Air preheaters 2 Heating equipment 2 Solar air heater Solar heating 2 Air flow-rate 1 Aspect ratio 1 Case-studies 1 Collector efficiency 1 Comparatives studies 1 Efficiency 1 Experimental investigations 1 Flow of fluids 1 Friction 1 Friction factor 1 Friction factors 1 Hydraulic efficiency 1 Jet diameter 1 Jet impingent 1 Nusselt number 1 Optimisations 1 Optimization 1 RSM 1 Relative roughness height 1 Relative roughness pitch 1 Response-surface methodology 1 Reverse orientation 1 Reynolds number 1 Roughness height 1 Solar equipment 1 Surface properties 1