Effective Solar Absorptance of Multilayer Insulation 2009-01-2392
Multi Layer Insulation (MLI) is very commonly used in all spacecraft for heat conservation. In most instances one has to deal with MLI facing space or other cold surfaces while protecting the thermally controlled surface at more moderate temperatures than the heat sink. But in some instances, either in steady state or transient modes, one has to deal with the MLI facing the sun. Examples of such situations are during spacecraft turns, deliberate or inadvertent, when the MLI is exposed to solar insolation for short or extended periods.
The effective emittance of MLI is commonly used to describe its heat loss behavior in the absence of solar incidence and is well documented in widespread literature based on measurements and rules of thumb from practice. However, when MLI faces the sun, its effective solar absorptance comes into play to determine its effectiveness in controlling the temperature of the object that it was designed to protect thermally. The heat loss from the controlled object is different when it is absorbing solar energy on the MLI's external surface versus when it is not. The effective absorptance of MLI then simply is this difference in the heat loss flux of the MLI as a fraction of the incident solar flux.
This is where it is analogous to the effective emittance of MLI and provides a simple metric of the MLI's performance when exposed to the sun. Knowledge of the effective solar absorptance, just like the effective emissivity of MLI, allows for a simple accounting for the effect of solar incidence on the MLI's performance for thermal design and analysis.
Since MLI is not simply a series of radiation shields with only radiation between them but also has heat transfer by conduction due to thermal contact between them, the effective absorptance cannot be easily characterized due to the non-linear complicated thermal coupling between the innermost and outermost layers. In this paper the effective solar absorptance of MLI is estimated analytically by utilizing the following three known properties of the MLI: effective emittance of the MLI blanket and the emissivity/absorptivity of the outermost surface of the blanket.
Two ways of making these estimates are presented in this paper and discussed in detail. The first way is bounding in nature by using two extreme assumptions to bracket the estimates: pure radiation or pure conduction between the innermost and outermost layers. A generalized relationship between the two extreme estimates as a function of the MLI's intrinsic properties (effective emittance, external layer emissivity an absorptivity) is presented, along with range of applicability, general trends and rules of thumb.
The second method is more rigorous which uses standard equations that break down the radiative and conductive components of heat flow through MLI which are based on measurements of MLI heat losses and effective emissivity as a function of temperature, layer density, number of layers and layer properties.
The two approaches are compared and traded off in terms of simplicity vs. rigor, accuracy vs. expediency.