Experimental investigation on solar thermal air heater with a combination of roughness elements for heat transfer improvement

This work attempts to determine the influence of the integration of a combination of roughness elements on the heat dissipation and frictional losses occurring within the airflow channel pathways of a counterflow double-pass solar thermal air heater. The investigation considers three configurations, namely (1) circular rings with a relative width ratio of 4 (Case A), (2) circular rings with a relative width ratio of 6 (Case B), and (3) combination of circular rings and ribs (Case C). All three configurations were studied at different operating conditions of the Reynolds number (in the range of 4000 to 10,000) and relative roughness height (0.026 and 0.043). During the experiments, the operating parameters considered as constants for comparison were heat input (1000 W m−2), aspect ratio (20), and hydraulic diameter (0.048). The resultant observations of the three configurations were also compared with that of the smooth-plate absorber configuration. The observations revealed that with an increase in relative roughness height, there was only a marginal improvement in Nusselt number. The experimental findings reported that Case C resulted in maximum improvement in Nusselt number with a penalty of increase in friction factor among the different configurations. For Case C, the calculated maximum value of the Nusselt number ratio and friction factor ratio obtained corresponding to relative roughness height of 0.043 was 3.19 and 1.46, respectively. The maximum enhancement obtained in the thermohydraulic performance parameter for Case C configuration corresponding to the relative roughness height of 0.026 and 0.043 was 2.91 and 3.01, respectively.