Spontaneous Nucleation Temperature Research Articles

Effects of outlet subcoolings and heat generation rates on transient critical heat flux for subcooled flow boling of water in a vertical tube

Critical heat fluxes (CHFs) for subcooled flow boiling of water in a vertical tube due to steady and exponentially heat inputs were measured. The platinum tube with an inner diameter of 2.0 mm and a length of 94.8 mm was placed vertically in the experimental water loop. The upward flow velocity was approximately 2.5 m/s and the outlet subcooling ranged from 18 to 48 K. The heat generation rate was varied exponentially to investigate the effect of e-folding time on the CHFs. As an experimental result, the CHFs increased with a decrease in the e-folding time. When the e-folding times were longer, the CHFs were almost constant, whereas the CHFs increased for shorter e-folding times. The CHFs were independent on outlet subcoolings at low flow conditions. Moreover, it was considered that the explosive-like CHF occurred when the inner surface temperature of the tube exceeded the lower limit of heterogeneous spontaneous nucleation (HSN) temperature.

Influence of boiling initiation surface superheat on subcooled water flow boiling critical heat flux in a SUS304 circular tube at high liquid Reynolds number

The subcooled boiling heat transfer and the steady-state critical heat flux (CHF) in a vertical circular tube for the liquid Reynolds numbers (Red=3.65×104–3.08×105) and the flow velocities (u=3.95–30.80m/s) were systematically measured by the experimental water loop comprised of a multistage canned-type circulation pump with high pump head. The SUS304 test tube of inner diameter (d=6mm) and heated length (L=59.5mm) was used in this work. The boiling initiation noise of outer surface of the test tube in the open air was simultaneously measured up to CHF point by the sound level meter (SLM) and the microphone of a video camera (MP). The outer surface temperatures of the SUS304 test tube with heating were also observed by an infrared thermal imaging camera (ITIC) and the color temperatures of outer surface of the test tube in the open air were observed by a video camera (VC). The subcooled boiling heat transfer and CHF for SUS304 circular tube were compared with the values calculated by authors’ and other researchers’ correlations for the subcooled flow boiling heat transfer. The influences of flow velocity on the boiling initiation surface heat flux, the boiling initiation surface superheat, the subcooled boiling heat transfer and the CHF were investigated into details based on the experimental data. At the flow velocities higher than 13.3m/s, boiling initiation surface heat fluxes were close to the CHFs and surface superheats at the CHF were over to the homogeneous spontaneous nucleation temperature as well as the lower limit of the heterogeneous spontaneous nucleation temperature. The dominant mechanism of the subcooled water flow boiling CHF on the SUS304 circular tube was discussed at high liquid Reynolds number.

Mechanism of critical heat flux during flow boiling of subcooled water in a circular tube at high liquid Reynolds number

The subcooled boiling heat transfer and the steady state critical heat flux (CHF) in a vertical circular tube for the liquid Reynolds numbers (Red=2.77×104–3.08×105) and the flow velocities (u=3.95–30.80m/s) are systematically measured by the experimental water loop comprised of a multistage canned-type circulation pump with high pump head. The SUS304 test tube of inner diameter (d=6mm) and heated length (L=59.5mm) is used in this work. The outer surface temperatures of the SUS304 test tube with heating are observed by an infrared thermal imaging camera and a video camera. The subcooled boiling heat transfers for SUS304 test tube are compared with the values calculated from correlations due to other researchers for the subcooled boiling heat transfer. The influence of flow velocity on the subcooled boiling heat transfer and the CHF is investigated in detail based on the experimental data. Nucleate boiling surface superheats at the CHF are close to the lower limit of the heterogeneous spontaneous nucleation temperature and the homogeneous spontaneous nucleation temperature. A suggestion as to what the dominant mechanism is for the subcooled flow boiling CHF on the SUS304 circular tube is made at high liquid Reynolds number. On the other hand, the RANS equations (Reynolds Averaged Navier–Stokes Simulation) with k–ε turbulent model in a circular tube of a 3mm in diameter and a 526mm long are numerically solved for heating of water on heated section of a 3mm in diameter and a 67mm long with various thicknesses of conductive sub-layer by using PHOENICS code under the same conditions as the experimental ones previously obtained and with temperature dependent thermo-physical fluid properties. The Platinum (Pt) test tube of inner diameter (d=3mm) and heated length (L=66.5mm) was used in this experiment. The thicknesses of conductive sub-layer from non-boiling regime to CHF are measured. The thicknesses of conductive sub-layer at the CHF point are predicted for various flow velocities. The experimental values of the CHF are also compared with the corresponding theoretical values of the liquid sub-layer dry-out models suggested by other researchers, respectively. A suggestion as to what the dominant mechanism is for the subcooled flow boiling CHF on the Pt circular tube is made at high liquid Reynolds number.

