Boundary-layer bypass

An important consideration in the design, optimization, and control of a chemical vapor deposition (CVD) reactor is the effective utilization of the reagent gases [324], For ex- 

One can write a macroscopic mass-flux balance that considers the inflow, creation, destruction, and outflow of all species, 

The first term represents the axially convected inflow through the manifold, the Ck and k terms represent the creation and destruction of species k by homogeneous reaction (cbk = Ck — k), the fourth term represents radially convected outflow, and the fifth term represents the consumption of a gas-phase species by heterogeneous reaction at the deposition surface. 

After dividing Eq. 17.5 by nr2, the following equation represents the mass balance per unit surface area  

Note that the radial-flux term now involves a factor V = v/r, which in the similarity formulation of the stagnation-flow problem is a function only of z. Thus, per unit-area, this mass balance is independent of r, so long as the similarity assumptions apply. 

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In an ideal stagnation flow, a certain amount of the flow that enters through the inlet manifold can leave without entering the thermal or mass-transfer boundary layers above the surface. For an axisymmetric, finite-gap, flow, determine how the bypass fraction depends on the separation distance and the inlet velocity. 

For the subcritical instability of the zero pressure gradient boundary layer shown in the experimental results of the previous section, the mean flow was given by the Blasius boundary layer that was destabilized by a convecting captive vortex. In Chattopadhyay (2001), Sengupta et al. (2001, 2003) the early stages of the bypass transition was computed by solving the full Navier-Stokes equation in two-dimensions. 

The shape of the flow passage that guides the air to the sensor element plays an important role in the function of an air-flow sensor (the bypass, Fig. 7.6.8). Cross section and length are the important factors. As an accelerated flow has the most stable boundary layers and therefore is less vulnerable to flow separation, it would be favorable to have a decreasing cross section (in the flow direction). To avoid a restriction of air flow in the bypass the cross section at the outlet should not be too small in relation to the inlet. Therefore a reduction of cross section for stable accelerated flow is only done in the vicinity of the sensor element and in curved parts of the bypass (Fig. 7.6.8). 

In the bypass flow concept (Figure 7), the reactor outlet gas flows in the inner pipe and a small quantity ( 1%) of the cooler reactor inlet gas flows in the same direction in the annular space between the inner pipe and outer pipe. The bypass gas flow rejoins the main gas flow at the turbine outlet. There may be a lined insulation layer within the inner pipe. Some type of support stmcture, such as stents, would be necessary in the annular spaces between the iimer pipe and outer pipe to maintain concentricity. Because the iimer pipe and liner are not pressure boundaries, they could be constmcted from either a nickel superalloy or a refractory metal depending upon other material selections in the SNPP. 

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