Plasma flow reactor studies of H2/O2/Ar kinetics

Abstract

In the present study, a plasma flow reactor (PFR) facility designed to perform both ex situ and in situ experiments of stable (H 2 and O 2) and intermediate (OH radicals) species detection was used to examine the plasma-assisted characteristics of hydrogen oxidation at 1 atm pressure for temperatures ranging from 420 K to 1100 K. Experiments were performed at nearly isothermal conditions, by heavily diluting reactive mixtures in argon, in an attempt to mitigate temperature changes from exothermic chemical reactions. This technique allows experimental results to be interpreted from a perspective that plasma and thermal (neutral) heat release effects are decoupled, essentially isolating the effects of the plasma-chemistry and its influence on the neutral-chemistry. Results showed no thermal reaction until 860 K, at which point hydrogen was rapidly consumed within the flow residence time associated with the reactor. With the plasma discharge, the onset of oxidation was extended to lower temperatures (T < 860 K), while exhibiting a steady increase in the rate of oxidation starting from 470 K, and eventually consuming all the initial hydrogen by 800 K. Absolute measurements of OH mole fraction reveal that at conditions well below the dominance of the thermal chain-branched chemistry (at T = 668 K), the plasma induced a chain-propagating effect on OH formation, which was entirely confined to the boundaries of the plasma discharge section of the reactor. Furthermore, temporal measurements were also performed, showing that the extent of OH formation and subsequently its global effect on the fuel consuming chemistry can be manipulated based on the plasma perturbation timescale (i.e., the time scale at which the high-voltage pulses are administered to the reactive flow to generate the plasma discharge). Experimental results are compared to modeling calculations and show relatively good agreement, with the model predicting similar kinetic trends as a function of temperature. These results demonstrate new insight into the kinetics governing plasma-assisted combustion, and provide new experimental data to facilitate the development and validation of PAC-specific kinetic mechanisms.

Kinetic modeling and sensitivity analysis of plasma-assisted oxidation in a H2/O2/Ar mixture

“…The aforementioned numerical model is used to describe the physics and the chemistry of the PFR experiments conducted by Tsolas et al [27], where the reactor assembly was placed inside an electric tube furnace that created a trapezoidal temperature profile along the reactor. The model only considers the 40-cm plateau (isothermal) region in the middle, of which the first 5 cm is the electrode assembly that generates the plasma.…”

“…To simplify any forthcoming discussion, a single discharge system is first considered and analyzed. The conditions were set to replicate the experiments of Tsolas et al [27], with the exception that only one high-voltage pulse is applied. The initial mixture of the gas is set at 668 K and 1 atm.…”

“…In this section, the single discharge analysis is extended to the analysis of a multi-discharge system that reflects the experimental conditions of [27] more appropriately. Figure 5 shows the time evolution of species profiles simulated in the isothermal region of the reactor at T = 668 K when the repetitive voltage pulses are applied at a rate of 1 kHz.…”

“…In Ref. [27], the plasma flow reactor experiments were performed for a wide range of temperatures where the extent of reaction was monitored by measuring the H 2 and O 2 species concentrations at the reactor outlet for a fixed mass flow rate. The simulations of these experiments are presented in Fig.…”

“…Together with species profiles and reaction flux analysis, in-depth insight into the governing kinetics is presented, including rate-limiting steps, competing steps, and time scales of the different sub-categories of chemistry. The modeling efforts presented in this paper are based on the experimental conditions presented in Tsolas et al [27], to which the readers are directed for comparisons between experimental and modeling results. 6 2 Numerical Framework…”