THE UNIVERSITY OF TRINIDAD & TOBAGO

Transcription

1 THE UNIVERSITY OF TRINIDAD & TOBAGO FINAL ASSESSMENT/EXAMINATIONS JANUARY/APRIL 2013 Course Code and Title: Programme: THRM3001 Engineering Thermodynamics II Bachelor of Applied Sciences Date and Time: Monday 22nd April pm-4pm Duration: 3 hrs PLEASE READ ALL INSTRUCTIONS CAREFULLY BEFORE YOU BEGIN THIS EXAMINATION Instructions to Candidates 1. This paper has _9_ pages and _5 questions. 2. An Equation Sheet is included. 3. Thermodynamics Tables are provided separately in an Appendix 4. You are required to answer TWO questions in Section A and ALL questions in Section B 5. The question paper and Appendix are to be returned with the Answer script. Key Examination Protocol 1. Students please note that academic dishonesty (or cheating) includes but is not limited to plagiarism, collusion, falsification, replication, taking unauthorised notes or devices into an examination, obtaining an unauthorised copy of the examination paper, communicating or trying to communicate with another candidate during the examination, and being a party to impersonation in relation to an examination. 2. The above mentioned and any other actions which compromise the integrity of the academic evaluation process will be fully investigated and addressed in accordance with UTT s academic regulations. 3. Please be reminded that speaking without the Invigilator s permission is NOT allowed. Page 1 of 9

2 Section A: Answer any two questions Question 1 a. There are many locations in the world where solar energy is plentiful (including Trinidad and Tobago), yet to date there is no widespread use of solar energy for electric power generation. What issues do you think have kept solar energy from being widely used? [3] b. Concentrating solar collectors are used to power a Rankine cycle with reheat. Water is the working fluid. Superheated vapor enters the turbine at 10 MPa 500 deg. C, and the condenser pressure is 10 kpa. Steam expands through the first stage turbine to 0.8 MPa and then is reheated to 500 deg. C. Determine for the cycle: i. The rate of heat addition from the solar collectors, in kj per kg of steam entering the first stage turbine ii. iii. The thermal efficiency The rate of heat transfer from the working fluid passing through the condenser to the cooling water, in kj per kg of steam entering the first stage turbine. [12] \ Page 2 of 9

3 Question 2 a. What is the purpose of the regenerator in the system shown in Figure 1, include a thermodynamic explanation on how it accomplishes its purpose. [3] b. Air enters the compressor of a regenerative air-standard gas turbine cycle at 0.95 bar, 20 deg C. The air enters the regenerator at 4.75 bar, 227 deg C and exits at 4.75 bar, 467 deg C. The air enters the turbine at 4.75 bar, 877 deg C and expands to 0.95 bar, 527 deg C. Determine: i. The isentropic compressor and turbine efficiencies ii. The regenerator effectiveness iii. The thermal efficiency of the plant. [for air: c p = kj/kgk, k=1.4] Figure 1: Regenerative Gas Turbine System [12] Page 3 of 9

4 Question 3 In the condenser of a power plant, energy is discharged by heat transfer at a rate of 836 MW to cooling water that exits the condenser at 40 o C into a cooling tower. Cooled water at 20 o C is returned to the condenser. Atmospheric air enters the tower at 25 o C, 1 atm, 35% relative humidity. Moist air exits at 35 o C, 1 atm, 90% relative humidity. Makeup water is supplied at 20 o C. For operation at steady state, determine the mass flow rate in kg/s of the (a) The entering atmospheric air (b) The makeup water Ignore kinetic and potential energy effects. Figure 2: Cooling Tower System [15] Page 4 of 9

5 Section B: Answer all questions Question 4 Benzene gas (C 6 H 6 ) at 25 0 C, 1 atm enters a combustion chamber operating at steady state and burns with 95% theoretical air entering at 25 0 C, 1 atm. The combustion products exit at 1000 K and include only CO 2, CO, H 2 O and N 2. Determine the mass flow rate of the fuel in kg/s, to provide heat transfer at a rate of 1000 kw. [15] Question 5 With explanation, indicate whether exergy would increase, decrease or remain the same for the following processes shown in figure 3: a) Process 1-2 [2] b) Process 3-4 [2] c) Process 5-6 [2] Figure 3: T-v diagram showing various processes Argon enters an insulated turbine operating at steady state at 1000 o C and 2MPa and exhausts at 350 kpa. The mass flow rate is 0.5 kg/s and the turbine develops a power output of 120 kw. Assuming ideal gas behavior for the argon and ignoring the kinetic and potential energy effects, determine Page 5 of 9

6 a) The temperature of the argon at the turbine exit in deg. C [4] b) The exergy destruction rate of the turbine in kw [5] P o = 1bar, T o =293 K, [for argon: c pargon = 0.52 kj/kg K, k argon = 1.667] Page 6 of 9

7 Equation Sheet 1. Enthalpy and Entropy definition for an ideal gas h h = cp( T ) T T s p 2 2 s1 = c LN( ) R ln T1 p p 2 1 (for all irreversible processes) 2. Enthalpy and Entropy definition for a mixture 3. Entropy evaluation of a component i within a mixture 4. Energy balance of a control volume with a mixture flowing through 5. Exergy balance of a Control Volume Page 7 of 9

8 6. Ideal gas laws pv= mrt Pressure ratio: r p = P 2 P 1 7. Efficiency Carnot Power Cycle Power Cycle η= W cycle Q in = 1 T L T H η= W net Q in Refrigeration Cycle Coefficient of Performance Wet Steam β = Q EVAP W COMPRESSOR v = v f + xv fg h = h f + xh fg s = s f + xs fg Page 8 of 9

9 8. Psychrometry Mixture enthalpy per unit mass of dry air h mix = Humidity ratio (moisture content) ω Relative humidity Φ or Page 9 of 9