Multi-bladed Pumps. Does the number of propellor blades affect the efficiency of a water pump?

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Pumps & Physics

Research and Rationale

What's new?

When I was thinking about which aspect of physics to investigate for my investigation, I knew it was a good idea to choose something that really interested me. At the time I was becoming more and more fascinated by subatomic particles. I liked the fact that much of it was new and not understood properly, unlike the classical physics that everyone associates the subject with. Unfortunately, high energy physics does not translate into good practical coursework. However, while reading Six Easy Pieces, a book adapted from Richard Feynman's famous textbook The Feynman Lectures on Physics, I noticed that a very common everyday phenomenon is still not properly understood by physicists. Encouraged by the prospect of discovering something new, I read on.

Chaotic ideas

Feynman wrote (on page 66) "There is a physical problem that is common to many fields, that is very old, and that has not been solved...It is the analysis of circulating or turbulent fluids...No-one can analyse it from first principles"

"Wow - something science can't explain" I thought.

I looked on the internet for further details and I found a poster from World Maths Year 2000 (http://www.newton.cam.ac.uk/wmy2kposters/march/), showing just the type of unpredictable fluid motion that Feynman was writing about. It's a new and exciting branch of maths called chaos theory and it is just beginning to be understood mathematically. The main idea is that simple systems can show very complicated behaviour that seems to have no repeating pattern. The sums that describe these systems are difficult to get your head round and appear to be way beyond my abilities as an A-Level maths student.

Despite this, I felt something chaotic was an excellent phenomenon to look into for this task - it's a chance to do some experimental work where there isn't a perfect formula or a flawless explanation in any textbook. I couldn't rely on distorting my results to fit a simple law, so my experimentation had to be rigorous.

Limitations

It was important to find a subject that was practical to investigate at school. While I was watching water swirl down the drain as I filled the kettle at home, I wondered how widely-used machines like ship's propellers cope with the unpredictable world of chaos. Propellers have an unusual and distinctive shape designed to reduce turbulence. I wanted to investigate why this particular shape works so well - and if it can tell us anything about turbulent flow. Conveniently, water and propellers are easy-to-use in school labs (or so I thought!).

Best of all, I thought, if I could model the situation but ignore the effect of turbulent water, I could look at the mechanics of the propeller, and then compare the theory with what happens in real life. It seemed like a good mix of fresh ideas and traditional physics problems.

I talked about my plans to some of my teachers and one of them mentioned that his son had done a PhD degree in the formation of bubbles by marine propellers - an effect called cavitation. This encouraged me to continue with this project, knowing that it relates to current areas of research and is an important and worthwhile topic.

Research

It turns out that one of the most interesting applications of pumps is in fire engines. As fire services are public organisations they make available plenty of high-quality, free information online. Engineering sites were also useful.

* The Physics Behind Firefighting

American high-school physics project

http://ffden-2.phys.uaf.edu/212_fall2003.web.dir/Matt_Taylor/Matt1.dwt

* How Fire Engines Work

General information

http://science.howstuffworks.com/fire-engine.htm

* Bedfordshire & Luton Fire and Rescue Service

My local fire brigade, who I actually went to visit to find out more

http://www.bedsfire.gov.uk/index.htm

* American Turbine: Pump Calculations

Web-based program for working out quantities in pumping

http://americanturbine.net/formulacalc/pump.htm

* Impeller Design

The engineering that goes into pumps

http://homepage.mac.com/mrbach/mixdesign.htm

* Firefighting.com

Useful data on pumps but uses frames so I can't give a full URL

http://www.firefighting.com

* How Things Work

A simple explanation of propellers and aerofoils

Lesley Firth, Kingfisher, 1983 p13

* The Physics of Firefighting

Some simple principles explained

Physics Teacher, vol 28, p 599

* Firefighting

Contains a bit of physics but interesting background information

Jack Gottschalk, Dorling Kindersley, 2002, ISBN: 0789489090, p128

* Go with the flow

Article about modelling granular and fluid motion

New Scientist, 2 August 2003, p38-39

Preliminary Experiments

I wanted to find the most efficient propeller design. From research I found out that propellers have different shapes for different tasks, so my first goal was to get a propeller up and working, and then look at what I could change to make it run more efficiently.

These are the variables I aimed to evaluate for their effect on power transfer efficiency in preliminary tests:

* The speed of rotation

* The size of the propeller

* Since speed of rotation is less time consuming to collect data for, I'll look at it first. I intend to plot a graph of speed of rotation vs. output flow rate.

Considering the shape of a ship's propeller, I expected to be looking at these variables later on:

* The number of blades on the impeller

* The shape of the blades

* The orientation of the blades (what angle they are in relation to the axis of rotation)

The physics principles that are important here are mechanical ones. The efficiency of the propeller depends on how much of its power goes into pushing water outwards and how much is wasted on heating the water up or causing it to form whirlpools.

New Scientist's article Go with the flow mentioned the Bernoulli Effect, which is observed on aircraft wings and on propeller blades.

Lower pressure

Higher pressure

A blade with a curved plane and a flat plane forces some air or water on a longer route over the curve, and the rest takes the shorter flat route. The longer journey over the curved plane causes a drop in pressure, which translated to lift in planes, and thrust in propellers.

According to all the textbooks, the optimum number of blades, the blade angle, the speed of rotation and the size of propeller all contribute to the efficiency. It seems like I've got my work cut out for me. I'm going to concentrate on rotation speed and its effect on water flow rate outwards. Let's see what the preliminary tests show.

Water flows in

Axle

Propeller

Watertight casing

Water flows out

Planning

Risk Assessment1,2

Apparatus or procedure

Hazard

Precautions

All apparatus

Accident or fire

Supervise the experiment at all times and clear away at the end of the session. Store all equipment safely and securely.

Boiling water for shaping polypropene propellers

Risk of scalding

Take care with boiling water, paying attention at all times. Stand well back from the saucepan and do not move it while the water is hot. Use a heat-insulating towel to manipulate the hot polypropene.

Electric circuit in general

Risk of fire from short circuiting etc.

Use insulated wires, keep connections clean and dry, and always supervise the apparatus while current is flowing.

Do not leave the set-up unattended without unplugging the mains supply.

Use wires of appropriate diameter to prevent overheating resulting in fire.

Rapidly rotating propeller

Possibility of injury from contact with rotating blades of propeller

Leave motor switched off until ready to record data. Take care to keep your distance from the propeller, especially fingers.

Heavy equipment (power pack, retort stands)

Falling equipment could injure

Ensure stands etc. are sturdily placed and avoid placing equipment near the very edge of the work bench.

Power pack

Output: 13V 5A DC
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Input: 230V mains AC

Risk of electrocution from mains input

(risk of injury from output voltage is minimal)

Keep power pack away from the wet part of the apparatus (to prevent conduction through water). In my experiment, I will keep all the electrics on a shelf above the level of the water-containing apparatus.

Ensure all water-containing equipment is as waterproof as possible, and have towels to hand to soak up spills.

Do not leave the set-up unattended without unplugging the mains supply.

Preliminary findings

In the research and rationale section, I ...

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