62nd Annual Report on Research 2017

The following objectives have been proposed for a two-year time period.

1. The principal objective is to study the effect of stretch and curvature on tubular flames at high pressures and validate the existing chemical kinetic mechanisms and molecular transport theory models.
2. Raman spectroscopy and LIF techniques will be used to obtain the temperature field, major and minor species concentrations.
3. During the first year, it was proposed that both premixed and non-premixed H₂/air flames will be studied using Raman spectroscopy for major species measurements and LIF for minor species measurements. Suitable experimental conditions will be surveyed using chemiluminescence imaging. Data obtained will be reduced and compared with numerical simulations performed using an in-house DNS code (Hall and Pitz, 2016). Performance of H₂/air chemical kinetic and transport models such as those proposed by Burke et al. (2012) will be evaluated. A Hencken burner will be used for calibrating the Raman and the LIF systems.
4. For second year, it was proposed that hydrocarbon/air premixed and non-premixed flames will be studied using the same techniques used in the first year. Reduced mechanisms for HC combustion will be validated and improved.
5. The high pressure experiments will be conducted in the high pressure duct located in the King Abdullah University of Science and Technology (KAUST) in the Kingdom of Saudi Arabia as a part of Clean Combustion Research Center. A graduate student will co-ordinate with the KAUST faculty for the same.

II. Completed and ongoing tasks

It may be noted that this project started six months after the original proposed start date. The timeline of the objectives has been modified to better accommodate them. During the first year, chemiluminescence imaging for both H₂ and HC flames will be undertaken by transporting the burner to KAUST in October, 2017. Premixed and non-premixed flames will be imaged to study cellular instabilities and extinction. Experimental conditions chosen are similar to those used by Shopoff et al. (2011a,b), Wang et al. (2007) and Hu et al. (2009). The effect of using different diluent gases on thermodiffusive instabilities will also be investigated. However, the number of experimental cases that will be studied will be subject to the time available and other practical difficulties such as burner assembly inside the high pressure duct, flame ignition etc.

Based on the chemiluminescence imaging from the first year, suitable candidates will be chosen for Raman and LIF experiments in the second year. Data obtained will be compared with numerical simulations to validate and improve chemical reaction mechanisms and transport models.

In preparation for the upcoming travel to KAUST in October 2017 for performing the high pressure chemiluminescence experiments, tasks as listed below have been completed and/or are ongoing.

1. The student has read about thermodiffusive instability, stretch and curvature effects and understood their importance in the formation of cells in tubular flames.
2. The student has burned flames in the tubular burner and got used to the tubular burner system.
3. Spark ignition in the high pressure duct works only up to 6 atm. For pressures beyond that, laser ignition is the only option. The tubular burner currently has two laser access ports with Brewster angle to facilitate Raman spectroscopy. Including these ports, the diameter of the burner exceeds 16 inches which is the inside diameter of the high pressure duct. Shorter arms with BK7 windows that permit 1064 nm ignition laser are being machined in the Vanderbilt University.
4. A comprehensive list of experimental conditions for imaging has been prepared.
5. We co-ordinated with the Vanderbilt Export Compliance (VEC) to assist with identifying an Export Control Classification Number (ECCN) for shipping the tubular burner outside of the United States of America. All relevant ECCN’s were disqualified which means that the tubular burner is not controlled.
6. We co-ordinated with KAUST and shipped the burner, obtained visa for the student to travel and booked the air ticket.
7. Student has attended the 2017 Princeton-Combustion Institute Summer School on Combustion at the Princeton University New Jersey in June, 2017. This has helped the student to augment her knowledge on topics such as combustion theory and laser diagnostic techniques.

III. Expected Outcome

Nishioka et al. (1991) have studied the effects of pressure on the structure and extinction of a premixed tubular flame. It provides a very useful insight into the behavior of these flames at high pressures. To the best of our knowledge, chemiluminescence imaging studies to be performed in KAUST in October 2017 is the first of its kind to characterize the effect of pressure on the formation of cells in a tubular flame. We expect to obtain meaningful insights into the thermodiffusive instabilities of these flames at high pressures which characterize realistic combustion environments such as those encountered in gas turbine combustors. This helps understand turbulent flames which are characterized by localized curvature and stretch effects although qualitatively. More importantly, the images obtained this year will be a valuable first step in identifying interesting candidates for next year’s Raman scattering experiments. In terms of deliverables, up to two conference and/or journal papers are expected to be published with the chemiluminescence data.

References

MP Burke, M Chaos, Y Ju, FL Dryer, and SJ Klippenstein. Comprehensive H₂/O₂ kinetic model for high- pressure combustion. International Journal of Chemical Kinetics, 44(7):444–474, 2012.

CA Hall and RW Pitz. Numerical simulation of premixed H₂–air cellular tubular flames. Combustion Theory and Modelling, 20(2):328–348, 2016.

S Hu, RW Pitz, and Y Wang. Extinction and near-extinction instability of non-premixed tubular flames.Combustion and Flame, 156(1):90–98, 2009.

M Nishioka, K Inagaki, S Ishizuka, and T Takeno. Effects of pressure on structure and extinction of tubular flame. Combustion and flame, 86(1-2):90–100, 1991.

SW Shopoff, P Wang, and RW Pitz. The effect of stretch on cellular formation in non-premixed opposed-flow tubular flames. Combustion and Flame, 158(5):876–884, 2011a.

SW Shopoff, P Wang, and RW Pitz. Experimental study of cellular instability and extinction of non-premixed opposed-flow tubular flames. Combustion and Flame, 158(11):2165–2177, 2011b.

P Wang, S Hu, and RW Pitz. Numerical investigation of the curvature effects on diffusion flames. Proceedings of the Combustion institute, 31(1):989–996, 2007.