Chapter Four Fluid Dynamic

Chapter Four Fluid Dynamic

Chapter Four Fluid Dynamic What are the types of flow equation (mass, momentum, and energy) Types of friction Boundary Layer Flow in non-circular pipe Multiple pipe system Unsteady state flow Conservation References: Streeter,V. Fluid Mechanic,3rd edition, Mc-Graw Hill, 1962. Frank M. White Fluid Mechanics 5th edition McGraw Hill.

Coulson, J.M. and J.F. Richardson, Chemical Engineering, Vol.I Fluid Flow, Heat Transfer, and Mass Transfer 5th edition, (1998). . Fluid mechanics is the study of fluids and the forces on them - The fluid motion is generated by pressure difference between two points and is constrained by the pipe walls. The

direction of the flow is always from a point of high pressure to a point of low pressure. - If the fluid does not completely fill the pipe, such as in a concrete sewer, the existence of any gas phase generates an almost constant pressure along the flow path. - If the sewer is open to atmosphere, the flow is known as open-channel flow and is out of the scope of this chapter or in the whole course. Types of Flow Flow in pipes can be divided into two different regimes, i.e. laminar and

turbulence. The experiment to differentiate between both regimes was introduced in 1883 by Osborne Reynolds (18421912), an English physicist who is famous in fluid experiments in early days. The velocity, together with fluid properties, namely density and dynamic viscosity , as well as pipe diameter D, forms the dimensionless Reynolds number, that is From Reynolds experiment, he suggested that Re < 2100 for laminar flows and Re > 4000 for turbulent flows. The range of Re between 2100 and 4000 represents transitional flows Types of Flow

Example Consider a water flow in a pipe having a diameter of D = 20 mm which isintended to fill a 0.35 liter container. Calculate the minimum time required if the flow is laminar, and the maximum time required if the flow is turbulent. Use density = 998 kg/m3 and dynamic viscosity . = 1.12103 kg/ms Governing Equations Mass cannot be created or destroyed

Continuity Equation F=ma (Newtons 2ndlaw) Momentum Equation Energy cannot be created or destroyed Energy Equation 1- Continuity Equation ( Overall Mass Balance) Its also called (conservation of mass)

For incompressible fluid (the density is constant with velocity) then 2 Momontume Equation and Bernoulli Equation Its also called equation of motion consider a small element of the flowing fluid as shown below,

Let: dA: cross-sectional area of the fluid element, dL: Length of the fluid element dW: Weight of the fluid element dL u: Velocity of the fluid element P: Pressure of the fluid element

Assuming that:p;;l the fluid is steady, non-viscous (the frictional losses are zero), incompressible (the density of fluid is constant) . The forces on the cylindrical fluid element are, 1- Pressure force acting on the direction of flow (PdA) 2- Pressure force acting on the opposite

direction of flow [(P+dP)dA] 3- A component of gravity force acting on the opposite direction of flow (dW sin ) ) Hence, the total force = gravity force + pressure force dP/ + udu + dz g = 0 ---- Eulers equation of motion

Bernoullis equation could be obtain by integration the Eulers equation dP/ + udu + dz g = constant P/ + u2/2 + z g = constant P/ + u2/2 + z g = 0 --------- Bernoullis P/ + P/ + u2/2 + z g = 0 --------- Bernoullis u2/2 + P/ + u2/2 + z g = 0 --------- Bernoullis z g = 0 --------- Bernoullis equation In general In the fluid flow the following forces are present: 1- Fg ---------force due to gravity

2- FP ---------force due to pressure 3- FV ---------force due to viscosity 4- Ft ---------force due to turbulence 5- Fc ---------force due to compressibility 6- F ---------force due to surface tension 3- Energy Equation and Bernoulli Equation The total energy (E) per unit mass of fluid is given by the equation: -

E1 + q + w1 = E2 + w2

where q represents the heat added to the fluid w1 represents the work added to the fluid like a pump w2 represents the work done by the fluid like the work to overcome the viscose or friction force E is energy consisting of: Internal Energy (U) This is the energy associated with the physical state of fluid, i.e. the energy of atoms and molecules resulting from their motion and configuration. Internal energy is a function of temperature. It can be written as (U) energy per unit mass of fluid.

Potential Energy (PE) This is the energy that a fluid has because of its position in the earths field of gravity. The work required to raise a unit mass of fluid to a height (z) above a datum line is (zg), where (g) is gravitational acceleration. This work is equal to the potential energy per unit mass of fluid above the datum line.

Kinetic Energy (KE) This is the energy associated with the physical state of fluid motion. The kinetic energy of unit mass of the fluid is (u2/2), where (u) is the linear velocity of the fluid relative to some fixed body. Pressure Energy (Prss.E) This is the energy or work required to introduce the fluid into the system without a change in volume. If (P) is the pressure and (V) is the volume of a mass (m) of fluid, then (PV/m PI) is the pressure energy per unit mass of fluid. The ratio (m/V) is the fluid density ().

