I. Fill in the blanks with the appropriate words: (1 points each, total 40 points) 1. The vacuum pressure of an airtight or clod container is 50 kPa. If the local atmospheric pressure is 100 kPa, the absolute pressure of the clod container is 50 kPa, and the gauge pressure of the clod container is -5.1 mH2O（the density of water is 1000 kg/m3）. 2. A kind of fluid flows through a circular straight pipe with outside diameter of 25mm and wall thickness of 2.5mm (i.e.Φ25 mm×2.5mm). The kinematic viscosity (not the viscosity or the absolute viscosity, ) of this fluid is 1×10-6 m2/s，and the average velocity of fluids is 1.0 m/s. Therefore, the Reynolds number, Re is 2×104 , and the flow region or the type of flow is turbulent flow . 3. A type of fluid flows through a circular straight pipe with laminar flow. If the flow velocity at the pipe center is 0.1 m/s, then the average flow velocity is 0.05 m/s. If the inner diameter of the pipe doubles with flow rate and other conditions unchanged, the new flow friction factor λ is 2 λ0 (λ0 is the flow Blasius or Darcy friction factor before the inner diameter being changed, and λ0=4f0 where f0 is Fanning friction factor). 4. In a two-dimensional flow field, the velocity component in x coordinate direction is, and the velocity component in y coordinate direction is. If the fluid is incompressible, a= 1 . 5. The fluid flows through two pipes in parallel with laminar, and the values of the friction factors of the two pipes are the same (). The ratio of pipe inner diameter is, and the ratio of pipe length (including all the equivalent lengths of local friction loss) is. Thus, the ratio of friction loss of the two pipes 1 , and the ratio of the volume flow rate of the two pipes 1/32. 6. In the orifice meter and venturi meter, the variation of flow rate through a constant area generates a variable pressure drop, which is related to the flow rate. Another class of meters, called area meters, consists of devices in which the pressure drop is constant, or nearly so, and the area through which the fluid flows varies with the flow rate. The pitot tube is a device ud to measure the local or point velocity along a streamline. 7. In order to avoid destroying the motor caud by the high starting electric current, the outlet valve of the centrifugal pump should be shut off when starting the pump. The outlet valve of the centrifugal pump should be shut down firstly when stop the pump to prevent destroying the impellers caud by the back flow of fluid in the outlet pipeline. 8. The existence of the air binding or air tied phenomenon (an amount of gas surrounds the impellers of the centrifugal pump) indicates that the centrifugal pump is not a lf-priming | device, and the pump 休斯顿火箭英文should be primed with liquid before it is started. 9. If the suction pressure of the pump is only slightly greater than the vapor pressure of liquid, some liquid may flash to vapor inside the pump, a process called cavitation, which will reduce/limits the suction lift and caus vere erosion. To avoid cavitation, the pressure at the pump inlet must exceed the vapor pressure and exceed it by a certain value called the net positive suction head (NPSH). The interction of the pump curve and the system resistance curve defines the operating point of both pump and process. 10. According to the affinity or proportionality laws of the centrifugal pump under certain conditions, the head H changes in direct proportion to the square of impeller rotational speed n ratio and the power consumed N changes in direct proportion to the cubic of impeller rotational speed n ratio. 11. Gas-moving machinery compris mechanical devices ud for compressing and moving gas. They are often classified from the standpoint of the pressure heads produced. Fans are for low pressures. Blowers are for intermediate pressures and compressors are for high pressures. 12. For a cubic particle with side length of 5 mm, its equivalent diameter of sphere that has the same volume of the cubic particle is 6.2 mm. 13. In the gravitational tting process, the gas capacity of the tting chamber depends on the length and width of the tting chamber and terminal velocity of the particle, and has nothing to do with the height of the tting chamber. 14. If the gas capacity of a standard cyclone parator increas with other conditions unchanged, the critical diameter of particles decreas, and the pressure drop increas. 15. In the plate-and-frame filter press, the washing water flows through the whole cake. The washing length is twice as the last filtrate, and the washing area is half of the filtration’s. This washing method is called the through washing. In a leaf filter and a rotary drum vacuum filter, wash water flows through the almost same path as the filtrate’s. This washing technology is called the displacement washing. 