流体力学课后习题答案第六章.doc

6-5 某蒸汽冷凝器内有250根平行的黄铜管，通过的冷却水流量q =8 l /s，水温为10oc，为了使黄铜管内冷却水保持为紊流（此时黄铜管的热交换性能比层流时好），问黄铜管的直径不得超过多少?解：查表1.3有10的水 由 及临界雷诺数 联立有 即为直径最大值 6.7 某管道的半径，层流时的水力坡度，紊流时的水力坡度，试求管壁处的切应力和离管轴轴处的切应力。解：层流时：紊流时： 6.9为了确定圆管内径，在管内通过为0.013 cm2/s的水，实测流量为35cm3/s，长15m，管段上的水头损失为2水柱，试求此圆管的内径。解: 设管内为层流校核 层流6-18 利用圆管层流，紊流光滑区和紊流粗糙区这三个公式，(1)论证在层流中，光滑区，粗糙区；(2) 在不计局部损失hm的情况下，如管道长度l不变，若使管径d增大一倍，而沿程水头损失hf不变，试讨论在圆管层流、紊流光滑区和紊流粗糙区三种情况下，流量各为原来的多少倍？(3) 在不计局部损失hm的情况下， 如管道长度l不变，通过流量不变，欲使沿程水头损失hf减少一半，试讨论在圆管层流、紊流光滑区和紊流粗糙区三种情况下，管径d各需增大百分之几？解：（1）由，有 即在层流 由 得 光滑区由 得粗糙区 （2）由，以上公式变为 q变为16倍 q变为6.56倍 q变为6.17倍（3）由以上公式计算可知分别19%，16%，14%6-19 两条断面面积、长度、相对粗糙高度都相等的风管，断面形状分别为圆形和正方形，试求（1）若两者通过的流量相等，当其管内流动分别处在层流和紊流粗糙区两种情况下时，两种管道的沿程水头损失之比hf圆/hf方分别为多少？（2）若两者的沿程水头损失相等，且流动都处在紊流粗糙区，哪条管道的过流能力大？大多少？解：（1） 当量直径 层流时 紊流粗糙区，相等 （2）=1.06此时圆管流通能力大，大6%6.20 水管直径为50，1、2两断面相距15 m，高差3 m，通过流量q6 l/s，水银压差计读值为250，试求管道的沿程阻力系数。解:6-23测定某阀门的局部阻力系数，在阀门的上下游共设三个测压管某间距l1=1m,l2=2m.若直径d=50mm,实测h1=150cm,h2=125cm,h3=40cm,流速v=3m/s,求阀门的值。解： 624 用突然扩大使管道的平均流速由v1减到v2，若直径d1及流速v1一定，试求使测压管液面差h成为最大的v2及d2是多少？并求最大h值。解: 6-25 速由变到的突然扩大管，如分两次扩大，中间流速取何值时局部水头损失最小？此时的局部水头损失为多少？并与一次扩大时比较。解：其中，要使最小，则 即 所以 时局部水头损失最小一次扩大时： 是两次扩大时的两倍6-26 水箱中的水通过等直径的垂直管道向大气流出。已知水箱的水深h，管道直径d，管道长l，沿程阻力系数，局部阻力系数之和为，试问在什么条件下：（1）流量q不随管长l而变化？（2）q随l的增加而减小？（3）q随管l的增加而增加？解（1）水箱水面进出口断面能量方程由得，因为，解得（2），即（3），即6-28 水池中的水经弯管流入大气中（题6-26图），已知管道的直径d=100mm,水平段ab和倾斜段bc的长度均为l=50m，高差h1=2m,h2=25m，bc段设有阀门，沿程阻力系数=0.035，管道入口及转弯的局部水头损失不计。试求：为使ab段末段b处的真空高度不超过7m,阀门的局部阻力系数最小应是多少？此时的流量是多少？解：取水池自由液面和 b处断面列伯努利方程：因为： =0 =-7 =0所以：=3.088m/s取b处断面和c处断面列伯努利方程：因为： = =0所以： 此时 6.29略631自水池中引出一根具有三段不同直径的水管，已知直径d50，d200，长度l100 m，水位h12 m，沿程阻力系数0.03，局部阻力系数阀5.0，试求通过水管的流量并绘总水头线及测压管水头线。解:由连续性方程列1、2断面伯努利方程hm=(入口+突扩+突缩+阀)6-32某闸板阀的直径d = 100mm，该阀门在开度e/d=0.125时局部阻力系数1=97.3，开度e/d=0.5时的2=2.06，该管道的沿程阻力系数为0.03。试求两不同开度情况下的折算长度lzh。解：acknowledgements my deepest gratitude goes first and foremost to professor aaa , my supervisor, for her constant encouragement and guidance. she has walked me through all the stages of the writing of this thesis. without her consistent and illuminating instruction, this thesis could not havereached its present form. second, i would like to express my heartfelt gratitude to professor aaa, who led me into the world of translation. i am also greatly indebted to the professors and teachers at the department of english: professor dddd, professor ssss, who have instructed and helped me a lot in the past two years. last my thanks would go to my beloved family for their loving considerations and great confidence in me all through these years. i also owe my sincere gratitude to my friends and my fellow classmates who gave me their help and time in listening to me and helping me work out my problems during the difficult course of the thesis. my deepest gratitude goes first and foremost to professor aaa , my supervisor, for her constant encouragement and guidance. she has walked me through all the stages of the writing of this thesis. without her consistent and illuminating instruction, this thesis could not havereached its present form. second, i would like to express my heartfelt gratitude to professor aaa, who led me into the world of translation. i am also greatly indebted to the professors and teachers at the department of english: professor dddd, professor ssss, who have instructed and helped me a lot in the past two years. last my thanks would go to my beloved family for their loving considerations and great confidence in me all through these years. i also owe my sinc