Читать книгу Hydraulic Fluid Power - Andrea Vacca - Страница 35

For liquids, the viscosity increases with pressure. While this effect can be negligible for limited pressure variations (<200 bar), it might be relevant for hydraulic systems working at high pressure (>200 bar). A formula that can be used to approximate the variation of fluid viscosity with pressure is given by

(2.21)

where μ₀ is the dynamic viscosity at atmospheric pressure (at a given temperature), p is the fluid pressure (in bar), and b is a coefficient that depends on the fluid. For mineral‐based oils, b = 1.7 · 10⁻³ bar⁻¹; for HFC oils, b = 3.5 · 10⁻⁴ bar⁻¹; and for HFD oils b = 2.2 · 10⁻³ bar⁻¹. Relation (2.21) is plotted in Figure 2.9. As the reader can notice from the figure, for a mineral oil, there is an increase in viscosity of about 40% from atmospheric conditions to 200 bar.

Figure 2.9 Viscosity as a function of pressure.