Flowing in pipe 2. The distinguish criterion of laminar flow and turbulent flow Definition As the pipe diameter d and fluid motion viscosity v are constant the average velocity of laminar flow becoming turbulent flow iS DC constant too. This velocity is called supercritical velocity which is expressed by D. The average velocity of turbulent flow becoming laminar flow if constant too and this velocity is called lower critical velocity which is expressed by U>U The flow state has relations with not only velocity of flow U but also pipe diameter d, fluid density and motion viscosity. According to the dimension analytic method in chapter 4 we can build up the above 4 parameters into a dimensionless number which is called reynolds number do ud R
11 2. The distinguish criterion of Laminar Flow and Turbulent Flow The flow state has relations with not only velocity of flow but also pipe diameter , fluid density and motion viscosity. According to the dimension analytic method in chapter 4 we can build up the above 4 parameters into a dimensionless number which is called Reynolds number. d v d d Re = = Definition： c c As the pipe diameter and fluid motion viscosity are constant d v the average velocity of laminar flow becoming turbulent flow is constant too . This velocity is called supercritical velocity which is expressed by . The average velocity of turbulent flow becoming laminar flow if constant too and this velocity is called lower critical velocity which is expressed by c > c
着空 二、层流和紊流的判别标准 定义: 当管径d及流体运动粘度V一定,则从层流变紊流时的平均 速度也是一定的,此速度称为上临界速度,以U表示;从紊流 变层流时的平均速度也是一定的,此速度称为下临界速度,以Ue 表示,b>D。 流动状态不仅与流速U有关,还与管径d,流体密度和 运动粘度有关。根据第四章的量纲分析方法可以将上述4个 参数组合成一个无量纲数雷诺数。 Re 12
12 二、层流和紊流的判别标准 流动状态不仅与流速 有关，还与管径 ，流体密度和 运动粘度有关。根据第四章的量纲分析方法可以将上述 4 个 参数组合成一个无量纲数——雷诺数。 d v d d Re = = 定义： c c 当管径 d 及流体运动粘度 v 一定,则从层流变紊流时的平均 速度也是一定的，此速度称为上临界速度，以 表示；从紊流 变层流时的平均速度也是一定的，此速度称为下临界速度，以 表示， c > c
Flowing in pipe Definition: The Reynolds number corresponding to the critical velocity of flow Uc is called critical Renaults number. Remembered as Re experiment proves: Though as pipe diameter and fluid medium are different U. is different, Re keeps in a definite range. Namely Re 2300 C C For the flow in round pipe, the distinguish conditions of the flow state are when Re< Re=2300 It belongs to laminar flow when Re Re =2300 It belongs to turbulent flow 13
13 Definition: experiment proves: For the flow in round pipe , the distinguish conditions of the flow state are c Rec Rec 2300 Though as pipe diameter and fluid medium are different is different , keeps in a definite range . Namely . c Rec The Reynolds number corresponding to the critical velocity of flow is called critical Renaults number . Remembered as . Re Re 2300 Re Re 2300 > = = c c It belongs to laminar flow It belongs to turbulent flow when when
着空 定义: 对应于临界流速U的雷诺数称为临界雷诺数,记作Re 实验证明: 虽然当管径或流体介质不同时,U不同,但Re基本上 保持在一个确定的范围。即Re≈2300。 对圆管流动,流态的判别条件是:当 Re<Re=2300 属于层流 Re > Re=2300 属于紊流
14 定义： 实验证明： 对圆管流动，流态的判别条件是：当 Re Re 2300 Re Re 2300 > = = c c 属于层流 属于紊流 c Rec Rec 2300 虽然当管径或流体介质不同时， 不同，但 基本上 保持在一个确定的范围。即 。 对应于临界流速 c 的雷诺数称为临界雷诺数，记作 Rec
Flowing in Pipe 3. Head loss regulation of laminar flow and turbulent flow in pipe On the experimental pipe segment Because as the fluid in the horizontal straight pipeline keeps stable we can write out that the on-way head loss equals the pressure head difference between two sections according to the energy equation. namely Change velocity to measure and the corresponding value h, under the circumstances of the laminar flow and turbulent flow the experiment result is shown in Figure 5--2 Ig h lg k, lgU lgu- Figure5--2 The header loss regulation of laminar flow and turbulent flow 15
15 3. Head loss regulation of laminar flow and turbulent flow in pipe p1 p2 hf − = Change velocity to measure and the corresponding value under the circumstances of the laminar flow and turbulent flow , the experiment result is shown in Figure 5—2. f h 0 45 lg f lg h 1 lg k 2 lg k c lg c lgC C Figure5—2 The header loss regulation of laminar flow and turbulent flow On the experimental pipe segment ,Because as the fluid in the horizontal straight pipeline keeps stable we can write out that the on-way head loss equals the pressure head difference between two sections according to the energy equation. namely