In the real welding arches the arch column is divided into two parts, significantly differing among themselves under the terms of existence: the arch column which is under the surface of the product and over its surface. These differences include not only available the cooling surface of the product crater for the column which is under the product surface, but also available the thermal effect on this column part of the reflected plasma flow covering this part of the column and protecting it from cooling with crater walls. The reflected gas flow as a result of the crater gas flow surface braking in a column is further heated and its temperature is higher, than the gas temperature in the column:
where Тт — the gas temperature at full braking, К; Т — the gas temperature in incoming flow, К; V — the gas velocity in incoming flow, m/sec; Ср — the gas specific heat at constant pressure, J/(kg*grad). It causes the heating of that part of the column which is under the product surface, as well as in close proximity to it, unlike other part of the column which is cooled with the flow of cold protective gas. Therefore, the part of the column which is under the product surface receive the additional heat rate from its surrounding reflected flow that allows to argue that in the lower part of the arch the energy consumed from the power supply, and the greatest conductivity of plasma is made the most efficient use.
The gas flow promotes the arch column expansion from the product surface, and cooling of the column top part with the protective gas flow results in compression of the part of the column. Both of these effects promote the taper column: to increase in its cross diameter as approaching the product surface.