Faculty of Engineering [PDF]

Zitiervorschau

Faculty of Engineering Alexandria University Mechanical Engineering Department



Submit to: Dr. Hamady Kandil Prepared by: Hafez Samy Mohamed Ghallab Section: 3 Seat number: 123 Date: 5/12/2017

Table of Contents Introduction: ..................................................................................................................... 3 Describe: ........................................................................................................................... 3 Theory of operation : ......................................................................................................... 4 Aircraft: ............................................................................................................................. 7 Applications of pitot tube: ................................................................................................. 8 Advantages of pitot tubes: ................................................................................................ 8 Disadvantages of pitot tubes: ............................................................................................ 9 Conclusion: ........................................................................................................................ 9 References: ..................................................................................................................... 10 Figure 1 ................................................................................................................................................... 3 Figure 2 ................................................................................................................................................... 4 Figure 3 ................................................................................................................................................... 6 Figure 4 ................................................................................................................................................... 8



Introduction: Pitot tubes can be used to indicate fluid flow velocities by measuring the difference between the static and the dynamic pressures in fluids. Basically, a pitot tube is used in wind tunnel experiments and on airplanes to measure flow speed.

Figure 1

Describe: It's a slender tube that has two holes on it. The front hole is placed in the airstream to measure what's called the stagnation pressure. The side hole measures the static pressure. By measuring the difference between these pressures, you get the dynamic pressure, which can be used to calculate airspeed. On an airplane, the pitot tube can be mounted in a number of ways, including jutting out from the edge of the wing or sticking up from the fuselage.

Figure 2





Theory of operation: The basic pitot tube consists of a tube pointing directly into the fluid flow. As this tube contains fluid, a pressure can be measured; the moving fluid is brought to rest (stagnates) as there is no outlet to allow flow to continue. This pressure is the stagnation pressure of the fluid, also known as the total pressure or (particularly in aviation) the pitot pressure. The measured stagnation pressure cannot itself be used to determine the fluid flow velocity (airspeed in aviation). However, Bernoulli's equation states:

Stagnation pressure = static pressure + dynamic pressure Which can also be written Solving that for flow velocity:

NOTE: The above equation applies only to fluids that can be treated as incompressible. Liquids are treated as incompressible under almost all conditions. Gases under certain conditions can be approximated as incompressible. where: •

is flow velocity to be measured in m/s;

•

is stagnation or total pressure in pascals;

•

is static pressure in pascals;

•

and

is fluid density in

.

The dynamic pressure, then, is the difference between the stagnation pressure and the static pressure. The dynamic pressure is then determined using a diaphragm inside an enclosed container. If the air on one side of the diaphragm is at the static pressure, and the other at the stagnation pressure, then the deflection of the diaphragm is proportional to the dynamic pressure. In aircraft, the static pressure is generally measured using the static ports on the side of the fuselage. The dynamic pressure measured can be used to determine the indicated airspeed of the aircraft. The diaphragm arrangement described above is typically contained within the airspeed indicator, which converts the dynamic pressure to an airspeed reading by means of mechanical levers. Instead of separate pitot and static ports, a pitot-static tube (also called a Prandtl tube) may be employed, which has a second tube coaxial with the pitot tube with holes on the sides, outside the direct airflow, to measure the static pressure.[4] If a liquid column manometer is used to measure the pressure difference Pt – Ps, or ∆p.

where: •

∆h is the height difference of the columns in meters.

•

is the density of the liquid in the manometer;

•

g is the acceleration due to gravity in



Therefore,



Figure 3







Aircraft: A pitot-static system is a system of pressure-sensitive instruments that is most often used in aviation to determine an aircraft's airspeed, Mach number, altitude, and altitude trend. A pitot-static system generally consists of a pitot tube, a static port, and the pitot-static instruments. Errors in pitot-static system readings can be extremely dangerous as the information obtained from the pitot static system, such as airspeed, is potentially safety-critical. Several commercial airline incidents and accidents have been traced to a failure of the pitot-static system. Examples include Austral Líneas Aéreas Flight 2553, Northwest Airlines Flight 6231, and one of the two X-31s. The French air safety authority BEA said that pitot tube icing was a contributing factor in the crash of Air France Flight 447 into the Atlantic Ocean . In 2008 Air Caraïbes reported two incidents of pitot tube icing malfunctions on its A330s. Birgenair Flight 301 had a fatal pitot tube failure which investigators suspected was due to insects creating a nest inside the pitot tube; the prime suspect is the Black and yellow mud dauber wasp. Aeroperú Flight 603 had a pitot-static system failure due to the cleaning crew leaving the static port blocked with tape.

Figure 4

Applications of pitot tube: The pitot tube is used in utility streams where high accuracy is not necessary. It is used in the air duct and pipe system. It is used in aircraft to measure air flow velocity. They are used for mapping flow profile in a channel or duct.

Advantages of pitot tubes: Economical to install Do not contain moving parts; this minimizes the frictional loss. Easy to install. Due to its small size, It can introduce to fluid flow without shutting down the flow. Loss of pressure is very small. Can be easily installed in extreme environment, high temperature and pressure conditions. Some types can also be easily removed from the pipeline.

Disadvantages of pitot tubes: Low sensitivity and Poor accuracy. It requires high-velocity flow. Not suitable for dirty or sticky fluid like sewage disposal. Sensitivity disturbed by flow direction. Pitot tubes have found limited applications in industries because they can easily become clogged with foreign materials in the liquid. There is no standardization for pitot tube. Change in velocity profile may cause significant errors. Due to change in velocity profile, it develops a very low differential pressure which is difficult to measure.

Conclusion: Pitot tube is one of the most important discoveries that has helped the world in many fields. It is an invention of hundreds of years and is used so far.



References: 1. Pitot, Henri : http://gallica.bnf.fr/ark:/12148/bpt6k35294.image.f543. langFR 2. Darcy, Henry : http://gallica.bnf.fr/ark:/12148/bpt6k408489d.image.f3 54 3. https://books.google.com.eg/books?id=0SkDAAAAMBAJ &pg=PA116&dq=Popular+Science+The+lads+who+fly+Br itain%27s+1944&hl=en&sa=X&ei=UHyfT6aEIYOs9ATwh8 C1AQ&redir_esc=y#v=onepage&q&f=true 4. http://www.mecholic.com/2017/05/applicationsadvantages-limitations-pitot-tube.html 5. https://www.engineeringtoolbox.com/pitot-tubesd_612.html