流体力学与传热习题参考解答(英文)10页

1、 . 1. Water is pumped at a constant velocity 1m/s from large reservoir resting on the floor to the open top of an absorption tower. The point of discharge is 4 meter above the floor, and the friction losses from the reservoir to the tower amount to 30 J/kg. At what height in the reservoir must the w

2、ater level be kept if the pump can develop only60 J/kg?U1=0 W=60j/kg 2. The fluid (density 1200 kg/m3 ) is pumped at a constant rate 20 m3 /h from the large reservoir to the evaporator. The pressure above the reservoir maintains atmosphere pressure and the pressure of the evaporator keeps 200 mmHg (

3、vacuum). The distance between the level of liquid in the reservoir and the exit of evaporator is 15 meter and frictional loss in the pipe is 120 J/kg not including the exit of evaporator, what is the pump effective work and power if the diameter of pipe is 60 mm? 3. Water comes out of the pipe (108x

4、4 mm), as shown in Fig. The friction loss of the pipeline which does not cover the loss at the exit of pipe can be calculated by the following equation: hf =6.5U2 where U is the velocity in the pipe, find a. water velocity at section A-A.b. water flow rate, in m3 /h. 4. Water passes through the vari

5、able pipe. The velocity in the small pipe is 2.5 m/s. The vertical glass tubes are inserted respectively at the section A and B to measure the pressure (see fig.) If the friction loss between two section is 15 J/kg, what is the water column difference between two glass tubes? By the way, draw the re

6、lative liquid column height of two tubes in the Fig. 5. A centrifugal pump takes brine (density 1180 kg/m3 , viscosity 1.2 cp) from the bottom of a supply tank and delivers it into another tank. The line between the tanks is 300 m of 25 mm diameter pipe (inner diameter). The flow rate is 2 m3 /h. In

7、 this line, there are two gate valves, four elbows (90o ) and one return bend, what is the friction loss if the roughness of pipe is 0.025 mm? 6. The orifice meter (diameter of orifice 0.0001 m) is installed for measuring the flow rate. The indicating liquid of orifice is mercury if U shape pressure

8、 gauge reading is 0.6 meter and orifice coefficient can be taken as 0.61, what is the flow rate of water? 7. Water flows through a pipe with a diameter di 100 mm as shown in figure.a. when the valve is closed, R is 600 mm and h equals 1500 mm. While the valve opens partially, R=400 mm and h=1400 mm,

9、 f=0.00625 (Finning factor) and kc =0.5 (contraction coefficient), what is the flow rate of water, in m3 /h?b. If the valve opens fully, what is the pressure of section 2-2, in N/m2 ? The equivalent length of the valve is 1.5 m and the Fanning factor f keeps the same?(rH2O=1000kg/m3, rHg=13600kg/m3)

10、(1) the valve opens partially ,for selection 1-1 and 2-2 , we have We can get Z1 from the valve closed (2) when the valve opens fully, for section 1-1 and 3-3, we have For section 1-1 and 2-2 8. The rotameter is installed to measure the water flow rate, as shown in figure. If the total length includ

11、ing equivalent length of pipeline A is 10 m and the reading of rotameter is 2.72 m3 /h, what is the flow rate for pipeline B? (fA =0.0075, fB =0.0045) For parallel pipe line 10. A flat furnace wall is constructed of 120 mm layer of sil-o-cel brick, with a thermal conductivity 0.08 w/(mo C), backed b

12、y a 150 mm of common brick, of conductivity 0.8 w/(mo C), the temperature of inner face of the wall is 1400 o , and that of the outer face is 200o C.a. What is the heat loss through the wall in w per square meter.b. To reduce the heat loss to 600 w/m2 by adding a layer of cork with k 0.2 w/(mo C) on

13、 the outside of common brick, how many meters of cork are requied?a. b. 600=(1400-200)/(0.12/0.08+0.15/0.8+x/0.2)x=0.0625m13. Air at the normal pressure passes through the pipe (di 20 mm) and is heated from 20o C to 100o C. What is the film heat transfer coefficient between the air and pipe wall if

14、the average velocity of air is 10 m/s? The properties of air at 60 o C are as follows:density 1.06 kg/m3 , viscosity 0.02 cp, conductivity 0.0289 w/(mo C), and heat capacity 1 kJ/kg-K 14. A hot fluid with a mass flow rate 2250 kg/h passes through a 25x2.5 mm tube. The physical properties of fluid ar

15、e as follows:k=0.5 w/(mo C), Cp =4 kJ/kg-K, viscosity 10-3 N-s/m2 , density 1000 kg/m3 Find:a. Heat transfer film coefficient hi , in w/(m2 -K).b. If the flow rate decreases to 1125 kg/h and other conditions are the same, what is the hi ?c. If the diameter of tube (inside diameter) decreases to 10 m

