Stagnant gas layers

The gas-phase diffusivity of sodium in helium Dvl may then be evaluated from the experimental data by using Equation 15. A value of 1.96 cm.2/sec. was obtained, which compares favorably with 2.11 cm.2/ sec. estimated from an equation given by Hirschfelder, Curtiss, and Bird (8), using Lennard-Jones parameters given by Chapman (5). The close agreement obtained here seems to justify the assumption of a stagnant gas layer through which both sodium and cesium diffuse. 

There are several types of situations covered by Eq, (21.16). The simplest case is zero convective flow and equimolal counterdiffusion of A and B, as occurs in the diffusive mixing of two gases. This is also the case for the diffusion of A and B in the vapor phase for distillations that have constant molal overflow. The second common case is the diffusion of only one component of the mixture, where the convective flow is caused by the diffusion of that component. Examples include evaporation of a liquid with diffusion of the vapor from the interface into a gas stream and condensation of a vapor in the presence of a noncondensable gas. Many examples of gas absorption also involve diffusion of only one component, which creates a convective flow toward the interface. These two types of mass transfer in gases are treated in the following sections for the simple case of steady-state mass transfer through a stagnant gas layer or film of known thickness. The effects of transient diffusion and laminar or turbulent flow are taken up later. 

Ammonia, NH3, is being selectively removed from an air-NH3 mixture by absorption into water. In this steady-state process, ammonia is transferred by molecular diffusion through a stagnant gas layer 5 mm thick and then through a stagnant water layer 0.1 mm thick. The concentration of ammonia at the outer boundary of the gas layer is 3.42 mol% and the concentration at the lower boundary of the water layer is essentially zero. 

The oldest description of CVD uses the boundary-layer model, which assumes that between the bulk gas phase (uniform in composition) and the substrate there is a stagnant boundary layer in which gradients develop in temperature (cold wall reactors) and in the partial pressures of the reactants and the gaseous reaction products. The boundary-layer model presents some difficulties. Stagnant gas layers have a variable thickness in the reactor as long as unmodified bulk gas concentrations exist, if they exist at all. Moreover the flow is always laminar. Conceptually, however, the model has its advantages.The boundary-layer thickness is an effective parameter by which to characterize the deposition regime. This model will be used here for a simple overview of reaction conditions. 

D. Gas phase diffusional impedance. This arc is observed mainly in high performance cathodes, and increases with cathodic polarization (negative applied potential). The observed impedance is in accordance with the presence of a stagnant gas layer close to the electrode surface. 

Internally insulated, counter flow, bypass flow, stagnant gas layer... 

In the stagnant gas layer concept (Figure 8), the hot reactor outlet gas flows in the inner pipe and a stagnant layer of He-Xe gas is contained in the annular space between the inner pipe and outer pipe. There may be a lined insulation layer within the inner pipe. Some type of support structure, such as stents, would be necessary in the armular spaces between the iimer pipe and enter pipe to maintain concentricity. Because the inner pipe and liner are not pressure boimdaries, they could be constmcted from either a nickel superalloy or a refractory metal depending upon other material selections in the SNPP. 

FIGURE 8. Stagnant Gas Layer Concept (Pictured with Inner Insulation). 

Thermal and hydraulic analyses were performed for the internally insulated and stagnant gas layer concepts. The counter flow and bypass flow concepts were excluded as discussed below in the omitted concepts section. The analyses were performed based on an arrangement which utilized a three Brayton configuration having a shared gas cooler with two Braytons operating. The concepts were compared based on thermal performance, hydraulic performance, past HTGR practice, and manufacturing simplicity. 

To assess the hot leg piping concepts, a detailed thermal analysis was performed for the internally insulated concept and the stagnant gas layer concepts (both with and without internal insulation). The thermal analysis was set up in a manner which equalized the thermal performance of the concepts by requiring that the outer pipe inner wall temperature not exceed 900K. As a result, only the insulating and/or stagnant gas layer thicknesses were varied, and the concepts could be directly compared by their hydrauhc performance. 

Stagnant Gas Layer Concepts with Internal Insulation For the stagnant gas layer concepts with internal insulation, there are seven heat transfer paths in series ... 

Stagnant Gas Layer Concepts without Internal Insulation... 

A space temperature sensitivity analysis was performed for the analyzed concepts in order to assess the impact of increasing space temperature on the hot leg inner diameter. As the space temperature increases, a larger thermal barrier to heat transfer is required to maintain the pressure boundary temperature at 900K. The increase in the thickness of the insulation and/or the stagnant gas layer decreases the diameter of the inner pipe for a fixed outer... 

The internally insulated concept continues to have the largest flow area of the analyzed concepts as the space temperature is increased. This is because, with other parameters fixed, the resistance to heat transfer of a conduction path is directly proportional to the length of the conduction path. To reduce the conductive heat transfer through the insulation or stagnant gas layer, the separation between the liner and pipe must be increased. 

Stagnant Gas Layer without Insulation Pressure Drop Space Temperature Sensitivity... 

Stagnant Gas Layer with Insulation Manufacturing Complexity N/A... 

---