The Experts below are selected from a list of 3582 Experts worldwide ranked by ideXlab platform
Robert Holyst - One of the best experts on this subject based on the ideXlab platform.
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a molecular dynamics test of the hertz Knudsen Equation for evaporating liquids
Soft Matter, 2015Co-Authors: Robert Holyst, Marek Litniewski, D JakubczykAbstract:The precise determination of evaporation flux from liquid surfaces gives control over evaporation-driven self-assembly in soft matter systems. The Hertz–Knudsen (HK) Equation is commonly used to predict evaporation flux. This Equation states that the flux is proportional to the difference between the pressure in the system and the equilibrium pressure for liquid/vapor coexistence. We applied molecular dynamics (MD) simulations of one component Lennard-Jones (LJ) fluid to test the HK Equation for a wide range of thermodynamic parameters covering more than one order of magnitude in the values of flux. The flux determined in the simulations was 3.6 times larger than that computed from the HK Equation. However, the flux was constant over time while the pressures in the HK Equation exhibited strong fluctuations during simulations. This observation suggests that the HK Equation may not appropriately grasp the physical mechanism of evaporation. We discuss this issue in the context of momentum flux during evaporation and mechanical equilibrium in this process. Most probably the process of evaporation is driven by a tiny difference between the liquid pressure and the gas pressure. This difference is equal to the momentum flux i.e. momentum carried by the molecules leaving the surface of the liquid during evaporation. The average velocity in the evaporation flux is very small (two to three orders of magnitude smaller than the typical velocity of LJ atoms). Therefore the distribution of velocities of LJ atoms does not deviate from the Maxwell–Boltzmann distribution, even in the interfacial region.
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transport of mass at the nanoscale during evaporation of droplets the hertz Knudsen Equation at the nanoscale
Journal of Physical Chemistry C, 2013Co-Authors: M Zientara, D Jakubczyk, Marek Litniewski, Robert HolystAbstract:The applicability of the Hertz–Knudsen Equation to the evolution of droplets at the nanoscale was investigated upon analysis of existing molecular dynamics (MD) simulations ( Holyst; Phys. Rev. Lett. 2008, 100, 055701; Yaguchi; J. Fluid Sci. Technol. 2010, 5, 180–191; Ishiyama; Phys. Fluids 2004, 16, 2899–2906). The Equation was found satisfactory for radii larger than ∼4 nm. Concepts of the Gibbs equimolecular dividing surface and the surface of tension were utilized in order to accommodate the surface phase density and temperature profiles, clearly manifesting at the nanoscale. The equimolecular dividing surface was identified as the surface of the droplet. A modification to the Tolman formula was proposed in order to describe surface tension for droplet radii smaller than ∼50 nm. We assumed that the evaporation coefficient for a system in and out of equilibrium may differ. We verified that this difference might be attributed to surface temperature change only. The empirical dependencies of the evaporat...
Chuanhua Duan - One of the best experts on this subject based on the ideXlab platform.
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exploring ultimate water capillary evaporation in nanoscale conduits
Nano Letters, 2017Co-Authors: Mohammad Amin Alibakhshi, Yihong Zhao, Chuanhua DuanAbstract:Capillary evaporation in nanoscale conduits is an efficient heat/mass transfer strategy that has been widely utilized by both nature and mankind. Despite its broad impact, the ultimate transport limits of capillary evaporation in nanoscale conduits, governed by the evaporation/condensation kinetics at the liquid–vapor interface, have remained poorly understood. Here we report experimental study of the kinetic limits of water capillary evaporation in two dimensional nanochannels using a novel hybrid channel design. Our results show that the kinetic-limited evaporation fluxes break down the limits predicated by the classical Hertz–Knudsen Equation by an order of magnitude, reaching values up to 37.5 mm/s with corresponding heat fluxes up to 8500 W/cm2. The measured evaporation flux increases with decreasing channel height and relative humidity but decreases as the channel temperature decreases. Our findings have implications for further understanding evaporation at the nanoscale and developing capillary eva...
