Study of radiative heat transfer in porous media for sintering applications

Powder sintering is a popular manufacturing method in the metalworking industry. It possesses the ability to fabricate high quality complex parts to close tolerances in an economical manner. Although there are numerous advantages for the powder sintering, sintering is a relatively slow process. Normally, the duration for sintering counts by days or weeks. This would greatly affect the productivity. In order to have an efficient sintering process, rapid sintering (non-isothermal sintering) is always implemented. Radiation is the dominant mode of heat transfer in this high temperature process. During this process, the relatively short heating time would induce a great thermal gradient within the sintering part. The temperature differences affect the microstructure of the product, which in turn leading to the occurrence of microstructure defects. As a result, understanding of the sintering process from the radiative heat transfer perspective is a must. However, the length scale of the temperature solution required for sintering application is beyond the existing method of radiative heat transfer analyses. This turns out to be the motivation for the present study. The objective of the work is to develop certain numerical methods to strike a balance on the length scale and the computational power for radiative heat transfer analysis in porous media. The present study is, in fact, the first attempt to develop a formulation to compute the packed bed temperature down to sphere’s size. In order to compute the temperature for each sphere in any three-dimensional packing structures, the Radiative Transfer Coefficient (RTC) was developed as a basic building block. By using the RTC scheme with a set of energy balance equations, the computational time for the radiant transmission (5000 spheres case) only requires 0.526% of the Monte-Carlo ray tracing method, providing same level of accuracy. Moreover, an experiment was developed for the transmittance (for RTC scheme validation), and emittance measurements (providing some more useful experimental data for future studies) of certain packed spheres system. The computed transmittance by using the RTC scheme agreed well with the experimental measurement. The maximum error is only 1.1%. As for the emittance measurements, different sintered glass packed beds were made and measured. Results showed that the emittance decreases with increasing coordination number. In addition to that, a Sphere Temperature Computation scheme (STC) was also developed for the sphere temperature solution. By using the STC scheme, the radiant conductivities were generated for various kinds of packing structures