The Measurement and Application of Electric Effects in Combustion
- Author(s): Murphy, Daniel Corrigan
- Advisor(s): Fernandez-Pello, Carlos
- et al.
The existence of interactions between flames and electric fields has been known for quite some time and has received experimental and theoretical study over the years but remain an active topic of research in the combustion community. The present work specifically investigates the sensitivity of premixed flames electric fields. Prior work has demonstrated that electric fields may be used to enhance and control combustion, but full potential and range of applications for this effect have not been explored. Multiple theories have been presented to explain the process, but there is not yet a truly complete understanding of how electric fields fundamentally change the process of combustion. This work serves to explore new applications of electric-flame interactions and to provide experimental measurements to support the development of detailed theoretical models.
Thermo-diffusive and acoustic instabilities in freely propagating flames can trigger the formation of wrinkled flames and turbulence, which may or may not be desirable in different scenarios. Electric fields present a means to interact with such instabilities either by generating direct hydrodynamic forces ('ionic wind') or by modifying the rate of combustion. Experiments performed on downward-propagating hydrocarbon flames have demonstrated that electric fields can be applied to excite or suppress existing instabilities and control the onset of turbulence. Numerical models following from these experiments indicate that subtle spatial variations in chemical reactivity can achieve similar control of inherently unstable flames.
A practical limitation of combustion is the phenomenon of quenching, where heat losses to the surroundings extinguish a flame. The enhancement achieved by electric fields is shown to dramatically increase flame propagation speed and reduce quenching in methane-air flames, which may permit the miniaturization of combustion-based power generation systems. In the course of these experiments, flames were found to exhibit different behavior depending on the direction of the applied field. This result is consistent with a proposed model of ion transport in laminar flames, although the experiments have shown a greater degree of enhancement than predicted.
Ultimately, modeling the electrical aspects of combustion should be based on detailed accounting of the ion species present in flames. To support the ongoing development of detailed ion-chemistry mechanisms, a non-intrusive microwave interferometer for use in shock tube studies of combustion kinetics was developed. This was used to measure the formation and consumption of free electrons in shock-induced combustion. The range of equivalence ratios tested and the variety of hydrocarbon fuels used provide a rich dataset. Comparison of these results to complementary chemical kinetics simulations have been used to validate proposed improvements to existing mechanisms.