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Practical Fluid Mechanics for Civil Engineers

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1

U nits and C onversio ns

1.1 Weight and Mass

This book uses US standard units. Most of the units and conversions discussed in this chapter relate to US units.

Weight is given in lbs.

1 ton = 2,000 lbs

To find mass is US units you have to work back from:

Weight = mass X gravity

W=mg

So m = W/g = lb/(ft/s2) = lb-s2/ft

Mass in US units is a called a slug.

So we say:

1 slug = 1 lb-s2/ft

1 lb = 4.448 N

1 N = 0.225 lb

1 N = 105 dynes

1 kg = 0.0685 slugs

1.2 Volume

1 ft3 = 7.48 gallons

1 gallon = 4 quarts

1 quart = 2 pints

1 pint = 16 ounces

1 acre-ft = 43,560 ft2 x 1 ft = 43,560 ft3

1 cubic yard = 27 ft3

1 gallon = 3.785 L

1 ounce = 29.57 mL

1 pint = 473.18 mL

Scott Lowe

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1.3 Pressure

1 lb/in2 (psi) = 144 lb/ft2 (psf)

1 N/m2 = 1 pascal (Pa)

1 atm = 14.7 psi at sea level

1 bar = 1000 Pa

1 mb (millibar) =1 Pa

1 atm = 1.01325 bar = 1013.25 mb at sea level

1.4 Power and Energy

Power is given by horsepower. This term was invented by James Watt to help sell his steam engines. The SI unit for power is named after Watt.

1 hp = the ability to move 33,000 lb by 1 ft in 1 minute

1 hp = 33,000 ft-lb/min

1 hp = 33,000 ft-lb/min divided by 60 s/min = 550 ft-lb/s

Energy = ft-lb

In SI units energy = Nm

1 Nm = 1 Joule (J)

Power is the rate of energy/time.

1 J/s = 1 Nm/s = 1 Watt (W)

Heat is a form of energy.

1 calorie = heat required to raise 1 gram of water by 1 oC

1 cal = 4.184 J

1 Btu (British thermal unit) = heat required to raise 1 lb of water by 1 oF

1 Btu = 252 cal = 1,054.35 J

1 Btu = 777.65 ft-lb

1.5 Length, Area and Velocity

1 mile = 5,280 ft

1 mile = 1609 m = 1.6 km

1 ft = 0.3048 m

1 inch = 25.4 mm

Practical Fluid Mechanics for Civil Engineers

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1 mile2 = 640 acres

1 acre = 43,560 ft2

1 hectare (ha) = 10,000 m2 = 100 m x 100 m

1 acre = 2.47 ha

1 ft2 = 144 in2

1 mph = 1.47 ft/s

1 football field = 360 ft long

1 football field area = 360 ft x 160 ft = 57,600 ft2 = 1.32 acres

1.6 Flow

1 ft3/s (cfs) = 448.8 gpm

1 million gallons per day (MGD) = 1.547 cfs

1.7 The Water Symbol

The symbol of the surface of water is an inverted triangle. This is the ancient alchemist symbol for water.

Fig. 1-1. The symbol for the surface of water.

Scott Lowe

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Practical Fluid Mechanics for Civil Engineers

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2 Fluid Properties

2.1 Property Tables for Water and Air

Table 2-1 below gives the physical properties of water at sea level. The

temperature spans the range water is in liquid form, 32oF to 212oF. During

this chapter we will discuss the properties listed in the table.

The actual value for viscosity at 32oF is read as 3.746 x 10-6 lb-s/ft2. A

similar convention is used in the other tables for properties that are either very

large or very small.

Note that water is at its maximum density around 40oF.

Note also that the unit of pressure for the vapor pressure is psia, not psi.

This is also discussed later.

