Mixed convection heat transfer from longitudinal fins inside a horizontal channel has been investigated in the natural convection dominated region for a wide range of Rayleigh numbers and different fin heights and spacings. An experimental parametric study was made to investigate effects of fin spacing, fin height and magnitude of heat flux on mixed convection heat transfer from rectangular fin arrays heated from below in a horizontal channel. The optimum fin spacing to obtain maximum heat transfer has also been investigated. During the experiments constant heat flux boundary condition was realized and air was used as the working fluid. The velocity of fluid entering channel was kept nearly constant (0.02 <= w(in) <= 0.025 m/s) using a flow rate control valve so that Reynolds number was always about Re = 250. Experiments were conducted for modified Rayleigh numbers 3 x 10(7) < Ra* < 6 x 10(8) and Richardson number 600 < Ri < 15,000. Dimensionless fin spacing was varied from S/H = 0.04 to S/H = 0.018 and fin height was varied from H-f/H = 0.25 to H-f/H = 0.80. The results obtained from experimental study show that the dimensionless optimum fin spacing which yields the maximum heat transfer is between S/H = 0.08 and S/H = 0.12. Results also show that optimum fin spacing depends on modified Rayleigh number and fin height. For high modified Rayleigh numbers (Ra* >= 5 x 10(8)) and low values of fin height (such as H-f/H = 0.25) the optimum fin spacing has taken greater values in comparison to the cases of smaller modified Rayleigh numbers. (C) 2009 Elsevier Ltd. All rights reserved.