Thermodynamics II MIDTERM MEH 35/ Fall 06 ONORDIA UNIVERSITY FAULTY OF ENGINEERING AND OMPUTER SIENE DEPARTMENT OF MEHANIAL ENGINEERING Student s Name: I.D.: I. [50 points] A steam power plant operates on an ideal rehear-regenerative Ranine cycle and has a net power output of 80 MW. Steam enters the high pressure turbine at 0 MPa and 550º and leaves at 0.8 MPa. Some steam is extracted at this pressure to heat the feedwater in a closed feedwater heater. The extracted steam leaves the feedwater heater as a saturated liquid. The rest of the steam is reheated to 500º and is expanded in the low pressure turbine to 0 Pa. Thermodynamic properties are given in the table below in order to save time. - Draw the cycle on a T-s diagram with respect to saturation lines - Determine the mass flow rate of steam through the boiler 3- Determine the thermal efficiency of the cycle. h 9.83 h 0.9 h3 7. h4 73.37 h5 3500.9 h6 8.9 h7 3480.6 h8 494.4 h9 73.37 h0 73.37 All in J/g /
Thermodynamics II MIDTERM MEH 35/ Fall 06 II.. A Diesel cycle, with a compression ratio of 8, operates on air with a low pressure of 00 Pa and a low temperature of 00º. Assume constant heat capacities. If the heat supplied is 800 J/g, determine: - The pressure, the temperature and the specific volume at each point. - The cutoff ratio. 3- The thermal efficiency of the cycle. For air at room temperature: =.4; p=.0035 J/g K; v=0.78 J/g K and R=0.87 J/ g K II.. Put the following sentences under the right cycle:. Uses a spar plug.. Heat is supplied at constant pressure. 3. Has a higher compression ratio. 4. ompresses fuel only. 5. Heat is supplied at constant specific volume. 6. ompresses air only. 7. Has a higher efficiency than a arnot cycle operating under the same conditions. 8. ompresses air+fuel. Otto cycle Diesel cycle /
Thermodynamics II MIDTERM MEH 35/ Fall 07 ONORDIA UNIVERSITY FAULTY OF ENGINEERING AND OMPUTER SIENE DEPARTMENT OF MEHANIAL ENGINEERING Student s Name: I.D.: I. [40 points]. onsider a regenerative vapor power cycle with one open feedwater heater. Steam enters the turbine at 8.0 MPa, 480 and expands to 0.7 MPa, where some of the steam is extracted and diverted to the open feedwater heater operating at 0.7 MPa. The remaining steam expands through the second stage turbine to the condenser pressure of 0.008 MPa. Saturated liquid exits the open feedwater heater at 0.7 MPa. The isentropic efficiency of each turbine stage is 85% and each pump operates isentropically. If the net power output of the cycle is 00 MW, determine: - Draw the T-s diagram. - The thermal efficiency. ANSWER.. - The mass flow rate of steam entering the first turbine stage, in g/s. ANSWER.. The following enthalpy is given: h 3s =46.3 J/g Turbine y 3 Boiler 6 Open Feedwater Heater 5 ondenser 7 4 Pump Pump /3
Thermodynamics II MIDTERM MEH 35/ Fall 07 II.. [40 points]. A Diesel cycle, with a compression ratio of 8, operates on air with a low pressure of 00 Pa and a low temperature of 00º. If the high temperature is limited to 00 K, determine, - Draw the P-v and the T-s diagrams - The thermal efficiency of the cycle. ANSWER.. - The mean effective pressure ANSWER.. - The efficiency of an Otto cycle operating with the same compression ratio. Discuss your result. ANSWER.. - ompare your efficiency to arnot efficiency under the same conditions. ANSWER.. For air at room temperature: =.4; p =.0035 J/g K; v =0.78 J/g K and R=0.87 J/ g K Diesel cycle efficiency: η th rc = r ( rc ) r compression ratio cut-off ratio r c Isentropic relations T v = T v s= cte T P = T P s= cte P v = P v s= cte p = v /3
Thermodynamics II MIDTERM MEH 35/ Fall 07 III. [0 points]. Put the following sentences under the right cycle:. Uses a spar plug.. Heat is supplied at constant pressure. 3. Has a higher compression ratio. 4. ompresses fuel only. 5. Heat is supplied at constant specific volume. 6. ompresses air only. 7. ompresses air+fuel. Otto cycle Diesel cycle 8. Which one of these cycles has an efficiency higher than arnot efficiency: ) Ranine cycle with reheat and regeneration ) Diesel cycle with a compression ratio of 30 and a cut-off ratio of higher than 5 3) Otto cycle with a compression ratio higher than 30 9. The exhaust process in the Otto and Diesel cycles is replaced with a constant-volume process for what primary reason? ) To simulate zero wor of the actual exhaust process ) To simulate zero heat transfer of the actual exhaust process 3) To restore the air to its original state 4) To ensure that the first law is satisfied 0. Someone says that your car s engine has more power when the ambient temperature is low. Do you agree? Why? 3/3