Table 2-1. Properties of Water at Sea Level

Temperature

Specific

Weight

Density Viscosity Kinematic

Viscosity

Surface

Tension

Modulus

of

Elasticity

Sat. Vapor

Pressure

T EV PV

oF lb/ft3 slug/ft3 x 10-6

lb-s/ft2

x 10-6

ft2/s

lb/ft x 103

psi

psia

32 62.42 1.94 3.746 1.931 0.00518 287 0.09

40 62.43 1.94 3.229 1.664 0.00614 296 0.12

50 62.41 1.94 2.735 1.41 0.00509 305 0.18

60 62.37 1.938 2.359 1.217 0.00504 313 0.26

70 62.3 1.936 2.05 1.059 0.00498 319 0.36

80 62.22 1.934 1.799 0.93 0.00492 324 0.51

90 62.11 1.931 1.595 0.826 0.00486 328 0.7

100 62 1.927 1.424 0.739 0.0048 331 0.95

110 61.86 1.923 1.284 0.667 0.00473 332 1.27

120 61.71 1.918 1.168 0.609 0.00467 332 1.69

130 61.55 1.913 1.069 0.558 0.0046 331 2.22

140 61.38 1.908 0.981 0.514 0.00454 330 2.89

150 61.2 1.902 0.905 0.476 0.00447 328 3.72

160 61 1.896 0.838 0.442 0.00441 326 4.74

170 60.8 1.89 0.78 0.413 0.00434 322 5.99

180 60.58 1.883 0.726 0.385 0.00427 318 7.51

190 60.36 1.876 0.678 0.362 0.0042 313 9.34

200 60.12 1.868 0.637 0.341 0.00413 308 11.52

212 59.83 1.86 0.593 0.319 0.00404 300 14.7

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Table 2-2 gives the physical properties of air at sea level for a large range

of temperatures. At 50oF the density of water is 62.41 lb/ft3. Air is ~0.08

lb/ft3. So water is 62.41/0.08 = 780 times more dense than air, roughly 800.

The viscosity of water at 50oF is 2.7 x 10-6 lb-s/ft2. Air, being a gas has a much

lower viscosity of 0.37 x 10-6 lb-s/ft2 – roughly 1/10th.

Table 2-2. Properties of Air at Sea Level

Temperature Specific

Weight

Density Viscosity

T

oF lb/ft3 slug/ft3 x 10-6

lb-s/ft2

-40 0.09460 0.002940 0.312

-20 0.09030 0.002807 0.325

0 0.08637 0.002684 0.338

10 0.08453 0.002627 0.345

20 0.08277 0.002572 0.350

30 0.08108 0.002520 0.358

40 0.07945 0.002470 0.362

50 0.07790 0.002421 0.368

60 0.07640 0.002374 0.374

70 0.07495 0.002330 0.382

80 0.07357 0.002286 0.385

90 0.07223 0.002245 0.390

100 0.07094 0.002205 0.396

120 0.06849 0.002129 0.407

140 0.06620 0.002058 0.414

160 0.06407 0.001991 0.422

180 0.06206 0.001929 0.434

200 0.06018 0.001871 0.449

250 0.05594 0.001739 0.487

Table 2-3 below gives the properties for air at various altitudes. This is known

as a standard atmosphere table – sort of a global average. For example, at sea

level the average temperature is only 59oF at certain places. Not in the tropics

or artic regions, for example.

In any engineering calculations you have ever done have you used

anything but 32.2 ft/s2 for gravity? It doesn’t change, right? Have a look at the

last column of Table 3. It does change – with altitude. This should make sense,

because if you keep going higher eventually you reach space and gravity goes

to zero. The atmosphere doesn’t abruptly end and space begin, it just

gradually gets thinner and thinner.

This table goes to 100,000 ft in altitude – far higher than any of us are

ever likely to get. Commercial planes fly just under 40,000 ft at their limit. But

Practical Fluid Mechanics for Civil Engineers

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realize that 100,000 ft is only about 19 miles – the straight line distance from

Manhattan College to JFK airport.

The distance to where the atmosphere ends and space begins is usually

taken as 62 miles. It takes a rocket about 3-10 minutes to reach space,

depending on the rocket. The space shuttle took about 2 ½ minutes.

