Jimmy Olofsson

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.

Optical Diagnostics of Laser-Induced and Spark Plug-Assisted Hcci Combustion

HCCI (Homogeneous Charge Compression Ignition), laser-assisted HCCI and spark plug-assisted HCCI combustion was studied experimentally in a modified single cylinder truck-size Scania D12 engine equipped with a quartz liner and quartz piston crown for optical access. The aim of this study was to find out how and to what extent the spark, generated to influence or even trigger the onset of ignition, influences the auto-ignition process or whether primarily normal compression-induced ignition remains prevailing. The beam of a Q-switched Nd:YAG laser (5 ns pulse duration, 25 mJ pulse energy) was focused into the centre of the cylinder to generate a plasma. For comparison, a conventional spark plug located centrally in the cylinder head was alternatively used to obtain sparks at a comparable location. No clear difference in the heat releases during combustion between the three different cases of ignition start could be seen for the fuel of 80/20 iso-octane/n-heptane used. However, with optical diagnostic methods, namely PLIF (Planar Laser-Induced Fluorescence), Schlieren photography and chemiluminescence imaging, differences in the combustion process could be evaluated.

Effect of Turbulence on HCCI Combustion

This paper presents large eddy simulation (LES) and experimental studies of the combustion process of ethanol/air mixture in an experimental optical HCCI engine. The fuel is injected to the intake port manifolds to generate uniform fuel/air mixture in the cylinder. Two different piston shapes, one with a flat disc and one with a square bowl, were employed to generate different in-cylinder turbulence and temperature field prior to autoignition. The aim of this study was to scrutinize the effect of in-cylinder turbulence on the temperature field and on the combustion process. The fuel tracer, acetone, is measured using laser-induced fluorescence (LIF) to characterize the reaction fronts, and chemiluminescence images were recorded using a high-speed camera, with a 0.25 crank angle degree resolution, to further illustrate the combustion process. Pressure in the cylinder is recorded in the experiments. Spatial and temporal resolved LES was used to gain information on the turbulence mixing, heat transfer and combustion process. It was shown that gas temperature in the piston bowl is generally higher than that in the squish, leading to an earlier ignition in the bowl. Compared to the disc engine, the square bowl engine has a higher temperature inhomogeneity owing to the turbulence wall heat transfer. The experimentally observed higher combustion duration and slower pressure rise rate in the square bowl engine as compared to the disc engine can be explained by the higher temperature inhomogeneity in the square bowl engine.

Development of high temporally and spatially (three-dimensional) resolved formaldehyde measurements in combustion environments

In the present article a multi-YAG laser cluster and a framing camera have been applied for ultrahigh framing rate and three-dimensional measurements of formaldehyde distribution in flames and engines. The measurement technique utilizes a laser/detection system which has been adopted for the generation of eight laser pulses at 355 nm. By combining these lasers with a framing camera, short movies showing the formaldehyde distributions in combustion phenomena can be recorded, by means of planar laser-induced fluorescence. The technique is successfully demonstrated in a laboratory flame as well as in an engine. In addition to the temporally resolved experiments also three-dimensional measurements are performed by sweeping the eight generated laser sheets across the flame by the use of a fast scanning mirror. By proper triggering of the laser sweep and the detector a three-dimensional image showing the formaldehyde distribution in the flame can be created, which is also demonstrated.

High-Speed LIF Imaging for Cycle-Resolved Formaldehyde Visualization in HCCI Combustion

High-speed laser diagnostics was utilized for single-cycle resolved studies of the formaldehyde distribution in the combustion chamber of an HCCI engine. A multi-YAG laser system consisting of four individual Q-switched, flash lamp-pumped Nd:YAG lasers has previously been developed in order to obtain laser pulses at 355 nm suitable for performing LIF measurements of the formaldehyde molecule. Bursts of up to eight pulses with very short time separation can be produced, allowing capturing of LIF image series with high temporal resolution. The system was used together with a high-speed framing camera employing eight intensified CCD modules, with a frame-rate matching the laser pulse repetition rate. The diagnostic system was used to study the combustion in a truck-size HCCI engine, running at 1200 rpm using n-heptane as fuel. By using laser pulses with time separations as short as 70 μs, cycle-resolved image sequences of the formaldehyde distribution were obtained. Thus, with this technique it is possible to follow the formaldehyde formation and consumption processes within a single cycle. The combustion evolution was studied in terms of the rate and spatial structure of formaldehyde formation and consumption for different engine operating conditions, e.g. different stoichiometries. Also, the impact on the rate of heat-release was investigated.

High-Speed PLIF Imaging for Investigation of Turbulence Effects on Heat Release Rates in HCCI Combustion

High-speed laser diagnostics was utilized for single-cycle resolved studies of the fuel distribution in the combustion chamber of a truck-size HCCI engine. A multi-YAG laser system consisting of four individual Nd:YAG lasers was used for planar laser-induced fluorescence (PLIF) imaging of the fuel distribution. The fundamental beam from the lasers at 1064 nm was frequency quadrupled in order to obtain laser pulses at 266 nm suitable for excitation of acetone that was used as fuel tracer. Bursts of up to eight pulses with very short time separation were produced, allowing PLIF images with high temporal resolution to be captured within one single cycle event. The system was used together with a high-speed framing camera employing eight ICCD modules, with a frame-rate matching the laser pulse repetition rate. The combustion evolution was studied in terms of spatial distribution and rate of fuel consumption for different engine hardware configurations as well as operating conditions, e.g., different stoichiometries and combustion phasing. Two different piston crown geometries were used for altering the degree of turbulence in the combustion chamber. In addition to the optical investigations, the impact of turbulence effects was also studied by calculating the rate of heat release and combustion phasing from the pressure trace. (Less)

Imaging Studies of Swirl-Stabilized Premixed Flames

Swirl-stabilized flames have been studied using imaging techniques, for LES code validation. Rapid framing PLIF has been combined with simultaneous PIV. Rapid imaging of chemiluminescent emission provided information on dynamic events such as flash-back.