課程名稱︰化工熱力學
課程性質︰必修
課程教師︰吳嘉文 教授
開課學院：工學院
開課系所︰化工系
考試日期（年月日）︰2012/10/23
考試時限（分鐘）：120
是否需發放獎勵金：是
（如未明確表示，則不予發放）
試題 :
1.(10%)
An insulated, electrically heated tank for hot water contains 190 kg of
liquid water at 333K when a power outage occurs. If water is withdrawn from
the tank at a steady rate of mass flow = 0.2kg/sec,how long will it take for
the temperature of the water in the tank to drop from 333K to 308K ? Assume
cold water enters the tank at 283K, and negligible heat losses from the tank.
For liquid water let Cv = Cp = C,independant of T and P.
2.(10%)
An open cylindrical tank with a base area of 2m^2 and a height of 10m
contains 4m^3 of water. As a result of corrosion, the tank develops a leak at
its bottom. The rate at which leaves the tank through the leak is
Leak rate(m^3 / s ) = 0.5 (delta P)^0.5
Where (delta P) is the pressure difference in bar between the fluid at the
base of the tank and the atmosphere.Determine the amount of water in the tank
at any time.
3.(10%)
A 40kg steel casting (Cp = 0.5KJ/(kg.K))temperature of 723K is quenched in
150 kg of oil (Cp =2.5KJ/(kg.K))at 298K.If there are no heat losses , what is
the change in entropy of (a)the casting ,(b)the oil,and(c)both considered
together?
4.(10%)
In a steady-state flow process,1mol/s of air at 600K and 1 atm is continuously
mixed with 2mol/s of air at 450K and 1atm.The product stream at 400K and 1 atm
Determine the rate of heat transfer and rate of entropy generation for process.
Assume that air is an ideal gas with Cp=(7/2)R,that the surroundings are 300K,
and the kinetic-and potential-energy changes are negligible.
5.(10%)
The following reaction occurs in air:
2NO + O2 --> 2NO2
At 293K the rate of this reaction is
d(C(NO))/dt = -1.4x10^-4 C^2(NO)C(O2)
For t in seconds and concentrations in kmol/m^3.The reaction occurs in a
constant-volume,1m^3 vessel,and the initial concentration of NO is 1kmol/m^3
and that of O2 is 3kmol/m^3.(a)If 0.5mol of NO reacts,how much NO2 is produced
(b)Determine how long it would take for 0.5 mol of NO have reated.
6.(10%)
A compressor is operating in a continuous,steady-state manner to produce a
gas at temperature T2 and pressure P2 from one at T1 and P1. Show that for the
time interval (delta t)
Q + Ws = (H2-H1)(delta M)
Where (delta M)is the mass of gas that has flowed into or out of system in
the time (delta t).
7.(10%)
Prove that it is impossible to construct a device that operates in a cycle and
whose sole effect is to transfer heat from a colder body to a hotter body.
8.(10%)
(a)Show that the rate at which shaft work is obtained or required for a
reversible change of state in a closed system at constant internal energy and
volume is equal to the negative of the product of the temperature and the rate
of change of the entropy for the system.
(b)Show that the rate at which shaft work is obtained or required for a
reversible change of state in a closed system at constant entropy and pressure
is equal to the rate of change of the enthalpy for the system.
9.(10%)
(a)A photovoltaic cell is used to convert solar energy to electric energy.
Estimate the maximum efficiency of a photovoltaic cell that is operating in
steady state.Assume that the temperatures of the sun and the photovoltaic cell
are 6500K and 300K,and the cell is losing heat by conduction to the
environment.
(b)
The current efficiency of the photovoltaic cells nowadays is usually around 10%
Do tou think what are the reasons for such low efficiency?How will you do to
improve the efficiency?
10.(10%)
Describe the following terms.
(a)Equilibrium
(b)Steady-state
(c)Reversible process
(d)Polytropic process
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