Reports: DNI650586-DNI6: Characterization of Turbulent Combustion Processes in Dimethyl Ether Flames Using Advanced Laser Diagnostics

Jeffrey A. Sutton, PhD, Ohio State University

Dimethyl ether (DME; CH3OCH3) is a promising oxygenated fuel that has the potential to be widely used as a substitute for diesel fuel in compression-ignition (CI) engines.  Realistic gas IC engine conditions are represented by highly turbulent environments and the turbulent combustion processes, including reaction chemistry and scalar transport, are not well understood under DME-fueled conditions.  The primary objective of this research is to use advanced laser-based imaging to measure species concentrations and temperature in highly turbulent DME flames and compare and contrast to well-characterized CxHy flames, thus elucidating the primary physio-chemical processes controlling DME-fueled combustion.

Two approaches are taken in this research project:  (1) formulate a series of canonical turbulent DME-fueled flames that are “equivalent” to well-characterized CH4-fueled flames and use the CH4-fueled flames as a “baseline” of which to compare to the DME flame structure and flame behavior.  In this manner we use the well-characterized DLR jet flames (.221 CH4/.332 H2/.447 N2) as a basis for forming similar DME flames.  Using a fuel mixture of .221 DME/.332 H2/.447 N2 results in a jet flame with a stoichiometric mixture fraction (ξs = 0.17), which is identical to the DLR flames, thus the two flame systems can be compared directly with laser-based imaging to examine mixing, reaction zone structure, and reaction chemistry.  (2)  Use combined 1D Raman scattering, Rayleigh scattering, and carbon monoxide (CO) laser-induced fluorescence (LIF) diagnostics to obtain simultaneous measurements of DME, N2, O2, H2O, H2, CO, CO2, and temperature in highly turbulent, piloted, partially-premixed DME/air flames.  This work is in collaboration with Sandia National Laboratories.

For the first component of this research project, we have performed CH2O and OH planar laser-induced fluorescence (PLIF) measurements in both the DLR and DME jet flames to analyze the differences in flame structure between the two different fuel systems.  Both instantaneous, average, and RMS fluctuation fields were used to show the differences in the CH2O and OH structure when replacing CH4 with DME as a fuel.  First, the CH2O PLIF signal in the DME A flame was approximately 200 times higher than the CH2O PLIF signal in the DLR A flames.  Second, the CH2O PLIF signal in DME A remained strong at all axial positions considered, whereas it decreased significantly (on a relative basis) as a function of increasing axial position in the DLR A flame.  Thirdly, the CH2O PLIF layers were found to be more continuous, smoother and preferentially aligned in the axial direction for the DME A flame, whereas for the DLR A flame, the CH2O is distributed within a somewhat broader space and randomly oriented at downstream positions.  Estimated radial mole fraction profiles showed that CH2O peaks in the low temperature region (~800 K) for the DME-based flames, whereas the CH2O mole fraction peaks near the peak temperature in the CH4-based DLR flames.  This indicates that the DME-based flame most likely has more active low-temperature oxidation processes than the methane-based DLR flame.  For the OH PLIF measurements, which are indicative of the reaction layer within non-premixed flames, it was noted that the DLR flames appear more wrinkled, with what appears to be significantly higher levels of local extinction (i.e., “holes” in the OH layer) than the DME-based flames.  Simply put, it appears that the local turbulence is affecting the DME-based flames much less than the DLR flames.  For example, at an axial position ten nozzle diameters downstream, the DME-based flame looks “laminar-like”, while the DLR flame is highly strained, wrinkled, and segmented. 

In addition to the laser-based planar imaging occurring at Ohio State, preliminary combined line-imaged Raman scattering, Rayleigh scattering, and carbon monoxide (CO) laser-induced fluorescence (LIF) diagnostics were applied at the Turbulent Combustion Laboratory (TCL) within the Combustion Research Facility (CRF) at Sandia National Laboratories to obtain simultaneous measurements of DME, N2, O2, H2O, H2, CO, CO2, and temperature.  From these variables, an important scalar, mixture fraction also is calculated.  Measurements were performed within the well-known Sandia piloted jet burner.  These measurements are used to characterize the thermo-chemical state of DME-fueled flames as well as providing new, high-fidelity databases for numerical model assessment and validation.  This work involves the participation of a post-doctoral researcher, Dr. Frederik Fuest, who travels to Sandia National Laboratories for data collection.  This not only allows collaboration, but allows the post-doc additional mentoring from a senior research scientist, Dr. Robert Barlow, and exposure to additional, unique measurement facilities. 

It should be noted that the preliminary work sponsored as part of the ACS DNI program was used to help win additional funding for the PI.  In February of 2012, Prof. Sutton was added to the Department of Energy Combustion Energy Frontier Research Center, headed by Princeton University to perform additional Raman/Rayleigh/LIF measurements in turbulent DME flames. Since this is one of the goals of ACS DNI grants, this aspect of the research project can be considered an overwhelming success.