Physiothermodynamics of lubricant application to hot die surfaces
The lubricant used in hot metalworking processes is applied to dies by spraying large numbers of atomized lubricant droplets through nozzles. An understanding of film formation and heat transfer of a single droplet colliding on a heated wall will result in precise control of spray to obtain required film patterns and heat transfer. The precision of spray control will, in turn, reduce energy consumption and pollution through reduction of billet pre-heat temperature and enhancement of lubricant wetting with the die. In this research, the spreading of droplets which impact on the die surface is modeled based on Navier-Stokes equations and this model is solved using a commercial CFD code, Flow-3D. A non-dimension analysis of the simulation results provides an insight into the physical phenomena and the model is validated through controlled experiments using a high speed camera. This study also presents an analytical model to predict the evaporation time of a droplet impacted on a die surface, heated to temperatures beyond Leidenfrost point, beyond which are the working temperatures for hot forging dies. Calculated evaporation times from the model are compared with experimental measurements and agree well in a right magnitude. A methodology for calculating the heat transfer coefficients in lubricant sprays is then presented. The heat transfer characteristics of the lubricants at various thermo-physical regimes on the hot surface are determined using single droplet experiments. The results of these experiments are used along with equations based on a non-dimensional and averaging approach to predict heat transfer coefficients for a complicated spray. Predictions by both the fluid dynamic model and the dryoff model agree well with the results obtained in experiments. Results from single droplet studies show that both Weber number (We) and die surface temperature play an important role on the lubricant wetting, dryoff and heat transfer on the die. Selection of lubricant dilution ratio has also been shown to be important on interactions between sprayed lubricants and the die surface.
School:The Ohio State University
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:die surface lubrication
Date of Publication:01/01/2005