Thermodynamics Exam for Mechanical Engineering Students: Autumn 2007, Exams of Physics Fundamentals

Download Thermodynamics Exam for Mechanical Engineering Students: Autumn 2007 and more Exams Physics Fundamentals in PDF only on Docsity! CORK INSTITUTE OF TECHNOLOGY Bachelor of Engineering (Honours) in Mechanical Engineering- Stage 2 (NFQ Level 8) Autumn 2007 THERMODYNAMICS Time: 3 Hours Answer any FIVE questions. All questions carry an equal 20 marks. Examiners: Dr. R.K. McMullan Mr. P. Clarke Prof. M. Gilchrist (1) Describe using sketches how the Carnot cycle can be adapted to overcome equipment operational constraints, and achieve the highest efficiency in practice. (6 marks) Regarding the regenerative cycle, explain why feedwater heaters are used. Describe the differences between open and closed feedwater heaters using a T-s diagram to assist. (4 marks) Consider a regenerative cycle using steam as the working fluid. Steam leaves the boiler and enters the turbine at 40bar and 400degC. After expansion to 4bar, some of steam is extracted from the turbine for the purpose of heating the feedwater in an open feedwater-heater. The pressure in the feedwater heater is 4bar and the water leaving is saturated liquid at 4bar. The steam not extracted expands to 0.1bar. Determine the mass-fraction of steam bled from the turbine for feedwater heating, and the cycle efficiency. Sketch the cycle on a T-s diagram. (10 marks) (2) Derive an expression for the air-standard Brayton Cycle that expresses efficiency as a function of isentropic pressure-ratio. (6 marks) Sketch the cycle on P-v and T-s diagrams, and show on the T-s diagram the effect of: • compressor inefficiency • pressure-drop between compressor and turbine • turbine efficiency (2 marks) An air standard Brayton cycle has air enter the compressor at 27degC and 100 kPa. The pressure ratio is 10 and the maximum allowable temperature in the cycle is 1350 K. Determine the compressor work, turbine work and the cycle efficiency per kilogram of air. Compare the results of the previous problem to the results obtained if the compressor and turbine efficiencies are 85% and a pressure drop of 27kPa exists between the compressor discharge and the turbine inlet. (12 marks) Assume constant for air Cp=1.005kJ/kgK, Cv=0.718kJ/kgK (3) Explain, in your own words, the 2nd Law of Thermodynamics, and discuss some of the consequences that follow from it. (4 marks) Starting with the Clausius and Kelvin-Planck statements of the 2nd Law, demonstrate either case where a machine that violates one statement also violates the other. (8 marks) Describe the construction of a Carnot cycle. Sketch the cycle on P-v and T-s diagrams, and using the latter, prove that the efficiency of a Carnot cycle can be expressed in the form H L T T −=1η (6 marks) What are the consequences for the construction of a steam power-plant, and how will the efficiency vary with the seasons? (2 marks) (4) Sketch an ideal vapour-compression refrigeration cycle on P-v, T-s and P-h diagrams, and describe the processes that make up this cycle. (6 marks) List four thermodynamic characteristics of an ideal refrigerant, and briefly explain the importance of each. (2 marks) An ideal vapour-compression refrigeration cycle uses Refrigerant-134a as a working fluid in an air- conditioning system. The refrigerant enters the compressor as a saturated vapour at 5degC and leaves the condenser as a saturated liquid at 55degC. The mass flow rate of the refrigerant is 0.7 kgs-1. Heat transfer to the refrigerant in the evaporator is from a reservoir at 15degC, and the refrigerant in the condenser is cooled by heat transfer to the environment, which is at a temperature of 45degC. Calculate: a) The heat-transfer rate in the condenser b) The heat-transfer rate in the evaporator c) The coefficient of performance of the cycle (12 marks) Tables of Refrigerant-134a properties are given at the back of this exam (5) A Rankine cycle steam plant expands the steam in two turbines, with re-heating between them. The isentropic efficiency of the high-pressure turbine is 83.6%, and that of the low-pressure turbine is 79.4%. The steam leaves the steam-generator at 6.2MN/m2, 540degC and expands to 5bar. Re-heat takes place at this pressure, taking the temperature of the steam to 510degC. The second stage of the expansion exhausts to the condenser at 4kPa. . Neglecting feed-pump work, a) sketch the cycle on a T-s diagram b) determine the thermal efficiency of the plant c) evaluate the flow-rate of steam required to produce 8.4MW (17 marks) How does the Re-heat cycle compare to the Regenerative cycle with regard to achieving a thermal efficiency close to the Carnot efficiency? (3 marks)