Evaporation Boundary Conditions for the Linear R13 Equations Based on the Onsager Theory.

Area of Science:

• Fluid dynamics
• Rarefied gas dynamics
• Computational physics

Background:

• Modeling rarefied gases and microflows is challenging due to continuum hypothesis limitations at higher Knudsen numbers.
• Macroscopic transport equations and particle methods like Direct Simulation Monte Carlo (DSMC) are used for accurate solutions.
• Understanding evaporation in microflows is crucial for applications like micro fuel cells.

Purpose of the Study:

• Derive novel evaporation boundary conditions for the R13 equations, suitable for rarefied gas regimes.
• Enhance the accuracy of macroscopic transport models in microflow simulations.
• Investigate evaporation phenomena in microfluidic devices.

Main Methods:

• Derived new evaporation boundary conditions for R13 equations using Onsager relations.
• Fitted derived boundary conditions to DSMC data for coefficient determination.
• Compared new R13 boundary conditions against kinetic theory and Navier-Stokes-Fourier (NSF) solutions.
• Implemented and tested new R13 boundary conditions in a 2D numerical code.

Main Results:

• The new phenomenological R13 boundary conditions demonstrated superior agreement with DSMC data compared to existing kinetic theory R13 boundary conditions.
• Numerical simulations in 2D geometries revealed distinct flow patterns between R13 and NSF models at higher Knudsen numbers.
• The derived conditions offer improved modeling capabilities for rarefied gas microflows.

Conclusions:

• The developed R13 evaporation boundary conditions provide a more accurate macroscopic approach for rarefied gas microflows.
• The findings highlight the importance of appropriate boundary conditions for capturing complex flow physics.
• This work advances the modeling of evaporation in microfluidic systems relevant to energy applications.