new_air_flow_rig_26022024

docx

School

University of Technology Sydney *

*We aren’t endorsed by this school

Course

CHNG3801

Subject

Mechanical Engineering

Date

Apr 3, 2024

Type

docx

Pages

13

Uploaded by ProfessorAtomChimpanzee38

Report
CHNG2801/CHNG9201 FLUID MECHANICS LABORATORY EXPERIMENT Air Flow Preparation and Prework Running the Equipment TAGL: March 2024 Air flow rig AR demo V2: https://view.vuforia.com/command/view-experience?url=https%3A%2F%2Funiversity-of- sydney-dev.es.thingworx.com%2FExperienceService%2Fcontent%2Fprojects %2Fair_flow_rig_v3%2Findex.html%3FexpId%3D1 Safety: You must wear safety glasses or spectacles, laboratory coat, pants or jeans and closed, covered footwear while in the laboratory area at all times. Failure to follow this requirement will result in your ejection from the laboratory for the rest of the laboratory session. You will not be given the opportunity to repeat the experiment on another day. This practice is intended to encourage the development of good safety discipline in industry as well as for your direct protection. Overall Learning Objectives Students should be able to: 1. Identify pipework and equipment used to make fluids move (fans and pumps). What do these pieces of equipment look like, and how do they work? (in laboratory) 2. Calculate flow rate using the Pitot tube, and the orifice plate flow meter to measure the flowrate in a duct. (in laboratory) 3. Construct fan and system curves from pressure and flow rate measurements. (in laboratory) 1
4. Operate chemical engineering equipment safely and extract relevant measurements and data. (in laboratory) 5. Synthesise findings with relevant fluid mechanics theory and report results concisely. (after laboratory) INTRODUCTION The aims of this experiment are (a) to compare two different flow measurement techniques, and (b) to measure the pump and system curves for the fan and the other equipment, respectively. We need to compare flow measurement devices, specifically Pitot tubes (and Pitot tube traverses) with orifice plate flow measurements. We need to create a set of pump and system curves. The pressure decrease through the system is the system curve (the pressure difference required by the system at a given flow rate), which starts at atmospheric pressure. This pressure drop must equal the pressure increase across the pump, which is the pump or fan curve). Each measurement is an operating point where a pump curve intersects a system curve. The system curve may be changed by altering a valve position, while a pump curve may be changed by altering the pump speed. EQUIPMENT Figure 1 shows a Piping and Instrumentation Diagram for the equipment. Figure 1: Piping and Instrumentation Diagram for the air flow experiment. DP DP Negative point for Pitot tube, static pressure: negative side of digital manometer Positive point for Pitot tube, impact pressure: positive side of digital manometer Negative point for orifice plate flow meter: negative side of digital manometer Positive point for orifice plate flow meter: in the atmosphere Pitot tube traverse From atmosphere Iris valve Orifice plate flow meter Fan 2
PREWORK CALCULATIONS (TO BE DONE BEFORE THE LABORATORY FOR THE EXPERIMENTS STARTING SOON) These calculations must be shown to the laboratory tutor on the day of the experiment and must be completed correctly before commencing. The next section 6 of these notes gives some examples that might be helpful. (a) The following pressure differences (between the impact and static tappings) were recorded at equispaced points in a 114 mm x 127 mm rectangular duct through which air (density, 1.2 kg m -3 ; viscosity, 1.8 x 10 -5 kg m -1 s -1 ) is flowing. Calculate the volumetric flow rate through the duct. The pressure differences (in Pa) are as shown in the following Table 1. Equation (1) in the next section will be important. Use a Pitot-tube coefficient ( C ) of 1 (unity). Table 1: Pressure differences recorded by the digital pressure meter (in Pa) in each section of the Pitot tube traverse for the air flow experiment. 75 100 125 100 100 125 150 125 75 150 175 100 50 75 125 75 Each rectangle in the grid above represents an equal area, so averaging the point velocities directly to give an average velocity is reasonable. 3
