Abstract:
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.