Nikolaos Soulopoulos

Turbulent premixed flames on fractal-grid-generated turbulence

A space-filling, low blockage fractal grid is used as a novel turbulence generator in a premixed turbulent flame stabilized by a rod. The study compares the flame behaviour with a fractal grid to the behaviour when a standard square mesh grid with the same effective mesh size and solidity as the fractal grid is used. The isothermal gas flow turbulence characteristics, including mean flow velocity and rms of velocity fluctuations and Taylor length, were evaluated from hot-wire measurements. The behaviour of the flames was assessed with direct chemiluminescence emission from the flame and high-speed OH-laser-induced fluorescence. The characteristics of the two flames are considered in terms of turbulent flame thickness, local flame curvature and turbulent flame speed. It is found that, for the same flow rate and stoichiometry and at the same distance downstream of the location of the grid, fractal-grid-generated turbulence leads to a more turbulent flame with enhanced burning rate and increased flame surface area.

Mixing and scalar dissipation rate statistics in a starting gas jet

We quantify the temporal development of the mixing field of a starting jet by measuring the mixture fraction and the scalar dissipation rate and their statistics in an isothermal, impulsively started, gaseous jet. The scalar measurements are performed using planar laser induced fluorescence and, with appropriate processing of the resulting images, allow scalar dissipation rate measurements within 20%. The probability density functions of the mixture fraction, measured within a region of the order of 3 times the Batchelor length scale of the flow, are bimodal and skewed around a well-mixed radial location, which depends on the downstream distance and the time after the start of injection. The instantaneous distributions of the scalar dissipation rate reveal regions of high mixing at the jet periphery and at the developing vortex ring. The normalised probability density function (pdf) of the scalar dissipation rate at various flow positions and times after the start of injection has the same characteristic shape but differs from the usually suggested lognormal distribution at both low and high dissipation values; the same, also, holds true for the pdf conditioned on different values of the mixture fraction. The mean of the scalar dissipation rate conditional on mixture fraction shows a variation across the mixture fraction range, which differs between flow locations and times after the start of injection; however, at later times and for larger downstream distances the conditional mean between flow locations has similar distributions. Implications of the measurements for the auto-ignition of gaseous jets are examined and demonstrate that near the nozzle exit or at earlier times conditions are un-favourable for auto-ignition.

Schlieren-based temperature measurement inside the cylinder of an optical spark ignition and homogeneous charge compression ignition engine

Schlieren [Schlieren and Shadowgraphy Techniques (McGraw-Hill, 2001); Optics of Flames (Butterworths, 1963)] is a non-intrusive technique that can be used to detect density variations in a medium, and thus, under constant pressure and mixture concentration conditions, measure whole-field temperature distributions. The objective of the current work was to design a schlieren system to measure line-of-sight (LOS)-averaged temperature distribution with the final aim to determine the temperature distribution inside the cylinder of internal combustion (IC) engines. In a preliminary step, we assess theoretically the errors arising from the data reduction used to determine temperature from a schlieren measurement and find that the total error, random and systematic, is less than 3% for typical conditions encountered in the present experiments. A Z-type, curved-mirror schlieren system was used to measure the temperature distribution from a hot air jet in an open air environment in order to evaluate the method. Using the Abel transform, the radial distribution of the temperature was reconstructed from the LOS measurements. There was good agreement in the peak temperature between the reconstructed schlieren and thermocouple measurements. Experiments were then conducted in a four-stroke, single-cylinder, optical spark ignition engine with a four-valve, pentroof-type cylinder head to measure the temperature distribution of the reaction zone of an iso-octane-air mixture. The engine optical windows were designed to produce parallel rays and allow accurate application of the technique. The feasibility of the method to measure temperature distributions in IC engines was evaluated with simulations of the deflection angle combined with equilibrium chemistry calculations that estimated the temperature of the reaction zone at the position of maximum ray deflection as recorded in a schlieren image. Further simulations showed that the effects of exhaust gas recirculation and air-to-fuel ratio on the schlieren images were minimal under engine conditions compared to the temperature effect. At 20 crank angle degrees before top dead center (i.e., 20 crank angle degrees after ignition timing), the measured temperature of the flame front was in agreement with the simulations (730-1320 K depending on the shape of the flame front). Furthermore, the schlieren images identified the presence of hot gases ahead of the reaction zone due to diffusion and showed that there were no hot spots in the unburned mixture.