In the case of: No heat added to the fluid The fluid is ideal There is no pump The temperature is constant along the flow Then P/ + u2/2 + z g = 0 --------- Bernoullis P/ + P/ + u2/2 + z g = 0 --------- Bernoullis u2/2 + P/ + u2/2 + z g = 0 --------- Bernoullis z g = 0 --------Bernoullis equation Modification of Bernoullis Equation 1- Correction of the kinetic energy term = 0.5 for laminar flow - = 1.0 for turbulent flow 2- - Modification for real fluid

Thus the modified Bernoullis equation becomes, P1/ + u12/2 + z1 g = P2/ + u22/2 + z2 g + F ---------(J/kg m2/s2) 3- Pump work in Bernoullis equation Frictions occurring within the pump are: Friction by fluid Mechanical friction Since the shaft work must be discounted by these frictional force (losses) to give net mechanical energy as actually delivered to the fluid by pump (Wp).

Thus, Wp = Ws where , is the efficiency of the pump. P1/ + u12/2 + z1 g + Ws = P2/ + u22/2 + z2 g + F ---------(J/kg m2/s2) By dividing each term of this equation by (g), each term will have a length units, and the equation will be: P1/ g + u12/2g + z1 + Ws /g = P2/ g + u22/2g + z2 + hf ---------(m) where hF = F/g head losses due to

friction. 4 Friction in Pipes Relation between Skin Friction and Wall Shear Stress dPfs = 4( dL/d) = 4 ( / ux2) (dL/d) ux2 where, ( / ux2) = =Jf =f/2 =f/2 (or Jf): Basic friction Factor f: Fanning (or Darcy) friction Factor f: Moody friction Factor

PPfs = 4f (L/d) (u /2) ---------------------(Pa) : The energy lost per unit mass Fs is then given by Fs = (PPfs/) = 4f (L/d) (u /2) -----------------(J/kg) or (m2/s2) The head loss due to skin friction (hFs) is given : by hFs = Fs/g = (PPfs/g) = 4f (L/d) (u /2g) ---------------(m) 2 2 2

Evaluation of Friction Factor in Straight Pipes Velocity distribution in laminar flow ux = [(-PPfs R2)/(4L )][1)][1 (r/R)2] velocity distribution (profile) in laminar flow umax = [(PPfs d2)/(16 L )][1)] ----------centerline velocity in laminar flow ux / umax = [1(r/R)2] ---------velocity distribution (profile)in laminar flow Velocity distribution in turbulent flow

ux / umax = [1(r/R)]1/7 Prandtl one-seventh law equation. (velocity distribution profile)in turbulent flow Average (mean) linear velocity in laminar flow u = umax/2 = [(P/ + u2/2 + z g = 0 --------- Bernoullis Pfs R2)/(8L )] = [(Pfs d)] = [(P/ + u2/2 + z g = 0 --------- Bernoullis Pfs d2)/(32 L )] = [(Pfs d)] PPfs = (32 L u) / d2 HagenPoiseuille equation

in Turbulent flow u = 49/60 umax 0.82 umax ------------average velocity in turbulent flow Friction factor in laminar flow f = 16 / Re Fanning or Darcy friction factor in laminar flow. Or F = 64/Re in Turbulent flow

for 2,500 < Re <100,000 Or

and, for 2,500 < Re <10,000,000 These equations are for smooth pipes in turbulent flow For rough pipes, the ratio of (e/d) acts an important role in evaluating the friction factor in turbulent flow as shown in the following equation

Graphical evaluation of friction factor Form Friction - Sudden Expansion (Enlargement) Losses Sudden Contraction Losses Form Friction

Losses in Fittings and Valves Butterfly valve Total Friction Losses Typical commercial valve geometries: (a) gate valve; (b) globe valve; (c) angle valve; (d) swingcheck valve; (e) disk type gate valve.

Example 4 Determine the velocity of efflux from the nozzle in the wall of the reservoir of Figure below. Then find the discharge through the nozzle. Neglect losses. Example 5 Oil, with 900 kg/m and 0.00001 m/s, flows at 0.2m3 /s through 500 m of 200mmdiameter cast-iron pipe. Determine (a) the head loss and (b) the pressure drop if the pipe slopes down at 10 in the flow direction. Example 6

A pump draws 69.1 gal/min of liquid solution having a density of 114.8 lb/ft3 from an open storage feed tank of large cross-sectional area through a 3.068I.D. suction pipe. The pump discharges its flow through a 2.067I.D. line to an open over head tank. The end of the discharge line is 50 above the level of the liquid in the feed tank. The friction losses in the piping system are F = 10 ft lbf/lb. what pressure must the pump develop and what is the horsepower of the pump if its efficiency is =0.65.

