Maldistributed flow within an automotive catalyst can cause reduced conversion
efficiency, high pressure loss, and premature deactivation. However, packaging
constraints often result in uneven flow distribution between the monolith
channels, thus compromising design and, inevitably, performance of the device.
Flow uniformity may be improved by the introduction of swirl upstream of the
catalyst assembly, and in turbocharged applications the residual swirl from the
turbine can serve that purpose. Indeed, low swirl has been shown to provide
favorable flow uniformity in the monolith substrate in an axisymmetric flow
setup. However, the automotive exhaust aftertreatment setups are seldom
axisymmetric, and the combined effects of inlet swirl and offset on the flow
profile through a monolith substrate are unknown.
To address this gap, this study provides the first systematic experimental
characterization of the coupled influence of inlet swirl and packaging-relevant
inlet offset on flow development and uniformity in a sudden expansion catalyst
assembly. Particle image velocimetry (PIV), wall pressure measurements, and
hot-wire anemometry (HWA) are combined to link the upstream separation and
recirculation structures to the velocity distribution downstream of the
monolith. The results reveal a previously unreported swirl-dependent sensitivity
to geometric asymmetry: under no-swirl and moderate-swirl conditions, flow
uniformity is robust to inlet offset, varying by no more than 1.4%, whereas at
low swirl the offset reduces uniformity by up to 8% at high mass flow rate.
Increasing mass flow rate reduces uniformity by up to 15%, while swirl improves
uniformity by up to 19% relative to axial flow. These findings demonstrate that
improvements observed for swirl in axisymmetric assemblies cannot be assumed to
transfer directly to offset geometries. Swirl intensity and inlet alignment must
instead be considered as coupled design variables. The measurements also provide
a benchmark dataset for validating computational fluid dynamics simulations
before their application to production-type systems.
