Application of triangular duct in solar-assisted air-heating system with surface roughness: a numerical investigation

The device called solar air heater (SAH) is used to collect and transfer solar-thermal energy to air that can further be used for space heating, drying, etc. The conventional air heater (solar-assisted) has poor performance, and with this work, an attempt has been made to improve its performance by providing surface roughness over the heated surface. The roughness employed over the surface has an elliptical cavity, and its distribution over the heated surface is defined with the three parameters (dimensionless): relative flow-wise distance (ranging from 6 to 14), relative cavity depth (ranging from 0.016 to 0.038), and relative crosswise distance (ranging from 6 to 14). A CFD code has been developed and validated with experimentation to do the parametric study for understanding the effect of the proposed surface roughness on the performance of the air heater. It is concluded that the proposed surface roughness promotes the local turbulence, flow separation, and strong vortices in the flow field resulting in comparatively higher thermal performance in the proposed air heater. But this higher thermal performance is achieved at the expense of higher-pressure loss in the passage. A substantial change in heat augmentation by 2.57 times (with 2.3 times higher pressure loss) which results in 1.75 times higher thermo-hydraulic performance has been noticed over conventional designs at a relative flow-wise distance of 10, relative cavity depth 0.038, and relative crosswise distance of 10.