Pressure

Topics: Pressure, Pressure measurement, Atmospheric pressure Pages: 6 (1469 words) Published: December 2, 2010
Module EG1083 Fluid Mechanics

PRESSURE , FLUID SYSTEMS and FLUID PRESSURES

SM(1)

Pressure
Definition of Pressure

Fluid pressure: Force per unit area exerted by a fluid in a solid wall. Force acts perpendicularly to the surface in contacts. Fluid is a co u d s common word for gas a d/o liquid. o od o and/or qu d Pressure is a scalar quantity. It has the units of: N/m2 or Pa (or kPa) in SI system of units psi in Imperial system of units Pressure can also be expressed in terms of height of a column of liquid List of units of pressure measurements & conversion of units Pascal s Pascal’s law

Scalar quantity

Units of Pressure

SM(2)

Pressure
Pressure measurements Absolute pressure Gauge Pressure

...divided into three different categories: 1. Absolute pressure – which is defined as the absolute value o pressure (force-per-unit-area) ac g o of p essu e ( o ce pe u a ea) acting on a surface by a fluid. su ace ud Abs. pressure = pressure at a local point of the surface due to fluid – absolute zero of pressure (see page 63 of lecture notes) 2. Gauge pressure – difference between abs. pressure and atmospheric pressure – is always positive 101.325 kPa or 14.7 psi

Equations

Pressure term relationships

– a –ve gauge pressure is vacuum ve vacuum. Pressure term relationships • Abs pressure = gauge pressure + atm pressure Abs. • Abs. pressure = – gauge pressure + atm pressure (vacuum) > atm < atm SM(3)

Pressure
Pressure measurements

Relation between abs., gauge and vacuum
Absolute pressure Gauge Pressure

gauge

Equations
gauge)

Pressure term relationships

SM(4)

Pressure
Pressure term relationships Hydrostatic pressure

3. Differential pressure – measurement of an unknown pressure minus the reference to a o e u e e e ce o another unknown p essu e o pressure. – it is used to measure differential pressure i.e. pressure drop (ΔP) in a fluid system

SM(5)

Fluid systems and Fluid pressures
Fluid systems

Two types of fluid systems: 1.Static system – in which fluid is at rest

Fluid pressures

Pressure measured i thi system i called static pressure P d in this t is ll d t ti

Static pressure system s stem

‘’The pressure at a given depth in a static liquid is The due to its own weight acting on unit area at that depth plus external pressure acting on the surface o the qu d of t e liquid’’ Gauge pressure = ρgh – which i d hi h is dependent j t only on fl id d d t just l fluid density ( ) it (ρ) and distance between below the surface of the liquid h. External pressure – is generally the atmospheric pressure

SM(6)

Fluid systems and Fluid pressures
Fluid systems

Fluid pressures

Example: A hydraulic pump used to lift a car: when a small force f is applied to a small area a of a movable piston it creates a pressure P = f/a. This pressure is transmitted to and acts on a larger movable piston of area A which is then used to lift a car.

Static pressure p

Lesson: Pressure along the horizontal line always remains the same for uniform singly fluid SM(7)

Fluid systems and Fluid pressures
Fluid systems

Fluid pressures

Example: If the height of the fluid's surface above the bottom of the five fluid s vessels is the same, in which vessel is the pressure of the fluid on the bottom of the vessel the greatest ? The amount of liquid in each vessel is not necessarily the same. y Answer: The pressure P is the same on the bottom of each vessel. Gauge pressure =F Force/Area /A = ρ(hA)g/A = ρgh ‘’For gases: the pressure increase in the fluid due to increase in height is negligible because the density (thus, weight) of the fluid is relatively much smaller compared to the pressure being applied to the system’’. In other words, p = ρgh shows pressure is independent of the fact that the wt. of liquid in each vessel is different. This situation is referred to SM(8) as HYDROSTATIC PARADOX.

Static pressure p

Fluid systems and Fluid pressures
Pressure term...

References: •Transport Phenomena by Bird, Stewart, Lightfoot •Fluid Mechanics and Hydraulic machines by R K Rajput R.K. •http://www.freescale.com/files/sensors/doc/app_note/AN1573.pdf (18 F 10) •http://www.ac.wwu.edu/~vawter/PhysicsNet/Topics/Pressure/Hydro Static.html (18 F 10)
SM(15)

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