Superheat Limit Research Articles

Heat transfer during quenching of high temperature surface by the falling cryogenic liquid film

Contact between a liquid and an extremely superheated solid surface is restored during quenching. The cryogenic experiment and numerical simulation were performed to study the peculiarities of heat transfer during quenching of hot vertical copper plate being much higher than the Leidenfrost temperature by the falling film of liquid nitrogen. An anomalous behavior of heat transfer has been found in the vicinity of the quench front. The results revealed that the maximum heat flux into the liquid during quenching is significantly higher than the average in the quasi-stationary conditions. The dynamic pattern of quench front propagation has been obtained in the numerical experiment. It was found that the quench front initialization occurs after lowering the surface temperature to the thermodynamic limit of superheat for liquid nitrogen. The correlation to determine the time of quench front stagnation is proposed. The numerical model allows us to quantify the quench front velocity and temperature fields in the heater, which are variable in space and time. The reliability of the simulation results has been confirmed by direct comparison with the experimental data on the quench front geometry and velocity as well as on the surface temperature change over time.

Comparison of ultrasound and LIB generated cavitation bubble

There has been tried many methods how to generate single cavitation bubble. The purpose of these experiments is to observe the surface impact of collapsing cavitating bubbles. The most of method are based on spark to induced superheat limit of liquid. Here we compare the one of these methods – laser induced breakdown (LIB) with more correct mechanical way using ultrasound as generator. There is described the setup for ultrasound cavitation effecting the sessile bubble as well as random generated and moving bubble. We visualize the cavitation bubble using high speed camera of speed 180 kHz. There are observed interface instabilities as the ultrasound response.

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.

The micro-explosion strength of emulsified heavy oil droplets in catalytic cracking process

The emulsified feedstock technique applies the secondary atomization mechanism to heavy oil catalytic cracking process, which can significantly improve the atomization of feedstock and result in a more desirable product distribution. A model of micro-explosion strength of emulsion droplets in riser reactor is proposed in this paper. Model calculation shows that the superheat limit of emulsified heavy oil is 307.7°C at atmospheric pressure, and the micro-explosion strength increases with the size of dispersed water sub-droplets. The theoretical prediction was validated experimentally through feedstock emulsification tests and catalytic cracking experiments. Results indicate that with an increase of the water content in feedstock emulsification, the size of dispersed water sub-droplets in emulsified oil rises, and thus the micro-explosion strength of emulsion droplets in riser reactor increases, which results in a higher light oil yield and lower coke and dry gas yields of catalytic cracking. Compared with pure heavy oil feedstock, the light oil yield rises by 2%, and the yields of coke and dry gas decline by 1.23% and 0.17% respectively for emulsified feedstock at 5% water content.

Predicting the superheat limit temperature of binary mixtures based on the quantitative structure property relationship

This study was devoted to develop the quantitative relationship model between the superheat limit temperature of binary mixtures and their molecular structures based on the quantitative structure-property relationship. The integral additive descriptors method was used to calculate the molecular descriptors of binary mixtures. The genetic algorithm combined with the multiple linear regression (GA-MLR) was used to select optimal subset of descriptors which had significant contribution to the superheat limit temperature. Three different external validations, which checked the stability and predictive capability of the obtained models, were employed to build the models. And the applicability domain for the models was also defined. The results showed the presented models were valid and predictive and there was strong linear relationship between the superheat limit temperature of binary mixtures and their molecular structures. This study can provide a new way to predict the superheat limit temperature of binary mixtures.

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.

Nanoscale dynamics of Joule heating and bubble nucleation in a solid-state nanopore.

We present a mathematical model for Joule heating of an electrolytic solution in a nanopore. The model couples the electrical and thermal dynamics responsible for rapid and extreme superheating of the electrolyte within the nanopore. The model is implemented numerically with a finite element calculation, yielding a time and spatially resolved temperature distribution in the nanopore region. Temperatures near the thermodynamic limit of superheat are predicted to be attained just before the explosive nucleation of a vapor bubble is observed experimentally. Knowledge of this temperature distribution enables the evaluation of related phenomena including bubble nucleation kinetics, relaxation oscillation, and bubble dynamics.

