COMBUSTION DYNAMICS IN A RIJKE TUBE (PULSED COMBUSTOR) Learning Objectives

Transcription

1 COMBUSTION DYNAMICS IN A RIJKE TUBE (PULSED COMBUSTOR) Rijke Tube Lab - 1 Learning Objectives 1. Familiarization with the characteristics of resonant systems, frequencies and mode shapes 2. Introduction to acoustic waves and combustion dynamics 3. Exposure to the use of piezoelectric pressure transducers, photomultiplier tubes and rotameters 4. Exposure to frequency analysis (power spectra) Rijke Tube Lab - 2 1

2 Lab Info Location ESM G2 When the next two weeks (only one week for each group) there will also be a 2 nd lecture next week Safety all experiments with combustion are potentially hazardous follow all the precautions outline in the lab manual regarding igniting and operating the combustor Rijke Tube Lab - 3 Pulsed Combustors, Combustion Dynamics Can produce pressurized outputs (thrust) without external machinery (e.g., compressors) Cause of spectacular failures in rockets (and gas turbines) Rijke Tube Lab - 4 2

3 Rijke Tube Acoustics Rijke tube is open ended pipe with internal, compact heat source x Acoustic (oscillating) modes p t p p t acoustic pressure 1 st Harmonic f a / L Fundamental = 2L f a / L p x,t A x sin ωt p (x,t) pressure at open ends ~same as surroundings; A(0) A(L) 0 L 2 nd Harmonic f a / L Pressure Standing Wave t Rijke Tube - A + Rijke Tube Lab - 5 Rijke Tube Acoustics Rijke tube is open ended pipe with internal, compact heat source p-node x Acoustic modes p x,t A x sin ωt acoustic velocity v p,v x,t B x sin ωt 90 Rijke Tube Lab - 6 for fundamental mode, velocity at center 0 L Rijke Tube in time, velocity out-of-phase with pressure t - A,B = 2L f a / L Pressure Standing Wave Velocity (+ up) + v-node 3

4 Rijke Tube Feedback Many ways to excite resonances in pipes Consider musical instruments most woodwinds: vibrating reeds brass instruments: vibrating lips flutes, organ pipes, coke bottles: blowing over an opening all of these are externally driven In a Rijke tube, a heat source supplies the energy and drives the acoustics the heat source must respond to the acoustics to produce the resonance feedback Rijke Tube Lab - 7 Rijke Tube Feedback In our experiment, the heat source is a flame (combustion) Fuel is introduced through a central pipe, air enters through (ideally) natural draft (chimney effect) Burning (combustion) can only occur when fuel mixes with air faster mixing of fuel and air means faster heat release mixing caused by velocity fluctuations L But heat release raises the temperature, lowers the density when the flame products expand, they compress the gases around them air pressure feedback fuel Rijke Tube Lab - 8 4

5 Piezoelectric Pressure Transducers Piezoelectric crystal releases or absorbs charge based on strain Combustor Allow fluid pressure to push on crystal mounted to solid base (via diaphragm); stress strain Piezo is current source Also sensitive to vibrations & temperature standoff mounting To Data Acquisition To Long Tube p Transducer Amplifier and Power Supply Rijke Tube Lab - 9 Sensors Spectral Intensity Chemiluminescence and PMT Chemiluminescence chemical reaction produces a molecule (usually a radical) in an electronically excited state which can decay by emitting a photon (light) PMT Photomultiplier tube few photons many e - ; low noise amplifier OH* H 2 :CO=50:50, =1.0, =1.2nm Methane, =1.22, =0.8nm CH* C 2 * photons e - CH+O 2 OH* + CO C 2 H+O CH* + OH (nm) Rijke Tube Lab

6 Experiment You will acquire acoustic pressure and PMT data at different burner heights You will make observations of flame behavior in resonance and out Rijke Tube Lab - 11 Lab Info Location ESM G2 When the next two weeks (only one week for each group) there will also be a 2 nd lecture next week Safety all experiments with combustion are potentially hazardous follow all the precautions outline in the lab manual regarding igniting and operating the combustor Rijke Tube Lab