Your preview ends here
Eager to read complete document? Join bartleby learn and gain access to the full version
  • Access to all documents
  • Unlimited textbook solutions
  • 24/7 expert homework help
(b) This rectangular duct is followed by a circular duct of 140 mm diameter in which there is an orifice plate flow meter. The orifice plate hole diameter is 108 mm and the pressure drop between the upstream tapping and the throat (the hole) is 216 Pa. Given a discharge coefficient of 0.6 for the orifice plate flow meter, calculate the volumetric flow rate as measured by this flow meter. Equation (2) in the following section will be important. = 0.25756 M^3/s Note: Useful information for these calculations (above) is given in the equations in the next section. EXPECTATIONS IN THE LABORATORY Suppose that we have a 140 mm diameter pipe, with an orifice plate flow meter having an orifice diameter of 108 mm. With a Pitot tube, in general, and here in this laboratory, the simple application of the Pitot tube equation is helpful: u = C pt 2 ∆ P ρ (1) Given that C pt ≈ 1 and ρ ≈ 1 kg m -3 , the pressure difference across a manometer for a velocity of 5 ms -1 through a test section is about (5 ms -1 ) 2 /2 = 12 Pa, and at 10 ms -1 , the pressure difference is (10 ms -1 ) 2 /2 = 50 Pa. In the experiment, we will use ρ ≈ 1.2 kg m -3 , but here we use ρ ≈ 1 kg m -3 for convenience. What should we expect in the orifice plate flow meter? Should the pressure differences be the same as those from the Pitot tube? Yes and no. Yes, the general form of the equation is similar for all devices, and all equations for these types of meter come from Bernoulli’s equation. For the Pitot tube, see equation (1) above. For the orifice plate flow meter, equations (2) and (3) apply: Q = C D A t 2 ∆ P ρ [ 1 ( A t A p ) 2 ] (2) u av = Q A p (3) 4
Where C D is a discharge coefficient (about 0.6 for the orifice plate flow meter), u av is the average pipe velocity, A t is the throat cross-sectional area, and A p is the pipe cross-sectional area. And no: The orifice plate flow meter is located in a 140 mm diameter pipe, so A p = π/4 (0.14 m) 2 = 0.0154 m 2 . For the orifice plate meter, A t = π/4 (0.108 m) 2 = 0.00916 m 2 , and rearranging equation (2), for the orifice plate flow meter, gives ∆ P = ( Q C D A t ) 2 ρ 2 [ 1 ( A t A p ) 2 ] (4) ∆ P = ( u av A p C D A t ) 2 ρ 2 [ 1 ( A t A p ) 2 ] (5) Hence, for an average air velocity of 5 ms -1 through the orifice plate flow meter, the pressure difference is: ∆ P = ( 5 × 0.0154 0.61 × 0.00916 ) 2 1 2 [ 1 ( 0.00916 0.0154 ) 2 ] = 61 Pa For the Pitot tube, the pressure difference at a velocity of 5 ms -1 is 12 Pa. For an average air velocity of 10 ms -1 through the orifice plate flow meter, the pressure difference is: ∆ P = ( 10 × 0.0154 0.61 × 0.00916 ) 2 1 2 [ 1 ( 0.00916 0.0154 ) 2 ] = 245 Pa For the Pitot tube, the pressure difference at a velocity of 10 ms -1 is 50 Pa. For the orifice plate flow meter, the relationship between the velocity and the pressure difference is affected by the pipe and throat cross-sectional areas and the discharge coefficients. TESTING PROCEDURE Orifice Plate: To measure the pressure difference between the upstream location and the throat in the orifice plate flowmeter, connect the plastic tube after the orifice plate to the negative side of the digital manometer. The upstream pressure is just atmospheric pressure, so the positive side of the manometer should just be open to the atmosphere. 5