Example 7 The siphon of Fig. 3.14 is filled with water and discharging at 2.80 cfs. Find the losses from point 1 to point 3 in terms of velocity head u2/2g. find the pressure at point 2 if two-third of the Losses occur between points I and2 Example 8 A conical tube of 4 m length is fixed at an inclined angle of 30 with the horizontal-line and its small diameter upwards. The velocity at smaller end is (u1 = 5 m/s), while (u2 = 2 m/s) at other end. The head losses in the tub is [0.35 (u1-u2)2/2g]. Determine the pressure head at lower end if the flow takes place in down direction and the pressure head at smaller end is 2 m of liquid.

Example 9 A pump developing a pressure of 800 kPa is used to pump water through a 150 mm pipe, 300 m long to a reservoir 60 m higher. With the valves fully open, the flow rate obtained is 0.05 m3/s. As a result of corrosion and scalling the effective absolute roughness of the pipe surface increases by a factor of 10 by what percentage is the flow rate reduced. )] = [(Pfs d= 1 mPa.s Example 10 A liquid of specific weight g 58 lb/ft3 flows by gravity through a 1-ft and a 1-ft capillary tube at a rate of 0.15 ft3/h, as shown in

Fig. Sections 1 and 2 are at atmospheric pressure. Neglecting entrance effects, compute the viscosity of the liquid. tank Example 11 630 cm3/s water at 320 K is pumped in a 40 mm I.D. pipe through a length of 150 m in horizontal direction and up through a vertical height of 10 m. In the pipe there is a control valve which may be taken as equivalent to 200 pipe diameters and also other fittings equivalent to 60 pipe diameters. Also other pipe fittings equivalent to 60 pipe diameters. Also in the line there is a heat exchanger across which there is a loss in head of 1.5 m H2o. If the main pipe has a roughness of 0.0002 m, what power must supplied to the pump if = 60%, )] = [(Pfs d= 0.65 mPa.s.

Example 12 A pump driven by an electric motor is now added to the system. The motor delivers 10.5 hp. The flow rate and inlet pressure remain constant and the pump efficiency is 71.4 %, determine the new exit pressure. Example 13 An elevated storage tank contains water at 82.2C as shown in Figure below. It is desired to have a discharge rate at point 2 of 0.223 ft3/s. What must be the height H in ft of the surface of the water in the tank relative to discharge point? The pipe is schedule 40, e = 1.5 x10-4 ft. Take that = 60.52 lb/ft3, )] = [(Pfs d= 2.33 x10-4 lb/ft.s. Example 14 Water, 1.94 slugs/ft and 0.000011 ft/s, is pumped between two reservoirs at 0.2 ft/s through 400 ft of 2-in-diameter

pipe and several minor losses, as shown in Fig.. The roughness ratio is /d 0.001. Compute the pump horsepower required. Example 15 The pump in Fig. E3.20 delivers water (62.4 lbf/ft3) at 3 ft3/s to a machine at section 2, which is 20 ft higher than the reservoir surface. The losses between 1 and 2 are given by hf =_ Ku 2 /(2g), where K _ 7.5 is a dimensionless loss coefficient. Take = 1.07. Find the horsepower required for the pump if it is 80 percent efficient. The Boundary Layer

Boundary layer for flow on flat plate The Boundary Layer Developing velocity profiles and pressure changes in the entrance of a duct flow The Boundary Layer For fully developed velocity profile to be formed in laminar flow, the approximate entry length (Le) of pipe having diameter d, is: -

Le/d = 0.0575 Re -------------------laminar In turbulent flow the boundary layers grow faster, and Le is relatively shorter, according to the approximation for smooth walls turbulent

------------------ The Boundary Layer Example A 0.5in-diameter water pipe is 60 ft long and delivers water at 5 gal/min at 20C. What fraction of this pipe is taken up by the entrance region? Flow in non-circular pipe

If the duct is noncircular, the analysis of fully developed flow follows that of the circular pipe but is more complicated algebraically. For laminar flow, one can solve the exact equations of continuity and momentum. For turbulent flow, the logarithm-law velocity profile can be used, or (better and simpler) the hydraulic diameter is an excellent approximation. Multiple pipe system Example: Given is a three-pipe series system. The total pressure

drop is pA-pB=150,000 Pa, and the elevation drop is zA-zB=5m . The pipe data are The fluid is water, . Calculate the flow rate Q in m3/h through the system. Example: Assume that the same three pipes in above example are now in parallel with the same total head loss of 20.3 m. Compute the total flow rate Q, neglecting minor losses.

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