Speakers
Description
Going into cryogenic fields is always challenging for reasons such as achieving and keeping desired temperatures and stabilities for suitable time or due to temperature dependent material- and thermo-optical properties. One of the crucial goals of Thermal tests is to qualify the radiator system and with it the cold chain, and other is to correlate the Thermal Mathematical Model. Therefore, the test consists of temperature plateaus several times on such low levels, considered as cryo, where surfaces’ emissivity and absorptivity may change by temperature. This can lead to issues when it comes to implementing two surfaces (e.g. radiator and it’s dedicated cold plate) facing towards each other on different cryotemperatures into thermal software, such as ESATAN. The problem is not a graybody problem anymore.
In flight analysis, radiator planes face towards deep space, represented by artificial node, in which the temperatures are nearly constant. Therefore, the above-mentioned problem does not exist.
On the other hand, the issue does very much exist during modelling a test. OHB recently encountered with such problems during Thermal Test where, on cryotemperatures, emissivity of a radiator panel (white) and of its dedicated coldplate (black) followed their wavelength dependent behaviour. The setup is even more complex as there is a third surface, a radiator shield with highly reflective coating. Thus, the problem translates essentially to three different surfaces with view factors towards each other, one with white, one with black coating with a third reflective surface taking away the possibility of having constant view factors with the value of 1. The setup is depicted on below sketch:
(PDF attached)
OHB have found two possible ways of solving, however, none of them could be considered as perfect solution, therefore both have their own advantages and disadvantages.
1.Taking higher uncertainties into account
2.Using ESATAN PWGBR subroutine
First method can lead to conservative approach and, therefore, to an overdesigned system which can lead the engineers to decisions which would increase cost significantly.
Second method leads to tremendous amount of effort and working hours (and, in the end, to extra cost), while the problem would use a grey body assumption still in the solve but only for a given wavelength band. The model uses an effective GR which is based on the temperature dependent emissivities and current temperatures of the emitting and receiving bodies to obtain the same net heat flux that would be calculated in a non-grey body process.
The presentation is dedicated to explaining the original test setup, possible resultant scenarios and the two above mentioned solution methods while such fascinating topics are going to be addressed as effect of coating thicknesses on thermo-optical properties or possible test modifications.
Zsolt Peterbencze, Niels Hendrik Schibilla
