Buoyancy-driven processes

Boundary layer similarity solution treatments have been used extensively to develop analytical models for CVD processes (2fl.). These have been useful In correlating experimental observations (e.g. fi.). However, because of the oversimplified fiow description they cannot be used to extrapolate to new process conditions or for reactor design. Moreover, they cannot predict transverse variations In film thickness which may occur even In the absence of secondary fiows because of the presence of side walls. Two-dimensional fully parabolized transport equations have been used to predict velocity, concentration and temperature profiles along the length of horizontal reactors for SI CVD (17,30- 32). Although these models are detailed, they can neither capture the effect of buoyancy driven secondary fiows or transverse thickness variations caused by the side walls. Thus, large scale simulation of 3D models are needed to obtain a realistic picture of horizontal reactor performance. 

In this section primary attention is directed at the turbulent processes that occur in the mixed layer as a result of the interaction between shear-and buoyancy-driven flows. The flux Richardson number Rf gives a measure of the relative importance of the buoyancy terms in the equations of motion, (g/T)w d , as compared to the shear production terms, u w dutbz. 

If a system lacks an interface between different fluids, such a monomer/air interface, then only buoyancy-driven convection will occur. If a free interface exists, then we will see that gradients in the interfacial tension can cause fluid motion — a process called Surface-Tension Induced Convection or Marangoni convection. This will be especially important in "microgravity". (How the condition of apparently zero gravity is achieved is discussed in chapter 2.)... 

Li et al. 48) studied how 1.0 pm particles of polystyrene and poly(methyl methacrylate) interacted when they were melt processed at 180 °C. They observed by confocal microscopy on a hot stage that there was a preferential motion for particles, which they attributed to a buoyancy-driven flow because of the 10% density difference between the polymers. Jang et al. had made a similar observation 49), Li et al. did not consider possible surface-tension induced convection or that droplets could migrate in a temperature gradient, as has been observed by Balasubramaniam et al. for the thermocapillary migration of bubbles 50),... 

Buoyancy-driven and surface-tension driven convection can affect a wide variety of polymer process. Often the role can be studied on earth by varying the viscosity or orientation or the system. Yet, performing experiments in weightlessness can be the only way to determine the relative effects of the two processes or if the viscosity can not be independently varied. 

DAMNA) (Figure 1). Frazier et al (6,7) reported the surface polymerization of polydiacetylene films from the photopolymerization of DAMt A monomer solutions. This unique photopolymerization process produces amorphous thin films of polydiacetylene, which can be used in the fabrication of waveguides and photonic devices. However films formed on earth are low quality and have defects that appear to be the result of buoyancy-driven convection. Microgravity experiments on the same system produced very high quality optical films. The lack of in-situ measurements limited the analysis of the effect of gravitational forces on the development of defects in the poyldiacetylene films. 

It is of interest to consider whether impairment of heat transfer would be encountered under the conditions likely to be achieved in a buoyancy-driven flow system of the kind which has been proposed for passively cooling a nuclear reactor containment vessel. In this connection, a further matter needs to be considered. Most of the experimental studies of mixed convection reported to date have been carried out with a thermal boundary condition of uniform wall heat flux. However, in the case of a severe accident in a pressurised water reactor, where steam is released from the core into a steel containment vessel and is condensing on its inside surface, the vessel will take up a uniform temperature. Since the nature of the thermal boundary condition could certainly affect the process of heat transfer to the air, there is a need to consider whether the behaviour with uniform wall temperature will be similar to that with uniform wall heat flux. 

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