Date of Award

Spring 1-1-2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

First Advisor

John W. Daily

Second Advisor

Barney Ellison

Third Advisor

Brian Argrow

Fourth Advisor

Oleg Vasilyev

Fifth Advisor

Peter Hamlington

Abstract

Thermal decomposition reaction is an interesting yet challenging subject in biomass gasification process. For a number of years, researchers at CU-Boulder and NREL have been studying the pyrolysis chemistry of typical biomass compounds by cracking them in a hyperthermal tubular reactor. Downstream of the reactor, matrix isolation/infrared spectroscopy (MI/IR) or photoionization mass spectrometry (PIMS) is used to diagnose products generated in the process. To study the pyrolysis reaction mechanism and kinetics, one needs to characterize the thermal and fluid properties in the system. The reactors are typically 2 to 4 cm in length with an i.d. of 0.5 to 1 mm. Direct experimental measurements of the thermodynamic states distribution are difficult to conduct due to the small geometry and high operating temperatures (up to 1800 K). Thus there has been little learned about the details of the internal flow field, the downstream molecular flow in the vacuum chamber or the chemical kinetics throughout the system. In this thesis, numerical methods of computational fluid dynamics (CFD) and direct simulation Monte Carlo (DSMC) are used to obtain the thermal and fluid information. Presented are simulation results within and downstream of the reactor under different operating conditions. We conclude that both continuum and non-equilibrium flows exist in the system. In order to solve the flow field transitions between the two flow regimes, hybrid CFD/DSMC algorithms are implemented and results are discussed in detail. The hybrid approach provides us with a complete picture of the distribution of the thermodynamic and flow properties for quantitative kinetics studies.

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