A molecular dynamics study on thin film liquid boiling characteristics under rapid linear boundary heating: Effect of liquid film thickness Kazi Fazle Rabbi, Saiful Islam Tamim, A. H. M. Faisal, K. M. Mukut, and Mohammad Nasim Hasan
Citation: AIP Conference Proceedings 1851, 020102 (2017); doi: 10.1063/1.4984731 View online: http://dx.doi.org/10.1063/1.4984731 View Table of Contents: http://aip.scitation.org/toc/apc/1851/1 Published by the American Institute of Physics
Articles you may be interested in Molecular dynamics study on evaporation and condensation characteristics of thin film liquid Argon on nanostructured surface in nano-scale confinement AIP Conference Proceedings 1851, 020105 (2017); 10.1063/1.4984734
A Molecular Dynamics Study on Thin Film Liquid Boiling Characteristics under Rapid Linear Boundary Heating: Effect of Liquid Film Thickness Kazi Fazle Rabbia), Saiful Islam Tamim b), A.H.M Faisal c), K.M. Mukut d), Mohammad Nasim Hasan e) Department of Mechanical Engineering, Bangladesh University of Engineering And Technology, Dhaka-1000, Bangladesh a)
[email protected] [email protected] c)
[email protected] d)
[email protected] e) Corresponding author:
[email protected] b)
Abstract. This study is a molecular dynamics investigation of phase change phenomena i.e. boiling of thin liquid films subjected to rapid linear heating at the boundary. The purpose of this study is to understand the phase change heat transfer phenomena at nano scale level. In the simulation, a thin film of liquid argon over a platinum surface has been considered. The simulation domain herein is a three-phase system consisting of liquid and vapor argon atoms placed over a platinum wall. Initially the whole system is brought to an equilibrium state at 90 K and then the temperature of the bottom wall is increased to a higher temperature (250K) within a finite time interval. Four different liquid argon film thicknesses have been considered (3 nm, 4 nm, 5 nm and 6 nm) in this study. The boundary heating rate (40x109 K/s) is kept constant in all these cases. Variation in system temperature, pressure, net evaporation number, spatial number density of the argon region with time for different film thickness have been demonstrated and analyzed. The present study indicates that the pattern of phase transition may be significantly different (i.e. evaporation or explosive boiling) depending on the liquid film thickness. Among the four cases considered in the present study, explosive boiling has been observed only for the liquid films of 5nm and 6nm thickness, while for the other cases, evaporation take place. Keywords. Molecular dynamics simulation; Thin liquid film; Evaporation; Explosive boiling; Boundary heating rate; Phase transition; Heat transfer efficiency
BACKGROUND Use of molecular dynamics to analyze phase change heat transfer have become popular in recent times. The ability to observe phase change phenomena at nano scale level intrigues researchers to expand their study. By simulating atomic and molecular motions atomistic insight into molecular structure and kinetics can be gained. Molecular behavior can be predicted and experimental observations can be compared or interpreted through this method. Studies have been conducted in this manner with a view to understanding nano scale heat transfer mechanisms and develop solutions to problems and questions related to evaporation/explosive boiling at nano scale surfaces. Yi et al. [1] simulated the vaporization phenomenon of an ultra-thin layer of liquid argon on a platinum surface for two different superheat temperatures. Yu and Wang [2] performed NEMD simulation to study the evaporation of the thin film, equilibrium vapor pressure as well as non-evaporating liquid layer in a nano scale triple-phase system. Morshed et al. [3] performed molecular dynamics simulation to study the effect of nanostructures on evaporation and explosive boiling of thin liquid films. Seyf and Zhang [4] performed non-equilibrium molecular dynamics to
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study effect of nano textured array of conical features on explosive boiling over flat substrate. Shavik et al. [5] performed molecular dynamics study to investigate the effect of nano-slotted surface under different wetting conditions on explosive boiling of thin liquid argon film. Wang et al. [6] monitored the space and time dependences of temperature, pressure, density number, and net evaporation to investigate the phase transition process on a flat surface with and without nanostructures. Hens et al. [7] investigated bubble formation on a platinum substrate with particular emphasis on the surface texture. Maroo and Chung [8] performed molecular dynamics simulation of platinum heater and associated nano-scale liquid argon film evaporation and colloidal adsorption characteristics. Hasan et al. [14] studied the effect of boundary heating rate on the phase change characteristics of thin film liquid. Hasan et al. [15] studied the effect internally recessed nanostructured surface on evaporation and explosive boiling of thin film liquid argon. Hasan et al. [16] studied the effect of wettability on evaporation characteristics of thin film liquid argon in nano-confinement. However, the effect of variation of liquid film thickness for constant heating rate has rarely been investigated in the previous literatures. Nomenclature r t b T TE Tw P
