Mixed convection heat transfer from longitudinal fins inside a horizontal channel has been investigated for a wide range of modified 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 (0.15 <= w(m) <= 0 16 m/s) using a flow rate control valve so that Reynolds number was always about Re = 1500. Experiments were conducted for modified Rayleigh numbers 3 x 10(7) < Ra-center dot < 8 x 10(8) and Richardson number 0.4 < Ri < 5. Dimensionless fin spacing was varied from S/H = 0 04 to S/H = 0 0 18 and fin height was varied from H-f/H = 0.25 to H-f/H = 0.80. For mixed convection heat transfer, the results obtained from experimental study show that the optimum fin spacing which yields the maximum heat transfer is S = 8-9 mill and optimum fin spacing depends on the value of Ra-center dot. (C) 2009 Elsevier Ltd. All rights reserved.