Use of Micro- and Nanocoating in the Evaporator to Enhance Heat Transfer in a Thermosiphon

Experimental data on the thermal resistance of thermosiphons designed with different structural junctions (“pipe in pipe,” “flat shelve”) are obtained, and the possibility is shown to decrease it as a result of heat transfer enhancement in the evaporator by using a combined coating in the form of longitudinal grooves 0.1 mm in radius, filled with aluminum oxide nanoparticles 100–200 nm in size. The effect the coating has on the thermosiphon thermal resistance was studied in an experimental section in the form of a 100 mm long pipe 38 × 3 mm in diameter made of grade AISI304 stainless steel. As a result of applying the coating, the thermosiphon thermal resistance is decreased by 2.4–3.0 times at heat fluxes in the range 200–1700 W/m2 and evaporator slope angles 0°–30°. Experiments with slope angles equal to 0°, 10°, and 20° were carried out, and it is pointed out that the maximal effect is reached at the evaporator horizontal and slightly inclined positions. For calculating the heat transfer in a thermosiphon, a procedure based on the Yu.A. Kuzma-Kichta formula is proposed. The predicted thermosiphon thermal resistance values agree with the experimental data within 20%, a circumstance that extends the model applicability area.