Simone Hochgreb

Rapid compression machines: Heat transfer and suppression of corner vortex

A method to suppress the piston corner vortex generated in compressing gas mixtures in rapid compression machines has been developed. A piston crevice was designed to swallow the thermal boundary layer along the wall, allowing better definition of core conditions in the reacting mixtures by confining the cold gases to the wall. Axisymmetric calculations of the flow and temperature fields show that the proposed design indeed suppresses the vortex formation, keeping the core reacting cases intact. A simple thermodynamic model based on the piston displacement history was formulated, incorporating the predicted heat transfer to the walls and mass transfer to the crevices. The model predictions agree very well with experimental pressure history under a range of initial pressures and types of different gases. The new experimental device and model allows the incorporation of complex chemical kinetics with early heat release without the need for experimental approximations for the heat transfer terms.

Time-resolved laser-induced incandescence of soot: the influence of experimental factors and microphysical mechanisms

We present a data set for testing models of time-resolved laser-induced incandescence of soot. Measurements were made in a laminar ethene diffusion flame over a wide range of laser fluences at 532 nm. The laser was seeded to provide a smooth temporal profile, and the beam was spatially filtered and imaged into the flame to provide a homogeneous spatial profile. The particle incandescence was imaged onto a fast photodiode. The measurements are compared with the standard Melton model [Appl. Opt. 23,2201 (1984)] and with a new model that incorporates physical mechanisms not included in the Melton model.

Time-Resolved Laser-Induced Incandescence and Laser Elastic Scattering Measurements in a Propane Diffusion Flame

Laser-induced incandescence (LII) and laser elastic-scattering measurements have been obtained with subnanosecond time resolution from a propane diffusion flame. Results show that the peak and time-integrated values of the LII signal increase with increasing laser fluence to maxima at the time of the onset of significant vaporization, beyond which they both decrease rapidly with further increases in fluence. This latter behavior for the time-integrated value is known to be characteristic for a laser beam with a rectangular spatial profile and is attributed to soot mass loss from vaporization. However, there is no apparent explanation for the corresponding large decrease in the peak value. Analysis shows that the peak value occurs at the time in the laser pulse when the time-integrated fluence reaches approximately 0.2 J/cm(2) and that the magnitude of the peak value is strongly dependent on the rate of energy deposition. One possible explanation for this behavior is that, at high laser fluences, a cascade ionization phenomenon leads to the formation of an absorptive plasma that strongly perturbs the LII process.

Hydrogen autoignition at pressures above the second explosion limit (0.6–4.0 MPa)

The investigation of high-pressure autoignition of combustible mixtures is of importance in providing both practical information in the design of combustion systems and fundamental measurements to verify and develop chemical kinetic models. The autoignition characteristics of hydrogen-oxygen mixtures at low pressures have been explored extensively, whereas few measurements have been made at high pressures. The present measurements extend the range of pressures up to 4 MPa, where few measurements have yet been reported. Using a rapid compression machine equipped with a specially designed piston head, hydrogen autoignition pressure traces were measured at pressures above the second explosion limit (p=0.6–4 MPa, T=950–1050 K). The measured pressure records show a more gradual pressure increase during induction time in this regime than in the low-pressure regime, indicating that the energy release becomes significant at conditions over the second explosion limit. By comparing the measurements and a thermodynamic model which incorporates the heat transfer and energy release, a modified reaction rate constant for H2O2+H=HO2+H2, one of the most important reactions for hydrogen oxidation at high pressure, and the reaction with the largest uncertainty, is suggested in this work as k17=2.3 . 1013exp(−4000/T) cm3/mol-s. The modeled pressure history with the modified reaction rate agrees well with the measured values during the induction period over the range of conditions tested.

A comprehensive study on CH2O oxidation kinetics

Abstract Results of calculations based on a detailed chemical mechanism for the oxidation of formaldehyde are compared with data from a variety of experimental conditions: flow reactor, shock tube, static reactor, and a lean flame. The range of conditions spans temperatures from 773 to 2500 K, equivalence ratios from pure pyrolysis to very fuel-lean and pressures from 0.3 to 1.5 atm. New experimental results from the oxidation of formaldehyde in flow reactors at 1095 K are reported. The original model, based on reaction rate constants obtained in the literature, was modified based on comparisons of calculated and experimental results. Recommended changes were guided by extensive sensitivity and flux analysis for each case, as well as on the current knowledge of uncertainties in reaction rate constant values. The comprehensive mechanistic features of formaldehyde oxidation at low and high temperatures are discussed. The results of the analysis reveal the key role of HO 2 radicals up to temperatures around 1100 K, through the cycle HCO + O 2 → CO + HO 2 , CH 2 O + HO 2 → HCO + H 2 O 2 and H 2 O 2 + M → OH + OH + M. Shock tube data are revisited using more recent rate constants to verify the model. Finally, the competition between formyl radical reactions HCO + M → H + CO + M and HCO + O 2 → CO + HO 2 , which determines the rate of production of H radicals, was found to be the primary controlling factor in the evolution of lean formaldehyde flames. A matrix of influences of the most important reaction rates on the calculated parameters for the different experimental conditions summarizes the conclusions.

