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Unformatted text preview: ENGRD 221: Engineering Thermodynamics
Problem Set 1 Due: Jan 30 2009, Upson Room 123 (6 Questions)
Question 1 (1.41): As shown in Fig. 1, air is contained in a vertical piston‐cylinder assembly fitted with an electrical resistor. The atmosphere exerts a pressure of 14.7 lbf/in2 on the top of the piston, which has a mass of 100lb and face area of 1 ft2. As electric current passes through the resistor, the volume of the air increases while the piston moves smoothly in the cylinder. The local acceleration of gravity is g=32.0 ft/s2. Determine the pressure of the air in the piston‐cylinder assembly, in lbf/in2 and psig. Figure 1 Question 2 (1.20): As shown in Fig. 2, a cylinder of compacted scrap metal measuring 2m in length and 0.5m in diameter is suspended from a spring scale ad a location where the acceleration of gravity is 9.78 m/s2. If the scrap metal density , in kg/m3, varies with position z, in m, according to p=7800‐360(z/L)2, determine the reading of the scale, in N. Figure 2 Question 3 (2.20): The drag force, Fd, imposed by the surrounding air on a vehicle moving with a velocity V is given by: Where Cd is a constant called the drag coefficient, A is the projected frontal area of the vehicle, and ρ is the air density. Determine the power, in kW, required to overcome aerodynamic drag for a truck moving at 110km/h, if Cd=0.65, A=10m2, and ρ=1.1 kg/m3. Question 4 (2.32): Air contained within a piston‐cylinder assembly is slowly heated. As shown in figure 3 , during this process the pressure first varies linearly with volume and then remains constant. Determine the total work, in kJ. Figure 3 Question 5 (2.53): Each line in the following table gives information about a process of a closed system. Each entry has the same energy units. Fill in the blank spaces in the table. Process a b c d e Q +50 +50 W ‐20 ‐60 ‐90 +150 E1 +20 +20 E2 +50 +50 +60 +50 ΔE +70 +20 0 Question 6 (2.56): As shown in Figure 4, 5kg of steam contained within a piston‐cylinder assembly undergoes an expansion from state 1, where the specific internal energy is u1=2709.9 kJ/kg, to state 2, where u2= 2659.6 kJ/kg. During the process, there is heat transfer to the steam with a magnitude of 80kJ. Also, a paddle wheel transfers energy to the steam by work in the amount of 18.5 kJ. There is no significant change in the kinetic or potential energy of the steam. Determine the energy transfer by work from the steam to the piston during the process in kJ. Figure 4 ...
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