Phase-change Lattice Boltzmann Research Articles

Enhancing flow boiling using a microchannel with pillar–cavity mixed structures: A lattice Boltzmann study

Enhancement of flow boiling in microchannels through adjusting surface structures has attracted much attention in recent years. However, most of the existing studies focus on homogeneous surface structures. In the present study, a novel vertical microchannel with pillar–cavity mixed structures is conceived to enhance flow boiling heat transfer. In the mixed microchannel, cavities and pillars are distributed on the vertical sidewalls of the upstream and downstream flow channel, respectively. A multicomponent phase-change lattice Boltzmann model is employed to investigate the flow boiling performance of the mixed microchannel. Numerical results show that the cavities in the mixed microchannel can supply effective nucleation sites for timely departure of bubbles, while the pillars in the mixed microchannel can suppress the expansion of the vapor film from the outlet toward the inlet. Moreover, the bubbles from the upstream cavities can entrain the cold liquid to disrupt the vapor film covering the downstream pillars for the rewetting of the heated surface. As a result, the flow boiling performance can be significantly enhanced by the synergistic effect of the pillar and cavity structures, and the best flow boiling performance can be achieved by controlling the ratio of the number of cavities to the total number of structures in the mixed microchannel to optimize the synergistic effect. The influences of the structural parameters of pillars and cavities on the flow boiling performance have also been studied. It is found that the height of the pillars and the depth of the cavities have important influences on the flow boiling performance, while the boiling performance is not sensitive to the width of the pillars.

Vapor condensation heat transfer on two-tier hierarchical microstructured surface

Condensation heat transfer on the hierarchical microstructured surfaces is simulated using a three-dimensional phase-change lattice Boltzmann method. Parametric effects raised by widely varying primary micropillar spacing, secondary micropillar position and height, surface wettability and subcooling on the condensation heat transfer are discussed in detail. The condensation heat flux first increases but then decreases with the increase of spacing between primary micropillars. Droplets are prone to nucleate on the top sides of primary micropillars with smaller pillar spacing, and they tend to maintain the Cassie‐Baxter state when secondary micropillars are arranged on the lateral sides of primary micropillars. As the height of secondary micropillar increases, the heat flux first increases but then decreases thereafter. When the contact angle rises from 57° to 130°, the state of condensate droplet changes from the Wenzel state to the partial wetting state. It is also found that there exist four types of droplet nucleation sites on the hierarchical microstructured surface: at the bottom of secondary micropillars, at the corner between primary micropillars and substrate, between two adjacent primary micropillars, and at the center of substrate between four primary micropillars. The increase of subcooling is favorable for nucleation, and the heat flux and condensation rate increase in accordance.

Numerical simulation on pore-scale pool boiling mechanisms of horizontal gradient porous metals

Horizontal gradient porous metals are proposed to solve the problem of nonuniform temperature distribution on the heating surface. The model of horizontal gradient porous metal is established based on the thermal response of metal skeletons. Pool boiling heat transfer performances of horizontal gradient porous metals are studied by phase-change lattice Boltzmann model. Horizontal porous metal layer thickness, wettability and thermal conductivity gradient effects on bubble behavior and heating surface temperature distribution uniformity are statistically analyzed. The results show that porous metals with horizontal thickness gradient can realize liquid-vapor channel separation, reduce the resistance of bubble rising, and improve the fresh liquid supplement. Compared with the uniform porous metal, the horizontal gradient porous metal with lateral hydrophobic surface or higher thermal conductivity has better boiling heat transfer performance and promotes heating surface uniform temperature distribution at low heat fluxes. The results provide a novel solution to improve the chip wall temperature uniformity for the stable operation of electronic equipment.

Pool boiling on micro-structured surface with lattice Boltzmann method

In this paper, pool boiling on micro-pillar structured surfaces is studied by using a three-dimensional pseudo potential phase-change lattice Boltzmann method (LBM). The joint enhancing effects on bubble nucleation and boiling performance are discussed in detail regarding various surface wettability and pillar geometrical parameters. Results show that on neutral and hydrophobic surfaces, enlarging the spacing of micro pillars delays the nucleation but can reduce the temperature inside vapor film and improve heat conduction. Despite that increasing the pillar height can improve heat flux, it is adverse to nucleation due to the increased cooling effect on the roots of micro pillars. In contrast, on hydrophilic surfaces, the impact of pillar spacing on nucleation is not monotonous and relatively much complicated. The heat flux is enhanced with increasing pillar spacing because of the extended three-phase contact line. And, the nucleation positions differ significantly by varying pillar geometrical parameters on hydrophilic surfaces.

