Imaging combustion using electrical capacitance tomography

It is well known that flames are rich in charged particles, ions and free electrons and such electrical activity will modify the permittivity and conductivity of the reaction zone. Work at UMIST over the past decade has led to the development of sensitive circuitry to detect such electrical changes within an enclosed volume. A commercially available electrical capacitance tomography (ECT) system, based on a switched capacitor circuit, was used in the investigations reported in this paper. Although nominally a capacitance measuring circuit, e.g. monitoring changes in permittivity, the ECT system also responds to changes in conductivity. In flames, the latter is normally the dominant effect. The ECT system has been connected to six electrodes inside a sealed combustion chamber, and used to monitor laminar methane/air flame propagation. The results were encouraging but inferior in spatial and temporal resolution to those obtained using high-speed Schlieren photography. A new (open) combustion can has been built, which has 12 electrodes, thus giving better spatial resolution. The new model is aimed at jet engine combustion studies. Here two factors are of particular importance: 1. Is the flame centrally located? and 2. Is the flame stable? The sensitivity of the ECT system increases rapidly as the flame approaches the electrodes and can thus be expected to highlight the two factors noted above