Performance enhancement of a double-pass solar air heater using perforated twisted tape turbulators

This study explores how the performance of a double-pass solar air heater (DPSAH) can be improved by introducing perforated twisted tape turbulators specially developed for this work. These tapes include a series of evenly spaced circular holes that allow part of the airflow to pass through them. By doing so, the perforations soften the wake behind the tape and maintain the swirling motion of the air, which helps promote better mixing without causing an excessive rise in pressure—an advantage over the more commonly used solid twisted tapes. Experiments were carried out for Reynolds numbers between 3,000 and 15,000. Several performance indicators were examined, such as absorber plate temperature, temperature rise parameter (TRP), thermal efficiency, pumping power, thermo-hydraulic efficiency, and the Comprehensive Thermo-Hydraulic Index (CTHI). With the perforated turbulators in place, the absorber plate temperature dropped by 1.5–2%, showing improved heat removal. At the lowest Reynolds number tested (Re = 3,000), the TRP increased by 45.7%, and the thermal efficiency rose from 31.69% to 46.26%, giving a 46% enhancement. The additional pumping power required remained relatively small at 7–11%. Overall, the modified system showed a thermo-hydraulic efficiency of 46.19% compared to 31.75% for the baseline, and CTHI values were up to 97% higher. These results indicate that perforated twisted tapes offer a simple, passive, and effective means to strengthen the thermal performance of DPSAH units with limited energy penalties.