Ling Li’s research while affiliated with University of Shanghai for Science and Technology and other places

This work investigates pool boiling heat transfer (BHT) and bubble dynamics from a porous medium. The influence of the porous media structural parameters, such as porosity, pore density, porous medium height, thermal conductivity, and wettability, are mainly investigated. The findings indicate that the presence of porous media can increase the critical heat flux (CHF) by an average of 3.75 times and the BHT coefficient by an average of 3.84 times when porosity varies between 57.5% and 98.0% as compared to the plain surface. It is also found that both the CHF and BHT coefficient increase as the porosity decreases if porosity ε≥71.4%. However, they drop with the porosity decreases if porosity ε≤71.4%. On the other hand, the number of nucleation sites, heat transfer area, and bubble escape resistance increase as pore density increases. In addition, increasing the porous media height may enhance BHT performance, but too high a porous media increases the bubble escape resistance and restricts the separation of bubbles. Moreover, the CHF value and the maximum BHT coefficient increase with the thermal conductivity of porous media linearly. Finally, the stronger the wettability, the faster the bubble detachment, and the stronger the BHT performance.

The saturated pool boiling heat transfer on micro-cavity and micro-fin surfaces is examined by the mesoscopic phase change lattice Boltzmann method. The important interfacial processes and boiling heat transfer performance are explored concerning the effects of micro-structure configurations, specifically fin and cavity, and micro-structure parameters including fin/cavity shape, height, length, and spacing between fins/cavities. It is discovered that both the micro-cavity and micro-fin surfaces are conducive to bubble nucleation and can enhance nucleate boiling heat transfer (NBHT) when compared with the smooth surface. By comparing fin and cavity surfaces, it is found that micro-cavity is more conducive to bubble nucleation whereas micro-fin is more conducive to bubble departure. As a result, micro-cavity surface has a higher NBHT while micro-fin surface has a higher critical heat flux (CHF). The saturated pool boiling heat transmission is significantly influenced by the micro-structure parameters as well, i.e., the boiling on the rectangular cavity/fin surfaces has an earlier nucleation time while that on the conical surfaces has a faster bubble escape speed. The mass of residual bubble left over after the bubble department increases with cavity/fin height, which leads to the advance of CHF. On the other hand, the CHF increases as the distance between micro-structures. Additionally, with the increase of micro-structure length, the CHF increases for the micro-cavity surface whereas decreases for the micro-fin surface.At last, a series of fitting equations between CHF and the micro-structure parameters are presented and an improved hybrid surface is developed based on the theoretical predictions.

As an inherent physical phenomenon of the contact-line motion, the contact angle hysteresis (CAH) has an important effect on dynamic behaviors of bubble/bubbles from a solid surface. In this study, boiling heat transfer considering the CAH is investigated by employing a phase change lattice Boltzmann (LB) model. Effects of the CAH on the bubble/bubbles dynamics during the boiling process and the boiling curves for both the hydrophilic and hydrophobic hysteresis windows are studied in detail. It is found that the bubble base, bubble height and phase change rate increase as the increase of the width of hysteresis window. On the other hand, the pinning of three-phase contact line is observed during the bubble growth processes. And the pinning times increase with the contact angle hysteresis. For the hydrophilic hysteresis window, the bubble completely departs from the heater if a small range of the hysteresis window is considered, while a residual bubble is left when the bubble departs from the heater in the cases of the hysteresis window larger than an approximate critical value of 45°. For the hydrophobic hysteresis window, a residual bubble can be observed regardless of the level of the contact angle hysteresis. The numerical results also indicate that the CAH affects the position of bubbles coalescence. The bubbles merge above the heater for the hydrophilic hysteresis window with a small level of hysteresis. While the bubbles merge on the heater for the hydrophilic hysteresis window with a large level of hysteresis as well as for the hydrophobic hysteresis window. Moreover, as the increase of the width of the hysteresis window, the critical heat flux (CHF) as well as the Leidenfrost temperature (the minimum temperature for film boiling) decrease, and the transition boiling regimes become shorter. Finally, we present the fitting equations between the width of the hysteresis window and the CHF/Leidenfrost temperature. The results show that the CHF and the Leidenfrost temperature decrease linearly as the width of the hysteresis window increases.

