Measurement of radiation in complex geometries and comparison with calculational techniques
thesisposted on 26.11.2010, 10:01 authored by Jose 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.
- Mechanical, Electrical and Manufacturing Engineering