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INTRODUCTION
In order to effectively utilize the high heat flux available through the
mechanism of nucleate boiling in forced convection heat transfer, it is of
primary importance that the maximum flux or "burnout" conditions be known for
the liquid under consideration. It is a characteristic of the boiling heat
transfer process that, as attempts are made to exceed the burnout heat flux,
the conditions at the heat transfer surface become such that the heat transfer
coefficient decreases with increasing temperature difference between the wall
and fluid. If the apparatus in which this process occurs is not of the type
in which a constant temperature is imposed, another equilibrium point will be
reached at a significantly higher wall temperature. In the case of water at
pressures of atmospheric and higher, the wall temperature assumed in the new
equilibrium state is high enough to cause failure in all but the most conservatively designed apparatus.
Because of the unstable nature of the boiling process beyond the maximum
vs. temperature difference on the q/A vs. L T curve, once the burnout temperature difference is exceeded, small power reductions will not save the heat
exchanger from the major portion of the incipient temperature jump. Power
must be reduced to a relatively low level to insure that excessive temperature will not be developed in the equipment. The time in which this power
reduction must be accomplished depends on the particular flux and the heat
capacity of the system being used; however, in most practical cases, this
time can be expected to be extremely short.
DETECTOR REQUIREMENT FOR MIT PROJECT
The boiling heat transfer project at M.I.T. utiliz0s water at a maximum
pressure of 2000 psia. flowing through a .18" I.D. nickel tube with a .015"
wall. Power is supplied by a Q.kw, 2000 amp generator system and is absorbed
by a 9" length of the nickel tube. In this system, calculations indicate that
approximately 6 millisecs are available between the time at which the burnout
A T is exceeded and the time at which "serious" overtemperature will occur.
Power must essentially be reduced to zero in this time. "Serious" overtemperature in this case is somewhat lower than that which would rupture the test
section because of the importance of an unaltered tube surface condition to
insure the consistency of subsequent data to be taken with the same tube.
It
is estimated that, in this particular configuration, a 50 degree overtemperature is tolerable at peak test conditions. It is on this temperature rise
that the 6 millisec requirement is predicated.
In order to avoid the delays involved in taking burnout data by actually
rupturing test sections, the project undertook to devise a mechanism which
would interrupt the heating current circuit within 6 millisec of the initiation of the burnout process. It is important that this particular apparatus
actually reach burnout flux, since it is being used for research on the burnout phenomenon. The circuit breaker must, therefore, be such as to insure
that interruption is not initiated before the actual burnout point is reached.
The system must be capable of "deciding" when a bona fide burnout condition
exists and performing the necessary sequence to insure circuit interruption
within the time alloted.
GENERAL FEATURES OF THE DETECTOR
The logical burnout criterion is test section wall temperature at the
Since the MIT projtube exit where thermodynamic burnout invariably occurs.
ect employs DC heating by passing current through the test section wall, ther-
mocouples must be electrically insulated from the wall. The necessary insulation reduces the response time of the thermocouple to an unacceptable level;
therefore, wall temperature is read indirectly through its effect on the resistivity of the tube wall. Specifically, the resistivity of the final 1/16"
of the test section is compared with that of the remainder with a rapidly increasing unbalance constituting a burnout signal. This unbalance, which is
measured with a Wheatstone bridge circuit, provides the input signal of a DC
Amplifier. The rate of change of tube resistivity with temperature is such
that, after biasing the circuit to exclude signals fran background noise
(generator ripple, nearby equipment, etc.) and internal noise generation within
the amplifier, the decision time of the system is relatively long. In view of
this anticipated decidion time, a design goal of 9.5 millisec was chosen as
the action time of the switch itself.
