Unsteady heat transfer of a monatomic gas between two coaxial circular cylinders
We consider a kinetic-theory treatment of the cylindrical unsteady heat transfer. A model kinetic equation of the BGK (Bhatnager-Gross-Krook) type is solved using the method of moments with a two-sided distribution function. We study the relations between the different macroscopic properties of the gas as the temperature, density, and heat flux with both the radial distance r and the time t. Also we study the problem from the viewpoint of irreversible thermodynamics and estimate the entropy, entropy production, entropy flux, thermodynamic forces, kinetic coefficients, the change in internal energy, and verify Onsager′s relation for nonequilibrium thermodynamic properties of the system.
The unsteady Boltzmann kinetic equation and non-equilibrium thermodynamics of an electron gas for the Rayleigh flow problem
In the framework of irreversible thermodynamics, the characteristics of the Rayleigh flow problem of a rarified electron gas extracted from neutral atoms is examined and proved to obey the entropic behavior for gas systems. A model kinetic equation of the BGK (Bhatnager-Gross-Krook) type is solved, using the method of moments with a two-sided dis- tribution function. Various macroscopic properties of the electron gas, such as the mean velocity, the shear stress, and the viscosity coefficient, together with the induced electric and magnetic fields, are investigated with respect to both distance and time. The distinction between the perturbed velocity distribution functions and the equilibrium velocity distribution function at different time values is illustrated. We restrict our study to the domain of irreversible thermodynamics proc- esses with small deviation from the equilibrium state to estimate the entropy, entropy production, entropy flux, thermody- namic force, and kinetic coefficient and verify the celebrated Boltzmann H-theorem for non-equilibrium thermodynamic properties of the system. The ratios between the different contributions of the internal energy changes, based upon the total derivatives of the extensive parameters, are predicted via Gibbs' equation for both diamagnetic and paramagnetic plasmas. The results are applied to a typical model of laboratory argon plasma.
The effect of collimation on image quality and radiation safety in digital radiography of small animals
Collimating the primary x-ray beam to the specific area of anatomic interest has traditionally been strongly recommended to produce high quality radiographic images, even though some small animal patients can fit their entire torso on a single radiographic casette or detector. However, modern digital radiography may allow for whole-body projections for these patients at similar quality to specific collimated projections. Lateral and ventrodorsal projections of 31 small dogs were obtained to evaluate the effect of collimation on the quality of the resulting digital radiograph. Each projection was performed using both a collimated and whole-body technique. Scatter radiation was detected during each exposure as well to determine the effect of collimation on radiation safety for veterinary personnel. The resulting images were cropped to portray only the thorax, and were evaluated by five radiologists blinded to the procedure used for each image. Visual grading characteristics analysis was performed. An overall trend towards higher image quality was found in the collimated approach compared to the whole-body, but in most cases this change was not significant. Overall image quality for the ventrodorsal images was found to be significantly better in the collimated approach compared to the whole-body. Evaluation of image histograms suggests that the application of digital processing and the look-up table differs between them, and this may account for a large portion of any quality difference. Over twice as much scatter radiation was detected during the whole-body images as during the collimated images. Overall image quality was rated as adequate to high for almost all images with both approaches. Adequate collimation remains recommended to provide the highest quality images with the lowest exposure to veterinary personnel.