Thin Liquid Water Film Research Articles

Effect of nanostructure on explosive boiling of thin liquid water film on a hot copper surface: a molecular dynamics study

ABSTRACT The explosive boiling characteristics of water on the copper surface are studied based on molecular dynamics simulation. The effects of the size, number and shape of nanostructures on the heat transfer performance of water explosive boiling are analysed. The results show that the heat transfer performance and boiling rate of liquid film increase with the increase of nanostructure size. At the same volume concentration of nanostructures, the boiling rate of the liquid film increases with the increase of the number of nanostructures. Compared with the spherical nanostructure, the water heating rate is higher and the heat flux of water is larger after the cylindrical nanostructure is adopted. Therefore, using cylindrical nanostructures and increasing, the uniformity of nanostructures on the surface can effectively improve the boiling heat transfer performance of water on the copper surface.

Numerical study of evaporation assisted hybrid cooling for thermal powerplant application

Abstract. This paper describes a numerical study of the effect of evaporative cooling on forced convective heat transfer in a system experiencing combined heat and mass transport. A multi-dimensional mathematical model has been developed to conduct simulations over a range of operating parameters to acquire insight into the dual mode cooling where convective heat transfer is augmented by evaporative heat and mass transfer process. The system being modeled consists of a thin liquid water film that undergoes evaporation as a result of being exposed to a prescribed heat flux and laminar convective flow condition. Predictions from the simulations indicate that in comparison to pure forced convection cooling, under convective-evaporative hybrid cooling conditions the overall heat transfer coefficient is increased by a factor of ~4 to 5, where evaporation alone contributes to 80−90% of the overall performance. A critical Reynolds number has been found beyond which no significant enhancement in the overall heat transfer is observed. Numerical results also identify a critical film thickness beyond which the thermal inertia across the liquid film becomes dominant and starts to suppress the overall heat transfer process. System size requirements and water consumption of hybrid cooling for power plant applications are compared with conventional water, and air- cooled systems and was found to be significantly lower.

Molecular dynamics study of rapid boiling of thin liquid water film on smooth copper surface under different wettability conditions

This work aims to present theory of heat transfer on rapid boiling for pure wettability surfaces and mixed wettability surfaces based on molecular simulation. The simulation results showed that the temperature of water increased obviously and the maximum evaporation rate of water reduced significantly with the increase of hydrophilic for pure wettability surfaces. Furthermore, a microfluidic layer which was formed on all cases except hydrophobic surface (surface 4) was conducive to critical heat flux (CHF) improvement. Meanwhile, the attraction of water molecules upon hydrophobic region was smaller than that on hydrophilic region which was conducive to boiling heat transfer coefficient (HTC) improvement. A mixed wettability surface enhances boiling heat transfer by regulating vapor spreading behaviors over the heating copper surface compared with pure wettability surfaces.

Liquid water film on the outer surface of a half-submerged rotating drum

In this study, a two-dimensional computational model describing the rise and development of thin liquid water film on a rotating drum is presented. The liquid water flow is predicted by solving the Navier–Stoke equation using computational fluid dynamics neglecting the end effects of the rotating drum side wall along width. The liquid film on a half-submerged rotating drum was modeled by volume of fluid to determine the thickness and quality of the liquid film on a rotating drum. Computational results show that the liquid film on the rising side is smooth, whereas the film on the plunging point is complex. For a 300 r/min of 75 mm rotating drum, it takes ∼300 ms to develop the film from the rising point to fall to the plunging point. Air is entrained in plunging side by the falling film from the rising side.

A numerical investigation on the conjugate heat transfer of thin liquid film of water in closed microcavity

The understanding of the conjugate heat transfer in an evaporating thin-film and an intrinsic meniscus is important for enhancing the thermal performance of phase-change heat transfer devices. In this paper, we numerically investigated the conjugate heat transfer in the thin film and the intrinsic meniscus under their own vapor in closed microcavities with different geometries. The results show that, with decreasing microcavity size, the percentage contribution of the thin liquid film to the overall heat transfer increases obviously. However, in order to optimally utilize the thin-film heat transfer, both the microcavity size and the length-height ratio need to be concerned for their effects on the thin-film contribution to the net heat transfer. With increasing superheats that are smaller than 5 K, the contribution of the thin film to the net heat transfer rate becomes somewhat smaller, although the variation seems to be less sensitive to the superheat than the microcavity size. Meanwhile, when the wettability improves, the contribution of the thin film to the net heat transfer rate increases, but the variation of the thin-film contribution is relatively less sensitive to the wettability than the microcavity size and the superheat.

