Alan Fletcher Member
  • Member since May 27th 2016
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Posts by Alan Fletcher

  • Mizuno Airflow Calorimetry

    Recommendations (using Mizuno's fan) :


    Make the outlet tube 15 diameters long, and measure 4 diameters from the outlet.


    Use the RPM counter (unused yellow wire) rather than watts, so that the results are linear (rather than squared).


    Correct the results for temperature.

  • Mizuno Airflow Calorimetry

    And : http://www.lth.se/fileadmin/ht…211/lab2b-pm-Eng-2009.pdf


    As a general rule-of-thumb, conventional hot wire anemometry should not be used if the
    local turbulence intensity is higher than about 35%, at least not for quantitative purposes.
    Apart from the mix-up of velocity components (cross-talk), it is evident that a hot wire
    can not distinguish between what is forward and backward, it behaves like a rectifier.

    (Turbulence intensity is based on the transition from laminar to turbulent flow. Others have reported that the outlet is well into the turbulent flow domain )

    See equations 18-19 :


    The relative error increases quadratically with Tu. For Tu = 10% and H = 1.05, K = 0.20
    the systematic error becomes +0.6%; +2.3% for Tu = 20%. Thus, to avoid excessive
    systematic errors, it is essential to have low enough turbulence intensities. During velocity
    calibrations, the rule-of-thumb is to have Tu < 5%, preferably Tu < 2%.

    The relative error in using the approximations ve(t) = u(t) and q ....
    instead directly proportional to the turbulence intensity. For Tu = 10%, with H and K
    as above, the absolute value of the relative error is less than 3%.

  • Mizuno Airflow Calorimetry

    I've given you substantial references to this. You obviously didn't (read or) understand them.

    In pharmaceutical terms it would be a black-box warning :

    WARNING : DO NOT USE HOT-WIRE ANEMOMETERS FOR QUANTITATIVE MEASUREMENTS IN TURBULENT FLOWS

    https://www.trutechtools.com/M…re-Anemometer_c_1001.html

    Measuring Airflow With a Hot Wire Anemometer

    The principal of a hot wire anemometer is based on a heated element from which heat is extracted by the colder impact airflow. The temperature of the hot wire is kept constant via a regulating switch, and the current (amp draw measured internally) is directly proportional to the air velocity. When using a hot wire in turbulent air streams the measured results can be impacted by turbulent airflow striking the measurement sensor from multiple directions. This could indicate a higher measured value than a vane probe. This characteristic is typically prevalent in ducts where turbulent airflow can occur even at very low velocities.

    Measurement Location and Selection

    All measurements should be made in a straight section of duct if possible. In an ideal location the duct will have a minimum of 10 diameters before the measuring spot and at least 4 diameters after before making a transition of turn. The airflow should not be inferred by dampers.

    Hot wires are calibrated to a specific air density and either require the density to be input to the meter or a correction to be made. Many are calibrated to standard air which is 68F 0%Rh and 29.92mmhg. Consult the manufacturer. Hot wires are best suited for low velocity measurements at or near standard air conditions. Care must be used when measuring conditioned and turbulent air. Hot wires are not recommended for air velocities exceeding 2000 FPM unless they are specifically designed for that purpose. Heavy duty models are available that can measure in excess of 6000 FPM.

  • Mizuno Airflow Calorimetry

    1. Show me the proof of that? I was asked a question, and said I didn't KNOW whether they were "HVAC Engineering" or "Scientific" accurate.

    2. For DIFFERENTIAL analysis. I've given TWO references saying they OVER-READ for TURBULENT FLOW.

    My Chart 4 shows that R20 doesn't follow "fan law" (see next post for some comments) where R19 DOES.
    Mizuno doesn't account for fan-law temperature dependence

  • Mizuno Airflow Calorimetry

    Quote from Curbina

    I think you are arriving late to the party and demanding a new keg of beer be opened, to a crowd that is already drunk, to use a fun analogy.


