In this work, the distribution of solar heat flux around receiver was calculated by Mont-Carlo statistical technique that has been written using matlab. The numerical investigations of convective heat transfer process, friction factor, and efficiency of LS-2 parabolic collector have been performed. Based on finite volume methods, the influence of Rayleigh number (Ra), diameter of plugs, and thermal conductivity of the tube were studied on Nusselt number, outlet temperature, and the efficiency of collector. Because of using several central plugs with different diameters, the amounts of flow velocity have been changed, as the mass flow rate of each case study was considered constant. The diameters of plug were as: 10, 15, and 25 mm, respectively. The diameter of LS-2 collector plug was 50.8 mm (r* = 0.765). So, in order to validate the numerical simulation method, the outlet temperature of LS-2 collector ( = 50.8 mm) was compared with Dudley et al. (Dudley, V., Kolb, G., Sloan, M., and Kearney, D., 1994, “SEGS LS2 Solar Collector—Test Results,” Report of Sandia National Laboratories, Report No. SANDIA94-1884) experimental results. Finally, the results show that, for m, the natural convection conquers to forced convection and for m the mixed convection is the dominant mechanism of heat transfer. Also, with the increase of plug diameter, friction factor decreases and the minimum amount of Nusselt number is occurred at m.