A thin circular disk (diameter 0.4 m, surface emissivity ε = 0.5) has been heated up to 400 K and then left inside a chamber to cool down. The chamber surface is kept at 400 K and the air inside is maintained at 300 K. Assume that the disk is held clamped on its curved side surface by an insulator, making heat conduction and heat dissipation from the curved side surface negligible. What is the total rate of heat transfer from the disk?