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流体力学局部阻力实验指导Experiment of Measuring Minor Head Loss 1 Introduction of experimental principle Considering the steady flow in pressure circular pipe, if the diameter of the pipe with length is a constant, the streamline of the flow contains stead. When the figure changes...

流体力学局部阻力实验指导
Experiment of Measuring Minor Head Loss 1 Introduction of experimental principle Considering the steady flow in pressure circular pipe, if the diameter of the pipe with length is a constant, the streamline of the flow contains stead. When the figure changes between Two sections of pipe, such as sudden contract, sudden expansion, gradual expansion, turning axial direction, conflux, diffluent or be blocked etc., the streamline will change and flow construction will be reformed. At the same time, vortices appear and then dissipate some energy. The minor head loss     is defined as the energy loss for unit weight fluid due to above changes. The minor head loss, deducing from energy equation between sections and is: Above two sections should be located in gradual changing flow. The minor loss coefficient is written as the ratio of minor loss and speed head: Different equations of minor loss use different speed head, including upstream speed head and downstream speed head. Generally, the downstream speed head is adopted.  It is difficult to gain the minor loss coefficient using theory method, except the condition of sudden expansion. The usual minor loss coefficients are determined by correspond experiments. Some minor loss coefficients have a limit of Reynolds Number for using. Now, let’s deduce the minor loss coefficient of sudden expansion. Take the just begin sudden expand section as section , the section locate at the section in which the flow construction have reformed. Two sections have same area , but averaged speed are and , respectively. Let the level of center of two sections are and . Assume the manometer head on section is constant. The minor loss for sudden expansion is: And the momentum equation: Assume the kinetic modify coefficients , momentum modify coefficients , then rewrite the momentum equation: Put above equation into the minor loss, Or write as upstream speed head: So, 2 Experimental Equipment A sudden expansion pipe is fixed serially in a closed pipe system, which include the pump, valves, pressure tank and many manometers.  3   Purpose and Requirement 1) Observe the surfaces of manometers at different sections on sudden expansion pipe. 2) Learn the measuring method of the minor loss coefficient, comparing the experimental value with academic value. 3) Learn to use manometer and measure flow rate with volumetric method. 4   Experimental Steps 1) Read carefully the experimental purpose, requirement, principle and attention. 2) Learn the operating method of manometer and measuring method flow rate with volume. 3) Realize the actual experimental equipment, prepare operation. 4) Check the surfaces of manometers have same level or not. Remove the gas in tube is need if they have different levels. 5) Check the numbers on tubes and affirm the diameter of sections etc. in the data recording list. 6) Open the downstream valve; choose correct sections and record relative data after the flow is stead. 7) Change the opening degree, so the flow rate changes. Repeat above steps for three different flow rate and record corresponding data. 8) Calculate the minor loss coefficient from experimental data, then comparing with academic value. Analysis the results and answer the problems. 5   Record Experimental Data Equipment No: __________ Experimental constants: , area of measuring water tank: Temperatures: ______(before experiment); ______(after experiment) Experiment No. Initial depth Final depth Tank volume Time 1               2               3               4               5                               No. Surfaces of manometers level data (cm) 1                             2                             3                             4                             5                                                           No. Speed Speed Real coefficient Academic coefficient Reynolds Number 1             2             3             4             5                           From the recording data, draw the qualitative curve for surfaces of manometers at maximum flow rate. 6   Problems 1) Why there is difference between real and academic values of minor loss coefficient in sudden expansion pipe? What is factor? 2) Why the sudden expansion pipe downstream section is chosen from the changing surface of manometers? How to find it?
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