John Mantzaras

An experimental and numerical investigation of homogeneous ignition in catalytically stabilized combustion of hydrogen/air mixtures over platinum

The gas-phase ignition of fuel-lean hydrogen/air mixtures over platinum was investigated experimentally and numerically in laminar channel-flow configurations. Experiments were performed at atmospheric pressure in an optically accessible catalytic channel combustor established by two Pt-coated parallel plates, 300 mm long (streamwise direction) and placed 7 mm apart (transverse direction). Planar laser induced fluorescence (PLIF) of the OH radical along the streamwise plane of symmetry was used to monitor the onset of homogeneous ignition, one-dimensional Raman measurements (across the 7-mm transverse direction) provided the boundary layer profiles of the major species and temperature, and thermocouples embedded beneath the catalyst yielded the surface temperature distribution. Computations were carried out using a two-dimensional elliptic fluid mechanical model that included multicomponent transport and elementary homogeneous (gas-phase) and heterogeneous (catalytic) chemical reaction schemes. Four homogeneous and three heterogeneous reaction schemes were tested in the model against measured homogeneous ignition characteristics. The differences between measured and predicted homogeneous ignition distances could be substantial (ranging from 8% to 66%, depending on the particular hetero/homogeneous schemes) and were ascribed primarily to the homogeneous reaction pathway. Sensitivity analysis indicated that the discrepancies induced by the gas-phase schemes originated either from the presence of heterogeneously-produced water due to its effectiveness as collision partner in the chain terminating reaction H O2 M HO2 M, or from an overall overprediction of the radical pool in the preignition zone. The heterogeneous schemes had significant differences in their surface coverage and radical fluxes, but these variations had practically no impact on homogeneous ignition. Sensitivity and reaction flux analyses have shown that this was attributed to the ability of all heterogeneous schemes to capture the measured mass-transport-limited fuel conversion and to the relative insensitivity of homogeneous ignition on the magnitude of the heterogeneous radical fluxes, provided that all radical adsorption reactions (OH, H, and O) were included in the heterogeneous schemes.

Two-dimensional modelling for catalytically stabilized combustion of a lean methane-air mixture with elementary homogeneous and heterogeneous chemical reactions

Abstract The catalytically stabilized combustion (CST) of a lean (equivalence ratio Φ = 0.4) methane-air mixture was investigated numerically in a laminar channel flow configuration established between two platinum-coated parallel plates 50 mm long and 2 mm apart. A two-dimensional elliptic fluid mechanical model was used, which included elementary reactions for both gaseous and surface chemistry. Heat conduction in the solid plates and radiative heat transfer from the hot catalytic surfaces were accounted for in the model. Heterogeneous ignition occurs just downstream of the channel entrance, at a streamwise distance (x) of 4 mm. Sensitivity analysis shows that key surface reactions influencing heterogeneous ignition are the adsorption of CH4 and O2 and the recombinative desorption of surface-bound O radicals; the adsorption or desorption of radicals other than O has no effect on the heterogeneous ignition location and the concentrations of major species around it. Homogeneous ignition takes place at x = 41 mm. Sensitivity analysis shows that key surface reactions controlling homogeneous ignition are the adsorption/desorption of the OH radical and the adsorption/desorption of H2O, the latter due to its direct influence on the OH production path. In addition, the slope of the OH lateral wall gradient changes from negative (net-desorptive) to positive (net-adsorptive) well before homogeneous ignition (x = 30 mm), thus exemplifying the importance of a detailed surface chemistry scheme in accurately predicting the homogeneous ignition location. The effect of product formation on homogeneous ignition was studied by varying the third body efficiency of H2O. Product formation promotes homogeneous ignition due to a shift in the relative importance of the reactions H + O2 + M → HO2 + M and HCO + M → CO + H + M.

