Critical Superheat Research Articles

Unit conversion in pseudopotential lattice Boltzmann method for liquid–vapor phase change simulations

Pseudopotential lattice Boltzmann (LB) model is an effective mesoscopic method for liquid–vapor phase change simulations. In LB methods, calculations are often carried out in lattice units. Thus, a correct mapping from the lattice unit system to the physical unit system is crucial for accurate simulations of practical problems. The unit conversion for liquid–vapor phase change problems is more complicated than single-phase problems, because an equation of state (EOS) for a nonideal fluid is introduced in the pseudopotential two-phase model. In this work, a novel unit conversion method for the pseudopotential LB model is proposed. The basic strategy is to obtain the conversion relations of fundamental units by mapping the surface tension and EOS parameters related to fluid properties, and thus, the unit conversion relations of other quantities are deduced. Numerical simulations of benchmark problems including the film evaporation and the bubble heterogeneous nucleation from a V-shaped cavity are carried out, and the simulation results are converted to the physical unit system by the proposed method. The numerical results demonstrate that the proposed method is able to recover the physical-unit latent heat of the fluid in the film evaporation problem. In the bubble nucleation from a V-shaped cavity problem, the conventional unit conversion method cannot derive the correct superheat temperature in the physical unit, whereas the proposed method based on the fundamental units recovers the critical superheat temperature which is consistent with the analytical result.

Numerical study of bubble behaviors and heat transfer in pool boiling of water/NaCl solutions using the lattice Boltzmann method

Through modification of the equation of state applicable to the non-volatile salt solution and characterization of the effect of solute concentration on the physical properties of the solution, a lattice Boltzmann pseudopotential model was developed and used to study the pool boiling processes of pure water and NaCl solutions. The model quantitatively verified the equilibrium vapor pressure reduction and boiling point rise of the NaCl solution with solute concentration. The complete pool boiling curves of pure water and the NaCl solution were simulated, and the differences in bubble behaviors and heat flux evolutions in pure water and the NaCl solution at different superheats were analyzed. The results revealed that the difference of heat flux between water and NaCl solution in the nucleate boiling regime is dominated by the nucleation site density and bubble departure frequency at lower superheats, while dominated by the bubble interactions at larger superheats. As a result, the NaCl solution presents a lower nucleate boiling heat flux than that of water when the imposed superheat is smaller than a critical superheat, while it exhibits a larger heat flux even a higher critical heat flux than water when the superheat is higher than the critical superheat. Furthermore, the lower bubble departure frequency in NaCl solution film boiling was captured by simulation. The simulated film boiling heat flux of the NaCl solution was smaller than that of water, and both the simulated results fitted the Berenson correlation well.

Numerical study of the boil-off gas (BOG) generation characteristics in a type C independent liquefied natural gas (LNG) tank under sloshing excitation

In this paper, a numerical model considering phase change and external heat leakage is established to study the thermodynamic and hydrodynamic of a type C LNG tank under sinusoidal sloshing excitation. The volume of fluid (VOF) method, coupled with the mesh motion treatment, is adopted to predict the movement of the vapor-liquid interface. The sinusoidal sloshing excitation is realized by a user-defined function (UDF). Compared with related fluid sloshing experiments, the feasibility of the numerical model is verified. The numerical results show that the sloshing excitation has great influences on the thermophysical process and the BOG generation of the LNG tank. The effects of different sloshing frequencies and amplitudes on the thermodynamic characteristics of the LNG tank are studied. In addition, the partial damage of the insulation system is also studied, and it is found that the sloshing has little effect on the critical superheat of the tank wall where the insulation layer is partial damaged, but it will delay the increase of the tank wall temperature. This study is significant to deeply understand the thermal behavior of the LNG sloshing and the characteristics of the BOG generation under sloshing condition during the actual marine transportation of LNG ships.

