Emissions sample probes are widely used in engine and vehicle
emissions development testing. Tailpipe bag summary data is used
for certification, but the time-resolved (or modal) emissions data
at various points along the exhaust system is extremely important
in the emission control technology development process. Exhaust gas
samples need to be collected at various locations along the exhaust
aftertreatment system. Typically, a tube with a small diameter is
inserted inside the exhaust pipe to avoid any significant effect on
flow distribution. The emissions test equipment draws a gas sample
from the exhaust stream at a constant volumetric flow rate
(typically around 10 SLPM). The sample probe tube delivers exhaust
gas from the exhaust pipe to emissions test equipment through
multiple holes on the surface of tube. There can be multiple rows
of holes at different axial planes along the length of the sample
probe as well as multiple holes on a given axial plane of the
sample probe. In a traditional sample probe design, there are
multiple planes of holes along the length and several holes evenly
distributed on a given plane with a constant hole size. It was
observed that the exhaust gas sample composition detected utilizing
a traditional sample probe design may not accurately represent the
gas composition in the exhaust system especially for samples taken
from a larger diameter exhaust pipe.
In this study, a systematic numerical investigation was
conducted to characterize the mass flow distribution for different
emissions sample probe designs used in 3.5\mi and 8\mi exhaust pipe
applications. First, the numerical investigation focused on the
effects of the number of holes in each axial plane (or row along
the circumference on the tube surface) and on the number of rows of
holes (along the tube length). Next, the effect of location and
orientation of the sample holes, as well as exhaust mass flow rate
effects were studied. Then, the effect of sample hole size on
sample mass flow rate distribution along the length of the
emissions sample tube was investigated. In the end, the sample hole
sizes were optimized for both 3.5\mi and 8\mi diameter exhaust pipe
applications. Numerical results showed significant improvement in
the mass flow rate distribution as the number of holes on a given
axial plane in an emissions sample probe tube was reduced from 3
holes to 1. An improvement in the mass flow rate distribution was
also found when the number of rows along a column was reduced.
Additionally, for a longer sample probe tube in a large diameter
exhaust pipe (8\mi), sample probe tube diameter also plays an
important role in achieving uniform mass flow rate
distribution.