Experiment based modeling of the mechanical expansion of tubes for the construction of heat exchangers

Tube heat exchangers are made by assembling metals tubes, which the fluid to be refrigerated is passed through, with fins where a refrigerating fluid (usually air) is flown over. The heat exchange between tubes and fins is obtained by exploiting their tight contact. This necessary very tight contact is obtained by means of brazing (typically in smaller equipment) or through the forced expansion of the tubes into the fins holes. The forced expansion can be hydraulic (by some fluid put in pressure in the assembly operation) or mechanic through the insertion of a sphere or an ogive with external diameter slightly larger than the internal diameter of the tube. The sphere or the ogive is pushed along the entire length of the tube so that the tube remains plastically forced into the fins holes. The process is then repeated for all the tubes of the heat exchanger. The present work concentrates on the mechanical expansion: to optimize the construction process it is necessary to have a model able to describe the mechanical phenomenon: that is, to evaluate the stress state in the tube during the insertion of the ogive, the residual stresses after the sphere/ogive passage, and the force required depending on the process and materials parameters (including the geometry of the tube, ogive, and fins, their material properties, friction, insertion speed etc.). The present work will describe an analytical model able to describe the process with a good level of predictability showing the effect of the main parameters involved in the process. The model is based and validated by means of experimental tests and numerical simulations at different levels and in different conditions and materials.