Homogeneous vapor nucleation of water in 3 M NaCl solution within a nanopore

Homogeneous vapor nucleation of water in the electrolyte solution within a nanopore at its superheat limit was studied using the bubble nucleation model based on molecular interaction. The wall motion of the bubble that evolved from the evaporated water was obtained using the Keller–Miksis equation and the distribution of temperature inside the bubble was obtained by solving the continuity, momentum and energy equations for the vapor inside the bubble. Heat transfer at the interface was also considered in this study. The nucleation rate of the 3M NaCl solution at 571K is estimated to be approximately 0.15×1028clusters/(m3s). With this value of the nucleation rate, the complete evaporation time of the 50nm radius of the electrolyte solution is approximately 0.60ns. The calculated life time of the bubble that evolved from the evaporated solution, or the time duration for the growth and subsequent collapse of the bubble, is approximately 32ns, which is close agreement with the observed result of 28ns. The bubble reaches its maximum radius of 301nm at 13.2ns after the bubble evolution.

Smoothed particle hydrodynamics simulations of evaporation and explosive boiling of liquid drops in microgravity.

The rapid evaporation and explosive boiling of a van der Waals (vdW) liquid drop in microgravity is simulated numerically in two-space dimensions using the method of smoothed particle hydrodynamics. The numerical approach is fully adaptive and incorporates the effects of surface tension, latent heat, mass transfer across the interface, and liquid-vapor interface dynamics. Thermocapillary forces are modeled by coupling the hydrodynamics to a diffuse-interface description of the liquid-vapor interface. The models start from a nonequilibrium square-shaped liquid of varying density and temperature. For a fixed density, the drop temperature is increased gradually to predict the point separating normal boiling at subcritical heating from explosive boiling at the superheat limit for this vdW fluid. At subcritical heating, spontaneous evaporation produces stable drops floating in a vapor atmosphere, while at near-critical heating, a bubble is nucleated inside the drop, which then collapses upon itself, leaving a smaller equilibrated drop embedded in its own vapor. At the superheat limit, unstable bubble growth leads to either fragmentation or violent disruption of the liquid layer into small secondary drops, depending on the liquid density. At higher superheats, explosive boiling occurs for all densities. The experimentally observed wrinkling of the bubble surface driven by rapid evaporation followed by a Rayleigh-Taylor instability of the thin liquid layer and the linear growth of the bubble radius with time are reproduced by the simulations. The predicted superheat limit (T(s)≈0.96) is close to the theoretically derived value of T(s)=1 at zero ambient pressure for this vdW fluid.

Direct observation of liquid nucleus growth in homogeneous melting of colloidal crystals.

The growth behaviour of liquid nucleus is crucial for crystal melting, but its kinetics is difficult to predict and remains challenging in experiment. Here we directly observed the growth of individual liquid nuclei in homogeneous melting of three-dimensional superheated colloidal crystals with single-particle dynamics by video microscopy. The growth rate of nucleus at weak superheating is well fitted by generalizing the Wilson–Frenkel law of crystallization to melting and including the surface tension effects and non-spherical-shape effects. As the degree of superheating increases, the growth rate is enhanced by nucleus shape fluctuation, nuclei coalescence and multimer attachment. The results provide new guidance for the refinement of nucleation theory, especially for the poorly understood strong-superheating regime. The universal Lindemann parameter observed at the superheat limit and solid–liquid interfaces indicates a connection between homogeneous and heterogeneous melting.

Measurement and Observation of Elementary Transition Boiling Process after Sudden Contact of Liquid with Hot Surface

