Estimation of temperature-dependent heat transfer coefficients in near-dry cutting

Abstract

Cutting fluid is the most common method to control the cutting temperature. However, it is usually harmful to the environment and personal health. Near-dry cutting could be an effective substitute of cutting fluid. Most researchers focus on the lubrication during near-dry cutting processes. Actually, the cooling ability of near-dry cutting is very important for the control of cutting temperature and effectiveness of coolant. In this study, heat transfer coefficients (HTCs) of different cooling processes, including nature cooling, compressed air cooling, compressed cold air cooling, minimal quantity lubrication (MQL) cooling, and atomizing water cooling, have been estimated. Surface temperature, temperature of coolant carrier, and air velocity of coolant carrier have a great effect on the cooling ability of near-dry cutting system. The finite element method (FEM) is employed to simulate the cooling process and modify the estimated heat transfer coefficients. It is found that HTC is from 9.3 to 53.9 W/m2/°C with different surface temperatures in nature cooling. HTC increases with the increase of surface temperature. HTC is from 48.1 to 346.0 W/m2/°C with different air velocity and surface temperature in compressed air cooling. HTC has an increased trend with the increase of air velocity. HTC is from 52.6 to 405.6 W/m2/°C with different air velocities and surface temperatures in compressed cold air cooling. Lower air temperature increases HTC by 10∼20 %. HTC is from 96.5 to 718.7 W/m2/°C with different oil quantities and surface temperatures in MQL cooling. HTC increases with the increase of oil quantity of MQL. As a comparison, HTC is from 133.7 to 519.6 W/m2/°C with different water quantities and surface temperatures in atomizing water lubrication (AWL) cooling. HTC of AWL is much lower than that of MQL in the same condition, especially when the surface temperature is high. The boiling point of water is much lower than oil; subsequently, less water mist arrives at the cooled surface, so less heat is removed with AWL cooling than that with MQL cooling in the same condition. HTC is from 282.1 to 1890.3 W/m2/°C with different oil quantities and surface temperatures in cold-air MQL cooling. Lower carrier temperature increases the HTC greatly, even increases the HTC by 150 % when the surface temperature is up to 700 °C. The reason is that the coolant prevents high-temperature deterioration before the coolant arrives at a cooled surface.