Heat transfer correlations for multilayer insulation systems
Introduction
The diurnal variation of solar, albedo (earth-reflected solar radiation) and earthshine (earth-emitted radiation) heat loads produces large temperature fluctuations in orbiting spacecrafts. Moreover, non-uniform heating of different parts of a spacecraft causes large thermal gradients in the spacecraft. In addition, appendages such as the boom, experimental packages, and sun sensor must be protected from high-temperature propulsion thrusters, plumes, external loads, etc. For these reasons, multilayer insulations (MLI) find application in spacecrafts as lightweight thermal isolation systems. The usual structure of a multilayer insulation system for spacecraft applications consists of a number of thin shields with very low infrared (IR) emittance, separated by low-conductivity spacers. The shields are generally made out of plastic (Mylar or Kapton) films (typically 1/4 mm thick), which have a high mechanical strength and a low thermal conductivity. For reflecting, the shields are coated with vacuum-deposited aluminium or gold (with a thickness of about 500 Å). The common materials for spacer are polyester or nylon netting or silk.
The heat transfer mechanism of a multilayer insulation system can be extremely complex. Presence of anisotropic conductivity, coupled conduction and radiation heat transfer, temperature dependent physical properties, three-dimensional temperature profile, complex geometries of the insulated system, etc. can make the heat transfer analysis in multilayer insulation quite difficult. Hence, for practical engineering design purposes, it is necessary to have simple procedures to evaluate the heat transfer rate in multilayer insulation. Models such as: (1) conductance model, (2) effective emittance model, (3) conduction–radiation model and (4) Cunnington and Tien [1] model may represent the heat flux through MLI. Predictions based on these four models are fitted with available experimental data on MLI performance [2], in order to develop correlations for heat flux through MLI.
Section snippets
Empirical models for heat flux
A brief description of the four heat transfer models is as follows.
Results and discussion
Results of curve fitting with the above models are presented in Table 1, Table 2, Table 3, Table 4. In these tables, the abbreviations nLSS and nLDS correspond to the number of single spacer layers and the number of double spacer layers, respectively. The observed experimental heat fluxes (unconnected filled symbols) are compared against the model-estimated heat flux curves (open symbols connected by continuous line) in Fig. 1, for a specimen of ten-layer single spacer MLI. Overall, the
Conclusion
The experimentally observed thermal performance data of MLI, reported in [2] have been correlated with conductance (h), effective emittance (εeff), conduction–radiation (CR) and Cunnington and Tien (CT) models. It has been found that the Cunnington and Tien model estimates heat flux sufficiently accurate for spacecraft MLI design purposes.
Acknowledgments
The authors wish to thank P.P. Gupta, Head, Thermal Testing Division, A. Ramasamy and V. Ramakrishnan, Thermal Properties Measurement Section, D.R. Bhandari, Head, Thermal Design and Analysis Division, H. Narayanamurthy, Group Director, Thermal Systems Group and Prof. A. V. Patki, Deputy Director, ISRO Satellite Centre for their constant support, encouragement and constructive suggestions during the course of this work.
References (3)
- Cunnington GR, Tien, CL. A study of heat transfer processes in multilayer insulation. In: Bevans JT, editor....