Effect of melt superheat on structural uniformity of lotus-type porous metals prepared by unidirectional solidification

Structural uniformity is an important parameter influencing physical and mechanical properties of lotus-type porous metals prepared by directional solidification of metal-gas eutectic (Gasar). The effect of superheat on structural uniformity as well as average porosity, pore morphology of porous metals was studied. The experimental results show that, when the superheat is higher than a critical value (ΔTc), the bubbling or boiling phenomenon will occur and the gas bubbles will form in the melt and float out of the melt. As a result, the final porosity will decrease. In addition, a higher superheat will simultaneously cause a non-uniform porous structure due to the pores coalescence and bubbling phenomenon. Finally, a theoretical model was developed to predict the critical superheat for the hydrogen to escape from the melt and the corresponding escapement ratio of hydrogen content. Considering the escapement of hydrogen, the predicted porosities are in good agreement with the experimental results.

Truncated versus Extended Microfilms at a Vapor−Liquid Contact Line on a Heated Substrate

The microstructure of a contact line formed by a liquid and its pure vapor on a perfectly wetted superheated smooth substrate, with the disjoining pressure most often in the form of a positive inverse cubic law (nonpolar case), is routinely considered to end up in a microfilm extended over adjacent "dry" parts of the solid surface. Invoking the spreading coefficient as an additional independent parameter within this framework, we argue however that a regime with a truncated microfilm is chosen instead if the spreading coefficient is decreased below a positive (still perfect wetting) critical value dependent upon the superheat, in which case the extended-microfilm thickness is surpassed by that of the "pancake" introduced by de Gennes and co-workers. Conversely, for a given positive spreading coefficient, there is a critical superheat above which the microfilm gets truncated, whereas for a negative one (partial wetting) the truncated regime should be preferred at any superheat. A parametric study of the apparent contact angle (a nonlinear eigenvalue of the steady microstructure problem) versus the spreading coefficient is carried out. When the latter is negative, Young's law is asymptotically recovered. Microfilm fronts on a bare surface are shown to be advancing or receding in accordance with the selected regime. A slightly more general class of disjoining pressures is also touched upon. The analysis is based in part upon thermodynamic considerations and in part upon a standard one-sided model of an evaporating liquid layer in the lubrication approximation.

Boiling regimes in a closed two-phase thermosyphon

Experimental results and an analytical model on the boiling mechanisms in a closed two-phase thermosyphon are presented in order to define the frontiers between the main boiling regimes and, in particular, the frontier between intermittent boiling and fully-developed boiling. The boiling regimes are classified on the basis of the frequency of bubble nucleation and the ratio of bubble diameter to device diameter. A criterion for the intermittent/developed-boiling frontier is based on the ratio of the bubble-nucleation waiting time and the bubble growth time. From this criterion a correlation between power throughput, working-fluid pressure and nucleation critical superheat are obtained. Experimental data on operating conditions, temperatures, and nucleation frequencies show the same functional dependence as the proposed correlation.

Critical superheat for flashing of superheated liquid jets

Critical superheat for flashing of superheated liquid jets

On the limit of superheat of cryogenic liquids

In an effort to test the empirical law governing the scaled superheat-limit, 1 we measured superheated neon and hydrogen with special emphasis on the critical superheat. Experimental results were compared with a Döring-Zeldovich-like nucleation theory and satisfactory agreement was found. Some deviation from the corresponding states of the scaled superheat-limit for classical fluids was apparent, thought to be caused by quantum effects on surface tension, and are appreciable for neon and hydrogen.

Sprays formed by flashing liquid jets

AbstractLiquids forced from a high‐pressure zone into a low‐pressure zone often cross the equilibrium pressure for the liquid temperature and disintegrate into a spray by partial evolution of vapor. The ordinary aerosol dispenser is a common example of this operation, and flash boiling is another.This paper reports on a study of the sprays formed by such a process and of the mechanism of spray formation. Sprays from water and Freon‐11 jets were analyzed for drop sizes, drop velocities, and spray patterns. The breakup mechanism was analyzed and data presented to show some of the controlling factors.A critical superheat was found, above which the jet of liquid is shattered by rapid bubble growth within it. The bubble‐growth rate was correlated with the Weber number, and a critical value of the Weber number was found to be 12.5 for low‐viscosity liquids. The mean drop size was also correlated with Weber number and degree of superheat.The spray from rough orifices and sharp‐edged orifices was compared with sprays produced from cold liquids by other techniques and was found to be comparable in all respects except temperature.