Table 2-3. Standard Atmosphere

Elevation Temperat

ure

Absolute

Pressure

Specific

Weight

Density Viscosity Speed of

sound

Gravity

T P c g

ft oF psia lb/ft3 slug/ft3 x 10-6

lb-s/ft2

ft/s ft/s2

0 59.0 14.7 0.076472 0.002377 0.37372 1116 32.174

5,000 41.2 12.2 0.065864 0.002048 0.36366 1097 32.158

10,000 23.3 10.1 0.056424 0.001755 0.35343 1077 32.142

15,000 5.5 8.3 0.048068 0.001496 0.34302 1057 32.129

20,000 -12.2 6.8 0.040694 0.001267 0.33244 1037 32.113

25,000 -30.0 5.5 0.034224 0.001066 0.32166 1016 32.097

30,000 -47.8 4.4 0.028573 0.000891 0.31069 995 32.081

35,000 -65.6 3.5 0.023672 0.000738 0.29952 973 32.068

40,000 -69.7 2.7 0.018823 0.000587 0.29691 968 32.052

45,000 -69.7 2.1 0.014809 0.000462 0.29691 968 32.036

50,000 -69.7 1.7 0.011652 0.000363 0.29691 968 32.020

60,000 -69.7 1.0 0.007218 0.000225 0.29691 968 31.991

70,000 -67.4 0.7 0.004448 0.000139 0.29836 971 31.958

80,000 -62.0 0.4 0.002737 0.000085 0.30182 978 31.930

90,000 -56.5 0.3 0.001695 0.000053 0.30525 984 31.897

100,000 -51.1 0.2 0.001057 0.000033 0.30865 991 31.868

2.2 States of Matter

A material may exist as a solid, a liquid, or a gas.

Liquids and gases are both considered fluids. Air is a fluid with a density

1/800th that of water, as noted previously. When you look at the motion of

the atmosphere in weather reports you can clearly see it behaves as a fluid.

Typically civil engineers work mainly with liquids. One liquid in

particular – water. Mechanical and chemical engineers work with both liquids

and gases. For example, a lot of mechanical engineering work is HVAC

(Heating, Ventilation & Air Conditioning) systems – this involves a gas – air.

For this reason it is not typical for civil engineers and mechanical engineers to

share a common fluids course.

Chemical and mechanical engineers do not often deal with open channel

flow, whereas this is a big topic in civil engineering. This is another reason

common fluids courses are not common.

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Liquid and Solid

A basic fluid properties question is – what is the difference between a solid and a liquid? One simple answer is the strength of the bonds between the molecules. A solid has stronger bonds than a liquid. This allows a solid to retain its shape. A liquid does not retain its shape and so takes the shape of whatever is containing it.

So a substance capable of retaining its shape is a solid.

For example water in a glass takes the shape of the glass. If you pour the water onto the floor it takes a random pattern related to surface tension and the friction bond with the floor material.

A less intuitive question is which form of a substance is the most dense? The obvious answer is probably a solid. However the actual answer is the liquid form. Consider water – is ice more dense than liquid water? No. It floats on the top. In fact it is about 8.5% less dense.

The reason for this has to do with the arrangement of the molecules. In a solid the molecules are arranged in a well-defined matrix. This also means there is a lot of space between the molecules. By comparison in a liquid the molecules tend to slide over each other, so have less space between them. This of course results in more density.

This also means that liquids are hard to compress.

Liquid and Gas

A gas has much weaker bonds than a liquid. Gases also expand indefinitely to fill a container. For example if I bring a small canister of helium into a classroom the helium will have expanded to fill the canister. If I open the canister the helium will expand to fill the classroom. If I open the door the helium will expand into the rest of the building, and so on.

Gases, in comparison with liquids, are also easily compressible. So in general when working with gas calculations this must be kept in mind.

2.3 Density

Density is the most basic fluid property. In fluids it is given by the symbol Density is mass per unit volume:

Where: m is mass in kg

Practical Fluid Mechanics for Civil Engineers

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V is volume in m3

So has units of kg/m3.

This is obviously the SI version. In US units this definition doesn’t work as you will end up with slugs/ft3. This is not typically used by civil engineers.

The density of water in SI units is around 1000 kg/m3. The density of the air at sea level is around 1.2 kg/m3. The density of water in US units is around 1.94 slugs/ft3.

2.4 Specific Weight (Density in the US)

Specific weight is defined as weight per unit volume and uses the symbol . It is defined as:

In SI units it is N/m3. In US units it is lb/ft3. In the US this is how density of fluids are commonly referred to. So if you ask a US civil engineer the density of water they will tell you 62.4 lb/ft3, the most common number for water.

Specific Gravity, s

The specific gravity of a liquid is simply the ratio of its density to water:

Use W = 62.4 lb/ft3.

