Have you considered a sensitive thermistor placed next
to a 1/4 watt resistor. The reistor would be placed outside the air flow path but close enough to the thermistor to
increase its sensitivity. As air flowed past the thermistor,
its resistance would change. The output from a bridge could
be...
If you can mount a strong ring magnet, at the end od the
pipe, you could use a Hall effect sensor to detect the
pipe end. I am not sure how the ring magnet would be affected by the high temperatures, though.