Still, that Sensocon device has the RTD surrounded by a horseshoe-shaped heating element. I would guess that high-speed particles would eventually clog the gap and cause errors.
Have you looked at magnetic sensors and magnetic meters (magmeters)? Their sensor elements are non-clogging, and go up to at least a 12" pipe diameter. However, you would have to find out if they are capable of measuring air velocity in the cornmeal airstream. Magmeters are normally used for conductive liquids, so may not be suitable. However they are also used for slurries and sludges. Whether a magmeter would measure airflow will depend on the conductivity of the airstream.
Is this device permanently mounted, or hand-held? If not permanent, then you could simply wipe it off when it gets clogged. If permanent, and under PLC control, you might build some type of cleaning device to periodically blow, wash, or wipe off the cornmeal. If this is a large duct, you could mount a funnel drain right under a thermal-type flow sensor, and a spray nozzle above it, and periodically hit it with a short blast of water to clear the clogs.