Radiant energy – Radiation controlling means
Reexamination Certificate
2000-11-30
2004-03-30
Lee, John R. (Department: 2881)
Radiant energy
Radiation controlling means
C250S492100, C250S493100, C250S494100, C250S503100, C165S133000
Reexamination Certificate
active
06713774
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a structure and method for changing or controlling the thermal emissivity of the surface of a radiating object in situ, and thus, for changing or controlling the radiative heat transfer between the object and its environment in situ. More particularly, changing or controlling the degree of blackbody behavior of the object is accomplished by changing or controlling certain physical characteristics of a structure defining a plurality of cavities on the surface of the object. As described herein, this cavity structure may be integral to the radiating object or added to the surface of the object to form a new radiating surface.
BACKGROUND OF THE INVENTION
Heat transfer between an object and its environment is achieved by up to three main processes: conduction, convection, and radiation. While conduction occurs at solid/solid and solid/fluid interfaces, the principal means of transferring heat into or out of many systems is by a combination of convective media and radiation. Terrestrial system designs typically exploit both convective and radiative heat transfer, however, heat management in many space (i.e., extraterrestrial) systems relies essentially on radiation because of the lack of a convective medium.
Convective heat transfer is provided by the natural or forced flow of a fluid over the surface of an object and can be controlled by changing parameters such as the fluid medium and/or its physical properties, flow rate, and surface roughness. In contrast, radiative heat transfer depends on the degree of blackbody behavior exhibited by the surface and the fourth power of surface temperature. Thermal energy radiated by a surface is expressed by the Stefan-Boltzmann equation:
Q
rad
=A
&sgr;&egr;(
T
b
4
−T
a
4
) (1)
where
Q
rad
=thermal power radiated (W)
A=area of radiating surface (m
2
)
&sgr;=the Stefan-Boltzmann Constant (5.67×10
−8
W/m
2
/K
4
)
&egr;=thermal emissivity factor of radiating surface
T
b
=temperature of the radiating surface (K)
T
a
=ambient temperature (K)
The thermal emissivity factor (&egr;) is the ratio of an object's radiative emission efficiency to that of a perfect radiator, also called a blackbody. The thermal emissivity factor of most materials ranges between 0.05 and 0.95 and is relatively constant over a significant temperature range. Therefore, the radiative heat transfer capability of an object is typically a predetermined, monotonic function of its temperature raised to the fourth power.
The following example illustrates the expected impact of changing the thermal emissivity, or degree of blackbody behavior, of an object that is transferring heat by free convection and radiation. In this example, the reference object is a horizontal cylinder 1 m long with a 10 cm outer diameter, rejecting heat to a 300K environment through free convection and radiation. A simplified equation for the laminar flow convective heat transfer coefficient, h, for the object is:
h
=1.32(&Dgr;
T/D
c
)
0.25
(2)
(Holman, J. P., Heat Transfer, Sixth Edition, McGraw-Hill) where
&Dgr;T=temperature difference between surface and ambient (K)
D
c
=diameter of cylinder (m)
Heat transferred by convection (Q
conv
) is expressed by:
Q
conv
=hA
(
T
b
−T
a
) (3)
where A, T
b
, and T
a
are the same variables as in Equation 1.
FIG. 1
shows the amount of heat rejected from the reference object by convection and radiation using Equations 1 and 3, respectively, over a &Dgr;T range of 1-1000 K, which covers a principal range of engineering interest. This figure shows the convection term (Q
conv
) to be approximately an order of magnitude larger than radiation (Q
rad
) from a surface with &egr;=0.1 for &Dgr;T up to about 100 K. Beyond this temperature, the T
4
dependence of radiation increases more rapidly, making the two modes of heat transfer approximately equal when &Dgr;T approaches 1000 K. In contrast, radiation from a surface exhibiting ideal blackbody behavior (i.e., &egr;=1.0) is always greater than convection and is at least an order of magnitude larger when &Dgr;T is near or above 1000 K. More importantly,
FIG. 1
illustrates the potential impact on the heat transfer capability of the reference object as the thermal emissivity of its surface changes, by changing the thermal emissivity factor from &egr;=1.0 to &egr;=0.1, and vice versa.
Thus, the ability to change or control the degree of blackbody behavior of a radiating object, while it is in service (i.e., in situ), analogous to changing or controlling the convective term in a fluid system during operation by altering the flow rate of the fluid, would enable a remarkable improvement in the thermal design and control of many systems where radiative heat transfer is important. For example, the surface of an object or system with controllable thermal emissivity could be activated at some limiting temperature as a thermal safety valve. In this mode of operation, the surface would be triggered to switch to a higher thermal emissivity that, in turn, radiates more heat to prevent the temperature of the object or system increasing above safe limits. Similarly, switching thermal emissivity to a lower value could protect against a system operating at less than a desirable temperature limit.
In addition, changing the thermal emissivity of an object will effectively change its thermal, or infrared (IR), signature. This is especially important in detection, recognition, and camouflage applications. For example, the ability to change or control the thermal emissivity of an object provides an opportunity for an object to match its thermal emission characteristics with those of other objects or structures in its vicinity, thereby enabling an IR camouflage effect.
In current systems where radiative heat transfer is important, the surface material and/or surface preparation of a radiating object is carefully selected to obtain the desired fixed thermal emissivity and resulting radiative heat transfer characteristic. Typical surface preparations include a variety of coating, etching, and polishing techniques. Etching techniques are also being used to create fixed surface textures for spectroscopic applications. For example, Ion Optics Inc. (Waltham, Mass.) has developed tuned infrared sources using ion beam etching processes that create a random fixed surface texture consisting of sub-micron rods and cones (http://www.ion-optics.com). Such a surface texture has a high emissivity over a narrow band of wavelengths and low emissivity in other bands and is an attractive alternative to IR light-emitting diodes.
Applying the emerging field of solid state microelectromechanical technology, tunable IR filters for IR spectral analysis are also being developed. An example of such a device is reported by Ohnstein, T. R., et al (“Tunable IR Filters With Integral Electromagnetic Actuators,” Solid State Sensor and Actuator Workshop Proceedings, 1996, pp 196-199, Hilton Head, S.C.). Such tunable IR filters comprise arrays of waveguides whose transmittance can be varied by changing the spacing between them using linear actuators. The wavelength cutoff range from 8 &mgr;m to 32 &mgr;m achieved by Ohnstein et al with this technology is typical of its narrowband selectivity. Such IR spectral analysis devices, like the devices developed by Ion Optics, Inc., are purposely designed with surface microstructures having dimensions comparable to specific wavelengths in the electromagnetic spectrum to be effective at wavelengths that are discrete or in narrow bandwidths. Consequently, these devices are ineffective for applications which require the changing or controlling of broader ranges of wavelengths important in radiative heat transfer.
Accordingly, there is a need for a capability to change or control broadband radiative heat transfer between an object and its environment while the object is in service.
BRIEF SUMMARY OF THE INVENTION
The present in
Antoniak Zenen I.
DeSteese John G.
Peters Timothy J.
White Michael
Battelle (Memorial Institute)
Lee John R.
Matheson James D.
May Stephen R.
Vanore David A.
LandOfFree
Structure and method for controlling the thermal emissivity... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Structure and method for controlling the thermal emissivity..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Structure and method for controlling the thermal emissivity... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3273111