Superheat Limit Research Articles

Analytical and experimental study on boiling vaporization and multi-mode breakup of binary fuel droplet

In this paper, the boiling vaporization and breakup of single droplets of n-butanol, n-hexadecane and their binary mixtures with 10-50 w.t.% n-butanol have been studied. The experiments of droplet evaporation with an elevated ambient temperature range of 537-609 K were conducted through the pendant drop method and high-speed camera technology. The mathematical method was also proposed to predict the heterogeneous superheat limit and nucleation rate within the droplet. The experimental results indicate that mono-component droplets only go through the transient heating and stable evaporation phases. While for bi-component droplets, the characteristics show strong two-phase flow instability. With increasing n-butanol concentration or ambient temperature, the heterogeneous nucleation will occur inside the blended droplet, which experiences fluctuation evaporation between the transient heating and stable evaporation phases. The droplet vaporization characteristics gradually change from normal evaporation to partial rupture or passive breakup and then to micro explosion which reduces the droplet lifetime considerably. The novelty of this work is that various ambient temperature conditions and compositions are adopted to study the droplet evaporation to identify the conditions that favor flash boiling, and a new analytical model of heterogeneous nucleation is proposed, which can be used to estimate the temperature at the instant of bubble nucleation.

Film boiling in quench cooling with high-temperature jets

In the steel industry, quench cooling is used in the Run Out Table (ROT) to define the steel microstructure and mechanical properties. Film boiling is the preferred boiling regime in the ROT since its characteristic homogeneous and constant heat flux leads to a stable and controllable operation. The collapse of the vapor layer formed during film boiling, or rewetting, leads to a sharp increase in heat flux resulting in process complications such as uneven cooling, poor product quality, and deformation of the steel strips. In this study, the use of high-temperature water jets during the quenching of hot steel plates resulted in the formation of a stable vapor layer and long periods of film boiling. High-speed recordings are used to observe the dynamics of the vapor layer collapse. Surface heat flux estimations during quenching indicate an increase of vapor layer thickness with increasing distance to the jet location. The vapor layer collapse occurs at a surface temperature comparable to the Thermodynamic Limit of Superheat of water (302°C). The effects of surface roughness, initial surface temperature, and water jet temperature in the stability of the film boiling regime are studied. Surface roughness is found to increase the rewetting temperature and the surface heat fluxes after the collapse of the vapor layer. Increasing water jet temperature leads to decreasing rewetting temperatures and surface heat fluxes and therefore has the potential to widen the ROT stable operation window.

Transcritical evaporation and micro-explosion of ethanol-diesel droplets under diesel engine-like conditions

A transcritical multi-component evaporation and micro-explosion sub-model is suggested to investigate the micro-explosion of blended ethanol-diesel droplets under diesel engine-like conditions. A homogeneous theory is used to qualitatively predict the bubble generation, and the subsequent bubble growth leads to the final explosion. The evaporation rate under subcritical state is calculated by a multi-component diffusion sub-model along with an isothermal-isobaric flash. The finite thermal conduction and mass diffusion equations are solved inside the multi-component droplets. The transcritical transition is defined by the time instant that the droplet surface attains the mixture critical temperature, then the finite regression rate is controlled by the thermal diffusion, and the critical mixing surface moves continuously inward. The suggested model is validated against the experimental data. Based on the numerical results, the heat transfer rate in ambient and the superheat limit are found to be the predominated factors on the micro-explosion under low pressures. However, with accelerating pressure, the net result of competition among the evaporation rate, heating rate, transcritical transition, as well as the bubble growth rate determines the micro-explosion for small micron droplets (<24 μm). When the ambient temperature is higher than 1000 K, the enhanced regression rate caused by the transcritical transition makes the droplet lifetime too short to explode. Finally, the suggested sub-model is implemented into the OpenFOAM code. The spray of blended ethanol-diesel under diesel engine-like conditions is simulated, and the occurrence of micro-explosion is confirmed and found to show large effects on the spray characteristics.

