Fundamentals of hydraulics system engineering

Fundamentals of hydraulics system engineering

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Why hydraulics is so important in the industry?

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What is an hydraulic fluid?

The hydraulic fluid represents one of the most important elements in the hydraulic system, with several tasks: ? Power/Energy transfer ? Generation and transmission of pilot signals for hydraulics ? Lubrication ? Heat transfer ? ...others (corrosion protection, particles carrying ...)

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How to define a fluid?

A lot of terms are usually associated with the description of a fluid:

Density

Compressibility

Viscosity

Thermal conductivity

Specific heat

Stability

Toxicity

Lubricity...

Electrical properties

Properties

Saturation pressure Surface tension

Vapor pressure Thermal expansion...

Flash and boiling points

Parameters

Aeration

Cavitation

Foaming...

However, only a few of them are used for dynamic hydraulic modelling.

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Phenomena

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Fluid properties

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Fluid density

The density r of a fluid is defined as the mass per unit volume:

rM

V

kg/m?

where, using SI units: M is the mass of the fluid [kg] V is the volume of the fluid [m?]

The density is a function of:

? pressure ? temperature ? type of fluid

r f P,T , fluid

Density includes mass, hence it is responsible for inertia effects

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Fluid compressibility

? Every substance is, to a certain extent, compressible!

? Similar to the the linear expansion caused by the pulling effort dF on a rod ...

V

... the volume reduction V is linked to the pressure increase dP

P pressure V volume

L 1 F

L EA

with E: Young's Modulus of the material

Note: this is a deformation due to traction. With a compression force, a minus sign appears.

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P + P V + V

(P,T )

V

p

Fluid

Bulk

Modulus

V

C fluid

V

( p,T ) Fluid compressibility

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Fluid viscosity

In everyday terms, viscosity is the inner molecular friction between two adjacent fluid elements.

According to Newton's law:

F A dv

dy

A: contact area between the two layers ?: absolute viscosity v: fluid velocity

? Viscosity results from an exchange of quantity of movement by molecular diffusion between 2 layers of fluid with different velocities.

? In this sense, the viscosity is a fluid property and not a flow property.

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