So if s < 1, then the liquid is lighter than water, and s>1 is a liquid denser than water. For example, oils will always have s<1 – oil floats on water. Mercury has an s~13.55 – much heavier than water – the only metal which is a liquid at room temperature. 2.5 Density of Water The density of water varies slightly with temperature. We typically use 62.4 lb/ft3 in calculations. This is water near its most dense, around 32oF: Scott Lowe 20 Liquids expand with temperature and become slightly less dense. So at 100oF: Water continues to become less dense right up to its boiling point at 212oF. Water is at its most dense at 40oF. This is because at 32oF water is already starting to expand as part of the transformation to becoming ice, which we know is significantly less dense than liquid water. So water actually contracts a little as you warm it above 32oF, reaching its maximum at 40oF. In deep reservoirs in cold regions we know the temperature at the bottom of the reservoir will be 40oF. As you descend into the ocean the water gets colder and if you got deep enough it will also be 40oF. The density of water only varies by 4% throughout its liquid temperature range. This does however have some important consequences in some larger systems, like lakes and reservoirs. The density variations cause lakes to “turn over” annually as surface water gets heated in summer then cools in the fall. When the surface water cools and becomes more dense than the underlying layer it plunges to the bottom and displaces the deeper water in a phenomenon known as “fall turnover”. Water is a very dense liquid. Most liquids on the planet are less dense than water. The densest liquid at room temperature is mercury. It is about 13.5 times denser than water and it the only metal which is a liquid at room temperature. For comparison concrete has a density of about 145 lb/ft3 so is only just over twice as dense. So if during a flood you have a wall of water moving at 30 mph it will destroy pretty much everything that gets in its way. 2.6 Compressibility Compressibility is the change in volume per change in pressure. For any material this is defined using the modulus of elasticity, EV. This is also called the bulk modulus. Assuming that the pressure on a fluid is increased, EV is defined as: more specifically: Practical Fluid Mechanics for Civil Engineers 21 Where: P1 = initial pressure (psi) P2 = final pressure (psi) V1 = initial volume (in3) V2 = final volume (in3) Note the volume units do not really matter as they cancel out. The volume is expressed as a fractional reduction in volume per psi of pressure. As such the units of EV are pressure, psi. As the volume will decrease under increasing pressure, V2 is less than V1, so the denominator is always negative. In engineering, constants are listed as positive, hence the negative sign in the numerator. Water is barely compressible and has an EV of 320,000 psi. Steel, the wonder material of the 20th century for engineering, has a remarkable EV of 26,000,000 psi. Hence why adding a small amount of steel to a reinforced concrete column adds a lot of compressive capacity. Example For a large increase in pressure, 1000 psi, how much will water compress? Is 1000 psi a large pressure? Yes. If you consider that the pressure in typical water pipes is between 40 – 80 psi, then 1000 psi can be considered a large pressure. Let EV = 320,000 psi, P1 = 0 psi and V1 = 1 ft3. So we have: solving for V2: Scott Lowe 22 So the water was compressed by 0.003 ft3 or 0.3%. This is a negligible value for most civil engineering fluid calculations and so we typically assume that water is incompressible. As I mentioned above, pressures in a water distribution system are in the 40-80 psi range so compressibility is on the order of 0.01% for changes of this magnitude. 2.7 Viscosity Viscosity is a measure of a fluids internal friction, i.e. how “sticky” it is. As a fluid moves its molecules slide over each other, creating friction. As the fluid moves over solid surfaces it also experiences friction. Viscosity is a measure of this friction. Friction is also defined as shear stress as layers of a fluid slide over each other. This creates a velocity profile as shown below. At the solid surface the fluid velocity is zero, as the surface is not moving. As you move away from the solid surface the velocity increases. u y solid fluid Velocity profile of a real fluid over a solid surface Fig. 2-1. Velocity profile of a fluid over a solid surface. Practical Fluid Mechanics for Civil Engineers 23 Experiments showed that the amount of shear stress, , in a fluid was proportional to the velocity gradient, i.e.: Having the relationship written this way is not useful for calculations, so a constant was introduced to make an equality: The constant, , is called the Coefficient of Viscosity, and is generally what engineers think of when they think of viscosity. This trick of introducing constants to make mathematically convenient relationships is standard practice in science and engineering. One consequence of this is having to deal with the units. The units must balance in the equation. So let’s find the units for . In SI units first. Stress is force per unit area, so has units of N/m2. So the above equation in terms of units is: or solving for One N/m2 is also called a Pascal or Pa, so SI units for are often expressed as Pa-s/m2. Scott Lowe 24 The US equivalent of N-s/m2 is lb-s/ft2. Note that these are nonphysical units that occur as a mathematical artifact. One cannot physically describe what a lb-s/ft2 is, unlike say velocity that has unit of ft/s – you move so many feet in one second. This is also a common occurrence with constants The larger is the more viscous the fluid. So syrup > oil > water >

gasoline > alcohol. Liquids have much higher viscosities than gases. Water has

a ~ 0.001 N-s/m2 while oil ~ 0.1 N-s/m2, 100 times greater. Air is ~

0.00002 N-s/m2.

The viscosity of some common liquids is given in Table 2-4 below.

