Solns to 2005级流体力学与传热考试题(双语班)(A卷)答案

(2) There are many industrial filters, give three of them:1) Cake filters (pressure filters, vacuum filters) ; 2) Clarifying filters ;3) crossflow filters (ultrafilter) .(3) The orifice meter belongs to B , C flow meter; while the rotameter is A , D flow meter.A) constant pressure drop; B) constant area; C) variable pressure drop; D) variable area(4) When the filter cake is incompressible, a cake resistance is not influenced by ( D ).A) the cake layer thicknessB) the specific cake resistanceC) the porosity (void fraction)D) the filtration rate(5) 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? ( C )A) increase vapor velocity; B) employ superheated vapor;C) increase air velocity; D) set up the baffles in the shell.(6) The removal of non-condensable gas in the condensing vapor will ( A )A) increase condensing film coefficient.B) decrease condensing film coefficient.C) not change condensing film coefficient.D) be uncertain.(7) Based on the temperature distribution as shown in theattached figure, which layer has the largest heat resistance?A , and which has the largest thermal conductivity?B , if the thickness of each layer is the same.A) layer A; B) layer B; C) layer C(8) When fouling (scale) and pipe wall resistances are ignored and the difference between h i and h o is very huge, overall heat transfer coefficient U is close to ( B ); and wall temperature is close to ( A ).A) larger h ; B) smaller h ; C) average value between h i and h o ; D) uncertain (9) For laminar flow, the average velocity in a pipe is 0.5 times the maximum velocity, for turbulent flow average velocity is 0.82~0.87 times the maximum velocity. (10) For laminar flow in a pipe, if the flow rate is constant, when diameter D i increases, the friction coefficient will increase , and friction loss will decrease .3. (15 points) It is proposed to pump 10000 kg/h of toluene at 114℃and 1.1 atm abs pressure from the reboiler of a distillation tower to a second distillation unit without cooling the toluene before it enters the pump. If the friction loss in the line between the reboiler and pump is 7 kN/m 2 and the density of toluene is 866 kg/m 3, how far above the pump must the liquid level in the reboiler be maintained to give a net positive suction head of 2.5 m? Solution.Use the equation for the suction lift H g .m NPSH g h g p p H fs v a g 32.35.280665.9803.8-=--=---'=ρ4. (20 points) A centrifugal pump takes brine from the bottom of a supply tank and delivers it into the bottom of another tank. The brine level in the discharge tank is 45 m above that in the supply tank, and both brine levels are open to the atmosphere. The line between the tanks is 180 m long of 100 mm inner diameter pipe. The flow rate is 90 m 3/h. In the line are 2 gate valves, 4 standard tees, and 4 elbows. What is the power of the motor required for running this pump? The density of brine is 1180 kg/m 3, and the viscosity of brine is 1.2 cP. The overall efficiency of pump and motor is 60 percent.Hint: Loss coefficient for gate valve is 0.17, for elbow is 0.75, for tee is 1.0 For laminar flow, f=16/Re or λ=4f=64/Re For turbulent flow, λ=0.3145/Re 0.25Solution.Bernoulli equation between station a and b becomesb fa b b b p a a ah Vgz p W V gz p -+++=+++2222ρηρ According to the given conditions,2)4()81.945(6.01)(1%60,,0452V K K K D L f h h h g z W p p V V mz z z f e c bfa b fa b fa p b a b a a b ⨯∑+++⨯=+⨯=+⋅∆=∴=====-=∆---ηη Here, from the given conditions,53221013.3102.11180183.31.0Re /183.336001.04904,1.0,180⨯=⨯⨯⨯==∴=⨯⨯⨯====-μρππV D sm D q V m D m LSo it is turbulent flow, and0133.0)1013.3(3145.0425.05=⨯==λf34.70.1475.0417.020.1,4.0=⨯+⨯+⨯=∑==f e c K K KkgJ V K K K D L f h f e c bfa /5.1652183.3)34.70.14.01.01800133.0(2)4(22=⨯+++⨯=⨯∑+++⨯=- kg J h h g z W b fa b fa p /1012)5.16581.945(6.01)81.945(6.01)(1=+⨯=+⨯=+⋅∆=∴--ηPower of motor W W mP p B 3109.2910123600118090⨯=⨯⨯==5. (20 points) In the shell-and-tube heat exchanger with the parallel-current flow, water is used to cool the oil. The temperature of entering water and leaving water are 15 o C and 40 o C, respectively. While the temperature of entering and leaving oil are 150 o C and 100 o C, respectively. Because of production condition changes, the leaving temperature of the oil is to be decreased to 80 o C. Suppose that the mass flow rate, the entering temperatures and the physical properties of fluids are not changed, if the tube length of the original heat exchanger is 1m, how long has the tube to be increased to meet the production requirement? The heat loss of the heat exchanger can be ignored. Solution.The logarithmic mean temperature difference is(15015)(10040)92.513560m t Cn---∆==︒ From the heat balance2112()40150.5()150100h Ph C Pc W C t t W C T T --===-- When the temperature of the oil drops to 80℃, from the heat balance2(15080)(15)h Ph c pc Q W C W C t '=-=- or2150.515080h PhC Pct W C W C '-==-From which 250t C '=︒And (15015)(8050)7013530m t C n ---∆==︒From the heat transfer rate equation,The former heat exchanger:(150100)(92.5)h ph m W C KS t Kn dL π-=∆= The new heat exchanger:(15080)(70)h ph m W C KS t Kn dL π'''-=∆=7092.5, 1.855070hence L m '=⨯=6. (15 points) How long will it take for the spherical particle of coal with diameter of 6 mm and density of 1300 kg/m 3 to settle, at its terminal velocity under free-settling conditions, through 3 m of water at 20℃ (Water viscosity and density are 1.05 cP and 998 kg/m 3 respectively)?Hint:Ifμρρ18)(,0.1Re 2-=<p p t p gD uIf ρρρ6.0Re )(269.0,1000Re 0.1pp p t p gD u -=<<If ρρρ)(75.1,200000Re 1000-=<<p p t p gD uSolution.Water properties at 20o C: 33/998,1005.105.1m kg s pa cp =⋅⨯==-ρμ. g=9.80665m/s 23/1300m kg p =ρ, m D p 3106-⨯=Find settling regime from criterion K . 9.681.83)(3/12>=⎥⎦⎤⎢⎣⎡-=μρρρp p g D KThis settling regime belongs to ,200000Re 1000<<p and Newton ’s law applies,s m gD u p p t /233.0)(75.1=-=ρρρThe settling time is s u L t t 88.12233.03===。