NTC-Affected Ignition of DME by Heated Counterflowing Air

摘要

An experimental study, supported by computation, was conducted on the coupling of NTC-chemistry and transport in low-temperature ignition, using the nonpremixed counterflow which provided a well-characterized flow time for comparison with the chemical ignition delay time. In particular we were interested to explore if ignition in a strained flow field with a finite residence time could still be fundamentally affected by the low-temperature NTC chemistry, as recently computationally demonstrated for the nonpremixed counterflow of heptane versus heated air. The fuel chosen in this study is dimethyl ether (DME). The presence of low-temperature reactivity was detected by using a photomultiplier tube (PMT) which measured the chemiluminescense from the radicals generated by the low-temperature chemistry. A filter combined with the PMT captured the spectrum around 400 nm, corresponding to the radiation emission of the HCHO molecules. The signal is further processed by a lock-in amplifier to diminish the noise. A highly sensitive infrared camera was also used to visualize the mixing reaction layer, with images clearly showing the diffusion cool flame. Experiments conducted under various fuel boundary concentrations and strain rates showed that ignition affected by the NTC chemistry, relevant at lower air boundary temperatures, was achieved at lower strain rates, and was found to be insensitive to a wide range of fuel concentrations. Both experimental findings agreed well with the computation using detailed chemistry, with the controlling mechanisms identified.