EFFECTS OF INSERTED POROUS SQUARE CYLINDER ON HEAT- AND MASS-TRANSFER ENHANCEMENT IN A CHANNEL

We numerically investigate the effects of an inserted square porous cylinder in a horizontal channel on flow structure and heat and mass transfer. Channel walls have a thin liquid water film and are heated with a constant heat flux density. Several blockage ratio (β) and gap distances (γ) between cylinder and channel wall are considered for study of geometric effects on heat and mass transfer. We perform a comparison between two configurations (with and without porous square cylinder) to highlight the effect of the addition. To achieve this, we solve the classical equations of forced convection and the Darcy−Brinkman−Forchheimer model. Our investigation finds an improvement in heat and mass transfer with the presence of porous cylinder. This improvement is greater with a decrease in Darcy number (Da) and when the obstacle is placed in the middle of the channel. The Sherwood number, which characterizes mass transport, is correlated by a relationship with Reynolds number and ratio blockage. We provide some design guidelines related to Da, β, and γ that can be used in an engineering environment.

Why is ice slippery?

Amit talks about the reason why ice is slippery. Ice is remarkably hard to get a grip on. The overwhelming consensus is that ice has low friction because of a thin film of liquid water coating its surface. Hence skaters balanced on thin metal blades can glide smoothly across the ice rink, but grind to a halt on the wooden floor beyond. The tricky part is how this liquid layer forms

Molecular dynamics simulations of Kapitza length for argon-silicon and water-silicon interfaces

A comprehensive understanding of heat conduction between two parallel solid walls separated by liquid remains incomplete in nanometer scale. In addition, the solid/liquid interfacial thermal resistance has been an important technical issue in thermal/fluid engineering such as micro electro-mechanical systems and nano electro-mechanical systems with liquid inside. Therefore, further advancements in nanoscale physics require an advanced understanding of momentum and energy transport at solid/liquid interfaces. This study employs three-dimensional molecular dynamics (MD) simulations to investigate the thermal resistance at solid/liquid interfaces. Heat conduction between two parallel silicon walls separated by a thin film of liquid water is considered. The density distribution of liquid water is discussed with the simulation results to further understanding of the dynamic properties of water near solid surfaces. Meanwhile, temperature profiles appear discontinuous between liquid and solid temperatures due to the dissimilarity of thermal transport properties of the two materials, which validates thermal resistance (or Kapitza length) at solid/liquid interfaces. MD results also investigate the temperature dependence of the Kapitza length, demonstrating that the Kaptiza lengths fluctuate around an average value and are independent of the wall temperature at solid/liquid interfaces. Our study provides useful information for the design of thermal management or heat dissipation devices across silicon/water and silicon/argon interfaces in nanoscale.

Flow reversal in combined laminar mixed convection heat and mass transfer with phase change in a vertical channel

This paper, deals with a numerical study of the effects of buoyancy forces on an upward, steady state, laminar flow of humid air in a vertical parallel-plate channel. The plates are wetted by a thin liquid water film and maintained at a constant temperature which is lower than that of the air entering the channel. A 2D fully elliptical model, associated with the Boussinesq assumption, is used to take into account axial diffusion. The solution of this mathematical model is based on the finite volume method and the velocity–pressure coupling is handled by the SIMPLER algorithm. Numerical results show that buoyancy forces have a significant effect on the hydrodynamic, thermal and mass fraction fields. Additionally, these forces induce flow reversal for high air temperatures and mass fractions at the channel entrance. It is established that heat transfer associated with phase change is, sometimes, more significant than sensible heat transfer. Furthermore, this importance depends on the mass fraction gradient. The conditions for the existence of flow reversal are presented in charts and analytical expressions specifying the critical thermal Grashof number as a function of the Reynolds number for different values of the solutal Grashof number and different aspect ratios of the channel.

Hydrodynamic Lubrication in a Nanoscale Bearing

Nonequilibrium molecular dynamics simulation is used to simulate the hydrodynamics lubrication in the nanoscale bearing. A physical model of the nanascale bearing that the thin liquid water film confined between two solid walls has been set up. The simulation results indicate that the hydrodynamic pressure profiles as the macroscopic state are formed in nanoscale bearings. The hydrodynamic pressure increases with increasing shear velocity, and corresponding load capacity also increases with increasing sliding velocity of the upper driven wall. These results are in accordance with classical Reynolds’ theory about hydrodynamic lubrication. On the other hand, the shear thinning behavior of the liquid water film appears, so that effective viscosity decreases under high shear velocity. Although effective viscosity of water film decreases under high shear velocity, this phenomenon doesn’t affect the hydrodynamic lubrication of the nanoscale bearing.