Pitot Tube: To measure the pressure difference for the Pitot tube, connect the bottom of the Pitot tube (connected to the impact pressure point) to the positive side of the digital manometer with a plastic tube, and also use a plastic tube to connect the side of the Pitot tube (connected to the static pressure point) to the negative side of the digital manometer. Positive point for orifice plate flow meter: in the atmosphere Pitot tube traverse points (distances from near wall): 2 mm 12 mm 19 mm 36 mm 69 mm 86 mm These distances represent (almost) equal cross-sectional areas, so the point velocities can be averaged to give the average velocity. 1. Flow meters: Set the flow to a high value by fully opening up the Iris valve (indicator at the blue line) after the entry of the duct and setting the variable speed drive (v.s.d.) on the fan to 50 Hz. 2. Flow meters: With the Iris valve fully opened (indicator at the blue line), carry out a Pitot Tube traverse of the circular pipe using the points given above (2 mm, 12 mm, etc). Without altering the flow, measure the pressure at the downstream tapping of the orifice plate flow meter. 3. Flow meters: Again, with the Iris valve fully opened (indicator at the blue line), repeat the experiment using two lower flow rates (e.g. 40 Hz, and 30 Hz on the v.s.d.), so that you carry out measurements for three flow rates. 4. Fan and system curves (valve setting 1): Start with the Iris valve fully opened (indicator at the blue line), set the variable speed drive (v.s.d.) on the fan to 30 Hz. Measure the static pressure before the fan. Also, measure the pressure downstream of the orifice plate flow meter, allowing us to calculate the flow rate. The static pressure will be less than atmospheric pressure, due to the pressure drop along the pipe. This combination of the static pressure and the flow rate from the flow meter is an operating point on both a system curve (for the system with the Iris valve fully open) and on a fan curve (for the fan at a frequency or speed of 30 Hz). The pressure drop down the pipe is equal to the pressure at the inlet to the fan. 5. Fan and system curves (valve settings 2, 3 and 4): Change the Iris valve setting (indicator at the blue line) to the numbers 4, 3, and the bottom line, and repeat the static pressure measurements before the fan for settings of the variable speed drive (v.s.d.) on the fan at 6
Your preview ends here
Eager to read complete document? Join bartleby learn and gain access to the full version
  • Access to all documents
  • Unlimited textbook solutions
  • 24/7 expert homework help
30 Hz, 40 Hz, and 50 Hz. You will have four valve settings and three fan speeds, so you will have 12 operating points) 6. Calculate flow rates and pressure differences (increases and decreases) on the spot. If your results are unexpected, measure them again, checking that your instrumentation is set up correctly. 7
RESULTS AND DISCUSSION Typical Data and Analysis Use C D = 0.6 for the orifice plate flow meter. Circular duct diameter : 105 mm Orifice plate hole diameter : 71 mm Comparison between Pitot tube traverses and orifice plate flow meters The raw data for the comparison between Pitot tube traverses and orifice plate flow meters are shown in Table 2. Table 2: Raw data for the pressure differences across the Pitot tube and the orifice plate flow meters, iris valve fully open Fan frequency Pitot tube Orifice plate flow meter Point 1, 2 mm Point 2, 12 mm Point 3, 19 mm Point 4, 36 mm Point 5, 69 mm Point 6, 86 mm (Hz) (Pa) (Pa) (Pa) (Pa) (Pa) (Pa) (Pa) 20 26 31 33 34 35 29 304 30 60 69 72 76 80 70 690 40 105 125 129 135 137 118 1200 50 170 195 201 202 209 186 1815 Processing the data and doing the calculations Sample calculation for the Pitot tube Row 1, 20 Hz fan frequency, iris valve