    So, if you think you have identified a previously overlooked potential source of error, could you state it in precise quantitative and qualitative terms in order to see the merit of your concern, and spare us of continuing the circular argument that has been already ongoing about this topic?


    Scattered over 2700 posts in 4 topics. Some of which are scheduled to be closed. I planned to read them and link to important posts.

    My concerns are in my original post : Mizuno reports increased excess heat

    a) Why R20 doesn't seem to follow fan laws

    b) No temperature correction (14% between 20C and 60C)

    c) Hot wire anemometry in turbulent flows

  • Mizuno reports increased excess heat

    Quote from RobertBryant

    This is strange.


    sources please and exact words..


    The source I linked to was a vendor, not a manufacturer
    https://www.trutechtools.com/M…re-Anemometer_c_1001.html

    Measuring Airflow With a Hot Wire Anemometer

    The principal of a hot wire anemometer is based on a heated element from which heat is extracted by the colder impact airflow. The temperature of the hot wire is kept constant via a regulating switch, and the current (amp draw measured internally) is directly proportional to the air velocity. When using a hot wire in turbulent air streams the measured results can be impacted by turbulent airflow striking the measurement sensor from multiple directions. This could indicate a higher measured value than a vane probe. This characteristic is typically prevalent in ducts where turbulent airflow can occur even at very low velocities.

    Measurement Location and Selection

    All measurements should be made in a straight section of duct if possible. In an ideal location the duct will have a minimum of 10 diameters before the measuring spot and at least 4 diameters after before making a transition of turn. The airflow should not be inferred by dampers.

    Hot wires are calibrated to a specific air density and either require the density to be input to the meter or a correction to be made. Many are calibrated to standard air which is 68F 0%Rh and 29.92mmhg. Consult the manufacturer. Hot wires are best suited for low velocity measurements at or near standard air conditions. Care must be used when measuring conditioned and turbulent air. Hot wires are not recommended for air velocities exceeding 2000 FPM unless they are specifically designed for that purpose. Heavy duty models are available that can measure in excess of 6000 FPM.

  • Mizuno Airflow Calorimetry

    I was wondering that.


    They are widely used in HVAC for sizing a fan from the nominal spec .. so maybe at least 10% accurate.


    Scientific? 1%?


    I'd expect the fan laws to break down at the low end, where friction, efficiencies come into play .. as in the M17/R19


    But the M19/R20 don't fit at all.

  • Mizuno Airflow Calorimetry

    The main areas of interest are as follows :

    1. The ambient situation (Jed has some posts on this)

    2. The INSIDE of a reactor : I don't think this relevant to the calorimetry, but it has been talked about

    3. The INLET to the calorimetry box. I don't think this needs to be analyzed, except to record the inlet temperature
    4 How the reactor(s) heat gets into the system (Conduction, convection, radiation).

    (I personally think this is irrelevant.

    5. The INSIDE of the calorimetry box. The flow may be laminar, rather than turbulent. Some heat may rise by convection to the top of the box.
    (Again, I think this is irrelevant)
    6. The outer WALLS of the calorimeter. Heat comes through the walls by conduction (R-value), and then escapes by convection and radiation. But with the reflective insulator this will probably be a small component of the total.
    (maybe need an array of inside/outside thermocouples -- IR camera to see if there ARE hotspots, temparature differences)

    7. The fan/blower outlet. I think this is the main one.
    As I point out, The M17/R19 reactor results look well-behaved: they follow the "fan laws".
    The M19/R20 is problematic (to me) : the calibration curve does NOT follow the fan laws. Why not?
    Recommend measuring the fan RPM instead of/as well as input power.

    The use of a hot-wire anemometer in turbulent flow might be problematic.

    8. Power calculations given all of the above.
    Temperature-dependence of fan air velocity, air density, specific heat ......
    Curve fitting (fan law, Mizuno equation ...)