Nanoparticles for improving the efficiency of heat recovery unit involving entropy generation analysis

Abstract

The present article aims to suggest new thermal unit as heat recovery system to augment the available energy. Hot gas flows inside the inner pipe and the operating fluid in annulus region is nanofluid (CuO-water). Helical tape with various revolution number (N) was employed to augment the fluctuations of fluid and reduce the boundary layer thickness. Range of Reynolds number (Re=4000 to 20,000) indicates turbulent regime. To involve such range of Re, K-ɛ approach has been utilized and its validity was reported. Achieving to greater useful energy happens if the irreversibility of system minimized. This function has two terms: frictional term (Sgen,f) and thermal term (Sgen,th). The thermal term is higher than frictional term and it should be considered as main source of irreversibility. With augment of rotational speed as a result of higher values of Re and N, velocity gradient goes up but thinner boundary layer will be generated. So, with rise of such parameters, temperature gradient declines and residence time enhances which results in better mixing of fluid. So, two scrutinized factors have favorite effect on Sgen,th and considering greater values of them leads to lower thermal irreversibility. Also, growth of these factors makes Sgen,f to increase which is unfavorable effect. As N augments from 3 to 5, the value of Sgen,f enhances about 47% but Sgen,th reduces about 42.47% when Re=5000. Also, with selecting higher N from 3 to 7 when Re=2000, frictional entropy augments about 128.95% while Sgen,th reduces by 35.97%. Sgen,th faces 39.14% decrement if Re goes up when N = 3 while with same change, the value of Sgen,f augments to 7.27 times greater value.