Date of Award

January 2013

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Mechanical Engineering

First Advisor

Forrest Ames

Abstract

Runway-independent aircraft are expected to be the future for short-haul flights by improving

air transportation and reducing area congestion encountered in airports. The Vehicle

Systems Program of NASA identified a Large Civil Tilt-Rotor, equipped with variable-speed

power-turbine engines, as the best concept. At cruise altitude, the engine rotor-speed will

be reduced by as much as the 50% of take-off speed. The large incidence variation in the

low pressure turbine associated with the change in speed can be detrimental to the engine

performance. Low pressure turbine blades in cruise altitude are more predisposed to develop

regions of boundary layer separation. Typical phenomenon such as impinging wakes

on downstream blades and mainstream turbulences enhance the complexity of the flow in

low pressure turbines. It is therefore important to be able to understand the flow behavior

to accurately predict the losses. Research facilities are seldom able to experimentally

reproduce low Reynolds numbers at relevant engine Mach number. Having large incidence

swing as an additional parameter in the investigation of the boundary layer development,

on a low pressure turbine blade, makes this topic unique and as a consequence requires a

unique facility to conduct the experimental research.

The compressible flow wind tunnel facility at the University of North Dakota had been

updated to perform steady state experiments on a modular-cascade, designed to replicate

a large variation of the incidence angles. The high speed and low Reynolds number facility

maintained a sealed and closed loop configuration for each incidence angle. The updated

facility is capable to produce experimental Reynolds numbers as low as 45,000 and as high

as 570,000 at an exit Mach number of 0.72. Pressure and surface temperature measurements

were performed at these low pressure turbine conditions.

The present thesis investigates the boundary layer development on the surface of an

Incidence-tolerant blade. The heat transfer approach is the method used to obtain knowledge

of the state of the boundary layer on the surface of the blade. Pressure and temperature distributions are acquired for Reynolds numbers of 50,000, 66,000, 228,000, and 568,000 at

an exit Mach number of 0.72, and Reynolds numbers of 228,000, and 568,000 at an exit

Mach number of 0.35. These experimental flow conditions are conducted at different flow

inlet angles of 40°, 34.2°, 28°, 18°, 8°, -2.6°, -12°, and -17°, and at two free-stream turbulence

levels. Results of the analyses performed show that as the incidence angle decreases,

a region of laminar separation bubble forms on the pressure surface and grows toward the

trailing-edge. It is also noted that the position of the leading-edge moves as the incidence

angle varies. A transitional flow is observed on both the pressure and suction surfaces,

mainly at the two highest incidence angles, for the high turbulence case. This investigation

also reveals that the Stanton number increases as the mainstream turbulence increases, and

that the Stanton number at the leading-edge increases as the Reynolds number decreases,

as it is documented in the literature.

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