MAE204 Thermodynamics 1

Problem 1 (10 pts)

A 2-m2 electric heater is used to heat the air above it. The surface of the heater is at 300?C while the surrounding air is at 40?C. If the heater draws 240 V and 22 A, what is the effificiency of the device? Assume that the convective heat transfer coeffificient is h = 10 W/(m2K).

Problem 3 (10 pts)

A thermodynamic system undergoes a cycle consisting of the following three processes:

1 → 2: Isochoric heating,

2 → 3: Isothermal expansion,

3 → 1: Isobaric cooling.

The following information is also known:

u1 = 2622.3 kJ/kg,

u3 = 3345.9 kJ/kg,

q12 = 718 kJ/kg into the system,

w23 = 288.05 kJ/kg out of the system,

ν1= 0.20602 m3 /kg,

ν3 = 0.41376 m3 /kg,

P1= 1 MPa.

Assume that only boundary work is possible and that the system is stationary.