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ME 322: Instrumentation Lecture 15

In this Instrumentation lecture, Professor Miles Greiner discusses the key concepts of pipe speed and volume flow rate, emphasizing the differences between laminar and turbulent flow. Students are reminded of important upcoming events, including the Career Fair, and are tasked with homework focused on flow relationships. The lecture covers measurements of centerline speed and volume flow in a wind tunnel, utilizing a Venturi tube and Pitot-static tube to analyze flow dynamics. This foundational knowledge prepares students for future internships and permanent employment opportunities.

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ME 322: Instrumentation Lecture 15

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  1. ME 322: InstrumentationLecture 15 February 27, 2012 Professor Miles Greiner

  2. Announcements/Reminders • HW 6 due Friday • No lab this week • Career Fair, Thursday, February 27, 2014 • Internships • Prepare for permanent employment next year • www.unr.edu/engineering/careerfair

  3. Pipe Speed and Flow Volume Flow Rate • Center line speed increases in the entrance region • In laminar flow (Re <~2000) fully developed flow is parabolic • In Turbulent flow (Re > 104) fully developed profile is flatter

  4. Speed and Flow Rate Consistency • Is there a unique centerline speed for every volume flow rate? • What does this relationship dependent on? • Is there a range of centerline speeds in which we expect VC to be for a given volume flow rate?

  5. Possible Centerline Speeds • At the pipe entrance and for fully developed turbulent flow, the velocity profile is relatively flat compared to fully-developed laminar flow • VC >~ VSlug = Q/A • For fully-developed laminar flow, we expect the velocity profile to be parabolic • where • is the pipe inner radius, and • is the centerline velocity • Relationship between speed and volume flow rate • In HW find that VP = 2VSlug • It’s reasonable to expect (VSlug= Q/A) < VC < (VP = 2VSlug) • In Lab 6 measure Q and VC in a small wind tunnel

  6. Lab 6 Air Volume Flow Rate and Centerline Speed in a Wind Tunnel • Plexiglas Tube and Schedule-40 Pipe have different diameters • Control flow rate using a variable speed blower • Cover blower exit for very low speeds • For a range of flow rates, measure • Volume flow Q rate using a Presso Venturi Tube (in pipe) • Centerline speed VC using a Pitot-Static Tube (in Plexiglas tube) • For both measure pressures difference using calibrated transmitters/digital multimeters • Both VC and Q increase with blower flow rate • Is VS < VC < VP?

  7. Venturi Tube • Inverted transfer function: • Need , (throat), • This expression needs pipe and throat dimensions • Presso Formulation: • = = • , ): Given by manufacturer • Only need D (pipe) and KPresso

  8. In Lab 6 use a Presso Venturi Tube • http://wolfweb.unr.edu/homepage/greiner/teaching/MECH322Instrumentation/Labs/Lab%2006%20Fluid%20Flow/Lab%20Index.htm • In Lab 6 use 2-inch schedule 40 Pipe, ID = 2.067 inch • PressoData Sheet – Page 10 • Venturi # 38, K = 0.3810 ± 2% (b = 0.6652, but don’t need to this) • Valid for 54,000 < < 137,000 (ReDor Red?)

  9. How to find VCand Q (and uncertainties)? • Pitot-Static Probe • (power product?) • Presso Venturi Tube • (power product?) • Both need air-density • (power product?) • RAir = 0.2870 kPa-m3/kg-K • Need to measure • Pressure differences PP, PV, and PStat • Air Temperature, T

  10. Instrument Schematic Variable Speed Blower Plexiglas Tube Pitot-Static Probe VC Barometer PATM TATM Venturi Tube Q Pipe • To measure PATM and TATM • Use hand-held digital-barometer • Is PStat< = or > than PATM? • Use 40-in-WC transmitter to find Gage Pressure PG = PATM – PS • PS = PATM - PG • To measure PP • Use 3-in-WC transmitter • To measure PV • Use 40-in-WC transmitter DTube DPipe PV - Static + 40 in WC Total PP PG Atm IV - - + + 3 in WC 40 in WC IG IP

  11. Inlet Pressure and Temperature • Fisher Scientific™ Traceable™ Hand-Held Digital Barometer • Barometric pressure, PATM • Uncertainty: = 5 mbar = 0.5 kPa = 500 Pa (95%?) • (1 mbar = 0.1 kPa = 100 Pa) • Atmospheric Temperature, TATM • = 1°C (95%?) • T[K] = T[°C] + 273.15 • Assume same in tunnel

  12. Pressure Transmitter Uncertainty • Pressure • = 998.7 kg/m3, g = 9.82 m/s2 • FS = (3 or 40 inch) • Manufacturer stated uncertainty: 0.25% Full Scale • (68%?) • For FS = 3 inch WC • PFS = rWghFS= (998.7 kg/m3)(9.81 m/s2) (3 inch) = 746.6 Pa • wP = 0.0025 PFS = 1.9 Pa • For FS = 40 inch WC • PFS = rWghFS= (998.7 kg/m3)(9.81 m/s2)(40 inch)= 9954 Pa • wP = 0.0025 PFS = 25 Pa

  13. Static Pressure • PStat = PATM – PG • Use for , RAir = 0.2870 kPa-m3/kg-K • Want kPa • Inputs • PATM • Measure using barometer • = 500 Pa = 0.5 kPa (68%) • PGAGE • Measure using 40 inch WC gage • = 25 Pa = 0.025 kPa(68%)

  14. Static Pressure Uncertainty • PStat = PATM – PG (power product?) • Square of absolute uncertainty in result is sum of squares of absolute uncertainty in inputs times coefficient.

  15. General Linear Sums

  16. Summary • Before Experiment • Use hand held barometer to measure • PATM • TATM • °C

  17. During Experiment • For each blower setting find the value and uncertainty of the • Static Pressure, PStat = Work on Board (WOB) • WOB • WOB • Air density WOB • WOB • Centerline speed WOB • WOB • Volume flow rate WOB • WOB

  18. Consistency Check • For a given volume flow rate Q • VS = Q/A • VP = 2VS • What area should we use • APipe or ATube ?

  19. Measured Results • Determine speed and flow rate uncertainty for a range of blower speeds

  20. Lab 6 Air Volume Flow Rate and Centerline Speed in a Wind Tunnel • To measure atmospheric inlet conditions use • Digital Barometer to measure PATM • In Millibar, 1 millibar = 0.1 kPa • wP(atm) = 5 millibar = 0.5 kPa = 500 Pa • Thermometer to measure TATM • T[K] = T[C] + 273.15 • wT = 1 K (68%)

  21. Wind Tunnel Schematic Variable Speed Blower Plexiglas Tube Pitot-Static Probe, VC Venturi Tube, Q Pipe DTube DPipe PV - Static + 40 in WC Total PP PG Atm IV - - + + 3 in WC 40 in WC IG IP

  22. During Experiment • For each blower setting find the value and uncertainty of the • Static Pressure, PStat = PATM – Pgage • Air density • Centerline speed • Volume flow rate

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