Optical measurement of local mass transfer coefficients in naturally convecting systems

• Main Author: Nebrensky, J. J.
• Published: University of Edinburgh 1996
• Subjects: 621.4022
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.659905

The present work covers three main areas. First, several suitable polymers (one-part silicone RTVs, including a certain silicone sealant and Dow Corning's DC 734) and swelling agents (aliphatic esters such as <I>iso</I>-Pentyl Ethanoate, <I>iso</I>-Butyl <I>iso</I>-Butanoate and <I>n</I>-Pentyl Propanoate; alkanes including <I>n</I>-Nonane and <I>n</I>-Decane) have been identified, along with effective methods of applying the rubber coatings to both flat and cylindrical substrates. The diffusion properties of these new solvent/polymer systems have also been measured. Also, various aspects of the holographic system have been examined. The fringe visibility problems mentioned above are found to stem from in the incorrect location of the optical diffusing screen and are exacerbated by an apparent design flaw in the thermoplastic holographic camera used by some workers causing the gradual re-localisation of the interference fringes after hologram development. Substantial improvements in the lifetime of the thermoplastic plates for the holocamera has resulted from surrounding the optical system with a simple plastic curtain to keep out airborne dirt and dust. The improved optical system has been used to look at natural convection mass transfer from a vertical plate, using a variety of swelling agents to cover a range of Rayleigh numbers (<I>Ra</I>). For the more volatile solvents the results are in fair agreement with the analytical prediction of Lorenz, <I>Sh</I>=0.411 <I>Ra</I><SUP>1/4</SUP>. For the less volatile swelling agents the measured values are significantly higher than predicted; this is believed to be due to background draughts increasing mass transfer rate. Natural convection from the cylindrical surface of a vertical rod has also been investigated. Other work performed in support of the main project includes the writing of a computer program to produce simulated fringe patterns in various geometries, the demonstration of a novel swollen polymer system using water and gelatin, and the investigation of temperature distributions in vertical rod arrays in free and mixed convection using both thermocouples and liquid crystal thermography.