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Airflow and Venturi Experiment Report

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Airflow and Venturi Experiment Report November 29th 2005 Airflow and Venturi Experiment Report Abstract While investigating air flow through a pipe, mass flow rate was to be calculated using two methods. One method was to measure the pressure difference of the flow across a venturi (a narrowing of the pipe) and the other was to measure the pressure at varying radii from the pipe's centreline to the pipe's wall using a pitot tube in the flow, with the recordings used to plot a velocity profile graph. The calculations showed that each of the two methods produce an answer for the mass flow rate that is within 10% of the other, despite the methods being very different in terms of their accuracy, ease and requirement of resources. It was found that for practicality the venturi method of calculating the flow rate was preferred over the pitot method and also that is was the more accurate of the two methods. It was also found that air velocity increases with distance from the pipe walls and decreases with the radius of the pipe. Contents Introduction Page 2 Background and Theory Page 3 Apparatus Page 6 Raw Results Page 7 Calculations Page 8 Analysed Results Page 11 Discussion Page 14 Conclusions Page 15 References Page 16 Introduction An experiment was performed in the aerospace laboratory to investigate air flow through a pipe. Air was moved through the pipe at an unknown constant speed using a vacuum pump and the calculation of mass flow rate of the moving air was to be performed by two different methods. The first method was to measure the pressure difference across a venturi - a narrowing of the pipe. The diameter of the pipe before and after the venturi was known. From this the velocity of the fluid could be calculated from which the mass flow rate can be easily calculated. The second method was to traverse a pitot tube across the pipe and calculate the pressure difference at varying diameters from the centre of the pipe. ...read more.


Once the manometer had settled around a particular value, a reading from the visible leg of the manometer against its scale was taken and recorded. The pressure tap was then set so the manometer was reading the pressure difference between the pitot tube and the pipe boundary. The pitot tube was traversed from the pipe centreline to the pipe boundary at intervals of 2mm. The paraffin level in the manometer was recorded as before for each value. The pitot was then returned to the centreline and the process repeated. The entire procedure was then repeated when the pump was set at a medium flow rate and once more at a low flow rate. The pump was then turned off and the equipment returned to its original state. Raw Results These are the recordings taken from the three experiments. High Flow Rate Pitot: figure 3 Venturi: ?H=0.24 mParaffin Medium Flow Rate Pitot: figure 4 Venturi: ?H=0.17 mParaffin Low Flow Rate Pitot: figure 5 Venturi: ?H=0.087 mParaffin Atmospheric Observations Atmospheric pressure = 769.25 mmHg Temperature on the Celcius scale = 20 K Calculations Thermodynamic temperature T /K = (273.15 + ? deg C) (eqn. 8) = (273.15 + 20) T = 293.15 K Density of Air: where V = Volume PV = nRT and ? = n / V So, ? = P / RT (reference 2) = 769.25 mmHg / 287.05 * 293.15 = 769.25 * 133.322 / 287.05 * 293.15 (reference 3) ?air = 1.218770336 kg m-3 Pipe Cross-sectional Areas: r1 = 1/2 * 108.3*10-3 = 54.15*10-3 m A1 = ? * (54.15*10-3) = 9.211848665*10-3 m2 r2 = 1/2 * 29.65*10-3 = 14.825*10-3 m A2 = ? * (14.825*10-3) = 6.904611969*10-4 m2 Sample Calculations Mass flow rate in the venturi using the results from the experiment with the high flow rate: " V1 = 2 ?paraffin g ?H " ?air A1 2 - 1 A2 (eqn. ...read more.


* The fact that the use of a pitot tube in the air flow causes turbulence which can affect the velocity * The fact that the pitot tube itself has dimensions and so it can never measure the air pressure at the pipe's wall * Varying atmospheric conditions in the laboratory * Plotting the graph incorrectly or errors in estimating the trend of the graph for the pitot method Conclusions If it is assumed that both methods lead to the correct value of the mass flow rate, then it must be concluded from this experiment that the venturi method is more accurate than the pitot method because it relies on fewer measurements and resources and the method does not involve any estimation. The venturi method is the better of the two because: * The venturi method requires fewer readings to be taken and calculations to be made than the pitot method leaving less scope for error and taking less time * The calculations required for the venturi method rely on fewer mathematical skills than those for the pitot method, which requires an understanding of differential calculus. This means that the venturi method leaves less room for mistakes to be made * Measuring the traversal of the pitot by eye could be inaccurate since the distance moved is minute * A venturi can be left permanently as a physical feature of a pipe since it requires no later user intervention - a pitot tube needs to be adjusted as measurements are taken. Performing these experiments has allowed me to conclude that: * Air velocity increases with distance from the pipe walls. This shows the existence of some type of turbulence at the wall which interferes with the fluid flow. * Air velocity decreases with the radius of the pipe. At the same flow rate, air moving through a wide pipe will have a lower velocity than air moving through a narrower one. ...read more.

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