Johan Sjöholm

Ultra-high-speed pumping of an optical parametric oscillator (OPO) for high-speed laser-induced fluorescence measurements

The feasibility of pumping an optical parametric oscillator (OPO) with an ultra-high repetition rate multi:YAG laser system, producing a burst of up to eight high-energy pulses, has been investigated. For this investigation an OPO with a bandwidth around 5 cm−1, together with a frequency doubling crystal, was selected. In some laser-induced fluorescence measurements the large linewidth from the OPO can be advantageous as several lines can be excited simultaneously avoiding the saturation effects of individual lines. The energy output from the OPO as a function of pulse separation was measured down to pulse separations of 400 ns and was found to be completely independent of the pulse separation. The efficiency of the OPO unit, when optimized for single-pulse operation, was measured to be around 25% for all pulses, giving over 80 mJ at 585 nm output when pumped with ~350 mJ at 355 nm. This is similar to the specified efficiency for the OPO. The system was found to give a slightly lower efficiency when double pulsing the Nd:YAG lasers. This is attributed to a somewhat elongated pulse length from the Nd:YAG lasers giving a lower pump energy density. The system was applied for measuring high-speed planar laser-induced fluorescence images of OH radicals in a Bunsen burner.

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.

Comparison of Three Schemes of Two-Photon Laser-Induced Fluorescence for CO Detection in Flames

Two-photon excitation laser-induced fluorescence of carbon monoxide suffers from interference from mainly C2 and strong pressure quenching. This paper presents an investigation of three excitation/detection schemes for two-photon excitation laser-induced fluorescence on carbon monoxide. The schemes are evaluated for pressure and quenching partner dependencies and C2 interference. Three different emission bands lie in the Hopefield-Birge system: The Angstrom B1Σ+ → A1Πu band, with two-photon excitation through B1Σ+ ← X1Π around 230 nm; the Herzberg band C1Σ+ → A1Πu, with two-photon excitation through, C1Σ+← X1Π, around 217 nm; and the third positive group b3Σ→a3Π, also with excitation of B1Σ ← X1Π around 230 nm. The measurements are performed in laminar premixed flames with various equivalence ratios as well as in a high-pressure cell, where pressure and species concentrations are varied in order to investigate the fluorescence quenching dependence.

Air-Entrainment in Wall-Jets Using SLIPI in a Heavy-Duty Diesel Engine

Mixing in wall-jets was investigated in an optical heavy-duty diesel engine with several injector configurations and injection pressures. Laser-induced fluorescence (LIF) was employed in non-reacting conditions in order to quantitatively measure local equivalence ratios in colliding wall-jets. A novel laser diagnostic technique, Structured Laser Illumination Planar Imaging (SLIPI), was successfully implemented in an optical engine and permits to differentiate LIF signal from multiply scattered light. It was used to quantitatively measure local equivalence ratio in colliding wall-jets under non-reacting conditions. Mixing phenomena in wall-jets were analyzed by comparing the equivalence ratio in the free part of the jet with that in the recirculation zone where two wall-jets collide. These results were then compared to φ predictions for free-jets. It was found that under the conditions tested, increased injection pressure did not increase mixing in the wall-jets. Comparisons with free-jet predictions further indicated that mixing in wall-jets is less effective than in free-jets for identical conditions and downstream distances. The confined nature of the wall-jet in the optical engine is suspected to be the reason for these observations. A rapid leaning-out of the jet after end of injection was observed for all cases, but this enhanced mixing was not transmitted to the wall-jet. (Less)

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)

Effects of Negative Valve Overlap on the Autoignition Process of Lean Ethanol/Air Mixture in HCCI-Engines

This paper presents a computational study of the effects of fuel and thermal stratifications on homogenous charge compression ignition (HCCI) combustion process in a personal car sized internal combustion engine. Stratified HCCI conditions are generated using a negative valve overlap (NVO) technique. The aims of this study are to improve the understanding of the flow dynamics, the heat and mass transfer process and the onset of auto-ignition in stratified charges under different internal EGR rate and NVO conditions. The fuel is ethanol supplied through port-fuel injection; the fuel/air mixture is assumed to be homogenous before discharging to the cylinder. Large eddy simulation (LES) is used to resolve in detailed level the flow structures, and the mixing and heat transfer between the residual gas and fresh fuel/air mixtures in the intake and compression strokes. Multi-Zone model based on a detailed chemical kinetic mechanism is then used to simulate the onset of auto-ignition in the combustion stroke near TDC, based on the mixtures predicted in LES. It is found that for low and moderate EGR rates (low and moderate NVO) the onset of ignition is more sensitive to the temperature of the mixture than to the fuel concentration. For the high EGR rate and large NVO case, there is a preferred mixture and temperature at which the first ignition occurs. Under similar operating conditions the moderate NVO and EGR rate case is found to have the earliest ignition, whereas the longest combustion duration is found in the lowest EGR rate and the lowest NVO case. (Less)

High-speed imaging of fuel/OH distributions in a gas turbine pilot burner at elevated pressure

Different laser visualization techniques were applied to a pilot burner at elevated pressure using relevant liquid fuels. Special care was spent to investigate the performance of the visualization techniques when using Jet-A as fuel compared to when using Bio-Jet as fuel at gas turbine relevant conditions. The burner, a centrally placed generic injector surrounded by a swirling co-flow, was mounted in a high-pressure combustion test rig with optical access from all four sides. Planar laser-induced fluorescence (PLIF) and Mie scattering was used for visualization of the fuel and OH distributions. For fuel PLIF, two different laser excitation wavelengths, 266 nm and 300 nm, were used to investigate the absorption of the laser sheets by different fuels. The Multi:YAG laser cluster, which can produce eight laser pulses in a rapid burst, and a optical parametric oscillator (OPO) was used for high-speed imaging. Furthermore, three-dimensional measurements of fuel PLIF were performed for the Bio-Jet fuel and the OPO laser was used to capture the flame front and burned gas regions using OH PLIF.