Superheat Limits Research Articles

Superheating of grain boundaries within bulk colloidal crystals

Whether grain boundaries (GBs) premelt is a longstanding question, because of the difficulty of direct experimental tests. Here, we focused an optical beam to locally heat single GBs within bulk hard-sphere colloidal crystals, observing the melting dynamics at single-particle resolution by video microscopy. The melting point is determined by analysing both the Lindemann parameter and the critical nucleus size for homogeneous nucleation. We found that all the GBs, including the high-energy GBs, can be superheated and melt via a heterogeneous nucleation mechanism. Based on the classical nucleation theory of GBs, we measured the incubation time and contact angle of the critical nucleus to compute all relevant kinetic factors, as well as the energy barrier, nucleation rate and the diffusion coefficient at the solid–liquid interface under weak superheating. The superheat limits of GBs with various misorientations have also been measured to further explore the instability mechanism. Under traditional uniform heating, premelting occurs only at triple junctions, whereas GBs retain their original structures up to the melting point. The premelted regions at triple junctions further interrupt high-energy GBs from superheating, through intrusion by uniform liquid layers. Overall, our experiments confirm the existence of superheating of GBs.

Explosion of metastable droplets in immiscible liquids

Evaporation of metastable droplets in an immiscible liquid can occur in a wide range of practical applications and in particular in cryogenic systems due to the low saturation temperature of the cryogenic liquid. In this article, we show experimentally that the incomplete explosion of a metastable droplet can lead to a ‘film evaporation’ configuration, where a remaining droplet is eventually separated from the host immiscible liquid by an established vapour layer. Based on a transient heat diffusion model, we identified that the necessary but not sufficient condition for the complete explosion of a metastable droplet is that the droplet must absorb, before explosion, an amount of heat larger than the latent heat required for a full evaporation, i.e. the global Jacob number Ja≥1. In addition, the measured superheat limits of pentane and isohexane are 420.65 K and 444.95 K respectively, which are very close to the theoretical Spinodal points (≤ 1.5 K) and other reported values in existing literature. Therefore, the experimental setup reported here can be used as an alternative way to quantify the superheat limit. The Droplet relaxation time was also found scaled to the square root of the diffusion characteristic time.

The spinodal of single- and multi-component fluids and its role in the development of modern equations of state

The spinodal represents the limit of thermodynamic stability of a homogeneous fluid. In this work, we present a robust methodology to obtain the spinodal of multicomponent fluids described even with the most sophisticated equations of state (EoS) available. We elaborate how information about the spinodal and its uncertainty can contribute both in the development of modern EoS and to estimate their uncertainty in the metastable regions. Inequality constraints are presented that can be exploited in the fitting of modern EoS of single-component fluids to avoid inadmissible pseudo-stable states between the vapor and liquid spinodals. We find that even cubic EoS violate some of these constraints.With the use of a selection of EoS representative of modern applications, we compare vapor and liquid spinodal curves, superheat and supersaturation limits from classic nucleation theory (CNT), and available experimental data for the superheat limit. Computations are performed with pure species found in natural gas, binary mixtures, as well as a multi-component natural gas mixture in order to demonstrate the scalability of the approach. We demonstrate that there are large inconsistencies in predicted spinodals from a wide range of EoS such as cubic EoS, extended corresponding state EoS, SAFT and multiparameter EoS. The overall standard deviation in the prediction of the spinodal temperatures were 1.4 K and 2.7 K for single- and multi-component liquid-spinodals and 6.3 K and 26.9 K for single- and multi-component vapor spinodals.The relationship between the measurable limit of superheat, or supersaturation, and the theoretical concept of the spinodal is discussed. While nucleation rates from CNT can deviate orders of magnitude from experiments, we find that the limit of superheat from experiments agree within 1.0 K and 2.4 K with predictions from CNT for single- and multi-component fluids respectively. We demonstrate that a large part of the metastable domain of the phase diagram is currently unavailable to experiments, in particular for metastable vapor. Novel techniques, experimental or with computational simulations, should be developed to characterize the thermodynamic properties in these regions, and to identify the thermodynamic states that define the spinodal.

Rapid evaporation at the superheat limit of methanol, ethanol, butanol and n-heptane on platinum films supported by low-stress SiN membranes

The bubble nucleation temperatures of several organic liquids (methanol, ethanol, butanol, n-heptane) on stress-minimized platinum (Pt) films supported by SiN membranes is examined by pulse-heating the membranes for times ranging from 1μs to 10μs. The results show that the nucleation temperatures increase as the heating rates of the Pt films increase. Measured nucleation temperatures approach predicted superheat limits for the smallest pulse times which correspond to heating rates over 108K/s, while nucleation temperatures are significantly lower for the longest pulse times. The microheater membranes were found to be robust for millions of pulse cycles, which suggests their potential in applications for moving fluids on the microscale and for more fundamental studies of phase transitions of metastable liquids.

