Cover Page for Lab Report Group Portion Compressible Flow in a Converging-Diverging Nozzle Prepared by Professor J. M. Cimbala, Penn State University Latest revision: 13 January 2012 Name 1: Name 2: Name 3: [Name 4: ] Date: Section number: ME 325. Group letter: (A, B,...) Score (For instructor or TA use only): Lab experiment and results, plots, tables, etc. - Procedure portion Discussion Neatness & grammar TOTAL / 45 / 15 / 10 / 70 Comments (For instructor or TA use only):
Procedure and Presentations of Results Safety Precautions: Wear safety goggles at all times the air in the wind tunnel is pressurized. Leave the protective Plexiglas cover on the test section except when performing flow visualizations. Wear hearing protection if the sound level is bothersome. A hearing protector is available for each lab member. A. Getting familiar with the schlieren optical technique Before visualizing shock waves in the test section of the supersonic wind tunnel, familiarize yourself with the schlieren system by looking at some other simpler flow fields: 1. Turn on the LED light. Be careful not to bump any of the optics stands. 2. A schematic diagram of the schlieren setup is sketched In Figure 10. Verify that the schlieren imaging system is operating correctly. An image of the test section should be visible through the viewfinder of the camera. If the light passing through the test section is out of focus, adjust the manual focus ring on the camera lens. If the system falls out of alignment, get help from your instructor or TA. Figure 10. Schematic diagram of schlieren setup, view from the top. The key dimensions for this system are that the slit and knife-edge are located at the focal point of lens 1 and lens 2, respectively. These lenses are 70mm diameter with a 500mm focal length. The LED, condenser lens, and slit are all mounted together here. The knife edge is replaced by a color filter to convert from black-and-white schlieren imaging to color schlieren imaging. 3. With one group member holding the soldering iron inline with the test section, just inside the field of view of the schlieren system (do not touch the soldering iron tip to anything), the other group members can observe the image through the camera viewfinder. Turn on the soldering iron and see if you can observe a thermal plume. Take turns so that every group member gets to see the schlieren image. 4. Visualize other compressible flow phenomena, such as compressed air streaming out of a spray can, shop air exiting a nozzle (you should be able to see shock diamonds in the jet), the thermal plume from your hand, etc. B. Visualization of shock waves using the schlieren optical technique The following procedure should be used to start the flow facility. Use Figures 8 and 9 of the Introduction as a guide. 1. Ask your instructor or TA to check that the compressor is running. 2. Open back pressure Valve 3 (the large one) at least two turns, and open back pressure Valve 4 (the small one) at least two turns. 3. Open Valve 2 slowly until fully open. You should hear the rush of air through the system. Let the system run for several seconds the sound should reach a steady state level. The facility is now operating, and the flow rate is controllable by the two back pressure valves, Valves 3 and 4. Use Valve 3 for gross adjustment and Valve 4 for fine adjustment. Never close both back pressure valves completely; otherwise high pressure builds up in the test section, and the optical glass can shatter. Note: If at any time the pressure relief valve blows, or the test section glass shatters, shut off Valve 2 immediately, and call your instructor or TA. 4. Remove the Plexiglas protective cover from the test section. Everyone in the lab group should wear safety goggles. 5. Slowly adjust back pressure valves 3 and 4 and observe the flow patterns in the camera viewfinder. Are you able to observe a shock wave? At this point, ask your instructor or TA to point out a few details about the shock wave, an example of which is shown in Figure 11.
(a) (b) Figure 11. Color schlieren images in the compressible flow rig: (a) back pressure such that a shock wave is observed just downstream of the throat, and (b) lower back pressure such that the shock wave is much further downstream and Mach lines are observed in the isentropic flow region upstream of the shock structures. Note that no compressible-flow phenomena are observed upstream of the nozzle throat. (5) 6. Briefly describe below what happens to the shock wave as you increase and decrease the back pressure. Do your observations agree with the theoretical description of Figure 1 of the Introduction? (5) 7. On a separate page, sketch the observed flow pattern for two or three flow conditions, including one with no shock present and one with a shock present in the nozzle. You may also record the images with the camera. To operate the camera, begin by turning it on (it should be powered either by an internal battery or plugged into the wall). The camera should be set to manual operation (M), with the f-stop set to 1.8 and the shutter to 2000. Adjust the f-stop by rotating the dial near the shutter button on the lens-side of the camera. The shutter is adjusted by the dial on the view-screen-side of the camera. The shutter may need to be adjusted to record a properly illuminated image depending on the knife-edge position. Keep the f-stop full-open at 1.8. Remove the camera memory card and use one of the computers to save the images. For more detailed camera instructions, see the instruction manual or ask your TA for assistance. See Figures. C. Measurement of mass flow rate The test rig contains two flow rate measuring devices: a) a diaphragm meter and b) a Venturi meter. Only the diaphragm meter will be used in this lab experiment. This device directly measures the volume of gas passing through the meter. It works by rotating a vane inside, which is attached by gears to a counter and a revolving pointer. You may have seen similar meters for measuring the volume flow of water or natural gas in your home. A similar meter with a cut-away casing has been placed under the flow rig for your viewing pleasure. The accuracy of this type of flow meter is around a couple
