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ABSTRACT
Artificially roughened solar air heater has been topic in research for the last 30 years. Prediction of heat transfer and fluid flow processes of an artificially roughened solar air heater can be obtained by three approaches: theoretical, experimental, and computational fluid dynamics (CFD). This article provides a comprehensive review of the published literature on the investigations of artificially roughened solar air heater. In the present article, an attempt has been made to present holistic view of various roughness geometries used for creating artificial roughness in solar air heater for heat transfer enhancement. This extensive review reveals that quite a lot of work has been reported on design of artificially roughened solar air heater by experimental approach but only a few studies have been done by theoretical and CFD approaches. Finally this article presents a comparative study of thermo-hydraulic performance of 21 different types of artificial roughness geometries attached on the absorber plate of solar air heater in terms of thermo-hydraulic performance parameter. Heat transfer and friction factor correlations developed by various investigators for different types of artificially roughened solar air heaters have also been reported in this article.
Nomenclature
| A | = | area of cross-section, mm2 |
| AP | = | surface area of absorber plate, mm2 |
| B | = | air gap, mm |
| Cp | = | specific heat of air, J/kg K |
| D | = | equivalent or hydraulic diameter of duct, mm |
| d | = | print diameter of dimple/protrusion or geometric parameter of broken rib, mm |
| e | = | rib height, mm |
| g | = | groove position/ width of gap, mm |
| Gt | = | global solar irradiance |
| H | = | depth of duct, mm |
| h | = | heat transfer coefficient, W/m2 K |
| I | = | intensity of solar radiation, W/m2 |
| k | = | thermal conductivity of air, W/mK |
| L | = | length of text section of duct, mm |
| l | = | longway length of mesh, mm |
| L1 | = | inlet length of duct, mm |
| L2 | = | test length of duct, mm |
| L3 | = | outlet length of duct, mm |
| m | = | mass flow rate, kg/s |
| P | = | pitch, mm |
| Pm | = | mechanical energy consumed for propelling air through collector, W |
| Ql | = | heat loss from collector, W |
| Qt | = | heat loss from top of collector, W |
| qu | = | useful heat flux, W/m2 |
| Qu | = | useful heat gain, W |
| S | = | length of discrete rib or shortway length of mesh, m |
| Ta | = | ambient temperature, K |
| Tam | = | mean air temperature, K |
| Ti | = | fluid inlet temperature, K |
| To | = | fluid outlet temperature, K |
| Tpm | = | mean plate temperature, K |
| UL | = | overall heat loss coefficient, W/m2 K |
| v | = | velocity of air in the duct, m/s |
| W | = | width of duct, mm |
| w | = | width of rib, mm |
| ΔP | = | pressure drop, Pa |
Dimensionless parameters
| = | average Nusselt number | |
| = | average Stanton number | |
| = | average friction factor | |
| B/S | = | relative roughness length |
| d/w | = | relative gap position |
| e/D | = | relative roughness height |
| e/H | = | rib to channel height ratio |
| e+ | = | roughness Reynolds number |
| f | = | fanning friction factor |
| Fr | = | collector heat removal factor |
| fr | = | fanning friction factor for rough surface |
| fs | = | fanning friction factor of smooth surface |
| g | = | heat transfer function |
| g/e | = | relative gap width |
| g/P | = | relative groove position |
| Gd/Lv | = | relative gap distance |
| L/D | = | test length to hydraulic diameter ratio of duct |
| l/e | = | relative logway length of mesh |
| l/s | = | relative length of grit |
| Nu | = | Nusselt number |
| Nur | = | Nusselt number for rough channel |
| Nus | = | Nusselt number for smooth channel |
| P/e | = | relative roughness pitch |
| P’/P | = | relative roughness staggering ratio |
| Pr | = | Prandtl number |
| R | = | roughness function |
| Re | = | Reynolds number |
| S/e | = | relative short way length of mesh |
| S’/S | = | relative roughness segment ratio |
| St | = | Stanton number |
| W/H | = | duct aspect ratio |
| W/w | = | relative roughness width |
Greek symbols
| (τα)e | = | effective transmittance-absorptance product |
| ɸ | = | wedge angle/chamfer angle, degree |
| α | = | angle of attack, degree |
| α/90° | = | relative angle of attack |
| δ | = | transition sub-layer thickness, mm |
| ε | = | dissipation rate, m2/s3 |
| ηeff | = | effective efficiency of solar air heater |
| ηm | = | efficiency of electric motor |
| ηp | = | efficiency of pump |
| ηth | = | thermal efficiency/efficiency of thermal conversion of power plant |
| ηtr | = | efficiency of electric transmission from power plant |
| κ | = | turbulent kinetic energy, m2/s2 |
| μ | = | dynamic viscosity, Ns/m2 |
| ρ | = | density of air, kg/m3 |
| ω | = | specific dissipation rate, 1/sec |
Subscripts
| a | = | ambient |
| am | = | air mean |
| b | = | bottom plate |
| c | = | collector |
| f | = | fluid |
| i | = | inlet |
| l | = | loss |
| m | = | motor |
| o | = | outlet |
| p | = | pump |
| pm | = | plate mean |
| r | = | roughened |
| s | = | smooth |
| t | = | top |
| th | = | thermal |
| tr | = | transmission |
| u | = | useful |