Feedback system monitors source temperature and makes necessary changes of area available for heat rejection by adjusting storage volume of noncondensible gas and position of vapor/gas interface

Bienert, W. B.

English

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1972	 Brief 72-10169 
NASA TECH BRIEF 
Ames Research Center	 4r 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Feedback Control of Variable Conductance Heat Pipes 
The results of theoretical studies suggest that the 
performance of variable conductance heat pipes used 
to regulate the temperature of a heat source can be 
improved by introduction of a feedback loop. Pre-
liminary designs have been completed for two dif-
ferent approaches to implement such a loop; one 
Temperature 
Sensor 
Heat
Source	
WteCl,al1iS
and regulation suffers if mass diffusion of vapor into 
the gas storage volume can occur. Under any of 
these conditions, a feedback that is keyed to the 
source temperature can effectively improve control 
of the source temperature. 
The typical variable conductance heat pipe is a 
Volume 
Control 
L.__ Rejection ......J Heat Pipe,,,,,,	 ,,/-Wick	 Zone
Gas 
Working Vapor	 Storage 
ace	 Volume 
I' Transport Zone	 Heat Fk
Heat 
Absorption	 4 Zone
I  
I	 Sink 
uses a passive feedback system and the other uses 
an active feedback system. 
The variable conductance pipe tends to maintain 
its own temperature constant by extracting heat from 
the source,, thereby maintaining the temperature of 
the source within limits. However, it will allow the 
temperature of the heat source to vary greatly if a 
high thermal impedance exists between the source 
and the pipe. Wide fluctuations in energy transport 
requirements or in sink temperature will also ad-
versely affect the temperature regulation of the source,
long, thin, sealed tube with an expanded region at 
the cooling end to act as a storage volume for a non-
condensable gas, as shown in the figure. The vapor/ 
gas interface is located somewhere in the heat re-
jection zone. The condensable working fluid moves 
from the heat absorption zone to the rejection zone 
as a vapor, and is returned as a liquid by the capil-
lary action of the wick lining the inside of the pipe. 
In principle, the feedback system monitors the 
source temperature and makes necessary changes of 
the area available for heat rejection by adjusting the 
(continued overleaf) 
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storage volume of the noncondensable gas and, hence, 
the position of the vapor/gas interface. For example, 
when the feedback system senses an increase in 
source temperature, the control action is such that 
the vapor/gas interface is caused to move down-
stream away from the evaporator, thus increasing the 
heat rejection area. 
The passive feedback system which has been de-
signed employs an extensible bellows as a storage 
volume. The feedback medium is an enclosed body 
of incompressible fluid that extends from a tempera-
ture-sensing bulb at the heat source through a slim 
tube to a second bellows attached to the first. Tem-
perature changes at the source cause volume varia-
tions of the medium which are coupled through the 
bellows to adjust the effective cooling area. 
The active feedback system is comprised chiefly 
of an electronic temperature sensor, an electronic 
controller, and an electrical device to heat the storage 
volume. Extension of the wick into the storage 
volume assures the presence of saturated vapor mixed 
with the noncondensable gas at all, times. Volumetric 
response of this mixture to temperature variation is 
much sharper than that of dry gas alone and, at the 
same time, mass diffusion is controlled. When the 
source temperature is too low, heat is applied to the 
storage volume; this results in a motion of the vapor/ 
gas interface in the direction which reduces the ef-
fective thermal rejection area. Consequently, the 
energy transport function of the pipe is reduced and 
the source temperature is allowed to rise. Stability 
analyses made for both of these designs and for the
general case showed that a stable system can always 
be achieved. A mathematical model has been devel-
oped in functional form to treat the various methods 
for obtaining thermal control; it may be applied to 
a wide variety of configurations. 
Notes: 
1. The following documentation may be obtained 
from: 
National Technical Information Service 
Springfield, Virginia 22151 
Single document price $3.00 
(or microfiche $0.95) 
Reference: NASA CR-73475 (N71-15602), 
Study to evaluate the Feasibility of a Feed-
back Controlled Variable Conductance Heat 
Pipe. 
2. No additional documentation is available. Specific 
questions, however, may be directed to: 
Technology Utilization Officer 
Ames Research Center 
Moffett Field, California 94035 
Reference: B 72-10169 
Patent status: 
No patent action is contemplated by NASA. 
Source: Walter B. Bienert of
Dynatherm Corporation 
under contract to
Ames Research Center 
(ARC-10460) 
Brief 72-10169
	 Category 03


Feedback control of variable conductance heat pipes

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