The Equation for Rise in Superheat by Adding Electrolyte

For many liquids, the limit of superheat is 0.89 Tc, where Tc = temperature Kelvin at critical point. The rise in the limit of superheat by addition of polymer has a formula, B’, derived by Jennings, and this paper makes use of a pattern to predict the formula for rise in superheat by adding electrolyte, B. The equations for boiling point elevation by addition of electrolyte, A, and polymer, A’, are known, so we make use of a proportion and solve for the unknown, B, thus: B= (A/A’) x B’ to get the equation.&#x0D; A, A’ and B’ all have the phase change temperature squared at 1 atmosphere pressure in the numerator times the ratio of the molecular weight solvent divided by the product of the density of solvent times molecular weight solute in the expression that equals lim c---&gt;0 (dT/dc) giving the limiting slope of rise in temperature versus concentration solute. The van’t Hoff factor is included. The proposed formula B for RISE IN SUPERHEAT BY ADDITION OF ELECTROLYTE is:&#x0D; &#x0D; lim c---&gt;0 (dT/dc)s, electrolyte = (3 k MW1 Ts 2 i) / (ρ1 σ1 a MW2(e)). &#x0D;

Classical Nucleation and Lattice Model Unite

Classical nucleation theory predicts the limit of superheat of liquids quite well. To come up with an equation for the limit of superheat of polymer solutions, the lattice model for polymer solutions was used to give the surface tension of polymer solutions. A formula for bubble nucleation in polymer solutions was derived by Jennings with the precursor equation dlnA/dK=1/(6K) where J=AexpK gives the nucleation rate for liquids. The aim of this paper was to show that the precursor equation holds for monomer in the polystyrene-cyclohexane system. Thus, the precursor equation is true for all molecular weight polymer. This happens because the surface tension of polystyrene is significantly more than cyclohexane and the influence of the surface tension dominates.

Experimental study on evaporation and micro-explosion characteristics of biodiesel/n-propanol blended droplet

The evaporation and micro-explosion characteristics of biodiesel/n-propanol blended droplets at 573, 673 and 773 K ambient temperatures are studied using high-speed backlight imaging technique. The results show that the droplet evaporation is relatively stable at 573 K. However, micro-explosions occur at 673 and 773 K, the micro-explosion intensity increases with ambient temperature. The calculated superheat limit of n-propanol is 490 K. The blended droplet micro-explosion occurrence time is associated with the n-propanol concentration. The lower content of n-propanol, the earlier blended droplet micro-explosion occurs, and vice versa. The micro-explosion intensity and evaporation rate of the blended droplet first increase and then decrease as the n-propanol concentration increases. The micro-explosion delay time of the blended droplet first decreases and then increases with the increase of n-propanol concentration. Interestingly, the micro-explosion intensity, evaporation rate and micro-explosion delay time of droplet all reach the optimum value when the n-propanol concentration is 50%. Moreover, the oil membrane formation mechanisms of the soluble blended droplet with two different blended structures (n-propanol in biodiesel and biodiesel in n-propanol) are proposed.

The nature of boiling during rewetting of surfaces at temperatures exceeding the thermodynamic limit for water superheat

Rewetting is the establishment of water-surface contact that occurs during quenching of high temperature surfaces by water jet impingement. Rewetting is an unexpectedly complex phenomenon that has been reported to occur at surface temperatures significantly higher than the superheating limit of water. The presence of intermittently wet and dry episodes, and in particular the occurrence of so-called explosive boiling, is one of the theories to explain the contact of water with high temperature surfaces. However, there is a lack of experimental data in the literature to prove the presence of explosive boiling and intermittent wetting due to the small duration and scale of the rewetting phenomenon. In this study, recordings of the jet stagnation zone during rewetting are provided at a frame rate of 81 kfps. The high-speed recordings show a flashing regime consisting of intermittent (dry) bubble-rich and (wet) bubble-free periods at frequencies up to 40 kHz when the rewetted surface temperature exceeds the water superheat limit. As far as the authors know, these are the first direct observations of intermittent dry-wet periods occurring in the jet stagnation zone during quenching by water jet impingement. The dependency of the flashing frequency on initial surface temperature is quantified. A correlation between the size of the rewetting patch and the flashing frequency is found. Finally, a hypothesis to explain the role of water subcooling in maintaining the water-surface contact at surface temperatures well above the superheating limit of water is presented.

Leidenfrost behavior in drop-wall impacts at combustor-relevant ambient pressures

Liquid-fueled combustion systems demand optimal performance over a range of operating conditions—requiring predictable fuel injection events, spray breakup, and vaporization across a range of temperatures and pressures. In direct injection combustors, these sprays impinge directly on combustion chamber surfaces. Although the outcome of fuel droplets impacting a wall is primarily driven by the wall temperature and the Leidenfrost effect, the shifting liquid-vapor saturation point with pressure may influence the droplet-wall heat transfer rate and transition from nucleate to film boiling. In this paper, the role of ambient pressure on the droplet impact regimes, spreading rate, and droplet rebound velocity during impact are explored for representative low boiling point and high boiling point pure hydrocarbon liquids (n-heptane and n-decane). High-speed image sequences of the drop-wall impact were acquired for ambient pressures of 1–20 bar and wall temperatures ranging from 35–300 ∘C with a drop Weber number of ~ 50. Droplet impact sequences were recorded using a high-speed CMOS camera and were processed to measure the droplet spread, droplet rebound velocity and track the droplet centroid motion. The dynamics of the drop spreading and rebound show similar behavior across a range of ambient pressures with the largest differences observed for wetted versus non-wetted cases (above the Leidenfrost temperature). For both fluids, the onset of drop rebound remains bounded by the saturation temperature (shifting with ambient pressure) and the thermodynamic limit of liquid superheat. This leads to a decrease in the superheat temperature above the saturation point as the critical pressure is approached.

