Theoretical and experimental investigation into the explosive boiling potential of thermally stratified liquid-liquid systems

The occurrence of a rapid phase transition, or so-called explosive boiling, when a cold volatile liquid comes into contact with a hot liquid or hot surface is a potential hazard in industry. This study was focused on the explosive boiling potential of thermally stratified liquid-liquid systems that result from a runaway reaction. The experimental runs were performed on both a non-reacting and a reacting system. The experimental results showed that under the analysed conditions, the cold phase was superheated but did not evaporate explosively, as the limits of superheat of the phase were not achieved. The response of the cold phase appeared to be completely controlled by the interface temperature between the hot and the cold phase. In general, based on the order of magnitude of temperature differences that result from a runaway reaction in a multi-phasic system and the fact that the system is pressurised by its own vapour pressure, the occurrence of explosive boiling under runaway conditions appears unlikely for these type of systems.     

IR thermographic investigation of nucleate pool boiling at high heat flux

Nucleate water pool boiling at high heat flux was investigated on the 25 µm titanium and stainless-steel heaters at atmospheric pressure. A high-speed IR thermographic camera was applied to measure the rapidly changing transient temperature field, which served as input data for calculating the transient local heat flux distributions by solving a 3-dimensional (3-D) inverse heat conduction problem (IHCP). A phenomenon of hot spot was observed at the irregular active bubble site characterized by a longer waiting time and a higher activation temperature compared to a regular active nucleation site. The results show that the temperature of the hot spot can significantly exceed the temperature of the heater in its surroundings and remains present on the boiling surface even after the bubble departure. The calculations have shown a strong reduction of the local heat flux at the spot, which represented a potential for the beginning of the boiling crisis.     

A possible mechanism of triggering a vapor explosion

Relations are derived which enable one to assess the size of fragments of a liquid-metal droplet after its fragmentation formed in the case of instantaneous contact between a hot metal surface and a coolant. The obtained experimental data demonstrate that the amplitude of pressure pulses generated during vapor film collapse (second boiling crisis) is several times lower than the values required for triggering a spontaneous vapor explosion. The assumption is validated according to which progress in studying the process of triggering a spontaneous vapor explosion is associated primarily with advances in understanding the mechanism of fragmentation of individual droplet.     

Transient solid-liquid He heat transfer and onset of film boiling

The transient heat transfer between single-crystalline Ge chips and liquid helium is investigated during the application of light pulses with different optical power to the Ge sample. The strong temperature dependence of the electrical conductivity of Ge conveniently serves for monitoring the temporal behaviour of the sample temperature during the input of optical energy. After a certain time interval following the beginning of the light pulse an abrupt rise of the sample temperature is observed. This time interval is much longer than the thermal time constant expected for the sample. This abrupt rise of the sample temperature can be understood in terms of the onset of film boiling. The observed onset time of film boiling and its dependence upon the heat transfer power density agrees reasonably with earlier results by Steward.     

Experimental study of temperature distribution in the vicinity of film boiling

The transition of nucleate to film boiling on a flat surface is studied experimentally. In the vicinity of the boundary of a propagating center of film boiling, the distributions of the temperature, of the heat flux to liquid, of the heat-transfer coefficient, and of the velocity of motion of isotherms along an exothermal surface are obtained. The experiments are carried out in liquid nitrogen. A sapphire plate with platinum temperature microsensors deposited on it by sputtering is used as a heater. The instability of the boundary of the change of the boiling modes due to the Taylor instability of the interface is found in the film boiling region, as well as an intermediate region between film and nucleate boiling, where the heat fluxes are almost three times the critical value.     

Experimental investigation of the film boiling heat transfer in He II: Heat transfer coefficient

He II film boiling is of both academic and applied interests. However, information about He II film boiling is still inadequate and further study is needed from both the technical application and the scientific aspects. In the present study, a thin stainless steel foil heater (10 μm thick) is employed to induce boiling in He II. The average heater surface temperature is measured to evaluate the heat transfer performance of He II film boiling under different thermal conditions. And meanwhile, the pressure and the temperature oscillations induced by the film boiling are also measured. It is found that the pressure oscillation and the temperature oscillation highly correlate with each other, which indicates that the vapor bubble is vibrating on the heater surface during film boiling. The heat transfer coefficient of the film boiling depends on both the pressure over the heater surface and the He II bath temperature. The heat transfer coefficients of three kinds of boiling states: noisy film boiling, transition boiling and silent film boiling, are measured in the present study. The visualization of the boiling process is also carried out.     

