1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Thermodynamics -- total entropy generation during cooling of tank of water...

  1. Oct 25, 2015 #1
    1. The problem statement, all variables and given/known data
    A 0.2 m^3 steel container that has a mass of 30 kg is filled with liquid
    water. Initially both the steel tank and the water are at 50°C. Now heat is transferred, and the entire system (i.e., steel and water) cools to the surrounding air temperature of 25°C. Determine the total entropy generation during this process.
    http://postimg.org/image/43k9ywc6v/
    2. Relevant equations


    3. The attempt at a solution
    I think that take the steel container and liquid water and air(at 25 °C) as an isolated system so from the equation Sin-Sout(Net entropy transfer by heat or mass out the isolated boundary)+Sgen=ΔSsystem so no net entropy changes across isolated system boundaries so Sgen=ΔSsystem and ΔSsystem=ΔSliquid water+ΔSsteel+ΔSair surrounding
    then find the mass of water by find the average density of water at 25°C and 50°C I got Density=992.5 kg/m^3 and also mass of water=992.5*0.2=198.5 kg
    ΔSliquid water=(198.5 kg)(S@25 °C - S@50 °C)=198.5*(0.3672-0.7038)=-66.8151 kJ/k
    and ΔSsteel(incompressible substances)=m*C*ln(T2/T1)=30*0.5*ln((25+273)/(50+273))=-1.2084 kJ/k
    and total heat transfer to an air surroundings is Qair=-Qliquid water-Qsteel
    find Qliquid=m*c*(T2-T1)=198.5*4.18*(25-50)=-20743.25 kJ
    Qsteel=m*c*(T2-T1)=30*0.5*(25-50)=-375 kJ
    Qair=-(-375-20743.25)=21118.25 kJ
    from ΔSair=Qair/T=21118.25/(25+273)=70.867 kJ/k
    then from ΔSliquid water and ΔSsteel and ΔSair I can find Sgen=ΔSsystem=ΔSliquid water+ΔSsteel+ΔSair surrounding =-66.8151 kJ/k-1.2084 kJ/k+70.867 kJ/k=2.8435 kJ/k
    Is my solution correct or not? please help me for my thermodynamics homework
    thankyou
     
  2. jcsd
  3. Oct 25, 2015 #2
    I didn't check your arithmetic, but your methodology is correct.

    Chet
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Thermodynamics -- total entropy generation during cooling of tank of water...
  1. Water Cooling (Replies: 3)

Loading...