Device for Measuring Low Flow Speed in a Duct
- Wednesday, 27 May 2009
Speed can be determined to within ±0.2 cm/s.
A multiple-throat venturi system has been invented for measuring laminar flow of air or other gas at low speed (1 to 30 cm/s) in a duct while preserving the laminar nature of the flow and keeping the velocity profile across the duct as nearly flat as possible. While means for measuring flows at higher speeds are well established, heretofore, there have been no reliable means for making consistent, accurate measurements in this speed range. In the original application for which this system was invented, the duct leads into the test section of a low-speed wind tunnel wherein uniform, low-speed, laminar flow is required for scientific experiments. The system could also be used to monitor a slow flow of gas in an industrial process like chemical vapor deposition.
In the original application, the multiple-throat venturi system is mounted at the inlet end of the duct having a rectangular cross section of 19 by 14 cm, just upstream of an assembly of inlet screens and flow straighteners that help to suppress undesired flow fluctuations (see Figure 1). The basic venturi measurement principle is well established: One measures the difference in pressure between (1) a point just outside the inlet, where the pressure is highest and the kinetic energy lowest; and (2) the narrowest part (the throat) of the venturi passage, where the kinetic energy is highest and the pressure is lowest. Then by use of Bernoulli’s equation for the relationship between pressure and kinetic energy, the volumetric flow speed in the duct can be calculated from the pressure difference and the inlet and throat widths.
The design of this system represents a compromise among length, pressure recovery, uniformity of flow, and complexity of assembly. Traditionally, venturis are used to measure faster flows in narrower cross sections, with longer upstream and downstream passages to maintain accuracy. The dimensions of the passages of the present venturi system are sized to provide a readily measurable pressure drop. Multiple throats are used to minimize the length needed to recover internal energy and enable the velocity profile to recover to near flatness. The venturi passages are defined by airfoil surfaces, two-dimensional configuration of which is dictated by the need to match the rectangular duct cross section.
The flow into and out of the venturi passages is guided by the airfoil surfaces. There are two half airfoils at the top and bottom of the inlet, and there are five full airfoils between them. A plenum downstream of the trailing edges allows the flow to even out prior to entering the screens and flow straighteners. To enable measurement of pressure in all six throats, tubes in three of the airfoils are connected to a manifold, and narrow holes connecting the tubes with the throats are drilled in these airfoils. The pressures sensed at the six throat measurement locations become averaged together in the manifold, which is connected to one side of a sensitive differential-pressure transducer. The other side of the transducer is exposed to the pressure just upstream of the inlet. It has been found that the speed-vs.-pressure calibration curve is highly repeatable, enabling measurement of flow speed to within an error of ±0.2 cm/s.
This work was done by Frank Quinn and Kevin Magee of ZIN Technologies, Inc. for Glenn Research Center.
Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Innovative Partnerships Office, Attn: Steve Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-18021-1.
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Device for Measuring Low Flow Speed in a Duct (reference LEW-18021-1) is currently available for download from the TSP library.
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