Home Random Page


CATEGORIES:

BiologyChemistryConstructionCultureEcologyEconomyElectronicsFinanceGeographyHistoryInformaticsLawMathematicsMechanicsMedicineOtherPedagogyPhilosophyPhysicsPolicyPsychologySociologySportTourism






Determination of Viscosity of Fuels

 

Theoretical information

 

Viscosity is a measure of a liquid’s resistance to flow under pressure, generated by either gravity or a mechanical source. “Thin” liquids, like water or gasoline, have low viscosities; “thick” liquids, like sugar syrup or motor oil, have higher viscosities. The viscosity of a liquid increases as its temperature decreases.

Viscosity describes a fluid's internal resistance to flow and may be thought of as a measure of fluid friction.

There are two related measures of fluid viscosity - known as dynamic (or absolute) and kinematic viscosity.

Dynamic viscosity μ is the resistance to flow encountered when one layer or plane of fluid attempts to move over another identical layer or plane of fluid at a given speed. Dynamic viscosity is also called absolute viscosity.

In the SI system the dynamic viscosity units are N·s/m2, Pa·s or kg/m·s.

The other quantity called kinematic viscosity (represented by the symbol ν "nu") is the ratio of the viscosity of a fluid to its density.

 

The SI unit of ν is m2/s. This unit is so large that it is rarely used. A more common unit of kinematic viscosity is the square centimeter per second [cm2/s], which is given the name stokes [St]. Even this unit is also a bit too large and so the most common unit is probably the square millimeter per second [mm2/s] or centistokes [cSt].

Kinematic viscosity is a measure of the resistive flow of a fluid under the influence of gravity. It is frequently measured using a device called a capillary viscometer — basically a graduated can with a narrow tube at the bottom. When two fluids of equal volume are placed in identical capillary viscometers and allowed to flow under the influence of gravity, a viscous fluid takes longer than a less viscous fluid to flow through the tube.

1. Jet fuel is exposed to very low temperatures both at altitude – especially on polar routes in wintertime – and on the ground at locations subject to cold weather extremes. The fuel must retain its fluidity at these low temperatures or fuel flow to the engines will be reduced or even stop.

2. Jet fuel at high pressure is injected into the combustion section of the turbine engine through nozzles. This system is designed to produce a fine spray of fuel droplets that evaporate quickly as they mix with air. The spray pattern and droplet size are influenced by fuel viscosity. If it is too high, an engine can be difficult to relight in flight. For this reason, jet fuel specifications place an upper limit on viscosity.

3. Fuel viscosity influences the pressure drop in the fuel system lines. Higher viscosities result in higher line pressure drops, requiring the fuel pump to work harder to maintain a constant fuel flow rate. Fuel viscosity also influences the performance of the fuel system control unit. It can also significantly influence the lubricating property of the fuel that, in turn, can affect the fuel pump service life.



4. At the same time, if the viscosity of a fuel is too low, it can produce an oversaturated air-fuel mixture in the engine causing a decrease in chemical completeness of combustion due to a lack of oxygen.

 

Fuel viscosity is determined using capillary viscometers at 20 º C. In the experiment the time required to discharge a certain volume of fuel through calibrated capillary is measured.

Fig.2. Viscometers

 

Viscometer (for low viscous liquids) (Fig. 2) consists of a U-shaped glass tube connecting a lower glass bulb with an upper glass bulb. One part of the U-tube is a capillary tube; The lower bulb is first filled with a liquid warmed to the desired temperature. The liquid is then sucked up through the upper bulb upto the mark M1 above it. The viscometer is then immersed in a water bath at the desired temperature and the time taken by the liquid to descend from M1 to the mark M2 on the capillary is noted.

 

Test Procedure

The thermostat is set at 20 ± 0,01 º C and maintains this temperature through the test. To determine the kinematic viscosity of fuels use a clean, dry viscometer with a numerical coefficient in the range 0,003-0,005 mm2/s2.

A fuel sample is filtered through a sieve, glass or paper filter. If there is water present in fuel, dewatering is conducted with anhydrous sodium sulfate or table salt, and the sample is filtered through a paper filter.

Viscosimeter is filled with a tested fuel and set to the thermostat so that the fluid level was a few inches above the upper extension of the viscometer.

Viscometer filled with the fluid is kept in the thermostat for 30 minutes. Then, the time of efflux of the fluid through a capillary of viscometer from M1 to the mark M2 is measured.

 

Kinematic viscosity of a fuel (ν) measured in mm2/s or cSt is calculated by the following formula:

C – is a viscometer constant, mm2/s2; - is arithmetic mean of efflux times measured, s.

The results of calculation are round to tenth.

In order to obtain correct results, it is needed to conduct two filling of viscometer. An efflux time is measured three times with every filling.

Calculate dynamic viscosity of the fuel.

 

Conclusions

1. State kinematic and dynamic viscosity values.

2. Is kinematic viscosity of the fuel tested in compliance with the standard for this fuel? If no, explain why. What should this value be?

3. What is Poise and Stokes?


Date: 2015-12-11; view: 1368


<== previous page | next page ==>
Illimité émotionnel | World Tour: North America
doclecture.net - lectures - 2014-2024 year. Copyright infringement or personal data (0.007 sec.)