VENTILASI TAMBANG

14 Oktober2014 Hendra Harisman, S.T., M.Eng.Sc.

JTP 505 2 SKS

FRICTION, DISCONTINUITY & OBSTRUCTIONCalculation of airway resistance and frictional losses

based on:

Shape and length of the airway or duct

Roughness of the surface

Flow regime (turbulent or laminar)

Quantity of air, gas or fluid in the airway or duct.

FRICTION, DISCONTINUITY & OBSTRUCTIONFriction loss:

pressure loss due to flow over rough surface

Discontinuity loss:

pressure loss when air flows through and around

discontinuities (e.g. bends).

Obstruction loss: Pressure loss due to objects

FRICTION, DISCONTINUITY & OBSTRUCTIONViscosityWhen a fluid begin to move, individual molecules or layers start to move over each other in an approximately linear fashion with retardation dependent on internal cohesion between molecules.

As the velocity of the fluid increases, molecules or layers move in increasingly random fashion as the magnitude of inertial forces increase and the flow become turbulent.

FRICTION, DISCONTINUITY & OBSTRUCTION

FRICTION, DISCONTINUITY & OBSTRUCTIONThe degree of turbulence in a moving fluid depends on: viscosity velocity

Less viscous fluids moving faster and more turbulent than more viscous fluids moving slower.

Turbulence is associated with energy or pressure loss.

FRICTION, DISCONTINUITY & OBSTRUCTION

Turbulence is associated with energy or pressure loss.

In laminar conditions pressure loss is proportional to velocity

In turbulent conditions pressure loss is proportional to velocity squared

FRICTION, DISCONTINUITY & OBSTRUCTION The limits of laminar and turbulent flow can be

determined from the dimensionless Reynolds number

Reynolds number:

= density(kg/m3)

L = characteristic dimension e.g. diameter(m)

V = Velocity(m/s)

= viscosity (kg/ms)

FRICTION, DISCONTINUITY & OBSTRUCTION The limits of laminar and turbulent flow can be

determined from the dimensionless Reynolds number

Generally, the upper limit of laminar flow is between

Reynolds number of 2,000 and 3,000.

in other words:

Turbulent flow starts between Re 2,000 to 3,000

FRICTION, DISCONTINUITY & OBSTRUCTIONReynolds number:

L = characteristic dimension e.g. diameter(m) In circular ducts or airways:

L = diameter In non-circular ducts or airways L = hydraulic diameter

A = AreaC = Circumference

FRICTION, DISCONTINUITY & OBSTRUCTIONContoh soal:Hitung kecepatan (velocity) dan quantity udara yang dapat menyebabkan turbulent flow (Re = 2500) di sebuahterowongan development yang lebarnya 5 m dantingginya 3 m.Assumsi:density udara = 1.16 kg/m3Viskositas udara = 18 x 10-6 kg/ms

FRICTION, DISCONTINUITY & OBSTRUCTIONFriction Loss on Rough Surface As gas or liquids flow over rough surfaces, there is loss of

pressure due to the interaction of the boundary layer with the surface.

This loss results in a reduction of static pressure, and in airways of constant cross sectional area, an equal reduction of total pressure.

In most applications, flow is assumed to be turbulent The frictional pressure loss is proportional to the square of

velocity.

FRICTION, DISCONTINUITY & OBSTRUCTIONFriction Loss on Rough Surface Frictional pressure loss represents a loss of energy to the

system- it results in a loss of static pressure which is not reversible- in turbulent regimes the loss is proportional to the square of velocity

FRICTION, DISCONTINUITY & OBSTRUCTIONFriction Loss on Rough Surface Atkinsons equation is the mathematical representation

of the relationship between frictional pressure loss occurring on rough surfaces and velocity.Atkinsons equation in terms of Velocity

Atkinsons equation in terms of QuantityDensity is incorporated asa ratio of actual fluiddensity to standarddensity of 1.2 kg/m3

FRICTION, DISCONTINUITY & OBSTRUCTIONFriction Loss on Rough SurfaceAtkinsons equation

k = roughness factor of the surface (Ns2/m4)C = Circumference/perimeter (m)L = Length(m)A = Area (m2) = density of the fluid/gas(kg/m3)V = Velocity (m/s)Q = Quantity (m3/s)

FRICTION, DISCONTINUITY & OBSTRUCTIONFriction Loss on Rough SurfaceAirway resistance (R)

K factors can be obtained from text books but should be checked by survey

K are range from 0.003 for smooth ducts to 0.015 for rough walls

FRICTION, DISCONTINUITY & OBSTRUCTIONContoh soal:Friction factor (k) suatu terowongan dengan tinggi 3 m danlebar 5 m adalah 0.012 Ns2/m4. Apabila densitas udara di terowongan tersebut adalah 1.17 kg/m3, berapakahresistance (R) dari terowongan tersebut per 100 m panjangnya?

THE END

INERTIA a property of matter by which it continues in its existing state of rest or uniform motion in a

straight line, unless that state is changed by an external force.

Inertia is a word we use when we talk about matter and movement. Basically, our idea of inertia goes back to Sir Isaac Newton's first two laws of physics:

1. An object at rest tends to stay at rest.

2. An object in motion tends to stay in motion.

Inertia is the quality in matter (matter is anything you can touch) that lets it stay still if it is still, or keeps it moving if it is moving.

If you want to overcome inertia, you have to apply a force. A force will make something that is still start to move, like flicking a wad of paper with a pencil will make it move. Also force, due to resistance, will slow or stop something that is already moving. The wad of paper will be slowed by resistance made by rubbing up against the air it is passing through.

Slide Number 1Friction, discontinuity & obstructionFriction, discontinuity & obstructionFriction, discontinuity & obstructionFriction, discontinuity & obstructionFriction, discontinuity & obstructionFriction, discontinuity & obstructionFriction, discontinuity & obstructionFriction, discontinuity & obstructionFriction, discontinuity & obstructionFriction, discontinuity & obstructionFriction, discontinuity & obstructionFriction, discontinuity & obstructionFriction, discontinuity & obstructionFriction, discontinuity & obstructionFriction, discontinuity & obstructionFriction, discontinuity & obstructionThe EndInertiaSlide Number 20