Study on fragmentation behavior of liquid lead alloy droplet in water

Fragmentation behavior of molten lead alloys droplet in water was investigated experimentally by releasing liquid LBE (45w%Pb–55w%Bi) and lead droplets into a pool of subcooled water. The fragmentation occurred when the temperature of the interface between a molten droplet and water was higher than the spontaneous nucleation temperature of water and lower than the minimum film boiling temperature. With increasing the droplet temperatures, the peak pressure in fragmentation of LBE droplet increased from 5 to 8 kPa, and for lead, the value remained around 2 kPa. With increasing the water subcooling, the peak pressure in fragmentation remained constant at 5 kPa for LBE droplet and at 2 kPa for lead droplet. The lead alloy fragmentation process in water was numerically simulated by embedding a semi-empirical fragmentation model for droplet fragmentation rate into the computer code of two-phase flow: JASMINE code. The corresponding results, such as pressure history and fragmentation peak pressure, agreed well with the experimental results.

Subcooled boiling heat transfer in a short vertical SUS304-tube at liquid Reynolds number range 5.19 × 10 4 to 7.43 × 10 5

The subcooled boiling heat transfer and the steady-state critical heat fluxes (CHFs) in a short vertical SUS304-tube for the flow velocities ( u = 17.28–40.20 m/s), the inlet liquid temperatures ( T in = 293.30–362.49 K), the inlet pressures ( P in = 842.90–1467.93 kPa) and the exponentially increasing heat input ( Q = Q 0 exp( t/ τ), τ = 8.5 s) are systematically measured by the experimental water loop comprised of a multistage canned-type circulation pump with high pump head. The SUS304 test tubes of inner diameters ( d = 3 and 6 mm), heated lengths ( L = 33 and 59.5 mm), effective lengths ( L eff = 23.3 and 49.1 mm), L/ d (=11 and 9.92), L eff / d (=7.77 and 8.18), and wall thickness ( δ = 0.5 mm) with average surface roughness ( Ra = 3.18 μm) are used in this work. The inner surface temperature and the heat flux from non-boiling to CHF are clarified. The subcooled boiling heat transfer for SUS304 test tube is compared with our Platinum test tube data and the values calculated by other workers’ correlations for the subcooled boiling heat transfer. The influence of flow velocity on the subcooled boiling heat transfer and the CHF is investigated into details and the widely and precisely predictable correlation of the subcooled boiling heat transfer for turbulent flow of water in a short vertical SUS304-tube is given based on the experimental data. The correlation can describe the subcooled boiling heat transfer obtained in this work within 15% difference. Nucleate boiling surface superheats for the SUS304 test tube become very high. Those at the high flow velocity are close to the lower limit of Heterogeneous Spontaneous Nucleation Temperature. The dominant mechanisms of the flow boiling CHF in a short vertical SUS304-tube are discussed.

Effect of surface property of molten metal pools on triggering of vapor explosions in water droplet impingement

Small-scale experiments have been conducted to investigate the triggering mechanism of vapor explosions. In order to attain good repeatability and visibility, a smooth round water droplet was impinged onto a molten alloy surface. This configuration suppresses premixing events prior to triggering. Six molten metal and alloys were used as the pool liquid. The lower limit of the contact temperature in the vapor explosion region closely agrees with the spontaneous bubble nucleation temperature of water. The upper limit of the initial molten alloy temperature decreases when an oxide layer forms on the surface causing an increase of the emissivity of thermal radiation that has a stabilizing effect on the vapor film. When an oxide layer was formed on the surface, a water droplet was occasionally entrapped into a molten alloy dome, since the oxide layer prevents the droplet from evaporating coherently. The vapor explosion region obtained for the mirror surface is a conservative estimate, since that for the oxide surface fell into the internal region of the vapor explosion for the mirror surface.

