Using plasmas for high-speed flow control and combustion control

The present dissertation discusses experiments on characterization of arc filament plasmas used in plasma actuators for high-speed flow control, as well as experimental studies of chemiluminescence and chemi-ionization for flame emission and combustion control.The Localized Arc Filament Plasma Actuators (LAFPA) used for high-speed flow control have been powered by an AC plasma generator, a pulsed DC plasma generator and a pulsed RF plasma generator. The custom-built 8-channel pulsed DC high-voltage plasma generator can power multiple plasma actuators, with independent control of pulse repetition rate (0--200 kHz), duty cycle, and phase for individual actuators. The pulsed RF plasma generator produces repetitive RF voltage bursts, with externally controlled burst frequency and burst duration. In the present experiments, two single-channel pulsed RF plasma generators, controlled individually, have been used to operate two actuators. Actuator current and voltage measurements demonstrated the time-averaged power of both pulsed DC and pulsed RF actuators to be low, 10-20 W at a 10% duty cycle. The main advantages of these actuators are high bandwidth and high forcing amplitude, resulting in flow control authority (mixing enhancement and noise reduction) in a wide range of operating conditions. Emission spectroscopy temperature measurements of the pulsed arc filament plasma, using nitrogen second positive system bands, showed rapid temperature increase over the first 5--20 mus of operation, at a rate of approximately 1000°C/10 mus. A pulsed RF plasma temperature is significantly higher than a pulsed DC plasma temperature at the same pulse/burst duration and discharge power. Modeling calculations using a time-dependent, quasi-one-dimensional arc filament model showed that rapid localized heating in the filament on a microsecond time scale generates strong compression waves. Presence of compression waves generated by a repetitively pulsed arc discharge has also been detected in the experiments.The effect of electrons present in chemi-ionized supersonic flows of combustion products on flow emission is studied experimentally. For this, a stable ethylene/oxygen/argon flame is sustained and nearly complete combustion is achieved in a combustion chamber at a stagnation pressure of P0=1 atm. Ultraviolet and visible emission is detected both from the combustion chamber and from a M=3 flow of combustion products. Temperature in the combustor, inferred from CH visible emission spectra, is T0=1900 +/-100 K. Electron density in M=3 flow of combustion products, ne &ap 3·108 cm-3, at an ionization fraction of ne/N &ap 10-9, has been measured using Thomson discharge. This corresponds to an electron density of ne0 &ap 5·109 cm-3 in the combustor. The results show that nearly all electrons can be removed from the supersonic flow of combustion products by applying a moderate transverse electric field. However, no effect of electron removal on CH and C2 emission has been detected. Also, electron removal did not affect NO beta band and CN violet band emission when nitric oxide was injected into the combustion product flow.Chemi-ionization current measured in the supersonic flows of combustion products has been used for feedback combustion control. The experiments showed that time-resolved chemi-ionization current is in very good correlation with the visible emission (CH and C2 bands) from ethylene--air and propane--oxygen--argon flames in the combustor at unstable combustion conditions. The experiments also demonstrated that in lean fuel-oxidizer mixtures at stable combustion conditions the current is nearly proportional to the equivalence ratio in the combustor. Chemi-ionization current signal from the combustion product flow has been used for feedback combustion control, to maintain the equivalence ratio in the combustor at the desired level and adjust it, if necessary. In particular, chemi-ionization current was used to control an actuator valve in the fuel delivery line and to vary the fuel mass flow rate. This approach has also been used to counter external perturbations used to deliberately change the equivalence ratio in the combustor. The results suggest that the present method can be used to operate a combustor at fuel lean conditions and to prevent flame extinction by increasing the fuel flow rate before the blow-off occurs. This approach can be used to develop a simple and straightforward combustion control technique.