空气纵掠平板时局部换热系数的测定.doc

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1、空气纵掠平板时局部换热系数的测定五、实验记录及数据处理U/v1.68I/A17.08板宽b0.065总长l0.66x离前缘距离温差t-tf局部换热系数hx227.224.59070341452923.064383892830.621.85840303513.532.920.330307992137.517.8364568763040.216.6384858924142.815.6277367495445.214.7979454177052.912.643991178953.612.4788644

2、1911055.712.00838658613552.912.6439911716452.612.71610518819950.813.16667584423450.113.35064137527547.114.201000698由以上的数据用excel软件画出折线图如下从图中可以看出距离平板前缘越远，局部换热系数越小，接着经过一段系数不变的距离后，局部换热系数又有上升的趋势。这是因为在平板的起始段，δ很薄。随着x的增加，由于壁面粘滞力的影响逐渐向液体内部传递，边界层逐渐增厚，但在，某一距离xc

3、以前会一直保持层流的性质。这时的边界层称为层流边界层。而层流边界层主要通过热传导导热，则，将冷却公式联立，得到以下关系式：由上式可知当平板温度和液体温度一定时，y越大，即δ越大，局部换热系数h越小。故距离平板前缘越远的，δ很厚，h局部换热系数h越小。但自距前缀xc处起，流动朝着湍流过渡，最终过渡为旺盛湍流，此时湍流产生不规则脉动，强化了对流传热，使局部换热系数h升高。五、误差分析及问题讨论本实验误差来源1、金属片纵向导热2、电热功率并不是均布在整个表面3、片面外界辐射散热的影响4、各种仪器本身的

4、误差，如热电偶的灵敏度问题1、由于测得的数据点少，故很难从实验数据得知流体从层流变为湍流的距离xc。2、由于本实验没有考虑片面外界辐射散热的影响，故可能给实验带来一定的不确定度。试材名称:水泥珍珠岩测量时间(s):1000试样厚度(mm):15.0试样重量(g):40.0试样容重(kg/m3):加热器电压(V):5.4012加热器电流(A):0.0855总采样次数:201时间冷面温度热面温度冷面与热面温度025250524.9250.1102525015252502025250252525030

5、25.125.103525.125.104025.125.104525.225.205025.125.105525.225.206025.225.206525.225.207025.225.207525.225.208025.225.208525.225.30.19025.325.309525.325.3010025.325.3010525.325.3011025.425.4011525.425.4012025.425.4012525.425.4013025.425.50.113525.425.

6、50.114025.425.4014525.525.5015025.525.5015525.525.5016025.525.60.116525.525.5017025.525.5017525.525.5018025.525.5018525.525.60.119025.625.6019525.525.60.120025.625.6020525.625.6021025.625.6021525.625.6022025.725.7022525.725.7023025.625.6023525.625.70

7、.124025.625.70.124525.725.7025025.725.7025525.725.7026025.725.7026525.725.7027025.725.7027525.725.7028025.725.80.128525.725.80.129025.725.80.129525.725.80.130025.725.80.130525.725.7031025.725.80.131525.725.80.132025.825.8032525.725.80.133025.725.80.1

8、33525.725.80.134025.825.8034525.725.90.235025.825.8035525.825.8036025.825.90.136525.825.90.137025.725.90.237525.725.90.238025.825.8038525.825.90.139025.825.90.139525.725.80.140025.825.90.140525.8260.241025.8260.241525.8260.242025.8260.242525.8260.243