Sample Questions from Dr Ahmed's ME 398 Finals from the last three years'" A heat pump, working in a reversed Carnot cycle and driven by a 1.5 kW electric motor, provides heating for a building whose interior is to be kept at 19 degrees Celsius. (i) On a day whentheoutsidetemperature is -5.35 degrees Celsius and energy is lost through the walls and the roof at a rate of 63,000 kJ/h, would the heat pump suffice? (ii) At 12 cents/kWh, estimate the electric bill for 8 hours of operation of this heat pump (whether or not the heat pump has the capacity to perform its function fully). The electronic components ofa computer consume 370 W of electrical power. The prevent overheating, a 20 W-fan blows in air from the room at 20 degrees Celsius. The air exits the computer enclosure at 35 degrees at nearly the same pressure. Heat transfer through the walls of the computer is negligible. For a steady state operation, determine: (a) the flow rates of air through the computer, in kg/so (b) the rate of entropy production in the computer, in kW/K. A turbine operates with steam entering at 1.5 MPa at the rate of 200 kg/so The steam exits as saturated vapor at 0.1 MPa. Assuming isentropic expansion in the turbine, estimate the power output. a) If a block of aluminum (p = 2700 kgim3, Cp= 0.903 kJ/kg. K) and a block of steel (p = 8930 kgim3, cp = 0.385 kJ/kg.K) having the same volume received the same amount of energy by heat transfer (oQ), which one would experience the greater temperature increase? b) What are some of the principal irreversibilities present during the operation of an automobile? c) A system consists of two gases, initially at two different temperatures, and separatedby a masslesspartition. The system is isolated from its surroundings. The partition is removed and the gasesmix with each other. Apply and simplify the conservation of energy equation to the system as it goes through the mixing process. Do the same with the equation describing entropy changes for the system. Briefly explain the implications of each of the final expressions. One kg of air behaving like an ideal gas operates in a Carnot cycle to produce power. The thermal efficiency of the cycle is 60%. The amount of heat added to the cycle (QH, at the high constant temperature, TH) is 40 kJ. At the end of this heat addition process the pressure and the specific volume of the air are 560 kPa and 0.3 m3, respectively. a) Show the process on the T-s plane. A 30 0. resistor is used for heating a stream of air flowing through an insulated duct. At steady state operation, the current flowing through the resistor is measured at 15 A, and the temperature is 400 K. The air enters the duct at 290 K and 1 atm, and exits the duct at 300 K, with negligible change in pressure. (a) Determine the volumetric as well as mass flow rates of air flowing through the duct. (b) For the resistor as the system, find the rate of production of entropy. (c) For a CV enclosing the air in the duct and the resistor, find the rate of production of entropy. One kg of air behaving like an ideal gas operates in a Carnot cycle to produce power. The thermal efficiency of cycle is 60%. The amount of heat added to the cycle (QH. at the high constant temperature, TH) is 40 kJ. At the end of this heat addition process the pressure and the specific volume of the air are 560 kPa and 0.3 m3, respectively. a) Show the process on the T-s plane. b) Determine THand TLo c) Determine the amount of heat (QJ that must be rejected at TLo d) Determine the works done during the heat addition and heat rejection processes. e) Determine the works done during the isentropic processes (assume constant specific heat). f) Find the net work produced by the air during one cycle. An engine working with 0.1 kg of air follows the Camot cycle. The high temperature reservoir is at 940 K, and at the beginning of isothermal expansion at this temperature the air pressure is 8.4 MPa. The heat added to the air in this expansion process is 8.4 kJ. The low temperature reservoir is at 300 K. Determine: (a) the pressures at the 3 other "comers" of the Camot cycle; (b) the net work developed per cycle, in joules; (c) the thermal efficiency of the engine. 1 kg of water initially at 3 bar and 200 degrees Celsius undergoes a process to its new state at 15 bar and 210 degrees Celsius while being compressed in a piston-cylinder assembly. Heat transfer occurs through a thin wall, with the surrounding at 22 degrees Celsius. The work done is measured to be (-)175 kJ. Neglecting the changes in KE and PE, determine whether the work measurement may be in error. The electronic components of a computer consume 300 kW of electrical power. The prevent overheating, a 50 W-fan blows in air from the room at 20 degrees Celsius. The air exits the computer enclosure at 35 degrees at nearly the ambient pressure. Heat transfer through the walls of the enclosure is negligible. For a steady state operation, determine: b) DetermineTHand TL. c) Detennine the amount of heat (QL) that must be rejected at TL. d) Detennine the works done during the heat addition and heat rejection processes. e) Detennine the works done during the isentropic processes (assume constant specific heat). t) Find the net work produced by the air during one cycle. (a) the mass as well as volumetric flow rates of the air; (b) the rate of entropy production in the enclosure. . The abovedo not includecorrectionsmadeduring thc cxamination.The type of questionsaswell asthe level of difficulty (along with the total numberof problems)can vary from semesterto semester.Usually thereare 3-4 problems that may requirethe full two hoursof the final exam.