Experimental assessment of presumed filtered density function models

Measured filtered density functions (FDFs) as well as assumed beta distribution model of mixture fraction and “subgrid” scale (SGS) scalar variance z′′2¯, used typically in large eddy simulations, were studied by analysing experimental data, obtained from two-dimensional planar, laser induced fluorescence measurements in isothermal swirling turbulent flows at a constant Reynolds number of 29 000 for different swirl numbers (0.3, 0.58, and 1.07). Two-dimensional spatial filtering, by using a box filter, was performed in order to obtain the filtered variables, namely, resolved mean and “subgrid” scale scalar variance. These were used as inputs for assumed beta distribution of mixture fraction and top-hat FDF shape estimates. The presumed beta distribution model, top-hat FDF, and the measured filtered density functions were used to integrate a laminar flamelet solution in order to calculate the corresponding resolved temperature. The experimentally measured FDFs varied with the flow swirl number and both axi...

Scalar dissipation rate statistics in turbulent swirling jets

The scalar dissipation rate statistics were measured in an isothermal flow formed by discharging a central jet in an annular stream of swirling air flow. This is a typical geometry used in swirl-stabilised burners, where the central jet is the fuel. The flow Reynolds number was 29 000, based on the area-averaged velocity of 8.46 m/s at the exit and the diameter of 50.8 mm. The scalar dissipation rate and its statistics were computed from two-dimensional imaging of the mixture fraction fields obtained with planar laser induced fluorescence of acetone. Three swirl numbers, S, of 0.3, 0.58, and 1.07 of the annular swirling stream were considered. The influence of the swirl number on scalar mixing, unconditional, and conditional scalar dissipation rate statistics were quantified. A procedure, based on a Wiener filter approach, was used to de-noise the raw mixture fraction images. The filtering errors on the scalar dissipation rate measurements were up to 15%, depending on downstream positions from the burner exit. The maximum of instantaneous scalar dissipation rate was found to be up to 35 s−1, while the mean dissipation rate was 10 times smaller. The probability density functions of the logarithm of the scalar dissipation rate fluctuations were found to be slightly negatively skewed at low swirl numbers and almost symmetrical when the swirl number increased. The assumption of statistical independence between the scalar and its dissipation rate was valid for higher swirl numbers at locations with low scalar fluctuations and less valid for low swirl numbers. The deviations from the assumption of statistical independence were quantified. The conditional mean of the scalar dissipation rate, the standard deviation of the scalar dissipation rate fluctuations, the weighted probability of occurrence of the mean conditional scalar dissipation rate, and the conditional probability are reported.

Ray tracing analysis of realistic atomizing jet geometries for optical connectivity applications

The optical connectivity technique has been proposed for the visualization of the continuous core of atomizing jets with little optical interference from the surrounding droplets. By introducing a laser beam through the nozzle, the high intensity laser light propagates along the liquid jet in the same manner as light propagates in an optical fiber. Beyond the breaking point the laser light diffuses and its intensity diminishes. The addition of a fluorescent dye in the atomizing liquid makes the intensely illuminated continuous jet volume luminous, while the poorly illuminated detached droplets remain dark. The technique has been demonstrated to perform well experimentally. However, there are losses of the laser intensity as the beam travels downstream the liquid jet, due to refraction through the liquid interface. As such, the maximum length of an atomizing jet that can be visualized is limited. For this reason the propagation of a laser beam inside a liquid jet and the subsequent fluorescent emission has been examined numerically in the past by means of ray tracing [1]. The investigation considered a number of parameters including the gas and the liquid refractive indices, the laser beam divergence at the nozzle exit, the concentration of the dye in the liquid jet and the geometry of the liquid jet as a sinusoidal wave. While considerable insight was obtained on the applicability of the optical connectivity technique and the importance of the initial laser beam divergence highlighted, the sinusoidal interface does not represent faithfully the geometry of an atomizing jet under intense atomization conditions. Here we extend the previous investigation to computationally derived atomizing liquid jet geometries for We=1040 and MR=336, in order to evaluate the propagation of the laser illumination under more realistic liquid jet geometries and assess the applicability of the optical connectivity technique.

