Hydrogen production via plasma reformers

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1996.Includes bibliographical references (p. 106).by Christopher J. O'Brien.M.S

Formation mechanisms of combustion chamber deposits

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2001.Includes bibliographical references.Combustion chamber deposits are found in virtually all internal combustion engines after a few hundred hours of operation. Deposits form on cylinder, piston, and head surfaces that are in contact with fuel-air mixture during the engine cycle. The effects of deposits include increased engine-out NOx emissions, octane requirement increase, and changes in flame speed and thermal efficiency. A framework is developed for examining the physical and chemical processes that contribute to the formation of combustion chamber deposits. First, a hypothesis for the general mechanism of deposit formation is developed from a review of previous work on this issue. The key features of this mechanism are formation of deposit precursor species from fuel and air as the flame quenches at the engine wall, diffusive and convective transport of these species to the wall, and condensation or adsorption at the wall surface. The experimental system and methodology developed in this work are meant to provide insight into the interactions between these processes, and in particular to study the chemical mechanisms that contribute to the formation of deposit precursor species. A cooled low pressure flat flame burner is used to produce steady-state propane-air flames doped with toluene, a known deposit forming species.(cont.) Profiles of concentrations and temperature are measured using infrared spectroscopy and gas chromatography techniques. In conjunction with the experiments, a one-dimensional numerical model is developed, capable of simulating flame quenching with deposition over a range of conditions extending from the low pressure, steady state burner experiments to high pressure, rapid transient engine conditions, using chemical mechanisms of precursor formation that may be determined experimentally. Modeling of deposition with simplified chemical mechanisms reveals that deposition by condensation can reproduce trends observed in experiments by other researchers; however, adsorption could still be a contributing factor. Experimental observations of toluene-doped flames show the formation of oxygenated compounds such as benzaldehyde and benzofuran, which are likely deposit precursor candidates. The methodology developed in this thesis shows promise for determining deposit precursor identities and formation mechanisms for important fuel components, and for clarifying the role of gas-phase processes in the formation of combustion chamber deposits.by Christopher O'Brien.Ph.D

A Comparison of Small Signal Modulation Parameter Extraction Techniques for Vertical-Cavity, Surface-Emitting Lasers

The small signal modulation characteristics of a vertical-cavity, surface-emitting laser (VCSEL) are determined using three different measurements: relative intensity noise, frequency response, and high resolution optical spectra. The resonant and damping frequencies were measured, and related rate equation parameters were extracted; excellent agreement was found both between experiment and theory, and amongst the different measurement techniques. The results and procedures are compared, and the findings are presented below

A Critical Comparison of High-Speed VCSEL Characterisation Techniques

This paper critically compares, for the first time, common microwave and optical procedures used for the high-speed characterisation of vertical-cavity, surface-emitting lasers (VCSELs). The intrinsic small-signal modulation characteristics of a VCSEL are measured, and the related rate equation parameters are extracted. Observed trends show excellent agreement with theory. The modulation characteristics of the VCSEL are determined by examining three different responses: relative intensity noise, S21 response, and high-resolution optical spectra. The various experimental techniques yielded consistent results. The relative strengths and weaknesses of each measurement are investigated below

RETROCAM: A Versatile Optical Imager for Synoptic Studies

We present RETROCAM, an auxiliary CCD camera that can be rapidly inserted into the optical beam of the MDM 2.4m telescope. The speed and ease of reconfiguring the telescope to use the imager and a straightforward user interface permit the camera to be used during the course of other observing programs. This in turn encourages RETROCAM's use for a variety of monitoring projects.Comment: 6 pages, 6 figures, Accepted by A

The Effect of Multiple Transverse Modes in Self-Mixing Sensors Based on Vertical-Cavity Surface-Emitting Lasers

In this work we investigate the effect of multiple transverse modes, such as those found in Vertical-Cavity Surface-Emitting Lasers, in self-mixing sensors. We show that the sensitivity of the system and the accuracy of the measurement changes periodically with target distance

Detected changes in precipitation extremes at their native scales derived from in situ measurements

The gridding of daily accumulated precipitation -- especially extremes -- from ground-based station observations is problematic due to the fractal nature of precipitation, and therefore estimates of long period return values and their changes based on such gridded daily data sets are generally underestimated. In this paper, we characterize high-resolution changes in observed extreme precipitation from 1950 to 2017 for the contiguous United States (CONUS) based on in situ measurements only. Our analysis utilizes spatial statistical methods that allow us to derive gridded estimates that do not smooth extreme daily measurements and are consistent with statistics from the original station data while increasing the resulting signal to noise ratio. Furthermore, we use a robust statistical technique to identify significant pointwise changes in the climatology of extreme precipitation while carefully controlling the rate of false positives. We present and discuss seasonal changes in the statistics of extreme precipitation: the largest and most spatially-coherent pointwise changes are in fall (SON), with approximately 33% of CONUS exhibiting significant changes (in an absolute sense). Other seasons display very few meaningful pointwise changes (in either a relative or absolute sense), illustrating the difficulty in detecting pointwise changes in extreme precipitation based on in situ measurements. While our main result involves seasonal changes, we also present and discuss annual changes in the statistics of extreme precipitation. In this paper we only seek to detect changes over time and leave attribution of the underlying causes of these changes for future work