Research on surface heat transfer mechanism of liquid nitrogen jet cooling in cryogenic machining

摘要

Since the control of liquid nitrogen (LN2) jet cooling plays an important role during cryogenic machining, especially for the through-tool cooling method, the characteristics of the surface heat transfer between LN2 jet flow and workpiece surface becomes particularly significant. In this work, an experimental platform using jet cooling was established. An inverse heat conduction theory was introduced and modified to calculate the temperature-dependent surface heat transfer coefficient (T-SHTC), which plays an important role in machining simulation and cutting thermal analysis. Mechanism of LN2 jet impingement cooling was clarified and the film boiling regime is found hard to exist in central LN2 impingement area since the generated nitrogen gas film would be flushed away by jet droplet, which minimizes the Leidenfrost effect. In addition, influencing factors such as the LN2 mass flow rate, jet distance and the phase of nitrogen were found to have effect on SHTC, but the SHTC is more sensitive to jet distance in comparison to the LN2 mass flow rate, while, a LN2 stream with big enough flow velocity and high liquid phase ratio is critical for an efficient cooling, which can provide a guidance for the process design in LN2 cryogenic machining.