Combustion Fundamentals At Engine-Relevant Conditions

The study of combustion as it relates to gas turbines and piston engines for aircraft development is an important topic to study. Unlike other fields of study, aviation, for the most part, relies purely on the use of fossil fuel combustion. This is due to the high-power density as compared to other alternatives such as battery powered. According to US patent US 6568633 B2 Fuel Cell Powered Electric Aircraft, despite electric engines being far more efficient than internal combustion engines, “gasoline still has nearly a 10 to 1 advantage of specific energy and energy density over rechargeable batteries” (Dunn, 2003). The extra weight that would be needed for a battery compared to fuel would hinder flight operations.

Two important characteristics of combustion that Professor Jagannath Jayachandran is studying are ignition delay and laminar flame speed. As the professor pointed out in his lecture, the understanding of these properties amongst others aid in the design, fuel screening, and chemical modeling of gas-turbines. This importance is collaborated by a 2008 Major Qualifying Project at Worcester Polytechnic Institute by Julie Buffam and Kevin Cox who studied laminar burning velocity of methane-air mixtures. In addition to mentioning the same three importance’s mentioned by Professor Jayachandran, they also stated that the study of laminar flame speeds is an essential part “in explosion protection and fuel tank venting” (Buffam, 2008). There is, however a gap in the research done on these properties. Most of the research being done at low pressures around 1 atm and the number of studies drops of rapidly the greater the pressure. One example of this was a study done on the ignition delay and laminar flame speed of 1,3-butadiene combustion. In this experiment ignition delay was tested at pressures up to 40 atm, however, the laminar flame speed was only tested at pressures up to 5 atm (Zhou et. al. , 2018). Another study was done on the combustion of monoxide and hydrogen using the spherical flame technique, described by Professor Jayachandran, at constant pressures up to 40 atm (Sun et. al. , 2007). These pressures, compared to what us usually seen in gas turbines, are small, presenting a problem in the design of gas turbines as most operate well above 40 atm in the supercritical region. Professor Jayachandran, using impinging flames, hopes to determine laminar flame speeds and visually see the flames at pressures greater than 40 atm.

Another important aspect that Professor Jayachandran is attempting to research is the effects of changing pressure on flame propagation. The previous examples discussed are either done in a constant pressure environment or assumed constant pressure as the moving flame is faster than the pressure change. This is important to study as in ignition and combustion engines are accompanied by changes in pressure. Part of conceptual study involved the study of thermal boundary layer ignition. Due to the way ignition delay varies with temperature it is possible to have ignition originate close to a cold wall. This is related to what is called knock in engines, resulting in a non-homogenous ignition. In a study done on knocking during the combustion in a spark-ignition engine, the importance of knocking research was heavily stressed as it directly relates to the durability and power outputs of the engine amongst others.

In-addition this study looks into the pressure oscillations that occur from this phenomenon (Wang et. al. , 2017). From Professor Jayachandran’s research, he showed that a change in pressure increases the burning flux, however he determined that this increase is not significant. However, he also determined that at different pressure changes is significant in determining whether autoignition will occur close to the wall or farther from the wall.

18 March 2020
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