专业外语 UNIT 1 Raeding B

导读:UNIT1ReadingmaterialbWaterTransmissionWatercanbetransportedfromeitheragroundorsurfacesupplysourcedirectlytoacommunityor,ifwaterqualityconsiderationsindicate,initiallytoawatertreatm

专业外语  UNIT 1  Raeding  B

UNIT 1

Reading material b

Water Transmission

Water can be transported from either a ground or surface supply source directly to a

community or , if water quality considerations indicate , initially to a water treatment facility, by different types of conduits , including:

Pressure conduits : tunnels , aqueducts and pipelines

Gravity-flow conduits : grade tunnels , grade aqueducts and pipelines

The location of the well field or river reservoir defines the length of the conduits,

while the topography indicates whether the conduits are designed to carry the water in open-channel flow or under pressure . the profile of a water supply conduit must generally follow the hydraulic grade line to take advantage of the forces of gravity and thus minimize pumping costs.

Service reservoirs are also necessary in the transmission system to help level out peak demands . In practice , intermediate reservoirs dose to the city , or water towers are sized to meet three design constraints:

hourly fluctuations in water consumption within the rice area

short term shutdown of the supply network for servicin

back-up water requirements to control fires

These distribution reservoirs are most often constructed as open or covered basins , elevated thanks , or , in the past , standpipes . if the service reservoirs are adequately designed to meet these capacity considerations , then the supply conduits leading to them generally

must only be designed to carry approximately 50 % in excess of the average daily demand of the system or subsystem.

Before discussing flow , it might first be useful to review some important fluid properties.

The density of a fluid is its mass per unit volume . in common units , density is expressed as slugs per cubic foot , or as ib . sec aft . density in the metric system is in terms

of grams per cubic centimeter . the density of water under standard conditions is 1 . 94 in common units and unity in the metric system .

Specific weight represents the force exerted by gravity on a unit volume of fluid and

therefore must be in terms of force per unit volume such as pounds per cubic foot . the specific weight is related to density as

W=pg

Wherew=specific weight , lb/ft^3

p=density , 1b . Sec^2/ft^4

g=gravitational constant , ft/sec^2

The specific weight of water is 62 . 4 pounds per cubic foot.

Specific gravity of a liquid is the ratio of its density to that of pure water at a standard

temperature . elo the metric system the density of water is one gram per cubic centimeter and hence the specific gravity has the same numerical value as the density.

The viscosity of a fluid is a measure of its resistance to shear or angular deformationand is defined as the proportionality constant relating the shear stress t to the rate of deformation du/dy . This proportionality constant is usually written as

τ=μ(du/dy)

The assumption inherent in this definition is that the shear stress is directly proportional to the rate of deformation . such a definition holds for many fluids , which are known as Newtonian fluids . fluids for which the shear stress is not proportional to the shear rate are known as non newtonian fluids . an example of a non - newtonian fluid is biological

sludge.The term viscosity , when applied to biological sludge , is therefore significant only if either the rate of deformation or the shear stress is also specified.

In the metric system, a unit of viscosity is a poise , with units of grams per centimeter - second . Most fluids have low viscosity and a more convenient unit is the centipoise or 0.01 poise . The viscosity of water at 20c ( 68 . 4f ) is i centipoise . in the English system

the unit of viscosity is pound seconds per square foot . One lb . Sec/ft^2 equals 479 poise. Kinematic viscosity is defined as the absolute viscosity , divided by the density of the fluid, or ν=μ/p. The dimensions of kinematic viscosity are square centimeters per second.

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