in Wiley Online Library (wileyonlinelibrary.com).The current work aims to develop a reliable theoretical model capable of simulating the depletion process of urea-water-solution (UWS) droplets injected in a hot exhaust stream as experienced in an automotive urea-based selective catalytic reduction system. A modified multicomponent vaporization model is presented and implemented in the current study to simulate the behavior of UWS droplet in heated environment. Although water depletion is modeled as a vaporization process, urea depletion is modeled using two different approaches: (i) vaporization and (ii) direct thermal decomposition. The suitability of both depletion approaches is assessed in the current study by comparison with experimental data of the decay of a single UWS droplet in a quiescent heated environment. The decay rate of UWS droplet is accurately predicted with the multicomponent vaporization model. The possibility of internal gasification is demonstrated. Based on the complex decomposition behavior of urea, the current study proposes a decomposition mechanism for UWS droplet. The suitability of implementing the rapid mixing approach is assessed through comparison with the diffusion limit approach at various operating conditions and initial UWS droplet sizes. V V C 2011 American Institute of Chemical Engineers AIChE J, 57: 3210-3225, 2011
The flow inside channels with periodic, wavy walls of arbitrary shape is considered numerically. Solutions are obtained using either a perturbation approach, for weak modulation amplitude, or a finite volume technique, for strong amplitude. The flow is examined for sinusoidal, arched and triangular modulation over a wide range of amplitude, wavelength and Reynolds number in the steady laminar regime. For weak wall modulation (ε<0.3, α<2, where ε and α are the dimensionless half-wave height and wavelength, respectively), it is found that the flow behavior along the modulated wall is of the boundary-layer type. As such, the critical Reynolds number, Rec, for separation for each modulation shape can be expressed as an explicit function of ε and α, while the location of separation and pressure distribution along the modulated wall scale with ε, α, and Rec. For strong modulations, the boundary layer model is no longer satisfactory to predict the flow behavior and deviations from the trends found for weaker modulations are observed. It is also shown that the driving force required to sustain a given flow rate increases as ε increases. For all modulation amplitudes, the sinusoidal wave shape is found to require the largest pressure gradient to maintain a given flow rate through the channel and, consequently, yields the highest friction factor. Finally, the existence of a stable recirculating flow regime is discussed in the light of earlier stability analyses.
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