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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 conser-

vatively 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 temper-

ature 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 tempera-

ture 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" overtemper-

ature 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 overtempera-

ture 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 initia-

tion of the burnout process. It is important that this particular apparatus

actually reach burnout flux, since it is being used for research on the burn-

out 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

tube exit where thermodynamic burnout invariably occurs. Since the MIT proj-

ect employs DC heating by passing current through the test section wall, ther-

mocouples must be electrically insulated from the wall. The necessary insula-

tion reduces the response time of the thermocouple to an unacceptable level;

therefore, wall temperature is read indirectly through its effect on the re-

sistivity 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 in-

creasing 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

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interruption. A level of performance which could only be achieved at the ex-

pense 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 in-

creased slowly. Therefore, any bridge unbalance due to varying tube temper-

ature 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 trigger-

ing 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

of the rest of the circuit is to charge the .10 f condenser and to provide

sufficient warmup time for the 4C35 cathode.

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The 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 cir-

cuit 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 vol-

tage divider and drives a 6AQ5 tube to establish ground at low impedance.

OPERATING EXPERIENCE

Without exception, the burnout detector has successfully prevented fail-

ure of the test section. The detector has permitted the actual attainment of

the burnout condition in the test section and no unexplained premature inter-

ruptions 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 fail-

ure of the interruptian system. In each of these cases, failure was attribu-

table to poor electrical connections or improper adjustment of the bridge cir-

cuit.

The reliability limiting electrical element of the system is the DC am-

plifier. The tolerance of this unit to variations in tube characteristics is

lower than the tube manufacturing tolerances. Consequently, electrical fail-

ures are difficult to isolate and trouble shooting the unit is time consuming.

Failure of the amplifier is usually characterized by wide fluctuations in out-

put 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 ac-

tuated by the erratic signal and the run aborted without failure of the test

section.

-5-

BURNOUT DETECTOR SWITCH

The actual interruption of the circuit, as mentioned previously, is ac-

complithed 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 insula-

tion 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 opbn-

ing 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.

-6-

On 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. The second point of wear is the rubber gaskets on the short

motion blocks. 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 in-

variably 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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Fig. 5

View showing orientation of leads to dynamite cap. downlead is grounded to case. Up leg is under entry point ofelectrode from capacitor.

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View showing the edge of one bus bar and rubbergasket around the bus bar contact area.

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