Investigation of Semi/Near-Dry Machining for Ecology in Thermal Engineering

微量润滑是半干式或近干式加工的一种。机械加工是一种生态技术。在该方法中，使用基于几cc/h的植物油混合的润滑剂切割材料。在加工过程中，液体也用作冷却剂。本研究以工业加工油与水为冷却介质，对油雾冷却之热传特性进行实验与分析。在此基础上，重点研究了液滴尺寸、液滴速度和液滴质量流量等因素对喷雾冷却换热特性的影响，并在室温下对纯铜圆柱体、工业机械油和纯水的雾流和空气进行了稳态传热实验。液体流量分别为0.3，0.9，1.8，4和l/hr，空气流量分别为0，40，75和120 lN/min。在本实验范围内的最大冷却速率为6 MW/m^2。油雾冷却的传热机理主要为液膜冷却，而非沸腾传热;水雾冷却的传热机理可分为非沸腾、液滴蒸发和液膜蒸发3个区域。以加热器上液体质量流量的最大蒸发率为研究对象，建立了细水雾流动的冷却极限（临界热流密度）模型。油雾中的换热系数可以用油滴对加热表面显热的排热能力来表示。最后，通过量纲分析和实验数据，得到了四个无因次的细水雾和油雾流动的传热关联式。

A kind of semi or near dry machining is Minimum Quantity Lubrication. The machining is expected as an Ecological technology. In the method, material is cut with lubrication blended on the basis of vegetable oils of a few cc/h. The liquid also serves as coolant during the machining. In this study, heat transfer characteristics in mist cooling with commercial machining oil and water were investigated experimentally and analytically. Especially, the heat transfer in the mist cooling was examined focusing on the effects of droplet size, droplet velocity and liquid mass flux on the heat transfer characteristics.Steady state experiments of heat transfer were conducted using a pure copper cylinder and mist flow of commercial machining oil or pure water with air at room temperature. Liquid flow rates were 0.3,0.9,1.8,4 and l/hr, respectively ; each air flow rate was 0,40,75 and 120 l_N/min.Experimental results showed that heat fluxes increased with an increasing liquid flow rate for every air flow rate. Maximum cooling rate in the present experimental range was 6 MW/m^2. The heat transfer mechanism in oil mist was only cooling of liquid film without boiling formed on a heated surface, whereas the heat transfer mechanism in water mist cooling was classified into three regions: non-boiling, evaporation of droplets and evaporation of the liquid film. A cooling limit (critical heat flux-CHF-) model of water mist flow was presented by focusing on maximum evaporation rate of liquid mass flux on a heater. The heat transfer coefficient in oil mist was well expressed by the heat removal capacity on sensible heat of supplying oil-droplets to the heated surface. Finally, four dimensionless correlations of heat transfer were derived from a dimension analysis and experimental data for both the water mist flow and the oil mist flow.