferential temperature of the drops (along the boundary line) is always higher than the temperature on the top. The temperature field on the surface of the drops is nonstationary and varies chaotically during water evaporation. It is found that the dynamics of histograms for albumin-containing and albumin-free liquids differ. Mechanisms behind the origination of thermocapillary liquid flows and their directivity in the drying drops are discussed.     

Numerical Simulation for Falling Film Flow Characteristics of Refrigerant on the Smooth and Structured Surfaces

Flow characteristics of a liquid film flowing over a smooth surface and structured surface with the Reynolds number range from 10 to 1121 are studied. The mixture of R21 and R114 refrigerants is used as the test liquid. The 3D transient simulations are taken to capture the liquid film’s dynamic characteristics and spatial distribution. Effects of the inlet dimension, inlet flow rates, surface tension, and surface structuring on the wettability, average velocity, and film thickness are studied systematically. The obtained results show that surface tension is essential for an accurate simulation, while inlet width has no effect on the liquid film parameters in the steady-state flow regime. For low flow rates, wetting area and film thickness both are small, and a suggested range of Reynolds number is chosen to simulate further heat transfer in order to balance the film thickness and dry spots generation. It is shown that a ripple surface structure hinders the liquid film movement, reflected in a lower velocity and a larger film thickness compared to the smooth surface. Lateral movement of a liquid film can also be observed at the structured surface.     

Dynamics of explosive boiling and third heat transfer crisis at subcooling on a vertical surface

Results of experimental studies on dynamics of explosive boiling and third heat transfer crisis under the conditions of liquid subcooling are presented for the vertical arrangement of the heat-transfer surface. Acetone was used in experiments at the pressure in the working volume from 20 to 46 kPa and subcooling from 0 to 20 K. The studied processes were recorded. Data on the velocity of evaporation front propagation at liquid subcooling were obtained. These data are compared with the results of calculations according to the models available in the literature. The effect of liquid subcooling on the regions of regime parameters corresponding to explosive boiling and third heat transfer crisis is studied.     

强化管管外升膜蒸发换热特性实验

升膜蒸发是利用微细槽道对液体的毛细抽吸作用,在强化管外表面覆盖一层薄液膜,进而以薄膜蒸发的形式实现强化换热.本文针对强化管在水中的浸入深度,蒸发压力,加热壁面过热度等因素对升膜蒸发换热性能的影响展开实验研究.实验结果表明随着管外液位的降低升膜蒸发换热系数明显提高,此外,蒸发压力和加热壁面过热度因素对升膜蒸发换热性能也有着显著的影响.     

Investigation of the third heat transfer crisis on a vertical surface

The process of development of the third heat transfer crisis for vertical orientation of the heating surface was studied experimentally. Experiments were carried out with acetone under the conditions of saturation for the pressures in the working volume from 20 to 28 kPa. In all experiments, the third heat transfer crisis was preceded by propagation of evaporation front along the heating surface. The threshold values of heat flux densities, above which a stable vapor film is formed on the whole heating surface, are lower for vertical orientation of this heating surface than for the horizontal one. Data on the threshold heat flux densities and overheating before boiling-up were obtained. Above these values, formation of evaporation fronts was observed. The range of operation parameters corresponding to formation of the sites of unstable film boiling on the heating surface after boiling-up was determined.     

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.     

Formation of thermocapillary structures in a heated liquid film

The conditions of formation of a three-dimensional thermocapillary structure on the surface of a liquid film flowing along a heater with the constant temperature were studied numerically based on the derived system of equations. Formation of the thermocapillary structure was modeled by periodic perturbations in the transverse direction superimposed on the two-dimensional flow. It is shown in calculations that transversal perturbations can develop into periodic rivulet structures on the film surface if the Marangoni number exceeds some threshold value. It is revealed that the rivulet structure develops when the period belongs to a certain range, which is determined by the value of Marangoni number. The results of calculations are in a good agreement with experimental data.     

Thermal crisis of a vortex source in a real gas flowing into vacuum with heat supply under constant pressure

Thermal crisis of a vortex source at energy input under constant pressure is investigated. The energy release intensities per unit mass and per unit volume are analyzed for the case of the vortex source flow into vacuum (regime I). Analytical solutions for gasdynamic parameters and an algorithm for constructing the heat release intensity function for different variants of vortex sources are studied. Limitations on the extension of a constant-pressure interval are determined, and the dependences of the energy input area critical coordinates, critical temperature, and energy input parameters on the mass flow, gas circulation, and initial coordinate of the energy input area are obtained. A small correction to the gas specific heat with rising temperature and a significant correction to the specific heat at a stagnation temperature of 1000 K are considered.     

Heat transfer performance of a toluene loaded two-phase thermosyphon

Large dimension thermosyphons are efficient heat transfer components in heat recovery systems. Their performance limits depend on the following parameters: geometrical (length, diameter, inclination angle), physical (fluid, fill charge), thermal (temperature, heat flux).An experimental investigation was carried out with a large dimension, closed, two-phase thermosyphon which correspond to a device used in industrial recuperators. A vertical or inclinded steel thermosyphon, 3 m long and 27 mm inner diameter, was tested at temperatures varying from 100°C to 300°C with toluene as the working fluid. The lower part of the pipe was electrically heated along a variable length and the upper zone was cooled with an air stream whose flow rate and temperature were controlled. The maximum heat flux was measured as a function of temperature for different liquid fill charges and inclination angles. From these experimental data, boiling and condensation heat transfer coefficients were deduced. It was observed that the critical heat flux depends little on the fill ratio unless the charge is less than 20% for which a local dry-out occurs. The optimal fill charge was found to be between 20% and 50%. Experimental data have been compared with existing theories. The inclination effects have been taken into account with an empirical formula.     

Intensification of cooling by a pulsed gas-droplet flow: Equipment,parameters, and results

A bench for studying heat exchange between a pulsed spray and a surface heated to a temperature below the boiling point is designed. A calorimeter to measure the integral heat exchange accurate to 1.5–3.0% and equipment generating the pulse spray are described. The main parameters of the forming gas-drop-let flow (the gas and liquid velocities, the variation of the hydropulse duration and the size of flow-constituting droplets when moving toward a heat exchanger, and the liquid concentration distribution over the cross section and along the flow) and respective measuring techniques are given. It is shown that the duration of a droplet train in the flow influences heat-and-mass transfer.