טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentBen Ami Yaron
SubjectEffect of Thermal Boundary Conditions on Heat and Mass
Transfer Processes in Rarefied Gas Flows
DepartmentDepartment of Aerospace Engineering
Supervisor ASSOCIATE PROF. Avshalom Manela
Full Thesis textFull thesis text - English Version


Abstract

Studies on rarefied gases consider gas flows where the characteristic length or time scale is of the order of the molecular mean-free path or time, respectively. Under such conditions, the hydrodynamic continuum description breaks down, and the microscopic properties of the medium must be taken into account. These scenarios prevail at small-scale mechanical systems applications, as well as at low-pressure conditions, encountered in outer space or vacuum chambers. In marked difference from incompressible flow fields, the coupling between the dynamic and thermodynamic descriptions of rarefied gas flows is normally inevitable, and may be induced through externally-imposed thermal excitation of a surface. These effects become more pronounced in highly rarefied gases, as the low molecular collision rate turns the system far from equilibrium. While the imposition of thermal boundary conditions is expected to have a significant effect on a given flow field, traditional studies of rarefied gas flows have been limited to gas-surface interactions where the surface temperature is prescribed. Such an assumption, however, may be of limited practical value, as in most experimental setups the direct imposition of boundary heat-flux is more easily achieved than prescription of surface temperature.

In view of the above, the goal of the present research is to explore the effect of thermal conditions at a solid boundary on heat and mass transfer phenomena in rarefied gas flows. The work examines a variety of non-equilibrium gas flow setups, where thermal boundary conditions have a significant effect, including vibro- and thermo-acoustic flow setups, nonlinear shear flows, and convective instabilities. In each of the problems considered, the analysis combines numerical simulations (based on the direct simulation Monte Carlo method) with analytical limit-case investigations, where the latter is carried out in both continuum and ballistic-flow limits. In the continuum limit, the effect of thermal conditions is manifested through a functional modification in the type of applied wall conditions. In the ballistic limit, the heat-flux condition is imposed by an integral equation for the unprescribed wall temperature. In the context of numerical computations, a novel non-iterative algorithm that directly imposes the wall heat-flux conditions (rather than the traditional fixed-temperature conditions) within the Monte Carlo simulations was derived and tested successfully in a variety of benchmark flow problems. The work demonstrates that the functional differences in the thermal constraints between temperature-prescribed and heat-flux-prescribed surfaces have a significant effect on the flow-field in the entire range of gas rarefaction rates.