High-pressure experiments and modeling of methane/air catalytic combustion for power-generation applications

The catalytic combustion of methane/air mixtures is investigated experimentally and numerically at gas turbine relevant conditions (inlet temperatures up to 873 K, pressures up to 15 bar and spatial velocities up to 3 × 10 6 h −1 ). Experiments are performed in a sub-scale test rig, consisting of a metallic honeycomb structure with alternately coated (Pd-based catalyst) channels. Simulations are carried out with a two-dimensional elliptic fluid mechanical code that incorporates detailed transport and heat loss mechanisms, and realistic heterogeneous and homogeneous chemistry description. The methodology for extracting heterogeneous kinetic data from the experiments is presented, and the effects of catalytic activity and channel geometry (length and hydraulic diameter) on reactor performance are elucidated. A global catalytic kinetic step provides excellent agreement (at temperatures below 950 K) between the measured and predicted fuel conversion, over a wide range of parameter variation (channel hydraulic diameter and length, pressure, and inlet temperature). It is shown that, under a certain combination of catalytic activity and channel length, the absolute temperature rise across the catalyst becomes essentially independent of pressure, a feature highly desirable for many practical systems. Even though the computed catalyst surface temperatures remain well below the decomposition temperature of PdO, a significant section of the catalyst—amounting up to 30% of the total reactor length—contributes minimally to the total fuel conversion, suggesting catalytic activity design improvements in the reactor entry.

Catalytic Combustion of Syngas

The catalytic combustion of syngas/air mixtures over Pt has been investigated numerically in a channel-flow configuration using 2D steady and transient computer codes with detailed hetero-/homogeneous chemistry, transport, and heat transfer mechanisms in the solid. Simulations were carried out for syngas compositions with varying H2 and CO contents, pressures of 1 to 15 bar, and linear velocities relevant to power generation systems. It is shown that the homogeneous (gas-phase) chemistry of both H2 and CO cannot be neglected at elevated pressures, even at the very large geometrical confinements relevant to practical catalytic reactors. The diffusional imbalance of hydrogen can lead, depending on its content in the syngas, to superadiabatic surface temperatures that may endanger the catalyst and reactor integrity. On the other hand, the presence of gas-phase H2 combustion moderates the superadiabatic wall temperatures by shielding the catalyst from the hydrogen-rich channel core. Above a transition temperature of about 700 K, which is roughly independent of pressure and syngas composition, the heterogeneous (catalytic) pathways of CO and H2 are decoupled, while the chemical interactions between the heterogeneous pathway of each individual fuel component with the homogeneous pathway of the other are minimal. Below the aforementioned transition temperature the catalyst is covered predominantly by CO, which in turn inhibits the catalytic conversion of both fuel components. While the addition of carbon monoxide in hydrogen hinders the catalytic ignition of the latter, there is no clear improvement in the ignition characteristics of CO by adding H2. Strategies for reactor thermal management are finally outlined in light of the attained superadiabatic surface temperatures of hydrogen-rich syngas fuels.

Homogeneous ignition of methane-air mixtures over platinum: Comparison of measurements and detailed numerical predictions

The homogeneous ignition of lean methane-air mixtures was investigated numerically and experimentally in a laminar plane channel flow configuration established by two externally heated catalytically active (Pt-coated) ceramic plates, 250 mm long by 100 mm wide, place 7 mm apart. Preheated fuel-air mixtures with equivalence ratios of 0.31 and 0.37 and uniform velocities of 1 and 2 m/s were examined, resulting in incoming Reynolds numbers ranging from 190 to 380. Planar laser-induced fluorescence (PLIF) was used to map the OH concentration field along the streamwise direction and thermocouples to monitor both catalyst plate temperatures. The numerical predictions included a two-dimensional elliptic model with detailed heterogeneous and homogeneous chemical reactions. The homogeneous ignition location strongly depends on the incoming velocity and mildly on the equivalence ratio. Following homogeneous ignition, a very stable V-shaped flame is formed in all cases. Measured and predicted flame sweep angles, OH levels, and the post-flame OH relaxation are in good agreement with each other, while the homogeneous ignition distance is predicted within 9% in all cases. The homogeneous ignition location is shown to be better identified with changes of averaged (over the channel cross section) quantities rather than with changes in local wall gradients. The overall model performance suggests that the employed surface scheme is capable of capturing the coupling between surface and gaseous chemistries leading to homogeneous ignition. Experiments and predictions were also carried out with noncatalytic plates. The resulting flame is unstable and asymmetric, clearly showing the stability advantages of catalytically assisted combustion.

