A. Dreizler

Simultaneous PIV/PH-PLIF, Rayleigh thermometry/OH-PLIF and stereo PIV measurements in a low-swirl-flame

The diagnostic techniques for simultaneous velocity and relative OH distribution, simultaneous temperature and relative OH distribution, and three component velocity mapping are described. The data extracted from the measurements include statistical moments for inflow fluid dynamics, temperature, conditional velocities, and scalar flux. The work is a first step in the development of a detailed large eddy simulation (LES) validation database for a turbulent, premixed flame. The low-swirl burner used in this investigation has many of the necessary attributes for LES model validation, including a simplified interior geometry; it operates well into the thin reaction zone for turbulent premixed flames, and flame stabilization is based entirely on the flow field and not on hardware or pilot flames.

SPARK IGNITION OF TURBULENT METHANE/AIR MIXTURES REVEALED BY TIME-RESOLVED PLANAR LASER-INDUCED FLUORESCENCE AND DIRECT NUMERICAL SIMULATIONS

By use of high-speed planar laser-induced fluorescence (PLIF) imaging, the evolution of turbulent reactive flows was recorded and studied in real time in a filmlike manner. The technique was used to track the concentration field of the OH radical, which was produced during spark ignition of a turbulent methane/air mixture. The results were compared qualitatively to a two-dimensional direct numerical simulation of the same system using a detailed chemical mechanism and a detailed transport model.

Gas compositional and pressure effects on thermographic phosphor thermometry

In the present study, the influence of gas compositional and pressure conditions on thermographic phosphor thermometry was investigated. A heatable pressurized and optical accessible calibration chamber was built to measure the phosphorescence decay time at different temperatures as well as at different partial and absolute pressures. At room temperature, the absolute pressure could be increased to 30 bar. To vary the gas composition, nitrogen, oxygen, carbon dioxide, methane, helium as well as water vapour were used. Three different phosphors were investigated: Mg4FGeO6:Mn, La2O2S:Eu and Y2O3:Eu. Phosphorescence was excited by the third and the fourth harmonics of a pulsed Nd:YAG-laser (355 nm and 266 nm, respectively) and recorded temporally resolved by a photomultiplier. Mg4FGeO6:Mn as well as La2O2S:Eu were not influenced significantly by varying partial and absolute pressures. In contrast, Y2O3:Eu showed a strong sensitivity on the oxygen concentration of the surrounding gas phase as well as irreversible changes in the phosphorescence decay time after increasing the absolute pressure.

Spark ignited hydrogen/air Mixtures : Two dimensional detailed modeling and laser based diagnostics

This study reports a detailed 2-D model which describes the spark ignition of an initially quiescent hydrogen/air mixture. The model includes the compressible Navier-Stokes equations, detailed chemistry and molecular transport in the gas phase as well as heat conduction to the electrodes. The spark is modeled for the phases subsequent to breakdown using the Maxwell equations for quasi-stationary conditions for the electric field. Initial and boundary conditions necessary for the simulations are chosen in accordance with experimental values. Heat conduction to the electrodes and different electrode shapes are investigated. The influence of these parameters on the shape of the initial flame kernel is discussed and compared qualitatively to experimental results. Spark ignition experiments are performed using a highly reproducible ignition system. Shapes of the early flame kernels are monitored by 2-D laser-induced fluorescence (PLIF) imaging of OH radicals produced during the ignition and the combustion process. The investigations are performed for different equivalence ratios. In addition, for a central position within the flame kernel, temperatures are measured at different times after ignition using vibrational coherent anti-Stokes Raman spectroscopy (CARS) of nitrogen.

Characterization of manganese-activated magnesium fluorogermanate with regards to thermographic phosphor thermometry

Apart from the temperature, four parameters were investigated for their impact on the phosphorescence characteristics of Mg4FGeO6:Mn with regards to phosphor thermometry: the dopant concentration, the laser pulse energy, gas compositional and pressure effects as well as irreversible changes due to heat treatments. Five specially produced phosphors with different dopant concentrations as well as commercially available Mg4FGeO6:Mn were investigated in the form of coatings and pure powder. The phosphorescence was excited by the third harmonic of a pulsed Nd:YAG laser (355 nm). The lifetime decay as well as the emission spectra of the subsequently emitted phosphorescence were determined. Generally, the decay time decreased with increasing dopant concentration, with increasing laser pulse energy and for coatings also with increasing maximum temperature and duration of heat treatments, whereas the impact of the laser power was minimized by a modified evaluation routine of the decay time. Gas compositional and pressure effects did not have a significant influence on the decay time of Mg4FGeO6:Mn. Neither the variation of the dopant concentration nor the exposure to heat treatments influenced the shape of the emission spectra in any way.

High-speed phosphor thermometry.

