Experimental investigation on the characteristics of inline dimple-based SAH with dimple pitch variation

In this work, the performance of an inline dimpled-based absorber plate solar air heater (IDPSAH) is studied in relation to pitch, a system parameter, and air flow rate (M_air), an operating parameter. In this regard, two alternative IDPSAH with pitch ratios 0.88 and 1.33 have been experimentally evaluated on alternative days by altering the M_air from 0.010 to 0.053 kg s^-1 (Proportional Re range: 1971.66–11,113.24) and the result are compared with flat plate solar air heater (FPSAH). For performance evaluation and comparative research, various parameters have been studied, including air temperature difference, efficiencies (η_inst, η_eff, η_daily), global heat loss coefficient, heat removal factor, collector efficiency, top loss, area averaged Nusselt number (Nu_avg), and friction factor (f_avg), among others. The effective efficiency reaches its maximum at a particular M_air and then begins to decline, but the instantaneous and daily efficiency of IDPSAH and FPSAH increases continuously. Due to a decrease top loss, the efficiency of IDPSAH is higher than FPSAH. The instantaneous efficiency of IDPSAH at pitch ratio 0.88 is 1.13–1.53 times greater than the instantaneous efficiency of IDPSAH at pitch ratio 1.33 and it is 1.21–1.86 times higher than FPSAH. With an increase in Re, the f_avg value decreases; while, the Nu_avg of both the FPSAH and IDPSAH increases. In comparison with IDPSAH with pitch ratio 1.33, the Nu_avg of IDPSAH at pitch ratio 0.88 is 1.23–1.41 times higher, and it is 1.68–2.12 times higher than FPSAH. In comparison with IDPSAH at pitch ratio 1.33 and FPSAH, the f_avg of IDPSAH at pitch ratio 0.88 is higher. However, there is a significant enhancement in overall performance of IDPSAH than the FPSAH.