Abstract
In this thesis, a novel design associated with a slot-liner in a sudden-expansion chamber is introduced. The slot-liner design effectively modulates the unsteady flow structures and combustion mechanisms of solid-fuel within a sudden-expansion chamber. For distinct slot-liner design, unsteady flow fields within the slotted sudden-expansion chamber were experimentally quantified via digital particle image velocimetry; a high speed camera was concurrently deployed to photograph flame structures, facilitating combustion status diagnosis. The experimental results reveal that for an uncombusted sudden-expansion chamber, alteration of the slot location is able to modulate the length of the main recirculation zone as well as the dimension of the corner eddy. Due to attraction and restriction effects of the slot on the vortex bubbles distributed along the shear layer immediately above the main recirculation zone, the vibration frequency of the reattachment point is significantly reduced. Air stream drawn into the slot was found to flow upstream and be ejected from the step corner, forming a feedback-jet. This feedback-jet augments the strength of the corner eddy and the main recirculation zone, lengthening the residual time of the air stream within the main recirculation zone, and consequently enlarging the amount of recirculating air. High-speed and high-temperature air stream (velocity U0=31 m/s, temperature T0 =800-850 oC, oxygen concentration [O2] ~ 11.7 %) was forced to convect over a solid-fuel (PMMA) plate placed downstream of a backward facing step, so as to examine unsteady ignition transients of the solid-fuel as well as the flame propagation modes in the combustion chamber. For cases without the slot arrangement, we found the flame-ignition point is situated downstream of the reattachment zone, incurring a counter-stream flame propagation. Since the counter-stream flame propagation is subject to a stronger heat loss due to flow convection, the counter-stream flame propagation occurs chronologically later than the co-stream flame propagation. By contrast, the slot-liner design devised in this work is capable of effectively modulating the flow structure of the main recirculation zone. The residual time of fuel gas within the main recirculation zone is elongated; accordingly, the extent of mixing of air and the fuel gas is enhanced. The possibility for the flame ignition to occur within the recirculation zone is increased as well. Flame-propagation dynamics associated with solid-fuel combustion is hence well controllable. The flame ignition delay is diminished and the combustion efficiency of the solid fuel is significantly improved. Research results in this work verify and demonstrate the modulation capability and potential diversity of the slot-liner design, which is beneficial for the development of control techniques associated with unsteady combustion of solid fuels. This work also provides a research foundation for future studies on solid fuels derived from wastes. To conclude, this work has contributions from both academic and engineering perspectives.