posted on 2010-11-26, 10:01authored byJose Sergio de Almeida
During the development of flight tests of a spacecraft, heat exchange occurs
among the many physically separated subsystem surfaces through the phenomenon of
thermal radiation. Considering the increasing complexity of the geometrical forms and
shapes in the design of such systems, the monitoring and control of the radiative heat
fluxes taking place in the multi-reflecting, absorbing and emitting heat transfer
environment are very critical. Because the analytical solution of thermal radiation in such
geometrically complex 3-dimensional systems is not practical, extensive numerical
modelling techniques are widely used to predict radiative heat fluxes on the many
thermally active surfaces. From experience, it is found that this can be very difficult and
not at all commensurate with fast feedback unless the analysis is from a simple system
layout.
Considering that a relatively new approach dedicated to the basic analysis of
radiative heat flux has been developed by the heat transfer community as a numerical
approximation called the Discrete Ordinates Method (DOM), a first question did arise in
terms of how well an enhanced and more comprehensive formulation based on this
concept would fulfil the task of achieving faster results whilst still accurately predicting
radiative heat transfer in 3-dimensional, more complex geometries. Since both the numerical modelling work and the applicability of the more
practical-to-use radiometers are actually intrinsically connected when considering
validation, a fundamental research program was undertaken in an orderly and sequential
fashion. For the theoretical part of the program, the analytical algorithm related to the
Discrete Ordinates Method started with the basic 2-dimensional formulation and was
enhanced and improved in a step-by-step manner, with results being compared with tests
from other published works. At the end of this phase, a comprehensive DOM-based
formulation was obtained, dedicated to the analysis of radiative heat transfer in complex
geometries. This included a number of internal boxes holding distinct temperature values
and having arbitrary characteristics of dimension, shape and system installation layout
and also with varying multi-emitting, absorbing and reflecting properties. This was
validated against experimental measurements. For the practical segment of the research
program, an extensive study was carried out in terms of the straightforward application,
installation and use of standard-built radiometers and, subsequently, a full-size system
thermal model representing all the requirements of arbitrary thermal, optical and
geometrical conditions was built. Results from this were compared with those from the
theoretical analysis. To access and analyse a direct comparison between the numerical modelling
predicted data and the experimental measurements, several thermo-geometrical situations
were proposed and subsequently reproduced in the laboratory. The theoreticalexperimental
comparison results showed that a consistent data correlation was
successfully obtained. The Discrete Ordinates Method proved to be fast and to accurately
predict radiative heat flux in complex geometries similar to those found in the design and
tests on actual spacecraft. Also, the proposed analytical and experimental approach gave
confidence that the installation and operation of standard radiometers could be
implemented in a straightforward way to produce the desired reliable practical results.
This work presents all the relevant details concerning the complete investigation
process that was undertaken during the research program.
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