fully open, column 1, point 1, 2 mm from wall, Δ P = 26 Pa on the Pitot tube. Equation (1) becomes the following equation, with C pt ≈ 1.29 at 20 o C, including the density of 1.2 kg m -3 at 20 o C: u = C pt ∆ P (1) Given that C pt ≈ 1.29 at 20 o C, includes the density of 1.2 kg m -3 at 20 o C. With Δ P = 26 Pa, u = 1.29 26 = 6.58 m s 1 (1) Pitot tube: For the first row, 20 Hz fan frequency, iris valve fully open, the average velocity for all the Pitot tube measurements is 7.21 m s -1 . The cross-sectional area of the pipe is π/4 (0.105 m) 2 = 0.008659 m 2 , and this cross-sectional area applies for all the fan frequencies and flow rates. Hence, the volumetric flow rate from the Pitot tube measurements for the first row, 20 Hz fan frequency, iris valve fully open, is 7.21 m s -1 × 0.008659 m 2 = 0.0624 m 3 s -1 ( Q Pitot ). 8
Sample calculation for the orifice plate flow meter Row 1, 20 Hz fan frequency, iris valve fully open, Δ P = 304 Pa for the orifice plate flow meter. Equation (2) for the orifice plate flow meter follows: Q = C D A t 2 ∆ P ρ [ 1 ( A t A p ) 2 ] (2) Where C D is a discharge coefficient (about 0.6 for the orifice plate flow meter), A t is the throat cross-sectional area, and A p is the pipe cross-sectional area (0.008659 m 2 , from above). The throat cross-sectional area ( A t ) is π/4 (0.071 m) 2 = 0.003959 m 2 . Substituting the values into equation (2) gives: Q orifice = 0.6 × 0.003959 2 × 304 1.2 [ 1 ( 0.003959 0.008659 ) 2 ] = 0.0601 m 3 s 1 (2) The comparisons between the volumetric flow rates from the Pitot tube traverses and the orifice plate flow meters are shown in Table 3. Table 3: Volumetric flow rate comparison between the Pitot tube and the orifice plate flow meters, iris valve fully open Pitot tube Fan freq. Point 1, 2 mm Point 2, 12 mm Point 3, 19 mm Point 4, 36 mm Point 5, 69 mm Point 6, 86 mm Pitot tube average velocity Q Pitot , Pitot tube volu- metric flow rate Q orifice , orifice plate volu- metric flow rate (Hz) (m s -1 ) (m s -1 ) (m s -1 ) (m s -1 ) (m s -1 ) (m s -1 ) (m s -1 ) (m 3 s -1 ) (m 3 s -1 ) 20 6.58 7.18 7.41 7.52 7.63 6.95 7.21 0.0624 0.0601 30 10.0 10.7 10.9 11.2 11.5 10.8 10.9 0.0941 0.0906 40 13.2 14.4 14.7 15.0 15.1 14.0 14.4 0.125 0.119 50 16.8 18.0 18.3 18.3 18.6 17.6 17.9 0.155 0.147 Comparison between Pitot tube traverses and orifice plate flow meters The raw data for the pump and system curves are shown in Tables 4 and 5. 9
Your preview ends here
Eager to read complete document? Join bartleby learn and gain access to the full version
  • Access to all documents
  • Unlimited textbook solutions
  • 24/7 expert homework help
Table 4: Raw data for fan and system curves, the pressure differences across the orifice plate flow meter (for calculating the volumetric flow rates) Valve position Fan frequency Fully open 4/5 open 3/5 open 2/5 open 1/5 open (Hz) (Pa) (Pa) (Pa) (Pa) (Pa) 20 293 274 225 143 38 30 663 612 505 326 92 40 1173 1079 891 578 158 50 1813 1670 1378 890 261 Table 5: Raw data for fan and system curves, the pressure at the inlet of the fan and the outlet of the measurement tube (below atmospheric pressure) Valve position Fan frequency Fully open 4/5 open 3/5 open 2/5 open 1/5 open (Hz) (Pa) (Pa) (Pa) (Pa) (Pa) 20 314 323 330 340 340 30 714 726 743 766 770 40 1265 1285 1316 1359 1360 50 1960 1990 2035 2100 2110 The raw data in Table 5 may be used directly, since the pressure below atmospheric pressure at the inlet to the fan and the outlet of the tube gives both the pressure drop across the tube and the pressure increase across the fan, for the following reasons. The pressure drops along the tube from atmospheric pressure to the pressure at the inlet to the fan, so this pressure is the pressure required for this volumetric flow rate on the system curve. The pressure increases across the fan from the pressure at the inlet to the