In this paper, the saturated pool boiling heat transfer from a heated circular surface is investigated numerically based on the popular lattice Boltzmann phase-change method. The crucial interfacial phenomena, including the bubble growth, departure, coalescence, and rising under different wall superheats, wettabilities, and heater sizes are studied, and the corresponding heat transfer characteristics within these bubble dynamics are also revealed. The numerical experiments indicate that at low wall superheats the bubble nucleation occurs only at the top of the heated circle. With the increase in the wall superheat, the sides and the bottom of the heated circle also become nucleate sites. As the wall superheats continue to increase, more and more nucleate sites are activated so that the heated circle is wrapped by the vapor film early in the boiling process. And the average heat flux varies periodically with time corresponding to bubble dynamics in nucleation boiling regime. It reaches a steady state of film boiling regime after the stable vapor film is generated from the heated circular. It is also found that hydrophobic surface is conducive to the onset of boiling. However, it leads to a low critical heat flux and incurs film boiling at a lower wall superheat, which is also observed in both experimental and numerical studies. On the other hand, the boiling regimes can undergo the transition from nucleate boiling regime to film boiling regime at the same wall superheat with the increase in the heater size. In addition, under various wall superheats and wettabilities, the bubble departure diameters from the circular surface are smaller than those from the flat surface, while the bubble departure periods from the circular surface are less than those from the flat surface one. Finally, the saturated boiling curves from the circular surface for different wall wettabilities, heater sizes, liquid–vapor density ratios, and heating modes are also achieved.

The hydrodynamics and thermal characteristics due to mixed convection in a vertical two-sided lid-driven differentially square cavity containing four hot cylinders in a diamond array are investigated by the lattice Boltzmann equation model. The moving walls of the cavity are cold while the others are adiabatic. The flow in the cavity is driven by both the temperature difference and the moving vertical walls. The influence of different flow governing parameters, including the direction of the moving walls (the left wall moves up and the right wall moves down (Case I), both the left and right walls are moving upward (Case II), both the left and right walls are moving downwards (Case III)), the distance between neighboring cylinders [Formula: see text] ([Formula: see text]), and the Richardson number [Formula: see text] ([Formula: see text]) on the fluid flow and heat transfer are investigated with the Reynolds number in the range of [Formula: see text], the Grashof number of [Formula: see text] and the Prandtl number of [Formula: see text]. Flow and thermal performances in the cavity are analyzed in detail by considering the streamlines and isotherms profiles, the average Nusselt number, as well as the total Nusselt number. It is found that the heat transfer efficiency is highest when [Formula: see text] for the cases of the walls moving in the opposite direction. When the walls move in the same directions, the heat transfer efficiency obtained by [Formula: see text] is maximum among the considered values of [Formula: see text]. On the other hand, compared with the cases of [Formula: see text] and [Formula: see text], the cylinder positions corresponding to the largest and the smallest Nusselt numbers are very sensitive to the moving direction of the walls for [Formula: see text]. Moreover, the results also show that in terms of the value of Nusselt number and the stability the case of both walls moving downwards works well. Besides, the effect of the distance between neighboring cylinders is also discussed, it is found that increasing or decreasing the spacing between cylinders could enhance heat transfer to different degrees for the range of [Formula: see text] number considered. Finally, the empirical relationships among [Formula: see text], [Formula: see text], and the spacing between the cylinders ([Formula: see text]) are given, and predictive results match with the computed values very well.

