Fluid Mechanics 3

Air

mixture of a number of gasses

Dry-bulb temperature

simply the air temperature

Wet-bulb temperature

measured with a thermometer whose bulb is covered with a thin, wet cotton sleeve.

Charles’ Law

states that the same rise of temperature produces in all gasses the same increase in volume, provided the pressure is kept constant

Boyle’s Law

states that the volume occupied by a gas is inversely proportional to the absolute pressure exerted upon it, provided the temperature is kept constant

Universal Gas Law

Charles’ law and Boyle’s Law can be combined mathematically to produce what is known as the

Dew Point Temperature

If air containing a certain amount of water vapor is cooled, a temperature will be reached where the air is unable to contain all of the water vapor.

Fogging

Condensation of water vapor can cause_____ and can sometimes be a problem in mine airways where warm, humid air is cooled below its dew point

Pressure

the force applied per unit area and in ventilating terms is usually expressed in Pascals.

Barometric Pressure

simply a description of the instrument used to measure the pressure of the atmosphere in which it is situated.

Static Pressure

the potential or stored energy of the air often called the “bursting” pressure and this term does in fact make it easier to visualize, because it is the pressure exerted by the air on the walls of the containing vessel.

Velocity Pressure

Moving air possess “kinetic energy” which is the energy associated with motion.

Total Pressure

the algebraic sum of the static pressure (potential energy) and the velocity pressure (kinetic energy) and is measured with a facing tube, parallel to the direction of flow.

Natural Ventilating Pressure

formed when there is a difference in air temperature and density between a vertical column of air inside a mine and the corresponding column of air outside the mine

frictional pressure losses

The losses in energy due to the roughness of the wall are termed

Shock pressure losses

The losses in energy due to a change in direction of the airflow are termed

Shear Stress

This force divided by the area of the layer

Laminar Flow

At low fluid velocities the streamlines of flow are almost parallel to each other. The shear resistance between these streamlines is caused by friction between the layers moving at different velocities.

Frictionless or ideal

If there is no resistance to movement, the fluid is called

Real

If the shear movement is resisted then the fluid is called

Turbulent Flow

As the velocity is increased, the streamlines become randomly arranged and the flow becomes turbulent with the additional eddy currents adding to the shear resistance.

Ventilation Equation

P=RQ²

1850

In around ____ Atkinson hypothesized that the value of R was dependent upon on certain characteristics of the airway or duct.

Atkinson’s k Factor.

In mine ventilation the term λ is usually replaced by ‘k’ known as the coefficient of friction or

Entrance

Shock losses occur at the entrance to any system due to the acceleration of air from zero velocity in the surrounding atmosphere to the velocity in the airway.

Outlet Losses

Air exiting a system decelerates from the airway velocity to zero velocity in the atmosphere.

Elbows

bends and miters are used to guide the change in direction of flow and restrict flow separation.

Junctions and Splitting

In the deflected branch of a split the pressure loss can be reduced considerably by using a converging taper between the main and the deflected branch.

Series Circuits

When one airway is followed by another airway and both have their own physical characteristics, the airways are said to be in

describe a combination of airways with different resistances where the air flows through each in turn.

Parallel Airway Circuits

Airways are in parallel when they have a common inlet and common outlet.

Kirchoff’s 1st Law

--The algebraic sum of all air quantities at a junction is zero

KIrchoff’s 2nd Law

The algebraic sum of all pressure losses in a closed mesh is zero. However if a fan is included in the mesh then the algebraic sum of all pressure losses is equal to the pressure provided by the fan.