流体力学与传热习题：ppt后习题答案

1、ppt11. The shear stress depends upon the ( viscosity of fluid) and ( velocity gradient )2. Pressure is a ( B ) quantity; at any given point its magnitude is ( D ) in all directions.A) vector , B) scalar, C) different , D) the same3. Shown in Figure, the pressure at point 1 is 1.5 atm abs, Pa is 101.

2、33103Pa. If the pressure Pa at the surface of liquid increases by 1 times, the pressure at this point increases by (101.33103Pa ), and is ( 2.5/1.5 )times original pressure.4. Incompressible fluid means that ( density is constant );compressible fluid means that( the density is sensitive to changes i

3、n both the temperature and the pressure)problem2 A manometer of the type shown in Fig1.33. is used to measure the pressure drop across the orifice. Liquid A is mercury and fluid B, flowing through the orifice and filling the manometer leads, is brine (density 1260 kg/m3). When the pressure at the ta

4、ps are equal, the level of the mercury in the manometer is 0.9m below the orifice taps. Under operating condition, the gauge pressure at upstream tap is 0.14bar, the pressure at the downstream tap 250mmHg below atmospheric. What is the reading of the manometer in millimeters?Solution:p1=0.14bar=0.14

5、105Pa, p2=-33332PaProblem 4 作业1.3ppt1-2In a Newtonian fluid ，the relationship between the shear tress and shear rate is（ ）l Keeping the pressure constant，the viscosity of a gas will （ increase ）as the temperature rising, and the viscosity of a liquid will （ decrease ）as the temperature risingl the e

6、ffect of the solid boundary on the flow is confined to a layer of the fluid immediately adjacent to （ wall of solid ） l At low flow rates the pressure drop in the fluid is proportional to the ( velocity ); at high rates it increases as the ( square of the velocity ). l Mechanical energy is finally c

7、onverted to ( heat ) when eddies in fluid are obliterated by viscous action, which may lead to a large ( friction ) loss in the fluid. l Potential flow has two important characteristics: (1) neither circulations nor eddies can form within the stream(2)friction cannot developppt1-3 Bernoullis equatio

8、n has following restrictions1) ideal fluid2) density is constant3) steady-state flow The total energy balance is not often used when (appreciable enthalpy) changes occur or (appreciable heat ) is exchanged between the system and the environment. Mechanic energy includes (work term ), (potential ener

9、gy ), (kinetic energy ), and (flow work part ) of the enthalpy term.4Experiment on the conversion of mechanical energy shown in Fig. 作业1.5Water from the reservoir flows through the pipe, which the throat diameter is d. the ratio of D to d equals 2, the vertical distance h between the tank A and axis

10、 of the pipe is 1m.How much the H is needed if the water at the tank A is left to the throat of the pipe? Assuming that fluid flow is a potential flow.作业1.85m4m3m1mABCDMoving the water from one container into the other place by means of a pipe of constant cross-sectional area is shown as in figure.

11、Find(1) The velocity at station A,B,C,D(2) The pressure at station A,B,C,D(3) The mechanical energy in station A,B,CAssuming that the water flows through the pipe without friction losses. Solution：1) to write Bernoulli equation 1.3-26 from station 1 to station Dsimplifying And givesVD=9.9m/sFrom con

12、tinuity equation VA=VB=VC=VD =9.9m/s2） gauge pressure PD=0writing Bernoulli equation between stations c-d gives，PC=-39.811000=-294300PaSimilarly PB=19.811000=9810Pa PA=-49.811000=-392400PaTotal mechanical energy maintain constant when the fluid flows from the container to the exit 9.815=49.05J/kgppt

13、1-4The average velocity is ( 0.5 ) times the maximum velocity at the center for laminar flow. The average velocity is ( 0.8 ) times the maximum velocity at center for turbulent flowThe maximum value of the momentum flux occurs ( at the interface between the fluid and the wall of solid) for a laminar

14、 flow.The maximum value of the velocity occurs ( at the center of the tube ) for a laminar flow in pipe.In laminar flow the velocity distribution with respect to radius is ( parabola ) with the apex at the centerline of the pipeThe pressure drop is ( proportional ) to the average velocity for the la

