Andreas Braeuer

Laser-induced fluorescence of ketones at elevated temperatures for pressures up to 20 bars by using a 248 nm excitation laser wavelength: experiments and model improvements

Laser-induced fluorescence of acetone and 3-pentanone for a 248 nm excitation wavelength was investigated for conditions relevant for internal combustion engines regarding temperature, pressure, and gas composition. An optically accessible calibration chamber with continuous gas flow was operated by using CO2 and air as a bath gas. According to the varying pressure and temperature conditions during the compression stroke of a spark ignition engine, fluorescence experiments were performed under isothermal pressure variations from 1 to 20 bars for different temperatures between 293 and 700 K. The ketone fluorescence behavior predictions, based on a model previously developed by Thurber et al. [Appl. Opt. 37, 4963 (1998)], were found to overestimate the pressure-related fluorescence increase for high temperature and small wavelength excitation at 248 nm. The parameters influencing the model only in the large vibrational energy regime were newly adjusted, which resulted in an improved model with a better agreement with the experiment. The models validity for excitation at larger wavelengths was not influenced. For the air bath gas an additional collision and vibrational energy sensitive quenching rate was implemented in the model for both tracers, acetone and 3-pentanone.

Two-dimensional Raman mole-fraction and temperature measurements for hydrogen-nitrogen mixture analysis

A two-dimensional laser Raman technique was developed and applied to directly probe the population number of selected rotational and vibrational energy levels of hydrogen and nitrogen. Using three cameras simultaneously, temperature and mole fraction images could be detected. Three different combinations of rotational and vibrational Raman signals of hydrogen and nitrogen were analyzed to identify the combination that is most suitable for future mixture analysis in hydrogen internal combustion engines. Here the experiments were conducted in an injection chamber where hot hydrogen was injected into room temperature nitrogen at 1.1 MPa.

Gas mixing analysis by simultaneous Raman imaging and particle image velocimetry

We report the combined application of Ramanography and particle image velocimetry (PIV) for the investigation of gas mixing processes. A droplet seeded hydrogen flow was injected as a free jet into pressurized nitrogen at 0.95 MPa. Ramanography and PIV were used to quantitatively image the mixture composition and the flow field of the mixing process, respectively.

Simultaneous Raman and elastic light scattering imaging for particle formation investigation

We used two-dimensional laser Raman scattering and elastic light scattering to analyze mixture driven particle formation in the supercritical antisolvent process. Using only one excitation laser, the partial density of the antisolvent CO(2), the composition of the solvent/antisolvent mixture, and the occurrence of particle formation can be analyzed simultaneously. Thus, all main parameters that influence the process of particle formation and, subsequently, the desired properties of the produced powder are accessible.

Simultaneous laser-induced fluorescence and Raman imaging inside a hydrogen engine

We report on the simultaneous and two-dimensional measurement of laser-induced fluorescence (LIF) and Raman scattering (Ramanography) applied inside a hydrogen internal combustion (IC) engine. Two different LIF tracer molecules, triethylamine (TEA) and trimethylamine (TMA), were used for the LIF experiments. The LIF and Raman results were found to be in very good agreement. The simultaneous application of Ramanography and LIF imaging indicated that TMA is the more suitable LIF tracer molecule, compared to TEA.

In situ monitoring of the acetylene decomposition and gas temperature at reaction conditions for the deposition of carbon nanotubes using linear Raman scattering

To understand the reaction mechanisms taking place by growing carbon nanotubes via the catalytic chemical vapor deposition process, a strategy to monitor in situ the gas phase at reaction conditions was developed applying linear Raman spectroscopy. The simultaneous determination of the gas temperature and composition was possible by a new strategy of the evaluation of the Raman spectra. In agreement to the well-known exothermic decomposition of acetylene, a gas temperature increase was quantified when acetylene was added to the incident flow. Information about exhaust gas recirculation and location of the maximal acetylene conversion was derived from the composition measurements.

Injection of ethanol into supercritical CO 2 : Determination of mole fraction and phase state using linear Raman scattering

For the pulsed injection of liquid ethanol into supercritical CO(2) inside an optically accessible chamber, for the first time to the best of our knowledge the spatially and temporally resolving linear Raman scattering technique was used to simultaneously determine the mole fraction and the corresponding phase state in the ethanol jet. The mole fraction was identified by calculating the ratio of the C-H band Raman signal (2950 cm(-1)) of ethanol and the CO(2) Raman signal. The magnitude of this ratio was found to be phase state sensitive. Thus, the phase state of the mixture of ethanol and CO(2) could be classified as being homogeneous liquid, homogeneous supercritical or not yet homogeneously mixed.

Liquid phase temperature determination in dense water sprays using linear Raman scattering.

Linear Raman scattering has been applied for the determination of the temperature of the liquid phase in water sprays under normal and superheated conditions. The envelope of the Raman OH-stretching vibration band of water is deconvoluted into five Gaussian peaks which can be assigned to five different intermolecular interactions (hydrogen bonding). The intensity of each of the peaks is a function of the temperature and the phase of the water under investigation. The interference of the Raman signals originating from the water vapor is eliminated from the Raman signals originating from the liquid water. Consequently the temperature of the liquid water droplets surrounded by water vapor is accessible which is favorable for the investigation of non-equilibrium sprays where the droplet temperature is different to the vapor temperature.

How Sodium Chloride Salt Inhibits the Formation of CO2 Gas Hydrates.

We present an experimental Raman study on how the addition of sodium chloride to CO2-hydrate-forming systems inhibits the hydrate formation thermodynamically. For this purpose, the molar enthalpy of reaction and the molar entropy of reaction for the reaction of weakly hydrogen-bonded water molecules to strongly hydrogen bonded water molecules are determined for different salinities from the Raman spectrum of the water-stretching vibration. Simultaneously, the influence of the salinity on the solubility of CO2 in the liquid water-rich phase right before the start of hydrate formation is analyzed. The results demonstrate that various mechanisms contribute to the inhibition of gas hydrate formation. For the highest salt concentration of 20 wt % investigated, the temperature of gas hydrate formation is lowered by 12 K. For this concentration the molar enthalpy and entropy of reaction become smaller by 50 and 20%, respectively. Concurrently, the solubility of carbon dioxide is reduced by 70%. These results are compared with data in literature for systems of sodium chloride in water (without carbon dioxide).

Raman mixture composition and flow velocity imaging with high repetition rates

A novel and completely tracer free strategy to image composition and velocity fields during the mixing process of two liquids is introduced. The achieved temporal resolution, spatial resolution and sampling rate of 30 ns, 54 x 54 µm2 and 10 kHz, respectively, are sufficient to resolve Kolmogorov time and length scales as well as transient mixing phenomena of many technical mixing processes. During the injection of liquid water into liquid ethanol, mixing was quantitatively observed by means of high repetition rate Raman imaging using a laser cluster for the excitation of the Raman process with 8 successive light sheet pulses. One high speed camera was used to detect the CH-vibration Raman band signal of ethanol, while a second one was used to detect the OH-vibration Raman band signal of water and ethanol. From the ratio of both, the mixture composition field was computed. The dense flow field was determined by processing the mixture composition images with a variational optical flow method.