Design and Development of a High-Pressure Flow Reactor with Continuous Temperature Ramping for Fuel Oxidation Studies

Abstract

To investigate the oxidation chemistry of fuels under more realistic conditions, with a particular focus on low-temperature chemistry, a high-pressure flow reactor with continuous temperature ramp control has been developed. Unlike conventional reactors which operate often at discrete temperature plateaus, this system enables a gradual and controlled temperature increase, allowing for a fast and detailed analysis of reaction kinetics and temperature dependent phenomena. The reactor is designed to operate at pressures between 2 and 20 bar and temperatures ranging between 473 and 1273 K, covering the key regimes relevant to low-temperature oxidation. To ensure stable operation, an automatically regulated valve continuously adjusts the reactor pressure in real time, maintaining the desired elevated-pressure conditions throughout the experiment. Oxidation occurs in a highly diluted environment (≥99% argon) to allow for fully premixed fed. The reactor geometry, flow characteristics, and control system were carefully designed, and analysis confirmed plug-flow behavior with residence times between 3 and 28 s. A specially designed expansion chamber connects the reactor to the electron-ionization molecular-beam mass spectrometry (EI-MBMS) system and is operated at a controlled pressure. EI-MBMS provides a comprehensive insight into the temporal evolution of species formed during the oxidation process, enabling the identification of key intermediates and the quantitative determination of product distributions. First validation experiments focused on the oxidation of oxymethylene ethers (OMEs), a promising class of synthetic fuels which were also studied systematically previously by MBMS in a laminar flow reactor at atmospheric pressure and in low-pressure flames. The new experiments presented here were conducted at 5 bar and reveal key oxidation intermediates such as formaldehyde (CH₂O) and methyl formate (CH₃OCHO). The results also highlight significant differences in low-temperature chemistry (LTC) reactivity among OMEs: OME1 exhibits low reactivity, whereas larger OMEs (OME2 and OME3) show enhanced LTC reactivity and negative temperature coefficient (NTC) behavior at elevated pressures. By enabling high-resolution electron ionization time-of-flight molecular-beam mass spectrometry (EI-TOF-MBMS) across a continuously varying temperature profile, the high-pressure reactor provides a powerful platform for detailed reaction kinetic such as low-temperature oxidation mechanisms. The modular design supports the integration of multiple diagnostic techniques, including MBMS, fourier-transform infrared spectroscopy (FTIR), and soot particle analysis, ensuring a comprehensive characterization of combustion intermediates and emissions.