Civil engineering level 1 Fluid Mechanics Lab report (oriphis and free jet flow) - Essay Example

INTRODUCTION Orifice is a term used for a small hole or opening in a vessel which enables the fluid to flow through it. Fluid keeps on flowing through the orifice as long as the head of fluid is above the level of orifice in the vessel. Orifice is not merely a hole but is considered as one of the fundamental equipment used in fluid mechanics as it is widely used to measure discharge rates and velocity coefficients(CodeCogs, 2013). Orifice can have any shape. It can be round, square, oval etc. Also it can be smooth of rough as well.

There are a number of applications for this device. Some of which are listed below(Nally, 2013): Used to increase the pressure of fluid line when needed. Used to restrict flow through the line. Used to control the velocity of fluid within the piping. Extensively used to measure flow rate within the piping. Orifice in the form of orifice plates is widely used for injecting constant flows to the apparatus. They are vital part of electronic fuel ignition systems where metered amount of fuel is injected within the combustion chamber through orifice, as they provide very stable flow.

Orifices used for such critical applications are designed with great effort. They must be smooth enough to reduce frictional effects which greatly affect the efficiency of orifice(Kinseler.com). Also while designing an orifice for a specific flow application, ratio of length vs orifice diameter are strictly monitored. Not only this, orifice is an integral part of jet flows as well which is in fact closely lying to orifice(Sarco, 2000). Measuring the jet flows is really important in calculating the steam acting upon the turbine in the power plant.

It is also integral part of many other chemical plant operations. Orifices are extensively being used in the fields of bioinformatics and botany. Gaseous flow for the processes of respiration and photosynthesis is measured by using orifice. In this case differential pressure at upstream and downstream is calculated and is directly proportional to the flow of fluid out of the orifice(Parkinson and .Day, 1979). This differential pressure is then maintained at certain level to get required results.

THEORY Vena contracta is the reduction of fluid jet area after the fluid emerges out of the orifice. It is the minimum diameter observed in the profile of the jet flow and lies close to the orifice opening along the axis of jet flow. According to the Bernoulli’s equation, the velocity of fluid at vena contracta is given by: Velocity of fluid is also affected by the frictional affects introduced by the reservoir walls and orifice, Reynolds number etc. These are compensated by introducing a constant, , the resulting expression is: In or der to determine the relation between horizontal distance,, and coefficient of velocity, , first the expression horizontal distance is determined as the flow of jet is in horizontal direction if no other force is being applied on it.

Due to the action of gravity, jet flow is in projectile motion and produces a parabolic profile. The effect of gravity is calculated in terms of the vertical displacement, given by: Value of t can be determined as: Substituting value of t in the horizontal displacement equation gives: This Implies, Putting value of v in the velocity equation gives: After simplification, we get the Cv-x relationship: This expression clearly indicates that is the slope of the , curve. This Implies, METHODOLOGY In this experiment we made use of F1-17, orifice and free jet flow instrument by armfield.

The apparatus is provided with a head tank, provided with a baffle plate. Constant head of fluid is maintained within the tank, for which adjustable over flow pipe is provided. Fluid to the tank is supplied by the piping connecting the tank with the hydraulic bench. At the bottom of the tank, required orifice is fitted along the vertical back frame. Needles are provided to track the flow trajectory from the orifice; simply adjusting the needles by unscrewing them. The position of the needles can then be marked on the graph paper pasted behind the needles on the vertical back frame.

Here is the diagram of the apparatus used. Figure 1: F1-17 - Orifice and Free Jet Flow Apparatus The fluid flowing through the orifice is collected in the tank at the end of the trajectory. This tank is shown in figure below: Figure 2: Fluid Collecting Tank The orifices used in the experiment were of 3 mm and 6 mm in diameter. Here is the figure of 3mm diameter orifice with shamble which helps in reducing friction and increases value of coefficient of velocity. Figure 3: Front of Orifice Plate with 3mm Diameter Here, the downstream side of the orifice provided with O-ring to provide better fit.

Figure 4: Back of Orifice Plate with 3mm Diameter We also made use of leveler to ensure accurate and reliable results. Leveler is used to ensure the back frame is perfectly horizontal on the plane according to the instrument. It is actually placed at the base of the back frame as shown in the figure below. Figure 5: Back Frame with Leveler Procedure Leveler was set to level the apparatus. Graph paper was fitted on the back frame. Orifice of required measurement was fixed in the tank bottom. Water was flowing to the tank from the reservoir at a constant rate.

Firstly the overflow pipe was adjusted at the height of 370mm and when the tank gets filled to that head, excess water was overflowed. The baffle plate provided at the base of the tank helps in restricting inlet flow of fluid as well as provides with a constant jet flow. When the fluid was allowed to flow through the orifice, flow profile was marked by first moving the needles down in such a way that their tip was at the center of the jet profile. Top of the needles was marked on the graph paper.

Then the graph paper was removed from the back frame and vertical as well as horizontal distances were calculated from the graph and the results were tabulated. Graph between and was drawn on the graph paper and finally coefficient of velocity was determined by determining the slope of graph. Same experiment was then carried out with the same orifice diameter of 3mm but by changing the head at 280mm. The orifice diameter was changed and reading was also taken at the same heights. CONCLUSION From this experiment we have learned that although the discharge of fluid through orifice satisfies Bernoulli’s equation but we need to consider the frictional effects and turbulence of the fluid on the upstream side of the orifice.

Therefore, coefficient of velocity was added to the velocity equation in order to remove the difference between calculated and observed readings. Also, if we keenly observe the tabulated data it is clear that variation in the head of fluid and orifice diameter strongly affects the coefficient of velocity. It is said that for a sharped edge orifice, the value of coefficient of velocity is about 0.98 percent(Lienhard, 1984) but our results does not fully satisfy this statement. With the orifice of 3mm and head set at 370mm, value of turns out to be 0.

919 which shows that the frictional effects and turbulence on the upstream side is high. Which increases more as the head was set to 280mm. But with the orifice of 6mm value of was closer to the expected one at 370 mm but much variation was observed at 280mm. Also with the reduced head, pressure on the fluid reduces and it also results in reduction in the value of and velocity as well. Also the variation in the orifice diameter directly affects the velocity of flow. With the reduced diameter, velocity of flow is much high and as a result value of decreases.

References CODECOGS. 2013. Orifice [Online]. Zyba Ltd. Available: http://www.codecogs.com/library/engineering/fluid_mechanics/orifice/index.php. KINSELER.COM. Orifice Theory. Available: http://www.kinsler.com/Cat_32_Pgs/Cat_32_4_09_Pg_202.pdf. LIENHARD, J. H. 1984. Velocity Coefficient for Free Jets from Sharp-Edge Orifice. Journal of Fluids Engineering, 106. NALLY, M. 2013. Approximate Flow Through an Orifice [Online]. Available: http://www.mcnallyinstitute.com/13-html/13-12.htm. PARKINSON, K. J. & .DAY 1979. The Use of Orifices to Control the Flow Rate of Gases.

Journal of Applied Ecology, 16, 623-632. SARCO, S. 2000. Orifice Plate Flowmeters for Steam, Liquids and Gases. Available: http://www.spiraxsarco.com/pdfs/sb/s66_11.pdf.