Laser-based temperature diagnostics in practical combustion systems

Abstract

Today’s energy supply relies on the combustion of fossil fuels. This results in emissions of toxic pollutants and green-house gases that most likely influence the global climate. Hence, there is a large need for developing efficient combustion processes with low emissions. In order to achieve this, quantitative measurement techniques are required that allow accurate probing of important quantities, such as e.g. the gas temperature, in practical combustion devices. Diagnostic techniques: Thermocouples or other techniques requiring thermal contact are widely used for temperature measurements. Unfortunately, the investigated system is influenced by probe measurements. In order to overcome these drawbacks, laser-based thermometry methods have been developed, that are introduced and compared in this work. Special emphasis is set on a recently developed multi-line technique based on laser-induced fluorescence (LIF) excitation spectra of nitric oxide (NO). This calibration-free temperature imaging method was optimized within this thesis such that accurate temperature measurements are possible in practical, harsh environments. Numerical and experimental studies were conducted to identify ideal spectral excitation and detection strategies. The limited accuracy of this time-averaging technique in turbulent systems was investigated. In cooperation with T. B. Settersten (Sandia, USA), energy transfer processes during quenching of NO LIF were quantified. These processes are not understood so far and hamper the application of saturated LIF spectroscopy. In collaboration with Prof. R. K. Hanson (Stanford University, USA) a two-line thermometry sensor based on tunable diode-laser absorption spectroscopy (TDLAS) of water was optimized. Applications: NO LIF and H2O TDLAS were applied to quantitatively measure the gas temperature over a wide range of pressures (3 – 500 kPa) and temperatures (270 – 2200 K). With multi-line NO-LIF thermometry, gas-temperature fields in spray flames were obtained that have been used to validate numerical models for spray combustion developed by Prof. E. Gutheil (Heidelberg University). In cooperation with the Robert Bosch GmbH, Germany, this technique was used to quantify the evaporative cooling in internal-combustion (IC) engine-relevant pulsed fuel-sprays. NO-LIF thermometry was compared to soot pyrometry, has been applied to sooting high-pressure flames, and the data was taken to calculate soot-particle sizes with laser-induced incandescence. In collaboration with Toyota Central R&D Labs, Japan, the temperature distributions in boundary layers of solid-wall quenched flames were measured. This data enables quantitative LIF species measurements and optimization of the IC engine thermal management. In a nano-particle flame-synthesis reactor, both techniques were applied to measure the gas temperature, which is taken to validate numerical simulation codes for nano-particle formation developed at the University of Duisburg-Essen. In cooperation with Shinko Electric Industries, Japan, and Prof. J. Warnatz (Heidelberg University), H2O TDLAS was applied to optimize a direct-flame solid-oxide fuel cell system. The versatile measurement techniques developed and improved within this thesis enable quantitative probing of the gas temperature in practical combustion devices. Accurate knowledge of this important quantity allows developing efficient power plants and engines with low emissions of green-house gases and toxic pollutants.