ME 333 Fluid Mechanics. Lab Session VISCOUS LOSSES IN PIPES

Transcription

1 ME 333 Fluid Mechanics Lab Session VISCOUS LOSSES IN PIPES Introduction Flow in pipes, laminar or turbulent, is subject to pressure losses that result from the viscous stresses on the wall of the pipe. These losses are dependent on the Reynolds number of the flow and the surface roughness of the pipe wall. This phenomenon is described in terms of the friction factor (non dimensional pressure loss per unit length of the pipe) in Moody s diagram (See back of the handout). Losses resulting from complex flow patterns appearing at some elements of piping systems, such as valves, expansions, contractions, elbows, etc., can be described in similar terms by means of a concentrated loss given by a non dimensional loss factor K. Objectives The objective of this lab session is to experimentally measure the pressure losses along a section of pipe containing a 90 turn, for different values of the Reynolds number. The different losses associated with different elements will be studied by measuring the pressure loss with a round elbow in the system and with a capped T fitting, which simulates a 90 sharp corner. This fitting can be positioned in two different ways, both of which will be measured. Experimental Procedure Before you begin the experiment make sure you understand the experimental setup, the instruments you are going to use in the measurements and what you need to do in order to carry out the experiment. Look at figure 1 and identify the different elements in the actual experiment. Follow the procedure: 1. Make sure the gate and ball valve under the table are fully open. 2. Turn on the system (main switch and manometer) and allow the system to warm up with the round elbow in the system. During this time, measure and record on your lab notebook: - barometric pressure - inside diameters of the pipe, elbow and capped T - distance between adjacent taps along pipe A, as well as the distance l 1 and l 2, defined in figure Leave the ball valve open in pipe A (valve A) and make sure the ball valve in pipe B (valve B) is fully closed. Adjust the flow rate with the gate valve under the table. The

2 flow rate is measured by the laminar flow meter and the inclined manometer (in mm H 2 O). Start the experiment by closing the ball valve under the table (bleeding air) and setting a flow rate of 50 mm H 2 O. Record the overall pressure drop between points 1 and 2 at the beginning and the end of the pipe loop. The reading from the electronic manometer is in mm Hg. Record also the temperature in the flow from the electronic display of the thermocouple unit. 4. Set the reference in the electronic manometer to the pressure at point R 1 and record the readings of the pressure drop between the different stations, 2 through 11, and the reference R Repeat steps 3 and 4, setting the flow rate to 20 mm H 2 O (open the ball valve under the table to bleed some air before closing the gate valve to reduce the flow rate. You don t want to choke the compressor.) 6. Repeat steps 3 to 5 replacing the round elbow with the capped T in both configurations showed in figure 1, T 1 and T 2. Measure the pressure drop in both configurations for the two different flow rates, 50 and 20 mm H 2 O. 7. When you are finished, turn of the power and leave everything as you found it.

3 Report Each group will present a single report with all the data and analysis. You can split the work but you are all expected to know what is in the report and to understand it well enough to explain it to others as if you had done it. Plot the pressure drop measurements as a function of linear distance from the reference R 1 for the different experimental conditions. Consider the elbow as a concentrated loss (that is of zero length). The distance between station 7 and 6 will then be just l 1 +l 2. Use a minimum squares fit to the data, together with equation 6.10 from White s Intro to Fluids book, 7 th Ed., or equivalent from Smits book, to calculate the friction factor in the pipe. Use the six independent measurements for the same flow rate to estimate the error. You can use Moody s diagram or equation 6.48 from White s Intro to Fluids book, 7 th Ed., or equivalent from Smits book) to calculate the roughness of the aluminum pipe. The pressure loss at the elbow can be measured from the jump between stations 6 and 7, discounting the loss due to the lengths l 1 and l 2. The expression to calculate the loss factor can be found in equation 6.74 from White s Intro to Fluids book, 7 th Ed., or equivalent from Smits book. You should estimate which measurements are more accurate and what are the sources of error. This is an important part of the report, not a little afterthought. The report should consists at the least of the following sections: Introduction Experimental setup Results (Raw data and calculations) Discussion Conclusions References You can include Appendices with any information you think is relevant but would not fit well in the other sections of the report, or would it make it difficult to read because is too lengthy or includes too much detail.

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