Table 2-4. Viscosity of Some Common Liquids

Liquid Specific

gravity

Viscosity

s

x 10-6

lb-s/ft2

Benzene 0.88 14.4

Carbon tetrachloride 1.59 20.4

Crude oil 0.86 150

Gasoline 0.68 6.1

Glycerin 1.26 31,200

Kerosene 0.81 40

Mercury 13.56 33

SAE 10 oil 0.92 1,700

SAE 20 oil 0.92 3,400

SAE 30 oil 0.92 9,200

SAE 40 oil 0.92 13,000

Water 1.0 21.0

Viscosity is typically measured by rotating a cylinder or blade in a beaker

of the liquid and recording the force required. The simplest way to measure

viscosity would be to drag a plate over a thin film of the liquid and measure

how much force it takes. Consider the set up below:

fluid

1 m2 plate at 1 m/s

1 N

0.01 m

Fig. 2-2. Simple viscosity measurement.

Practical Fluid Mechanics for Civil Engineers

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We know the size of the plate (1 m2) and the force applied (1N), so we know (N/m2). We know the velocity of the plate (1 m/s) and the thickness of the fluid (0.01). With this we could compute the only unknown, . Conceptually this is what commercial viscosity meters do.

2.8 Newtonian Fluids

For most fluids the viscosity does not change as the fluid flows. This is a great advantage in engineering calculations as it simplifies the analysis a lot. It is for this reason that most fluid calculations do not involve a viscosity term. Specifically viscosity does not change with pressure for most fluids. These fluids are called Newtonian fluids. Water is a Newtonian fluid, for example.

In some fluids the viscosity does change with pressure. This means that a basic fluid property is changes as the fluid is moving. These are called Non-Newtonian fluids. Calculations with Non-Newtonian fluids are very complex and beyond the scope of this book.

Examples of Non-Newtonian fluids include:

paints

sludge – like wastewater sludge from a treatment plant

mudflow – like a mud slide

wet concrete

slurries

ketchup

putty

some gels

gypsum paste

Although this is a diverse list there is one common component for most of these– these are not pure liquids but a mix of solids and liquids. Gels may be an exception.

If you perform a series of viscosity experiment like the one described above and plot the results you would have:

Scott Lowe

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1

du/dy

Fig. 2-3. Newtonian and non-Newtonian fluids.

So if the results are linear you have a Newtonian fluid and the slope of the line

is . A non-linear result indicates a Non-Newtonian fluid and a varying .

2.9 Viscosity and Temperature

While viscosity does not change for Newtonian fluids as it flows, viscosity is

strongly effected by temperature. As you heat a liquid up it loses viscosity:

1

du/dy

Fig. 2-4. Viscosity and temperature.

Practical Fluid Mechanics for Civil Engineers

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This effect shows up most noticeably in engine oil. This was a design problem for engines. Engines use oil for lubrication and will seize in a few seconds without it. At a cold start up the engine required very light grade oil to quickly coat the moving parts. When the engine is at running temperature a more viscous oil is needed whose viscosity wont degrade at high temperatures.

Oils are graded based on their viscosity from 1 (lightest) to 100+ (most viscous). The solution is to use a blend of oils. The picture below shows typical engine oil. The numbers indicate the range of the blend – 10W30 in this case. “W” stands for winter. So the lightest oil is 10 and the heaviest is 30. As the range gets bigger the price goes up. SAE is the Society of Automotive Engineers that rates the oil.

In oil commercials this is the meaning of slogans like “protects against thermal viscosity breakdown”.

The best engine oil is synthetic which is a custom blend at the molecular level. This is also the most expensive. It comes standard in many luxury and performance vehicles. It is used in jet engines.

Fig. 2-5. Engine oil.

Example – viscosity and engine oil

At the heart of internal combustion engines are the pistons and cylinders. Oil lubricates the metal to metal interaction of the pistons and cylinder walls. Calculate the force required to move the piston through the oil.

