Fluid Flow Measurements

Transcription

1 Fluid Flow Measurements Introduction The flow of fluid through a network of pipes has many engineering applications. Systems such as the heat exchanger, turbine, heat pump, air conditioner, and experimental test section of a wind tunnel are a few examples. The proper selection and use of experimental instrumentation used to characterize the flow of a fluid is a critical part of the overall design verification process. Furthermore, the measurement of flow properties in a process is often required to produce and control a desired product. The objective of this experiment is to introduce a means of determining the flow of fluid in a pipe. Dynamic pressure measurements will be made using a differential pressure transducer, total head tube, and a static pressure port. Calibration of the pressure transducer will be accomplished using an incline manometer. Mean velocity measurements will be integrated over the cross sectional area of the pipe resulting in the flow rate. A turbine flow meter will used to verify results obtained from the pressure transducer. The angular dependence of the total head tube will also be examined. Pre-Lab Assignment 1. A Pitot-static tube measures the stagnation and static pressure of air at 8.5" and 1.5" of H2O respectively. What is the velocity of the fluid at the measurement point in units of feet/second assuming standard atmospheric conditions? 2. A gage pressure transducer with a sensitivity of 0.01 volts/inch of H 2 O is connected to an open vessel containing air. The output voltage from the transducer is 0.53 volts. When the vessel is closed and pressurized the output from the transducer increases to 5.5 volts. Calculate the pressure in the vessel in psig. 3. The velocity profile across a pipe with an ID of 2.5 inches will be measured in this experiment. In order to efficiently measure the velocity profile and calculate the volume flow rate, a measurement grid should be developed prior to conducting the experiment. Select one of the two grids in figure 2 while discussing the reason why the grid pattern was chosen. 4. Read sections 9.1 and 10.2 of Wheeler and Ganji 2 nd Ed. 1

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3 Assignment #1 Transducer Calibration Calibration of the pressure transducer is required prior to making measurements. The transducer being used is a differential pressure transducer capable of sensing two pressures simultaneously while generating one output voltage which is proportional to the difference between pressures being measured. Calibration of the pressure transducer will be accomplished using a medium of air blown through a 2.5" ID tube. The flow of air will be controlled using a variac attached to a blower. As the flow is increased the corresponding pressure increase is measured with an incline manometer. Refer to figure 3 for a schematic representing the calibration setup used in this experiment. Manometer Variac Traverse Low High Transducer Switch Flow Pitot Tube Multimeter Solid Thick Line Flow Dash Thick Line--Signal Blower Flow Meter Figure 3 Transducer Calibration Safety Considerations The blower is a rotating piece of machinery capable of causing a severe injury. Use caution when operating the variac and blower. Keep fingers and loose clothing away from the blower intake. Do not look into the flow without eye protection! Procedure 1. Connect the transducer BNC output located on the power supply to the input of the multimeter. Switch the button to on the switch box. The pressure transducer generates a direct current output. 2. Turn the pressure transducer power supply on. After the signal stabilizes, record the offset voltage generated with both pressure ports open to the atmosphere. 3. Using the adjustment knob on the bottom right side of the incline manometer, adjust the fluid to the zero position. 3

4 4. Connect the total head tube to the manometer and high pressure port of the pressure transducer using the tygon tube and tee connector supplied. Allow the manometer and pressure transducer low pressure ports to vent. 5. Set the traverse to 42.8 cm. The probe should be aligned with the axis of the 2.5" ID tube 6. To prevent damage to the turbine flow meter, turn the blower up slowly. Set the variac to approximately 50 and allow the system to stabilize. Record the output voltage and pressure indicated on the manometer. Repeat this process varying the speed of the blower while taking measurements at enough intervals to properly calibrate the transducer. 7. Adjust the variac to zero. Record the output voltage and pressure indicated on the manometer. Disconnect the tygon tubing. Assignment #2: Velocity Profile and Flow Rate Measurements The calibrated pressure transducer may now be used to measure the velocity profile across the pipe. The transducer will be connected in the differential pressure mode allowing the dynamic pressure to be sensed directly. As a means of verifying the flow rate determined from the velocity profile and integration method, a turbine flow meter will be used simultaneously. The turbine flow meter generates an output frequency that is proportional to the flow rate. External signal conditioning is used to convert the frequency to a linear direct current output. Procedure 1. Connect the total head tube to the high port of the pressure transducer and the static pressure tap to the low port of the pressure transducer. Refer to figure 4 for proper equipment configuration. Manometer Variac Traverse Low High Transducer Switch Flow Pitot Tube Multimeter Solid Thick Line Flow Dash Thick Line--Signal Blower Flow Meter Figure 4 Velocity Profile Measurement 4

5 2. Set the traverse so that the total head tube just touches the bottom of the Plexiglas tube. Be careful not to bend the total head tube by forcing it against the bottom of the pipe. 3. Switch the button to Flow on the switch box. The pressure transducer output should be connected to the digital multi-meter. 4. Set the blower to produce an intermediate to high flow rate. After the system has reached steady state, measurements can be made. 5. Record the output voltage from both transducers. The flow meter output should be used to verify that the flow rate has remained constant throughout the velocity profile measurement. 6. Adjust the total head tube using the grid pattern that was chosen as part of the pre-lab assignment. Measure the output voltage from the pressure transducer at each position along the grid. Verify that the turbine flow meter output remains constant. 7. Return the probe to the 42.8 cm position. 8. Bring the blower speed down and shut it off. Assignment #3: Total Head Tube Angular Dependence The orientation of either the total head tube, or Pitot-static tube with the streamlines of the flow is of great importance for making accurate flow measurements. It should be evident that if the axis of a total head probe is oriented perpendicular to the streamlines, static pressure will be measured. However if the probe is aligned with the streamlines, stagnation pressure is measured. An adjustment of the probe between these two positions will result in a component of the resultant velocity vector being measured. Therefore, the determination of the tubes angular dependence relative to the streamlines is necessary in order to predict the limit of accurate measurements as a result of probe orientation. Procedure 1. Connect the total head tube to the high pressure port the pressure transducer. The low port on the pressure transducer should vent to the atmosphere. 2. Adjust the variac to produce an intermediate to high flow rate. 3. Record the output voltage from the pressure transducer and the corresponding angular position. 4. Vary the angle of the total head tube. Record the angle and output voltage. 5. Repeat step #4 as many times as necessary to determine the angular dependence of the total head tube. 6. Shut off the variac (blower), transducers, and all bench power. 5

6 Post-Lab Analysis 1. Determine the pressure transducer sensitivity. Compare the regression analysis result to the manufacturer's nominal sensitivity. 2. Plot the manometer reading and output voltage from the pressure transducer. 3. Discuss the effect of the DC offset on measurements made with the pressure transducer. 4. Discuss the reasons why the velocity profile grid pattern used was chosen. 5. What is the sensitivity of the turbine flow meter? 6. Plot the velocity profile measured in assignment #2. 7. Determine the flow rate by numerically integrating the velocity profile. 8. Compare the computed flow rate with the turbine flow meter measured value. Comment on the difference. 6