Homogeneous combustion of fuel-lean H2/O2/N2 mixtures over platinum at elevated pressures and preheats

The gas-phase combustion of H2/O2/N2 mixtures over platinum was investigated experimentally and numerically at fuel-lean equivalence ratios up to 0.30, pressures up to 15 bar and preheats up to 790 K. In situ 1-D spontaneous Raman measurements of major species concentrations and 2-D laser induced fluorescence (LIF) of the OH radical were applied in an optically accessible channel-flow catalytic reactor, leading to the assessment of the underlying heterogeneous (catalytic) and homogeneous (gasphase) combustion processes. Simulations were carried out with a 2-D elliptic code that included elementary hetero-/homogeneous chemical reaction schemes and detailed transport. Measurements and predictions have shown that as pressure increased above 10 bar the preheat requirements for significant gas-phase hydrogen conversion raised appreciably, and for p = 15 bar (a pressure relevant for gas turbines) even the highest investigated preheats were inadequate to initiate considerable gas-phase conversion. Simulations in channels with practical geometrical confinements of 1 mm indicated that gas-phase combustion was altogether suppressed at atmospheric pressure, wall temperatures as high as 1350 K and preheats up to 773 K. While homogeneous ignition chemistry controlled gaseous combustion at atmospheric pressure, flame propagation characteristics dictated the strength of homogeneous combustion at the highest investigated pressures. The decrease in laminar burning rates for p P 8 bar led to a push of the gaseous reaction zone close to the channel wall, to a subsequent leakage of hydrogen through the gaseous reaction zone, and finally to catalytic conversion of the escaped fuel at the channel walls. Parametric studies delineated the operating conditions and geometrical confinements under which gas-phase conversion of hydrogen could not be ignored in numerical modeling of catalytic combustion. 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Three-dimensional simulations of premixed hydrogen/air flames in microtubes

The dynamics of fuel-lean (equivalence ratio φ=0.5) premixed hydrogen/air atmospheric pressure flames are investigated in open cylindrical tubes with diameters of d=1.0 and 1.5 mm using three-dimensional numerical simulations with detailed chemistry and transport. In both cases, the inflow velocity is varied over the range where the flames can be stabilized inside the computational domain. Three axisymmetric combustion modes are observed in the narrow tube: steady mild combustion, oscillatory ignition/extinction and steady flames as the inflow velocity is varied in the range 0.5≤ U IN ≤ 500 cm s -1 . In the wider tube, richer flame dynamics are observed in the form of steady mild combustion, oscillatory ignition/extinction, steady closed and open axisymmetric flames, steady non-axisymmetric flames and azimuthally spinning flames (0.5 ≤ U IN ≤ 600 cm s -1 ). Coexistence of the spinning and the axisymmetric modes is obtained over relatively wide ranges of U IN . Axisymmetric simulations are also performed in order to better understand the nature of the observed transitions in the wider tube. Fourier analysis during the transitions from the steady axisymmetric to the three-dimensional spinning mode and to the steady non-axisymmetric modes reveals that the m = 1 azimuthal mode plays a dominant role in the transitions.

Effects of finite rate heterogeneous kinetics on homogeneous ignition in catalytically stabilized channel flow combustion

Abstract The homogeneous (gas-phase) and heterogeneous (catalytic) ignition of fuel-lean premixtures is investigated analytically and numerically in two-dimensional laminar channel-flow configurations with uniform incoming properties and isothermal catalytic walls. First-order matched activation energy asymptotics are employed, along with a one-step catalytic reaction and a one-step large activation energy gaseous reaction. Parametric description of the chemically frozen state leads to a closed-form heterogeneous ignition criterion in terms of non-dimensional variables that are relevant to confined flows. Formulation of the weakly reactive state yields a closed-form homogeneous ignition criterion that includes explicitly the homogeneous and heterogeneous reactivities through the relevant gaseous and surface Damkohler numbers ( Da g and Da s , respectively). Both ignition criteria are valid over the range 0.002 ≤ x /( bRePr ) ≤ 0.16, 1.5 ≤ T W / T IN ≤ 3, and 0.9 ≤ Le ≤ 2.0, where x is the streamwise distance, Re the incoming Reynolds number based on the channel half-height b , Pr the Prandtl number, T W / T IN the wall-to-inlet temperature ratio, and Le the Lewis number of the fuel. Numerical simulations have shown good agreement between the numerically and the analytically predicted homogeneous ignition distances. A reduction of the surface reactivity ( Da s ) promotes homogeneous ignition due to the associated increase of the near-wall fuel levels, and this effect is manifested in the homogeneous ignition criterion via a corresponding increase of the characteristic transverse diffusion time scale with decreasing Da s . It is shown that there exist infinite combinations of surface and gaseous reactivities yielding the same homogeneous ignition distance, suggesting caution in the interpretation of catalytically stabilized combustion (CST) experiments. Moreover, the homogeneous ignition distance is much more sensitive to the gaseous rather than to the surface reaction pathway, thus exemplifying the importance of validated homogeneous reaction schemes under CST-relevant conditions.

