Radiation-assisted internal heat transfer enhancement with fiber arrays

Heat transfer enhancement in heat exchanger devices is not only important towards achieving improved performance, but can be crucial in extending equipment lifetime in highly corrosive environments via lower operating temperature. This paper explores radiation-assisted internal heat transfer enhancement with fiber arrays, a technique which has potential for novel heat exchanger designs where temperature reduction is a primary concern. In this technique, small diameter fibers (approx. 100 μm) are inserted longitudinally within an externally heated tube. Radiative interaction between fibers and tube wall and convective transport from fibers to the fluid drive the heat transfer augmentation. In this work, coupled radiation, conduction and convection within an externally heated tube containing uniform fiber arrays have been numerically modeled for steady state, laminar flow (Re_D = 1000) via a multilevel modeling approach. At the macroscopic scale, volume-averaged porous media equations (Darcy-Brinkman-Forchheimer flow) have been utilized to model the fluid flow and heat transfer within the highly porous fiber arrays (porosity ≥ 0.9800). Simplified local modeling is used to define globally-based porosity parameters and radiative extinction coefficients, which are functions of fiber material, size, and orientation. For the given conditions, results show that the optimum heat transfer rate occurs within a narrow porosity band ranging from 0.9950 to 0.9980, whereby wall temperatures are reduced by up to 30%. The increased pressure drop due to the presence of the fibers rises monotonically as the porosity is reduced.