fan to atmospheric pressure, so this pressure is the pressure increase across the fan at this volumetric flow rate on the fan or pump curve. The pressure difference data in Table 4 must be converted to volumetric flow rates as shown in the sample calculation applying equation (2) to the flow meter comparison. Taking the pressure difference in the first row, first column, fan frequency 20 Hz, fully open iris valve, Δ P = 293 Pa for the orifice plate flow meter, and using equation (2), gives: Q orifice = 0.6 × 0.003959 2 × 293 1.2 [ 1 ( 0.003959 0.008659 ) 2 ] = 0.0590 m 3 s 1 (2) The remaining data in Table 4 have been calculated as above and placed into Table 6. 10
When plotting the fan (pump) and system curves, each point is a point on both fan and system curves. The x-coordinate (abscissa) comes from a row, column location in Table 6, for the volumetric flow rate. The y-coordinate (ordinate) comes from a row, column location in Table 4, for both the pressure drop across the tube (system curve) and the pressure increase across the fan (fan curve). The resulting fan and system curves are shown in Figure 2. Table 6: Calculated volumetric flow rates corresponding to the pressure differences across the orifice plate flow meter (from Table 4) for the fan and system curves Valve position Fan frequency Fully open 4/5 open 3/5 open 2/5 open 1/5 open (Hz) (m 3 s -1 ) (m 3 s -1 ) (m 3 s -1 ) (m 3 s -1 ) (m 3 s -1 ) 20 0.0590 0.0571 0.0517 0.0412 0.0213 30 0.0888 0.0853 0.0775 0.0623 0.0331 40 0.1181 0.1133 0.1029 0.0829 0.0433 50 0.1468 0.1409 0.1280 0.1029 0.0557 0.0000 0.0500 0.1000 0.1500 0 1000 2000 3000 Fully open 4/5 open 3/5 open 2/5 open 1/5 open 20 Hz 30 Hz 40 Hz 50 Hz Air flow rate, m3/s Pressure increase or decrease, Pa Figure 2: Fan (pump) and system curves for the air flow experiment. 11
12
Your preview ends here
Eager to read complete document? Join bartleby learn and gain access to the full version
  • Access to all documents
  • Unlimited textbook solutions
  • 24/7 expert homework help
Discussion 1. You must prepare a concise report to your manager summarising the key findings. Your support calculations and raw data should be included in the Appendices that should be attached to your report. 2. Calculate and compare the flows given by the two methods of measurement. 3. Calculate the Reynolds Number for each flow rate and comment on the likely state of flow (laminar or turbulent). Do not plot Reynolds number against flow rate because they are directly proportional by definition! 4. Discuss the uncertainties in the measurements and how these uncertainties propagate through to affect the results. The appendix has notes on how to do this. What effects do the uncertainties in all the input numbers (diameters, pressure differences, densities) for the calculations have on the final results (e.g. flow rates)? 5. The fan/pump and system curves show that there are two ways to control the flow rate through the equipment (below the maximum flow rate). One way is to reduce the fan frequency using the variable speed drive. The other way is to close the valve. Given that the power required to pump the fluid (air) is the product of the pressure rise across the pump (fan), in Pa, and the volumetric flow rate, in m 3 s -1 , what is the lowest energy cost way to achieve a volumetric flow rate that is half the maximum volumetric flow rate? At this halfway value of the maximum volumetric flow rate, what is the difference in power requirements between reducing the volumetric flow rate by (a) closing the valve, or (b) reducing the fan/pump speed? REFERENCES Coulson, J M, Richardson, J F, Backhurst, J R, Harker, J H, (1999) Chemical Engineering. Volume 1: Fluid Flow, Heat Transfer and Mass Transfer , 6th Edition, Pergamon, Oxford. There are many other books on fluid mechanics, fluids engineering in the library. 13