Lattice Boltzmann simulation of surface evaporation in porous media

In this study, the phase-change lattice Boltzmann method was used to simulate the evaporation phenomenon on the surface of a porous medium. A new wetting boundary scheme was proposed to consider the surface wettability. The effect of surface wettability on the evaporation in the porous medium was studied. It is found that when the porous medium is hydrophilic, the surface evaporation rate can be increased by reducing the contact angle of the porous media skeleton. However, as the contact angle of the skeleton decreases, the benefit of decreasing the contact angle to improve the evaporation rate decreases.

Melting Processes of Phase Change Material in Sidewall-Heated Cavity

Melting processes of phase change material (PCM) confined in a rectangular cavity with an isothermal vertical wall are investigated to quantify the transition criterion between different melting regimes. A series of numerical simulations are conducted via the phase-change lattice Boltzmann method, and the results show that the temperature field in the liquid PCM region changes from the structure with two thermal boundary layers to the structure with two thermal boundary layers plus a convection region. Moreover, the results also indicate that the heat transfer mechanism undergoes a transition from conduction to convection when the relative thickness between the convention region and the thermal boundary reaches a critical value. This value (transition criterion) can be quantified by the critical melted volume fraction, and its dependence on Rayleigh number, Prandtl number, and aspect ratio of cavity is theoretically derived in this study. Then, based on the transition criterion, a piecewise correlation of melted volume fraction is proposed, which considers the effect of different melting regimes and is proven to predict the literature’s result.

Electric field enhancement of pool boiling of dielectric fluids on pillar-structured surfaces: A lattice Boltzmann study

In this paper, by using a phase-change lattice Boltzmann (LB) model coupled with an electric field model, we numerically investigate the performance and enhancement mechanism of pool boiling of dielectric fluids on pillar-structured surfaces under an electric field. The numerical investigation reveals that applying an electric field causes both positive and negative influences on the pool boiling of dielectric fluids on pillar-structured surfaces. It is found that under the action of an electric field, the electric force prevents the bubbles nucleated in the channels from crossing the edges of the pillar tops. On the one hand, such an effect results in the bubble coalescence in the channels and blocks the paths of liquid supply for the channels, which leads to the deterioration of pool boiling in the medium-superheat regime. On the other hand, it prevents the coalescence between the bubbles in the channels and those on the pillar tops, which suppresses the formation of a continuous vapor film and, therefore, delays the occurrence of a boiling crisis. Meanwhile, the electric force can promote the departure of the bubbles on the pillar tops. Accordingly, the critical heat flux (CHF) can be improved. Based on the revealed mechanism, wettability-modified regions are applied to the pillar tops for further enhancing the boiling heat transfer. It is shown that the boiling performance on pillar-structured surfaces can be enhanced synergistically with the CHF being increased by imposing an electric field and the maximum heat transfer coefficient being improved by applying mixed wettability to the pillar-structured surfaces.

Vapor condensation on micropillar structured surface with lattice Boltzmann method

Pure vapor condensation process and heat transfer on micropillar structured surfaces with various wettabilities are simulated numerically based on an improved three-dimensional phase-change lattice Boltzmann method (LBM). Condensed droplet dynamic behaviors containing nucleation, growth and departure are mainly explored. Two preferable droplet nucleation sites are detected successfully: at the corner between micropillar sides and subcooled substrate, and at the center of substrate. The effects of contact angle, micropillar spacing and height on droplet nucleation time, departure diameter and departure frequency are investigated quantitatively. Results indicate that as the contact angle increases, nucleation of condensate is delayed, while the departure diameter is reduced and departure frequency is increased. The increase in micropillar spacing, height and contact angle changes droplet from Wenzel state to partial wetting state. Moreover, it is also shown that the highest temperature and maximum local heat flux appear at the three-phase contact line. Despite the very complex roles of micropillar structures and wettability on condensation heat transfer, an optimal heat transfer surface with configurations of contact angle of 90°, micropillar spacing of 12 and height of 18 is presented. This study provides a fundamental understanding in regards to vapor condensation on microscale structured surface from numerical view.