Distance between molecules (Å) Time heating rate Temperature (K) Initial Temperature at stage three (K) Final Temperature at stage three (K) Pressure
Greek Symbols ε Energy parameter of LJ potential (eV) σ Length parameter of LJ potential (Å) ϕ Energy (eV) Subscripts Ar Argon Pt Platinum
In the present work, a comprehensive molecular dynamics investigation was conducted to improve the understanding the effect of different film thickness on the phase change heat transfer from the thin liquid argon film during explosion boiling. Explosion boiling was simulated by increasing the surface temperature very quickly from 90 K to 250 K. Four different cases were considered all of which included increasing of surface temperature from 90 K to 250 K with film thickness of 3nm, 4 nm, 5nm and 6nm. In this study from the molecular dynamics simulations the surface heat flux normal to the surface, evaporation rate of liquid argon, temperature and pressure variation of argon region with time, spatial temperature and pressure distribution etc. were obtained and compared to study the variation and the reason of such variation in these four cases.
SIMULATION METHOD The three-phase system domain consisting of platinum plate at the bottom and argon region above it, is shown in Fig. 1. The argon region was liquid argon near the bottom plate and vapor argon molecules above it. The domain was enclosed within a dimension of 7.5nm(x) × 70nm(y) × 7.5nm (z). The platinum plate had eight monolayers with the bottom layer fixed to the boundary and the next two layers were used as a source of heat. The remaining five layers conducted heat to the liquid argon. Approximately 5476 platinum atoms were arranged in FCC (1 0 0) lattice structure for each plate with a density of 21.45 × 10 3 kg/m3. Liquid argon layer height on top of them were changed from initially 3 nm to 6 nm corresponding to its density of 1.367 × 10 3 kg/m3. The rest of the space were filled with argon vapor atoms whose number varied with the variation in the number of liquid molecules. A non-periodic fixed boundary condition was applied at the top in Y direction. This allowed for the containment of the gaseous atoms within the simulation domain. Also, periodic boundary conditions were applied to the x and z directions. In order to calculate the intermolecular forces for performing the molecular dynamics simulation, the Lennard-Jones potential [10] was used:
ª§ V ·12 § V ·6 º I (r ) 4H «¨ ¸ ¨ ¸ » «¬© r ¹
© r ¹ »¼
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(1)
Here, ε is the energy parameter and σ is the length parameter. According to Hens et al. [7], εAr-Ar < εAr-Pt for hydrophilic substrate was considered throughout this study. Their corresponding values in this study are listed below: TABLE 1. LJ potential interaction parameters
Interaction Argon-Argon
Energy parameter (Joule) εAr-Ar= 1.67 ×10
−21
σAr-Ar = 3.405
−21
Argon-Platinum
εAr-Pt= 3.27 × 10
Platinum-Platinum
εPt-Pt = 8.32 × 10−20
Length parameter (nm) σAr-Pt= 2.94 σPt-Pt= 2.475
In this study 4σAr-Ar cutoff distance for L-J potential was used. Time step of 5 fs was chosen for whole simulation. These parameters were chosen in a way to describe the mutual or reciprocal actions between atoms. First the entire system was maintained at 90K under equilibrium molecular dynamics using langevin thermostat and the system was run for 0.5 ns. Once the whole system was in equilibrium the thermostat was removed only for the fluid domain and the system was again run for 0.5ns for equilibration. Then the wall temperature was raised from 90K to 250K within a finite heating period of 4ns. Now th being the time within which bottom wall temperature increases from equilibrium temperature TE (90K) to the target temperature Tw (250K), the heating rate (b) can be obtained from the following equation:
Tw=TE+bth
(2) 9
This provided a boundary heating rate of b=40x10 K/s which was kept constant for all the cases studied. In Hasan et al. [14] eight different boundary heating rates were considered for fixed liquid film thickness. Here in 9 this study four different liquid thickness for fixed heating rate of b=40x10 K/s was considered. In the final stage the system was run for some more time to reach a steady condition. In total, each of the simulations took place for 6ns. To check whether the argon is in equilibrium state the temperature, pressure was monitored during the equilibration period. The simulations were performed using LAMMPS [11] and visualization was done by VMD (Visual Molecular Dynamics) [12] and OVITO (Open Visualization Tool) [13].
