John W. Daily

Laser induced fluorescence spectroscopy in flames

The use of laser induced fluorescence spectroscopy (LIF) is described for making measurements in the combustion environment. Some background in molecular structure, spectroscopic concepts, thermodynamics and combustion chemistry is first presented. This is followed by application of the rate equations to model and demonstrate the response of atomic and molecular systems to laser excitation. Next, experimental aspects are explored, including excitation, optical signal collection, dispersion and detection, signal conditioning, uncertainty and detectability, recovering population distributions from spectra, and sources of systematic error. Finally, specific strategies for measuring concentration, temperature, pressure and velocity are discussed. An example of selecting a collisional rate set for modeling purposes is given in an Appendix.

Fabrication of SiCN MEMS by photopolymerization of pre-ceramic polymer☆

This paper describes the use of photopolymerization of a liquid polysilazane as a novel, versatile and cost-effective means of fabricating SiCN ceramic MEMS. SiCN is a new class of polymer-derived ceramics whose starting material is a liquid-phase polymer. By adding a photo initiator to the precursor, photolithographical patterning of the pre-ceramic polymer can be accomplished by UV exposure. The resulting solid polymer structures are then crosslinked under isostatic pressure, and pyrolyzed to form an amorphous ceramic capable of withstanding over 1500°C. By adding and curing successive layers of liquid polymer on top of one another, multi-layered ceramic MEMS can be easily fabricated. The use of photopolymerization can also be used to make thin, membrane-like ceramic structures. Key issues concerning the fabrication process are discussed. By combining photopolymerization with other in-house developed techniques such as polymer-based bonding and flip-chip bonding, three SiCN MEMS devices for high-temperature applications have been fabricated: an electrostatic actuator, a pressure transducer, and a combustion chamber. These represent a wide range of MEMS, demonstrating the versatility of this technique.

Saturation effects in laser induced fluorescence spectroscopy

Laser based spectroscopic diagnostic tools offer the possibility of spatially and temporally resolved measurements of species concentrations in complex reacting gas flows of engineering interest. The major problem associated with such measurements is the effect of quenching reactions on the fluorescence signal. To overcome this difficulty operating in the saturation mode is proposed. For suitable systems the fluorescence signal is then no longer a function of quenching rates or laser power. Very low detectability limits appear possible.

Saturation of fluorescence in flames with a Gaussian laser beam.

The method of saturated fluorescence for measuring species concentrations in flames is usually performed with laser beams that do not provide a constant intensity distribution across the focal volume. Because of the intensity distribution across the beam, the fluorescence signal does not depend on laser power or intensity in the same manner as for uniform illumination. This leads to anomolous apparent saturation intensities. In the following, the effect is considered for atomic fluorescence. Relations for the fluorescence signal under two common excitation geometries are derived and uncertainty relations used to consider the benefits of high laser intenstiy.

Direct Detection of Products from the Pyrolysis of 2-Phenethyl Phenyl Ether

The pyrolysis of 2-phenethyl phenyl ether (PPE, C(6)H(5)C(2)H(4)OC(6)H(5)) in a hyperthermal nozzle (300-1350 °C) was studied to determine the importance of concerted and homolytic unimolecular decomposition pathways. Short residence times (<100 μs) and low concentrations in this reactor allowed the direct detection of the initial reaction products from thermolysis. Reactants, radicals, and most products were detected with photoionization (10.5 eV) time-of-flight mass spectrometry (PIMS). Detection of phenoxy radical, cyclopentadienyl radical, benzyl radical, and benzene suggest the formation of product by the homolytic scission of the C(6)H(5)C(2)H(4)-OC(6)H(5) and C(6)H(5)CH(2)-CH(2)OC(6)H(5) bonds. The detection of phenol and styrene suggests decomposition by a concerted reaction mechanism. Phenyl ethyl ether (PEE, C(6)H(5)OC(2)H(5)) pyrolysis was also studied using PIMS and using cryogenic matrix-isolated infrared spectroscopy (matrix-IR). The results for PEE also indicate the presence of both homolytic bond breaking and concerted decomposition reactions. Quantum mechanical calculations using CBS-QB3 were conducted, and the results were used with transition state theory (TST) to estimate the rate constants for the different reaction pathways. The results are consistent with the experimental measurements and suggest that the concerted retro-ene and Maccoll reactions are dominant at low temperatures (below 1000 °C), whereas the contribution of the C(6)H(5)C(2)H(4)-OC(6)H(5) homolytic bond scission reaction increases at higher temperatures (above 1000 °C).