Direct numerical simulations of moisture transport in porous media by a multi-component/phase-change lattice Boltzmann method

In this paper, simultaneous heat and mass transport in porous media is studied numerically using a 2D multicomponent/multiphase phase-change lattice Boltzmann scheme. This newly developed LB model requires no correlation equations as inputs for physical properties of the porous medium (such as relative permeability, heat capacity, thermal conductivity and mass diffusivity) that is filled with a multi-component fluid of air/vapor mixture. Natural convection and condensation of humid air in an enclosure filled with and without a porous medium that is heated differentially by vertical walls are compared. The same numerical scheme is applied to simulate capillary rise of liquid in semi-infinite packed hydrophilic beds with different structures that are filled initially with a mixture of unsaturated vapor and air above a water reservoir. It is shown that evaporation in the packed hydrophilic bed plays an important role in capillary rise of liquid. Effects of contact angles as well as the influence of the directionality and heterogeneity of the packed bed are investigated. The directional and the heterogeneous packed beds are shown to be able to suck a larger amount of liquid in a shorter time period compared to homogeneous packed beds with the same porosity under the same condition.

The study of nucleation site interactions on the mixed wettability rough surface

In order to achieve the best heat transfer performance on a mixed wettability surface, engineering of the wettability pattern is often used as heat transfer performance is significantly dependent on pitch distance between hydrophobic spots, which is maybe linked to the nucleation site interaction effects. In this work, the effects of nucleation site interactions between two hydrophobic cavities on a hydrophilic surface (a mixed wettability rough surface) are investigated, using a two-dimensional pseudopotential phase-change lattice Boltzmann method. The results of the mixed wettability rough surface are compared with completely hydrophilic rough surface and mixed wettability smooth surface to understand the effects of wettability and surface roughness. The focus is placed on the hydrodynamic interaction (which has promotive effect on the bubble formation activity) and thermal interaction (which has inhibitive effect). There exists an optimum pitch distance at which the heat flux and heat transfer coefficient are highest for mixed wettability surface, which are due to larger intensity of hydrodynamic interaction and nearly zero intensity of thermal interaction. Moreover, heat flux is found to be larger for the mixed wettability rough surface than the mixed wettability smooth surface. The bubble nucleation mechanism and time are found to be different for all surfaces.

Investigation on the effect of convective outflow boundary condition on the bubbles growth, rising and breakup dynamics of nucleate boiling

In this work, a phase-change lattice Boltzmann model was employed to simulate the effects of two computational domains with confined space and free outlet boundary considering the conjugate heat transfer on entire nucleate boiling. This model was validated by Laplace's law and droplet wetting process, especially in the improved convective outlet boundary for multiphase flow in detail. The film boiling was employed to verify the phase change model quantitatively, and the results exhibited a good agreement with a theoretical model so that the accuracy and robustness of the current model were ensured. The dynamic behaviors of the entire nucleate boiling including nucleation embryo, embryo growth, twin and triple vapors coalescence, departure, rise and subsequent rupture were investigated comprehensively. The temperature and velocity fields, as well as pressure propagations associated with nucleate boiling were investigated. The automatically sharp growth of nucleation vapor pushes away the vicinity of vapor outwardly, while the bottom fluid still flows into the root of the vapor contact line, which coincides with experimental observation. The flow dynamic behaviors of double and triple bubbles coalescence are almost the same, whereas more isolated bubbles merge is beneficial to the growth and departure dramatically. The sliding growth on the heating substrate was found clearly, which is mainly due to the non-uniform temperature gradient below the two side of the vapor bubble. The curvilinear rising trajectory and large deformation of rising vapor bubbles were observed, leading to a temperature wake in the rear of the rising bubble. The increasing volume of the rising bubbles occurred in the free outlet domain was found, which is mainly caused by the combined pressure influence between the inside and vicinity of the bubble. Moreover, the comparisons of nucleate boiling in both two computational domains were also studied.

