In this work a two-phase forced flow in channels of small geometric dimensions is analyzed. The Navier-Stokes equation was used to create a theoretical model describing the phenomenon of a evaporation of a flowing vapor and the Maxwell equation was used to describe the flow of a thin liquid layer. Both equations are coupled and overall create an original theoretical model describing the phenomenon of the evaporation of a liquid layer. This model of a liquid forced convection cooling the flat element with distributed internal heat sources is theoretically and experimentally analyzed. A flow of the cooling and evaporating liquid is forced by a suction of the vacuum pump. The cooling process is investigated theoretically for two specific cases with both, a constant and a cyclic heat emission. The impact of the liquid layer thickness on the cooling intensity is analyzed for both cases.