Exercises on thermodyanmics, Esercizi di Termodinamica

Scarica Exercises on thermodyanmics e più Esercizi in PDF di Termodinamica solo su Docsity! Fundamentals of Engineering Thermodynamics and Heat Transfer Tutorial 4 ________________________________________________________________________ ________________________________________________________________________ Campagnoli 2016-2017 4. GAS POWER AND REFRIGERATION CYCLES EXERCISE 4.1 In an air standard (k=1.4, R=287 J/kgK) Otto cycle the temperature at the beginning of the compression phase is 20 °C and the pressure is 1 bar. The maximum temperature during the cycle is 1200 °C and the compression ratio is equal to 8. If the mass within the piston- cylinder assembly is 1 kg, determine: 1. the temperature and the pressure at the end of each process of the cycle 2. the cycle thermal efficiency [T2= 673 K; p2= 18.4 bar; p3= 40.3 bar; T4= 641 K; p4= 2.2 bar; =0.565] EXERCISE 4.2 In an air standard (k=1.4, R=287 J/kgK) Otto cycle it is given that: a) at the beginning and at the end of the expansion process the temperatures are T3=3260 K and T4=1420 K respectively b) the heat transfer per unit mass during the combustion phase is mQin =1850 kJ/kg c) the pressure at the beginning of the compression phase is p1=1.2 bar Sketch the cycle on the (p,v) and (T,s) planes and hence determine the net work per unit mass developed during the cycle and the cycle thermal efficiency. [ mWcycle =1044.2 kJ/kg; =0.546] EXERCISE 4.3 At the beginning of the compression process of an air-standard Diesel cycle operating with a compression ratio of 18, the temperature is 300 K and the pressure is 0.1 MPa. The cut- off ratio for the cycle is 2. Determine: 1. the temperature and the pressure at the end of each process of the cycle 2. the cycle thermal efficiency [T2= 953.3 K; p2= 57 bar; T3= 1907 K; p3= 57 bar; T4= 793 K; p4= 2.64 bar; =0.631] EXERCISE 4.4 One kilo of standard air (k=1.4, R=287 J/kgK) undergoes a Diesel cycle. Known that: a) the minimum specific volume is v2=0.111 m³/kg b) the maximum specific volume is v4=0.767 m³/kg c) heat is transferred to the system from a reservoir at TH=900 K (combustion process) and from the system to the surroundings at TL=300 K (heat rejection), being the minimum temperature differences between the system and the reservoirs are equals to 10 K. Draw the cycle on the (p,v) and (T,s) planes and then determine the cycle thermal efficiency and the entropy production. [=0.51;  =114 J/K] EXERCISE 4.5 In a Brayton-Joule cycle, a standard air mass flow rate (6 kg/s) enters the compressor at 100 kPa and at 300 K. The compressor pressure ratio is 10 and the turbine inlet temperature is 1400 K. Given that the turbine and the compressor each have an isentropic efficiency of 80%, determine, 1. the thermal efficiency of the cycle Fundamentals of Engineering Thermodynamics and Heat Transfer Tutorial 4 ________________________________________________________________________ ________________________________________________________________________ Campagnoli 2016-2017 2. the back work ratio 3. the net power developed [=0.244; bwr= 65%; cycleW  = 1.1 MW] EXERCISE 4.6 Assuming that a regenerator with effectiveness of 80% is present in the cycle in the exercise 4.5, determine the thermal efficiency of the cycle. [=0.314] EXERCISE 4.7 An inverse ideal Joule cycle uses air (ideal gas) as working fluid and it operates between p1 = 50 bar and p2 = 200 bar. Air enters the compressor and the turbine respectively at 5 °C at 20 °C. During the cycle a heat transfer rate of 2000 kcal/h from the cold ambient is obtained. Determine: 1. the refrigerant mass flow rate 2. the cycle coefficient of performance [  m= 0.0286 kg/s; =2.058] EXERCISE 4.8 An air mass flow rate of 8000 kg/h is used in order to obtain a cooling capacity of 30 kW in an inverse Joule cycle that works between p1=1 bar and p2=6.5 bar. During the cycle the working fluid is subjected to the following four processes:  adiabatic compression: the compressor inlet temperature is 4 °C and its isentropic efficiency is 0.9;  constant pressure cooling: the rate of heat transfer is 130 kW;  adiabatic expansion;  constant pressure heating until the working fluid reaches again the compressor inlet temperature. Determine: 1. The temperature values at the cycle vertexes 2. The coefficient of performance [T2= 494.5 K; T3= 436.2 K; T4= 263.5 K; =0.3] EXERCISE 4.9 An inverse Joule cycle uses air (ideal gas) as working fluid. During the cycle the working fluid is:  adiabatically compressed from 1 bar and 4°C to 5 bar (the compressor isentropic efficiency is 0.75);  constant pressure cooling until the fluid reaches a temperature of 25 °C;  adiabatic expansion (the turbine isentropic efficiency is 0.8);  constant pressure heating until the working fluid reaches again the compressor inlet temperature. Determine: 1. The temperature values at the cycle vertexes 2. The coefficient of performance [T2= 492.6 K; T4= 210.2 K; =0.523]