Phase-change lattice Boltzmann simulation of condensate falling and heat transfer on hybrid-wettability surface in the presence of non-condensable gas

• Condensate falling off hybrid-wettability surfaces is studied under non-condensable gas effect. • Pseudopotential MRT LB method is coupled with finite difference for phase-change multicomponent flow. • Droplet nucleation, growth and departure time prolong as non-condensable gas fraction increases. • Hydrophilic-superhydrophobic hybrid surface exhibits higher heat transfer performance. A hybrid phase-change lattice Boltzmann model is employed to investigate the condensate falling from subcooled surface in the presence of non-condensable gas. The effects of hydrophilic length ratio, the non-condensable gas fraction ( w NCG ) and the hybrid-wettability of the subcooled surface on condensate droplets dynamics and heat transfer characteristics are investigated systematically. It shows that the average heat flux at the subcooled surface first increases and then decreases with the increase of hydrophilic length ratio. Owing to the increased thermal resistance by the thicker non-condensable gas layer, the nucleation time, growth time as well as departure time of condensate droplets are prolonged as the non-condensable gas fraction increases. The nucleation time of w NCG = 0.4 is 1.5 times higher than that of w NCG = 0.1 , and the growth time and departure time of w NCG = 0.4 are 1.7 times higher than those of w NCG = 0.1 . As the surface wettability changes from hydrophilicity-superhydrophobicity ( θ y = 60 o - θ g = 150 o ) to hydrophilicity-more hydrophilicity ( θ y = 60 o - θ g = 30 o ), the condensation pattern transits from dropwise to filmwise mode. Moreover, the local heat flux at hydrophilic-superhydrophobic ( θ y = 60 o - θ g = 150 o ) and hydrophilic-hydrophobic ( θ y = 60 o - θ g = 105 o ) surface is obviously higher than that at hydrophilic-hydrophilic ( θ y = 60 o - θ g = 75 o ) and hydrophilic-more hydrophilic ( θ y = 60 o - θ g = 30 o ) surface. The present simulations clarify the mechanism of condensate droplets falling off subcooled surface with the existence of non-condensable gas and are conducive to the design of hybrid-wettability surfaces for condensation heat transfer enhancement, which is of great significance to energy resources conservation.

Lattice Boltzmann study of nucleation site interaction and nucleate boiling heat transfer on a hybrid surface with multiple cavity-pillar structures

In this work, nucleation site interaction and nucleate boiling heat transfer are studied on a hybrid rough surface, a surface with multiple cavities and a pillar on sides of each cavity, using a two-dimensional pseudopotential phase-change lattice Boltzmann method (LBM). To elucidate the effects of incorporation of pillars, the results of the hybrid rough surface are compared with a simple rough surface (a surface with multiple cavities). First, bubble nucleation, growth and departure processes are investigated above the simple and hybrid rough surfaces with three cavities and bubble interactions and their effects on cavity activation/deactivation at low superheat are discussed. The results showed a complete suppression of the central cavity at small pitch distance and left/right cavities at large pitch distance, which are caused by combined suppression effect of adjacent cavities and edge effects of the heater, respectively, for the simple rough surface. But, with the incorporation of pillars the heat transfer area is increased and the effects of induced flow are reduced, resulting in the activation of suppressed cavities, and thus, the higher heat flux is observed for the hybrid rough surface than the simple rough surface. The cavity suppression is also observed for unequal cavity depths or widths. The complete suppression of the central cavity occurs, when the central cavity is narrow or left/right cavities are deep. On the other hand, the highest heat flux is observed for the hybrid rough surface with central deep cavity and left/right shallow cavities due to the activation of all nucleation sites and additional heat transfer from the central deep cavity. Increasing pillar height or decreasing cavity-pillar spacing results in the stability and growth of residual bubbles to the next cycle without waiting periods, which increase the heat flux. Next, heat transfer in the nucleate boiling region is investigated by comparing saturated boiling curves obtained through simulations. The heat flux in the nucleate boiling including the critical heat flux (CHF) is higher for the hybrid rough surface than the simple rough surface. The heat flux also found to be increased in developing region of nucleate boiling, when pillar height is increased. The CHF increases with pillar height up to a certain height. But, with tall pillars at high wall superheats, the excessive nucleation of bubbles can also occur in the inter-pillar spacings, in addition to cavities, increasing the susceptibility of merger of closely spaced bubbles, which reduce the CHF.