15、minar flowppt1-5 For laminar flow, the average velocity in a pipe is maximum velocity, for turbulent flow average velocity is maximum velocity. For laminar flow in a pipe, the flow rate is constant, when di increases, the friction coefficient will ( increse), and friction loss will (reduce ). When t

16、he pipe installation changes from horizontal to vertical position and the velocity keeps unchanged, the frictional loss will be C . A）. increased; B）. decreased; C）. the same; D）. uncertain In laminar flow ,the velocity distribution with respect to radius is ( ) at the centerline of the pipe Roughne

17、ss has ( no effect ) on the friction factor for laminar flow unless k is very large . For laminar flow, the friction coefficient is proportional to a ( reciprocal of Re ) For turbulent flow, the friction coefficient is a function of both (roughness and Re ) For completely turbulent flow, the frictio

18、n coefficient is a function of ( relative roughness )1250 cm3/s of water is to be pumped through a steel pipe, 25mm diameter and 30m long, to tank 12m higher than its reservoir. Calculate approximately the power required. What power motor (in kW) would you provide? The roughness of a steel pipe will

19、 be taken as 0.045mm; the efficiency of the pump is 50%Solution: turbulent flow from moody figure作业1.9Liquid flows through the tube, shown in Fig. the length of ab equals cd，and the diameter and roughness of tube are same. Find:（1）the expressions of pab/( g), hfab, pcd/( g) and hfcd,respectively.（2）

20、the relationship between readings R1and R2 in the U tube. 两水池的水位差H=24m，l1=l3=l4=100m，d1=d2=d4= 100 mm，d3=200mm，沿程阻力系数1= 2 = 4= 0.025 ，3=0.02，除阀门外，忽略其他局部阻力。（1）阀门局部阻力系数=30，求管路中的流量；（2）关闭阀门，其他条件不变，求管路中的流量。解：假设2、3的l相等1） 由得流量得总摩擦阻力损失故u3=0.575m/s u1=3m/s Q=0.024m3/s2）得u1=2.5m/s Q=0.0196m3/sppt1-7l orifice

21、and venturi meters measure the average velocity of the entire stream of fluidl The pitot tube measures the local velocity of fluid.l Rotameter must be vertically installed in the pipe line, and the fluid must flow upward through the tapered tube and suspends freely a float.v In the orifice and ventu

22、ri meter, the variation of velocity through ( constant ) area generates a (variable) pressure drop.v In the rotameter, the variation of flowrate through ( variable ) area generates a ( constant ) pressure drop.v For a orifice , nozzle and venturi meter, when the Reynolds number is greater than criti

23、cal Re, then orifice coefficient Co is (only dependent) of the Reynolds number for a given Sulphuric acid of specific gravity 1.3 is flowing througha pipe of 50 mm internal diameter. A thin-lipped orifice, 10mm, is fitted in the pipe and the differential pressure shown by a mercury manometer is 10cm

24、. Assuming that the leads to the manometer are filled with the acid, calculate (a)the weight of acid flowing per second, and (b) the approximate pressure drop caused by the orifice. The coefficient of the orifice may be taken as 0.61, the specific gravity of mercury as 13.6, and the density of water

25、 as 1000 kg/m3Solution:(a) the mass flow rate m is given by Or, where is approximately unity, m=0.268kg/s (b) the approximate pressure drop caused by（13600-1300）9.810.1=12066.3PaWater flows through an orifice of 25mm diameter situated in a 75mm pipe at the rate of 300cm3/s. what will be the differen

26、ce in level on a water manometer connected across the meter? Take the viscosity of water as 1mN s/m2.Solution:The velocity flowing through hole velocity flowing pipe the ratio of area : m=0.11so Co=0.61R=51.2mmWater is transported by a pump from reactor, which has 200 mm Hg vacuum, to the tank, in w

27、hich the gauge pressure is 0.5 kgf/cm2, as shown in Fig. The total equivalent length of pipe is 200 m including all local frictional loss. The pipeline is 573.5 mm , the orifice coefficient of Co and orifice diameter do are 0.62 and 25 mm, respectively. Frictional coefficient is 0.025, Find: Develop

