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 thickness

B) the specific cake resistance

C) 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 the

attached 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-=--=---'=ρ