Air, with a mass of 1.5 lbm, is initially at 2000°F. In a cooling process, the internal energy of the air is reduced by 350 Btu. Determine the final temperature of the air if (a) the specific heat is considered constant, with its value taken at 2000°F, (b) the specific heat is considered constant, with its value taken at 100°F, and (c) considering the specific heats to be variable.
Given: m = 1.5 lbm; T1 = 2000oF; ?U = -350 Btu
What will be an ideal response?
For constant specific heats:
(a) At 2000°F, take cv = 0.217 Btu/lbm-R.
?U = m cv (T2 – T1) , so T2=?u/mcv + T1 = 924.7 °F
(b) At 100°F, cv = 0.172 Btu/lbm-R, so T2 = 643.4°F
(c) With variable specific heats, the values of the initial specific internal energy can be found from the air tables or on-line: u1 = 464.71 Btu/lbm
So, u2 = U/m + u1 = 231.38 Btu/lbm. Then, using air tables or on-line properties, this can be found to correspond to T2 = 1319 R = 859°F
With such a large temperature change, it is difficult to know which temperature should be used for the value of the specific heats if one wishes to approximate the calculation with constant specific heats.
You might also like to view...
Which of the following best describes the Ethical Leader?
A) Honest B) Truthful C) Leads by example D) Responsible E) All of the above
A large block of text in a sans serif font is difficult to read because the characters in sans serif fonts do not have:
a. hats b. hands c. feet d. shoes
SIMMs have ________ pins
Fill in the blank(s) with correct word
All variable displacement compressors ___________________
a. adjust piston stroke by an internal pressure valve or solenoid b. have no clutch c. change vapor refrigerant into liquid d. control condenser pressure by varying their input speed