Study On Heat Transfer Characteristics Of Falling Film Evaporation On Vertical Plates With Expanded Surfaces

The Enhancement of heat transfer of falling film evaporation process has significant economic and social benefits, because it can increase the effectiveness of heat exchanger, decrease the heat transfer area and save investment. And it is one of the main ways to enhance heat transfer effectiveness by changing the surface structure of the falling film plate. Basing on this, the characteristics of falling film evaporation of vertical plate with expanded surfaces is studied in this paper, and its main contents are introduced in the following paragraphs.At first, the heat transfer principle of falling film process, with tube or plate as heat part, is studied. Some factors effecting on the heat transfer coefficient are analyzed. The results indicate that the heating part made by combining the tube and plate is proper for a better heat transfer surface. And the similarities of the effect of these factors on tube and plate prove that it is feasible to study in this way.The second, numerical simulations of falling film evaporation on vertical flat plate and vertical plate with expanded surfaces are made. It is found that the interfacial shear stress, the properties of the liquid and the mass transfer have significant effects on the falling film evaporation process; the wire diameter and the distance between the wires have a strong effect on the flow field, and the optimal diameter and distance are determined by its extent of disturbance and immersion and the sprinkling density.Finally, the experiment of falling film evaporation on vertical plate with expanded surfaces is carried out. Its results indicate that the heat transfer can be enhanced by adding wires to the plate. On the one hand, the wires homogenize the film thus preventing it to break up and form dry patches on the heating plate, on the other hand, the wires strengthen the turbulence of the film flow, and thus cause a transition to turbulence even at lower Reynolds numbers so that high heat transfer rate can be realized at lower flow rate. Also, it is found that the vertical plate with expanded surfaces, with wire diameter d=0.64mm and distance between the wires â–³L=4mm, can increase the heat transfer coefficient by 16% higher than the vertical flat plate, and its heat transfer coefficient formula is imitated by using the least square error method.