percent, except when the volume flow rate falls below 5% of rated capacity. For our meter, one revolution of the pointer corresponds to 5 cubic feet of air, which is equivalent to a volume of 0.142 m 3. To compute mass flow rate, use its definition, along with the ideal gas law, i.e. P V m Q (1) RT t where Q is the volume flow rate, measured as V t, where V is the volume of air passing through the meter in a measured time period t. Gas density is computed from the ideal gas law in Equation (7). Pressure P and temperature T must be measured just upstream of the flow meter. On the control panel, these correspond to the pressure toggle switch labeled Flow Meter Inlet, and thermocouple T 1 (number 1 on the dial). The control panel permits measurement of any of the pressure taps (see Figures 8 and 9 of the Introduction), with the pressure being read from the large Heise absolute pressure gauge. When measuring pressure, only one of the pressure toggle switches should be open at any time. The toggle switches merely connect the line from the chosen pressure tap directly into the input line for the Heise pressure gauge. Temperature is measured by thermocouples and a digital display unit. The temperature dial should be turned to the desired position to read the chosen temperature in degrees Centigrade. 1. Turn both Valve 3 and Valve 4 to their fully open (counterclockwise as far as possible) condition so that the imum flow rate is achieved through the system. Measure the pressure and temperature just upstream of the flow meter, as discussed above. Record (in the spaces provided below) these readings: P flow meter inlet = psia T flow meter inlet = T 1 = C (5) 2. With a stopwatch, measure the volume flow rate through the diaphragm meter, and calculate m using Equation (1). Show all your calculations in the space provided below. Show all units in your calculations, and be careful that the units combine correctly into kg/s. Note that 1 psia = 6894.8 N/m 2 = 6894.8 Pascals. m = kg/s (2) 3. For this same set of flow conditions (imum flow rate), measure both P 0 and P b. On the control panel, these are labeled P 2 and P 10 respectively. Again, note that only one pressure toggle switch should be open at a time; otherwise the pressure reading will be in error. Calculate the back pressure ratio for these conditions. Record your measurements in the spaces provided below: P 0 = P 2 = psia P b = p 10 = psia P b /P 0 =
(2) 4. The theoretical imum flow rate for air is given by * 0.6847PA m o (2) 1/2 RT o In our experimental test rig, the stagnation temperature T 0 is equal to T 3 on the control panel, and throat area A * is equal to 4.03 10-5 m 2. Calculate m for this same set of flow conditions. Show all your work (include units) in the space provided below: m = kg/s. (5) 5. Repeat Steps 1 through 4 above for several back pressures by turning Valves 3 and 4 slowly clockwise. Using the schlieren image as a guide, be sure to include cases ranging from fully supersonic throughout the entire diverging part of the nozzle (valves fully open) to fully subsonic everywhere (Valve 3 fully closed and Valve 4 nearly closed). Be sure to include several cases where a shock wave is present somewhere in the diverging part of the nozzle, and at least one case of fully subsonic flow (no shock). Also make sure that you include at least one case where the shock wave is just barely visible near the throat. For each case, measure P 0, P b, P flow meter inlet, T o, T flow meter inlet, and the volume flow rate using the flow meter and stopwatch as in Step 2 above. Also sketch the flow conditions in the converging-diverging nozzle for each case, pointing out the shock wave (if one exists). Calculate P b /P 0, m, m, and m / m for each case as well. Put all your results into a neatly labeled table. See Table. (5) 6. Plot m / m as a function of back pressure ratio P b /P 0. Compare your results with Figure 1c. Have you observed choking? Explain. See Figure.
D. Examination of choking and acoustic noise Two microphones are mounted in the test rig, one upstream and one downstream of the converging-diverging nozzle, as sketched in Figure 8 of the Introduction. Most of the flow-induced noise is generated downstream of the nozzle, where the flow is highly turbulent. When the flow is subsonic, sound can propagate upstream as well as downstream, and both microphones will pick up lots of noise. As discussed previously, however, when the flow becomes choked, noise can no longer propagate upstream past the throat. The upstream microphone should therefore suddenly be exposed to a much quieter flow whenever the nozzle chokes. (Ask your instructor or TA for help). (3) 1. Turn on the power supply (gray box) for the amplifier (black box), and turn up the volume on the amplifier. A toggle switch on the amplifier allows you to select either the upstream or downstream microphone. Vary the back pressure so that the flow goes from unchoked (subsonic everywhere) to choked. With the toggle switch pointed upstream (i.e. with the upstream microphone connected), describe your observations below. You may keep the schlieren system on to aid in your observations if you so desire. Does choking affect the sound recorded by the upstream microphone? Explain. (3) 2. Repeat the above with the downstream microphone. Does choking affect the sound recorded by the downstream microphone? At this point, ask the instructor or TA for assistance in the interpretation of your observations. E. Measurement of pressure distribution in the converging-diverging nozzle (5) 1. For several back pressure conditions (going the full range again from fully subsonic to fully supersonic), record pressures P 2 through P 10. Tabulate your results neatly in a table which should also include nondimensional pressure ratios P/P 0 for pressure tap locations 2 through 9, where again, P 0 is taken as pressure P 2. See Table. (5) 2. Plot P/P 0 as a function of streamwise coordinate x for all your cases in Step 1 above on the same plot. Include on your plot both the subsonic and supersonic theoretical calculations of P/P 0 from Table 1. If a shock wave is present, indicate its location on your plot. Note: The values of streamwise coordinate x can be obtained from Table 1. See Figure. 3. Shut off the flow in reverse order from the procedure to turn on the flow. Make sure the schlieren light bulb is turned off. Turn off the digital thermometer display unit. Make sure the Plexiglas safety cover has been reinstalled on the test section for protection.
Discussion (5) 1. Briefly summarize what you have learned about choking, and what it means. (5) 2. How well do your measurements of pressure agree with those of Figure 1 of the Introduction for the various back pressure ratios? Discuss possible reasons for any disagreement. (5) 3. When a shock wave is present in the diverging part of the nozzle, does the pressure jump up suddenly as indicated in Figure 1b of the Introduction? Why or why not?