Fluid Mechanics in Multi-Bypass Air Flow Sensor with Wide Dynamic Range

This paper describes an advanced bypass flow passage configuration for a thermal air flow sensor which increases the dynamic flow-rate range, and describes its basic fluid mechanic relations in terms of Bernoulli' s principle.

This configuration consists of a main flow path and two bypasses located inside the main flow path. Thermal and temperature compensation probes are installed only in the primary bypass. The secondary bypass connects with the primary bypass downstream of the probes. The flow velocity, and consequently the pressure, in the primary bypass is affected by this conflux. The conflux loss coefficient increases as the flow rate in the secondary bypass increases. As a result, the flow velocity in the primary bypass decreases as the total air flow rate increases.

In tests, the dynamic range is shown to be increased by about 30% for flow velocities in the primary bypass of between 0.5 m/s and 50 m/s.