Thermal Activation of Superheated Lipid-Coated Perfluorocarbon Drops

This study explored the thermal conditions necessary for the vaporization of superheated perfluorocarbon nanodrops. Droplets C3F8 and C4F10 coated with a homologous series of saturated diacylphosphatidylcholines were formed by condensation of 4 μm diameter microbubbles. These drops were stable at room temperature and atmospheric pressure, but they vaporized back into microbubbles at higher temperatures. The vaporization transition was measured as a function of temperature by laser light extinction. We found that C3F8 and C4F10 drops experienced 90% vaporization at 40 and 75 °C, respectively, near the theoretical superheat limits (80-90% of the critical temperature). We therefore conclude that the metastabilty of these phase-change agents arises not from the droplet Laplace pressure altering the boiling point, as previously reported, but from the metastability of the pure superheated fluid to homogeneous nucleation. The rate of C4F10 drop vaporization was quantified at temperatures ranging from 55 to 75 °C, and an apparent activation energy barrier was calculated from an Arrhenius plot. Interestingly, the activation energy increased linearly with acyl chain length from C14 to C20, indicating that lipid interchain cohesion plays an important role in suppressing the vaporization rate. The vaporized drops (microbubbles) were found to be unstable to dissolution at high temperatures, particularly for C14 and C16. However, proper choice of the fluorocarbon and lipid species provided a nanoemulsion that could undergo at least ten reversible condensation/vaporization cycles. The vaporization properties presented in this study may facilitate the engineering of tunable phase-shift particles for diagnostic imaging, targeted drug delivery, tissue ablation, and other applications.

The influence of nitrogen on an expander in a carbon dioxide transcritical heat pump

In this study, an expander was employed in a heat pump to replace the throttle valve in order to recover the expansion work. The influence of a non-condensable gas, nitrogen (N2), on the performance of the expander in a carbon dioxide transcritical heat pump is investigated. The phase change delay during the expansion process can affect the recovery work and the superheat limit will influence the phase change of the fluid. The superheat limits of carbon dioxide with and without N2 are calculated. It is found that the superheat limit and the critical radius of the bubble could decrease with the presence of nitrogen (N2). Then the required energy of bubbling can be lowered as the phase change occurs. The performance of the expander was investigated when the N2 concentration increased from 1% to 2.9% in a carbon dioxide transcritical cycle. The experimental results show that the recovery work of the expander with the presence of N2 is higher than that without N2. However, the efficiency of the expander decreases with increasing N2 concentration.

PHYSICAL AND MATHEMATICAL MODELS OF BIO-OIL COMBUSTION

This article discusses classical analytical models with respect to their ability to model bio-oil droplet combustion and, especially, the heating period before the droplet microexplosion. Following an analysis of the droplet combustion histories published in several literature sources, a diffusion limit model with a radiative heating term and Stefan boundary condition was identified as the most adequate for the droplets' preheating period. Considering the multiphase nature of the bio-oil, a bubble-shell model was developed. The basic assumption of this model is that the bio-oil volatile matter forms a core with an expanding radius and growing pressure, while the solids and the low-volatile matter form a membrane (shell). Special attention was paid to the assumption that droplet microexplosions are due to the superheating of droplet kernel liquid. In this regard, several approaches to calculate the limiting temperature, and consequently the droplet preexplosion time, were discussed. These approaches are mainly based on the assumption that impurities in the kernel play roles of nuclei in microbubble growth. An idea for how to define the bio-oil superheat limits via intersection of asymptotes of bio-oil surface temperature evolution with temperature was conceived and a method defining the bio-oil superheat limit based on surface tension variation with temperature was developed. An alternative shrinking-core model of bio-oil droplet explosive disruption was also discussed. A scale analysis defined the parameters that must be determined in preliminary tests of bio-oils, enabling an estimation of the significance of the preexplosion controlling parameters and testing of the results with the published data.

Numerical analysis of heat and mass transfer in the capillary structure of a loop heat pipe

The heat and mass transfer in the capillary porous structure of a loop heat pipe (LHP) is numerically studied and the LHP boiling limit is investigated. The mass, momentum and energy equations are solved numerically using the finite element method for an evaporator cross section. When a separate vapor region is formed inside the capillary structure, the shape of the free boundary is calculated by satisfying the mass and energy balance conditions at the interface. The superheat limits in the capillary structure are estimated by using the cluster nucleation theory. An explanation is provided for the robustness of LHPs to the boiling limit.

Explosive boiling of liquid droplets at their superheat limits

The maximum temperature limit at which liquid boils explosively is called the superheat limit of liquids. Using the droplet explosion technique, the superheat limit of several hydrocarbons and their mixtures was measured. The dynamic behavior of the bubble formed from the fully evaporated droplet at its superheat limit was investigated by measuring the far-field pressure signal from the bubble and by visualizing its behavior. The evaporation process of a droplet and the evolution of the bubble from the evaporated droplet were also studied theoretically by using an appropriate bubble nucleation and dynamics model. It was found that the superheat limit of liquids could be accurately predicted when correct vapor–liquid equilibrium data for the liquids were provided. The calculated far-field pressure signals, which indicate thermal damping, are in good agreement with observed one until the bubble disintegrates.

