... friction factor in a solar air heater. Solar Energy. 1988, 41(6), 555-560. [3] Prasad B.N., Saini J.S. Optimal thermohydraulic performance of artificially roughened solar air heaters, Solar Energy. ... for a solar air heater duct roughened artificially with Multiple V-ribs. Solar Energy. 2010, 84, 898-911. [31] Mittal, M.K., Varun, Saini R.P., Singal S.K. Effective efficiency of solar air heaters ... friction factor correlations for solar air heater duct roughened artificially with multiple V- ribs. Mittal et al. [31] studied the effective efficiency of solar air heaters having different types...