The 2013-2014 Academic Year NSF AY-REU Program

PROJECT SUMMARY REPORT

Study of Separated Flow in Low-Pressure Turbine

Submitted To

The 2013-2014 Academic Year NSF AY-REU Program

Part of

NSF Type 1 STEP Grant

Sponsored By

The National Science Foundation

Grant ID No.: DUE-0756921

College of Engineering and Applied Science

University of Cincinnati

Cincinnati, Ohio

Prepared By

Josh Combs, Junior, Aerospace Engineering

Devon Riddle, Junior, Aerospace Engineering

Report Reviewed By:

Dr. KirtiGhia

REU Faculty Mentor

Professor of Aerospace Engineering

School of Aerospace Systems

University of Cincinnati

September 9, 2013 – December 5, 2013

Study of Separated Flow in Low-Pressure Turbine

NSF Type 1 Step Grant

Grant ID No.: DUE-0756921

September 9, 2013 – December 5, 2013

Josh Combs, Junior, Aerospace Engineering, University of Cincinnati

Devon Riddle, Senior, Aerospace Engineering, University of Cincinnati

Goals

This research project is comprised of two-phases. In this first phase, we aimed to achieve two main goals. The first goal is to understand the characteristics of a low-pressure turbine (LPT). The second goal is to understand the methodologies of controlling flow separation.

Objectives

In order to reach our goals, we needed to meet certain objectives. Since the first phase is primarily a focus on understanding and comprehending our topic, a huge emphasis was put on the literature review. Therefore the first objective was to gain as much information as possible through a variety of technical sources such as reports, articles and theses pertaining to low-pressure turbines and flow separation. The second objective was to simply organize this information in a logical and thoughtful manner, i.e. in a technical paper.

Research Tasks

Below is a set of research tasks that were used to accomplish the goals and objectives that were mentioned above.

1. Conduct literature review.
2. Attend weekly workshops/seminars.
3. Learn basic modeling in computational fluid dynamics software (CFD) (if time allows).
4. Model selected control strategy using CFD. (if time allows)
5. Prepare findings in a technical paper.
6. Present research at the UC Undergraduate Research Conference in April

Methods

The literature review was conducted using methods shown in Workshop #1 “Online Literature Search”. This proved to be one of the most useful workshops since the goals of this project are to understand LPT characteristics and methods of flow separation control. We used many of the online resources found on the College of Engineering and Applied Science library website. These included technical resource databases such as Aerospace Research Central and SCOPUS. Many of the technical documents found on these databases are accessible using university privileges.

Results

Modern engine design aims to reduce manufacturing costs and fuel consumption by reducing the overall engine weight. The low-pressure turbine accounts for 20-30% of the total weight in most engines, making it a prime choice for weight reduction. This is done by reducing the blade count, which increases the aerodynamic loading on each remaining blade. As the air flows inside the blade passage along the suction surface of the blade, the pressure decreases first up to the shoulder and then gradually increases in the flow direction, which is known as an adverse pressure gradient. If the flow does not have enough momentum to overcome this pressure gradient, then it will separate from the surface, generating a wake that is proportional to drag.

The adverse pressure gradient is not the only issue with the aerodynamic efficiency of LPT blades. High altitude long endurance (HALE) UAVs operate at around 60,000 feet so the Reynolds number (Re) is significantly lower than that of a typical aircraft at lower altitude. The value of the Reynolds number also indicates whether streamlines along a body are smooth and regular, or random and erratic. Intuitively, it seems that the latter flow is undesirable for any streamlined body, but actually this turbulent flow is preferred because it delays and reduces the effects of flow separation. When the flow is laminar, that is, steady and regular, it does not have enough momentum to overcome the adverse pressure gradient. As discussed by Anderson (2011), or less for external flow exhibits laminar behavior, while or greater exhibits turbulent behavior. However, Re is not only dependent on air density (altitude) but also the characteristic length of the body. Also, the Re for which flow transitions from laminar to turbulent will depend on the body shape.

