Heat Transfer In Enclosures: Ovens

The use of natural convection to effect heat transfers in enclosures is limited principally because of its low heat transfer coefficients. This study aimed to establish a better understanding of the heat transfers occurring in enclosures which are heated by cylindrical heat sources and to identify preferred heating arrangements for improving the rates of natural convection heat transfers. Experimental investigations including interferometric observations were carried out on a square-sectioned enclosure containing a rectangularsectioned, isothermal inner body and cylindrical, sheathed electrical heating elements. The locations of the heaters were varied and the effects on natural convection heat transfers established. The findings conformed to intuitive expectations, concluding that the maximum position for natural convection heat transfer was with the heaters located in the lower half of the sidewalls. The flow patterns and heat transfer characteristics of a single horizontal cylindrical (9.5mm dia) heater in a square-sectioned enclosure were studied by traversing the heater both horizontally and vertically across the enclosure at distinct Rayleigh numbers (7.5x104 and 1.1x105). Optimal positions for maximum and minimum Nusselt numbers were established with respect to heater diameter and vertical height, and horizontal offset from a vertical wall. Flow and Interferometric observations were also carried out on an enclosed single tubular 55mm dia. heater, and two 55mm dia. heaters. Improved arrangements for enhanced heat losses by natural convection from horizontal tubular arrays were deduced, applicable to arrays either totally enclosed or In the proximity of a vertical wall. A square-sectioned (35Ox35Oxl7OOmmtu)n nel oven was designed which would withstand operations under vacuum. Within this enclosure, the heating arrangements were located and varied essentially in the lower half of the enclosure, flow and thermal profiles obtained, a multi-surface radiation model developed and the convective heat transfers validated by evacuating the enclosure. At oven temperatures, over 40% of the heat input was achieved by natural convection