07流体力学与传热期末考试题（含答案）.
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华南理工大学期末考试
2007《流体力学与传热英文》试卷
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一．(38 points) True or false or fill the black or choose the correct answer
(1)In SI system, the dimension of viscosity is ( M/Lq ), and its unit is ( kg/ms ).
(2) ( T ) If the volume flow rate Q increases, the developed head H of centrifugal pumps will decrease.
(3) ( F )When the gas temperature increases, its viscosity will decrease.
(4) ( F ) Water need not to be added during the start of centrifugal pump because it has priming function.
(5) If water with density r=1000kg/m3, viscosity μ=0.001Pa.s and u=1m/s flows through a pipe with di=0.001m, its Re=(1000), f (Fanning
(maximum velocity)=( 2 m/s); If the pipe factor)=( 0.016), u
max
length is 2m, its pressure drop Dp=(128000 Pa).
(6) A single spherical solid particle settles freely in static liquid, in which the settling obeys Stoke's Law (laminar flow). If the diameter of particle increases, the settling velocity u
will ( be up ); when the
t
will ( be down ).
temperature decreases, u
t
(7) The revolution n of centrifugal pump increase, its volumetric flow rate Q will increases , its developed head H will increase .
(8) Orifice meter measures ( C ) to get mass flow rate of fluid in
a pipe, rotameter measures ( A ) to get mass flow rate of fluid in a pipe. The former ( E ) keeps constant, and the later ( D ) keeps unchanged.
A Volume flow rate;
B Local velocity;
C Average velocity;
D Pressure drop;
E cross section area
(9) The pressure of 10 m H
2
O is equal to 735.7 mmHg. If
atmospheric pressure is 1 atm and the reading of centrifugal pump outlet gauge is 5000Pa(N/m2)= 106300 N/m2 (absolute pressure).
(10) When the flow rate is fixed, the maximum suction lift Z
a will increase if the diameter of suction pipe increases. When
the flow rate is fixed, Z
a
will decreases if a valve and some fittings are set up in the suction pipe.
(11) The expression of Newton's Law of cooling is ( hADt ).
(12)
If the temperature drop DT o between the saturated vapor and pipe wall is double in a vertical tube, the condensation heat transfer film coefficient h will increase to ( A ) of original value.
A) 2-1/4 ; B) 21/4 ; C) 23/4 ; D) 2
(13) Based on the temperature distribution as shown in figure, which has largest temperature drop A , and which has the largest thermal conductivity C if the thickness of each layer is the same?
A) layer A; B) layer B; C) layer C
(14) The removal of uncondensable gas in the condensing vapor will ( B )
A increase condensing film coefficient h;
B decrease condensing film coefficient h;
C do not change condensing film coefficient h;
D be uncertain.
(15) A flat wall thickness b=0.1m and heat transfer area A=2m2 consists of some material with conductivity k=0.1 w/m×K, heat transfer rate q will be 200 (w) when the temperature difference between both sides of wall is 100 K. If heat resistance is double and temperature drop between both sides of wall keeps constant, heat transfer rate q will be 100 (w).
(16) If h
i is much smaller than h
o
and wall heat resistance is ignored,
overall heat transfer coefficient U will be approximately equal to ( A ); in order to enhance heat transfer, the best way is to increase ( ).
A h
i ; B h
o
; C average of h
i
and h
o
; D uncertain
(17) If the side of a square is 0.5m, its equivalent diameter is (0.5 m).
(18) Saturated vapor is cooled by air in a shell-tube exchanger. When air flow rate increases, the outlet temperature of air will ( B )
A increase;
B decrease;
C be the same;
D be uncertain
(19) Water flows fully turbulently through a pipe and the velocity changes from 1.5m/s to 3m/s. If inlet and outlet temperatures of water keep the same, heat transfer coefficient of water hi is ( A )
A．1.74 of origin; B．2 of origin; C．4 of origin; D. 1/2 of origin
2 (12 points) A slurry is filtered by a 1 m2 filter press at constant pressure. The cake is incompressible. The filter basic equation is as follows:
(V + 1)2 = 30 (t + 2)
where V is filtrate volume, liter; t is filtration time, min
Find 1) how much filtrate is obtained after 28 min, in liter?
2) if the pressure drop decreases to 50% of original value, how much filtrate can be got after 28 min, in liter?
3) The filtration rate in the beginning and at 28 min in case 1, in liter/min
Solution: 1) V=29 L; 2) V=20.2 L
3) dV/dt=KA2/2(V+Ve)=30/(2*1)=15 l/min; and
dV/dt=KA2/2(V+Ve)=30/2(29+1)=0.5 l/min
3 (25points) Water (r=1000kg/m3) flows from the top tank to the low one with a velocity 2m/s, as shown in Fig. There are one orifice and a gate valve in the pipeline with f57′3mm. The total pipe length including all local frictional equivalent length is 60 m. If the frictional coefficient l=4f is 0.036 and K
v
(local loss coefficient of valve) is 7.5 (wide open), Find
(1) mass flow rate in pipeline, in kg/s.
(2) liquid surface height difference DZ between top and low tanks.
(3)pressure drop through the valve and reading R
2
of U-shape pressure gauge.
(4) if the valve is closed a little bit, there is 20% decreases of reading
R 1for U-shape pressure meter. Assume orifice coefficient C
o
keeps
unchanged, what is the mass flow rate in the pipeline, in kg/s?
Hint: velocity equation in orifice u
o =C
o
[(2R
1
g(r
o
-r)/r)1/2 , and r
o
is
13600kg/m3.
Solution: (1) m=2*1000*0.785*(0.051)2=4kg/s DZ=4fL*u2/2dg=0.036*60*22/2*0.051*9.81=8.63m
(2)
(3) Dp=R
2(r
o
-r)g and Dp/r=Ku2/2=7.5*22/2=15
Dp=15000= R
2(13600-1000)*9.81 R
2
=0.12m
(4) m'/m= C
o [(2R'g(r
o
-r)/r)1/2 / C
o
[(2Rg(r
o
-r)/r)1/2 =(R'/R)1/2=0.81/2=0.4
m'=0.4*m=0.4*4=3.58kg/s
4. (25points) Heated oil (heat capacity Cp=3.35kJ/kg·℃) is cooled by water (heat capacity Cp=4.187kJ/kg·℃) and they flow countercurrently through a double pipe exchanger with inner pipe dimension φ180×10mm. Water at 15℃ of inlet temperature goes through the pipe and leaves at 55℃. Oil passes through the annulus with mass flow rate 500kg/h and inlet and outlet temperatures of oil are 40℃ and 90℃, respectively. The heat
transfer coefficients h
i and h
o
for water and oil are 1000W/m2·℃ and
299W/m2·℃，and heat resistances of pipe wall and fouling as well as heat loss can be ignored. Find
(1) water flow rate, in kg/h.
(2) overall heat transfer coefficient U
o
based on outer surface of pipe.
(3) LMTD Dt
m
and pipe length of pipe, in m.
(4) if inlet temperature of water becomes 20℃, what happens to that?
(5) in order to enhance heat transfer, what ways can be employed? Solution: (1) w=500*3.35*50/(4.187*40)=500kg/h
(2) Uo=1/(180/1000*160+1/299)=223.7w/m2K
(3) LMTD=75-15/ln75/15=37.3oC
L=q/U
o pd
o
LMTD=500*3.35*50/(3600*p*0.18*37.3)=4.9m
4.oil outlet temperature decreases
5.increase oil velocity and change 1-1 pass to 1-2 pass, decrease oil
heat resistance.