Thermo-hydraulic performance analysis of a solar air heater roughened with reverse NACA profile ribs

Abstract

The solar air heater is subjected to a low convective heat transfer coefficient on account of the formation of the laminar sub-layer between the absorber plate and flowing air. To increase the heat transfer coefficient, the laminar sub-layer can be interrupted with the use of artificial roughness (turbulator) on the absorber plate. In the present analysis, the effects of novel type of roughness element in the form of NACA 0040 profile ribs in the reverse position are studied on the performance of the solar air heater. The effects of relative roughness pitch (P/e) and relative roughness height (e/D) were studied for the Reynolds number (Re) ranging from 6000 to 18,000. Among the various roughness parameters, NACA 0040 profile ribs provided in the reverse direction with P/e as 5 and e/D as 0.065 was found to be the optimum. The thermo-hydraulic performance parameter for this case was found to be 2.53 at a Reynolds number of 6000. The experimental results validated the numerical results, and the maximum deviation was found to be 4.84%. An empirical correlation was developed for prediction of Nu and f in terms of Re and e/D and found to conform within ±3% of the corresponding experimental values.