8.8 Venturi tube
A Venturi tube is a device used to measure the flow rate of a fluid in a pipe. The device exploits the Venturi effect4, i.e. that pressure reduces when the fluid flows through a restriction.
The volumetric flow rate is calculated according to
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(8.2) |
For , is calculated accurately using values between 0.97 and 1,5 but, for , suitable values of decrease significantly below 1 in order to account for pressure losses due to viscous forces.
A simulation was performed to calculate for a Venturi tube, shown in the figure, with and an inlet . The inlet velocity was specified using the quadratic profile in Sec. 8.7 with a mean cross-sectional speed , corresponding to a maximum speed .
The simulation used the steady-state algorithm in Sec. 5.12, with an incompressible fluid with uniform . The mesh contained 57,600 cells, with a near-wall cell height of 1.5mm, which resolved the velocity profiles as shown below. The flow recirculates near the wall downstream of the Venturi throat, causing inflow at the outlet boundary. Consequently, the total pressure and inlet-outlet-velocity boundary conditions, described in Sec. 4.7 and Sec. 4.15 respectively, were applied at the outlet to maintain stability.
The flow was laminar so no turbulence modelling was required. The solution converged to within an absolute tolerance , see Sec. 5.4 , in 292 iterations.
The pressure drop was between the centres of the inlet and throat sections, with a corresponding .