The problem of failure prevention then became one of devising a system
which would satisfy the above requirements. It was soon learned that the
fastest $echanical circuit breakers available fell far short of the necessary
performance. The best mechanical breaker considered required 2.$ millisec for
-3interruption. A level of performance which could only be achieved at the expense of extreme wear of the breaker components. Consequently, explosives
were investigated as a means of rapid circuit interruption. After extensive
testing of various interrupting and firing configurations, a satisfactory
system was designed around a DuPont X-98-N blasting cap (#6 strength, RDX
loaded for 350 degrees F temperature stability) inside of a 1/2" OD, .047"
wall copper tube carrying the test section current. This cap is fired by a
7000 volt discharge from a 0.1/A f farad condenser.
ELECTRICAL COMPONENTS
The detection of incipient burnout is accomplished by the Wheatstone
Bridge. The resistance of the final 1/16" of tube and the remainder of the
tube form two legs of the circuit. The remaining two bridge circuit legs are
made up of fixed and variable resistances (see Fig. 1). This configuration
is ideal for its present use. During a run, the test section current is increased slowly. Therefore, any bridge unbalance due to varying tube temperature can be readily compensated for by manual adjustment of the variable re-.
sistors. At high heat flux, in the neighborhood of burnout, the test section
is normally almost isothermal, so that, while the test section resistance may
vary, the resistances of the two portions of the tube remain in a relatively
constant ratio to each other.
The net voltage at the reference point in the bridge circuit is fed to a
four stage DC amplifier consisting of two 12AY7 and two 12AX7 stages. The
voltage across any one or combination of the four amplification stages may be
monitored and manually adjusted.
The output of the DC amplifier is used to fire a triggering circuit (Fig.
2) which discharges the 7000 volt .1014,f capacitor through the spark gap in
the dynamite switch. A 4035 thyratron tube is required to perform this function.
The 4C35 will not, however, tolerate the relatively slow buildup of the triggering pulse in the amplifier. It is, therefore, isolated from the amplifier by
another pulse circuit consisting of a 2D21 thyratron, pulse transformer, and
capacitor which will tolerate relatively slow pulses and provide an output
signal with a rise time of approximately one microsec.
This circuit is shown
in Fig. 1 as part of the amplifier circuit.
The pulse generator physically contains somewhat more circuitry (see Fig.
2) than required for a functional explanation of its operation. The function
f condenser and to provide
of the rest of the circuit is to charge the .10
sufficient warmup time for the 4C35 cathode.
-4The pulse generator physically contains somewhat more circuitry (see
Fig. 2) than required for a functional explanation of its operation. The
function of the rest of the circuit is to chatge the .10 f condenser and to
provide sufficient warmup time for the 4C35 cathode.
In an effort to minimize the susceptibility of the DC amplifier to drift,
a power supply is provided which accurately regulates the reference voltages
(Fig. 3). This is achieved in part by a series tube voltage regulator circuit using gas-discharge reference tubes and amplified feedback. The supply
is, in reality, two regulated supplies; one for +180v and the other for -90v
with respect to ground. The OB2, 6AU6, 12AU7 circuit produces a regulated
270v ungrounded. The important ratio of the + and - voltages is determined
by a 12AX7 difference amplifier which reads the signal from a precision voltage divider and drives a 6AQ5 tube to establish ground at low impedance.
OPERATING EXPERIENCE
Without exception, the burnout detector has successfully prevented failure of the test section. The detector has permitted the actual attainment of
the burnout condition in the test section and no unexplained premature interruptions have occurred. The fact that the detector is not firing prematurely
is checked by periodically allowing a test section to fail while observing the
relation between the discharge of the capacitor and the appearance of steam
or other evidence of failure. These two events invariably occur simultaneously.
There have been a few instances of unintentional test section loss due to failure of the interruptian system. In each of these cases, failure was attributable to poor electrical connections or improper adjustment of the bridge circuit.
The reliability limiting electrical element of the system is the DC amplifier. The tolerance of this unit to variations in tube characteristics is
lower than the tube manufacturing tolerances. Consequently, electrical failures are difficult to isolate and trouble shooting the unit is time consuming.
