The present work investigates the exergy and effective efficiency of the multi-pass solar air collector with longitudinal fins by analysis approach and multi-objective optimization. The effect of 0.01-0.02 kg/s air flow rate, 15-35 mm collector depth, 1-3 m collector length, and 24.21-30.67 mm fin pitch was considered. The optimization was analyzed by the Preference Selection Index (PSI) method, with three maximum criteria: thermal efficiency, effective efficiency, and exergy efficiency. Mathematical models were solved by EES software. Results indicated that the multi-pass (TPLF and DPLF) type was better than the SPWF type by three criteria. The highest exergy efficiency of the TPLF and DPLF types was 6.696% and 5.636%. The greatest effective efficiency of the TPLF and DPLF types was 69.09% and 66.17%. Furthermore, the optimization results indicated that the three efficiency criteria of the DPLF type were 58.38%, 58.22%, and 4.491% for the best case; the three efficiency criteria of the TPLF type were 60.97%, 60.85%, and 5.439% for the best case. The worst configuration was the model with a short collector length, large collector depth, and large fin pitch. The collector efficiency decreased with decreased fin pitch for the configuration with the large collector length, short collector depth, and high mass flow rate.
This study investigates the entropy generation and heat transfer of water density inversion region in a square cavity with the different relative positions of hot and cold walls by numerical simulation. Mathematical models are solved using the EES (Engineering Equation Solver) program. A physical model is a 38 mm square cavity filled with water, with cold and hot wall temperatures maintained at 0oC and 10oC. Results indicated that the Nusselt number of cases 1 (cold and hot walls on both sides) is higher than case 2 (hot wall above, cold wall below) and case 3 (cold wall above, hot wall below) by 1.582 and 1.059 times, respectively. The average total generation per unit volume in case 1 increases by 1.577 times and 1.059 times, respectively, compared to cases 2 and 3, which almost correspondingly increase with heat transfer capacity. Entropy generation by fluid friction is negligible
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