16、m, and the velocity u keeps the same as that of case a, calculate hi .d. When the average temperature of fluid and quantity of heat flow per meter of tube are 40 o C and 400 w/m, respectively, what is the average temperature of pipe wall for case a?e. From this problem, in order to increase the heat

17、 transfer film coefficient and enhance heat transfer, what kinds of methods can you use and which is better, explain?Hint: for laminar flow, Nu=1.86Re Pr1/3 for turbulent flow Nu=0.023Re0.8 Pr1/3 (1) (2) (3) (4) (5) there methods : increase u or hi or decrease d The first is better 15. In a double p

18、ipe exchange (23x2 mm), the cold fluid (Cp=1 kJ/kg, flow rate 500 kg/h) passes through the pipe and the hot fluid goes through the outside. The inlet and outlet temperatures of cold fluid are 20 and 80 o , and the inlet and outlet temperatures of hot fluid are 150 and 90o , respectively. The hi (fil

19、m coefficient inside pipe) is 700 w/(m2 oC)and overall heat transfer coefficient Uo (based on the outside surface of pipe) is 300w/(m2 oC), respectively. If the heat loss is ignored and the conductivity of pipe wall (steel) is taken as 45 w/(moC), find:(1) heat transfer film coefficient outside the

20、pipe ho?(2) the pipe length required for counter flow, in m?(3) what is the pipe length required if the heating medium changes to saturated vapor(140 oC) and it condenses to saturated liquid and other conditions keep unchanged? (4) When the exchanger is used for a year, it is found that it cannot me

21、et the need of production (the outlet temperature of cold fluid cannot reach 80 oC), explain why? (a) 1/h0=1/U0-(do/hidi+bdo/kdm)=1/300-23/700*19-0.002*23/45*21 Q=UoAoDTm=mcCp(Tcb-Tca) 300*2p*0.023*70L=500/3600*1000*(80-20) L=5.4m (c) (d) scale is formed on the outside ,V0 is decreased16. Water flow

22、s turbulently in the pipe of 25x2.5 mm shell tube exchanger. When the velocity of water u is 1 m/s, overall heat transfer coefficient Uo (based on the outer surface area of pipe) is 2115 w/(m2 oC). If the u becomes 1.5 m/s and other conditions keep unchanged, Uo is 2660 w/( m2 oC ). What is the film

23、 coefficient ho outside the pipe? (Heat resistances of pipe wall and scale are ignored) (1) (2) (1)-(2)= C=2859 ho=8127W/(m2K)17. Water and oil pass parallelly through an exchanger which is 1 m long. The inlet and outlet temperatures of water are 15 and 40 oC, and those of oil are 150 and 100 oC, re

24、spectively. If the outlet temperature of oil decreases to 80 oC, and the flow rates and physical properties and inlet temperatures of water and oil maintain the same, what is the pipe length of new exchanger? (Heat loss and pipe wall resistance are neglected) 18. Air which passes through the pipe in

25、 turbulent flow is heated from 20 to 80 oC. The saturated vapor at 116.3 oC condenses to saturated water outside the pipe. If air flow rate increases to 120% of the origin and inlet and outlet temperatures of air stay constant, what kind of method can you employ in order to do that? (Heat resistance

26、 of pipe wall and scale can be ignored) Th=118.5oC 19. Water flows through the pipe of a 25x2.5 mm shell-tube exchanger from 20 to 50 oC. The hot fluid (Cp 1.9 kJ/kgo C, flow rate 1.25 kg/s) goes along the shell and the temperatures change from 80 to 30 o C. Film coefficients of water and hot fluid

27、are 0.85kw/(m2 oC) and 1.7 kw/(m2 o C). What is the overall heat transfer coefficient Uo and heat transfer area if the scale resistance can be ignored? (the conductivity of steel is 45w/(mo C).W=1.25Kg/s Cp=1.9Kj/kg 20. A spherical particle (density 2650 kg/m3) settles freely in air at 20 o C (densi

28、ty of air 1.205 kg/m3 , viscosity 1x10-5 Pa.s). Calculate the maximum diameter of particle if the settle obeys the Stokes Law?Re1 =3.85x10-521. A filter press(A=0.1 m2 ) is used for filtering slurry. The vacuum inside the filter is 500 mm Hg. One liter filtrate can be got after filtering of 5 min and 0.6 more liter filtrate is obtained after 5 more min. How much filtrate will be got after filtering of 5 more min?for filter press 5 min (1) 10min (2) From (1) (2),we can see Ve=0.7 K=4815 min V=2.07m3/h2