Control of Temperature Nonuniformity Based on Line-of-Sight Absorption

This paper describes the development and demonstration of a sensing approach for control of temperature nonuniformity (pattern factor) based on diode laser line-of-sight water absorption near 2 μm. A temperature nonuniformity variable is defined based on the difference in temperatures measured with two pairs of water lines. To reduce the complexity of the system the sensor relies on the ratio of the absorption line peaks rather than the more common, integrated absorption. Selection of appropriate lines and factors influencing sensor performance are described. Open loop control with this sensor is demonstrated in the exhaust plane of a stratified methane-air combustor. The exit temperature profile can be altered by changing the flow rates to two upstream injectors or with a synthetic jet mixing actuator. In both cases, the nonuniformity variable tracks the mixidness of the flow. I. Introduction New gas turbine engine designs are driven by the need for increased engine efficiencies, reduced environmental impact, and improved maintainability. This is true for both ground-based and aero-engines. To reach the desired levels of fuel economy, emissions and lifetime, control systems will likely have to be employed at all levels of operation. These intelligent engine systems can help reduce emissions, ensure safer operation and enable real-time engine system health monitoring. Sensing and control strategies are especially important at off -design operating conditions and as the engines ages. Pattern factor sensing and control is one element of the intelligent engine concept. For example, pattern factor (temperature profile) control can be used to increase turbine-blade lifetime. It may also help decrease combustor emissions through control of mixing in the combustor, and can be used for health monitoring. Development of sensors is a key step required for effective control systems. Optical sensing techniques are a natural choice for combustion environments because they are non-intrusive, fast, and can be used in the extreme conditions associated with combustion. Diode laser based, active sensing approaches offer many advantages. Because of their commercial applications in telecommunica tions, diode lasers are relatively cheap and widely available, making them ideal light-sources for optical combustion sensors. Other advantages of diode lasers are their compatibility with fiber optics, fast response time, narrow spectral features, and good sensitivity. They are also small, rugged, and tunable Diode laser absorption-based combustion sensors for temperature, pressure, velocity, and species concentrations have been demonstrated both in the laboratory and in limited in situ applications. 1-4 Generally these sensors use the easy tuning ability of diode lasers to resolve spectral features and compute path-averaged quantities of interest in combustion flows. In this paper, we describe and demonstrate an optical sensing approach for temperature nonuniformity based on line-of-sight water absorption near 2 μm. Our goal is to develop a sensor for pattern-factor control that is fast and relatively simple to implement in gas-turbine engines. Water was chosen as the marker species because it is a major combustion product that has relatively strong absorption lines throughout a large portion of the spectrum (Fig. 1). Moreover, it is easy to find water lines with little or no interference from other combustion products. It is important to note that the sensor is not intended to provide especially accurate temperature measurements, but rather is designed for sensing changes in the temperature profile with good time response. The rotational-vibrational