Molecular dynamics study of wettability and pitch effects on maximum critical heat flux in evaporation and pool boiling heat transfer

Molecular dynamics simulations were employed to investigate the effects of wettability (contact angle) and pitch on nanoscale evaporation and pool boiling heat transfer of a liquid argon thin film on a horizontal copper substrate topped with cubic nano-pillars. The liquid–solid potential was incrementally altered to vary the contact angle between hydrophilic (～0°) and hydrophobic (～127°), and the pitch (distance between nano-pillars) was varied between 21.7 and 106.6?Å to observe the resultant effect on boiling heat transfer enhancement. For each contact angle, the superheat was gradually increased to initiate nucleate boiling and eventually pass the critical heat flux (CHF) into the film boiling regime. The CHF increases with pitch, and tends to decrease substantially with increasing contact angle. A maximum overall heat flux of 1.59?×?10⁸?W/m² occurs at the largest pitch investigated (106.6?Å), and as the contact angle increases the superheat required to reach the CHF condition also increases. Finally, in certain cases of small pitch and large contact angle, the liquid film was seen to transition to a Cassie–Baxter state, which greatly hindered heat transfer.     

MOLECULAR DYNAMICS SIMULATION OF THERMAL CONDUCTIVITY OF NANOSCALE THIN SILICON FILMS

Molecular dynamics simulations are performed to explore the thermal conductivity in the cross-plane direction of single-crystal thin silicon films. The silicon crystal has diamond structure, and the Stillinger-Weber potential is adopted. The inhomogeneous nonequilibrium molecular dynamics (NEMD) scheme is applied to model heat conduction in thin films. At average temperature T = 500 K, which is lower than the Debye temperature Θ_D = 645 K, the results show that in a film thickness range of about 2–32 nm, the calculated thermal conductivity decreases almost linearly as the film thickness is reduced, exhibiting a remarkable reduction as compared with the bulk experimental data. The phonon mean free path is estimated and the size effect on thermal conductivity is attributed to the reduction of phonon mean free path according to the kinetic theory.     

Effects of film evaporation on laminar mixed convection heat and mass transfer in a vertical channel

A numerical study of finite liquid film evaporation on laminar mixed convection heat and mass transfer in a vertical parallel plate channel is presented. The influences of the inlet liquid mass flow rate and the imposed wall heat flux on the film vaporization and the associated heat and mass transfer characteristics were examined for air-water and air-ethanol systems. Predicted results obtained by including transport in the liquid film are contrasted with those where liquid film transport is neglected, showing that the assumption of an extremely thin film made by Tsay and Yan (Wärme- und Stoffübertragung 26, 23–31 (1990)) is only valid for a system with a small liquid mass flow rate. Additionally, it is found that the heat transfer between the interface and gas stream is dominated by the transport of latent heat associated with film evaporation. The magnitude of the evaporative latent heat flux may be five times greater than that of sensible heat flux.     

Evaporative heat transfer characteristics of a water spray on micro-structured silicon surfaces

Experiments were performed to evaluate the evaporative heat transfer characteristics of spray cooling of water on plain and micro-structured silicon surfaces at very low spray mass fluxes. The textured surface is made of an array of square micro-studs. It was found that the Bond number of the microstructures is the primary factor responsible for the heat transfer enhancement of evaporative spray cooling on micro-structured silicon surface in the present study. A qualitative study of evaporation of a single water droplet on plain and textured silicon surface shows that the capillary force within the microstructures is effective in spreading the deposited liquid film, thus increasing the evaporation rates. Four distinct heat transfer regimes, which are the flooded, thin film, partial dryout, and dryout regimes, were identified for evaporative spray cooling on micro-structured silicon surfaces. The microstructures provided better cooling performance in the thin film and partial dryout regime and higher liquid film breakup heat flux, because more water was retained on the heat transfer surface due to the capillary force. Heat transfer coefficient and temperature stability deteriorated greatly once the liquid film breakup occurred. The liquid film breakup heat flux increases with the Bond number. Effects of surface material, system orientation and spray mass flux were also addressed in this study.     

