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ABSTRACT
The performance characteristics of a solar air heater can effectively be improved by providing artificial roughness in the form of protrusions and dimples of various shapes, sizes, and orientations on the underside of heated surface. An extensive literature review on artificial roughness elements has been carried and the correlations developed for heat transfer and friction factor for roughened solar air collector have been discussed and presented in this paper. The performance parameter has also been computed and compared with various kinds and shapes of roughness geometries using correlations developed by various investigators. The optimum values of the roughness parameters obtained by several investigators have also been discussed.
Nomenclature
| ṁ | = | Mass flow rate of air, kg/s |
| A | = | Plate area, m2 |
| H | = | Channel height, m |
| f | = | Friction factor |
| FR | = | Heat removal factor |
| W | = | Channel width, m |
| v | = | Air velocity, m/s |
| h | = | Heat-transfer coefficient, W/m2 K |
| T | = | Temperature, °C |
| D | = | Hydraulic mean diameter of duct, m |
| k | = | Thermal conductivity of air, W/m K |
| l | = | Length of the duct |
| PUL | = | Pitch of the roughness pattern, m Overall loss coefficient, W/m2 K |
| e | = | Rib height, m |
| Re | = | Reynolds number |
| PrQu | = | Prandtl number Useful heat gain, W |
| qu | = | Specific heat gain |
| NuΔP | = | Nusselt number Pressure drop, Pa |
Greek symbols
| ρ | = | Density, kg/m3 |
| ηth | = | Thermal efficiency |
| η | = | Thermo-hydraulic performance factor |
| µ | = | Dynamic viscosity, N s/m2 |
| ν | = | Kinematics viscosity, m2/s |
| α | = | Angle of attack, ° |
Subscripts
| am | = | Average fluid |
| pm | = | Average plate |
| out | = | Exit |
| in | = | Entry |
| s | = | Smooth |
| d | = | Duct |
| p | = | Plate |
Acknowledgments
The first author (Chandra Prakash) expresses sincere thanks to Alternate Hydro Energy Centre (AHEC) and Quality Improvement program (QIP) Centre, IIT Roorkee, India, for providing research facilities.