60th Annual Report on Research 2015

For the majority of combustor systems, auto-ignition underpins the operational envelope, whether it is targeted for combustion initiation such as in diesel or HCCI engines or actively prevented as in gas turbine or spark-ignition engines. To date, there has been little experimental research examining the fundamental details of auto-ignition under highly turbulent conditions and in particular, little is known about the time-dependent coupling of the flow turbulence, scalar mixing of inhomogeneous reactants, and finite-rate kinetics that lead to the observed auto-ignition topology. The primary objective of this research is to use time-resolved (10 to 50 kHz), multi-scalar imaging of mixture fraction, temperature, and species (CH₂O and OH) fields to quantify the auto-ignition dynamics of highly turbulent fuel jets issuing into hot, vitiated, oxidizing environments.

Our original target was to investigate vaporized n-heptane fuels, but we have altered our original course of action to look at two simpler fuels before investigating n-heptane: (1) n-butane (C₄H₁₀) and dimethyl ether (DME; C₂H₆O). The fuels are in the gas phase and represent a more amenable system for investigating auto-ignition dynamics initially. In addition, n-butane still retains complexity beyond very simple fuels such as methane. C₄H₁₀ also shares several operational similarities and flame characteristics as n-heptane. DME represents an oxygenated fuel of which there is little auto-ignition information and is a suitable fuel for joint testing. Following successful investigation of these fuels, we will return to the investigation of C₇H₁₆.

In the first year we have developed a stable auto-ignition platform as shown in Fig. 1, as well as a unique operating methodology targeted to perform parametric studies of the rate-limiting parameters influencing auto-ignition under turbulent flow conditions. Figure 1 shows the Jet in Hot Co-flow (JiHC) facility fabricated for this project. Pulsed fuel issues from a 5-mm-diameter nozzle into a 150-mm-diameter vitiated, co-flow using a fast-actuating solenoid valve. The duration of the fuel jet injection can be varied from less than 2 ms to continuous, although no ignition dependence on injection duration has been noted for valve opening times. In the current program, the solenoid valve will be opened (and the fuel will issue) for 100 ms, which is sufficient to auto-ignite and establish a stably-burning flame, but short enough such that system heating does not occur. Since system heating does not occur, boundary conditions remain constant for successive fuel injection/auto-ignition sequences, enabling repeated “bursts” of multi-kHz-rate data collection. The annular co-flowing stream consists of the combustion products of lean, premixed H₂/O₂/N₂ combustion, which is generated by a series of 3200 1.0-mm-diameter holes which stabilize the individual flames. The high open area of the coflow (holes per area = 18/cm²) was designed to produce a very uniform coflow temperature distribution. Second, it is noted that the operating conditions have been specifically designed to parametrically study the influence of mixture composition and temperature individually. Unlike previous vitiated burners, the coflow stream consists of products from H₂/O₂/N₂ flames and not H₂/air flames. The utility of this approach lies in the ability to fix one parameter and vary another desired parameter. As one example, we have developed a series of test conditions such that the equivalence ratio of the premixed H₂/O₂/N₂ co-flow varies between 0.25 and 0.50 and the O₂/N₂ ratio is adjusted at each equivalence ratio to yield the same product gas temperature (e.g., 1500 K for all cases). In this manner, the vitiated coflow temperature is fixed, but the O₂ content in the vitiated stream varies and thus the resultant stoichiometric mixture fraction (single fuel blend + vitiated stream) is varied by a factor of three. Similarly, we have designed cases where the equivalence ratio varies within the same 0.25 to 0.50 range, but with different O₂/N₂ ratios such that the stoichiometric mixture fraction is fixed and the temperature varies from 1000K to 1800K. This novel approach allows a detail parametric study of individual mixture ratio and temperature effects that has not been available previously. It is noted that the burner platform was designed to allow the use of liquid fuels (such as C₇H₁₆) as originally proposed, so that these measurements will be facilitated.

Additional year 1 work in this program has focused on finalizing a data processing method previously established as well as further analysis of previous auto-ignition imaging studies which help lay the foundation for experimental targets in this work. This program has been used to support the Ph.D. research on one student, Rajat Saksena. Because of the topic of his research, Mr. Saksena was sent to participate in the Combustion Summer School hosted by Princeton University. This unique opportunity allowed Mr. Saksena to interact with leaders in the field and participate in a week-long course that has advanced his understanding on combustion and energy-conversion processes. Finally, it is noted that the PI has submitted a grant proposal to the National Science Foundation to follow on this preliminary work. Since this is one of the goals of ACS DNI grants, this aspect of the research project can be considered a success.

Fig. 1 – Schematic and photograph of the Jet-in-Hot Coflow (JiHC) facility.