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71-000731-CLN-01

***TO: DISTRIBUTION***

***FROM: C NEUMEYER***

***SUBJECT: ANALYSIS OF OH WATER OUTLET TEMPERATURE DATA***

***References:***

\[1\] 71-000302-CLN-01, "Study of OH Cooling vs. Repetition Rate"

\[2\] 71-000627-CLN-01, "Guidance on OH Protection Settings\"

\[3\] D-NSTX-ISTP-270, "NSTX OH Rep Rate Interlock System Commissioning"

\[4\] NSTX-CALC-13-3, "OH Coil Thermal Behavior"

This memo documents the analysis of the recent OH outlet water
temperature measurements and the adjustment of the simulation of OH coil
thermal behavior to fit the measurements. This is a required step of the
ISTP \[3\] which commissions the OH repetition rate interlock system.
The details have been incorporated into rev. 2 of the formal calculation
\[4\].

Data was taken from shot #102190 by P. Sichta from the EPICS data and
provided in spreadsheet form. The waveforms of the eight temperature
measurements are shown in figure 1. A magnification during the beginning
of the cool down period is shown in figure 2. A plot showing the
simulated result versus the minimum, maximum and average of the path
measurements (minimum and maximum at each time sample) is shown in
figure 3. The simulated result shown differs from that originally
presented in references \[1\] and \[4\] in that the average of the eight
winding path lengths was used instead of the longest, and the actual
flow was used instead of the low flow drop out setpoint. These changes
were found to produce a better match to the observed measurements.

The following points are noted:

1\) From figure 2 it is noted that there seem to be four distinct pairs
of cool down waveforms which correspond to the four layers of the coil
wound two-in-hand. However, the numbering of the measurements (1X, 1Y,
etc.) does not match the expected behavior. It appears that 1Y and 4Y
seem to be from layer 1, 2Y and 3X seem to be from layer 2, 2X and 3Y
seem to be from layer 3, and 1X and 4X seem to be from layer 4. This
would correspond to 1X and 4Y being swapped, and 2X and 3X being
swapped. Testing recently performed in which the cooling hoses were
pinched off at the point where they are attached to the machine and
labeled, causing the flow detector on the pinched path to trip, confirm
this finding. At the next opportunity the hoses will either be
reconnected or the naming of the paths will be swapped to correct this
discrepancy. Also, clear unmistakable labeling of both the hose ends and
the connection points needs to be implemented to avoid this problem in
the future.

2\) From figure 1 it is noted that there appears to be a radial gradient
in temperature, with the innermost layer being the coolest. This could
be explained by inward radial cooling which could be expected to be most
significant on the inner layer which faces the OH tension tube which in
turn faces the TF inner legs. The TF inner legs cool down very rapidly
after a pulse due to the large flow of water and the short water path
length. Further, it is noted that little outward radial cooling would
take place because the outer surface of the coil is covered by the
Microtherm insulation.

3\) As shown in figure 3, using the average OH winding length instead of
the maximum winding length, and the actual flow instead of the low flow
setpoint, the waveshape of the simulated temperature fits the
measurements reasonably well.

4\) As seen on figure 3, the simulated temperature during the plateau
period, when the cooling wave has not yet propagated through the coil,
exceeds the measurements by about 5^o^C. Possible explanations for this
include:

a.  errors in the thermocouple measurements, individual or systemic

b.  cooling of the water between its point of exit from the winding,
    through the hose, to the manifold where the thermocouples are
    located

c.  error in the ∫i^2^(t)dt used in the simulation

d.  error in the conductor dimensions used in the simulation

e.  error in the copper or water properties used in the simulation

f.  simulation neglects heat absorbed by epoxy/fiberglass insulation

g.  simulation neglects radial cooling

Concerning a. above, the measurements performed for the ISTP show that
at 50^o^C the t/c instrumentation reads about 2 ^o^C on the low side.
Concerning b., since the thermal resistance of the rubber hose material
and the surrounding air is large, this should be minimal in the
judgement of the writer. Concerning c., the writer has performed this
calculation very carefully using a spreadsheet with actual data from the
MDS Tree. Concerning d., the writer has double-checked this number and,
besides, the calculated resistance of the coil agrees with the
measurement. Concerning e. and f., a simple energy balance spreadsheet
which takes the energy dissipated during the pulse in the copper, and
then distributes it in the combined heat capacity of the copper,
entrained water, and epoxy glass insulation does indeed yield a plateau
temperature in between the simulation and the measurement (measurement
average = 46 ^o^C, spreadsheet = 49 ^o^C, simulation = 54 ^o^C).
Concerning g., there is in fact evidence of radial cooling as noted in
2) above.

Based on the above discussion, it is concluded that the simulation
overestimates the water temperature rise over the inlet (13 ^o^C )
during the plateau period by (54-(46+2))/(46+2-13)\*100=17%, due mainly
to the fact that the it does not include the heat capacity of the epoxy
glass insulation or the inward radial cooling.

The reader is reminded, however, that *during* a pulse, the heat
dissipated in the copper does not immediately equilibrate with the
entrained water and the epoxy glass insulation, so that the actual peak
*copper* temperature is considerably in excess of the plateau *water*
temperature (for the shot analyzed, with a starting temperature of 13
^o^C the copper temperature at the end of the pulse under adiabatic
conditions is calculated to be 61^o^C). So the key features of the
protection are that 1) the SOP temperature measurement feeding the
interlock is accurate, and 2) that it is properly coordinated with the
RIS ∫i^2^(t)dt setting. The fact that the simulated plateau temperature
is higher than the actual is not really very important. It simply means
that the actual temperatures will fall below the interlock value a bit
sooner than predicted by the simulation.

Based on this analysis the recommended approach is to adjust the
∫i^2^(t)dt and Start Of Pulse (SOP) temperature allowables previously
stated \[1,2\] for the three repetition periods based on the modified
simulation results which use the average winding path length instead of
the maximum and the actual flow instead of the low flow setpoint. Due to
the fact that the simulation does not include the heat capacity of the
epoxy glass insulation, or the inward radial cooling, the actual
cooldown will occur more rapidly then predicted by the simulation. This
represents a slightly sub-optimal utilization of the capacity of the
system but is judged to be a reasonable compromise. The revised
settings, which result in a nominal EOP copper temperature of 90 ^o^C,
maximum EOP copper temperature of 100^o^C in case of fault at 24kA, are
given in the following table.

  ----------------- ----------------- ----------------- -----------------
  Repetition Period 300 sec           450 sec           600 sec

  Allowable         160 kA^2^-sec     265 kA^2^-sec     280 kA^2^-sec
  ∫i^2^(t)dt                                            

  Allowable         57%               95%               100%
  ∫i^2^(t)dt                                            

  T~sop~            42^o^C            14 ^o^C           12.4 ^o^C
  ----------------- ----------------- ----------------- -----------------

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Figure 1: Eight Cooling Water Measurements

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Figure 2: Zoom of Eight Measurements During Cool Down

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Figure 3: Comparison of Simulation Against Min, Max and Avg At Each Time
Step

cc:

E Baker M Bell R Herskowitz M Kalish R Marsala D Mueller

M Ono G Pearson S Ramakrishnan P Sichta A Von Halle M Williams

NSTX File