Prediction of superheat limit temperatures for fuel mixtures using quantitative structure-property relationship model

In this work, a novel QSPR model for the prediction of hydrocarbon mixtures' superheat limit temperatures (SLTs) was developed solely based on chemical structures and mole fractions of pure compounds. Experimental SLTs of 274 blended fuels were collected from existing references and molecular descriptors for the 25 pure materials were obtained by Gaussian 09 software package. Then the integral additive method was employed for the calculation of mixtures' descriptors and support vector machine (SVM) analysis was carried out to establish the nonlinear QSPR model. The most rigorous way of external validation, “compounds out”, was applied to test the true external predictive capabilities of the model and compared with the latest existing model of Lulu Zhou et al. this model performs better in both internal and external validations. The applicability domains (ADs) of this model were discussed and almost all the points are within the AD area. The use of this model only requires the knowledge of chemical structures and the SLTs of the individuals needn't to be known. The accuracy of the proposed model coupled with the ease of its implementation would definitely ensure quick estimation of SLTs of mixtures.

Eliminating the Pre-exponential Factor in Classical Nucleation Theory

Blander and Katz give a formula in classical nucleation theory, J = A exp K, for homogeneous nucleation (liquid--&gt;gas). Jennings proved that dlnA/dK = 1/6K for all pure liquids by combining two theories, taking the limit as polymer concentration--&gt;0. This gives lnA = (1/12)ln(K2) + C, where C is the integration constant. The conjecture is that C is a constant for fluids of low molecular weight. We used data for 7 sample solvents, and solved for C. The surface tension drops out in C, which makes C more accurate, as the surface tension is difficult to get at 0.89Tc, the limit of superheat. Tc = critical point in Kelvin. All quantities are evaluated at the limit of superheat, which is approximately 0.89Tc for solvents. C = 74.77 ± 0.33 for the 7 solvents (not all alkanes). This eliminates the prefactor A, streamlining J: ln J = (1/12)ln(K2) + 74.77 + K is the exact new equation. A computer can more easily be used to calculate J, the nucleation rate.

Early moments of BLEVE: From vessel opening to liquid flashing release

The boiling liquid expanding vapour explosion (BLEVE) is well known but not well understood. Some still argue about what comes first, the BLEVE or the vessel rupture. Some believe the BLEVE is triggered by some pressure transient inside the vessel and this causes a superheat limit explosion which causes the vessel to rupture. Others believe it is the vessel rupture by some weakening process that leads to the BLEVE. This paper will provide evidence that the latter description that is correct for most, if not all BLEVEs observed in practice.This paper describes small scale experiments of aluminum tubes that were weakened by machining a thinned wall area over a specified length. The tubes were filled to a desired level with liquid propane and then the propane was uniformly heated electrically until the tubes failed. The failure pressures ranged from 10 to 33bar.The tube was instrumented to capture failure characteristics (pressure, temperature) and consequences: blast overpressure and imaging of the propane cloud and shock around the vessel; ground force under it; transient pressure and imaging of the boiling process inside the vessel. The work was done to improve our understanding of the fluid – structure interactions during the fire heat induced failure of a pressure vessel holding a pressure liquefied gas. We were specifically interested in the near field hazards including blast overpressure and ground force. This paper will focus on the early milliseconds of the process where the vessel begins to open and a shock wave is formed and moves out into the surroundings. The imaging reveals presence of a Mach shock at the exit of the vessel at the early stage of the opening. A chronology of the event also shows that the lead shock is generated early in the explosion process, and is long gone before the liquid starts boiling, arguing that vapour expansion is the main contributor to the first shock overpressure.