RESULTS AND DISCUSSION In this study after the equilibration period from 0.5 to 1 ns of the simulation the wall temperature was increased from 90K to 250K within the next 4ns at the heating rate of 40x10 9 K/s. As a result for the four different liquid film thickness, varying change in the argon region was observed. This variation is demonstrated in Fig. 2
FIGURE 1. Initial configuration of the simulation
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FIGURE 2. Temperature History of argon region
Here it can be seen that, from 0.5 to 1ns in the equilibration period the temperature is constant at 90K proving that the system was in equilibrium. After the first 1ns, the argon region temperature starts to rise for all four cases, with varying rapidity with the increase of the wall temperature. Temperatures rise faster for smaller liquid thickness of 3nm and 4nm than the larger thickness of 5 nm and 6nm. At 5 ns, when the wall reaches 250 K and remains steady afterwards, none of the four cases of film thickness reaches steady state, and temperature keeps on increasing. After 6 ns the temperatures for 3nm and 4nm liquids were in the scale of 220K, while the temperatures for 5nm and 6nm liquids were in the scale of 160K. It can be interpreted from the Figure 2 that for higher liquid film thickness (5nm and 6nm) comparatively more time is required to reach the wall temperature. This phenomenon occurring for the higher liquid film thickness is more clearly explained using the snapshots of the simulation domain as shown in Fig. 3.
(b)
(a)
(c) (d) FIGURE 3. Snapshots from the simulation domain for liquid thickness (a) 3nm (b) 4nm (c) 5nm (d) 6nm at b=40x10 9 K/s boundary heating rate
Snapshots from the simulation domain are displayed in Fig. 3 which gives molecular insight into the phase transition for dissimilar cases. Snapshots of Fig. 3(a) and 3(b) illustrates the boiling phenomena occurring with number of argon atoms moving from liquid to vapor phase. Figures 3(c) and 3(d) depicts cluster of argon liquid atoms moving upwards. This indicates the explosion boiling phenomena. When the temperature of the wall is increased very high very quickly, the liquid argon nearest to the wall exceeds the critical temperature and vaporizes, with the argon atoms just above it still in liquid phase. This vapor creates a pressure from below the liquid layers and forces them upwards. Of course, in all cases lower layers, which are adjacent to platinum wall, absorbs heat quicker than the layers immediately above them. For large enough film thickness, i.e. for 5nm and 6nm the lower layers absorb enough heat to evaporate even before the heat reaching the top layers. This is also why bigger liquid
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molecule clusters are pushed upwards in case of thicker liquid films. This cluster formation also helps explain the disparity in temperature rise for different film thickness, discussed for Fig. 2. With more molecules forming clusters, heat is transferred slower on to all of them, and temperature rise, as a consequence, is slower. Figure 4 shows, since system volume remains constant, increase in temperature results in the increase in pressure following similar trends discussed for Figure 2. In the equilibration period from 0.5 to 1ns the pressure is constant around 3 to 5 bar following the constant temperature proving the system was in equilibrium. Figure 5 shows the spatial number density of argon along the y axis at 4ns of the simulation. The number density, which is otherwise quite constant and close to low values of 0.001, rises abruptly at heights around 20nm. This perfectly corresponds with our observation of explosive boiling and resulting cluster formation for 5nm and 6nm films at the height of 20nm during 4ns of the simulation. These clusters forming at 15-20 nm heights from the platinum wall results in a high number of argon atoms found at those heights. Also, a higher value of number density indicates bigger clusters being formed, which corroborates with Fig. 3(c) and 3(d).