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Exploring Ultimate Water Capillary Evaporation in Nanoscale Conduits
2017Co-Authors: Mohammad Amin Alibakhshi, Yihong Zhao, Chuanhua DuanAbstract:Capillary evaporation in nanoscale conduits is an efficient heat/mass transfer strategy that has been widely utilized by both nature and mankind. Despite its broad impact, the ultimate transport limits of capillary evaporation in nanoscale conduits, governed by the evaporation/condensation kinetics at the liquid–vapor interface, have remained poorly understood. Here we report experimental study of the kinetic limits of water capillary evaporation in two dimensional nanochannels using a novel hybrid channel design. Our results show that the kinetic-limited evaporation fluxes break down the limits predicated by the classical Hertz–Knudsen Equation by an order of magnitude, reaching values up to 37.5 mm/s with corresponding heat fluxes up to 8500 W/cm2. The measured evaporation flux increases with decreasing channel height and relative humidity but decreases as the channel temperature decreases. Our findings have implications for further understanding evaporation at the nanoscale and developing capillary evaporation-based technologies for both energy- and bio-related applications
D Jakubczyk - One of the best experts on this subject based on the ideXlab platform.
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a molecular dynamics test of the hertz Knudsen Equation for evaporating liquids
Soft Matter, 2015Co-Authors: Robert Holyst, Marek Litniewski, D JakubczykAbstract:The precise determination of evaporation flux from liquid surfaces gives control over evaporation-driven self-assembly in soft matter systems. The Hertz–Knudsen (HK) Equation is commonly used to predict evaporation flux. This Equation states that the flux is proportional to the difference between the pressure in the system and the equilibrium pressure for liquid/vapor coexistence. We applied molecular dynamics (MD) simulations of one component Lennard-Jones (LJ) fluid to test the HK Equation for a wide range of thermodynamic parameters covering more than one order of magnitude in the values of flux. The flux determined in the simulations was 3.6 times larger than that computed from the HK Equation. However, the flux was constant over time while the pressures in the HK Equation exhibited strong fluctuations during simulations. This observation suggests that the HK Equation may not appropriately grasp the physical mechanism of evaporation. We discuss this issue in the context of momentum flux during evaporation and mechanical equilibrium in this process. Most probably the process of evaporation is driven by a tiny difference between the liquid pressure and the gas pressure. This difference is equal to the momentum flux i.e. momentum carried by the molecules leaving the surface of the liquid during evaporation. The average velocity in the evaporation flux is very small (two to three orders of magnitude smaller than the typical velocity of LJ atoms). Therefore the distribution of velocities of LJ atoms does not deviate from the Maxwell–Boltzmann distribution, even in the interfacial region.
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transport of mass at the nanoscale during evaporation of droplets the hertz Knudsen Equation at the nanoscale
Journal of Physical Chemistry C, 2013Co-Authors: M Zientara, D Jakubczyk, Marek Litniewski, Robert HolystAbstract:The applicability of the Hertz–Knudsen Equation to the evolution of droplets at the nanoscale was investigated upon analysis of existing molecular dynamics (MD) simulations ( Holyst; Phys. Rev. Lett. 2008, 100, 055701; Yaguchi; J. Fluid Sci. Technol. 2010, 5, 180–191; Ishiyama; Phys. Fluids 2004, 16, 2899–2906). The Equation was found satisfactory for radii larger than ∼4 nm. Concepts of the Gibbs equimolecular dividing surface and the surface of tension were utilized in order to accommodate the surface phase density and temperature profiles, clearly manifesting at the nanoscale. The equimolecular dividing surface was identified as the surface of the droplet. A modification to the Tolman formula was proposed in order to describe surface tension for droplet radii smaller than ∼50 nm. We assumed that the evaporation coefficient for a system in and out of equilibrium may differ. We verified that this difference might be attributed to surface temperature change only. The empirical dependencies of the evaporat...
Suresh K Bhatia - One of the best experts on this subject based on the ideXlab platform.
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the low density diffusion coefficient of soft sphere fluids in nanopores accurate correlations from exact theory and criteria for applicability of the Knudsen model
Journal of Membrane Science, 2011Co-Authors: Mauricio R Bonilla, Suresh K BhatiaAbstract:Molecular transport in confined spaces is of central importance to many traditional and emerging applications, such as gas separation and storage, catalytic and non-catalytic fluid–solid reactions and nanofluidics. The classical method to model the influence of solid–fluid particles collisions in transport comes from the early works of Knudsen and Smoluchowski, in which van-der-Walls interactions are neglected. While such assumption may be adequate for wide channels or high temperatures, it has been shown to lead to significant overprediction of the low-density diffusion coefficient in narrow pores at low and moderate temperatures, when compared to that obtained through molecular dynamics simulations. However, while molecular dynamics offers a much more accurate route for the calculation of diffusivities, its implementation and use is time-consuming and the Knudsen formulation remains preferred, particularly for interpretation of experimental permeation data. On the other hand, the oscillator model, developed in recent years in this laboratory, provides a straightforward method to estimate the low-density diffusivity, taking into account the dispersive forces exerted by the wall through rigorous statistical–mechanical considerations. The computational demand of the oscillator model is several orders of magnitude lower than that of a typical molecular dynamics run, though much greater than that of the simple Knudsen formulation. In order to facilitate its use, we provide several simple correlations for the fast estimation of the oscillator model-based diffusion coefficient for LJ fluids in simple geometries, as a function of the pore size and the LJ fluid–solid interaction parameters. Moreover, the parameter values for which the Knudsen Equation supplies a reasonable estimation of the diffusion coefficient are presented for various degrees of accuracy.