Heat exchange processes near the boundary of a film boiling nucleus

Results are presented of an experimental investigation of the change in temperature, heat flux to the liquid, and rate of displacement of the isotherms near a film boiling nucleus propagating over a plane surface. The experiment was carried out in a liquid nitrogen bath at atmospheric pressure on the saturation line. The heater was a sapphire plate 1.2 mm thick having a heat transfer surface area of 77×22 mm². The following facts were established: 1) near the boundary of the film boiling nucleus a new heat exchange mechanism takes place caused by the instability of the liquid microlayer; 2) the maximum heat flux to the liquid is considerably greater than the critical heat flux; 3) the vapor film in the film boiling region grows gradually with increasing distance from the boundary, i.e., there is a smooth transition in terms of heat exchange intensity before the equilibrium film boiling level is reached. Pis’ma Zh. Tekh. Fiz. 25, 39–46 (November 12, 1999)     

The boiling suppression of liquid nitrogen

When He gas is injected from room temperature into boiling liquid N₂, boiling is suppressed, leaving liquid surface flat like a mirror. Although the qualitative explanation for this phenomenon is known [Minkoff GJ, Scherber FI, Stober AK. Suppression of bubbling in boiling refrigerants. Nature 1957;180(4599):1413-4], it has not been studied quantitatively and comprehensively yet. In this report, we made careful simultaneous measurements of temperature and weight variation of the liquid. The results clearly indicate that the boiling suppression is caused by cooling of the liquid with “internal evaporation” of N₂ into the He bubbles.     

Heat transfer in nucleate boiling of different low boiling substances

Heat transfer coefficients for nucleate boiling of methane, ethane, ethylene, argon and carbon dioxide were determined using an apparatus for the precise investigation of pool boiling heat transfer in the low temperature range. The apparatus used a horizontal cylinder as the heating element. The influence of the thermophysical properties of the boiling liquid was established by comparing the absolute values of the heat transfer coefficients in a normalized boiling state, i.e. a saturation pressure equal to 10% of the critical pressure and a heat flux density equal to 2 × 10⁴ W m⁻². By including the results for a number of higher boiling liquids, which were investigated previously under similar experimental conditions, and using literature data for three very low boiling liquids, an empirical correlation is established which allows an approximate prediction of the absolute value of the heat transfer coefficient at nucleate boiling for substances of different molecular structure.     

Effect of orienting the hot surface with respect to the gravitational field on the critical nucleate boiling of a liquid

Laws are established according to which the critical boiling mode changes with the orientation of the hot surface. The existence of an extremum temperature is shown. Formulas are proposed which account for the surface orientation during boiling in a free volume.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 24, No. 1, pp, 59–66, January, 1973.     

High heat flux transport by microbubble emission boiling

In highly subcooled flow boiling, coalescing bubbles on the heating surface collapse to many microbubbles in the beginning of transition boiling and the heat flux increases higher than the ordinary critical heat flux. This phenomenon is called Microbubble Emission Boiling, MEB. It is generated in subcooled flow boiling and the maximum heat flux reaches about 1 kW/cm²(10 MW/m²) at liquid subcooling of 40 K and liquid velocity of 0.5 m/s for a small heating surface of 10 mm×10 mm which is placed at the bottom surface of horizontal rectangular channel. The high pressure in the channel is observed at collapse of the coalescing bubbles and it is closely related the size of coalescing bubbles. Periodic pressure waves are observed in MEB and the heat flux increases linearly in proportion to the pressure frequency. The frequency is considered the frequency of liquid-solid exchange on the heating surface. For the large sized heating surface of 50 mm length×20 mm width, the maximum heat flux obtained is 500 W/cm² (5 MW/m²) at liquid subcooling of 40 K and liquid velocity of 0.5 m/s. This is considerably higher heat flux than the conventional cooling limit in power electronics. It is difficult to remove the high heat flux by MEB for a longer heating surface than 50 mm by single channel type. A model of advanced cooling device is introduced for power electronics by subcooled flow boiling with impinging jets. Themaxumum cooling heat flux is 500 W/cm² (5 MW/m²). Microbubble emission boiling is useful for a high heat flux transport technology in future power electronics used in a fuel-cell power plant and a space facility.     