Buoyancy effects on upward and downward laminar mixed convection heat and mass transfer in a vertical channel

PurposeThe objective of the present study is to investigate numerically the effects of thermal and buoyancy forces on both upward flow (UF) and downward flow (DF) of air in a vertical parallel‐plates channel. The plates are wetted by a thin liquid water film and maintained at a constant temperature lower than that of the air entering the channel.Design/methodology/approachThe solution of the elliptical PDE modeling the flow field is based on the finite volume method.FindingsResults show that buoyancy forces have an important effect on heat and mass transfers. Cases with evaporation and condensation have been investigated for both UF and DF. It has been established that the heat transfer associated with these phase changes (i.e. latent heat transfer) may be more or less important compared with sensible heat transfer. The importance of these transfers depends on the temperature and humidity conditions. On the other hand, flow reversal has been predicted for an UF with a relatively high temperature difference between the incoming air and the walls.Originality/valueContrary to most studies in channel heat and mass transfer with phase change, the mathematical model considers the full elliptical Navier‐Stokes equations. This allows one to compute situations of flow reversal.

Is Liquid Water a Hot Quantum Fluid? Anomalies of Water in Thin Liquid Films and in Biological Systems

The anomalies that appear at every multiple of 15 ℃ in the viscosity of a thin liquid film of water and of water near solid interfaces are explained in this paper by comparing the thermal wavelength and molecular free volume of water, and quantum numbers are found. The possibility that these anomalies are related to the preferred and/or lethal temperatures of organisms is considered. The toxicity of heavy water (D₂O) can also be explained with this approach.

Gravity-driven flows of viscous liquids over two-dimensional topographies

Using phase-stepped interferometry, we have measured full two-dimensional maps of the free-surface shape of a thin liquid film of water flowing over an inclined plate with topography. The measurement technique allows us to image automatically the shape of the free surface in a single field of view of about 2.4 by 1.8 mm, with a lateral resolution of 3.1 μm and a height resolution of 0.3 μm. By imaging neighbouring regions and combining them, complete two-dimensional free-surface profiles of gravity-driven liquid films with a thickness ranging between 80 and 120 μm are measured, over step, trench, rectangular and square topographies with depths of 10 and 20 μm, and lateral dimensions of the order of 1 to several mm. The experimental results for both one- and two-dimensional flows are found to be in good agreement with existing models, including a recent two-dimensional Green's function of the linearized problem by Hayes et al. This extends the applicability of simple models to cases with a high value of topography steepness and low-viscosity liquids as in our experiments. A corollary of the agreement with the linear two-dimensional model is that our experimental results behave linearly, a convenient property that allows the free-surface response to complex topographies to be worked out from knowledge of the response to an elementary topography like a square.

Nitrous oxide emissions from soil during freezing and thawing periods

In a laboratory investigation, the processes of N 2O emissions during freezing/thawing periods were studied. Four undisturbed soil columns from an agricultural site were subjected to two freeze/thaw cycles. Two periods of higher N 2O emissions were detected, a period of elevated N 2O emissions during continuous soil freezing and a period of brief peak emissions during thawing. Soil respiration indicated that microorganisms were still active in both periods. We concluded that N 2O was produced by microorganisms during continuous soil freezing in an unfrozen water film on the soil matrix. This thin liquid water film was covered by a layer of frozen water. The frozen water in form of an ice layer represents a diffusion barrier which reduces oxygen supply to the microorganisms and partly prevents the release of the N 2O. Peak emissions during soil thawing were explained by the physical release of trapped N 2O and/or denitrification during thawing.

Transfer of a pollutant molecule through a water film on a single crystal surface

We study the transfer of a CO2 molecule from the gas phase through a thin liquid water film supported on ionic single crystal surfaces NaCl and MgO (001). The free energy profile for the CO2 is calculated at 300 K using constrained molecular-dynamics simulation, and the detailed analysis of the competition between interaction and entropic contributions can help to the understanding of the pollution kinetic process of a water covered solid surface. It is shown that the CO2 solvation and its adsorption directly on the solid surface is easier for a NaCl supported film than for a MgO one.

