Amelia sung vsim

August 2015 ME495 - Pipe Flow Losses Page 2

ME 495—Thermo Fluids Laboratory ~~~~~~~~~~~~~~

PIPE FLOW LOSSES ~~~~~~~~~~~~~~

PREPARED BY: GROUP LEADER’S NAME

LAB PARTNERS: NAME

NAME

NAME

TIME/DATE OF EXPERIMENT: TIME, DATE

OBJECTIVES The objectives of this lab are to:

a) measure head losses through bends, transitions, and fittings,

and use these measurements to estimate the loss coefficients for

each transition or fitting.

b) illustrate flowrate measurement by measuring the pressure

drop across a gate valve (i.e. orifice plate).

INTRODUCTION Fluids are usually transported through pipes from one location to

another using pumps. In order to size a pump for a given application, it

is necessary to predict the pressure drop which results from friction in

the pipe and fittings. Also, once a pipe system is built,

measurements of the amount of fluid flowing through the pipes

have to be performed, to ensure that the desired flow rate is

delivered by the system.

PRESSURE LOSSES

Head Loss in Pipe Flows Pipe flows belong to a broader class of flows, called internal

flows, where the fluid is completely bounded by solid surfaces. In

contrast, in external flows, such as flow over a flat plate or an

airplane wing, only part of the flow is bounded by a solid surface.

The term pipe flow is generally used to describe flow through

round pipes, ducts, nozzles, sudden expansions and contractions,

valves and other fittings. In this experiment we will study only

flow through round pipes.

When a gas or a liquid flows through a pipe, there is a loss of

pressure in the fluid, because energy is required to overcome the

viscous or frictional forces exerted by the walls of the pipe on the

moving fluid. In addition to the energy lost due to frictional

forces, the flow also loses energy (or pressure) as it goes through

fittings (i.e. valves, elbows, contractions and expansions). This

loss in pressure is mainly due to the fact that flow separates locally

as it moves through such fittings.

The pressure loss in pipe flows is commonly referred to as

head loss. The frictional losses are referred to as major losses (hf)

while losses through fittings are called minor losses

(hi). Together they make up the total head losses (hL) for pipe

flows. Hence:

 

 n1i

ifL hhh 

(1)

Head losses in pipe flows can be calculated by using a special

form of the energy equation that is discussed in the next section.

Energy Equation for Pipe Flows Consider a steady, incompressible flow through a piping

system. The energy equation between two points, 1 and 2, in the

flow can be written as:

Lh g

V z

p

g

V z

p 

22

2

2 2

2

2

1 1

1

 (2)

In the above equation, the terms in the parenthesis represent the

mechanical energy per unit mass at a particular cross-section in

the pipe. Hence, the difference between the mechanical energy at

two locations, i.e. the total head loss, is a result of the conversion

of mechanical energy to thermal energy due to frictional effects.

The significant parameters in equation 2 are:

 Z - the elevation of the cross section, taken to be positive

upwards.

 V - the average velocity at a cross section.

 hL - the total head loss between cross-sections 1 and 2.

Details on how to calculate the head loss are given in the next

section.

An examination of Equation 2 reveals that for a fixed amount

of mechanical energy available at point 1, a higher head loss will

lead to lower mechanical energy at point 2. The lower mechanical

energy can be manifested as a lower pressure, lower velocity (i.e.

lower volumetric flow rate), a lower elevation or any combination

of all three. It should also be noted that for flow without losses,

hL = 0, and the energy equation reduces to Bernoulli’s Equation.

Calculation of Head Losses Major Losses

The major head loss in pipe flows is given by the equation:

g

V

D

L fh f

2

2

 (3)

where L and D are the length and diameter of the pipe,

respectively, V is the average fluid velocity through the pipe and f

is the friction factor for the section of the pipe. In general, the

friction factor is a function of the Reynolds number and the non-

dimensional surface roughness, e/D. The friction factor is

determined experimentally and is usually published in graphical

form as a function of Reynolds number and surface

roughness. The friction factor plot, shown in Fig. 2, is usually

referred to as the Moody plot, after L. F. Moody who first

published such data in this form.

Flow in a pipe is considered laminar if Reynolds number,

ReD < 2000, in which case the friction factor is only a function of

the Reynolds number and is given as:

e

arla R

f 64

min  (4)

Minor Losses

The minor head losses can be expressed as:

g

V Kh ii

2

2

 (5)

where K is a loss coefficient that must be determined

experimentally for each situation. In some cases, such as short

pipes with multiple fittings, minor losses are actually a large

percentage of the total head loss.

Another common way to express minor head loss is in terms

of frictional (major) head loss through an equivalent length, Le, of

a straight pipe. In this form, the minor head loss is written as: