Transient heat transfer analysis and radiative properties measurements of porous thermal insulation

A combined radiative and conductive heat transfer model that uses the two-flux equations for radiation exchange has been applied to analyze the transient heat transfer data obtained with the flat-screen tester at ORNL. The data included those for the National Bureau of Standards (NBS) transfer standard and the NBS 1450b fiberglass board. Predictions that deviated by no more than 0.2C of the measured screen temperature rise were obtained. Results indicate that the model can be used in conjunction with the transient data to determine the thermophysical properties of porous insulations. Transient heat transfer through porous insulations with one diurnal temperature boundary condition has been investigated using the same combined heat transfer model. By comparing the accumulated heat flow at the boundaries to those predicted using a purely conductive model, differences as large as 40% were found for some conditions. Results for a thermal resistance defined in terms of time-integrated quantities were also obtained and compared with that defined for steady-state conditions. Radiative properties for the NBS transfer standard, NBS 1450b, Cab-O-Sil S-17 and the Degussa R-974 have been measured. Properties reported included the extinction coefficient, single scattering albedo and the back-scattered fraction for the former two materials and the extinction coefficient for the latter two materials.