Preliminary studies of sperm cryopreservation in the mushroom coral, Fungia scutaria

Coral species throughout the world are facing severe environmental pressures. Because of this, we began cryobiological studies on the sperm of the mushroom coral, Fungia scutaria. We determined that F. scutaria sperm had a mean length of 56 μm and head diameter of 2.5 μm, and a mean spontaneous ice nucleation temperature of −37.2 ± 1.7 °C. When the sperm were exposed to the cryoprotectant glycerol for 5 or 20 min (at 10% v/v), no fertilized larvae were produced. However, when sperm were exposed for 20 min to propylene glycol (10% v/v), fertilizations were produced at the same rate as untreated control eggs and sperm ( P > 0.05), but slightly less for dimethyl sulfoxide (10% v/v) ( P < 0.05). Regardless, dimethyl sulfoxide caused less osmotic damage to the sperm membrane than did propylene glycol. Therefore, we used the dimethyl sulfoxide (10% v/v) to develop cryopreservation protocols that yielded good post-thaw morphology and motility (>95%) for coral sperm.

Coral larvae conservation: Physiology and reproduction

Coral species throughout the world’s oceans are facing severe environmental pressures. We are interested in conserving coral larvae by means of cryopreservation, but little is known about their cellular physiology or cryobiology. These experiments examined cryoprotectant toxicity, dry weight, water and cryoprotectant permeability using cold and radiolabeled glycerol, spontaneous ice nucleation temperatures, chilling sensitivity, and settlement of coral larvae. Our two test species of coral larvae, Pocillopora damicornis (lace coral), and Fungia scutaria (mushroom coral) demonstrated a wide tolerance to cryoprotectants. Computer-aided morphometry determined that F. scutaria larvae were smaller than P. damicornis larvae. The average dry weight for P. damicornis was 24.5%, while that for F. scutaria was 17%, yielding osmotically inactive volumes ( V b) of 0.22 and 0.15, respectively. The larvae from both species demonstrated radiolabeled glycerol uptake over time, suggesting they were permeable to the glycerol. Parameter fitting of the F. scutaria larvae data yielded a water permeability ⩾2 μm/min/atm and a cryoprotectant permeability = 2.3 × 10 −4 cm/min while modeling indicated that glycerol reached 90% of final concentration in the larvae within 25 min. The spontaneous ice nucleation temperature for F. scutaria larvae in filtered seawater was −37.8 ± 1.4 °C. However, when F. scutaria larvae were chilled from room temperature to −11 °C at various rates, they exhibited 100% mortality. When instantly cooled from room temperature to test temperatures, they showed damage below 10 °C. These data suggest that they are sensitive to both the rate of chilling and the absolute temperature, and indicate that vitrification may be the only means to successfully cryopreserve these organisms. Without prior cryopreservation, both species of coral settled under laboratory conditions.

Instability of subcooled boiling film on a vertical wall

Consider a vertical plate with a leading edge. The temperature of the plate is above the spontaneous nucleation temperature, so that vapor completely covers the plate. Two-dimensional quasi-parallel theory is used to examine the stability of the two-phase system. Numerical calculations show that the main parameter determining the thickness of the vapor film and its stability is the difference between the temperature of the heated vertical plate and the saturation temperature of the liquid. As the thickness of the vapor film is made smaller, the nose of the neutral curve approaches that of the corresponding one-phase liquid. The overall temperature difference between the plate and liquid bulk does not strongly influence stability properties.

