| During the machining of difficult-to-machine materials, most of the consumed energy converts to heat and accumulates in the machining zone. If the heat cannot be transferred out in time, the temperature in the machining zone will rise quickly and make tools disabled and workpiece useless. If plenty of coolant is used to transfer the heat out of machining zone, other problems will emerge, such as the increase of production cost, environment pollution, the harm to worker's wealth and etc. It becomes a handicap to the healthy development of manufacturing. Although there have been some kinds of green cooling technologies, they all have limitations of themselves. They cannot attain the effect of lower cost and higher efficiency. Here an original idea about Cryogenic Pneumatic Mist Jet Impinging (CPMJI) Cooling based on the conception of Green Manufacturing is put forward in this paper. The kernel of the idea is that CPMJI that involves two-phase heat convection, jet impinging and full vaporization is used to enhance the heat transfer potential of cooling medium and obtain the best cooling effect. According to this idea, some works have been done in this paper:1. Experimental equipment of CPMJI has been set up, which can produce cryogenic mist jet. Then a heat-transfer experimental platform was established based on the CPMJI set-up. Transient and steady-state heat-transfer experiments can be completed on the platform.2. The parameters about gas and liquid flow rate, temperature of the mist, jet velocity and the average droplet size under different conditions have been measured. And an optimum combination of these parameters was obtained for the following heat-transfer experiments. The experimental result indicates that employing withФ1.2mm jet nozzle, under the working condition of air pressure 0.5MPa and hydraulic pressure 0.4MPa, can obtain good atomizing effects with the jet speed above 150m/s and the mean particle size bellow 20μm in the impingement distance of 10-40mm.3. Critical heat flux (CHF) and heat transfer coefficient, which are characteristics used in expression of heat-transfer efficiency about CPMJI, were acquired through transient and steady-state experiments respectively. And the high efficiency cooling ability of CPMJI cooling technology was validated by comparing with cold air jet and flood cooling. The experimental result shows that the CHF and heat transfer coefficient of CPMJI increases by 70 and 25 times repectively in comparison to the pool saturated boiling. And the CHF and heat transfer coefficient of CPMJI is 1.7 and 3 times of flood cooling method, more than 10 and 22 times of cold air jet cooling method.4. Machining effects of CPMJI have been studied compared with other two cooling methods in the turning of titanium alloy. Lower cutting temperature, longer tool life, better surface quality and better chip-breaking performance can reach employing with CPMJI. Then the temperature field on cutting tool with different cooling methods was simulated by means of the finite element method. The simulation result shows good consistency with the experimental retult. It provided further evidence on the cooling ability of CPMJI during the machining of titanium alloy.5. Finally, CPMJI has been employed in the grinding and high speed milling of titanium alloy. The application results indicate that CPMJI have greater applied potentialities in high efficiency machining of difficult-to-machine materials compared with other two cooling methods.
|
PDF Full Text Request