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ABSTRACT
The solar cavity receiver performance is considerably influenced by wind specifications in the parabolic dish system. This paper performs experiments to study the effects of wind direction and speed on the heat losses from a bicylindrical cavity receiver at different inclinations. A range of wind speeds from 0.3 to 5.7 m/s and the three head-on, back-on, and side-on wind directions for the side-facing and downward-facing cavity situations are examined. The cavity surface temperature distribution shows a jump due to the particular shape of the cavity and the sharp corner between two cylinders. The results show that the head-on wind yields a higher combined convection heat loss in contrast to the back-on and side-on wind directions for all wind speeds and cavity inclinations except 90° (vertically downward-facing) where the effects of wind directions are the same. For the lower side-on wind speeds, increasing cavity inclination results in decreasing combined convective loss, while on the contrary the change in the combined convective loss is very small regardless of the cavity inclination at higher wind speeds above 4 m/s. Moreover, for the back-on wind, comparing to the other wind directions, the combined convective loss is lower. For all wind directions and speeds, the horizontal cavity receiver gives lower forced convective loss compared with the natural convective loss. For the downward-facing cavity, an empirical correlation has been developed for the combined Nusselt number with respect to the Reynolds number, Grashof number, cavity inclination, wind direction, and the absolute temperature ratio. The proposed correlation supports the experimental results by a deviation within ±20%. Moreover, the uncertainty analysis revealed that the relative expanded uncertainty of combined Nusselt number varies between 2.1% and 16.3%, depending upon the cavity inclination, wind speed and direction.
Nomenclature
| At | = | total inner surface area of cavity, m2 |
| A | = | surface area, m2 |
| B | = | systematic error |
| d | = | aperture diameter, m |
| D | = | cavity diameter, m |
| Eb | = | blackbody emissive power, W/m2 |
| g | = | gravitational acceleration, m/s2 |
| Gr | = | Grashof number for cavity with constant heat flux |
| hc | = | combined convective heat transfer coefficient, W/m2 · K |
| I | = | current, A |
| J | = | radiosity, W/m2 |
| k | = | thermal conductivity, W/m · K |
| kins | = | thermal conductivity of insulation material, W/m · K |
| Lc | = | characteristic length, m |
| Nuc | = | combined Nusselt number |
| Nu0 | = | natural Nusselt number |
| q | = | heat loss, W |
| q” | = | heat flux, W/m2 |
| Re | = | Reynolds number |
| Ri | = | Richardson number |
| S | = | random error |
| T | = | temperature, °C |
| u | = | wind speed, m/s |
| U | = | uncertainty |
| Greek letters | = | |
| β | = | thermal expansion coefficient, 1/K |
| α | = | wind direction, degree |
| δ | = | thickness of the insulation, m |
| ε | = | emissivity of cavity surface |
| θ | = | cavity inclination, degree |
| ν | = | kinematic viscosity, m2/s |
| Subscripts | = | |
| a | = | ambient |
| ins | = | insulation layer |
| cond | = | conductive |
| c | = | combined convective |
| rad | = | radiative |
| t | = | total |
| w | = | wall |
Acknowledgments
This work was partially funded by the Deputy of Research and Technology of the University of Sistan and Baluchestan.