Solution-to-Midterm-Examination-for-Fluid-Mechanics-and-Heat-Transfer

1,Midterm Examination for Fluid Mechanics and Heat Transfer,1. (10 points) True or false(1) ( ) When a fluid passes through a pipe, the total mechanical energy in section 1 (inlet) is always equal to that in section 2 (outlet).,Key: False.,2,1. (10 points) True or false(2) ( ) When the developed head H is the same, one pump can not develop more volume flow rate than that two same pumps which work in parallel.,Key: True.,3,1. (10 points) True or false(3) In a laminar flow, the frictional loss is proportional to velocity square.,Key: False.,4,1. (10 points) True or false(4) A reciprocating pump is not subject to air binding.,Key: True.,5,1. (10 points) True or false(5) There are three types of flow, that is, laminar, transitional and turbulent flow.,Key: False.,6,2. (30 points) Fill in the blanks or choose the correct answer(1) The orifice meter belongs to , flow rate meter; while the rotameter is , flow rate meter.A) constant pressure drop; B) constant area; C) variable pressure drop; D) variable area,Key: B, C; A, D.,7,2. (30 points) Fill in the blanks or choose the correct answer(2) ( ) In order to avoid the cavitation, which of the following methods is feasible? A) increase the operating temperature; B) decrease the diameter of suction pipe; C) install the valve or fittings in the suction pipeline; D) reduce the frictional loss in the suction pipeline,Key: D.,8,2. (30 points) Fill in the blanks or choose the correct answer(3) The orifice meter is used to measure ; the rotameter is utilized to measure ; the Pitot tube is used to measure .A) average velocity in the orifice B) volume flow rate in the orificeC) average velocity in annular space between the float and tube wallD) volume flow rate in annular space between the float and tube wallE) local velocity in the pipeF) average velocity in the pipe,Key: A or B; D; E.,9,2. (30 points) Fill in the blanks or choose the correct answer(4) The methods of adjusting volume flow rate of centrifugal pump are1) ; 2) ; (3) .The most common used way in practice is ( ).,Key: (1) changing pump revolution ; (2) changing valve opening ; (3) changing diameter of impeller ;The most common used way in practice is (method 2).,10,2. (30 points) Fill in the blanks or choose the correct answer(5) To avoid cavitation, the pressure at the centrifugal pump inlet must exceed the vapor pressure by a certain value called the .,Key: net positive suction head (NPSH).,11,2. (30 points) Fill in the blanks or choose the correct answer(6) 1atm= mH2O= N/m2； 1cp= P= Ns/m2.,Key: 1atm= 10.33 mH2O= 1.0133 105 N/m2； 1cp= 0.01 P= 0.001 Ns/m2.,12,2. (30 points) Fill in the blanks or choose the correct answer(7) When the temperature of fluid to be transported increases, the maximum suction lift will ( ). If the velocity of suction pipe decreases, the maximum suction lift will ( ).,Key: decrease; increase.,13,2. (30 points) Fill in the blanks or choose the correct answer(8) The viscosity of a gas with temperature increasing, and viscosity of a liquid usually significantly when the temperature is raised.,Key: increases; decreases.,14,2. (30 points) Fill in the blanks or choose the correct answer(9) In the boundary layer, there exist . Outside the boundary layer, potential flow survives.,Key: velocity gradients.,15,2. (30 points) Fill in the blanks or choose the correct answer(10) When the pipe installation changes from horizontal to vertical position and the velocity keeps unchanged, the frictional loss will be .A. increased; B. decreased; C. the same; D. uncertain,Key: C.,16,3. (30 points) A liquid with density 900 kg/m3 and viscosity =30 cp flows from tank 1 to tank 2 through the pipeline of inner diameter 40 mm. The levels of liquid surfaces in two tanks keep constant. When the valve is closed, the readings of gauges A and B are 8.82104 N/m2 and 4.41104 N/m2, respectively. When the valve is open, the total length of pipeline is 100 m including all local frictional equivalent length, find:the volumetric flow rate in the pipe, in m3/h?when the valve is open, what happens to the readings of gauges A and B, explain simply why?Hint: for laminar flow, f=16/Re or =4f=64/Re For turbulent flow, =0.3145/Re0.25,17,3. Solution.,(1) When the valve is closed,18,Between section a and section b, Bernoulli equation is,19,Between section a and section b, Bernoulli equation is,Assume the flow is laminar,(*),20,Substitute into Eq.(*),Eq.(*) gives,(*),So, it is laminar flow.,21,(2) when the valve is open, the reading of gauges A will be decreased, and B will be increased, because of frictional loss and pApB.,22,4. (30 points) The characteristics of a centrifugal pump are measured under certain rotational speed. The inside diameters of the suction and discharge pipelines are 70mm and 50mm respectively. When the volumetric flow rate of water is 30m3/h, the readings of the vacuum gauge at the suction station and the pressure gauge at the discharge station of the pump are 40kPa and 215kpa respectively. The vertical distance between the pressure tap (pressure measurement point) at the suction station and the pressure tap at the discharge station is 0.4m. The shaft power is 3.45kW. Calculate the developed head and efficiency of the pump.,23,4. Solution.,Between Station 1 and Statio