The bubble dynamics and their effects on flow boiling heat transfer in a horizontal channel are investigated under constant wall temperature and constant wall heat flux conditions. It is found that both the bubble departure time and departure diameter decrease with increasing the inertial force for all the considered heating modes. Moreover, the bubble departs from the heater with a residual for the constant wall temperature condition so the heat transfer efficiency for the constant wall temperature condition depends mainly on the bubble growth rate and the vapor thickness. While for the constant heat flux condition, the bubble slides away from the heater accompany with waiting periods. So the heat transfer performance for this condition is mainly affected by the growth and waiting times. Besides, the results also show there were two contradictory trends of heat transfer efficiency under different wettabilities for the considered heat modes.

In this study, a phase–change LB model was adopted to simulate the pool boiling on a cylinder. The simulation results showed that the phase-change phenomenon occurred firstly at the top of the cylinder and produced the bubble at a low heating temperature on hydrophilic surface. With the increase of heating temperature, new nucleation sites appeared on both sides of the cylinder. And under the action of buoyancy, the bubbles on both sides risen to the top and merged and then departed. When the heating temperature reached a certain level, a bubble appeared at the bottom of the cylinder and grown steadily. Eventually, the bottom bubble would wrap the cylinder completely so the cylinder entered the film boiling. For hydrophilic surface, the temperature and heat flux over the cylinder surface would vary with the formation of bubble which was similar with flat heater. And we conducted the boiling curve (from the natural convection to the film boiling) on the cylinder by the numerical simulation.

Classical lattice Boltzmann method (LBM) can recover Fourier's law. Since Fourier's law results in a parabolic heat conduction equation, the classical LBM is called the parabolic Boltzmann method (PLBM). But the Fourier’s law is based on the absurd assumption that the heat transfer rate is infinite, so whether the classical LBM can be used to observe the non-Fourier heat transfer problem remains controversial. In this paper, a hyperbolic lattice Boltzmann method (HLBM), which can be used to analyze the non-Fourier effect, was derived based on the Cattaneo-Vernotte (CV) model. To verify the accuracy of the HLBM, the process of a gold film irradiated by the laser was simulated using the HLBM and the PLBM and the results were compared with the experimental data. Because of the non-equilibrium heat transfer between electron and lattice during the laser irradiation, the two-step HLBM/PLBM models were proposed according to the two-temperature model. The results show that the electron temperatures simulated by the two models are not much different from each other, and both of them coincide with the experimental values. However, the thresholds obtained by the two models are different, and the results of HLBM are closer to the experimental values.

A new incompressible gas-liquid two-phase flow model for non-Newtonian power-law fluid is proposed based on an incompressible lattice Boltzmann model. And the fundamental physical mechanism of Newtonian fluid displacing non-Newtonian power-law fluid liquid in porous medium is studied by using the proposed model. The effects of capillary number Ca, dynamic viscosity ratio M, surface wettability θ, porous medium geometry, and power law index n on the displacement process are investigated. The comprehensive results show that with the increase of capillary number, the displacement process turns faster, the fingering phenomenon becomes more obvious and the displacement efficiency decreases. However, for different values of power-law index n, the effects of the Ca on the displacement process have some differences. Specially, the decrease rate of displacement efficiency becomes slow if the displaced fluid is shear thickening fluid as compared with that if the displaced fluid is shear thinning fluid. On the other hand, the displacement efficiency decreases as dynamic viscosity ratio M increases. And the effect of the viscosity ratio on the displacement process becomes more obvious for the low value of the power-law index n. Moreover, the effect of the surface wettability of the porous medium on the displacement process is also related to the size of the power-law index. With the increase of the contact angle of the porous medium, the fingering phenomenon turns less obvious, and the displacement efficiency increases. However, with the increase of power-law index n, the influence of the contact angle on the displacement process decreases. Besides, the displacement processes with different geometric types of the porous media are also studied in the work. The results show that comparing with the case of porous medium denoted by circle shape and square shape, the fingering phenomenon obtained by the case of triangular shape is most obvious, and the displacement efficiency is lowest.