28、ed head H of pump, in m .(the reading R of U pressure gauge in orifice meter is 168 mm Hg)Solution:Pr=-20013.69.81= -26683.2Pa Pt=49035Pa，Pressure drop in Up=0.168(13600-1000)9.81=20765.8Pafrom continuity equation: 1.6m/sskin friction lossdeveloped headppt 2-1l If a centrifugal pump operates on air,

29、 then, it cannt draw liquid from an initially empty suction line。l Positive-displacement pumps are not subject to air binding.l Before a centrifugal pump starts up, close the valve in discharge in other to ( reduce ) the start-up power requiredl When the temperature of fluid to be transported increa

30、ses, the maximum lift of suction will ( B ). If the velocity of suction pipe decreases, the maximum suction lift will ( A ). If the flow rate remains constant but the diameter of suction pipe decreases, the maximum suction lift will ( B ) A） increase; B ）decrease; C）keep the same D） be uncertain l W

31、hen the suction pressure is ( close ) to the vapor pressure, the cavitation will occurl The difference between the theoretical and actual curves results from( shock loss) ; ( friction loss); and (recirculation loss) . When higher developed head is required, the best selection is ( B ).A) operation i

32、n series B) multistage centrifugal pump C) throttled by valve l ( )One pump can develop more H than that two same pumps which work in series. l The methods of adjusting volume flow rate of centrifugal pump are (1) changes in revolution of pump; (2) change of diameter of impeller ; (3) adjust the ope

33、ning of valve in discharge line; The commonest way in practice is ( 3 ). 作业2.3练习 ?前面ppt3-1l Settling of a spherical particle in a air is followed Stokes law, if the temperature changes from 25 to 50 , the terminal velocity will ( decrease ); if settling in liquid, the terminal velocity will ( increa

34、se ) v The terminal velocity is the velocity that the acceleration that a particle moves through the fluid approaches to ( 0 )v A single spherical particle settling freely in the fluid and it is laminar flow, when the particle diameter increases, the terminal velocity u will increase ; when the visc

35、osity of fluid increases, u will decrease ; if the fluid is a gas, what happens to u if the temperature increase?v As shown by equations (3.2-16 ) and (3.2-19 ), the terminal velocity ut varies with the square of diameter of particle in the (laminar flow ) range, whereas in the( Newton law ) range i

36、t varies with 0.5 power of the diameter of particle v For a given packed bed, Blake-Kozeny equation indicates that the flow is ( proportional ) to the pressure drop and ( inversely ) proportional to the fluid viscosity.vppt3-2l For given dust system the capacity of gravity settling chamber depends o

37、n ( C )A) The height and length of chamberB) height of chamberC) The bottom area of chamberD) Volume of chamber Settling separation is based on ( density difference ) between the fluid and particles. For coarse solid particles, larger than about 325-mesh（43m diameter), which may be removed by （A ） A

38、) a gravity settling； B) a centrifugal settling; C) press filter; D) deep-bed filter l For given dust system the critical diameter of particle removed by a cyclone varies ( with cyclone size, velocity, viscosity, and density of solid. ) ppt3-4l Fluid passes through a packed bed in laminar flow, the

39、basic equation applied to packed bed for relating the pressure drop with the width of bed, average interstitial velocity, diameter of tortuous channels, and the properties of fluids is (Blake-Kozeny equation )l Hagen-Poiseuille modified by substituting equivalent diameter of channel for the diameter

40、, and superficial velocity for the interstitial velocity gives a( Blake-Kozeny ) equationl Basic filtration equation comes from (Blake-Kozeny ) equation, and constant k depends on the ( porosity ) of packed bed and (specific surface ) of particle, and specific cake resistance r is ( a reciprocal of

41、) kl For an incompressible cakel When the filtrate increases, the rate of filtration will ( decrease )l When the filtration area increases, the rate of filtration will ( increase )l When the pressure difference applied to the cake increases, the rate of filtration will (increase )l When the temperat

42、ure of the suspension increases, the rate of filtration will ( increase )l When the concentration of the suspension increases, the rate of filtration will ( decrease )ppt3-5l For constant pressure filtration, if the filtering medium (septum) resistance is ignored, the filtering time is double and ot