A Fast response surface temperature measurement technique has been developed to investigate transient transition boiling phenomena after ethanol droplet impact on a nickel hot surface. Our motivation is to make clear the mechanism of boiling transition between nucleate and film boiling which is closely related the recovery of wetting on the hot surface being higher than the Leidenfrost temperature. In this study transient boiling phenomena during sudden contact of an ethanol sessile droplet on a hot solid surface has been investigated to understand what governs whether the solid surface keeps wet or not. The droplet impact boiling system is very simple and easier to observe but it includes the elementary transition boiling process. Since the transient transition took place within a couple ten milliseconds, observation with high speed video system and fast response surface temperature measurement technique are essential for better understanding of the transition boiling phenomena.The experiments were conducted for single and multiple droplet impacts. The multiple droplet impacts simulates liquid and solid contact situations on a hot surface during spray or laminar jet quench. The nickel disk of 50mm in diameter and 5mm in thickness was used as the hot surface. The hot surface was inclined from 0 deg (horizontal) to 40 deg. The fast-response film thermocouple with a typical minimum response time of 80 microseconds was developed and fabricated on the nickel disk at the depth of 3 micrometers from the surface. The temperature histories just beneath the surface were recorded at the sampling frequency of 200kHz. The boiling phenomena beneath a sessile droplet impacted on the hot surface were observed by using a microscope equipped with the high speed video camera at the maximum frame rate of 22.5 kfps. The surface temperature and surface heat flux were estimated with 1D inverse heat conduction analysis. Histories of local surface temperature and local heat flux were compared with the boiling video image. The experiments were done for different initial wall temperatures up to 250°C, different liquid subcoolings from 33 to 53K. The multiple impact frequency was changed from 280 to 840Hz.In case of the single droplet impact tests, the observation results showed that wetting situation was maintained for very short time even though the surface temperature was beyond the liquid superheat limit temperature. We defined the vapor film generation time as the characteristic boiling transition time scale from wetted nucleation boiling regime to dry film boiling regime. The film generation time were measured for the extensive initial surface temperature range, different subcoolings and different impact velocities. As the initial surface temperature increased, the film generation time decreased to order of microsecond and film boiling situation seemed to be established via spontaneous nucleation (vapor explosion) process. In case of the multiple droplets impact tests, film generation times were evaluated every droplet impact during continuous cooling from the initial solid temperature. As long as the generation time was less than 1 millisecond and much shorter than the impact period, stable film boiling was observed. Then the film generation time increased with the droplet impact times and approached to the droplet impact period as the cooling time elapsed. This region was recognized as the transition boiling region. After the generation time reached to the droplet impact period and then stable wetted situation, namely nucleate boiling was observed. As compared with the single impact tests, the generation time for the multiple impact became shorter than that for the single droplet impact at same initial surface temperature and the lower limit surface temperature observed vapor film generation was reduced. This fact may imply the effect of transient heat conduction on the wetting temperature shifts to higher wall superheat temperature beyond liquid superheat limit temperature during quenching with jets or sprays.

Measurement of the bubble nucleation temperature of water on a pulse-heated thin platinum film supported by a membrane using a low-noise bridge circuit

Abstract This study describes the performance of stress-minimized platinum (Pt) films supported by thin membranes (200 nm thick) to promote bubble nucleation of water near its theoretical superheat limit. The membrane configurations consist of Pt films deposited on 200 nm thick SiN films over bulk Si, with membranes being formed by etching Si from the back side of the films. Results are compared with more conventional Pt films supported by SiO2 and Si substrates. The average metal temperature is monitored by a bridge circuit with capacitive and inductive filtering to reduce noise in the output signal. Voltage pulses with durations ranging between 0.5 and 10 μs are imposed on the bridge to electrically heat the Pt film. The paper includes discussion of fabrication of the films, their treatment prior to using them as temperature sensors, the bridge circuit design for monitoring the change in electrical resistance during the power pulse, the calibration process of the films, and results of the bubble nucleation temperatures for the range of pulse durations examined. The results show that significantly less power is needed to trigger bubble nucleation on a membrane-supported platinum film compared to a platinum film on a bulk Si substrate. The nucleation temperatures which were closest to the theoretical limit of water were realized at heating rates of nearly 109 C/s. The potential for employing back-side etched devices is suggested for fundamental studies of phase transitions of highly superheated liquids and in applications where bubble nucleation is an important process such as for ink-jet printers and microscale bubble pump concepts.

A unified thermodynamic framework for LNG rapid phase transition on water

Rapid phase transition (RPT) is a phenomenon which frequently occurs after an LNG release on water. Its effects are potentially hazardous mainly because of the very fast rate of high energy release, in addition to fire and explosion. A significant case history and various experimental campaigns provide evidence which has allowed assessing different aspects of this event. This paper aims at offering a unified thermodynamic analysis of RPT. The thermodynamic and the kinetic limits of liquid superheat have been fully reviewed and specifically applied to LNG, within the homogeneous nucleation theory for multi-component liquids. Thermal and thermo-mechanical interface properties, such as interface temperature, evaporation rate, surface properties and liquid fragmentation have also been investigated. The importance of LNG composition has been analysed with respect to the experimental data. Finally, on the basis of the well known Shepherd and Sturtevart test, bubble growth rate has been modelled according to Mikic, Rohsenow and Griffith (MRG) equation and a new rigorous method has been set up to predict RPT overpressure, in line with Lighthill's acoustic theory, which removes the existing uncertainty and some subjectivities of the available models and possibly increases the thermo-fluid dynamic understanding of the phenomenon.