16. In the turbulent convection, the thermal resistance of heat transfer largely exists in the laminar sublayer, and the main route to enhance turbulent convection is to reduce the thickness of the laminar sublayerszy. 17. If the emissivity of a gray-body at temperature of 27 ℃ is 0.8, its absorptivity is 0.8 . If its temperature is incread to 327℃, the new total radiating power W2 equals to 16 W1. 18. Generally there are three methods to enhance the heat transfer process: enlarging the heat transfer area, increasing the overall heat transfer coefficient and raising the logarithmic mean temperature difference (LMTD). 19. In the evaporation process, the loss of the heat-transfer temperature drop is mainly caud by: solution boiling point elevation, the depth of liquid over the heating surface or liquid head and the frictional loss in the pipes. 20. According to the methods of feeding, plea write two kinds of flow charts of multiple-effect evaporators: forward/backward/parallel-/mixed-feed multiple-effect evaporators or flow charts. | |||||||||||||||||||||
. (20 points) Water is pumped from a storage tank A to a storage tank B by a centrifugal pump. The levels of the surfaces of the liquid in both tanks are kept constant. The elevation of the level of the surface of the liquid in the tank B is 10m higher than that in the tank A. The vacuum pressure of the space above the level of the surface of the liquid in the tank A is 50 kPa, and the gauge pressure of the space above the level of the surface of the liquid in the tank B is 50 kPa. The dimension of the suction pipe of the pump is Φ108 mm×4 mm, and the length of the suction pipe is 50 m; the dimension of the discharge pipe of the pump is Φ89 mm×4.5 mm, and the length of the discharge pipe is 100m. The length values of the pipes mentioned above include all the equivalent lengths of local friction loss except for local friction loss of inlet and outlet of pipelines. The friction factors of all the pipes are 0.02. The reading R of the orifice meter in the discharge pipe is 100 mm, and the indicator liquid is the mercury. The pump efficiency is 75%. The density of the water at the operation temperature is 1000 kg/m3. The density of the mercury is 13.6×103 kg/m3. The orifice meter coefficient英语培训学校网站 Co=0.6. The orifice diameter is 45 mm. The local atmospheric pressure is 100 kPa. The acceleration of gravity g=10 m/s2. The velocity through orifice. (1) What is the power supplied to the pump N (kW); (15 points); (2) When the opening of the outlet valve of the pump is reduced in certain degree, plea point out the variation trend of the vacuum pressure in the inlet of the pump and that of the gauge pressure in the outlet of the pump by concrete analys. (5 points) | |
. (20 points) Filtration of a certain suspension solution of water is carried out at a constant pressure difference of 10 kPa in a BMP26/810-25 type plate-and-frame filter press, with 20 frames 810mm square and 25mm thick. The ratio of the incompressible cake volume to the filtrate volume per cycle is 0.04. The filtration constant K=410-5 m2/s. The filtration medium constant qm is 0.01 m3/m2. The ratio of the volume of washing water to the volume of filtrate collected per cycle is 0.08. The auxiliary operation time including dismantling and refitting of the press time is 8 min. (1) What is the capacity of the filter press (m3 filtrate /h)? (15 points); (2) It is decided to u a rotary drum filter in place of the filter press, with drum filtration area of 5m2. The filtration in the rotary filter is carried out at a constant pressure difference. The filter cake is incompressible and the filtration constant K of 3×10-5 m2/s. The fraction of the drum submerged is 1/3. The filtration medium resistance is negligible. Plea calculate the speed of rotation of the drum which will result in the same capacity of the filter press (r/s). (5 points) | |
. (20 points) In a 1-1 pass shell-and-tube exchanger, the low viscosity water solution pass fully turbulently through the tubes and is heated from 20 ℃ to 100 ℃ by the saturated vapor at 160 oC which goes through the shell and condens to saturated liquid. The shell-and-tube exchanger consists of 100 tubes with dimension of 252.5 mm and tube length of 3 m. The mass flow rate of the water solution is 2×104 kg/h, and specific heat of the water solution is 3.0 kJ/(kg·℃). The convection heat-transfer coefficient of vapor condensation is 1.1×105 W/（m2·℃）. The heat loss of the exchanger, tube wall heat resistance and fouling resistance are all negligible. (1) What is the convection heat-transfer coefficient of water solution side in tube? (12 points) (2) What is the new outlet temperature of the water solution if the volume flow rate of the water solution doubles with heat-transfer coefficient of vapor condensation and other conditions maintain unchanged? (8 points) | |
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