On the mechanism of aluminum ignition in steam explosions

An available theory [Epstein, M., Fauske, H.K., 1994. A crystallization theory of underwater aluminum ignition. Nucl. Eng. Des. 146, 147–164] of the ignition of aluminum melt drops under water, which is based on the assumption that the aluminum oxide (Al 2O 3) drop-surface skin first appears in a metastable molten state, is compared with existing experimental data on the ignition of aluminum drops behind shock waves in water [Theofanous, T.G., Chen, X., DiPiazza, P., Epstein, M., Fauske, H.K., 1994. Ignition of aluminum droplets behind shock waves in water, Phys. Fluids 6, 3513–3515]. The predicted and measured ignition temperature of about 1770 K coincides approximately with the spontaneous nucleation temperature of supercooled liquid Al 2O 3 (1760 K). This suggests that the crystallization of the oxide layer represents a strong ‘barrier’ to aluminum drop ignition under water. Apparently a similar interpretation is applicable to aluminum drop ignition in gaseous oxidizing atmospheres. We conclude from the theory that the low-temperature aluminum ignitions (in the range 1100–1600 K) that have been observed during steam explosions are a consequence of the short aluminum drop oxidation times in this environment relative to the characteristic time for Al 2O 3 crystallization. Several aspects of the aluminum drop/shock interaction experiments besides ignition are discussed in the paper. In particular, the experiments provide strong evidence that during the course of a vapor explosion metal fragmentation occurs via a thermal mechanism at low pressure and precedes the development of a high-pressure shock.

Numerical analysis of fragmentation mechanisms in vapor explosions

Fragmentation of molten metal is the key process in vapor explosions. However, this process is so rapid that the mechanisms have not yet been clarified in experimental studies. In addition, numerical simulation is difficult because we have to analyze water, steam and molten metal simultaneously with boiling and fragmentation. The authors have been developing a new numerical method, the moving particle semi-implicit (MPS) method, based on moving particles and their interactions. Grids are not necessary. Incompressible flows with fragmentation on free surfaces have been calculated successfully using the MPS method. In the present study, numerical simulation of the fragmentation processes using the MPS method is carried out to investigate the mechanisms. A numerical model to calculate boiling from water to steam is developed. In this model, new particles are generated on water–steam interfaces. A two-step pressure calculation algorithm is also developed. Pressure fields are separately calculated in both heavy and light fluids to maintain numerical stability with the water and steam system. The new model and algorithm are added to the MPS code. Water jet impingement on a molten tin pool is calculated using the MPS code as a simulation of collapse of a vapor film around a melt drop. Penetration of the water jet, which is assumed in Kim–Corradini’s model, is not observed. If the jet fluid density is hypothetically larger, the penetration appears. Next, impingement of two water jets is calculated. A filament of the molten metal is observed between the two water jets as assumed in Ciccarelli–Frost’s model. If the water density is hypothetically larger, the filament does not appear. The critical value of the density ratio of the jet fluid over the pool fluid is ρ jet/ ρ pool=0.7 in this study. The density ratios of tin–water and UO 2–water are in the region of filament generation, Ciccarelli–Frost’s model. The effect of boiling is also investigated. Growth of the filament is not accelerated when the normal boiling is considered. This is because normal boiling requires more time than that of the jet impingement, although the filament growth is governed by an instant of the jet impingement. Next, rapid boiling based on spontaneous nucleation is considered. The filament growth is markedly accelerated. This result is consistent with the experimental fact that the spontaneous nucleation temperature is a necessary condition of vapor explosions.

Experimental determination of the spontaneous nucleation temperature of sodium

The superheat limit of sodium was determined using a submillisecond laserpulse heating technique. Specimens of 1 g of liquid sodium were encapsulated and heated with a laser pulse through a thin tungsten window. A pyrometer measured the increase in surface temperature. Rates of the temperature rise were between 2000 and 7000 K· sm−1. Incipient boiling of the sodium resulted in a break in (lie slope of surface temperature. Using a numerical solution of the heat equation, the measurements gave a value of 2128 K for the spontaneous nucleation temperature with a standard deviation of ±61 K. Systematic errors contribute ±28 K to the total inaccuracy.

Cold destabilisation of enzymes.

