Proceedings of the International scientific and practical conference ―Science, technology and art in global context (July 8-10, 2025) / OP website: www.naukainfo.com. – Dresden, Germany, 2025. - 140 p.

120 These packed-bed systems absorb heat from hot exhaust gases and release it to cooler air or liquid, operating either in recuperative (continuous counterflow) or regenerative (cyclic flow reversal) modes. The next section explores the principles and potential of such systems, focusing on annular channels filled with sand. PHYSICAL PRINCIPLES OF HEAT TRANSFER IN ANNULAR CHANNELS WITH POROUS FILLINGS An annular channel with a porous filling is formed by the gap between two coaxial cylindrical walls, filled with a granular material (e.g., sand). When hot gas flows through this bed, it passes through the pores and efficiently transfers heat to the filler due to the large contact surface area. Heat transfer mechanisms include convection from gas to solid surfaces, conduction within the solid layer and walls, and thermal storage in the filler. Compared to smooth-wall channels, porous beds provide significantly higher surface area and turbulence, enabling strong heat exchange even at low velocities. Modeling typically uses porous media flow equations, such as Darcy–Brinkman and coupled energy equations for gas and solid. Two regimes are considered: local thermal equilibrium (gas and solid have equal local temperatures) and non- equilibrium (microscopic temperature differences). In dense packings, near- equilibrium is usually achieved due to the small particle size. Heat transfer performance is often described via Nusselt (Nu) and Reynolds (Re) numbers. Experiments show a power-law dependence Nu ~ Reⁿ, with n ≈ 0.5– 0.6 for random packings like sand [6]. A widely used empirical relation is the Wakao–Kaguei correlation: Nu = 2 + 1.1 Re⁰.⁶ Pr¹ ᐟ ³ (1) This equation effectively predicts heat transfer in packed beds for Re ~10 to several thousand [6].