Thermodynamics (20 points)

ngai_manchung

2008-03-11 11:38 am

Q1. A room has the dimensions 20.0 m long, 5.0 m, wide and 3.40 m high. Assuming the air in the room is 14°C, 104104 Pa and has a gas constant of 0.2870 kJ/kg K, Calculate

i) the volume of air in the room (m3)
ii) the mass of air in the room (kg).

Q2. Assume now that the temperature in Q1 has changed to 20°C - the gas constant remains constant, calculate

i) the mass of air in the room (kg)
ii) the pressure of air in the room (Pa)

Q3. What is the specific enthalpy change (kJ/kg) when oxygen, which may be considered a perfect gas (molecular weight 32), is heated from 70°C to 290°C ?

ANSWER and explaination in English(optional) please! =]

更新1:

0.2870 k = 287 and its not 2870, right?

回答 (1)

志仁

2008-03-11 11:17 pm

✔ 最佳答案

1)
V=20 x 5 x 3.4=340m^3
PV=mkT, m=PV/kT=[(104104)(340)]/[(2870)(273+14)]=42.972kg

2)
By the conservation of mass, the mass is kept unchange, 42.972kg
PV=mkT, P=mkT/V=(42.972)(2870)(273+20)/340=106280.4Pa

3)
delta T=290-70=220K
specific enthalpy change =k delta T=0.2870 (220)=631.4kJ/kg

👍 0 👎 0