Some Experiments on Thermo - Acoustics of RIJKE Tube with Geometric Modifications and Forced Vorticity S.D. Sharma Aerospace Engineering Department IIT Bombay

Scope of Study To develop a test setup consisting of a horizontal Rijke tube driven by a flame for experimental investigation of the thermo-acoustics. To develop a test setup consisting of a horizontal Rijke tube driven by a flame for experimental investigation of the thermo-acoustics. To study the effects of introduction of streamwise vorticity and certain geometric modifications including concentric tubes on thermo-acoustic behavior of the Rijke tube. To study the effects of introduction of streamwise vorticity and certain geometric modifications including concentric tubes on thermo-acoustic behavior of the Rijke tube.

Preamble Thermo-acoustic instability appears inside chambers with heat source and mean flow when unsteady heat release is coupled in phase with pressure fluctuations. Such instability gives rise to excitation of acoustic modes resulting in noise. Thermo-acoustic instability appears inside chambers with heat source and mean flow when unsteady heat release is coupled in phase with pressure fluctuations. Such instability gives rise to excitation of acoustic modes resulting in noise. Typical examples include Rocket Motors, Pulsed Combustors, Noisy Industrial Burners and Heat Exchangers. Typical examples include Rocket Motors, Pulsed Combustors, Noisy Industrial Burners and Heat Exchangers.

Rijke Tube Rijke tube is the simplest possible device that demonstrates the thermo-acoustics instability. It is a vertical tube with open ends having its length to diameter ratio of about 10. When a wire gauge placed inside at about one fourth the tube length from its lower end is sufficiently heated with flame, a loud noise is produced. The reason for this noise is excitation of acoustic mode due to the coupling between the unsteady heat release and the pressure fluctuations that is enabled by the low speed flow driven by the convective currents. Rijke tube is the simplest possible device that demonstrates the thermo-acoustics instability. It is a vertical tube with open ends having its length to diameter ratio of about 10. When a wire gauge placed inside at about one fourth the tube length from its lower end is sufficiently heated with flame, a loud noise is produced. The reason for this noise is excitation of acoustic mode due to the coupling between the unsteady heat release and the pressure fluctuations that is enabled by the low speed flow driven by the convective currents.

Schematics of Rijke Tube Present Test Setup Rijke Model Heated metal mesh / wire gauze

Rijke Tube Demonstration

Rijke Tubes: Two Different Frequencies with Phase Difference

Development of Test Setup Various Tube Configurations Used: Various Tube Configurations Used: L/D=9.23 Port with optical window for viewing flame and LDV measurements

L = 705 mm 100 Φ 75 Φ 65 Φ 50 Φ 40 Φ L /D: Concentric tubes arrangement: 40 mm Φ and 65 mm Φ tubes each with L=200 mm and 300 mm inside the 75 mm Φ tube.

10.5 mm 31 mm 72 o sweep Delta Fin Vortex Generator at 30 degree angle of attack Stepped collar for fixing vortex generators 6 Contra-rotating 6 Co-rotating 8 Contra-rotating 8 Co-rotating vortex generators vortex generators Vortex Generators Vortex Generators

Preliminary Design

Improved Design

Interior Details of Plenum Chamber

Improved Design with Insulated Tube

Plenum Chamber with Cooling

Suction-end of the Plenum Chamber ThermocoupleTwin Blowers Cooling air Flow Orifice meter

Instrumentation Pressure Transducers K Type Thermocouple Rotameter

Visualization of Vortex Flow

Flow Through Vortex Generators

Some Observations from Preliminary Experiments A certain flow velocity for a fixed fuel mass flow rate triggers the acoustic instability that results in intense noise. A certain flow velocity for a fixed fuel mass flow rate triggers the acoustic instability that results in intense noise. Hysteresis effects on flame position inside the tube. Hysteresis effects on flame position inside the tube. Introduction of vorticity advances the instability even when the flame is lean and closer to the entry of the tube. Introduction of vorticity advances the instability even when the flame is lean and closer to the entry of the tube. With increase in the equivalance ratio marginal increase in peak pressure and frequency was observed. With increase in the equivalance ratio marginal increase in peak pressure and frequency was observed.

Temperature Profile at Various Axial Locations

Wall Pressure Distribution (improved setup)

Wall Pressures with Vortex Generators

Wall Pressures Comparison

Wall Pressure Spectra

Vorticity Effect on Temperature

Frequency Spectra of Pressures Tube=40 Φ, A/F=220, Burner x/L=0.128 P1 P7 P1 P7 Tube=40 Φ, A/F=220, Burner x/L=0.624

Burner x/L=0.128 Burner x/L=0.624 Tube=75 Φ, A/F=530 Burner x/L=0.128 Burner x/L=0.624 Frequency Spectra of Pressures Tube=75 Φ, A/F=280 Burner x/L=0.128 Burner x/L=0.624

Frequency Spectra of Pressures Tube=100 Φ, A/F=530 at P1 Burner x/L=0.128 Burner x/L=0.624 Tube=100 Φ, A/F=280 at P1 Burner x/L=0.128 Burner x/L=0.624

Frequency Spectra for Concentric Tubes 40 Φ, Longer 40 Φ, Shorter A/F=530, Burner x/L= Φ, Longer 50 Φ, Shorter A/F=530, Burner x/L=0.128

Wall Pressure Distribution Tube = 40 Φ, A/F=220Tube = 50 Φ, Burner x/L=0.128 Tube = 50 Φ, Burner x/L=0.624 Tube = 65 Φ, Burner x/L=0.128

Wall Pressure Distribution Tube = 65 Φ, Burner x/L=0.624Tube = 75 Φ, Burner x/L=0.128 Tube = 75 Φ, Burner x/L=0.624Tube = 100 Φ, Burner x/L=0.128

Pressure and Temperature Distributions in Concentric Tubes Axial Pressure Distribution: A/F ratio = 530, Burner x/L=0.128 Radial temperature profile at T3, A/F = 530, Burner x/L=0.128

Hysteresis Effect: Vortex Generators on Burner

Hysteresis Effects: Vortex Generators on Burner

Hysteresis Effect: Vortex Generators on Burner

Effect of Vortex Ring over Flame

Some Remarks 40 and 50 mm diameter tubes had long spatial range of instability for burner positions up to x/L=7. 40 and 50 mm diameter tubes had long spatial range of instability for burner positions up to x/L=7. This range was found to reduce for increasing diameter of the tube. This range was found to reduce for increasing diameter of the tube. Concentric tubes tend to produce thermo-acoustics earlier compared to the plain Rijke tube. Concentric tubes tend to produce thermo-acoustics earlier compared to the plain Rijke tube. In concentric tube, the flame was always blue and the noise levels were amplified for all the cases when burner was inside up to x/L=0.5. In concentric tube, the flame was always blue and the noise levels were amplified for all the cases when burner was inside up to x/L=0.5.