الثلاثاء، 9 نوفمبر، 2010

1)      In the turbulent flow , from the previous plot (Figure 6 ) we notice that there is no relation between Reynolds number and the loss coefficient .
The values of loss coefficient K for various types of valves and fittings are tabulated in Figure(7) ,we compare our experimental ‘k’ values with these found as the flowing:
1)   
  
From the previous comparison Table we used Figure (9) by dividing the diameter of the pipe over the diameter of enlargement and contraction (D1/D2 =0.2 ). We find that our experimental values are slightly different ( about 50% ) than the values from figure (9).
From the previous comparison table,(15). we can clearly notice that the loss coefficient values for the Gate valve 25% open are very close to the value from Figure 7 ( about 4% different) . While the loss coefficient values for Gate valve 50% open differ from the values in Figure 7 ( about 50% different ) .
3)       For standard Elbow 90ο
From Figure 7 , we found that the loss coefficient for the standard elbow equal 0.75 , and our average experimental value was 0.2034 , which is very different than 0.75 .
4)       Meter and Short bends
For these two types of fitting we didn’t find  enough resources ,that provides the loss coefficient K .   
As we notice in the previous comparison and when we compare our results with the values in Figure (7) and (9), we got different values of  loss coefficient K, because in reality the value of K varies with the size of the fitting , the level of turbulence (Reynolds number ), and the dimension of the fitting ,such as the diameter and radius for curvature[4].

energy loss coefficent for different fittings and some common valves

  • skin friction due to the roughness in the inner part of the pipe where the fluid comes in the contact of the pipe material
  • Form friction due to the obstructions present in the line of flow, it may be due to a bend or a control valve or anything which changes the course of motion of the flowing fluid.
Our experiment will show the effect of introducing bends and fittings into a fluid flow and find the friction losses due to different fittings.
An important and widely used equation that is used to calculate the head loss due to friction within a given length of pipe is the Darcy-Weisbach equation [2].

The energy loss which occurs in pipe fitting is commonly expressed in terms of head loss (h, meters) in the following form:


Where k= the loss coefficient and v = mean velocity of flow into the fitting.
The coefficient (k) is usually determined by experiment due to the complexity of the flow in many fittings. When getting two manometer readings, the head loss can be calculated, then (k) for the pipe fitting experiment can be determined as




Enlargement and contraction causes changes in the pipe cross-sectional area, which additionally causes change in the static pressure through the system. This change can be calculated as


K can be considered to be independent of both friction factor and Reynolds Number because they are related to the pipe friction. consequently, K is treated as a constant for any given valve or fitting .we can get K from the following equation:


To remove the affects of this area change on the measured head losses, this value should be added to the head loss reading for the enlargement, and subtracted from the head loss reading for the contraction.
For the gate valve experiment, pressure difference before and after the gate is measured directly using a pressure gauge. This can then be converted to an equivalent head loss using the equation:
1 bar = 10.2 m water
The loss coefficient may then be calculated as above for the gate valve.
Friction loss can have many applications; one of the most common is in the realm of firefighting. With the advent of modern power-takeoff (PTO) fire pumps, pressures created can sometimes overwhelm the ability of water to flow through a hose of a given diameter. As the velocity of water inside a hose increases, so does the friction loss. This resulting increase occurs as an exponential rate, thus an increase in the flow by a factor of X will result in an increase in friction loss by a factor of X2. For example, if you double the flow you will quadruple the friction loss. Ultimately, as the pressure created by a fire pump goes higher and higher the amount of water actually flowing through a hose to a given point lessens, threatening firefighting operations.[