Microwave Resoant Cavity Transducer for Fluid Flow Sensing*

We are developing a microwave cavity-based transducer for applications in high-temperature fluid flow sensing. This sensor is a hollow metallic cylindrical cavity, which can be fabricated from stainless steel, and expected to be resilient to ionizing radiation, high temperature (above 500C) and corrosive environment of molten salt cooled and liquid sodium cooled nuclear reactors. Currently, limited options exist for flow sensing in such fluids, particularly for salts such as FLiBe (mixture of lithium fluoride and beryllium fluoride), with melting point at ambient pressure above 450C. The principle of sensing consists of making one flat wall of the cylindrical cavity flexible enough so that dynamic pressure, which is proportional to fluid velocity, will cause microscopic membrane deflection. Cavity volume change due to membrane deflection leads to a measurable shift in the resonant frequency. Membrane thickness is determined according to the constraint that maxium stress, which is on the membrane circumference, should remain an order of magnitude below material yield strength. A cylindrical resonator prototype with 0.875in diameter and 0.01in membrane thickness was fabricated from brass for microwave K-band operation. Microwave field was coupled from a waveguide through a subwavelength-size aperture on the side of the cavity. A leak-proof assembly consisting of a piping Tee with a bulkhead WR-42 microwave waveguide was developed for cavity insertion into a fluid stream. Preliminary tests were conducted in a water flow loop at room temperature and ambient pressure. Optimal response to flow sensing was observed when the cavity was excited in TE011 resonant mode.