The objective of this study is to investigate the radiation-affected wave characteristics in continuum and molecular flow regime. To this ends, the first part of this thesis deals with the continuous medium while the molecular flow regime is considered in the second part.
In the continuum regime, the theoretical analyses are carried out to study the acoustic wave propagation through a hot planar gas-particle two-phase medium with emphasis on nongray two-phase radiation effect. The relaxation model is used to describe the temporal momentum interaction between gas and particles, and the differential approximation for nongray two-phase radiation is newly formulated. Then, the final mathematical expression describing the wave behavior is derived for the most general case of nongray two-phase radiation. However, in order to perform the systematic analyses, the first part is divided again into three sections according to the assumptions about the radiant energy transfer through gas-particle two-phase medium.
In the first section, it is assumed that the particles did not participate the radiation and the gas is gray. It is found that the radiation could induce the attenuation mode of which position is varied with the gas absorption coefficient in addition to the immovable mode by suspended particles. The attenuation due to radiation is greatly influenced by the absorption coefficient of the radiative medium while the dispersion remained almost unchanged. As the gas absorption coefficient increases, the attenuation mode due to radiation shifts to the higher frequency zone. This radiation effect is significantly reduced as the particle mass loading increased, since the convection became much more dominant.
The next section considers the radiative absorption and scattering by particles as well as gas. As a result, the particle radiation alone without gas radiation is able to incur the radiation-induced wave attenuation. Moreover, once the gas radiation is involved, the wave attenuation due to radiation is apparently influenced by the particles as well as gas radiation. As the gas absorption coefficient increases, the additional wave attenuation appeared only when the gas absorption coefficient became larger than the given particle absorption coefficient. The radiative absorption by particles also affects the dispersion of wave. In addition, the wave attenuation in low frequency region disappears as the scattering coefficient increases. It is also observed that the forward scattering augments the wave attenuation in low frequency region for a fixed scattering coefficient while the backward one reduces it. Finally, in a higher medium temperature the wave attenuation by radiation is seen to be enhanced.
In the last section, nongray gas radiation is taken into account while the radiative contribution by suspended particles is assumed to be negligible. The nongray radiative property of gas is modeled using WSGGM and SGM and the results are compared and discussed based on the gray gas approximation. The results show that the effects of various radiation models appeared only in the radiation-induced attenuation mode. It is the biggest for the gray gas approximation whereas the smallest for the nongray model with WSGGM. The acoustic characteristics in the induced attenuation by nongray radiation models is also found to occur through nonlinear interaction by a combination of each gray band effect.
In the molecular flow regime, the non-equilibrium effects at the stagnation wall in one-dimensional unsteady micro-flow responding to the external sinusoidal wave are investigated. First, the fluctuating behavior of the rarefied medium is simulated by employing the unsteady DSMC method. Then, another wall heat flux can be converted from the transient DSMC temperature profiles through the Maxwell-Smoluchowski relation as well as its higher order modifications. The converted fluxes are compared with the direct DSMC wall fluxes in order to evaluate the Maxwell-Smoluchowski formula. And, this part is also divided into two sections according to the medium temperature and the kind of the medium gas. In the first section, the monatomic argon is considered at room temperature, and then, the unsteady behavior of hot nitrogen medium is investigated.
In the first section, six cases are considered for various system sizes and medium densities while the excitation frequency and amplitude remains constant. Since the frequency and the amplitude are fixed, the changes of characteristic length and the medium density correspond to the variations of the acoustic Reynolds number and the Knudsen number, respectively. Consequently, It is found that there exists the more non-equilibrium effect at the stagnation wall especially when the number of reflected molecules from the diffuse wall is small. Also, due to such a non-equilibrium effect in fluctuating medium, the system behavior is found to be more sensitive to the change of characteristic length, i.e. Knudsen number. However, the Knudsen number alone is not enough to identify a given unsteady situation, therefore the acoustic Reynolds number is additionally introduced in the present study. For a sufficiently small system, the wall heat fluxes from the linearized free molecular approximation agree very well with those from DSMC except slight phase differences incurred by the molecular collision. Such facts prove that the accuracy of present DSMC output is satisfactory and mean that the linearized free molecular approximation can still describe the microscale wave propagation in some situations. Moreover, the analysis based on free molecular approximation enables the concrete discussion in mathematical aspect.
In the second section, the medium temperature is so high that the molecular vibrational energy should be involved. Similar to the first section, four cases are taken into account. Inside the medium, since the vibrational process has the larger relaxation time, the vibrational mode does not follow the fluctuation very well while the rotational mode has the definite oscillating property after some initiation period. However, the wall heat flux through molecular vibration contains the fluctuation since the oscillations are embedded in the incident number flux onto wall.