Rewet Temperature Correlations for Liquid Nitrogen Boiling Pipe Flows Across Varying Flow Conditions and Orientations

Abstract In many convective liquid–vapor phase-change heat transfer engineering applications, cryogenic fluids are widely used in industrial processes, spacecraft and cryosurgery systems, and so on. For example, cryogens are usually used as liquid fuels such as liquid hydrogen, liquid methane, and liquid oxygen in the rocket industry, liquid nitrogen and helium are frequently used to cool superconducting magnetic device for medical applications. In these systems, proper transport, handling, and storage of cryogenic fluids are of extreme importance. Among all the cryogenic transport processes performed in room temperatures, quenching, also termed chilldown, is an unavoidable initial, transient phase-change heat transfer process that brings the system down to the cryogenic condition. The Leidenfrost temperature or rewet temperature that signals the end of film boiling is practically considered the completion point of a quenching process. Therefore, rewet temperature has been considered the most important parameter for the engineering design of cryogenic thermal management systems. As most of the previous correlations for predicting the Leidenfrost temperature and the rewet temperature have been developed for water, they are shown to disagree with recent liquid nitrogen pipe chilldown experiments in upward and downward flow directions over a wide range of flow rates, pressures, and degrees of inlet subcooling. In addition to a complete review of the literature, two modified correlations are presented, one based on bubble growth and another based on the theoretical maximum limit of superheat. Each correlation performs well over the entire dataset.

Analytical investigation on homogeneous nucleation of bi-component fuels

A modified model of bubble point pressure for binary mixtures based on the simplified Wilson equation was proposed to study the homogeneous nucleation of binary fuel mixture. The model was verified for four binary systems and well agreed with the available experimental results. On the basis of the bubble point pressure model, the mole fraction of each component in the vapor phase was obtained. The homogeneous nucleation model combined with the classical nucleation theory and the bubble point pressure model was used to predict the critical bubble radius and the nucleation rate of dimethyl ether (DME)/n-decane and DME/n-dodecane binary systems. The influence of blending ratio on homogeneous nucleation was discussed under several ambient pressures. The superheat limit temperature (SLT) was also calculated for the tested binary systems. The results showed that even a small amount of light component could obviously reduce the temperature needed to trigger the homogeneous nucleation. Consequently, the intensities of homogeneous nucleation and flash boiling could be also greatly improved. The calculation revealed that depressurization could promote the performance of homogeneous nucleation and flash boiling of the binary mixtures.

Quenching by Falling Cryogenic Liquid Film of Extremely Overheated Plate with Structured Capillary-Porous Coating

The paper presents results of a computational experiment simulating rapid cooling by falling liquid nitrogen film of an overheated vertical copper plate with a structured capillary-porous coating. A dynamic pattern of the running quench front was obtained, and it correlates satisfactorily with that observed in the experiments. The features of the heat transfer and quench front dynamics in the transient process are studied. The maximum density of the heat flux carried away into the liquid turned out to exceed by far that in quasi-stationary conditions. The presence of capillaryporous coating significantly affects the dynamics of quenching and temperature fields and makes it possible to reduce the total quenching time more than threefold. Initialization of a quench front on a plate with a structured capillary-porous coating occurs at a temperature much higher than the thermodynamic limit of liquid superheat. The reliability of the numerical simulation results was confirmed via direct comparison with experimental data on the variation of the plate temperature, as well as on the velocity and geometry of the quench front.

The gamma ray detection threshold temperature of different superheated droplet detectors

The response of superheated droplet detectors (SDD) loaded with four different active liquids, R-12 (CCl2F2), R-114 (C2Cl2F4), R-134a (C2H2F4) and R-610 (C4F10) have been studied to obtain the gamma ray detection threshold temperature (Tγ) of the respective detectors. A 137Cs gamma ray source is used for this study. To obtain Tγ from the experimental data a phenomenological model has been used. Result indicates that Tγ maintains a linear relationship with the limit of superheat (Tlim) of different active liquids used in SDD. It indicates that the limit of superheat of a liquid can be used for the prediction of its gamma ray detection threshold temperature.

Superheat limit and micro-explosion in droplets of hydrous ethanol-diesel emulsions at atmospheric pressure and diesel-like conditions

Micro-explosion of emulsified fuels has been studied on promoting the secondary break-up and improving fuel-air mixing. However, most studies are limited to atmospheric conditions, and experimental observation of micro-explosion under diesel-like conditions is rarely reported. The predictive model for the superheat limit of hydrous ethanol-diesel emulsion needs to be re-examined. In this study, firstly, with the molecular kinetics theory a mathematical model for predicting the superheat limit of hydrous ethanol diesel emulsions are established. Then, the model is verified by the experimental results of micro-explosion in the emulsion droplets. The superheat limit is independent of volume fraction of hydrous ethanol in the emulsions. The intensity of micro-explosion obeys the parabolic rule when changing volume fraction of the hydrous ethanol. The intensity of micro-explosion with the homogeneous nucleation is higher than with the heterogeneous nucleation. The emulsions of 30% and 40% hydrous ethanol exhibit the higher probability for the homogeneous nucleation than the other emulsions. Finally, the droplet behaviors of the emulsions under the diesel-like high-pressure conditions are experimentally observed. While the micro-explosions can be clearly observed for some droplets, neither the micro-explosion nor puffing are observable for most other droplets. The mechanism behind the phenomena is discussed.