Model of intensive inter-phase interaction between high temperature melt and cold volatile fluid

The fragmentation of the molten metal drop in the cold volatile liquid was studied. Two cases of water surface temperature were investigated: a—lower than its critical meaning during direct contact with melt, and b—higher than the critical one. The criterion determining existence of the fragmentation was presented for the first case. In the second case, that is being more complicated, possibility of the direct contact between the surrounding liquid and the melted metal surface and development of the instability on both surfaces were studied. It was shown that the fragmentation mechanism for the second case is strongly dependent on the characteristic time of the direct water-melt contact. The characteristic time must be sufficient enough for liquid water to interact with the melt before generation of the vapor layer between two surfaces. The process that follows the contact of water with the melted metal and a high pressure region formation was considered. The fragmentation mechanism, based on the analogy with the known problem, when a body with a flat circular nose impacts upon a flat liquid surface, was presented. Mean velocity and mean width of the generated jets from the melt surface were calculated.     

New type of dry out crisis in free falling boiling liquid films

Here we present measurements of heat-transfer rate and critical heat flux as well as high-speed visualization at intensively evaporating and boiling falling liquid film flow. Research were carried-out at heated surfaces 67 mm wide and 20, 42, 64 mm long along the film flow over the range of inlet Reynolds number from 100 to 2000. Appreciably different crisis phenomena scenarios are observed depending on the heated surface length. Direct experimental measurements and visualization have shown the existence of previously unexplored surface dry out crisis development regime which is characterized by the upstream extrusion of the bubble boiling from the heated surface with the drying front. This type of crisis occurs under the certain range of the operating conditions and heated surface lengths. When the critical heat flux density occurs, large dry spots merge at the lower part of the stream. As a result of the dry spots merging, unstable temperature disturbance is formed. Subsequently it spreads over the whole surface, causing its drying, and critical heat flux is no longer determined by known calculation dependencies and is characterized by significantly smaller values. At such regimes critical heat flux is controlled not by hydrodynamic boiling crisis, but by equilibrium heat flux at which dry spots become unstable. This undesired critical heat flux reduction is potentially avoidable if measures can be taken for artificial liquid redistribution in transversal direction in order to decrease dry spots initial size (thus increase equilibrium heat flux).     

The flow of liquid past a plate with boiling and injection of gas

A classical model boundary layer problem is considered for the flow of liquid past a plate in view of injection of a vapor-gas mixture from its surface. The obtained self-similar solutions enable one to estimate the typical values of thickness of the vapor-gas layer, the value of heat-transfer coefficient as a function of temperature of liquid, intensity of injection and composition of mixture being injected, and the velocity of flow past the plate. In addition, the problem is considered of reducing the hydrodynamic drag owing to vapor and vapor-gas “lubrication” because of boiling of liquid and injection of vapor-gas mixture from the plate surface. The possibility is analyzed of the emergence of vapor film due to viscous friction forces in the case where the liquid is in the vicinity of the boiling point.     

Bubble behaviors in subcooled boiling of wetting surface under low gravity

Stainless steel plate with 30mm in length, 1 mm in width and 0.1 mm in thickness is employed for a heating surface in subcooled quasi-pool boiling of water under low gravity performed by a parabolic flight. Testing liquid subcooling is about 10K at atmospheric pressure. The wetting heating surfaces are coated with ceramics materials which have been developed by a certain glass company. DC power is applied directly into the test heating surface and the bubble behaviors are observed by a high-speed video camera. Contact angle of water droplet is about 77–96 degree for the stainless surface and 30 degree or less for the wetting surface. In the ground experiment, the size of detaching bubbles from the wetting surface is smaller than those of stainless surface and the detaching period is shorter at same heating power. The burnout heat fluxes of wetting surfaces are about 50 percent higher those of stainless surfaces. In the low gravity experiment, DC power is applied into the surface at 10 second before start of low gravity and increases slightly until burnout. A single large bubble grows on the stainless surface and finally, the surface is burned out in a short period. For wetting surface, several large coalescing bubbles appear and they move rapidly on the surface, then one of the large bubbles grows and the burnout occurs. The burnout heat fluxes are higher than those of stainless surface. The wetting ceramics surface is considered to accelerate the liquid supply and the bubble moving.     