Superheating and homogeneous single bubble nucleation in a solid-state nanopore.

We demonstrate extreme superheating and single bubble nucleation in an electrolyte solution within a nanopore in a thin silicon nitride membrane. The high temperatures are achieved by Joule heating from a highly focused ionic current induced to flow through the pore by modest voltage biases. Conductance, nucleation, and bubble evolution are monitored electronically and optically. Temperatures near the thermodynamic limit of superheat are achieved just before bubble nucleation with the system at atmospheric pressure. Bubble nucleation is homogeneous and highly reproducible. This nanopore approach more generally suggests broad application to the excitation, detection, and characterization of highly metastable states of matter.

Insight into instabilities in burning droplets

The complex multiscale physics of nano-particle laden functional droplets in a reacting environment is of fundamental and applied significance for a wide variety of applications ranging from thermal sprays to pharmaceutics to modern day combustors using new brands of bio-fuels. Formation of homogenous nucleated bubbles at the superheat limit inside vaporizing droplets (with or without nanoparticles) represents an unstable system. Here we show that self-induced boiling in burning functional pendant droplets can produce severe volumetric shape oscillations. Internal pressure build-up due to ebullition activity ejects bubbles from the droplet domain causing undulations on the droplet surface and oscillations in bulk. Through experiments, we establish that the degree of droplet deformation depends on the frequency and intensity of these bubble expulsion events. In a distinct regime of single isolated bubble residing in the droplet, however, pre-ejection transient time is identified by Darrieus-Landau evaporative instability, where bubble-droplet system behaves as a synchronized driver-driven system with bulk bubble-shape oscillations being imposed on the droplet. The agglomeration of nanophase additives modulates the flow structures within the droplet and also influences the bubble inception and growth leading to different levels of instabilities.

F212 高温面上への液滴列衝突時の非定常遷移沸騰過程(OS9 熱・流動(4))

Transient transition boiling process during impingement of multiple ethanol droplets on a nickel hot surface has been investigated to get our better understanding wetting phenomena during repeating liquid-solid contacts. In this study, we focused on vapor film generation time which is required to develop a stable vapor film on the liquid-solid interface. Experiments has been done for initial surface temperature from 160 ℃ to 210 ℃ including liquid superheat limit temperature of ethanol, two different subcoolings of 53 K and 33 K, fixed droplet impact velocity of 3 m/s, and impact frequency range from 280-840 Hz

Phase transformation research of fusion reactor first wall material tungsten

Tungsten is remarkable for its robustness; especially it has the highest melting point of all the non-alloyed metals. Tungsten and tungsten alloys have been widely used in aerospace, weapon, nuclear industries and fusion reactor. Tungsten is expected to become fusion reactor first wall material for this reason. In this paper, phase transformation processes of fusion reactor first wall material tungsten have been investigated via molecular dynamics simulation based on the modified embedded atom model. Surface melting velocities at different temperatures are calculated as V(T) = −5.082 + 0.00136T and thermodynamic melting point is determined by fitting front advance velocities. Structure changes, thermal expansion coefficient, radial distribution function, static structure factor and average atomic energy for uniform melting processes are studied to simulate plasma thermal shock heating to superheat state of tungsten in fusion reactor. The superheat limit of tungsten crystals can be gotten according to simulation results. The superheat limit for tungsten crystal melting is about 27.2%. Tungsten is the best plasma-facing material because of its highest melt point and highest limiting superheating of all the non-alloyed metals.

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.

Limit of Superheat of Polystyrene-Cyclohexane Solutions: Theory

A theory for the surface tension of polymer solutions is used along with classical nucleation theory to predict the temperature rise above the solvent limit of superheat due to addition of polymer. Polystyrene, of 2000 and 4000 molecular weight, was added to cyclohexane and it produced a linear rise in the superheat to an extent that depends upon both molecular weight and concentration. The theory is used in the athermal case (dT = 0) to give the additional temperature rise with respect to weight fraction.

Limit of Superheat of Polystyrene-Cyclohexane Solutions: Theory

Limit of Superheat of Polystyrene-Cyclohexane Solutions: Theory