An investigation into the suitability of several types of velocity probes in current use for measurement of nonuniform flow in large ducts is described. An experimental duct was fabricated with a test section near the discharge of a centrifugal fan. Different flow rates were obtained by using an adjustable throttle plate at the duct outlet. The flow rate in the duct was also measured at a downstream section located nine diameters from the fan. A flow straightener was located between the fan and this measuring station; the flow rate measured at this section was assumed to be the true flow rate in the duct. Six different velocity probes of five different designs were used to measure the flow at the fan discharge. The probes were 1) Pitot-static tube, 2) Kiel (Pitot-venturi) tube, 3) Forward-reverse tube (2 tubes), 4) Fechheimer (three hole yaw probe), and 5) turbine meter probe. Of these, only the Fechheimer is capable of measuring the direction of the flow. The best results were produced by the Fechheimer probe. This is felt to be due to this probe’s ability to measure direction as well as speed. It is recommended that only probes which are capable of sensing flow direction be used in highly nonuniform flows.

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