Role of buoyancy in fuel-thermal-stability studies

A unique methodology is used to investigate the effects of gravity on fuel flowing through the small-bore heated tubes that are often used in the study of fuel-thermal- stability characteristics. The copper block that houses the fuel tube (or test section) is located on a swivel, and experiments are conducted for different tube orientations, namely; horizontal, vertical with flow from bottom to top and vice versa. Results obtained for different fuel-flow rates and block temperatures are discussed. An axisymmetric, time-dependent numerical model is used to simulate the flow patterns in the test section. This model solves momentum, energy, species, and k-e turbulence equations. The buoyance term is included in the axial-momentum equation. Natural flow resulting from buoyancy was found to have a significant effect on heat transfer and oxygen consumption for fuel-flow rates up to 100 cc/min (Reynolds numbers up to 2.3 x 10 3). Flow instabilities were observed when the fuel was flowing downward in a vertically mounted test section. The effect of block temperature and flow rate on these instabilities was also studied.