Lattice Boltzmann study of bubble dynamics and heat transfer on a hybrid rough surface with a cavity-pillar structure

In this work, a two-dimensional phase-change lattice Boltzmann model is used to investigate the pool boiling from a hybrid rough surface with a single cavity and one or two pillars on the superheated plate for an entire ebullition cycle. The bubble growth and departure dynamics on the hybrid pillared cavity surfaces, namely a single-pillared cavity surface (a surface with one cavity and one neighbouring pillar) and a double-pillared cavity surface (a surface with one cavity and one pillar on its both sides), are also compared with a simple cavity surface (a surface with only one cavity). The results showed that the incorporation of the pillar to the simple cavity surface increases the surface area and distance between the pinning locations of the three-phase contact line (TCL), and thus, enhances the bubble departure diameter and heat flux. It is found that bubble departure diameter and heat flux increase with cavity-pillar spacing up to a certain value, in this case sp∗ = 0.50, beyond which they start to decease and approach the simple cavity surface case at sp∗ = 1.0, while the opposite trend is observed for bubble departure frequency. The bubble departure diameter, frequency and heat flux also increase with pillar width. Compared to uniformly hydrophilic surfaces, the mixed wettability surfaces exhibit large heat flux with relatively small increase in bubble departure diameter.

Pore-Scale Simulation on Pool Boiling Heat Transfer and Bubble Dynamics in Open-Cell Metal Foam by Lattice Boltzmann Method

AbstractBased on the newly developed geometrical model of open-cell metal foam, pool boiling heat transfer in open-cell metal foam, considering thermal responses of foam skeletons, is investigated by the phase-change lattice Boltzmann method (LBM). Pool boiling patterns are obtained at different heat fluxes. The effects of pore density and foam thickness on bubble dynamics and pool boiling heat transfer are revealed. The results show that “bubble entrainment” promotes fluid mixing and bubble sliding inside metal foam. Based on force analysis, the sliding bubble is pinned on the heating surface and cannot lift off completely at high heat flux due to the increasing surface tension force. Pool boiling heat transfer coefficient decreases with increasing pore density and foam thickness due to high bubble escaping resistance.

Pore scale simulation of evaporation in a porous wick of a loop heat pipe flat evaporator using Lattice Boltzmann method

The evaporation heat transfer of a loop heat pipe (LHP) flat evaporator with porous wick is numerically simulated at pore scale using an advanced phase-change Lattice Boltzmann Method (LBM). The effects of heat flux and surface wettability (contact angle) on the patterns and dynamics of liquid-vapor interface, liquid volume fraction, temperature distribution at wick-fin and wick-groove interfaces, as well as effective heat transfer coefficient of the evaporator are investigated. It is found that liquid-vapor interface will appear five kinds of patterns or dynamics sequentially with the increase of heat flux, i.e., (a) entirely saturated wick, (b) partially saturated wick with periodical nucleation and vanishing of bubbles, (c) partially saturated wick with periodical growth and shrink of bubbles, (d) partially saturated wick with stable liquid-vapor interface, and (e) fully dry-out at wick-fin interface. Generally, the higher the heat flux and the larger the contact angle, the smaller the steady-state liquid volume fraction in the wick. However, at a certain range of heat flux, the liquid volume fraction oscillates periodically corresponding to periodical oscillations of liquid-vapor interface, and its amplitude and period increases with increasing heat flux and contact angle. Due to the random pore size distribution of porous wick, the temperature distributions are not symmetric between the left and the right outlets. This discrepancy is especially significant for partially saturated wicks, revealing the remarkable impacts of local pore structure on evaporation characteristics. With the increase of heat flux, the effective heat transfer coefficient at first increases owing to the inception and development of bubble nucleation within the wick, and then decreases due to the occurrence and enlargement of dry out. The larger the contact angle, the smaller the heat flux to trigger the bubble nucleation, and therefore the earlier to achieve the maximum effective heat transfer coefficient. Nevertheless, contact angles studied in this paper have no obvious effect on the maximum value of effective heat transfer coefficient.

3D simulations of pool boiling above smooth horizontal heated surfaces by a phase-change lattice Boltzmann method