Application of microforging to SiCN MEMS fabrication

Ceramics and polymers are attractive candidate materials for MEMS applications because of the wide range of properties that can be obtained, and the promise of improved performance as compared to the existing materials set for MEMS. A challenge in the fabrication of ceramic MEMS is prohibiting cracking that can occur during processing. For example, this is significant in the development of a microcasting fabrication technique from a polymer precursor for silicon carbonitride (SiCN) MEMS. In this case, shrinkage mismatch between the SiCN structure and the microfabricated mold during thermal processes leads to significant stresses that can crack the ceramic structure. Here, we propose an approach to overcome this problem that relies on demolding prior to the large shrinkage mismatch thermal processes, which itself is a nontrivial challenge. To this end, we propose and describe a microforging process that facilitates demolding and show representative results for numerous SiCN ceramic microstructures.

Laser excitation dynamics of OH in flames

The response of OH in flames to laser excitation is studied in some detail. The population balance equations are solved numerically using an empirical model for the rotational relaxation rates. The empirical model parameters are fit to experimental spectra using a linear regression procedure and the resulting description of OH behavior is shown to be satisfactory to within the precision of the data. The model is then used to recover branching ratios for a number of flame conditions.

Measurement of temperature in flames using laser induced fluorescence spectroscopy of OH

A technique for measuring translational flame temperature utilizing the laser induced fluorescence spectrum of OH is demonstrated. The method is based on matching the observed spectrum to a numerical model in which the detailed balance temperature is a parameter. The precision of the method exceeds that of the line reversal technique. Accuracy is limited by the calibration source and the validity of the numerical rotational relaxation rate model.

The properties of a micro-reactor for the study of the unimolecular decomposition of large molecules

A micro-reactor system (approximately 0.5–1 mm inner diameter by 2–3 cm in length) coupled with photoionization mass spectrometry and matrix isolation/infrared spectroscopy diagnostics is described. Short residence time flow reactors (roughly ≤ 100 μs) combined with suitable diagnostic tools have the potential to allow observation of unimolecular decomposition processes with minimum interference from secondary reactions. However, achieving the short residence times desired requires very small micro-reactors that are difficult to characterise experimentally because of their size. In this article the benefits of using these micro-reactors are presented along with some details of the systems employed. This is followed by some general flow considerations and then some simple analyses to illustrate particular features of the flow. Finally, computational fluid dynamics simulations are used to explore the flow and chemical behaviour of the reactors in detail. Some findings include: (1) The reactor operates in the laminar domain. (2) Heating and large pressure differences across the reactor result in a compressible flow that chokes (meaning the velocity reaches the sonic condition) at the reactor exit. (3) When helium is the carrier gas, under some circumstances there is slip at the boundaries near the downstream end of the reactor that reduces the pressure drop and heat transfer rate; this effect must be accounted for in the simulations. (4) Because the initial reactant concentration is held to less than 0.1%, secondary reactions are minimised. (5) Although the fluid dynamical residence time from entrance to exit ranges from 25 to 150 μs, in practice the period over which reactions take place is much shorter. In essence, there is a ‘sweet spot’ within the reactor where most reactions take place. In summary, the micro-reactor, which has been used for many years to generate radicals or study unimolecular decomposition chemical mechanisms, can be used to extract kinetic information by comparing simulations and measurements of reactant and product concentrations at the reactor exit.

Cycle-to-Cycle Variations: A Chaotic Process?

Abstract A simple spark ignition engine cycle model has been used to illustrate the concept that cycle-to-cycle variations are an inherent consequence of non-linear combustion kinetics. Sample calculations show highly chaotic behavior when the burn time occupies an excessive fraction of the cycle time. The results are consistent with the well known fact that the engine designer must strive to shorten the burn time as much as possible to minimize variations.