FIGURE 4. Pressure history of argon region
FIGURE 5. Spatial density distribution of argon region along y axis at 4ns
Figure 6 shows the spatial temperature distribution during 4ns of the simulation domain for all the four cases of liquid film thickness. Lower argon temperature for higher film thickness can be seen in Fig. 6. This happens because of the formation and presence of cluster argon which slows down the heat transfer, and thus resulting in low temperatures. It can be seen that temperatures drop quite sharply in case of 5nm and 6nm liquid films at heights from 10nm to 20nm. This is the exact region of the simulation domain where, according to Fig. 3, formation of molecule clusters is observed.
FIGURE 6. Spatial temperature distribution for argon region along y axis at 4ns of simulation
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Figure 7 illustrates net evaporation number increasing and becoming steady after a period. The highest liquid film thickness (6nm), has the highest evaporation number and takes the most time (after 3ns) to reach this value. On the contrary the lowest liquid film thickness (3nm), has the lowest evaporation number and takes the least time (after 3ns). This is because the higher film thickness has higher number of argon and it takes longer for all of the argon to evaporate. The increase of evaporation number for the highest film thickness is very sharp compared to the gradual increase of the lowest film thickness case. This occurs primarily due to explosive boiling which takes place for the higher value of film thicknesses (5nm and 6nm). Here, with the liquid layers at the lowest level pushing the ones above it upwards resulting in explosive boiling, more liquid argons reach the vapor region. With no explosive boiling occurring, steady and gradual increase of evaporation number takes place for 3nm and 4nm liquid film.
FIGURE 7. Net evaporation number
FIGURE 8. Heat flux through the bottom plate
In Fig. 8, heat fluxes normal to the bottom plates are shown. Here it can be seen that for the cases with smaller liquid film thickness (3nm and 4nm) the heat flux drops to zero at 3ns and 3.5ns respectively. This can be explained using Figure 3. In Fig. 3(a) and 3(b) which illustrates that for these cases almost all the liquid has evaporated after this time resulting in drop of heat flux. For 4nm and 6nm cases most liquid evaporates from the surface slightly after 3.5 ns and 4 ns respectively and hence the drop in heat flux for these two cases is illustrated in Figure 8 at the corresponding time. The heat flux profiles are in good agreement with previous studies conducted by Shavik et al. [5], Maroo and Chung [8], Yamamoto and Matsumoto [9], and Hasan et al. [14].
CONCLUSION In this study, explosive boiling occurred when the pressure built at liquid-solid interface was high enough to force the upper liquid layers upward for the different film thicknesses at the heating rate of 40x109 K/s. When the liquid thickness is not high enough, the liquid layers gets uniformly heated resulting in gradually evaporation. Hence the rise of evaporation number is sharp in cases of film thicknesses high enough for explosion boiling, on the other hand gradual in case of lower film thicknesses. In case of the higher film thicknesses explosion boiling forms clusters and with formation of clusters, heat is transferred slower to the liquid argon due to discontinuity. Changes in pressure, temperature and spatial number density distribution is also in agreement with the occurrence of the explosive boiling phenomena. Here in this present study for the heating rate of 40x109 K/s explosive boiling occurred in case of thickness 5nm and 6nm. But did not occur in case of 3nm and 4nm. Further studies can be pursued to determine the critical film thickness required for the occurrence of explosive boiling with variation heating rates.
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