Forced convection enhancement of air flowing inside circular pipe with varying the pitch (P) of wire-mesh porous media

The forced convective heat transfers on a circular pipe with varying the pitch (P) of stainless wire-mesh porous media for the constant case of surface temperature have been investigated experimentally. The examined parameters were Nusselt number (Nu), friction factor (f), and performance evaluation criteria (PEC). The circular test pipe’s wall temperature was maintained at a constant 25 °C. The porous inserts were made of SUS 304 stainless wire mesh with a pore per inch (PPI) of 8, and the working fluid was airflow rate expressed as Reynolds number (Re) changing from 6000 to 55,000. Five pitch distances (P) were examined: 25, 50, 75, 100, and 125 mm. The test section consisted of two horizontal concentric pipes through which heated air flowed in one direction (a test pipe or inner pipe) and cold water flowed in the opposite direction through the annulus. The dimension of the circular test pipe yielded 100 mm long, 25.4 mm inner diameter (Di), and 3 mm thick were adopted. From the experiment, the Nu increased with Re, but the result was reversed by raising P. As decreasing Re and P, the f decreased. It was described by a higher material volume of the solid skeleton of porous media leading to gain more energy absorption from hot air due to the higher effective thermal conductivity (ETC) and, then, to generate higher pressure drop with many the obstruction. The maximum PEC was given at P=75 mm because this may be a proper condition for enhancing forced convective heat transfers on a circular test pipe with stainless wire-mesh porous media placed for the constant situation of surface temperature.