An experimental study of rupture dynamics of evaporating liquid films on different heater surfaces

Experimental data were obtained to reveal the complex dynamics of thin liquid films evaporating on heated horizontal surfaces, including formation and expansion of dry spots that occur after the liquid films decreased below critical thicknesses. The critical thickness of water film evaporating on various material surfaces is measured in the range of 60–150 μm, increasing with contact angle and heat flux while decreasing with thermal conductivity of the heater material. In the case of hexane evaporating on a titanium surface, the liquid film is found resilient to rupture, but starts oscillating as the averaged film thickness decreases below 15 μm.     

Thermal protection with liquid film in turbulent mixed convection channel flows

In this numerical study, a channel flow of turbulent mixed convection of heat and mass transfer with film evaporation has been conducted. The turbulent hot air flows downward of the vertical channel and is cooled by the laminar liquid film on both sides of the channel with thermally insulated walls. The effect of gas–liquid phase coupling, variable thermophysical properties and film vaporization are considered in the analysis. In the air stream, the k–ε turbulent model has been utilized to formulate the turbulent flow. Parameters used in this study are the mass flow rate of the liquid film B, Reynolds number Re, and the free stream temperature of the hot air T_o. Results show that the heat flux was dramatically increases due to the evaporation of liquid water film. The heat transfer increases as the mass flow rate of the liquid film decreases, while the Reynolds number and inlet temperature increase, and the influences of the Re and T_o are more significant than that of the liquid flow rate. It is also found that liquid film helps lowering the heat and mass transfer rate from the hot gas in the turbulent channel, especially at the downstream.     

On the dynamics and heat transfer of bubble train in micro-channel flow boiling

The dynamics and heat transfer characteristics of flow boiling bubble train moving in a micro channel is studied numerically. The coupled level set and volume of fluid (CLSVOF) is utilized to track interface and a non-equilibrium phase change model is applied to calculate the interface temperature as well as heat flux jump. The working fluid is R134a and the wall material is aluminum. The fluid enters the channel with a constant mass flux (335 kg/m² 1 s), and the boundary wall is heated with constant heat flux (14 kW/m²). The growth of bubbles and the transition of flow regime are compared to an experimental visualization. Moreover, the bubble evaporation rate and wall heat transfer coefficient have been examined, respectively. Local heat transfer is significantly enhanced by evaporation occurring vicinity of interface of the bubbles. The local wall temperature is found to be dependent on the thickness of the liquid film between the bubble train and the wall.     

Molecular Dynamics Simulation of Thin Film Evaporation of Lennard-Jones Liquid

Evaporation of a thin liquid film is of significant fundamental importance for both science and engineering applications. This work investigates the evaporation of a thin liquid argon layer into vacuum employing molecular dynamics simulation based on the Lennard-Jones potential. The simulation results demonstrate that the net evaporation rate of an ultra-thin liquid film into vacuum in a closed system may be modeled by the balance of evaporation and condensation based on the Schrage model. The evaporation/condensation coefficient and the non-Maxwellian factor may thus be evaluated. As for the open system, the simulation results demonstrate a constant evaporation rate for each leakage probability. The corresponding evaporation heat transfer coefficient is very high and increases with increase in leakage percentage. Such a high heat transfer coefficient demonstrates very high heat transfer capability of evaporation from an ultra-thin liquid film.     

A study on the thermal resistance over solid–liquid–vapor interfaces in a finite-space by a molecular dynamics method

Molecular dynamics of argon atoms in a nano-triangular channel which consists of (111) platinum walls were studied. The molecular dynamics simulations aim to gain understanding in the heat transfer through the channel including the influence of the contact resistances which become important in small-scale systems. The heat transfer properties of the finite-space system were measured at a quasi-steady non-equilibrium state achieved by imposing a longitudinal temperature gradient to the channel. The results indicate that the total thermal resistance is characterized not only by the thermal boundary resistances of the solid–liquid interfaces but also by the thermal resistance in the interior region of the channel. The overall thermal resistance is determined by the balance of the thermal boundary resistances at the solid–liquid interfaces and the thermal resistance attributed to argon adsorption on the lateral walls. As a consequence, the overall thermal resistance was found to take a minimum value for a certain surface potential energy. A rich solid–liquid interface potential results in a reverse flow along the wall which gives rise to a stationary internal flow circulation. In this regime, the nanoscale-channel functions as a heat-pipe with a real steady state.     