The thermal stability profile of chymotrypsinogen has been investigated in the temperature range 230-340 K, with special emphasis on the phenomenon of cold instability. Differential scanning calorimetry was used to study the heat capacities of the native and denatured protein in undercooled solution and the results were combined with those obtained by spectrophotometry at ordinary temperatures. The partial heat capacities of both forms decrease with decreasing temperature, assuming negative values. In the experimentally accessible temperature range (above the spontaneous nucleation temperature of ice) the heat capacity difference delta C is found to be positive with a non-linear temperature dependence. delta C is predicted to change sign at some low temperature which cannot, however, be reached by experiment for chymotrypsinogen. In contrast to earlier studies, covering a much more limited temperature range and having to employ an additional destabilisation by means of pH and/or chaotropes, the present findings permit the construction of a more reliable thermodynamic stability profile and related properties. These differ in important details from those reported for other proteins, but based on measurements only in the neighbourhood of the heat-denaturation temperature. The thermodynamic characteristics are, however, in good agreement with earlier predictions and with recent low-temperature measurements on the tetrameric enzyme lactate dehydrogenase.

Effects of system pressure on minimum film boiling temperature for various liquids

The minimum temperatures of saturated pool film boiling on horizontal cylinders of various diameters in various liquids such as water, Freon-11, Freon-113, isopropanol, ethanol, liquid nitrogen, and liquid argon were measured for a wide range of system pressures. A correlation for minimum film boiling temperature on solid surfaces with various thermal physical properties in various liquids is presented based on the experimental results. The experimental minimum film boiling temperatures and the lower limits of heterogeneous spontaneous nucleation temperatures measured on the same cylinder in liquid nitrogen at various pressures agreed well with each other. The vapor film collapse in film boiling seems to occur at the lower limit of heterogeneous spontaneous nucleation temperature. Minimum film boiling temperatures on pure and oxidized Zircaloy-4 in water are estimated by using the correlation at system pressures ranging from 101 kPa to 7 MPa.

A MODEL TO EXPLAIN COMPOSITION EFFECTS IN SMELT-WATER EXPLOSIONS

Abstract A salt bridge model, which takes into account mass transfer of components between the two fluids, was developed and proved successful in interpreting the effects of composition on explosiveness in the smelt-water system. The model is based on the fact that the main smelt constituent, sodium carbonate, is not soluble in water at temperatures approaching the critical point, while certain other constituents are soluble. The soluble substances allow a substantial increase in the critical temperature of the solution, which in turn shifts the range of contact interface temperatures at which spontaneous explosions can occur (between the spontaneous nucleation temperature and the critical temperature of the coolant) to higher values. The model was able to provide an explanation of why sodium chloride, sodium hydroxide, and sodium sulfide act as smelt sensitizers and a semiquantitative definition of the concentration ranges in smelt and in the quench solution where spontaneous explosions are likely

Thermal Interaction of Water Droplet with Molten Tin

Thermal interaction which is caused by explosive boiling of water droplet in contact with hot molten tin was investigated, paying attention on the influences of the tin temperature T tin and the water temperature T water together with the impact Weber number We. The results were summarized as follows. (1) T tin of lower boundary of thermal interaction zone (TIZ) is almost constant at about 290°C for a larger We than about 70, independent of T water. (2) The corresponding contact interface temperature is around 275°C and almost agrees with (27/32) T crit, or the spontaneous nucleation temperature, where T crit = 647K (= 374°C). (3) T tin of upper TIZ boundary increases from around T crit to above 500°C as T water decreases. (4) T tin of upper TIZ boundary is about 80°C. (5) For the woodsmetalwater system, in spite of a very low melting point of 78°C, the woodsmetal temperature of lower TIZ boundary rises up to about 350°C because of a low thermal conductivity of woodsmetal. (6) The Leidenfrost temperature is about 195°C for water droplet on solid tin, which has a melting point of 232°C, and appreciably lower than T tin of lower TIZ boundary. In conclusion, the present results support the spontaneous nucleation temperature concept by Fauske & Henry, as far as extremely energetic thermal interaction is concerned.

Thermal interaction of water droplet with molten tin.

