An experimental set-up has been developed for the investigation of flow inside an inclined cylindrical open thermosyphon. A glass cylinder, whose bottom is sealed and whose top is open to a reservoir, is heated from the sides, generating natural convection currents between itself and the reservoir. The set-up simulates the flow inside evacuated tubular solar absorbers. The cylinder walls are heated in two different ways: (1) uniform wall temperature; and (2) differential wall heating, in which one temperature is applied on the upper surface of the cylinder wall, and another temperature on the lower surface. The second scheme of heating attempts to approximate the manner in which absorbers are usually heated under exposure to solar radiation, with the upper surface normally getting more heat than the lower. Axial velocity measurements are done with a laser Doppler anemometer at the central vertical plane of the orifice. Correlations between the velocity and the independent control parameters (Rayleigh number, aspect ratio and mode of heating) are performed.