Technical Feasibility of Electric Field Control for Turbulent Premixed Flames

Abstract

The utilization of electric fields for the control of technical combustion processes (EFCC, Electric Field Controlled Combustion) provides an interesting approach in present-day discussions regarding reductions in CO 2 emission levels and improved fuel efficiency. The technical feasibility of electric field control of turbulent premixed flames is demonstrated for the first time at technically relevant operating conditions. The technical criteria addressed include the efficiency of control at thermal powers above 50 kW, operating pressures in the order of 10 bar, air preheating, increased mixture exit velocities as well as swirl-stabilized flames. Capacitive coupling of electric fields is explored against the background of preferably non-intrusive integration into existing gas turbine systems. It is demonstrated that electric fields can be transmitted through ceramic burning chamber walls using capacitive coupling by electrodes deposited as coatings on the walls, thus avoiding sensitive mechanical parts as weak spots within the combustion chamber. No reasonable limitations are observed that might hinder a possible realization of flame control for technical combustion processes using EFCC. IntroductionThe influence of electric fields on flames is known for almost 200 years and was mentioned for the first time in 1814 [1]. In the 1960's and 1970's, several different studies attempted to characterize the observed effects and to utilize them for technical applications. The two main effects observed that are of technical importance are an increase in flame stability and a reduction of pollutant emissions. Therefore, the utilization of electric fields for the control of technical combustion processes (EFCC, Electric Field Controlled Combustion) provides an interesting approach in present-day discussions regarding reductions in CO 2 emission levels and improved fuel efficiency.Most of the previous studies in the area were limited to simple atmospheric laboratory flames with low thermal power and laminar or low-turbulence flow fields [2][3][4][5][6][7][8][9][10][11]. In order to extend the scope of research to technically relevant conditions, a new study was recently begun, dealing with the technical feasibility of flame control by electric fields in stationary gas turbine combustors. This study was conducted at the Institute for Engineering Thermodynamics (LTT) at the FriedrichAlexander-University of Erlangen-Nuremberg in close cooperation with the Siemens AG company within the framework of the research initiative "Power Plants for the 21 st Century" (KW21). It was shown that the application of electric fields for flame control can broaden the regime for stable operation as well as decrease overall pollutant emissions at relevant operating conditions, i.e., increased pressure, high exit velocity and air preheating as well as swirl stabilization. Moreover, in order to avoid the requirement for the installation of metallic electrodes inside the combustion chamber, the coupling of an electric field in a capacitive manner t...

Overview of Electric Field Applications in Energy and Process Engineering

“…However, also capacitive coupling of electric fields was explored in Ref. [35] for non-intrusive integration of mechanical parts into existing technical combustors (e.g., of gas turbines). Electric fields could be transmitted through ceramic walls of the combustion chamber for possible realization of flame control.…”

“…In this case, Many fundamental experiments using lab-scale burners have been carried out for studying electric field enhanced combustion using premixed (see, e.g., [22][23][24][25][26][27][28][29][30]) and non-premixed flames [31,32]. Attempts for a technical application at higher thermal load were conducted by Weinberg et al for a meso-scale burner [33], by Sakhrieh et al for a Bunsen flame array [34] and Altendorfner et al [35] for a swirl flame that is usually applied in gas-turbine burners. Usually, in these studies the electrode was positioned close to the flame inside the combustion chamber.…”

“…The flame stability can be improved in terms of increased lean blow-off-limit of (ultra) lean flames [34,35] and by reducing the lift-off height of non-premixed flames [36]. In general, premixed flames stabilize at that position where the incoming velocity of the gas mixture and the flame speed are in balance.…”

“…In case that the gas velocity becomes too high or the fuel/air mixture becomes to lean (followed by a reduced flame speed), the flame tends to lift off the burner and/or will finally blow off. Altendorfner et al [35] showed that an electric field could lead to an increase of the lean blow-off limit of up to 8%. The maximum AFR without electric field was 1.14.…”

“…Sun et al [35] used high-speed photography for studying the flame response of a flat-flame in an electric field. They observed first flame luminosity changes 2 ms after activation of the e-field, whereas the total flame response duration was about 36 ms. Laser-based experiments were conducted for laminar premixed Bunsen flames in a weak DC electric field.…”