Karthik Ramanathan

Light-off criterion and transient analysis of catalytic monoliths

A one-dimensional two-phase model is used to derive an analytical light-off criterion for a straight channeled catalytic monolith with washcoat, in which the flow is laminar. For the case of uniform catalyst loading and a first order reaction, the light-off criterion is given by Here, Tf,in is the inlet fluid temperature, ΔTad is the adiabatic temperature rise, is one-half the channel hydraulic radius (, , cross-section area, perimeter), L is the channel length, ū is the fluid velocity, De is the reactant effective diffusivity in the washcoat, δc is the effective washcoat thickness, kf is the fluid thermal conductivity and kv(Tf,in) is the first order rate constant per unit washcoat volume at the inlet fluid temperature. NuH,∞ is the asymptotic Nusselt number in the channel. The function f accounts for diffusional limitations in the washcoat and is given by f(ϕ)=1 for ϕ 0.5. The factor g(Peh) depends on the solid conductivity, or more precisely, the heat Peclet number, , where δw(kw) is the effective wall thickness (thermal conductivity). The function g(Peh) decreases monotonically from 2.718 for Peh=0 to unity for Peh=∞. We also show that if the second term is negligible and the first exceeds unity, then ignition occurs at the back-end. If the second term exceeds unity then ignition occurs at the front-end. If the sum exceeds unity with the second term less than unity and not negligible compared to the first term then ignition occurs in the middle of the channel. This analytical ignition criterion is verified by numerical simulations using an accurate transient model that uses position dependent heat and mass transfer coefficients. We show that the plot of exit concentration versus time consists of two regions: kinetically controlled transient region and the mass transfer controlled steady-state asymptote. For the case of high solid conductivity, we present an analytical expression for the transient time at which the monolith shifts from the kinetically controlled to the mass transfer controlled regime. We also determine the influence of various parameters such as the washcoat thickness, channel dimensions, catalyst loading and initial solid temperature on this transient time and the cumulative emissions. Examination of the influence of solid conduction and channel geometry on cumulative emissions showed that designs that are optimum for steady-state operation lead to higher transient emissions and vice versa. Finally, we discuss the transient and steady-state behavior of the monolith for the special case of Lef<1 (hydrogen oxidation).

Post-processing of vehicle emission test data for use in exhaust after-treatment modelling and analysis

The accurate measurement of transient engine-out exhaust compositions, flow rate and temperature is critical for analysing and understanding the performance of exhaust system after-treatment components. This paper describes procedures for processing the transient emission data measured at vehicle test sites. These procedures include estimation of the hydrogen, oxygen and water concentrations and correction of the engine-out exhaust gas flow for the first few seconds of the drive cycle for non-oxygenated and oxygenated fuels. Both secondary air injection and non-secondary air injection applications are discussed. A procedure for accurately measuring the exhaust gas temperature during transient operations is also described with a specific thermocouple recommended for best results. A checklist for handling and processing the measured data is also provided. In addition to providing better data for further analysis and understanding of exhaust after-treatment components, the procedures listed in this work allow consistent and reliable input data to be generated for the mathematical model and model-based application studies using the measured vehicle emission data.