Failure of the amplifier is usually characterized by wide fluctuations in output voltage.
This type of failure is usually detected during warm-up of the unit. If
the amplifier should become unstable during a run, the circuit breaker is actuated by the erratic signal and the run aborted without failure of the test
section.
-5BURNOUT DETECTOR SWITCH
The actual interruption of the circuit, as mentioned previously, is accomplithed by setting off an explosive cap which is contained in a hollow
copper tube (Fig. 4).
The tube is 3-1/32" long and has a 1/2" OD and .047"
wall.
It is fitted with a nylon end piece which holds the blasting cap leads
at right angles to the tube axis and provides electrical and thermal insulation between the tube and cap leads.
One of the cap leads is grounded to the
switch body (Fig. 5) while the other is held under an entry for the electrode
from the .1A.-f condenser.
Contact between the electrode and the cap lead is light and is maintained
by the lead bearing on the electrode under its own elastic restoring force.
The tube is held by two pairs of steel clamping blocks, each block being 2" by
3" by 1" thick. One pair of these blocks is shown holding a spent tube in
Fig. 6. The mating faces of each pair form an oval hole with a circumference
slightly larger than that of the tube. The change in tube cross section during
clamping insures a strong grip on the tube and low contact resistance. The
clamping force on the blocks has been measured at approximately 2-1/2 tons,
and is provided by a heavy toggle linkage mounted on the switch body.
To avoid arcing between the clamping blocks and the copper tube on opbning or closing the clamping blocks, an auxiliary pair of contacts is used to
make the connection with the bus bars.
The clamping blocks may be thought of
as "long" and "short" motion blocks. One pair of these is shown in Fig. 6.
The sequence of events in closing the switch is as follows:
a) As the motion starts, the long motion block (2-1/4" travel) shown
to the left of the copper tube in Fig. 6, moves to make contact with
the tube.
b) Further motion causes the clamping block-tube assembly to compress
the rubber gasket between the bus bar and the rear face of the short
motion block shown to the right of the copper tube in Fig. 6. After
1/16" of travel, the short motion block comes into contact with the
bus bar. The 1/16" compression of the rubber gasket requires a force
of approximately 100 lbs.
c) Further motion of the toggle mechanism causes deformation of the
copper tube between the clamping blocks.
-6On opening the switch, the reverse takes place with the contact between
the short motion block and the bus bar being broken before the clamping blocks
separate from the tube.
The separation force is provided by the rubber gasket
on the bus bar.
The entire assembly is enclosed in a box of 1/4" steel plate (Fig. 8 and
9).
The outer container is insulated from the current carrying components
inside the unit by 1/4" of high strength glazed micarta.
Micarta is also used
as insulation around all bolts.
SWITCH OPERATION
The mechanical components have an excellent service record.
The switch
shown in the photographs in this report has fired approximately 150 times.
It can be seen that the clamping blocks show little evidence of wear.
The mi-
carta insulation is eroded in the blast area, but penetration of blast effects
into the micarta has not yet warranted any replacement.
There are, however,
two areas in the mechanical switch which are prone to wear.
The portion of
the case directly opposite the open end of the copper tube must be protected
from erosion with a small (1" x 1") steel plate which must be replaced at 50
shot intervals.
motion blocks.
The second point of wear is the rubber gaskets on the short
These eventually acquire a permanent set and must also be re-
placed at approximately 50 shot intervals.
CONCLUSION
The burnout detector design outlined above is well suited for research
work because of its rapid action and accuracy.
The nature of the burnout
process under study mates further refinements in its operation unnecessary.
The detector and switch have excellent operating records and burnout is invariably prevented quickly enough to avoid measurable effects on the test
section.
The largest single factor effecting the utilization rate of the
unit is the DC amplifier which is somewhat lacking in stability and should be
changed to a more stable, commercially available design in future detectors.
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View showing the edge of one bus bar and rubber
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