Chemiluminescence Based Sensors for Turbine Engines

Abstract

This work focuses on the use of naturally occurring optical emissions, specifically chemiluminescence, for sensing applications in active control and health monitoring of combustors. First, monitoring local equivalence ratio (φ), at the reaction zone, has been demonstrated using the ratio of CH to OH chemiluminescence. This ratio (CH*/OH*) increases monotonically with equivalence ratio, and the dependence on equivalence ratio has been shown to be a universal function for combustor configurations ranging from unconfined jet flames to swirl and dump stabilized combustors. There is essentially no difference between the CH*/OH* ratio for methane and natural (city) gas, but the ratio has a lower sensitivity to φ for n-heptane compared to methane or natural gas. The ratio was found to increase almost linearly with pressure for natural gas/methane combustion above 3 atm. Second, chemiluminescence emission from the combustor was used to detect precursor events to blowout, using a robust thresholding method. This method was shown to be successful in jet flames and swirl/dump stabilized combustors using premixed methane/air and nonpremixed Jet-A/air. This method gives the kind of information on proximity to blowout that can be used by an active control system to prevent lean blowout in low NOx turbine engine combustors. INTRODUCTION Both active control and health/performance monitoring systems for turbine engine combustors require knowledge of the state of the combustion processes within the combustor. For example, active control can be an efficient way to expand turbine engine combustor operating limits without loss of performance and safety. In the area of emissions control, it is known that NOx can be reduced by use of low fuel-air ratios in the flame region. However, operation under these conditions also makes the combustor prone to lean blowout (LBO) problems. Thus sensors that could give advance warning of LBO, in conjunction with an active control system, would *Graduate Research Assistant, Student Member AIAA † Undergraduate Research Assistant ‡ Associate Professor, Associate Fellow AIAA permit lower emissions operation. In a similar way, significant variations in local equivalence ratio in premixed or partially premixed combustors can lead to temperature nonuniformities that will increase NOx emissions and decrease the useful life of the turbine. Thus active control or engine health monitoring systems would be aided by sensors that could monitor fluctuations in local flame zone equivalence ratio inside a combustion chamber. In general, reliable and versatile sensors are required. For most engine applications, they must also provide measurements of conditions at locations away from the combustor wall, thus nonintrusive methods are preferred. In addition for active control systems that use state feedback, the sensor time response is also an important issue. Optical sensors offer the benefit of being able to gather data from extremely hostile environments (e.g., the combustion zone), and to do so over large regions of space. With the rapidly growing capability of these technologies for sensor hardware, there is an increased interest and need to develop data interpretation strategies that will allow optical flame emission data to be converted to meaningful combustor state information, such as heat release rate, proximity to LBO, and local flame zone equivalence ratio. There are a number of optical methods that can give information about the combustion process nonintrusively, e.g., optical emission, absorption, fluorescence and other spectroscopic methods. The focus of this work is on the simplest of all these techniques, viz., observing the naturally occurring, optical emissions from the combustor. While there are a number of sources for optical radiation from a combustor, the source most directly connected to the combustion reactions is chemiluminescence. This radiation is from high energy states of molecules (typically electronically excited states) that are produced by chemical reactions. Once produced, the excited molecules will transfer to lower energy states, in part by emitting light. This is known as chemiluminescence. Since the intensity of emission is proportional, in part, to the chemical production rate of the particular molecule, the chemiluminescence intensity can be related to (specific) chemical reaction rates. For this reason, chemiluminescence has been used previously as a rough measure of reaction rate and heat release rate. Thus chemiluminescence can