Governance by those who do the work.

Wednesday, January 21, 2015

Wiring the Plate

For those new to these postings, in 2011 through 2013 I developed a complete theory for single-phase mixed convection from a rough (or smooth) rectangular plate at uniform temperature:
  http://people.csail.mit.edu/jaffer/SimRoof/Convection

In 2014 I started assembling the equipment necessary to test this
theory:
  http://people.csail.mit.edu/jaffer/SimRoof/Convection/Measurements

Previous blog postings document the progress in building the wind tunnel and measurement and control electronics:
  http://voluntocracy.blogspot.com/2014

John Cox milled the front of the plate to have a precision rough surface with 3 mm average height of roughness.  This particular surface also has 3 mm root-mean-squared height of roughness.



John Cox milled slots in the back of the plate for the heaters (TO-220 resistors).  I drilled 11.mm deep pilot holes and tried to hand tap the holes for #6-32 screws.  On the first hole, the tap started to lean.  Carl Mikkelsen suggested that I remove the drive belt from my drill press, clamp the tap in the chuck, and tap (turning by hand) with the drill press enforcing vertical alignment.  Having only two hands, I used an elastic cord to counteract the spring in the press, as can be seen in the photograph.



 Tapping an 11.mm blind (not through) hole is not reliably acheived by machine, but is quite doable by hand.  I tapped each hole in 3 stages, backing out the tap and blowing out the hole with a (sports) ball inflation pump after each stage.  The resistance to turning the tap when the hole is clogged with chips feels different than the hard stop when the tap has reached the bottom of the hole.  Per Carl's suggestion I used vegetable oil when tapping, which eased cleanup and degreasing.



I wired the heaters (resistors) in series with solid AWG 22 wire (red).  The external drive points (top terminal of the top left and top right heaters) are also connected to conductors of the 6-wire ribbon cable (gray).  This allows measurement of the voltage across the heater chain to be unaffected by voltage drop in the drive wires.  Three gray wires connect to an electronic temperature sensor in the hole right of center.  The gray wire connected to the center screw grounds the plate in order to shield the temperature sensor from electrical noise.

In my initial calculations I neglected the tap taper length, so I shortened each screw by 1.6 mm.  I painted "Silver Alumina" thermal grease on the back of each heater and screwed it and a compression washer into its slot.  I used the TO-220 recommended torque limit on the screws.  But the screws were tight in the holes even without the compression washers, so I turned until the grease squeezed out.

To test the thermal coupling of the heaters to the plate, I jumpered the electronics board to run 1.83 Amps (33 Watts) through the string of heaters.  Applying the "finger test", the warmth of the plastic on the heaters was barely discernable, yet the plate temperature sensor registered a 1.K rise after several minutes.

Once the electronics board is programmed to record measurements of the sensors and heater power, I will encase the plate in polyisocyanurate foam board (insulation) and measure the power required to maintain a 5 K or 10 K temperature between the plate and ambient air.  Half of the ratio of this power to temperature difference is the (leakage) thermal conductivity of the back side of the plate.  The back of the plate will remain insulated while the front will be bare for convection measurements.

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