reversible. (Note: The process is not externally reversible, since temperature is transferred across a finite temperature difference). In this case the following approach may be applied to find the entropy change in the system:212121genSTQS(1)Because the process is internally reversible, so in the systemSgen= 0. The equation (1) can be simplified to:TQS2121(2)In equation (2), T denotes the temperature of the system, since we are just considering the entropy change in the system and not the total entropy generated throughout the process (in which case we would use the temperature of the surroundings). (See example 7-21 in the textbook for more information).ΔS1-2= m·(s2- s1)(3)The relevant properties can be found on table A-4.s2= sg at 120 oC =7.1292 kJ/kg·Ks1= sf at 120 oC + x1·sfg at 120 oC= 1.5279 + 0.65 × 5.6013kJ/kg·K= 5.168745 kJ/kg·KPut the values of m, s2ands1into the equation (3):ΔS1-2= 8kg × (7.1292 –5.168745)kJ/kg·K = 15.68364 kJ/K
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