THERMODYNAMIS II MIDTERM, OTOBER 0, 008 MEH 35/ ONORDIA UNIVERSITY FAULTY OF ENGINEERING AND OMPUTER SIENE DEPARTMENT OF MEHANIAL ENGINEERING Student's Name: I.D.: Notes: State clearly any assumptions you mae. If applicable, draw a setch of the problem. Return the questions paper with the answers boo. The maximum grade is 00. Question no. (60 Points) onsider a steam power plant that operates on the ideal regenerative Ranine cycle with a closed feed water heater as shown in figure. The plant maintains the turbine inlet at 3000 Pa and 350 o ; and operates the condenser at 0 Pa. Steam is extracted at 000 Pa to serve the closed feed water heater, which discharges into the condenser after being throttled to condenser pressure. Properties of selected states are given in table. a. Setch the cycle on a T-s diagram. b. alculate the wor produced by the turbine. c. alculate the heat rejected by the condenser. d. alculate the thermal efficiency of the cycle. /
THERMODYNAMIS II MIDTERM, OTOBER 0, 008 MEH 35/ Property h h h3 h4 s4 h5 h6 h7 Table Value 5.40 J/g 54.45 J/g 763.53 J/g 36.0 J/g 6.745 J/g.K Determine it 3.0 J/g Determine it Question no. (40 Points) An air-standard Diesel cycle operates with a compression ratio of 6 and a cutoff ratio of. At the beginning of compression process the conditions are 37, 0.0 MPa, respectively. Assuming constant specific heats for air at room temperature (p =.005 J/g.K; v = 0.78 J/g.K and R=0.87 J/g.K), determine: (a) The maximum temperature in the cycle. (b) The pressure after the isentropic expansion. (c) The net wor per cycle and thermal efficiency. (d) The mean effective pressure. (e) The volume flow rate of air in m 3 /min, measured at conditions at the beginning of compression, needed to produce 00 KW. Isentropic relations T v T v s cte T P T P s cte P v P v s cte p v /
THERMODYNAMIS II MIDTERM, FEB 7TH, 0 MEH 35/4 Student's Name: I.D.: Notes: ONORDIA UNIVERSITY FAULTY OF ENGINEERING AND OMPUTER SIENE DEPARTMENT OF MEHANIAL ENGINEERING State clearly any assumptions you mae. If applicable, draw a setch of the problem. Return the questions paper with the answers boo. The maximum grade is 50. Question no. (30 Points) A Ranine steam cycle modified for reheat, a closed feewater heater, and an open feedwater heater is shown below. The high-pressure turbine receives 00 g/ s of steam from the boiler. The following data tables give the saturation data for the pressures and data for h and s at selected states.. Setch the T-s diagram for the ideal cycle.. Determine the net power output of the cycle, in MW. 3. If cooling water is available at 5º, what is the minimum flow rate of the cooling water required in g/ s (i.e., for a maximum possible temperature difference for the cooling water)? Tae p (water)= 4.8 J/ g. K. Process states and selected data
THERMODYNAMIS II MIDTERM, FEB 7TH, 0 MEH 35/4 State P (Pa) T (º) h (J/ g ) s (J/ g. K) 0 60. 5.4 400 3 400 53 4 400 830 5 5000 6 5000 700 3894 7.504 7 400 3400 7.504 8 00 3349 7.504 9 00 600 369 7.938 0 45 354 7.938 0 60 7.938 53 3 NOTE: the wor of all pumps is negligible compared to the wor of the turbines. ANSWERS Net power output Minimum flow rate of the cooling water Question no. (0 Points) An ideal Diesel cycle has a maximum cycle temperature of 000 o and a cutoff ratio of.. The state of the air at the beginning of the compression is P = 95 Pa and T = 5 o. This cycle is executed in a four-stroe; eight-cylinder engine with a cylinder bore of 0 cm and a piston stroe of cm. The minimum volume enclosed in the cylinder is 5% of the maximum cylinder volume. Determine the power produced by this engine when it is operated at 600 rpm. Use constant specific heats at room temperature: p =.005 J/g.K and V = 0.78 J/g.K. ANSWER Power produced by the engine T v T v s cte T P T P s cte P v P v s cte p v