Assumptions in Thermodynamics Air Cycle

University:
Swansea University
Course:
EG-086 | Engineering Science
Academic year:

2023
Views:

402

Pages:

3
Author:

Lloyd R.

Assumptions in thermodynamics Air Cycles 1) Perfect gas (It obeys the gas laur heats) and has constant specific (Without internal, friction 2) Compression and Expansion process are reversible nadiabat 3) Combustion process is replaced by heat addition procen from an external soulle. 4) Exhaust procen is replaced by heat rejection procen from an external source. 5) It Should be a remaining closed seandey che with the same 'air' always in the cylindest to repeat the cycle. 6) No chemical reaction takes place takes place in the cylinder. 7) Molecular Cycle weight of cylinder gas is 29. Cp = 1 b=1-005 kJ/kg.K, Cr: 0-718 A cycle is a repeated series of operations occurring in Certain order. The former imaginary perfect enfine engine is a is called ideal cycle and the latter actual Called actual cycle. Air standard Efficiency The efficiency of engine using an as medium is known nown as s the working 1. " Are standard efficiency. This efficiency is: Oftenly called icleal efficiency The actual efficiency of a cycle is always less than u als standard the efficiency of that cycle, under idial Conditions. This is taken into account by introducing. new term" Relative efficiency" a 4 Actual thermal efficiency revo in relativeAir Standard efficiency Cylinder Terminologies. The point 1 represent that cylinder is full of air with Volume VI, pressure P. and absolute temperatiude T. Line 1-2 represents the adiabatic Compression of ais duc to which Pi, Vi and I, Ch Change to 3, 2 and I respectively. Line 2-3 shows the supply of heat to the air at constant Volume so that B. and to changes to Ps and is (Ys being the bame as V₂). Line 3-4 represents the adiabatic expansione of the air. During expansion of P P3, Vs and Is Change or. Vi and 74 hespectively. to a final value. •Line 4-1 Show the rejection of heat by air Volume till orginal State (point 1) reaches. at Constant • maximum temperature is at point 3. I Consider I kg of our I working subrtana): Heat Supplied at Constant Volume Cv (T3-13). Heat rejected at Constand Volume CVCTY-TI). But, work done Heat Supplied Heat rejected =Cv (7-7)-C(Th-T) *Efficiency= Werk done Heat Supplied C (13-T) 1- (14-71) 13-Tr Let Compression ratio, kc = and 22 Expansion ratio, re (These two ratios are same in thi's cycle) as Similarly,) giving@andin we get, this expression is known as efficiency of the Otto cycle the air standard It is cliar from the above enpression that efficiency Preveases with the increase in the value of r, deu to prachtal difficulties value of r is limited to about 8. انال

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