Flow Visualization Study on the Effect of a Gurney Flap in a Low Reynolds Number Compressor Cascade

Abstract

IntroductionThe first century of heavier-than-air powered flight has been dominated by the desire to fly "faster, higher, and farther." Fulfilling this desire led to the development of the gas turbine driven turbojet. Since their introduction more than six decades ago, gas turbine driven engines have become quite powerful in terms of thrust produced 1 while achieving significant improvements in fuel efficiency 2 and reductions in maintenance costs 3 . Today, gas turbine driven turbofans and turboprops dominate the field of aero-propulsion for aircraft varying from "moderate" size general aviation planes to large passenger jets. This domination of gas turbine engines is true even in some "smaller" aircraft like Unmanned Aerial Vehicles (UAV) including the Air Force RQ-4A Global Hawk and HQ-9A Predator B. Since gas turbine engines are so prevalent in aviation, it is important to learn what affects their performance. Gas turbine 2 American Institute of Aeronautics and Astronautics driven engines are designed for, and can typically be operated at, optimum performance in medium to large size aircraft applications because their flight envelopes are so well defined. However, recent advances in UAV applications have pushed the flight envelope into the lesser known low Reynolds number flight regime.A typical general aviation aircraft may involve wing chord Reynolds number on the order of 10 6 to 10 7 while large passenger aircraft might be in the 10 8 range. 4 Recently, however, there has been a strong interest in very small reconnaissance vehicles called Micro Air Vehicles (MAV) which typically have a wing chord length on the order of 6 inches (0.15 m). MAV's involve chord Reynolds number on the order of 20,000 to 200,000, and very little aerodynamic data exists in this range.5 Similar issues exist in the gas turbine engine research area as well. A microscale engine (i.e., micro-fabricated gas turbine) for application on MAV involves design blade chord Reynolds numbers of about 20,000 or less.6 In addition to small-scale effects, extreme operating conditions on conventional gas turbine engines often result in low Reynolds numbers. The low-pressure turbine blades on UAV engines, which are in operation today, involve blade chord Reynolds numbers less than 25,000 under high-altitude operating conditions. 7 However, currently existing data for turbomachinery blades only go as low as Reynolds numbers in the 40,000 range. 8,9 Thus, there is a strong need to obtain data for turbomachinery blades at the low Reynolds number range.Modern gas turbines oftentimes employ a multi-stage axial compressor where the pressure rise across each compressor stage is relatively small. Since compressor blades operate in an adverse pressure gradient environment, it is relatively easy to cause flow separation (i.e., stall). The consequences of compressor blade stall can simply be poor performance or, in the worst case scenario, significant engine damage.3 In order to prevent compressor blade stall, gas turbine engines are normally operated close ...

Application of Gurney Flaps on a Centrifugal Fan Impeller

“…There is a significant increase in lift with only a small increase in drag as long as the flap height scales with the local boundary layer thickness. Despite their widespread use in aeronautics (Lee [1], Lee and Su [2] and Wang et al [3]), very little research work has been done on turbomachinery (Byerley et al [4], Chen et al [5], Greenblatt [6], Janus [7] and Myose et al [8]). An effort has been made here to find the effectiveness of Gurney flaps on the performance of centrifugal fan.…”

Effect of Gurney flap Height and Mounting Position on the Performance of a Centrifugal Fan

“…The experimental static pressure distribution and total pressure loss coefficient agreed well with the computational results. Myose et al (1996) conducted flow visualization study with Gurney flap in compressor cascade at low Reynolds number and showed that Gurney flap delayed the stall at large incoming flow angles. Greenblatt (2011) used large Gurney flaps in an axial fan and studied about the effect of height and thickness of Gurney flap.…”