Characteristics of subcooled water boiling on structured surfaces

Characteristics of the process and heat transfer of subcooled water boiling on mesostructured surfaces obtained by microarc oxidation of titanium foil with formation of a TiO₂ layer and deposition of Al₂O₃ particles from boiling nanofluid have been experimentally investigated. The experiments have been carried out in the forced flow of deaerated water in a vertical rectangular channel, 21 × 5 mm in size. The ranges of regime parameters are as follows: water mass velocity is up to 650 kg/(m2 s), subcooling is 30–75°C, pressure is ~10⁵ Pa, and heat flux rate is 0.7–5.0 MW/m². It is established that the number of active nucleation sites is (70–80) × 105 1/(m² s) at the heat flux of 1.5–2.0 MW/m². Significant subcooling of the liquid and good wettability of the structured surface provide intense deactivation and lead to random spatial distribution of the nucleation sites. The characteristic size of vapor bubbles is about 200–250 μm and the bubble lifetime is 200–500 μs. Application of the coating prepared by microarc oxidation enhances heat transfer by 20–30%. At high subcoolings of liquid, the characteristics of boiling on smooth surfaces and surfaces with the coating were fairly close.     

Impact of selected parameters on refrigerant flow boiling heat transfer and pressure drop in minichannels

This paper presents results concerning flow boiling heat transfer in a rectangular minichannel 1 mm deep, 40 mm wide and 360 mm long. The refrigerant flowing in the minichannel, Fluorinert FC-72, was heated by a thin foil microstructured on the side in contact with the fluid. Two types of microstructured surfaces were used: one with evenly distributed microcavities and the other with non-uniformly distributed minicavities. Liquid crystal thermography was applied to determine the temperature of the smooth side of the foil. The paper analyses mainly the impact of the microstructured heating surface and orientation of the minichanel on the heat transfer coefficient and two phase pressure drop. This required calculating the local values of heat transfer coefficient and measuring the pressure drop for different positions of the minichannel with enhanced heating wall. Moreover, the effects of selected thermal and flow parameters (mass flux density and inlet pressure), the geometric parameters, and the type of cooling liquid on the nucleate boiling heat transfer is studied. From the measurement results it is evident that applying a microstructured surface caused an increase in the heat transfer coefficient, which was approximately twice as high as that reported for the smooth surface. The highest values of the coefficient were observed for position 90° (the vertical minichannel) and position 0° (the horizontal minichannel), whereas the lowest were reported for position 180° (the horizontal minichannel). The experimental data concerning the two-phase flow pressure drop was compared with the calculation results obtained by applying nine correlations known from the literature. It is reported that most of the correlations can be used to predict the two-phase flow pressure drop gradient within an acceptable error limit (±30%) only for positions 90° and 135° (the vertical and inclined minichannels, respectively). The lowest agreement between the experimental data and the theoretical predictions was reported for the horizontal positions of the minichannel.     

Critical heat flux in a boiling aqueous dispersion of nanoparticles

The effect of nanoparticles in an aqueous dispersion (nanofluid) on the critical heat flux (CHF) removed by boiling liquid from a heat-exchange surface has been studied. It is shown that a nanoparticles layer formed on the heated surface in the course of boiling possesses a hierarchical structure. Hydrophilic properties of this layer and its high permeability facilitating the supply of liquid to vapor bubbles lead to an increase in the CHF density.     

Forced flow boiling heat transfer of liquid hydrogen for superconductor cooling

Heat transfer from inner side of a heated vertical pipe to liquid hydrogen flowing upward was first measured at the pressure of 0.7 MPa for wide ranges of flow rates and liquid temperatures. The heat transfer coefficients in non-boiling regime for each flow velocity were well in agreement with the Dittus–Boelter equation. The heat fluxes at the inception of boiling and the departure from nucleate boiling (DNB) heat fluxes are higher for higher flow velocity and subcooling. It was found that the trend of dependence of the DNB heat flux on flow velocity was expressed by the correlation derived by Hata et al. based on their data for subcooled flow boiling of water, although it has different propensity to subcooling.     

A comparison of the finite element and control volume numerical solution techniques applied to timber drying problems below the boiling point

Drying is a process which involves heat and mass transfer both inside the porous material, where a phase change in moisture occurs from the liquid to the gaseous state, and in the external boundary layer of the convected hot dry air, which heats the porous medium. The equations which govern this process consist of three tightly coupled, highly non-linear partial differential equations for the unknown system variables of moisture content, temperature and pressure. Due to the inherently complex boundary conditions and intricate physical geometries in any practical drying problem, an analytical solution is not possible. In order to obtain a transient drying solution it is necessary to resort to a numerical technique. The numerical solution techniques which were employed in this research were the finite element method and the control volume method. The transient numerical results were compared and contrasted for two timber drying problems, first, at a dry bulb temperature of 50°C, and secondly, at 80°C, both cases being below the boiling point of water.