Thermal gradient tube

Figure 6.3. Deposition profiles for iodine and cesium in the thermal gradient tube (Collins et al., 1988 Copyright 1988 by the American Nuclear Society, La Grange Park,...

As can be seen from the summary paper of Collins et al. (1988), in the ORNL high-temperature experiments about 40% of the iodine released from the fuel specimen was plated out in the thermal gradient tube, with a condensation maximum at about 500 °C when a steam—helium—hydrogen atmosphere was used. Since cesium showed a quite similar distribution in the thermal gradient tube (see the example... 

The steep thermal gradient along the tube means that any variation in the sample position (e.g. because of pipetting, or spreading due to surface tension and viscosity effects) will alter the atomization peak shape. Peak area integration will help to minimize this problem, as will a rapid heating ramp and isothermal operation (see Sections 3.6.2 and 3.6.3). 

Due to the existence of a high thermal gradient along a pyrolysis fiimace tube, it is difficult to pinpoint the concept of residence time. A frequent solution is to define an... 

TG, a null-point mechanism is to be preferred, as this ensures that the sample remains in the same part of the furnace, even when the mass changes. Thus any effects due to thermal gradients within the furnace tube remain constant. 

To Overcome this diflBculty and to resolve the sequential processes leading to the first soot particles, the reflected shock region of a conventional shock tube has been used. The core region of the reflected shock provides a largely homogeneous reaction zone where concentration and thermal gradients are reduced greatly compared with atmospheric flames, for example. 

The Clusius-Dickel column is shown schematically in Figure 2. A wire is mounted at the axis of a cylinder. The wire is heated electrically and the outer wall is cooled. This sets up a radial thermal gradient which leads to a thermal diffusion separation in the x direction. As a result of the radial temperature gradient, a convection current is established in the gas, which causes the gas adjacent to the hot wire to move up the tube with respect to the gas near the cold wall. The countercurrent flow leads to a multiplication of the elementary separation factor. For gas consisting of elastic spheres, the light molecules will then concentrate at the top of the column, while the heavy molecules concentrate at the bottom. The transport theory of the column has been developed in detail (3, iS, 18) and will not be presented here. In a later section we shall discuss the general aspects of the multiplication of elementary separation processes by countercurrent flow. 

---