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some pitfalls in the use of the Knudsen Equation in modelling diffusion in nanoporous materials
Chemical Engineering Science, 2011Co-Authors: Suresh K Bhatia, D NicholsonAbstract:Abstract The Knudsen model of diffusion in small pores, originally verified in macropores, is widely applied at the mesopore scale with adsorption effects neglected, largely based on linearity of the T / M correlation. Here, we show that this approach is misleading, and that the correlation masks inconsistencies arising from neglect of van der Waals forces in the Knudsen model. We examine the tortuosity for diffusion of light gases in nanoporous carbons using the Oscillator model of low pressure transport developed in the first author’s laboratory, which incorporates van der Waals interactions. Pore network effects are considered through a hybrid correlated random walk-effective medium theory approach. It is shown that in the presence of a pore size distribution the apparent tortuosity is not a porous medium property alone, but depends also on the temperature and on the diffusing molecule, because of the temperature and the gas-dependent short circuiting effects associated with pores that have high conductance. This short circuiting effect leads to a complex and rich variety of behaviour with respect to pore size, temperature and diffusing gas, which is consistent with experimental evidence, but is absent when the Knudsen model is used with adsorption effects neglected. It is shown that when effects of adsorption on equilibrium and transport are overlooked the commonly used correlation of diffusivity with T / M is deceptive, as the product of the adsorption equilibrium constant and diffusivity (or pore conductance) also approximately scales linearly with the Knudsen diffusivity (i.e. with T / M ). Such behaviour is found for diffusion in mesoporous carbons and silica as well as in silicon. Consequently, claims of validity of the Knudsen model based on such a correlation may be misconceived.
Daniel C Allgood - One of the best experts on this subject based on the ideXlab platform.
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analysis of flame deflector spray nozzles in rocket engine test stands
46th AIAA ASME SAE ASEE Joint Propulsion Conference & Exhibit, 2010Co-Authors: Jai Sachdev, Vineet Ahuja, Ashvin Hosangadi, Daniel C AllgoodAbstract:The development of a unified tightly coupled multi-phase computational framework is described for the analysis and design of cooling spray nozzle configurations on the flame deflector in rocket engine test stands. An Eulerian formulation is used to model the disperse phase and is coupled to the gas-phase Equations through momentum and heat transfer as well as phase change. The phase change formulation is modeled according to a modified form of the Hertz-Knudsen Equation. Various simple test cases are presented to verify the validity of the numerical framework. The ability of the methodology to accurately predict the temperature load on the flame deflector is demonstrated though application to an actual sub-scale test facility. The CFD simulation was able to reproduce the result of the test-firing, showing that the spray nozzle configuration provided insufficient amount of cooling.
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analysis of flame deflector spray nozzles in rocket engine test stands
46th AIAA ASME SAE ASEE Joint Propulsion Conference & Exhibit, 2010Co-Authors: Jai Sachdev, Vineet Ahuja, Ashvin Hosangadi, Daniel C AllgoodAbstract:The development of a unified tightly coupled multi-phase computational framework is described for the analysis and design of cooling spray nozzle configurations on the flame deflector in rocket engine test stands. An Eulerian formulation is used to model the disperse phase and is coupled to the gas-phase Equations through momentum and heat transfer as well as phase change. The phase change formulation is modeled according to a modified form of the Hertz-Knudsen Equation. Various simple test cases are presented to verify the validity of the numerical framework. The ability of the methodology to accurately predict the temperature load on the flame deflector is demonstrated though application to an actual sub-scale test facility. The CFD simulation was able to reproduce the result of the test-firing, showing that the spray nozzle configuration provided insufficient amount of cooling.
