Lin
Shi
,
Chengzhi
Hu
*,
Changli
Yi
,
Minli
Bai
,
Jizu
Lyu
and
Linsong
Gao
Key Laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of Education, School of Energy and Power Engineering, Dalian University of Technology, Dalian 116024, China. E-mail: shixdcs@163.com; Fax: +86-0411-84706305; Tel: +86-0411-84706305
First published on 13th December 2022
Non-equilibrium molecular dynamics simulations of liquid flow through the surface were performed to investigate the flow resistance and thermal resistance under conditions of different solid–liquid interactions and surface temperatures. A novel phenomenon was observed in the simulation, namely the rise of surface temperature increases the flow resistance when solid–liquid interaction is weak, but decreases the flow resistance when solid–liquid interaction is strong. A higher density of the boundary layer brings a larger friction force to increase the flow resistance. For heat transfer, it is innovative to calculate the heat conduction and convection of the boundary region discretely. The results showed that the heat transfer performance of the interface is not positively correlated with the boundary liquid density, and the structure of the boundary liquid is also crucial. We believe that this research can improve the existing theory of flow heat transfer and provide a more effective method for analyzing the flow heat transfer of the solid–liquid interface.
(1) |
For the heat transfer in the solid–liquid interface, due to the mismatch of the electronic characteristics and vibration characteristics of the solid and liquid, scattering occurs when a carrier (phonon or electron, depending on the properties of the material) attempts to pass through the contact interface; hence there will be a thermal resistance in the interface.2–4 Similar to velocity slip length, thermal resistance can be represented by Kapitza length lK, which is shown in Fig. 1(b) and predicted as5
(2) |
In recent years, the problems of flow resistance and heat transfer have become increasingly prominent in various industrial fields. Surface wettability not only has a significant effect on flow resistance,6–12 but can also affect heat transfer.13,14 In general, the hydrophobic surface favors drag reduction15 but worsens heat transfer,16 while the hydrophilic surface improves heat transfer16,17 but deteriorates drag reduction. This phenomenon is attributed to the different surface energies under various wettability conditions, which result in different structures or phases of fluid in the boundary region. However, conventional macro-research methods are inapplicable to studying the microcosmic structure of fluid in the interface. Therefore, it is vital and urgent to study the flow resistance and heat transfer in the microscale.
As an alternative to experiments, molecular dynamics simulation (MDS) has been widely accepted as a powerful tool for interfacial studies due to its low cost and high resolution. In 1989, it was proved by Koplik18 that MDS can reproduce the interfacial properties of solid–liquid at a low Reynolds number. In recent years, many researchers have proved that wettability, which can be represented by solid–liquid interaction in MDS, is one of the important factors affecting flow resistance by MDS. Cao19 pointed out that a liquid can easily slip over poorly wetted surfaces. Sofos20 calculated the effective slip length using MDS and found that as the hydrophilicity increases, the effective slip length decreases. Zhang21 found that the extent of velocity slip is related to the solid–liquid interaction, and it is believed that there are different fluid structures in the solid–liquid boundary region, which bring about a change in energy dissipation and affect velocity slip. Nagayama22 and Zhang23 pointed out that when the solid–liquid interaction is weak, there is a low-density layer in the boundary region that can lead to slip. It is easy to find that a large slip length can be induced by a weak solid–liquid interaction, and most researchers attribute it to the low-density layer adjacent to the surface.
The above studies provide effective evidence for the effects of wettability on flow resistance by MDS. In addition to flow resistance, heat transfer at the solid–liquid interface has also attracted much attention. The interest in thermal transport through the solid–liquid interface began with the early findings of Kapitza24 who found a temperature discontinuity between the solid surface and liquid helium, but to date, there has been no theory that fully explains the mechanism of the thermal resistance at the interface. Thermal resistance is affected by several factors, including wettability,25 the internal structure of the solid surface,26 surface structures27,28 and others. Regarding the wettability (solid–liquid interaction), Kim5,29 investigated heat conduction between parallel plates separated by a thin layer of liquid argon using MDS, and the results showed that the thermal resistance decreases with increasing wettability. Jabbari30 investigated the heat transfer between liquid and carbon-based particles, and the results showed that the thermal resistance decreases with increasing solid–liquid interaction. Cao31 also pointed out that the hydrophilic surface enhances heat transfer in the solid–liquid interface by MDS. Generally, the influence of wettability on heat transfer is attributed to the change in liquid density at the interface.32 Furthermore, the study of heat transfer in non-equilibrium MDS usually needs to generate heat flux or temperature gradient in the system; hence the effects of temperature on heat transfer must also be explored. Barisik26 found that the effect of temperature on thermal resistance is related to the solid–liquid interaction; when the solid–liquid interaction is strong, the interface thermal resistance increases as the temperature rises, but under weak solid–liquid interaction conditions, the thermal resistance decreases as the temperature of the surface increases. In summary, the change in fluid density/structure near the surface affects the heat transfer properties of the interface.
