Volume 1, Issue 1
Real and Effective Heat Transfer Coefficients (HTCs) Used for Computer Simulation of Transient Nucleate Boiling Processes during Quenching
In this paper, real and effective heat transfer coefficients (HTCs) are considered. The real HTC is determined by dividing the heat flux density by the superheat of the liquid. The effective HTC is obtained by dividing the heat flux density by the underheat of the liquid plus the superheat of the boundary layer. In practice, especially in the heat treating industry, effective HTCs and their average values are widely used. In many cases that is not correct, because bubble formation (critical diameter) depends on the superheat ΔT = T − TS only and does not depend on the bath temperature. It is shown that due to this incorrectness, many problems arise, and misunderstanding of the quenching process often exists. So as to understand correctly the involved thermo-physical processes and receive correct data for computer simulation, in this paper, the real and effective HTCs are compared with each other. Several contradictions are shown between real and effective HTCs, such as the fact that the real HTCs do not depend on the size of a steel part and the thermal properties of a material. The effective HTC depends proportionally on the thermal properties of a material and is inversely proportional to the size of steel parts that contradict each other. In order to solve this problem, in this paper it is shown that the effective HTC is a mathematical value that can be used only for simplified cooling rate and cooling time calculations at the core of steel parts, and it cannot be used for temperature field calculations and residual stress prediction. The results of this paper can be used for computer simulation of the process of quenching and for the development of new intensive quenching technologies.