Flow and forced convection heat transfer characteristics of developed laminar flow in the octahedral channels of octo-square asymmetric particulate filters

Abstract

The flow and forced convection heat transfer characteristics for fully developed laminar flow in an octahedral channel with alternating sides of different length has been investigated. Such channels occur in octo-square asymmetric diesel and gasoline particulate filters (PF). The cross section of these channels resembles a square with the corners “cut off”. As the size of the cut off is increased, the channel cross section varies from square (zero cut off) to a regular octahedron. The velocity and temperature fields have been solved using the point matching and least squares methods. These give an analytical expression for the velocity and temperature field in the form of algebraic-trigonometric series. The least squares method was preferred as it resulted in smaller and more evenly distributed deviations from the intended boundary condition at the channel wall.As the channel shape is varied from square to a regular octahedron, the velocity profile becomes slightly flatter, resulting in a small decrease in the momentum flux correction factor, kinetic energy flux correction factor and the maximum velocity; the friction factor and Nu increase, but the viscous loss coefficient (F) and the product of the channel perimeter and heat transfer coefficient, which actually appear in the momentum and energy balance equations of a 1-dimensional model of a PF, decrease. However, when all factors are considered, there is very little difference in the along-channel pressure drop and heat transfer predicted for octahedral and square channels of the same width for typical commercially available octo-square PFs. Keywords: Asymmetric particulate filter, Diesel or gasoline particulate filter (DPF or GPF), Friction factor, Nusselt number, Octahedral channel