A lot of flow and heat transfer phenomena often occur simultaneously, and heat transfer can be affected by flow, so it is necessary to combine the two phenomena for study. Khare33 investigated the relationship between velocity slip and temperature jump in laminar shear flows, and found that temperature jump could exist without velocity slip, and velocity slip enhances temperature jump. Sun34,35 found that the slip length and the Kapitza length can be correlated by a piecewise function that is separated at a critical solid–liquid interaction, the Kapitza length grows as a power function of the slip length when the interaction is weak, while it remains almost constant when the interaction is strong. Although the above research has summarized the relationship between velocity slip and temperature resistance, the physical mechanisms are still unclear.
In summary, the existing studies have tended to focus on the effects of wettability on velocity slip or thermal resistance independently. Although a few studies have combined flow and heat transfer, the physical mechanisms have not been fully explored. In particular, the influences of surface temperature on flow and heat transfer also need to be explored. Moreover, most existing studies pay more attention to the mathematical dependence between surface properties and flow characteristics, the discussion of mechanism only stays in the stage of speculation or qualitative, and the way the surface properties affect the flow characteristics is still unclear. Therefore, the purpose of this paper is to investigate how the solid–liquid interaction and the surface temperature affect the flow resistance and interfacial thermal resistance. By calculating the potential energy of the fluid in the boundary region, the physical mechanism of the influence of the fluid density on the flow resistance is explored. More importantly, by discretely studying the heat conduction and convection, the influence of the fluid structure in the boundary region on the thermal resistance is investigated in detail. We believe that this work can provide new ideas and methods for the study of flow heat transfer in micro/nano scales.
In the simulations, the 6-12 Lennard-Jones potential with cutoff distance 3.5 σll was applied between the fluid atoms:
(3) |
(4) |
(5) |
In this paper, density ρ and temperature T were represented by reduced units as shown below:
ρ* = ρσ3 | (6) |
T* = kT/ε | (7) |
The simulation system was relaxed for 300 ps, and the temperature of system was held at T* = 0.73 by using a Nose–Hoover thermostat. Then the temperature of the lower surface was turned up to , and the NVE assemble was applied in the fluid. The upper surface started moving with U = 0.25 Å ps−1. The shearing and temperature gradients were imposed simultaneously. After a few picoseconds the system was stable as shown in Fig. 3, and the data statistic was obtained in the last 2000 ps. In addition, the energy dissipation induced by the friction is ignored, which is discussed in S1 (ESI†). All simulations were carried out using a Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS).
Fig. 4 Density profile adjacent surface with different β and Th and coexistence curve for the LJ fluid.36 (a) β = 0.2; (b) β = 0.6; (c) β = 1; (d) coexistence curve. |
The amplitude and range of the density oscillations increase significantly as the β increases in Fig. 4(a)–(c), which is consistent with the previous research.22,23,44 The density profile is also dependent on surface temperature. It can be seen that the density near the surface increases as the temperature rises when β = 0.2. However, when β = 0.6, surface temperature has little influence on the density profile. When β = 1, the density near the surface decreases as the temperature rises.
Fig. 4(d) is the coexistence curve for the LJ fluid, it is easy to judge the phase of fluid by calculating the dimensionless temperature (T*) and density (ρ*) of Ar, and this method is widely used in enclosed systems.45,46 Based on Fig. 4(d), (the density of fluid near the surface), (the temperature of fluid near the surface) and the phase states of fluid near the lower surface are summarized in Table S1 (ESI†). When β = 0.2, the phase of fluid near the lower surface is a mixture of gas and liquid or pure liquid, the density of which is lower than the bulk liquid. When β > 0.6, the phase of fluid adjacent to the surface is solid.
Fig. 5 shows in a more intuitive way the change in the liquid density near the surface at different solid–liquid interactions and temperatures. In Fig. 5(a), there are only 2 layers with higher density. The light blue part in the figure is the low-density region where the distribution of liquid atoms is sparse and random. In Fig. 5(c), the number of fluid layers with higher density increases obviously, and the distribution is more regular. In Fig. 5(e), the solid–liquid interaction is stronger, and an ordered liquid structure (solid-like layer) appears adjacent to the lower surface. The result is similar to previous studies.43 In essence, the distribution of fluid near the surface is the result of the competition between liquid–liquid and solid–liquid interactions. When β is small, the attraction between solid–liquid is relatively small, and the fluid atoms form a low-density layer near the surface under the attraction of the bulk liquid. When β is large, the attraction between solid and liquid atoms increases, the distribution of fluid adjacent to the surface will affected by the force field of solid surface, and the lattice arrangement of Cu leads to a regular force field, inducing the regular arrangement of fluid atoms near the surface.