In this specific application, modern LPT blades are susceptible to flow separation due to an adverse pressure gradient and relatively low Reynolds number. There are several methods to delay the point of separation and reduce the negative effects. These methods are generally divided into two main categories, passive and active techniques. Passive techniques are permanent devices that are fixed on the body surface, and although they are beneficial for high altitude conditions, they create unnecessary drag in other modes of operation. Active techniques are devices that may be “turned off” and are likewise beneficial at high altitudes, but they usually add a considerable amount of weight to the aircraft and will require an energy source. The focus of this research project will be on a relatively new active device known as a plasma actuator.

Glancing further into active flow separation control methods, plasma actuators became the main focus of the research partially due to the fact they were designed for aerodynamics flow control. Three types of plasma actuators that were analyzed include single dielectric barrier discharge (SDBD) plasma actuators, glow discharge plasma actuators and plasma synthetic jet actuators.

SDBD plasma actuators are made of two separated layers of electrodes that are placed on the opposite side of the dielectric material. There is a slight overlap between the dielectric barrier materials, where the dielectric material is sandwiched between two electrodes. A voltage source is used to power the electrodes and has the capability of ionizing the air surrounding the electrodes. This means that the actuator pulses at a varied frequency which is what creates the plasma downstream of the actuator.As the plasma forms and builds, it creates a body force on the fluid flow, helping it to move downstream. The force built up behind the fluid flow is what accelerates the reattachment and has little effect on the airflow once the reattachment occurs. It is noted that SDBD plasma actuators have a plasma discharge containing a unique property where it can sustain a large volume discharge at atmosphere pressure without arcing.The plasma discharge is self-limiting by preventing this arc and maintaining its connection with the airfoil.

An experiment performed by W. Shyy (2002) showed the effect of plasma actuators on a cylinder. The cylinder was investigated using SDBD plasma actuators for landing gear noise reduction. Through this experiment it was found that the plasma will only stay on the airfoil if the voltage travelling through the actuator is continuously increasing. We want to use plasma actuators to control the flow between an airfoil and keep it from creating a wake and therefore creating drag.

Glow Discharge plasma actuators are similar to SDBD plasma actuators, but unlike SDBD plasma actuators, glow discharge plasma actuators can be placed directly behind the propeller immediately attaching the flow to the airfoil. The glow discharge plasma actuator is placed upstream from the flow separation developing plasma that forces the fluid through similar to how the SDBD plasma actuators develop plasma before forcing the fluid flow downstream.Glow discharge actuators create pulses that are sent to the electrodes with opposite polarities with different periods.This creates a beat frequency of the glow discharge plasma. This allows for a wide frequency range and promotes a swifter transition into the shear layer of the separation bubble leading to an earlier reattachment.

In plasma synthetic jet actuators, the flow is described as quiescent flow where a circular plasma region is shown to generate a vertical zero-net mass flux jet.This is where the name plasma synthetic jet actuator developed from.As the actuator pulses, it creates a vortex ring ahead of the jet while another is created near the actuator surface. With a varied frequency, multiple vortex rings are created close to the airfoil in the fluid flow which increases the velocity and the force acting on the fluid flow.

Explained in the Shyy Trial, the buildup of plasma in glow discharge plasma actuators results from the amount of energy added to a molecular gas. The gas will then split resulting from the collisions between the particles that have enough kinetic energy to exceed the molecular binding energy creating the buildup plasma behind the fluid flow.

Conclusion

The characteristics of a low-pressure turbine, at high altitude, were determined. It was found that an adverse pressure gradient was created due to a fewer amount of turbine blades doing work. This is coupled witharelatively low Reynolds number which generates laminar flow that does not have enough momentum to overcome the adverse pressure gradient. This creates a need for flow separation control.

Although there are a variety of passive and active devices for flow control, we focused on a relatively new active device known as a plasma actuator. This device actually ionizes the air within the boundary layer, creating a body force that accelerates flow along the suction side of a turbine blade, thus delaying the point of separation or eliminating it all together. The most notorious experiments were conducted by Shyy using SDBD plasma actuators on a cylinder, in which the trailing wake was successfully decreased in size. Unfortunately time did not permit any experimentation using computational fluid dynamics (CFD), but this will the main focus of phase two in this project.

Image 1: Devon Riddle conducting research

Image 2: Josh Combs conducting research