THE UNIVERSITY OF TRINIDAD & TOBAGO

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1 THE UNIVERSITY OF TRINIDAD & TOBAGO FINAL ASSESSMENT/EXAMINATIONS DECEMBER 2012 Course Code and Title: Programme: Date and Time: Duration: THRM3001_Thermodynamics II B.A.Sc./M.Eng. Utilities Engineering Wednesday 19 th December 2012 [9.00am 12.00noon] 3hours PLEASE READ ALL INSTRUCTIONS CAREFULLY BEFORE YOU BEGIN THIS EXAMINATION Instructions to Candidates 1. This paper has 4 pages and 6 questions. 2. You are required to answer any 2 questions in Section A. 3. You are required to answer all questions in Section B. 4. Each question is worth 15 marks Question paper must 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. (THRM3001) Page 1

2 Section A: Answer any two questions 1. Superheated steam at 18 MPa, C, enters the turbine of a vapor power plant. The pressure at the exit of the turbine is 0.06 bar and liquid leaves the condenser at bar, 26 0 C. The pressure is increased to 18.2 MPa across the pump. The turbine and pump have isentropic efficiencies of 82 and 77% respectively. For the cycle determine: a. The net work per unit mass of steam flow, in kj/kg b. The heat transfer to steam passing through the boiler in kj/kg of steam flowing c. The thermal efficiency 2. Steam at 10 Mpa, 600 deg C enters the first stage turbine of an ideal Rankine cycle with reheat. The steam leaving the reheat section of the steam generator is at 500 deg C and the condenser pressure is 6 kpa. If the quality at the exit of the second-stage turbine is 90%, determine the cycle thermal efficiency. Figure 1 Rankine cycle with reheat (THRM3001) Page 2

3 3. Figure 2 illustrates a gas turbine power plant that uses solar energy as the source of heat addition. Operating data are given on the figure. Modeling the cycle as a Brayton cycle and assuming no pressure drops in the heat exchanger or interconnecting piping determine: a. the thermal efficiency b. the air mass flow rate in kg/s for a net power of 500 kw. [c p = kj/kgk and R=0.287 kj/kgk] Figure 1: Gas Turbine power plant that uses solar energy Figure 2: Solar driven Brayton cycle 4. Air enters the compressor of an ideal air-standard Brayton cycle at 1.01 bar, 310K. The compressor ratio is 12 and the temperature at the turbine exit is 770K when the isentropic efficiency of the turbine is 100%. If the isentropic efficiency of the turbine is 87% and the isentropic efficiency of the compressor is 84% Determine: a) The back work ratio b) The Thermal efficiency [c p = kj/kgk and R=0.287 kj/kgk] (THRM3001) Page 3

4 Section B: Answer all questions 5. 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. 6. Figure 3 shows a turbine operating at steady state with steam entering at p 1 = 30 bar, T 1 = 350 o C and a mass flow rate of 30 kg/s. Process steam is extracted at p 2 = 5 bar, T 2 = 200 o C. The remaining stream exits at p 3 =0.15 bar, x 3 = 90% and a mass flow rate of 25 kg/s. Stray heat transfer and the effects of motion and gravity are negligible. Let T 0 =25 o C, p o =1 bar. The accompanying table provides property data at key states. For the turbine, determine the power developed and rate of exergy destruction, each in MW. Also devise and evaluate an Exergetic efficiency for the turbine. State P (bar) T ( o C) h (kj/kg) s (kj/kgk) x = 90% Figure 3: Steam Turbine (THRM3001) Page 4

5 Equation Sheet 1. Enthalpy and Entropy definition for an ideal gas h h cp( T ) T T s p 2 2 s1 c LN( ) Rln T1 p p 2 1 (for an irreversible process) 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 6. Ideal gas laws (THRM3001) Page 5

6 pv= mrt 7. Efficiency Carnot Power Cycle η Power Cycle η Refrigeration Cycle Coefficient of Performance Wet Steam (THRM3001) Page 6