An asymptotic and numerical investigation of homogeneous ignition in catalytically stabilized channel flow combustion

The gas-phase ignition of a fuel-lean premixed combustible gas is investigated in a forced convection two-dimensional laminar channel flow configuration established by two catalytically-active parallel plates placed at a distance 2b apart. The gaseous mixture has uniform inlet properties and both plate temperatures are constant and equal. First-order matched activation energy asymptotics are used to describe the reactive gaseous flow in conjunction with the boundary layer approximation, a one-step large activation energy gaseous reaction, and an infinitely fast (mass-transport-limited) catalytic reaction. A closed form ignition criterion is obtained for the gas-phase ignition distance in terms of nondimensional groups that are relevant to confined flows. The characteristic chemical and transverse diffusion time scales are included explicitly in the ignition criterion clearly demonstrating the competition between gaseous and catalytic fuel conversion while the effect of flow confinement (b) is included implicitly. The ignition criterion is valid over the range 0.002 < x/(bRePr) < 0.16, with x the streamwise distance, Re the flow Reynolds number based on the channel halfwidth b and the uniform inlet properties, and Pr the Prandtl number. The temperature and transport parameter ranges of applicability are 1.5 < TW/TIN < 3 (with TW/TIN the ratio of the catalytic wall to the inlet temperature) and 0.9 < Le < 2.0 (with Le the Lewis number) respectively, rendering the ignition criterion of particular interest to hydrocarbon catalytically stabilized combustion (CST) applications. Numerical simulations are performed for channel flow catalytic combustion of a fuel-lean (equivalence ratio 0.32) propane–oxygen–nitrogen mixture using the same underlying chemistry assumptions as in the analytical asymptotic approach. The analytically calculated ignition distances are in good agreement with those numerically predicted. The effect of flow confinement (finite b) on gaseous ignition is examined by comparing ignition distances with the corresponding ones of the unconfined (flat plate) case. Flow confinement (decreasing b) increases the ignition distances due to the resulting increase in the channel surface-to-volume ratio. Moreover, the effect of flow confinement is important already from x/(bRePr) = 0.002.

Catalytic combustion of methane/air mixtures over platinum: Homogeneous ignition distances in channel flow configurations

The homogeneous ignition of fuel-lean methane/air mixtures is investigated numerically in laminar plane channel configurations with platinum-coated isothermal walls and uniform incoming properties. Parametric studies are carried out to determine the dependence of the homogeneous ignition distance (χ ig ) on the fuel-to-air equivalence ratio (), the wall temperature ( T W ), the inlet temperature ( T IN ), the inlet velocity ( U IN ), and the channel wall separation (2 b ). Computations are performed with elliptic and parabolic two-dimensional numerical codes, both with elementary heterogeneous and homogeneous chemical reaction schemes. The applicability of the parabolic approach (boundary layer approximation) in assessing homogeneous ignition is investigated. The elliptic approach yields shorter x ig compared to those of the parabolic approach, but as U IN increases, their difference diminishes, and for U IN greater than a minimum value U m, IN , both computations give the same x ig . U m, IN depends strongly onand ranges from 8 m/s (=0.35) to 15.5 m/s (=0.55) at atmospheric pressure. An analytical homogeneous ignition criterion based on activation energy asymptotics, a one-step gaseous reaction and a mass-transport-limited surface reaction, is presented for catalytic channel configurations and adapted to lean methane/air combustion. The mass-transport-limited assumption is shown to be valid only at atmospheric pressure. A one-step gaseous reaction with a methane order of −0.33 and an activation energy of 243.4 kJ/mol yields, in conjunction with the analytical ignition criterion, homogeneous ignition distances at atmospheric pressure within 9.2% of those numerically predicted over a wide range of operating conditions (0.35≤≤0.55, 1380 K≤ T W ≤1600 K. 623 K≤ T IN ≤743 K and 1.5 mm≤ b ≤15 mm). The negative methane reaction order (methane self-inhibition) results in shorter x ig for the leaner mixtures. The apparent activation energy is higher than that of purely homogeneous combustion (≈200 kJ/mol) due to catalytic inhibition via radical adsorption.