h c Convective heat transfer coefficient W m −2 K −1 h(x) Instantaneous ribs position along the x-axis h(y) Instantaneous ribs position along the y-axis k Thermal conductivity W m −1 K −1 L Solar collector length (m) m Mass flow rate Nu Nusselt number P Spacing between ribs (m) Re Reynolds number (ρUD h /µ) T Temperature (K) t Instant time (s) U Incoming flow velocity (m s −1) U Mean velocity in channel (m s −1) Greek letter η Thermal enhancement factor = ((Nu/Nu 0)/(f/f 0)) 1/3 μ Dynamic viscosity ρ Density
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