The Effect of External Disturbances on Thermal Entry Length

The thermal entry length at high Reynolds numbers ($\mathrm{Re}=300$ and $500$) in a water film flowing down a vertical plate with a heater in the absence and in the presence of artificial disturbances with the “most dangerous wavelength” has been studied experimentally. The field of film thickness at different values of heat flux has been measured using the fluorescent method. The data obtained using a high-speed infrared camera have shown that the thermal entry length increases under the influence of external disturbances in the interrivulet areas, which are formed between the crests of developed three-dimensional synchronous waves, and decreases for the rivulets.