Philip Varghese - US grants

This work will develop an expert system for assisting and controlling high resolution diode laser spectroscopic measurements, integrating data acquisition, numeric processing and graphics capability of a laboratory micro-computer to provide rapid feedback to a spectroscopist on experimental conditions. Successful extensions of this work will accelerate the transfer of newly developed measurement technologies from research laboratories to industry.

Accurate measurements of temperature and species concentrations are important for characterization of high temperature gas flows, particularly in the area of combustion. Optical measurements are desirable since they are nonintrusive, nonperturbing, and the probe is not subject to a hostile environment. An absorption measurement provides a simple quantitative definition of the concentration of the absorbing specie along a line-of-sight; this is sometimes adequate information, but more often spatially resolved measurements are needed. In principle, two-dimensional temperature and concentration fields should be recoverable from a limited number of line-of-sight measurements using tomographic reconstruction, a method widely used in medical imaging. However, accurate quantitative reconstructions of temperature and concentration fields have been difficult to achieve because reconstruction algorithms are very sensitive to errors in the input data. The principal objective of this program is to measure temperature and concentration fields simultaneously using a reconstruction technique that exploits the tuning capability of infrared diode lasers to greatly reduce the sensitivity of the reconstruction methods to both random and systematic errors in the input data. The study will begin with reconstruction of radial temperature and concentration profiles (CO and CO2) in an axisymmetric flow, with subsequent extension of the procedures to reconstruction of two-dimensional sections of three-dimensional flows. The application of quantitative laser diagnostics to industrial scenarios has been hampered by the typical high cost of equipment involved and by the user sophistication required for achievement of quantitative results. Success with the these lower-cost, more user-friendly systems may well lead to wider usage in industry; the software and hardware developed should enable implementation of tomographic absorption diagnostic techniques in a wide variety of applications. Potentially, further development of this technology, with use of inexpensive laser diode sources, may allow instrumentation packages cheap and robust enough for use in in-plant process control systems.

Abstract Varghese Funds are awarded to develop a new laser velocimeter that allows continuous measurement of velocity at rates ranging from 10-100 kHz. The instrument will utilize a novel variation on filtered Rayleigh scattering called Modulated Filtered Rayleigh Scattering. This velocimeter will be an improvement on typical laser doppler velocimetry because of the following features: only a single beam is required, rather than two beams, by viewing in opposite directions, two components of velocity can be measured using a single laser beam; velocity measurements every 20 microseconds are feasible; multi-point measurements can be made; measurement sensitivity can be maximized without optical realignment; measurements will be possible in luminous and sooting flames; measurements can be made in unseeded flows. The system will make flow-mapping studies more efficient than current LDV methods.

Proposal Number: CTS 9977481

Principal Investigator: N. Clemens

This Major Research Instrumentation (MRI) award supports the development of a spontaneous Raman scattering instrument. Spontaneous vibrational Raman scattering has proven to be one of the most important diagnostic techniques in combustion research owing to its ability to provide spatially resolved information on the concentration of several species simultaneously. However, conventional Raman scattering systems employ high-power pulsed lasers where the rate at which they can acquire data is limited by the pulse repetition rate (typically 5 to few hundred Hz). The problem with low sampling rates is that they restrict measurements to essentially random sampling, so that statistical information can be extracted, but important time-correlation information cannot be captured.

It is proposed to develop a spontaneous Raman scattering instrument that will enable essentially continuous single-point measurements of all major species (hence mixture fraction) and temperature simultaneously in a turbulent flame, at a rate of 10 kHz or more. A core element of this effort will be the development of a high-repetition rate laser system that operates at visible wavelengths. In order to accomplish this, the proposes will work with a partner in industry to develop a laser that meets the demanding requirements for the proposed instrument. The proposed laser will be a diode-pumped Nd:YAG laser that will be frequency doubled to a visible (green) wavelength. It will operate at an average power of 150 W in the green with a repetition rate of 10 kHz and above; such a laser would be the first of its kind developed commercially.

Once the laser is delivered, substantial work will be required to develop an instrument that can be used to make meaningful measurements in actual large-scale laboratory flames. Despite the fact that the average power of the laser is quite high, the energy per pulse is not (15 mJ at 10 kHz), and thus substantial effort will be required to maximize the light collected. This will be accomplished by utilizing a multi-pass cell so that the laser energy in the probe volume is increased by an order-of-magnitude. The system will employ separate custom-designed fast collection optics and high efficiency interference filters. This system will yield excellent signal-to-noise ratios on a single-shot basis, for all major species in an atmospheric pressure turbulent methane flame.

PI: Varghese, Philip L.
Proposal Number: 1438530

The goal of the proposed research is to develop an improved method to compute flows under conditions where the conventional continuum equations are invalid. If successful, the work would result in solution strategies for a broad range of applications where continuum equations fail. The work proposed will advance the state-of-the-art for hybrid flow simulations that are important in design and simulation of wafer fabrication machines, plasma processing equipment, and micro-sensors.

The continuum approach to fluid mechanics reaches its limitations in rarefied flows and in micro- and nanoflows. A commonly used approach for computing non-continuum flows is the direct simulation Monte Carlo (DSMC) method, which can be thought of as providing a statistical representation of solutions of the Boltzmann equation by modeling the behavior of a large number of the molecules in the flow. This proposal is about a novel scheme to solve the Boltzmann equation numerically that has the potential to alleviate many of the limitations of DSMC, including feasibility for large scales. The proposed work is based on a new idea on how to compute the collisional integral, which appears in the Boltzmann equation and has been the reason for inefficient solutions and approximations to-date, using a projection into a discrete yet conservative velocity space. This approach can be viewed as a variation of DSMC that uses variable-mass fixed-velocity quasi-particles, rather than fixed-mass variable-velocity particles. Preliminary results show good agreement with theoretical predictions. The software developed during this project will be given to other academic and government laboratory users on request. The research center within the Institute for Computational Engineering and Sciences at UT Austin will be utilized for dissemination - it has established high standards for software documentation and code verification and validation.

This award by the Fluid Dynamics Program of the CBET Division is co-funded by CIF 21 Software Reuse Venture Fund Program of the CISE/ACI Division.