Page 195 - MEMS and Microstructures in Aerospace Applications

Osiander / MEMS and microstructures in Aerospace applications DK3181_c009 Final Proof page 186 1.9.2005 12:07pm

186 MEMS and Microstructures in Aerospace Applications

For a simple case, the concept of conductance is equivalent and thermally
describes the plate. For a complicated case, such as an interface or a complicated
structure, the conductance is measured or modeled and can be used to describe the
thermal transport. For example, the thermal conductance for multilayer systems is
calculated with the same laws as electrical conductance. The concept of thermal
conductance is important for the spacecraft thermal design with numerous struc-
tures and materials. In a model, they all can be treated as a conductance.

9.2.2 CONVECTION
Convection is heat transport in a fluid or gas by the macroscopic movement of matter.
Convective heat transfer is classified as free convection or forced convection accord-
ing to the nature of the flow. Forced convection employs a pressure gradient (e.g.,
from a fan, a mechanical pump, or a capillary wick) to drive the fluid motion, as
opposed to free convection in which density gradients driven by gravity induce fluid.
Free convection is of little importance for heat transport in space where as forced
convection can be very effective. Important applications that make use of convective
transport are heat pipes and related capillary-driven devices. These devices rely on
the latent heat associated with a change of phase. When a substance changes phase —
from vapor to liquid, liquid to solid, solid to liquid, or liquid to vapor — there is a
significant change in the energy state of the material. Typically, this is associated with
the addition or loss of thermal energy. For spacecraft, two-phase heat transfer
commonly involves vapor or liquid transformations (i.e., vaporization and liquifac-
tion); although, a few applications involve liquid to liquid or solid to solid transform-
ations. External energy is absorbed into a two-phase device when the liquid
evaporates (taking heat away from the evaporator area) and is released when it
condenses. Therefore, the gas flow not only carries the heat related to its specific
heat (C p ) and temperature (T), but also the ‘‘latent heat’’ (L), which typically is much
higher. Such two-phase heat transfer is extremely efficient; several orders of magni-
tude more effective than normal convection and also offers the benefit of isotherm-
ality. Heat pipes play a very important role in spacecraft thermal control; however,
two-phase systems tend to be more challenging to design.

9.2.3 RADIATION
The third and last form of heat transfer process is radiation. Contrary to conduction
and convection, thermal radiation does not rely on any type of medium to transport
the heat. Radiation heat transfer depends on the characteristics and temperature of
the exposed, radiating surfaces and the effective sink temperature of their views to
space. With these unique characteristics, radiation heat transfer becomes the most
critical heat transfer process in space.
Any object at a temperature above absolute zero emits electromagnetic radi-
ation. This thermal radiation is the dominant form of heat transfer in space, since the
thermal radiation emitted from the outside surface of a spacecraft is the only means
of losing heat.

© 2006 by Taylor & Francis Group, LLC