Fluid Velocity Sensors Made by Thermal Spray

Thermal spray technology was used to make rugged surface sensors to measure the velocity of fluid flowing over a metal plate. The sensor consisted of a spiral-shaped nichrome (NiCr) heater coil, approximately 17 mm in diameter, and a thermocouple, both made by wire arc spray. Plasma spray was used to deposit an electrically insulating layer of alumina, ~ 300 µm thick, on a 316 stainless steel block. A 17-mm-diameter heater coil was deposited on top of the alumina layer by spraying nichrome from a twin wire arc spray system through a 3D printed polymer mask. The resulting NiCr line was ~ 50 µm thick and 104 mm long, with an electrical resistance of approximately 20 Ω. A thermocouple junction was built next to the heater by inserting an insulated constantan wire through a vertical hole drilled in the steel block and spraying steel on the top of the hole to close it and form an electrical connection between the wire and the surrounding substrate. The junction of the wire and the steel formed a thermocouple whose output voltage was calibrated as a function of temperature A flow loop was built to calibrate the sensor by passing either air or ethylene glycol over it at velocities of up to 4.4 m/s. A series of 1- to 6-min-long voltage pulses were applied to the heater, increasing its temperature by approximately 5-10 °C each time, before letting it cool. The surface temperature cooling rate was recorded by the surface thermocouple. A calibration curve was developed of fluid velocities as a function of time constants for cooling of the sensor.