| Popis: |
At the moment, very little knowledge is available about the flow of shear-thinning media in safety valves. In this paper, the flow of aqueous solutions of polyvinylpyrrolidone with a zero-shear viscosity between 0.3 and 3 Pa s in a LESER Type 441 DN 25/40 safety valve with a lift of 2.2 mm for relieving pressures up to 6.5 bar is computed using ANSYS FLUENT 12. According to these calculations, the lowest values of the dynamic viscosity are reached in the slit between the seat and the disk due to the large local velocity gradients. A more moderate increment in the fluidity of the solutions is observed also in the recirculation zone outside the disk when the stream from the rear of the valve meets the one from the front. A preliminary study suggests that the experimental flows may be turbulent in the slit and then evolve into a laminar profile in the discharge pipe. For this reason, the calculations are carried out assuming laminar, turbulent and transitional flows. The predicted mass flow rates are close to each other and to the measured values, except at relieving pressures close to ambient. The major cause of deviation seems to be the entrainment of air microbubbles, whose elongation may be responsible for additional shearing in the solutions with large polymer weight. |
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