43、her operating conditions keep unchanged, the filtrate volume will be(C)（A) double; (B) half of original; (C) 20.5 of original; (D) uncertainl Which the statement is correct in following( B )A) Filtration rate is proportional to filtration area (A).B) Filtration rate is proportional to the square of

44、filtration area (A2).C) Filtration rate is proportional to the volume of filtrate (V).D) Filtration rate is inversely proportional to the resistance of filter medium.作业3.9ppt4-1l The reason for causing natural convection is the result of buoyancy force generated by (density difference of fluid), so

45、the properties of fluid affecting natural convection are ( density, thermal expansion coefficient and viscosity)l Heat transfer coefficient depends on ( type of flow ),(properties of fluid), (thermal properties of fluid ), and (geometry of surface)l The rate of heat transfer by a radiation is propor

46、tional to (fourth power) of absolute temperaturel A metal tube with outside radius of r1 covered with two layers of insulated materials having different thickness(bBbA), the temperature distributions of two layers are shown as in figure, the conductivity of material A is ( C ) that of material B.A)

47、Less than; B) can not judge; C) larger than; D) the same as温度分布BAl For a steady-state heat transfer through a series of cylindrical layers, the heat-transfer rate q will be ( A ) for each layer and the heat flux q/A will be ( B ) for each layer.l For a steady-state heat transfer through a series of

48、flat layers, the heat-transfer rate q will be ( A ) for each layer and the heat flux q/A will be ( A ) for each layer.A) the same B) different C) analogous D) uncertainppt4-2l A trap is a device that allows (the condensate flowing out) but (holds back the vapor).l Parallel flow is rarely used in a s

49、ingle-pass exchanger because it is (impossible) with this method of flow to bring the exit temperature of one fluid nearly to the entrance temperature of the other and the heat transferred is ( less ) than that possible in countercurrent flow. l ( ) If the inlet and outlet temperatures of fluids are

50、 fixed, the LMTD of countercurrent flow is always larger than that of parallel-current flow without phase heat transfer l air flows along the tube and saturated vapor passes through the shell in a shell-tube exchanger. In order to enhance heat transfer, which way is feasible in practice as follows.（

51、C）A）increase vapor velocity; B) employ superheated vapor; C) increase air velocity. D) set up the baffle in the shell.l If ho is very small in comparison with both b/k and the other coefficient hi，it is sufficiently accurate to equate the overall coefficient to the individual coefficient（ho）.l Heat

52、transfer by two fluids，if one of the fluids is at constant temperature, the mean temperature difference exists between countercurrent flow and parallel flow.( )l ho is a film coefficient of shell side and hi is a film coefficient of tube side, if ho is much larger than hi the temperature of the meta

53、l wall will close to ( that of ho side)l The Prandtl number of gases is almost （independent）of temperature because the viscosity and thermal conductivity both increase with temperature at about the same rate.l Assuming that fluid flows through a pipe in turbulent flow, the film coefficient will be(

54、20.8 ) times that of original as the flowrate increased 2 times. Problem 1,2 3 上课时讲解了A cooling coil, consisting of a single length through which water is circulated, is provided in the reactor vessel, the contents of which are kept uniformly at 360 K by means of stirrer. The inlet and outlet tempera

55、tures of the cooling water are 280 K and 320 K respectively. What would the outlet water temperature become if the length of the cooling coil were increased 5 times ? Assume the overall heat transfer coefficient to be constant over the length of the tube and independent of the water temperature.Solu

56、tion: when A，=5A solving for t=357.35KIn an oil cooler, 60g/s of hot oil enters a thin metal pipe of diameter 25mm. An equal mass of cooling water flows through the annular space between the pipe and a larger concentric pipe, the oil and water moving in opposite directions. The oil enters at 420 K and is to be cooled to 320 K. If the water enters at 290 K。1）what length of pipe will be required? Take coefficients of 1.6kW/m2K on the oil side and 3.6kW/m2K on the water side and 2.0kJ/kgK for the specific heat of the oil. 2）What would