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Improved Design of Self Actuated Valve for Pulsejet Combustors


									          Improved Design of a Self-Actuated Valve for Pressure Gain Pulsejet Combustors
                                                                                                  Kyle Gaiser, Case Western Reserve University, Department of Physics
                                                                                                            Dr. Daniel Paxson, NASA Glenn Research Center
                                                                                         Dr. James T’ien, Case Western Reserve University, Department of Aerospace Engineering

Background                                                                                                 Project
Jet Engine Combustion                                                                                      Overview                                                        Impact Dynamics Model
     A jet engine operates on a Brayton cycle, which                                                                                                                            An energy based model is used to predict the life of
incorporates three engine components: the compressor,                                                                                                                      the valve. The impact energy is calculated from the
the combustor, and the turbine, as illustrated below. The                                                                                                                  velocity of the valve upon impact, which is obtained from
compressor and combustor increase the fluid pressure                                                                                                                       the Global Dynamics model. This energy is also the area
and temperature, respectively, resulting in hot gases from                                                                                                                 under the stress-strain curve, which is shown below. By
which useful energy is extracted by the turbine or utilized                                                                                                                interpolation, the valve’s stress and strain is determined
in the nozzle for additional thrust.                                                                                                                                       from the area underneath the stress-strain curve.
     The efficiency of the Brayton cycle is determined by
the product of the compressor and combustor pressure                                                             Pulsejet reed valves have notoriously short life spans.        Given basic material properties of the valve, the
ratios, also known as the overal pressure ratio, PB/PA:                                                    When subject to the extreme temperatures, pressures,            following equations are estimated and plotted.
                                                                   1                                     and high rates of impact characteristic of the pulsejet tests        1. Ramberg-Osgood Equation for stress-strain curve:
                                          PB                     
                                                                                                           performed at NASA Glenn, the valves last approximately
                                    1  
                                          PA                                                             fifty seconds. This is detrimental to pulsejet performance
                 PB/PA – overal pressure ratio; γ– specific heat ratio; η - efficiency
                                                                                                           and hinders research efforts. Below are two figures
A larger pressure ratio increases efficiency, resulting in                                                 depicting a failed valve (left) and the valve head (right)
more useful work and thrust. However, a 4 – 8 percent
pressure loss across the combustion chamber alone is                                                       The project’s objectives are threefold:
typical of current jet engines. One method to remedying                                                        1. Global dynamics: understand the valve’s motion
this problem is through pressure gain combustion.                                                                 and correlation to pressure variation.
                                                                                                               2. Impact dynamics: understand causes of failure and
                                                                                                                  predict cycles to failure.                                   2. Manson-Coffin Equation (blue) and Universal
                                                                                                               3. Mitigation Techniques: evaluate geometry, valve                 Slopes Method (red) for cycles to failure:
                                                                                                                  materials, and valve arrangement, and provide
                                                                                                                  suggestions for better designs.

                                                                                                           Global Dynamics Model
                                                                                                                To model the valve’s motion, a computer simulation
Pressure Gain Combustion and Pulsejets
                                                                                                           simplifies the valve to a spring covering an inlet, as shown
     Pressure gain combustion is a method that increases
                                                                                                           below on the left. The motion is complex, governed by the
the pressure across the combustion chamber, resulting in
                                                                                                           valve’s material properties, the mass flow rate, and
higher efficiency engines. This can be achieved via a high
                                                                                                           pressure variations on the valve. Applying Bernoulli’s
frequency, resonant, pulsed combustion process such as
                                                                                                           equation and Newton’s second law, a set of coupled
that utilized in pulsejets.                                                                                                                                                      Graph 2 depicts two methods used for predicting the
                                                                                                           equations is numerically integrated. The figure below and
                                                                                                                                                                           life of the valve. While literature says the Universal Slopes
                                                                                                           to the right illustrates the relationship between the valve’s
                                                                                                                                                                           curve is typically more accurate, the Manson-Coffin
                                                                                                           position, in blue, and velocity, in green, obtained by the
                                                                                                                                                                           method works better for the spring steel valve used in the
                                                                                                                                                                           pulsejet. Nevertheless, the two methods combined give a
                                                                                                                                                                           range of confidence, within an order of magnitude, of the
                                                                                                                                                                           actual number of cycles to failure.
     The pulsejet is a mechanically simple engine with
one moving part: a reed valve. This valve controls the                                                                                                                     Future Work
injection of the air-fuel mixture into the combustion                                                                                                                      • Account for high temperature fatigue on the valve
chamber, opening and closing at the same high frequency                                                         To validate the program, a high speed pressure             • Develop mitigation techniques for future valve designs
as the combustion. The valve’s movement is completely                                                      transducer and an infrared fiber optic sensor are placed
                                                                                                           next to the valve during a test of the pulsejet. The data is    Acknowledgments
self-actuated; it is governed by the mass flow and                                                                                                                         Dr. John Lewandowski, Dr. Kenneth Singer, Dr.
pressure variations on of the valve.                                                                       used to compare the experimental pressure and motion of
                                                                                                           the valve to that of the computer model.                        Mark De Guire, Dr. Robert Mullen

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