管翅式换热器胀接参数对换热器性能的影响

对于管翅式换热器,铜管与翅片接触热阻占到换热器总热阻的10%—20%,并且随着翅片热疲劳的加剧,接触热阻不断增大。通过对不同胀接工艺参数的管翅式换热器换热量进行测试,分析得出胀接工艺参数胀头直径对换热器性能的影响。运用数值计算及可视化试验方法,从局部特性分析胀接对管翅接触状态和管内内螺纹形变的影响。结论如下:胀头直径过小会出现欠胀问题,管与翅片胀接程度达不到要求,会影响换热器性能;胀头直径过大会出现过胀现象,接触间隙大大增加且损伤内螺纹齿,同样会降低翅片换热器的换热性能。     

沸腾回流法制备纳米NiZn铁氧体的研究

Nanometer NiZn ferrite powder with spinel structure was prepared by a new chemical method —— refluxing method. Refluxing time and pH value of coprecipitation were discussed based on the XRD analysis and pH value measurement. The optimum refluxing time and pH value of coprecititation were 6 hours and 9.5~11.0, respectively. Ni_0.5Zn_0.5Fe₂O₄ powder prepared by refluxing method was about 20 nm by TEM analysis. And VSM results revealed that the prepared Ni_0.5Zn_0.5Fe₂O₄ powder had typical soft magnetic characteristics.