Quantifying the mass discharge rate of flashing two phase releases through simple holes to the atmosphere

Many toxic industrial chemicals (TICs) such as chlorine and ammonia are stored as liquids at ambient temperature which flash when released to atmospheric pressure. Sponsored by the Chemical Security Analysis Center (CSAC) of the U.S. Department of Homeland Security, the Defense Threat Reduction Agency (DTRA) of the U.S. Department of Defense, and Transport Canada, the Jack Rabbit II tests were designed to release liquid chlorine at ambient temperature in quantities of 4,500–18,000 kg (5 to 20 T) to quantify the hazards of catastrophic release at scales represented by rail and truck transport vessels. All trials were conducted releasing chlorine through a 15.2 cm (6 in) circular breach in the 9,100 kg (10 T) capacity test vessel (disseminator) at different orientations (vertically downward, 45° below horizontal, and vertically up). A final 18,000 kg (20 T) release was made vertically downward using a truck vessel. This article analyzes disseminator load cell data to determine the mass release rate. Along with this data, previously published data are also considered to quantify the impact on release rate of flash fraction and superheat limit temperature for various materials. This work develops a general approach to quantifying the mass release rate of flashing two phase releases of TICs and other flashing liquids such as cryogenics. © 2018 American Institute of Chemical Engineers Process Process Saf Prog 37:382–396, 2018

Homogeneous melting near the superheat limit of hard-sphere crystals.

A defect-free crystal can be superheated into a metastable state above its melting point and eventually melts via homogeneous nucleation. Further increasing the temperature leads to the metastable crystal becoming unstable and melting catastrophically once beyond its superheat limit. The homogeneous melting is not well studied near the superheat limit and this limit is difficult to measure accurately, even for the simplest model of hard-sphere crystals. Here our molecular-dynamics simulations identify its superheat limit at volume fraction φlimit = 0.494 ± 0.003, which is higher than the previous theoretical estimations. We found that the hard-sphere crystal at the superheat limit does not satisfy Born's melting criterion, but has a vanishing bulk modulus, i.e. a spinodal instability, which preempts other thermodynamic or mechanical instabilities. At the strong superheating regime, the nucleation deviates from the assumptions in the classical nucleation theory. In contrast to crystallization which often develops nuclei with various intermediate structures, the melting of face-centered cubic (fcc) hard-sphere crystal does not produce intermediate structures such as body-centered cubic (bcc) crystallites although bcc is more stable than fcc at the strong superheating regime. Moreover, we found that the time evolutions of the order parameters and the pressure all exhibit a compressed exponential function, in contrast to the stretched exponential relaxation of supercooled liquids. The compressed exponential functions have the same exponent, which poses a new challenge to theory.

Comparison of cavitation bubbles evolution in viscous media

There have been tried many types of liquids with different ranges of viscosity values that have been tested to form a single cavitation bubble. The purpose of these experiments was to observe the behaviour of cavitation bubbles in media with different ranges of absorbance. The most of the method was based on spark to induced superheat limit of liquid. Here we used arrangement of the laser-induced breakdown (LIB) method. There were described the set cavitation setting that affects the size bubble in media with different absorbance. We visualized the cavitation bubble with a 60 kHz high speed camera. We used here shadowgraphy setup for the bubble visualization. There were observed time development and bubble extinction in various media, where the size of the bubble in the silicone oil was extremely small, due to the absorbance size of silicon oil.

Dynamic behavior with rapid evaporation of an inkjet water droplet upon collision with a high-temperature solid above the limit of liquid superheat

The dynamic behavior of the rapid evaporation of an inkjet droplet of water upon collision with a high-temperature silicon substrate has been investigated experimentally. The colliding droplet spreads into a thin disk-shaped liquid film and then splashes away. The splashing behavior depends on the solid temperature, which varies significantly due to nucleation and evaporation while spreading along the solid surface. Even when the temperature of the contact interface of the substrate, as estimated by transient heat conduction, significantly exceeds the kinetic limit of liquid superheat, numerous fine bubbles are simultaneously generated, presumably by spontaneous nucleation, beneath the liquid film and break up the film, which is dispersed into fine droplets. The surface temperature at the collision point is measured using a small platinum film sensor and exhibits a significant decrease compared to the heat conduction to the liquid for this peculiar nucleation.