Marangoni convection,evaporation and interface deformation in liquid films on heated substrates with non-uniform thermal conductivity

Marangoni convection plays an important role in hydrodynamics of liquid films on heated or cooled substrates. In this paper a model describing Marangoni convection, interface dynamics and evaporation in liquid films on composite substrates or substrates of functionally graded materials is developed. Non-uniform thermal conductivity of the substrate causes non-uniformity of temperature distribution at the liquid–gas interface which leads to appearance of Marangoni stresses, convective vortices and film deformation. The film dynamics is described in the framework of long-wave theory. The substrate thermal conductivity non-uniformity has a pronounced effect on transport processes in the liquid film.     

Interface Structure Influence on Thermal Resistance Across Double-Layered Nanofilms

This work investigated the interface influence on the thermal resistance across double-layered thin films by non-equilibrium molecular dynamics (NEMD) with Lennard-Jones potential. Layer A is a solid argon with a face-centered cubic structure and Layer B is obtained by changing atomic mass only. A flat interface is formed when each of the contacting atomic planes from the two layers has the same kind of atoms. A staggered interface is obtained by mixing atoms A and B around the interface region. The temperature profile, vibration amplitude, and structure factor are studied to observe the interface effects. It is found that the thickness of the staggered atomic layer has significant influences on the normal thermal conductivity. With a staggered interface thickness of two atomic planes, the normal thermal conductivity is sharply increased. Further increasing the staggered thickness will gradually decrease the normal conductivity. This result suggests a possibility to control the thermal conductivity of the double-layered structure by engineering its interface condition.     

Thermal Analysis of Nanofluids on the Thin Film Evaporation of Meniscus

A mathematical model for predicting evaporation in the thin film region was developed and its analytical solutions were obtained for thin‐film thickness, the heat transport per unit length and the total heat flux transport in the thin‐film region. These analytical solutions show that the higher heat flux through the thin film region occurs due to the higher superheat. The maximum evaporative rate occurs when the effects of the increase in the temperature difference and in the thin film thickness on the heat flux q stay equal. A nanofluid, which is a colloidal mixture of nanoparticles (1 nm to 100 nm) and a base liquid (nanoparticle fluid suspensions), is employed as the working fluid. In a certain range, increasing the volume fraction of nanoparticles in the base fluid leads to decreasing the kinematic viscosity of the nanofluid and increasing the thermal conductivity, which influences the evaporation in the thin film region. The heat transfer rate per unit length and the total heat flux in the thin film region display various characteristics among the different type of nanofluids due to the differences of the kinematic viscosity and the thermal conductivity.     

The drying of liquid films on cylindrical and spherical substrates

The drying of liquid films from binary solid–solvent mixtures on cylindrical and spherical substrates is investigated by solving the diffusion equation on the cylindrical or spherical domains with account for the shrinkage of the systems due to the solvent evaporation. As a main result of the gas phase analysis, d²-laws for both the cylindrical and the spherical cases are found. The spatio-temporal evolution of the component mass fractions in the liquid are obtained for varying initial film thickness and drying conditions. Large mass fraction gradients, formed at high drying rates, lead to a skin-core morphology with low product quality. Initially very thin films dry very uniformly, which is analogous to the flat temperature profiles in unsteady heat conduction at small Biot numbers. The maximum initial film thickness allowable to ensure flat solute mass fraction profiles in the films, and thus high quality of the dry films, is computed for varying drying rates. From these results a guideline for the drying of liquid films on cylindrical and spherical substrates is deduced.     

Experimental investigation of falling film evaporation on horizontal tubes

This paper presents the results of an experimental investigation relating to heat transfer during evaporation of thin liquid films falling over horizontal tubes. Experiments were conducted using 25 mm o.d. copper tubes heated by internal electrical cartridge heaters so that a uniform heat flux was generated on the outside tube surface. Five heated tubes were arrayed on a vertical plane with a pitch of 50 mm. Freon R-11 preheated to the saturation temperature at 0.2 MPa was supplied to the topmost heated tube through feeding tubes. Heat transfer characteristics on each heated tube were clarified in a range of film Reynolds number from 10 to 2000 and the measured data are presented in the form of correlations. Deterioration of heat transfer due to film break down was also considered. © 1999 Scripta Technica, Inc. Heat Trans Jpn Res, 27(8): 609–618, 1998     