On the other hand, surface temperature also affects the distribution of the fluid near the surface. However, the effects of on the distribution of fluid near the surface are not the same when β changes. Fig. 6 shows how the fluid structure near the lower surface changes with increasing temperature under conditions with different solid–liquid interactions. The gap between the fluid and the solid surface becomes smaller as rises; when β = 0.2, the Brownian motion of the atoms increases in the bulk liquid with rising temperature, and more fluid atoms enter the gap in the boundary region and lead to the increase of density. When β = 0.6, the change in has little influence on the density, which is conjected to be due to the pressure of the bulk liquid and boundary region being always in equilibrium as rises. Comparing Fig. 5(e) with Fig. 5(f), it can be found that the ordered structure is destroyed as rises; when β = 1, the liquid with higher density acquires enough kinetics to get rid of the bonds of the solid atoms, resulting in a decrease in the density.
Furthermore, to quantitatively describe the influence of solid–liquid interaction and surface temperature on fluid structure, the radial distribution function (RDF) g(r) was calculated by eqn (8) to discuss the structure of fluid adjacent to the surface:
(8) |
The curve in Fig. 7(a) agrees with the characteristics of the short-range order of amorphous structure. However, the curves in Fig. 7(b) and agree with the long-range order of crystal structure, indicating that there is a crystallized structure adjacent to the surface when β = 1. For the effects of surface temperature, there is a slight increase in the first peak of g(r) as rises when β = 0.2 in Fig. 7(a), indicating that the distribution of atoms is more compact. When β = 1, the fluctuation degree of the curve decreases significantly as rises, and the solid-like structure is destroyed to a certain extent.
Fig. 8 shows the effects of β and on velocity profiles and velocity slip length. The velocity near the lower surface decreases dramatically when β increases, and the change in the fluid structure near the surface is the main reason for this phenomenon. Based on the velocity profile, velocity slip length ls can be obtained easily by eqn (1), which is shown in Fig. 8(b). The results show that ls exceeds 200 Å when β = 0.2, and the negative slip occurs when β = 1, which is a typical feature of super-hydrophilic surface. This part of the results is consistent with the conclusion of other studies.20,43 In addition to the effects of solid–liquid interaction, the more interesting part in Fig. 8 is the effect of the surface temperature on the velocity and slip length. When β = 0.2, it can be found that ls decreases as rises, but ls increases When β = 1. For a detailed discussion of velocity and velocity slip length please refer to S3 (ESI†).
(9) |
Fig. 11 shows q, qk and qc when β = 1 are significantly higher than that when β = 0.2. The percentage of kinetic part decreases with the increase of β, and heat conduction plays a more important role in the whole heat transfer process, suggesting that the low-density layer has relatively weak heat transfer capability, which blocks heat transfer from heat source to fluid, and the crystallized fluid near the surface has the characteristics similar to lattice vibration, thus improving its internal thermal conductivity near the surface.
The heat flux at different surface temperatures is also compared in Fig. 11. Although it is clear that q increases as rises under both weak and strong solid–liquid interaction conditions, the mechanisms of the two conditions are different. When β = 0.2, due to fluid near the surface is gas–liquid mixed phase at , a small increase in density can significantly improve both qk and qc. When β = 1, it is noted that there is a slight increase in qc despite the density of the fluid in the boundary region decreasing as rises, which is conjected to be due to the melted solid-like structure improving the contact between the fluid and solid as shown in Fig. 6(b). Furthermore, qk increases remarkably as rises when β = 1; the fluidity of the liquid atom is significantly enhanced due to high temperature, which enhances heat convection. Therefore, the main reason for the improved heat transfer by increasing the surface temperature under strong solid–liquid interaction is the enhancement of heat convection, which is caused by the change in the fluid structure in the boundary region.
(1) For flow resistance, the low-density layer reduces the flow resistance when the solid–liquid interaction is weak. With the increase of solid–liquid interaction, the fluid density in the boundary region increases, and the arrangement of fluid atoms is more regular, resulting in a greater flow resistance. The influence of surface temperature on flow resistance should be divided into two conditions. When the solid–liquid interaction is weak, the rise in surface temperature leads to the increase of fluid density in the boundary region then the resistance increases; when the solid–liquid interaction is strong, the higher surface temperature destroys the solid-like structure in the boundary region and reduces the resistance.
(2) For heat transfer, the thermal resistance decreases as the solid–liquid interaction increases. The larger fluid density in the boundary region mainly enhances heat conduction as the solid–liquid interaction increases. More importantly, under both the strong and weak interaction conditions, the increase of surface temperature reduces the thermal resistance, which is inconsistent with the change in density. When the solid–liquid interaction is weak, the increase in the surface temperature leads to the increase of fluid density in the boundary region and both heat conduction and heat convection are enhanced; when the solid–liquid interaction is strong, despite the density of the fluid in the boundary region decreasing, the destroyed solid-like structure will enhance both heat conduction and heat convection.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2cp03905b |
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