Experimental investigation for heat and flow characteristics of solar air heater having symmetrical gaps in multiple-arc rib pattern as roughness elements

ABSTRACT

The working medium is air in a solar air heater (SAH) and because of its low heat-transfer coefficient, SAH results in poor performance. By placing artificial-roughness over the heat-collector plate, the performance of a SAH can be improved significantly. The improvement in thermal and hydraulic performance by providing multiple-arc-rib roughnesses patterns with the gap is discussed and generalized correlations are developed for Nusselt number and friction factor based on results collected from the experimentation (under steady condition). In total seven parameters were considered to study heat transfer and friction characteristics, that is, Reynolds number, rib-pitch, rib-height, number of gaps, arc-attack angle, rib, and gap width. In comparison to smooth collector-plate type of solar air heater, the 5.76 and 6.05 times higher Nusselt number and friction factor obtained in the proposed roughened SAH. Besides this, the developed mathematical expression for Nusselt number and friction factor shows close-fit with the experimental data under the error band of only 10 and 12%, respectively.

KEYWORDS:

• Solar air heater
• ribs
• symmetric gap
• heat transfer
• enhancement

Nomenclature

D: Hydraulic diameter (m)

e: rib-height (m)

f: friction factor

w_g: gap width (m)

h: Coefficient of convective heat transfer (Wm-2K)

k: Thermal conductivity (Wm-1K)

N: Gaps in number

Nu: Nusselt number

P: Rib-pitch (m)

W: Width of heat-collector plate (m)

w: Rib-width (m)