Phosphor thermometry is a semi-invasive surface temperature measurement technique utilising the luminescence properties of doped ceramic materials. Typically, these phosphor materials are coated onto the object of interest and are excited by a short UV laser pulse. Up to now, primarily Q-switched laser systems with repetition rates of 10 Hz were employed for excitation. Accordingly, this diagnostic tool was not applicable to resolve correlated temperature transients at time scales shorter than 100 ms. This contribution reports on the first realisation of a high-speed phosphor thermometry system employing a highly repetitive laser in the kHz regime and a fast decaying phosphor. A suitable material was characterised regarding its temperature lifetime characteristic and its measurement precision. Additionally, the influence of laser power on the phosphor coating was investigated in terms of heating effects. A demonstration of this high-speed technique has been conducted inside the thermally highly transient system of an optically accessible internal combustion engine. Temperatures have been measured with a repetition rate of 6 kHz corresponding to one sample per crank angle degree at 1000 rpm.

High-speed micro particle image velocimetry studies of boundary-layer flows in a direct-injection engine

Mass and energy exchange processes near the in-cylinder walls have become increasingly important for internal combustion engine research and development. Heat-transfer and fuel-deposition processes are not well enough understood to fully predict their magnitude and temporal and spatial variation. To a large extent, improvements in modeling are limited because of a lack of experimental guidance on the dynamics of boundary-layer flows in engines. High-speed particle image velocimetry and micro particle image velocimetry were used to study the boundary-layer flow field at the cylinder head of a motored internal combustion engine for three different engine speeds (400, 800 and 1100 r/min). Velocity measurements were taken throughout the compression stroke and the beginning of the expansion stroke, and comparisons to the frequently used law-of-the-wall were made. The low-resolution particle image velocimetry images showed that the average bulk flow maintained a tumble flow throughout the compression and expansion strokes and velocities scaled with engine speed. Adequate spatial resolution of the boundary-layer flow required the use of high-resolution (micro particle image velocimetry) measurements which showed that the log-law does not accurately describe the experimental boundary-layer structure in the outer layer ( 30 < y + < 50 ). Using the log-law, the velocity for earlier crank angles (180–260 crank angle degrees) was over predicted while the velocity for later crank angles (300–330 crank angle degrees) was under predicted, as engine speed increased. However, the experimental velocity profiles near the wall ( y + < 4 ) matched the description of the viscous sublayer ( u + = y + ). Furthermore, it was observed that the thickness of the viscous sublayer decreased as the engine speed increased. The viscous sublayer could not be adequately resolved at 1100 r/min because of the limitations in adequate seeding density near the surface and the resulting loss of data points closest to the wall. The capability of high-speed particle image velocimetry measurements is demonstrated by the ability to identify and track vortical structures that move through the outer and buffer layers. Future work must address the evolution of such flow structures, along with measurements of temperature, to enable development and validation of boundary-layer models that are not restricted by the assumptions necessary for the law-of-the-wall such as steady bulk flow conditions and properties.

Thermal grating effects in infrared degenerate four-wave mixing fro trace gas detection

Abstract Degenerate four-wave mixing (DFWM) in the infrared region using a single mode CO 2 laser has been used for the detection of ethylene, sulfur hexafluoride and mixtures of these gases with nitrogen and argon in the pressure range up to one atmosphere. The investigations showed that for the low pressure regime ( 3 Pa) the formation of the DFWM signal was dominated by population gratings whereas for high pressures (>25 × 10 3 Pa) mostly contributions from thermal gratings gave rise to the observed DFWM signal. This behaviour is reproduced by a simple model prediction for these relaxation processes.

Experimental characterization of onset of acoustic instability in a nonpremixed half-dump combustor

This paper reports work on a nonpremixed half-dump combustor, in which methane is injected at the backward-facing step, and mixes and burns with the air flowing past the step in the unsteady recirculation zone. The flow and geometric parameters are widely varied, to gradually change from conditions of low-amplitude noise to excitation of high-amplitude discrete tones. The purpose of the work is to focus on the transition from the former condition to the latter, and to mark the onset of instability. Dimensionless groups such as the Helmholtz and Strouhal numbers are formed based on the observed dominant frequencies, whose variation with the air flow Reynolds number is used to identify the oscillations as those due to the natural acoustic modes or the vortex shedding process. High-speed chemiluminescence imaging reveals shedding of vortical structures in the flame zone. With variation in the conditions, flow-acoustic lock-on and transition from one vortex shedding mode to another is marked by nonlinearity in the corresponding amplitude variations. Such conditions are identified as the onset of instability in terms of the ratio of the flow time scale to the acoustic time scale and mapped against the operating fuel-air equivalence ratio of the combustor.

Time and spatially resolved LIF of OH A2Σ+(v′=1) in atmospheric-pressure flames using picosecond excitation

One dimensional imaging of fluorescence lifetimes was performed directly for the first time in a premixed, atmospheric-pressure ethylene/air flame. A picosecond laser system based on the microscope-DFDL (Distributed-Feedback Dye Laser) was used for excitation of OH. The method involves imaging a section of the flame onto the streak camera and evaluating a series of windows, each with a certain number of channels of the CCD-multichannel plate. In this way, changes in quenching efficiency can be monitored along a horizontal line through the flame. These direct lifetime measurements confirm the observations of relative lifetimes previously obtained for this flame by saturated 2D-LIF.