Abstract

Film Cooling Heat Transfer Mechanism and Applications

Hao-Ming Li, Ph.D.
Concordia University, 2021

Film cooling is a mainstay in cooling technology that is imperative in modern gas turbines. However, the improvement in film cooling performance research has leveled at a plateau for decades. Hence, it is urgent to clarify the film cooling heat transfer mechanism.
Recently, the effect of the counter-rotating vortex pair (CRVP) intensity on film cooling effectiveness was found to be more critical than the velocity related factor like momentum flux ratio. The present dissertation began with that factor of CRVP and concluded that the essential source of CRVP is the velocity gradients in the mainstream-coolant shear layer; the CRVP is mostly mainstream flow direction; and its main components are the relevant velocity gradients.
Along these findings, a mechanism of the film cooling heat transfer has been proposed: Film cooling heat transfer is mostly advection; strong velocity gradients at the mainstream-coolant interface produce secondary flow that penetrates the interface, hence drastically increases advection. These secondary flows are classified into mainstream entrainment, in which the flow is from the mainstream to the coolant, and coolant expansion, in which the flow is from the coolant to the mainstream. Mainstream entrainment drastically decays film cooling effectiveness, while coolant expansion is likely to increase film cooling effectiveness.
Therefore, two advanced film cooling schemes, named comb scheme and High Aspect Ratio (HAR) scheme, have been developed. The comb scheme was aimed to eliminate the mainstream entrainment by trapping the CRVP in its blind slot, while the HAR scheme was aimed to exploit the coolant expansion by drastically prolonging the coolant supply.
The new schemes were investigated numerically and experimentally, with transient thermochromic liquid crystal technique and steady RANS simulations coupled with realizable κ-ε turbulence model. Addition to demonstrating the so-called ‘ideal performance’, it has been shown that CRVP was trapped in the blind slot of the comb scheme, and coolant dominated the vicinity of the HAR exit trailing edge. They matched the designs. It convincingly proved the proposed film cooling heat transfer mechanism, established a solid fundamental for the film cooling research.