Multi-Scale Modelling of Monolith Honeycomb Substrates

• Author / Creator

  Cornejo Garcia, Ivan

• This thesis reports the study of flow inside a honeycomb type substrate. Honeycomb monoliths are extensively used in the automotive industry, as substrates in the exhaust gas after-treatment system. The flow approaching a honeycomb monolith is usually highly turbulent; However, once entering the channels, turbulence dissipates because of a dramatic reduction of the Reynolds number. The first part of the honeycomb channels can be very active in terms of chemical reactions and can impact the performance of the entire system significantly; Therefore, it is important to study the flow regime transition and other phenomena occurring in that area. The flow regime, pressure drop and convective heat transfer when flow enters, passes through and leaves the substrate are analyzed. Computational models at a channel and a converter scale are used. At a converter scale, the honeycomb is modelled as a continuum, meanwhile, for channels, a discrete model is used. Laminar and turbulent flow approaching the monolith are considered. The cases with turbulent flow are modelled with Reynolds-Average Navier-Stokes (RANS) models and Large Eddy Simulation (LES).

  According to the results, the turbulence approaching the honeycomb in fact dissipates inside the channels, but does not lead to a steady flow, instead of that, the flow becomes laminar unsteady. When turbulence effectively enters the channels, it enhances the convective heat transfer in the entrance length; However, in the laminar unsteady region, both the pressure drop and the convective heat transfer coefficient are similar to those for steady laminar flow. Regarding the exit of the substrate, when flow leaves the channels behaves like a jet, and under certain conditions, it generates turbulence, even when the flow inside the channels is steady. The generation of turbulence is promoted by a higher channel velocity, remaining turbulence inside the substrate and pulsating flow inside the channels, and it is also affected by the channel shape. A new strategy to obtain a realistic decay and generation of turbulence in converter scale simulations using the continuum approach is proposed. The new strategy corrects nonphysical flow regime transitions observed in previous models. A new pressure drop model for flow through a honeycomb is presented. The model accounts for the effects of flow entering, developing and leaving the substrate, and can be applied for circular, square, triangular and hexagonal channel cross-sections. A new correlation for the apparent permeability of a continuum modelling a monolith is reported. The correlation is based on the friction factor inside a monolith channel and accounts for a realistic inlet velocity profile and the hydraulic entrance length.

  New correlations for the convective heat transfer coefficient at a constant wall temperature and a constant wall heat flux when laminar flow is entering into a circular cross-section monolith channel are reported. The correlations consider temperature-dependent fluid properties. For the case with a constant wall temperature, when the heating rate is high, the curve of the convective heat transfer coefficient along the channel has a minimum significantly lower than the asymptotic value. Correlations available in the literature are only valid for monotonically decreasing curves; Hence, a new mathematical expression that combines a decreasing function with a sigmoidal one is used. A methodology to calibrate such a model is also presented. A methodology for modelling the effect of the upstream turbulence when it enters the channels on the convective heat transfer coefficient is proposed.

  Finally, a wall-flow filter, which is a particular type of honeycomb with porous walls, is analyzed. The developing of the flow inside the filter is investigated. A criterion to consider the flow as fully developed is presented. It is found that the friction factor in the filter channels is different from that of pipes with non-porous walls, as usually assumed. An improved pressure drop model, based on first principles, is proposed.

• Subjects / Keywords

  • Converter
  • CFD
  • Honeycomb
  • Monolith

• Graduation date

  Spring 2020

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-58vg-ec89

• License

  Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.