Friction and Minor losses in Pipes

ME 309 Fluid Mechanics

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SAFETY INSTRUCTIONS

All practical work areas and laboratories should be covered by local safety regulations which must be followed at all times.

Hot Surfaces and Liquids The unit incorporates a pumped electric water heater, and is capable of producing temperatures that could cause skin burns.

Before disconnecting any of the pipes or tubing:

Stop all the pumps.

Leave time for the water to cool

Check that the temperature is at a safe level

Do not touch any surfaces close to ‘Hot Surfaces’ warning labels, or any of the interconnecting tubing, whilst the equipment is in use.

General Instructions

If a spill occurs, turn off the pumps (if possible without injury) and immediately

get in touch with the Laboratory Instructor or Technician.

Ensure that protective clothing (LAB coat) and gloves are worn when handling any of the substances used in the reactor.

Shorts or skirts should not be worn to the lab.

Sandals, high heels, or open-toe shoes are not acceptable.

Safety glasses are a required item to be worn in all areas of the laboratories.

Electrical - Burn / Shock: Care with electrical connections, particularly with grounding, and not using frayed electrical cords, can reduce hazard.

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Contents Objective ........................................................................................................................................................ 4

Introduction .................................................................................................................................................... 5

Theory............................................................................................................................................................. 6

Rig Specifications: ........................................................................................................................................... 8

Procedure: ...................................................................................................................................................... 8

Analysis: .......................................................................................................................................................... 8

Data tables:..................................................................................................................................................... 9

References ...................................................................................................................................................... 9

List of tables: Table 1 data calculations regarding smooth pipes ......................................................................................... 9

Table 2 data calculations regarding rough pipes ............................................................................................ 9

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Objective

This unit (Figure 1) is designed to study the behavior of internal flows. It makes it possible to study pressure drops in pipes as well as in different hydraulic accessories. The losses by friction in straight pipes of different sizes can be studied on a certain range of Reynolds number. The objective of this experiment is to determine the pressure loss and the friction factor f for the following:

Rough pipe (PVC covered in sand) with an internal diameter

of 23 mm (3).

Smooth pipe (PVC) with an internal diameter of 26.5 mm (6). The loss coefficient of two valves is determined as well, namely:

Angle seat valve (7).

Gate valve (8).

The experimentally determined values will be compared to theoretical and existing values. Please refer to the schematic sketch (figure. 2) in the appendix section to locate the different segments and devices according to the numbers given above.

Learning Outcomes & relationship to ABET Student Outcomes (SO)

Experiment Name Learning Outcomes ABET Student Outcomes

Friction and Minor Losses in Pipes

To confirm the head loss

predicted by a pipe friction

equation associated with flow of

water through a smooth pipe.

b, g, i

* b: An ability to design and conduct experiments, as well as to analyze and interpret

data.

* g: An ability to communicate effectively.

* i: Recognition of the need for, and an ability to engage in life-long learning

Performance indicators for SO; Students are expected to be able to

SO (b) 1. Design an experiment plan

2. Identify variables and acquire data related to the experiment

3. Compare obtained data and results to appropriate theoretical models

4. Explain observed differences between model and experiment

SO (g) 1. Deliver an organized written document

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Introduction

Energy losses in pipe flows are the result of friction between the fluid and the pipe walls and internal friction between fluid particles. Minor (secondary) head losses occur at any location in a pipe system where streamlines are not straight, such as at pipe junctions, bends, valves, contractions, expansions, and reservoir inlets and outlets. In this experiment, you will measure minor head losses through a pipe section that has several bends, transitions, and fittings.

Figure 1 Experimental apparatus.

Figure 2 Schematic sketch of the apparatus.

