Sample Exam 3 (B) Solutions

Sample Exam 3 (B) Solutions - EML 3100 — Exam 3'—...

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Unformatted text preview: EML 3100 — Exam 3 '— Spring 2012 Friday, April 20 my ' I _ Name H825, -— mg UF—ID Problem 1 (20 points) Consider the ideal Rankine cycle with superheating that is plotted in T —s coordinates below.>1dentify how the thermal efficiency of the cycle changes under the following conditions: Choose ONE of the following to complete each statement: INCREASE, DECREASE, or STAY THE SAME; (A) Increasing the exhaust pressure, P4, slightly will cause the thermal efficiency to Wag/t A35 . . (B) Increasing the boiler exit temperature, T 3, slightly will cause the ' thermal efficiency to l“ Clem/3E Z (C) Increasing the mass flow rate through the system slightly will cause the thermal efficiency to g’M‘,’ W15 $44442 . (D) Adding a reheat process to the cycle at an intermediate pressure, P4 < P < P3, such that x4' is unchanged from its current value, will cause the thermal efficiency to D ECREA’SE ‘ . (E) Adding a regeneration process to the cycle whereby fluid is extracted fi'om the tLu'bine at an intermediate pressure, P4 < P < P3, and reintroduced through the use of an open feedwater heater, will cause the thermal efficiency to lN (Next/Ki: Problem 3 (20 points) spark-ignition automobile engine can be modeled as an Otto cycle with compression ratio I“, = 9. For operation at full throttle the following conditions are found: P1=1001<Pa, Tlé3201{, _T3=3lOOK. Assume that the effective cOnstant specific heat ratio is k = 9/7 and that-the heating value of the fuel is FHV= 42000 kJ/kg-fuel. The combustion process is modeled as an equivalent input of heat given by Q.”. = m F H V. Determine the constant pressure specific heat, c,,, the pressure after the compression stroke, P2, and theair to fuel ratio AF =_ mil/nu. ' ' cm co We ‘ 2 W987) : ms . .14 k , ' (07/7) I I ' ‘Pp IVS . , I I ‘ " ; I I H ' ‘ Fw C (T > CV<7§~T§> ‘ W)A V «Ta M i! teal “flows “ZN ' Tl Va Treats k 50, AFA I Ll«0€‘t§)( 3250 1, Wei/S) '- [(0-79] ‘Cp = l-QOlLS “704344 P2: [£80 uPa AF=' “of—)R’ Problem 4 (20 points) A basic turbojet engine with an afterburner is shown in the schematic below. The work generated by the turbine is just sufficient to drive the compressor. In addition to, m = 80 kg/s, WC = 25 MW, ' Cp = 1.148 l<J/l<g-K and k = 4/3, the following conditions are known: T2=6SOK, T3=13OOK, P5=4001<Pa, T5=1800K,_P,-=1OOkPa. Determine T 4, 7}, and the exit velocity, Vj. QHl T4 = [0957 R 1:27 3 k 1m m/s :73 || xii ll Problem 5 {20 points) An ideal Rankine cycle is shown in the schematic below. (A) Derive an expression for the thermal efficiency in terms of only the variables 17 1, 172, 173, and m. (B) Calculate the specific heat loss in the condenser 1'ij = 25 lcPa and X4 = 0.87. QM r that w _ We 1%: w ‘ QH CiH U"3‘"’"w) * (hark!) T. / Ur“) R=9€L¢Pa ,X‘EO -—> hi: thwart/oat, xszom ——=? 1“: seems We)”; lurid.» —' 30%;.3’733 W/Kglfiv '§i( @- he ...
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