The present work considers the innovative design of absorber tube of solar water heater having circular rings inserts inside the riser tubes to provide optimal thermohydraulic performance of solar water heater having flat reflector beneath the absorber tubes. To produce substantial effect on the heat transfer rate, three different cases of absorbers with pitch ratio of 2, 3, and 4 have been considered. The influence of circular rings on heat transfer and fluid flow characteristics were experimentally evaluated and presented for laminar flow conditions. The data were collected at varying mass flow rates of 0.0056, 0.0097, 0.0138, 0.0207, 0.0277, and 0.0346 kg/s from 11:00 a.m. to 3:00 p.m. at intervals of 10 min in the month of March and April 2023 for all the three pitches considered. The results demonstrated that the maximum thermal efficiency and optimal performance evaluation factor of the solar water heater with ring inserted risers were found 83.92% and 1.7, respectively, for the pitch ratio 2. The Nusselt number and friction factor increased by 73.35%–91.88% and 29.72%–41.98%, respectively, in comparison to plain riser tube.
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