Pool boiling heat transfer from smooth horizontal hydrophilic and hydrophobic heaters having a finite thickness under constant bottom wall temperature conditions is simulated numerically with a three-dimensional liquid-vapor phase-change lattice Boltzmann method. For a small heater, single bubble dynamics and temporal variations of heater’s top wall temperature in contact with the bubble are investigated. It is found that top wall temperature drops during initial bubble growth period at low wall superheats on a hydrophilic heater due to the microlayer evaporation process. Because of a residual bubble is left on the top of the heater after bubble departure, no wall temperature drop is found on a hydrophilic surface at high wall superheats nor on a hydrophobic surface at any wall superheats. 3D pool boiling curves in dimensionless forms for smooth horizontal heaters of infinite extent from nucleation to critical heat flux through transition boiling to stable film boiling are presented. It is found that 2D and 3D simulated heat fluxes are identical in nucleate boiling and stable film boiling regimes, although transition boiling heat fluxes differ greatly. It is also found that the 3D simulated CHF is in better agreement with existing theories and correlation equations than 2D simulated CHF. Detailed 3D multiple bubbles dynamics above hydrophobic and hydrophilic surfaces and temporal variations of dry spots distribution on these surfaces at CHF are presented. Effects of wettability and thickness of the heater on boiling curves for smooth heaters are investigated. It is founded that only wettability play predominate roles in nucleate boiling regime from smooth superheated surfaces while all factors influence the transition boiling regime (including maximum and minimum heat fluxes) greatly.

Numerical Simulation of Complete Pool Boiling Curves: From Nucleation to Critical Heat Flux Through Transition Boiling to Film Boiling

The latest version of the newly developed liquid-vapor phase-change lattice Boltzmann method, with a conjugate thermal boundary condition imposed at the solid-fluid interface, is applied to simulate numerically pool boiling from smooth, infinitely long, upward-facing, horizontal heated surfaces under controlled wall temperature conditions. A parametric study is carried out to investigate effects of wettability as well as heater and fluid physical properties on pool boiling curves, from onset of nucleation to critical heat flux (CHF) through transition boiling to stable film boiling. It is found that although a heater’s wettability has no effect on film boiling, it has important effects on nucleate boiling and transition boiling. Decreasing heater wettability shifts the nucleate and transition boiling curves to the left, decreases the maximum heat flux, decreases the minimum heat flux (MHF), and lowers the Leidenfrost temperature. With the increase of the heater’s thermal conductivity, both the MHF and the Leidenfrost temperature decrease, but this has no effect on nucleate boiling, CHF, or film boiling. On the other hand, increasing the vapor’s thermal conductivity has no effect on nucleate boiling, but it increases the MHF and decreases the Leidenfrost temperature in transition boiling as well as in film boiling.

Simulations of saturated boiling heat transfer on bio-inspired two-phase heat sinks by a phase-change lattice Boltzmann method

Pool boiling heat transfer from four types of micro-pillar heat sinks with different wettability patterns is simulated numerically with the latest version of liquid-vapor phase-change lattice Boltzmann model. Effects of pillar geometry and wettability on bubble dynamics are investigated. It is found that bubbles will nucleate either on the hydrophobic pillar top or on the hydrophilic cavity bottom between micro-pillars, depending on wettability and local wall temperature. Among the four types of micro-pillar heat sinks with hybrid wettability patterns, it is found that the bio-inspired heat sink (with hydrophobic pillar tops and hydrophilic base) has the best boiling heat transfer performance with the following desirable features: (i) unique characteristics of orderly separation of vapor and liquid paths at low superheats, (ii) hydrophobic surface characteristics where residual bubbles on hydrophobic pillar tops provide faster bubble departure frequency, (iii) triple phase lines are pinned at corners of micro-pillar, restricting expansion of bubbles into film boiling at high superheats. Simulated results show that geometry of micro-pillars and wettability patterns greatly influence transition boiling regime including the maximum heat flux (CHF) and the Leidenfrost temperature, resulting in pool boiling curves with widely different shapes.

Numerical investigation on boiling mechanism in porous metals by LBM at pore scale level

In the present study, the boiling mechanism in porous metals is investigated by Gong-Cheng phase-change lattice Boltzmann method. The complex geometry of porous metals is implemented in this study, which is different from the previous upscaling studies of boiling in porous metals, such as the method of representative elementary volume (REV). The detailed ebullition process at pore scale level is revealed, and the effects of wettability, porosity and porous structures on bubble behaviors and heat transfer are further analyzed. We found that bubble coalescence helps bubbles to be squeezed out of pores and bubbles accumulate more closely at the solid surface with increasing static contact angle. A thin liquid film forms between the bubble and solid surface with the static contact angle of 46.2°. Bubbles at the bottom of the porous metals are hard to escape, and bubbles coalesce frequently at the bottom of porous metals with lower porosity. Bubbles escape more easily in gradient porous metals than uniform porous metals in flow boiling simulation.