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Non-uniform heat flux profiles on circular tubes are found in a number of heat transfer applications, including solar heating. In this numerical study the influence of the circumferential angle spans of non-uniform heat flux distributions are considered on the secondary buoyancy-driven flow, internal fluid heat transfer coefficients, and friction factors in horizontal absorber tubes in parabolic trough solar collector applications for water heating in the laminar flow regime. Inlet Reynolds numbers ranging from 130 to 2200 for 10 m long tubes with different inner diameters were considered. Sinusoidal type incident heat flux distributions, tube-wall heat conduction and heat losses were taken into account. It was found that due to buoyancy-driven secondary flow, overall and local internal heat transfer coefficients were increased significantly due to the non-uniformity of the incident heat flux. Average internal heat transfer coefficient increased with the heat flux intensity, the incident heat flux angle span and the inlet fluid temperature. The effective friction factor decreased with an increase in the absorber tube inlet fluid temperature. It was found that improved thermal efficiencies can be achieved for low mass flow rate water heating applications by employing parabolic trough collector systems compared to flat plate systems. [ABSTRACT FROM AUTHOR]