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Theory

A quantity of interest in the analysis of pipe flow is the pressure drop ∆P since it is directly related to the power requirements of the fan or pump to maintain flow. For laminar flow, the pressure drop can be Expressed as:

A pressure drop due to viscous effects represents an irreversible pressure loss, and it is called pressure Loss ∆PL to emphasize that it is a loss (just like the head loss hL, which is proportional to it). Note from Eq. 1 that the pressure drop is proportional to the viscosity µ of the fluid, and P would be zero if there were no friction. Therefore, the drop of pressure from P1 to P2 in this case is due entirely to viscous effects, and Eq. 1 represents the pressure loss ∆PL when a fluid of viscosity flows through a pipe of constant diameter D and length L at average velocity Vavg. In practice, it is found convenient to express the pressure loss for all types of fully developed internal flows (laminar or turbulent flows, circular or noncircular pipes, smooth or rough surfaces, horizontal or inclined pipes) as:

For fully developed laminar flow in circular pipes, the Darcy-Weisbach friction factor f can be found using Eq. 3. Where Re in Eq. 3 is the Reynolds number and is given by Eq. 4

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The flow can be treated as laminar so long as Reynolds number is less than 2300. For RE > 2300 (i.e. for transitional and fully turbulent regions) the Colebrook equation is used to find the friction factor. Where ε/D is the relative roughness of the pipe (ε/D = 0 for smooth pipes). As seen from Eq. 5, the friction factor ƒ appears on both sides of the equation; that's, the equation is implicit in ƒ and must be solved iteratively. Alternatively, Eq. 7 is an approximate expression of the Colebrook equation and can be used to solve explicitly for ƒ. The resulting values are within 2 percent of those obtained using Colebrook equation.

The fluid in a typical piping system passes through various fittings, valves, bends, elbows, tees, inlets, exits, enlargements, and contractions in addition to the pipes. These components interrupt the smooth flow of the fluid and cause additional losses because of the flow separation and mixing they induce. In a typical system with long pipes, these losses are minor compared to the total head loss in the pipes (the major losses) and are called minor losses. Minor losses are usually expressed in terms of the loss coefficient KL (also called the resistance coefficient), defined as:

The loss coefficient K can be indirectly found by measuring the pressure drop ∆Ptotal across the whole pipe line (including the fitting/valve) and then subtracting the pressure loss due to friction yielding the loss due to the fitting only. In Eq. 8, L refers to the pipe length excluding the fitting in question.

P.S. Please refer to Fluid Mechanics textbook by Fox and McDonald (Chapter 8-Section 8.1 & 8.2)

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Rig Specifications:

The device includes the FME00 hydraulic bench and has a rotameter incorporated (range: 600-6000 ƪ/h). The experiment offers a multitude of possibilities and the reader is advised to refer to the schematic drawing (Fig. 2) to acquaint him/herself with them. The circuits have 7 ball valves (V1-V7), required to distribute the flow through the different elements tested. The equipment has differential anti-obturant pressure sensors, located at the beginning and at the end of every element studied. Each one of them connects easily to Bourdon tube pressure gauges (24) and to differential water manometers (25). The Bourdon tube will be used to measure large differences of pressure, while the water manometer will be used to measure small differences of pressure.

Procedure:

N.B. Make sure to carry out the necessary calibration before performing the experiment. The steps of this experiment go as follows: 1. Open only the ball valve that would direct the flow rate through the pipe segment or the fitting desired (keep the rest closed). 2. Switch on the pump on the hydraulic bench. 3. Carefully open both valves until you reach the desired flow rate (directly read on the rotameter). 4. Wait until a steady flow is established throughout the pipe line 5. Plug in the pressure probes on both ends of your test segment (use the differential manometers for small pressure differences). 6. If the manometers go o_-bound in the previous step, unplug them and attach the Bourdon tube probes in their places. 7. Measure the length of the test segment, excluding the fitting, for use in equations 2 and 8. 8. Take different readings for different Reynolds numbers. 9. Repeat the above steps for a different test segment and/or fitting.

Analysis:

1. Compare the experimental loss-coefficient values for different fittings to those found in a fluid mechanics text book (or another source). Be sure to cite the source of the published values.

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2. Does the static pressure increase or decrease for the enlargement and contraction? Explain the increase or decrease in static pressure.

3. What is the advantage of expressing the friction factor as a function of the Reynolds number rather than as a function of the flow rate?

4. Why are two different pressure transducers used to measure the head loss? (The answer isn't explicitly in the lab manual!)

Data tables:

Table 1 data calculations regarding smooth pipes

Table 2 data calculations regarding rough pipes

References

1. Fluid Mechanics: Fundamentals and Applications, Cengel, Y. and Cimbala, J., Third Edition in. (n.d.). (Edibon AFT User's Manual.)

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