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Failure Pressure Model and Validation for Aluminum R744 Micro-Channel Tube
Published May 20, 2007 by Institution of Mechanical Engineers in United Kingdom
Operating conditions for components on the high-pressure side of R744 (CO2) climate control systems are 17.6 MPa and 180°C. Test/design requirements for the failure pressure at that temperature may be 2 to 3 times greater. Proper design of critical components, such as the gas cooler micro-channel tube, requires consideration of internal tube dimensions, alloy and post-braze material properties. To aid in the design of micro-channel tube for this application, a model that predicts the failure pressure is presented. The model presupposes (typical) ductile failure at the center of an internal tube wall. The state of stress at this location and the internal pressure at plastic instability are derived. The maximum pressure occurs when the differential pressure is zero (the instant of plastic instability). It is also shown that material properties for use in the model can be readily determined from typical room temperature tensile testing. Two common alloys, AA3102 and AA3003, are evaluated. True stress-true strain tensile properties of extruded tube in the post-braze condition are presented for use in the model. Pressure test results of micro-channel tube with circular and elliptical channels are presented to validate the model. Testing was also performed at elevated temperatures (to 200°C) in order to predict the decrease in failure pressure. The reduction in failure pressure with temperature is approximately 0.2% per °C, and it is fairly linear. The model exhibits a potential accuracy of about 7%.