Forced Convection Heat Transfer Characteristics of Developed Laminar Flow in the Octagonal Channels of Octo-Square Asymmetric Particulate Filters

The forced convection heat transfer characteristics of fully developed laminar flow in an octagonal channel with alternating sides of different lengths have been investigated. Such channels occur in octo-square asymmetric diesel and gasoline particulate filters (PFs). The cross section of these channels resembles a square with the corners “cut off”. As the size of the cutoff is increased, the channel cross section varies from square (zero cutoff) to a regular octagon. The temperature field has been solved, and Nu was determined for the constant wall heat flux (H2) and constant wall temperature (T) boundary conditions (BCs) for the full range of possible channel geometries from a square to a regular octagon. This work compliments earlier work by the author where the temperature field for the H1 BC, as well as the velocity field, was solved. Together, these two papers provide all the parameters required in the momentum and energy balance equations of a 1-dimensional model for an octo-square PF (Nu, friction factor, momentum flux correction factor, etc.). Heat transfer is more effective for a square channel than a regular octagonal one with the H1 and T BCs (where the wall temperature is constant), but the converse is true for the H2 BC. For typical commercially available octo-square PFs, the error induced in a 1-dimensional model by incorrectly assuming that the octagonal channels are square is relatively small for the constant wall temperature BCs (H1 & T) but is potentially significant for the constant wall heat flux (H2) BC.