Data: Piston is 3” diameter and 3” long.

Oil = 0.02 lb-s/ft2 at 250oF

Scott Lowe

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10 ft/s

piston

Cylinder wall

Piston rings

oil

0.1”

So we assume that the same exists everywhere in the oil as it is a thin

film (0.1” thick). So u=0 ft/s at the cylinder wall and u=10 ft/s at the piston.

So the du/dy term is known and we know so we can solve for the shear

stress in the oil:

From here we can solve for the force: because stress is always force/area:

In this case the area is the area of oil in contact with the piston:

Practical Fluid Mechanics for Civil Engineers

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One final note on viscosity and temperature. While in liquids the viscosity decreases with increasing temperature, in gases the opposite occurs – gas viscosity increases with temperature. The gas molecules interact more with each other as the gas is heated – resulting in more friction – more viscosity.

But remember that gases start with very low viscosity compared to liquids (~ 1/10th) and the increase is small. See the viscosity in Table 2 for air. It goes from 0.0000003 lb-s/ft2 at 0oF to 0.0000005 lb-s/ft2 at 250oF.

2.10 Vapor Pressure

All liquids evaporate, or vaporize or volatilize. Rapid vaporization is called boiling. Vapor pressure is the pressure caused by molecules leaving the liquid phase. It has units of pressure, e.g. psi.

Saturation vapor pressure is the pressure at which a liquid will boil. When engineers talk about vapor pressure they are nearly always referring to saturation vapor pressure. When the saturation vapor pressure reaches the ambient air pressure then a liquid will boil.

The saturation vapor pressure increases with temperature. This is intuitive as you expect to drive more liquid into the gas phase as you heat it up.

Saturation vapor pressure is also a good indication of how volatile a liquid is. For example the saturation V.P of mercury at 60oF is 0.000025 psi. So mercury is not volatile and is incredibly stable. The VP of water @60oF is 0.339 psi – not particularly volatile but it will still evaporate over time. This is the bane of water supply systems with large surface area reservoirs – Lake Mead in the desert of Nevada being a prime example.

The VP of gasoline is 8.0 psi – extremely volatile. This is why vehicles have sealed fuel tanks and the underground tanks at gas stations are sealed – to prevent fuel escaping into the atmosphere. This not only prevents losses but also helps air quality.

The simple example below helps show the relationship between VP, temperature and boiling.

Scott Lowe

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Example – boiling water

Will water in the glass boil?

Water @50oF

Air – 14.7 psi

The solution to the question is to compare the VP of water at 50oF to the

air pressure at sea level, 14.7 psi. From Table 2-1 we get VP = 0.18 psi. This is

less than 14.7 psi so the water will not boil.

At what temperature will the water boil? Again go to Table 2-1 and see the

temperature where the VP = air pressure. At 212oF the VP is 14.7 psi so that’s

why water boils at this temperature.

Example – boiling blood

If you were to travel at high altitudes without a pressure suit your blood

would literally boil. Assuming blood has basically the same VP properties as

water, at what altitude would this occur?

At 10,000 ft the air pressure is 10.1 psi. Our body temperature is ~100oF

so a VP of ~0.95 psi. So 0.95<10.1 psi so our blood would not boil at 10,000 ft. From Table 3 we see at 60,000 ft the air pressure is 1.0 psi. So if we were to go just over 60,000 ft the air pressure and our blood VP would be ~same, and our blood would begin to boil. 2.11 Boiling Water Everyone knows that water boils at 212oF, right? But water boils when its VP>air pressure. At higher altitude air pressure decreases, so water will boil at

a lower temperature. At 20,000 ft the air pressure is 6.8 psi. The VP of water

is 6.8 psi somewhere between 170-180oF, so this is the temperature it will boil

Practical Fluid Mechanics for Civil Engineers

31

at.

This relationship was first noted by Charles Darwin, who noticed food in boiling water didn’t cook well in the Andes mountains.

Can you make water boil at room temperature? Yes. If you put water in a container and draw a vacuum over it you can decrease the pressure to the point where water will boil, but will not be hot. You have to create a strong vacuum, as water at 70oF has a VP of 0.36 psi.

Can you make water boil >212oF? Yes. You have to increase the pressure over the water surface. The easiest way to do this is to heat the water in a closed container. This is how pressure cookers work and why they are useful.

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