Presentation on theme: "MECH 221 FLUID MECHANICS (Fall 06/07) Chapter 9: FLOWS IN PIPE"— Presentation transcript:
1 MECH 221 FLUID MECHANICS (Fall 06/07) Chapter 9: FLOWS IN PIPE Instructor: Professor C. T. HSU
2 9.1 General Concept of Flows in Pipe As a uniform flow enters a pipe, the velocity at the pipe walls must decrease to zero (no-slip boundary condition). Continuity indicates that the velocity at the center must increase.Thus, the velocity profile is changing continuously from the pipe entrance until it reaches a fully developed condition. This distance, L, is called the entrance length.
3 9.1 General Concept of Flows in Pipe For fully developed flows (x>>L), flows become parallel, , the mean pressure remains constant over the pipe cross-section
4 9.1 General Concept of Flows in Pipe Flows in a long pipe (far away from pipe entrance and exit region, x>>L) are the limit results of boundary layer flows. There are two types of pipe flows: laminar and turbulent
5 9.1 General Concept of Flows in Pipe Whether the flow is laminar or turbulent depends on the Reynolds number, where Um is the cross-sectional mean velocity defined byTransition from laminar to turbulent for flows in circular pipe of diameter D occur at Re=2300
6 9.1 General Concept of Flows in Pipe When pipe flow is turbulent. The velocity is unsteadily random (changing randomly with time), the flow is characterized by the mean (time-averaged) velocity defined as:Due to turbulent mixing, the velocity profile of turbulent pipe flow is more uniform then that of laminar flow.
7 9.1 General Concept of Flows in Pipe Hence, the mean velocity gradient at the wall for turbulent flow is larger than laminar flow.The wall shear stress, ,is a function of the velocity gradient. The greater the change in with respect to y at the wall, the higher is the wall shear stress. Therefore, the wall shear stress and the frictional losses are higher in turbulent flow.
8 9.2 Poiseuille FlowConsider the steady, fully developed laminar flow in a straight pipe of circular cross section with constant diameter, D.The coordinate is chosen such that x is along the pipe and y is in the radius direction with the origin at the center of the pipe.yxDb
9 9.2 Poiseuille FlowFor a control volume of a cylinder near the pipe center, the balance of momentum in integral form in x-direction requires that the pressure force,acting on the faces of the cylinder be equal to the shear stress acting on the circumferential area, henceIn accordance with the law of friction (Newtonian fluid), have:since u decreases with increasing y
10 9.2 Poiseuille Flow Therefore: when is constant (negative) Upon integration:The constant of integration, C, is obtained from the condition of no-slip at the wall. So, u=0 at y=R=D/2, there fore C=R2/4 and finally:
11 9.2 Poiseuille FlowThe velocity distribution is parabolic over the radius, and the maximum velocity on the pipe axis becomes:Therefore,The volume flow rate is:
12 9.2 Poiseuille FlowThe flow rate is proportional to the first power of the pressure gradient and to the fourth power of the radius of the pipe.Define mean velocity asTherefore,This solution occurs in practice as long as,Hence,
13 9.2 Poiseuille FlowThe relation between the negative pressure gradient and the mean velocity of the flow is represented in engineering application by introducing a resistance coefficient of pipe flow, f.This coefficient is a non-dimensional negative pressure gradient using the dynamic head as pressure scale and the pipe diameter as length scale, i.e.,Introducing the above expression for (-dp/dx),so,
14 9.2 Poiseuille Flow At the wall, So, As a result, the wall friction coefficient is:
15 9.3 Head Loss in PipeFor flows in pipes, the total energy per unit of mass isgiven by where the correction factor isdefined as,with being the mass flow rate and A is the cross sectional area.
16 9.3 Head Loss in PipeSo the total head loss between section 1 and 2 of pipes is:hl=head loss due to frictional effects in fully developed flow in constant area conduitshlm=minor losses due to entrances, fittings, area changes, etcs.
17 9.3 Head Loss in PipeSo, for a fully developed flow through a constant-area pipe,And if y1=y2,
19 9.4 Turbulent Pipe FlowFor turbulent flows’ we cannot evaluate the pressure drop analytically. We must use experimental data and dimensional analysis.In fully developed turbulent pipe flow, the pressure drop, , due to friction in a horizontal constant-area pipe is know to depend on:Pipe diameter, DPipe length, LPipe roughness, eAverage flow velocity, UmFluid density,Fluid viscosity,
20 9.4 Turbulent Pipe Flow Therefore, Dimensional analysis, Experiments show that the non-dimensional head loss is directly proportional to L/D, hence
21 9.4 Turbulent Pipe Flow Defining the friction factor as, , hence where f is determined experimentally.The experimental result are usually plotted in a chart called Moody Diagram.
22 9.4 Turbulent Pipe FlowIn order to solve the pipe flow problems numerically, a mathematical formulation is required for the friction factor, f, in terms of the Reynolds number and the relative roughness.The most widely used formula for the friction factor is that due to Colebrook,This an implicit equation, so iteration procedure is needed to determine.
23 9.4 Turbulent Pipe FlowMiller suggested to use for the initial estimate,That produces results within 1% in a single iteration
24 9.5 Minor Loss The minor head loss may be expressed as, where the loss coefficient, K, must be determined experimentally for each case.Minor head loss may be expressed aswhere Le is an equivalent length of straight pipe
25 9.5 Minor Loss Source of minor loss: 1. Inlets & Outlets 2. Enlargements & Contractions3. Valves & Fittings4. Pipe Bends
26 9.6 Non-Circular DuctsPipe flow results sometimes can be used for non-circular ducts or open channel flows to estimate the head lossUse Hydraulic Diameter,A - Cross section area; P - Wetted perimeterFor a circular duct,For rectangular duct,where Ar =b/a is the geometric aspect ratio
27 9.6 Non-Circular Ducts Effect of Aspect Ratio (b/a): For square ducts: For wide rectangular ducts with b>>a:Thus, flows behave like channel flowsHowever, pipe flow results can be used with good accuracy only when:baa=bAr=1Dh=abaArDh2ababa1/3<Ar<3