Coupled heat and mass transfer of a stagnation point flow in a heated porous bed with liquid film evaporation

An analytical solution is presented for the study of coupled heat and mass transfer in a stagnation point flow of air through a heated porous bed with thin liquid film evaporation. The analysis indicates that the coupled heat and mass transfer process in the air stream induced by the evaporation of a thin liquid water film embedded in the porous medium of a given porous material is governed by the Peclet number Pe, the Lewis number Le, the porosity of the porous bed φ, as well as the ambient temperature T ∞ and relative humidity φ of air. It is shown that the heat transfer between the heated plate and the air stream is predominated by the transport of the latent heat associated with the thin liquid film evaporation. The analysis also demonstrates that the air stream with a lower relative humidity, a higher temperature, and a higher Peclet number lead to an increase of heat transfer rate. For the case when Le>1, the mass transfer rate is enhanced while the heat transfer rate is reduced. These findings are of significance for the design of a cross-flow indirect evaporative air cooler.

The Motion of Rapidly Rotating Curling Rocks

We present a physical model that accounts for the motion of rapidly rotating curling rocks. By rapidly rotating we mean that the rotational speed of the contact annulus of the rock about the centre of mass is large compared with the translational speed of the centre of mass. The principal features of the model are: (i ) that the kinetic friction induces melting of the ice, with the consequence that there exists a thin film of liquid water lying between the contact annulus of the rock and the ice; (ii ) that the curling rock drags some of the thin liquid film around the rock as it rotates, with the consequence that the relative velocity between the rock and the thin liquid film is significantly different to the relative velocity between the rock and the underlying solid ice surface. Since it is the former relative velocity which dictates the nature of the motion of the curling rock, our model predicts some interesting differences between the motions of slowly versus rapidly rotating rocks. Of principal note is that our model predicts, and observations confirm, that rapidly rotating curling rocks stop moving translationally well before rotational motion ceases. This is in sharp contrast to the usual case of slow rotation, where both rotational and translational motion cease at the same instant. We have verified this and other predictions of our model by careful comparison with the motion of actual curling rocks.

The motion of a curling rock

We present a plausible physical model that accounts for the motion of a curling rock. The principal features of the model are (i) that the kinetic friction induces melting of the ice with the consequence that the curling rock experiences both "dry friction," when encountering solid ice, as well as "wet friction," for contact areas that pass over the thin film of liquid water lying above the ice; (ii) that the wet friction is velocity dependent; and (iii) that the curling rock is able, in its last stages of motion, to drag some of the thin liquid film part way around the rock, which significantly enhances the curl of the rock. We compare the model to actual trajectories of curling rocks.

Ice friction during speed skating

During speed skating, the external power output delivered by the athlete is predominantly used to overcome the air and ice frictional forces. Special skates were developed and used to measure the ice frictional forces during actual speed skating. The mean coefficients of friction for the straights and curves were, respectively, 0.0046 and 0.0059. The minimum value of the coefficient of ice friction was measured at an ice surface temperature of about −7°C. It was found that the coefficient of friction increases with increasing speed. In the literature, it is suggested that the relatively low friction in skating results from a thin film of liquid water on the ice surface. Theories about the presence of water between the rubbing surfaces are focused on the formation of water by pressure-melting, melting due to frictional heating and on the ‘liquid-like’ properties of the ice surface. From our measurements and calculations, it is concluded that the liquid-like surface properties of ice seem to be a reasonable explanation for the low friction during speed skating.

Film shapes for deformable drops at liquid-liquid interfaces. II. The mechanisms of film drainage

The light interference pattern produced by illuminating the thin liquid film of water trapped between a rising oil drop and the bulk oil/water interface with white light was observed for various sizes of oil drops for four oil/water systems. Various concentrations of a surfactant, sodium lauryl sulfate, were used in the water phase. The interference patterns are presented and are broken into mechanisms to explain the different patterns observed. These mechanisms are based on analyses of the shear stress at the bulk (surfactant-bearing) interface and of the interfacial concentration gradient of the surfactant and consequent interfacial tension gradient that this produces in the interface. The mechanisms are rapid approach of the drop to the interface, dimple formation, even drainage, uneven drainage, and film rupture. Uneven drainage is discussed in detail, with particular reference to toluene and anisole drops at the bulk interface. Measurements gave 300–500 Å as the film thickness at rupture.