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الكلية كلية الهندسة
القسم الهندسة الميكانيكية
المرحلة 2
أستاذ المادة قصي رشيد عبد الامير الحجاج
5/25/2011 1:32:07 PM
Q1\ A gas contained within a piston–cylinder assembly is shown in Fig. 1. Initially, the piston face is at x = 0 and the spring exerts no force on the piston. As a result of heat transfer, the gas expands, raising the piston until it hits the stops. At this point the piston face is located at x = 0.06 m, and the heat transfer ceases. The force exerted by the spring on the piston as the gas expands varies linearly with x according to F=k.x, where k = 9000 N/m. Friction between the piston and the cylinder wall can be neglected. The acceleration of gravity is g =9.81 m/s2.
(a) What is the initial pressure of the gas?
(b) Determine the work done by the gas on the piston,
(c) If the specific internal energies of the gas at the initial and final
states are 210 and 335 kJ/kg, respectively, calculate the heat transfer.
Ans.( 112.6 kPa, 68.89 J, 131.39 J)
Q2\ As shown in Fig.2, 5 kg 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 80 kJ. 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.
Ans. (350 kJ)
Q3\ Fig.3 shows an object whose mass is 50 lb attached to a rope wound around a pulley. The radius of the pulley is 3 in. If the mass falls at a constant velocity of 3 ft/s, determine the power transmitted to the pulley, in horsepower, and the rotational speed of the pulley, in RPM. The acceleration of gravity is g = 32.0 ft/s2
Q4\ 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 100 lb and a face area of 1 ft2. Electric current passes through the resistor, and the volume of the air slowly increases by 1.6 ft3 while its pressure remains constant. The mass of the air is 0.6 lb, and its specific internal energy increases by 18 Btu/lb. The air and piston are at rest initially and finally. The piston–cylinder material is a ceramic composite and thus a good insulator. Friction between the piston and cylinder wall can be ignored, and the local acceleration of gravity is g =32 ft/s2. Determine the heat transfer from the resistor to the air, in Btu, for a system consisting of
(a) the air alone, (b) the air and the piston. Ans. (15.4Btu, 15.4Btu
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