Thermal interaction which is caused by explosive boiling of water droplet in contact with hot molten tin was investigated, paying attention on the influences of the tin temperature T tin and the water temperature T water together with the impact Weber number We. The results were summarized as follows. (1) T tin of lower boundary of thermal interaction zone (TIZ) is almost constant at about 290°C for a larger We than about 70, independent of T water. (2) The corresponding contact interface temperature is around 275°C and almost agrees with (27/32) T crit, or the spontaneous nucleation temperature, where T crit = 647K (= 374°C). (3) T tin of upper TIZ boundary increases from around T crit to above 500°C as T water decreases. (4) T tin of upper TIZ boundary is about 80°C. (5) For the woodsmetalwater system, in spite of a very low melting point of 78°C, the woodsmetal temperature of lower TIZ boundary rises up to about 350°C because of a low thermal conductivity of woodsmetal. (6) The Leidenfrost temperature is about 195°C for water droplet on solid tin, which has a melting point of 232°C, and appreciably lower than T tin of lower TIZ boundary. In conclusion, the present results support the spontaneous nucleation temperature concept by Fauske & Henry, as far as extremely energetic thermal interaction is concerned.

Small-scale Vapor Explosions on a Surface of Stationary Molten Tin Cooled by Flowing Water

A special experimental system in which a surface of stationary molten tin is cooled by flowing water is employed, and the trigger mechanism of vapor explosion is investigated by measuring the transient tin temperature and by observing the aspects of phenomena with cinecamera. Tests are repeated varying water temperature as well as initial tin temperature over a wide range. In the cooling process of molten tin, four characteristic tin temperatures are detected which are associated with four peculiar phenomena: splashing, quenching, swelling and vapor explosion. The vapor explosion takes place only when the tin temperature is in the range limited from upper side by the so-called quench temperature and from lower side by the spontaneous nucleation temperature of water. From these results, some of the characteristics of small-scale vapor explosion in a previous paper can be explained well.

Fuel-Coolant Interactions in a Shock Tube Geometry

Shock tube experiments with a variety of liquids have been conducted in which large pressures were obtained for systems of water-Wood's metal, butanol-Wood's metal, and water-molten salt. With the water-Wood's metal system, three separate regions were observed. When the hot liquid temperature was below 210/sup 0/C (which can be identified as the spontaneous nucleation temperature), no thermal interaction occurred, and the cold liquid column only bounced if vapor were present initially (region A). When the hot liquid temperature was greater than the spontaneous nucleation temperature but the contact interface temperature was less than this value (region B), the low rate of vaporization resulted in bouncing of the liquid column, which in turn produced high pressures on the order of the theoretical water hammer pressure. Those hydrodynamic pressures are larger than the vapor pressure corresponding to the bulk temperature of the hot liquid and larger than the maximum pressure that may be generated from single-phase pressurization. The third region, observed when the hot liquid temperature was above the spontaneous nucleation temperature upon contact (region C), resulted in fast production of vapor and impulses larger than the theoretical impulse for stopping the liquid column. The mechanism for producing the high pressures inmore » region C is a combination of hydrodynamic impact and thermal interaction. Since pressures produced in region C are also on the order of impact pressures, the only indication for thermal interaction is a considerable increase in the resulting impulse of pressure pulses with short rise time (<1.0 ms).« less

RELATIONSHIP OF THE LOW TEMPERATURE EXOTHERM TO APPLE AND PEAR PRODUCTION IN NORTH AMERICA

Twig pieces from the current season’s growth of three crabapple (Malus sp.), eight apple (Malus pumila Mill.) and seven year (Pyrus communis L.)cultivars, and limb pieces from 6- to 7-yr-old branches of Starking Red Delicious apple and Bartlett pear were taken during mid-winter and-subjected to a preconditioning treatment to induce maximal cold hardiness. The apple and pear cultivars studied included many of the major commercial cultivars grown in North America. Freezing tests and differential thermal analysis (DTA) were performed on the twig and limb pieces after preconditioning. Xylem was the most susceptible in stem tissue of both preconditioned apple and preconditioned pear. The temperature at which apple and pear xylem of commercial cultivars became injured (−35 to −40 C and −30 to −35 C, respectively) was related to the initiation of the low temperature exotherm on the DTA profile (−37 to −40 C and −33 to −38 C, respectively) and in turn was related to the average annual minimum temperatures at the northern limits of commercial production (−34.4 to −40 C and −28.9 to −34.4 C, respectively). The low temperature exotherm was previously shown to be produced by freezing of supercooled water in the xylem. The spontaneous nucleation temperature of supercooled water in xylem of the cultivars studied appears to be related to the northern limit of pear and apple production in North America.