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To achieve very low NOx emission levels, lean-premixed gas turbine combustors have been commercially implemented which operate near the fuel-lean flame extinction limit. Near the lean limit, however, flashback, lean blowoff, and combustion dynamics have appeared as problems during operation. To help address these operational problems, a combustion control and diagnostics sensor (CCADS) for gas turbine combustors is being developed. CCADS uses the electrical properties of the flame to detect key events and monitor critical operating parameters within the combustor. Previous development efforts have shown the capability of CCADS to monitor flashback and equivalence ratio, and progress has been made on detecting and measuring combustion instabilities. In support of this development, a highly instrumented atmospheric combustor has been used to measure the pressure oscillations in the combustor, the ultraviolet flame emission, and the flame ion field at the premix injector outlet and along the walls of the combustor. This instrumentation allows examination of the downstream extent of the combustion field using both the ultraviolet (mostly OH*) emission and the corresponding electron and ion distribution near the walls of the combustor. During testing the combustion dynamics were controlled using a fuel feed impedance control technique. This provided flame ionization measurements for both steady and unsteady combustion, without changing the operating parameters of the combustor. Previous testing in this combustor had fewer data acquisition channels, and did not include a full implementation of a CCADS centerbody. This testing included both the guard and sense CCADS electrodes installed on the nozzle centerbody, and an array of 14 wall mounted spark plugs to monitor the flame ionization downstream along the walls of the combustor. This paper reports the results of this testing, focusing on the relationship between the flame ionization, ultraviolet flame emission, and pressure oscillations. Tests were run over a matrix of equivalence ratios from 0.6 to 0.8, with inlet reference velocities of 20 and 25 m/s. The acoustics of the fuel system for the combustor were tuned using an active-passive technique with an adjustable quarter-wave resonator. Data processing included computing the logarithm of the real-time current signal from the guard electrode, to compensate for the exponential decay of the potential field from the electrode. The data show the standard deviation of the guard current to be the most promising statistic investigated for correlation with the standard deviation of the chamber pressure. This correlation could expand the capabilities of CCADS to allow for dynamic pressure monitoring on commercial gas turbines without a pressure transducer.Copyright © 2006 by ASME |