Atomic insights of thin‐film evaporation over biphilic surfaces: Effect of philic‐phobic patterning and wettability contrast

Molecular dynamics simulations have been carried out to investigate the effect of philic‐phobic patterning and wettability contrast of biphilic (surface with interlaced hydrophilic and hydrophobic segments) surfaces on thin‐film evaporation and compare its performance with the surfaces with uniform wettability, ie, hydrophilic/hydrophobic. Thin liquid film of argon with 3 nm film thickness is placed over platinum surface. After equilibrating the system at 90 K, the temperature of the platinum wall is raised to two different temperatures 110 and 130 K to study the effect of wall superheat temperature. The results obtained in this study indicate that biphilic surface offers wall heat flux and evaporative mass flux close to the hydrophilic surface. With the decrease of philic‐phobic pattern bandwidth of biphilic and superbiphilic surface, the maximum value of evaporative mass flux and wall heat flux increases. Also higher wettability contrast among hydrophilic and hydrophobic region offers higher value of wall heat flux and evaporative mass flux. The boiling inception time decreases with the decrease of philic‐phobic pattern bandwidth and increase of wettability contrast therefore, the nonevaporating layer appears earlier in case of lower philic‐phobic pattern bandwidth and higher wettability contrast.     

Unified description of size effects of transport properties of liquids flowing in nanochannels

The present work connects transport properties of simple fluids flowing in nanochannels: the diffusion coefficient, the shear viscosity and the thermal conductivity. We used non-equilibrium molecular dynamics (NEMD) simulations of liquid argon flowing in a nanochannel formed by krypton walls, macroscopically equivalent to the planar Poiseuille flow, at constant temperature. All properties approach bulk values as the channel width increases. As the channel width decreases, the transport properties change dramatically: the diffusion coefficient decreases with the channel width, the shear viscosity increases with the channel width and the thermal conductivity increases with the channel width. The novel result of this work is that, if the size effect of one of the transport properties (say the diffusion coefficient) is known, then all the others can be estimated from results that apply to classic fluid mechanics.     

Shear-driven flows of locally heated liquid films

This paper considers the flow of a liquid film sheared by gas flow in a channel with a heater placed at the bottom wall. A one-sided 2D model is considered for weakly heated films. The heat and mass transfer problem is also investigated in the framework of a two-sided model. The exact solution to the problem of heat transfer is obtained for a linear velocity profile. The double effect of Marangoni forces is demonstrated by the formation of a liquid bump in the vicinity of the heater’s upper edge and film thinning in the vicinity of the lower edge. The criterion determining the occurrence of “ripples” on the film surface upstream from the bump is found. Numerical analysis reveals that evaporation dramatically changes the temperature distribution, and hence, thermocapillary forces on the gas–liquid interface. All transport phenomena (convection to liquid and gas, evaporation) are found to be important for relatively thin films, and the thermal entry length is a determining factor for heaters of finite length. The thermal entry length depends on film thickness, which can be regulated by gas flow rate or channel height. The influence of the convective heat transfer mechanism is much more prominent for relatively high values of the liquid Reynolds number. The liquid–gas interface Biot number is shown to be a sectional-hyperbolic function of a longitudinal axis variable. Some qualitative and quantitative comparisons with experimental results are presented.     

Stability of an evaporating thin polymer film

The paper demonstrates existence of a purely thermal instability mechanism in a thin evaporating film that can be responsible for formation of mesoscopic patterns in thin polymer films. The instability is caused by the increasing evaporation with the film thinning due to higher heat flux to the film surface.     

Falling film evaporation of ternary mixture under three different modes of heating

This numerical study deals with heat and mass transfer by evaporation under mixed convection in three different configurations of a ternary liquid film in a vertical channel. The ternary liquid mixture water-methanol-benzene falls along the right plate of channel while the other plate is kept thermally insulated. In the first configuration, a heat flux density is applied to the wall carrying the trickle film, while in the other configurations this same amount of heat is used to preheat the liquid film or the air at the inlet of the channel. The implicit finite difference scheme is used to solve the system of equations in both liquid and vapor phases. According to this study, it was observed that the evaporation efficiency is high when the mass fraction of volatile components is high or in the preheating state of ternary film.