ABSTRACT This paper proposes direct steam generation by solar radiation falling on an evacuated tube. Partially filled mass flow of acetone is controlled in a solar evacuated tube collector (ETC) of an Organic Rankine Cycle (ORC), with the purpose of ensuring the generation of vapor at all times irrespective of the irradiance from the sun during the day and night. A system of single pipe in which the subcooled undergoes heating and evaporation process is analyzed controlled by the variable flow. Mathematical equations are derived based on energy and mass balances for system components. A Scilab code is built to simulate the flow of acetone inside the absorber tube and determine properties of acetone along the pipe. Empirical correlations and mathematical models of mass flow, for single and two-phase flow, and heat transfer are used in the simulation. Simulation are run from a subcooled temperature of 60 °C to a saturation temperature of 120 °C, vapor qualities from 1 - 99 percent and solar radiation from 600 - 1000 W/m². Two-phase flow observations showed that the simulation data covered only the fully stratified regimes. The Kattan-Thome-Favrat flow boiling model is used to obtain the two-phase local heat transfer coefficients along the tube. The flow boiling model is also successfully compared to the experimental result of ammonia obtained by Zürcher, O., et al [1].