Shock Tube and Kinetic Study on the Effects of CO₂ on Dimethyl Ether Autoignition at High Pressures

Abstract

First-stage and overall ignition delay times of dimethyl ether (DME) were measured in a high-pressure shock tube for various mixtures containing high amounts of CO2. The data are available for DME/air mixtures diluted with 40% CO2 as well as for the mixture of DME and oxidizer containing 20.5% O2 and 79.5% CO2. As a reference, data were also collected for mixtures containing the corresponding amounts of N2. Measurements were conducted for pressures of 15, 35, and 50 bar and a range of temperatures (744–1316 K). For the DME/air mixture diluted with CO2, the equivalence ratio was varied in the range of 0.5–2.0. The results demonstrate that CO2 dilution has a strong effect on ignition delay times in the NTC (negative temperature coefficient) region. The kinetic study that was conducted showed that this phenomenon can be attributed to a thermal effect resulting from the high heat capacity of CO2. In low and high temperature ranges, the effect of CO2 is less pronounced. Additionally, a chemical effect was identified. At high temperatures this effect can be attributed mostly to the influence of CO2 on third-body reactions and leads to slight acceleration of ignition. Various chemical-kinetic models available were evaluated with respect to their accuracy in prediction of ignition delay times for mixtures containing DME and large amounts of CO2.

Ignition delay and chemical–kinetic modeling of undiluted mixtures in a high‐pressure shock tube: Nonideal effects and comparative uncertainty analysis

“…This was already observed for n-heptane 42 and for CO 2 -diluted DME-air mixtures. 39 Mittal et al 25 report a weak dependency on oxygen mole fraction, which is not observed here, as the oxygen mole fraction varies less compared to the mixtures investigated in Ref. 25.…”

“…43 Hence, emission at 407 nm can be associated with first-stage ignition. 39 Chemiluminescence and pressure signals are recorded by a spectrum M2i4912-exp card with a data rate of 10 M samples/s into eight channels with a resolution of 16 bit. To reduce the size of the acquired data to the necessary only, the data acquisition is triggered by a piezoelectric pressure sensor (PCB-113B03) mounted in the driven section close to the diaphragm.…”

“…For estimating the sensitivity functions in Equation 13the chemical-kinetic model AramcoMech 3.0 is chosen, as it shows good agreement with measurement data. 39 The resulting uncertainty in IDT is estimated to a maximum of 56.2% and 16.7% on average. Note that large uncertainty is present for short IDTs, which is caused by the relatively high extraction uncertainty component.…”

“…Additionally, excited formaldehyde () emissions are recorded with a filter centered around 407 nm to detect the first stage of ignition, as already done in Ref. 39. Excited formaldehyde emission is observed during low‐temperature oxidation of hydrocarbons 40–42 .…”

“…Excited formaldehyde emission spectrum overlaps excited and , but these species are not intrinsic to low‐temperature oxidation 43 . Hence, emission at 407 nm can be associated with first‐stage ignition 39 …”