The effect of fuel volatility on sprays from high-pressure swirl injectors

The effect of fuel volatility on the spray distribution of a pressure-swirl atomizer of the type used in direct-injection gasoline engines was investigated in a firing optical engine. Planar laser-induced fluorescence (PLIF) and planar Mie scattering were used to visualize the fuel spray. Experiments were performed at three loads (0.3, 0.6, and 0.9 bar intake pressure) and two head temperatures (30 °C and 90 °C). Fuel mixtures consisted of doped and undoped isooctane and indolene. Dopants were ketones of varied volatility: acetone (Tb=56°C), 2-butanone (Tb=80°C), and 3-pentanone (Tb=102°C). At high head temperature and low pressure with volatile, fuel species, the spray characteristics changed from the hollow-cone structure observed under cold conditions to a solid-cone distribution. This transition was observed with both the PLIF and Mie-scattering images, suggesting that not only vapor was being drawn to the interior of the cone, but droplets as well. The observed solid-cone structure can be explained by flash boiling of the highly volatile species, followed by entrainment of the smaller droplets into the cone center by the airflow induced by the spray. The observed transition temperatures and pressures agree well with bubble-point calculations and suggest that a superheat of about 20 K is necessary for flash boiling to be vigorous enough to noticeably affect the spray structure. Similar results were obtained with a full-boiling-range gasoline.

Spray Combustion Characteristics of Palm Biodiesel

The potential of palm methyl esters (PME) as an alternative fuel for gas turbines is investigated using a swirl burner. The main air flow is preheated to 623 K, and a swirling spray flame is established at atmospheric pressure. The spray combustion characteristics of PME are compared to diesel and Jet-A1 fuel under the same burner power output of 6 kW. Investigation of the fuel atomizing characteristics using phase Doppler anemometry (PDA) shows that most droplets are distributed within the flame reaction zone region. PME droplets exhibit higher Sautermean diameter (SMD) values than baseline fuels, and thus higher droplet penetration length and longer evaporation timescales. The PME swirl flame presents a different visible flame reaction zone while combusting with low luminosity and produces no soot. NOx emissions per unit mass of fuel and per unit energy are reduced by using PME relative to those of conventional fuels.

The Effects of Small-Scale Mixing Models on the Prediction of Turbulent Premixed and Stratified Combustion

Reynolds-Averaged Navier-Stokes calculations of turbulent premixed and stratified flames in two different configurations are performed to assess the effect of small-scale mixing rate models. The small-scale mixing rate is commonly known as the scalar dissipation rate. The Libby–Williams model involving delta functions is used to calculate the mean reaction rate. The classical model for the scalar dissipation rate underpredicts the mixing rate and gives poor agreement with mean values of velocity, temperature, and species mass fractions. Two other recently proposed dissipation rate models give good predictions at various stratification levels. Further analysis shows that despite adequate mean predictions, there are substantial differences in the mixing rates, fuel mass fraction variance, turbulence kinetic energy, and turbulence frequency predicted by these models. Additionally, the turbulence quantities predicted by the models show opposite trends with increasing stratification. Further Direct Numerical Simulation or experimental data is required for a thorough understanding of these differences.

Quantitative shearography in axisymmetric gas temperature measurements

This paper describes the use of shearing interferometry (shearography) for the quantitative measurement of gas temperatures in axisymmetric systems in which vibration and shock are substantial, and measurement time is limited. The setup and principle of operation of the interferometer are described, as well as Fourier-transform-based fringe pattern analysis, Abel transform, and sensitivity of the phase lead to temperature calculation. A helium jet and a Bunsen burner flame are shown as verification of the diagnostic. The accuracy of the measured temperature profile is shown to be limited by the Abel transform and is critically dependent on the reference temperature used.

Autonomous extraction of optimal flame fronts in OH planar laser-induced fluorescence images

The location of a flame front is often taken as the point of maximum OH gradient. Planar laser-induced fluorescence of OH can be used to obtain the flame front by extracting the points of maximum gradient. This operation is typically performed using an edge detection algorithm. The choice of operating parameters a priori poses significant problems of robustness when handling images with a range of signal-to-noise ratios. A statistical method of parameter selection originating in the image processing literature is detailed, and its merit for this application is demonstrated. A reduced search space method is proposed to decrease computational cost and render the technique viable for large data sets. This gives nearly identical output to the full method. These methods demonstrate substantial decreases in data rejection compared to the use of a priori parameters. These methods are viable for any application where maximum gradient contours must be accurately extracted from images of species or temperature, even at very low signal-to-noise ratios.