Local density, changes

The pressure inside the heated chamber may also vary as a result of the local density changes produced by thermal expansion or phase changes resulting from the heating. For example NaCl may expand, melt, and thereby increase the local pressure, while pyrophyllite, a layer-lattice-type aluminum silicate, may transform into a denser assembly of coesite and kyanite, thereby reducing the local pressure. It follows that experimental results in high-pressure, high-temperature work must be interpreted with care. 

As a result of heating through dissipation of kinetic energy, local temperature and therefore local density changes can exist in adiabatic flows. A flow, in which the density of a volume element of the material changes in the course of its motion is known as compressible. In the following we will discuss of heat transfer phenomena in these compressible flows. We will restrict ourselves to steady flows. 

The inert ceramic matrix which holds the electrolyte in place between the cathode and the anode serves two purposes first, it holds the electrolyte by capillary action and prevents the molten salts from completely flooding the porous electrodes second, the membrane acts to prevent the bulk diffusion of gases between the cathode and the anode side of the cell. If the electrolyte was not in chemical equilibrium with the process gas, localized density changes in the electrolyte caused by reaction (20) would cause the membrane to crack and allow bulk mixing of the process and sweep gas streams. 

A nitrogen sweep was applied to both the process and sweep sides of the cell and the ceU was loaded into the furnace for heat-up. The binder from the MgO tapes was volatilized out at 376 0 overnight. The temperature was then ramped up to the run temperature and the electrolyte wicked into the MgO powders and zirconia doth at process temperature. Process gas was then supplied to the cell and the electrolyte was aUowed to reach the equilibrium composition described by reaction (20). Since the ceramic matrix was no longer a rigid sintered structure, localized density changes in the electrolyte did not cause the cracks seen with the more rigid stmctures. 

If the surface is impenetrable and the heterogeneous reaction is not accompanied by a local density change, then the condition of sticking, u = 0 must be satisfied on the surface. Otherwise, near the surface, there arises a convective flux of reactant, which is normal to the surface and directed toward it. This flux is known as Stefan s flux. As a rule, it does not exert a noticeable influence on chemical and biochemical heterogeneous reactions, and can be disregarded. However, in problems involving intense melting, evaporation, or condensation of substance, Stefan s flux may not be small and thus should be taken into account. 

Projection radiography is widely used for pipe inspection and corrosion monitoring. Film digitisation allows a direct access to the local density variations by computer software. Following to a calibration step an interactive estimation of local wall thickness change based on the obtained density variation is possible. The theoretical model is discussed, the limitations of the application range are shown and examples of the practical use are given. The accuracy of this method is compared to results from wall thickness measurements with ultrasonic devices. 

In fig, 4 local corrosion by erosion is shown in a pipe with a bore of 100 mm behind a welding. In this case only the nominal wall thickness of the pipe is known (6.3 mm). To calibrate the obtained density changes into wall thickness changes a step wedge exposure with a nominal wall thickness of 13 mm (double wall penetration in the pipe exposure) and the same source / film system combination was used. From this a pcff = 1-30 1/cm can be expected which is used for the wall thickness estimation of the pipe image according to equation (4). 

Most often, the Mach number is calculated using the speed of sound evaluated at the local pressure and temperature. When M = 1, the flow is critical or sonic and the velocity equals the local speed of sound. For subsonic flowM < 1 while supersonic flows have M > 1. Compressibility effects are important when the Mach number exceeds 0.1 to 0.2. A common error is to assume that compressibihty effects are always negligible when the Mach number is small. The proper assessment of whether compressibihty is important should be based on relative density changes, not on Mach number. 

The hydrogen frequency appropriate to the H—B pair was supported by the local-density pseudopotential calculations of Chang and Chadi (1988)... 

---