Heat transfer coefficient in serpentine coolant passage for CCDTL Page: 4 of 6
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HEAT TRANSFER COEFFICIENT IN SERPENTINE COOLANT
PASSAGE FOR CCDTL*
P. Leslie, R. Wood, F. Sigler, A. Shapiro, A. Rendon
Los Alamos National Laboratory, Los Alamos, NM 87545, USA
A series of heat transfer experiments were conducted to
refine the cooling passage design in the drift tubes of a
coupled cavity drift tube linac (CCDTL) . The
experimental data were then compared to numerical
models to derive relationships between heat transfer rates,
Reynold's number, and Prandtl number, over a range of
flow rates. Data reduction consisted of axisymmetric
finite element modeling where the heat transfer
coefficients were modified to match the experimental
data. Unfortunately, the derived relationship is valid only
for this specific geometry of the test drift tube.
Fortunately, the heat transfer rates were much better
(approximately 2.5 times) than expected.
The objective of this experiment was to use
experimental results combined with numerical simulation
to measure heat transfer rates in drift tube coolant
passages for the cavities in the Accelerator Production of
Tritium (APT) , Low Energy Demonstration
Accelerator (LEDA)  CCDTL Hot Model. The hot
model is a full scale, copper brazed structure that will be
exposed to full RF fields, but will not have beam through
it. A goal of the experiment is to refine the design of the
cooling passages and coolant systems for the LEDA
CCDTL. The results of this experiment were used to give
a better estimate of the heat transfer rates within the drift
tube coolant passages and are just a first look at the drift
tube thermal problem. Since the experiment is not error-
free, the Nusselt equation coefficients determined are
probably not an exact representation of all the physics of
the problem, but a match with this empirical data using
the specific geometry of the test item.
In the CCDTL, the drift tube is located within an RF
cavity and provides a region of no electric field which
shields the beam when the electric field would decelerate
the beam (for an in-depth description, see ). A great
deal of RF power is dissipated on the outer surface of the
APT drift tubes. A method was developed to form an
elaborate network of cooling passages within the body of
each drift tube . The coolant passages within the drift
tubes are rectangular, short, and curved, a situation which
is not well covered in the literature.
In the literature , the heat transfer coefficient in long,
straight, circular passages is given as
he = kwater * 0.023 * Re" * PrO4
where k, is the thermal conductivity of water, Re is the
Reynold's number, Pr is the Prandtl number. Since these
drift tube passages are not round, the convention is to use
the equivalent hydraulic diameter for a rectangular cross
section which is given by
D = -P
where A is the flow area and P is the wetted perimeter. It
is much more difficult to account for the passages being
short and curved. The complex three dimensional
geometry of the drift tube coolant passages make it
difficult to determine an effective heat transfer coefficient
directly from published data. It was necessary to use a
finite element, thermal/structural model to extract an
approximate value for the heat transfer coefficient. From
that data, an approximate relationship between the
Nusselt number, the Reynold's number, and the Prandtl
number for this geometry was derived.
The test setup consisted of a water chiller,
approximately 5 gallon reservoir, a flow meter with range
of 0 to 2 gpm, water filter, 17 heater cartridges, rheostat,
IOOX amplifier, a modified drift tube slug placed on a
styrofoam base with styrofoam "popcorn" completely
over it, tubing to connect these components together, two
thermocouples to measure drift tube temperatures,
another thermocouple to measure coolant inlet
temperature, a two pass thermopile to measure the
coolant temperature rise through the drift tube, and a data
acquisition system to record the data. Figure I shows a
schematic of the setup. For data reduction purposes, the
flow rate was determined from the heater power and the
temperature rise within the coolant from inlet to outlet.
Much depends on this measurement, so a two pass
thermopile was used to increase the sensitivity of the
measurement and lessen the effect of noise in the data.
Fiter Fbw Moter
5 - a on
m .- s Rheosla
- - - -
Figure 1. Schematic of the experiment.
The drift tube slug used in the heat transfer experiments
was a three passage drift tube that is identical to those
from which CCDTL hot model drift tubes were made.
Figure 2 shows the cross sectional drawing showing the 3
* Work supported by the US Department of Energy, Defense Programs
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Leslie, P.; Wood, R.; Sigler, F.; Shapiro, A. & Rendon, A. Heat transfer coefficient in serpentine coolant passage for CCDTL, report, December 31, 1998; New Mexico. (digital.library.unt.edu/ark:/67531/metadc684844/m1/4/: accessed August 14, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.