An investigation of the effect of post-injection schemes on soot reduction potential using optical diagnostics in a single-cylinder optical diesel engine

This work employs a combination of pressure trace analysis, high-speed optical measurements and laser-based techniques for the assessment of the effects of various post-injection schemes on the soot reduction potential in an optical single-cylinder light-duty diesel engine. The engine was operated under a multiple injection scheme of two pilot and one main injection, typical of a partially premixed combustion mode, at the lower end of the load and engine speed range (ca 2.0 bar IMEP at 1200 r/min). Experiments considering the influence of the post-injection fuel amount (up to 15% of the total fuel quantity per cycle) and the post-injection timing within the expansion stroke (5, 10 and 15 CAD aTDC), under a constant total fuel mass per cycle, have been conducted. Findings were analysed via means of pressure trace and apparent rate of heat transfer analyses, as well as a series of optical diagnostic techniques, namely, high-speed flame natural luminosity imaging, CH*, C 2 * and OH* line-of-sight chemiluminescence, as well as planar laser-induced incandescence measurements at 31 and 50 CAD aTDC. The combination of post-injection fuel amount and timing has substantial effects on charge reactivity and soot oxidation potential. The analysis reveals that an amount of fuel (7% of the total fuel mass per cycle) injected more than 10 CAD after the main combustion event leads to higher levels of soot emissions, while a larger amount of fuel (15% of the total fuel mass) injected 5 CAD after the main combustion event appears to have a beneficial effect on the soot oxidation processes. Overall, results indicate that a post-injection scheme close to the main combustion phasing could reduce soot levels and improve engine performance, that is, higher IMEP levels at the same fuel consumption rates, although it could increase engine noise.

Towards Identifying Flame Patterns in Multiple, Late Injection Schemes on a Single-Cylinder Optical Diesel Engine

ABSTRACT The work investigates the effect of various post-injection strategies on the flame patterns in a Ricardo Hydra optical single-cylinder, light-duty diesel engine, operated in a partially premixed combustion mode, under low load (IMEP: ca. 2.3 bar), low speed (1200 rpm) conditions. The effect of post-injection fuel amount (12% and 24% of the total fuel quantity per cycle) and post-injection timing (0, 5, 10 deg aTDC) are investigated via pressure trace analysis and optical measurements. Flame propagation is captured by means of high-speed flame natural luminosity imaging and of CH*, C2*, and OH* line-of-sight chemiluminescence measurements. Results suggest that post-injections suppress mixture reactivity but enhance oxidation, and that a larger amount of fuel and/or later post-injection, leads to higher levels of natural luminosity, indicating possible higher soot-out emissions, while post-injection close to the main combustion event appears to have a beneficial effect on the soot oxidation processes.

Using Infrared Laser Absorption to Measure Hydrocarbon Concentration in a Lean-Burn, Stratified-Charge, Spark-Ignition Engine

The operating range of lean-burn spark-ignition (SI) engines is limited by the cycle-to-cycle variability of the fuel concentration at or near the spark plug at ignition timing. An experimental investigation was undertaken to measure the temporal and spatial distribution of hydrocarbon (HC) concentration in a spark-ignition engine, using the infrared (IR) laser absorption at 3392 nm. The purposes were to establish whether there is a correlation between time-resolved HC measurements for a range of global air-to-fuel (A/F) ratios (A/F = 15.5–23) with the strength of the firing stroke and to establish how this varies with fuel port-injection strategies against either open (injection timing 30° crank angle (CA) after intake top dead center) or closed-valves (injection timing 180°CA after intake top dead center), respectively resulting in stratified and near-homogeneous charge distributions. The results showed that IR line-of-sight (LOS) averaged A/F ratio measurements yielded a good agreement with the global A/F ratio readings obtained by the linear air-to-fuel (LAF) zirconia-based sensor. Furthermore, the cyclic variability of the measurements of the fuel concentration increased with increasing A/F ratio. At A/F = 23, closed-valve injection strategy resulted in small spatial stratification of the fuel charge with an ensemble-averaged correlation coefficient of fluctuations of the IR LOS A/F ratios with fluctuations of peak in-cylinder pressure of 0.37. For open-valve injection strategy, which resulted in axial fuel mixture charge-stratification but no radial charge-stratification, a maximum correlation coefficient of the IR LOS A/F ratios with fluctuations of peak pressure of 0.34 was measured (at the measurement location closest to the spark). This correlation was reduced to 0.17 at locations furthest from the spark plug. Finally, results showed that at A/F = 23, fuel stratification can be used to control lean-burn SI combustion, while at A/F = 15.5, no significant difference was observed between the two injection timings. The consequences of these findings on the fuel distribution in the pentroof and the control of the lean limits of SI engines are discussed.