액체의 과열한계 측정과 과열한계에 달한 액적의 거동

The maximum temperature limit at which liquid boils explosively is called the superheat limit of liquids. The superheat limits of hydrocarbon liquids and their mixtures were measured by the droplet explosion technique. Also the fully evaporated droplet at the superheat limit and subsequent bubble evolution from the fully evaporated droplet were visualized. The pressure wave emanating from the evaporating droplet and subsequent bubble evolution process were measured by a piezoelectric transducer.

A generalized corresponding states method for predicting the limits of superheat of mixtures: Application to the normal alcohols

The limits of superheat of the normal alcohols from methanol to octanol and of ethanol/ n-propanol, n-propanol/ n-butanol, and n-butanol/ n-pentanol mixture were measured at atmospheric pressure. The results were predicted using a new method based on the generalized corresponding states principle (GCSP) in which properties of two reference substances were used to predict the superheat limits of the fluids studied. Ethanol and n-butanol, and the two components of each mixture studied, were used as reference fluids for predicting the superheat limits of the pure alcohols and mixtures respectively. Results showed that it is possible to predict superheat limits to well within the accuracy of measured values ( < 1%). The method requires only accurate vapor pressure correlations and acentric factors of the reference fluids, and an accurate method for predicting the variation of the true critical mixture temperature with composition. It is shown that the GCSP reduces to an expression for the superheat limit of a mixture which is a mole fraction weighted average of the limits of superheat of the individual components in solution when the mixture and its components satisfies certain conditions. In the general case, both linear and nonlinear variations of mixture superheat limit with mole fraction can be predicted with the proposed method. Considerable simpiflication using the GCSP over the approach based on classical homogeneous nucleation theory is derived from the fact that no mixture surface tension or bubble point pressure data are required.

Homogeneous Vapor Nucleation and Superheat Limits of Liquid Mixtures

Two features distinguish vapor nucleation in multicomponent liquids from the single component case. Both result from the unequal volatilities of the species. One is that the vapor phase may contain several components; the other is that nucleus formation alters the composition of the nearby liquid. These two features are incorporated into the classical theory of homogeneous nucleation to yield a general theory applicable to multicomponent liquids. The theory is applied to binary hydrocarbon mixtures by using an equation of state extrapolated into the metastable region. Superheat limits thus calculated are compared with published experimental results.

Experimental Study on Bubble Nucleation in the Oscillating Pressure Field

In bubble nucleation under the oscillating pressure field, when the oscillation period τ is of the same order of magnitude as the characteristic time τn of bubble nucleation, it is expected that the distribution of radius of bubble embryo in liquid will be largely affected by the pressure oscillation and the degree of superheat limit may change. In order to clarify this point, superheat limits of homogeneous nucleation under the oscillating pressure field generated by ultrasonic oscillators are measured for propane with and without dissolved carbon dioxide by the floating droplet method. From the experimental results it is found that when τ &gt; τn the measured superheat limit agrees with that calculated by the conventional theory where the quasi-steady state is assumed, but the bubble nucleation occurs at temperature lower than that preducted by the theory when τ nearly equals τn. It is also found that the characteristic time of bubble nucleation is changed by the amount of dissolved carbon dioxide.

Bubble nucleation in superheated liquid—liquid emulsions

An investigation of the process by which bubbles form in superheated liquid-liquid emulsions is reported. A theoretical model for the rate of bubble nucleation is developed that is valid for any binary liquid system. Because of the interesting “microexplosion” phenomenon postulated to occur during combustion of fuel/water emulsions, special attention is focused on hydrocarbon/water/surfactant mixtures. The model predicts that in these systems nucleation takes place at the interface between the two liquids but that the vapor bubbles rest entirely within the hydrocarbon phase. The maximum superheat limits of emulsions containing 83% by volume hydrocarbon (benzene, n-decane, n-dodecane, n-tetradecane, and n-hexadecane), 15% water and 2% surfactant were measured experimentally at 1 atm and found to be in good agreement with the predictions of the theoretical model. The superheat limits of these emulsions are lower than those of either pure liquid. However, for microexplosion of an emulsified fuel droplet, the superheat limit of the fuel/water emulsion must be less than the boiling point of the fuel. Only the n-hexadecane/water emulsion satisfies this criterion at atmospheric pressure.

Limits of superheat and explosive boiling of light hydrocarbons, halocarbons, and hydrocarbon mixtures

AbstractThe limits of superheat of thirteen light hydrocarbons and four light halocarbons were experimentally measured at one atmosphere using a superheating column. While there was some variation in the number of degrees to which a compound could be superheated before boiling explosively, in all cases the reduced limits TL/Tc were close to 0.88. The measured superheat limits of three binary hydrocarbon systems and several ternary mixtures were close to mole fraction averages of the limits of the pure components. The results of both pure components and mixtures paralleled predictions of Beegle (1973), based on thermodynamic stability theory. Limits of pure components agreed closely with limits predicted from homogeneous nucleation theory.