Using Gibb\'s Rule estimate the number of state variables that is needed to full

Question

Using Gibb's Rule estimate the number of state variables that is needed to fully specify the system 1. CO2 at triple point 2. Subcooled liquid 3. Supercritical fluid b) Lower heating value (LHV) and higher heating values (HHV): 1. What is the difference between lower and higher heating values? 2. Why is HHV higher that LHV? 3. For a gas turbine application which value would you use? c) For which one of the ideal flow devices does the enthalpy of the flow drops? 1. Compressor 2. Nozzle 3. Combustor 4. Valve d) For which of the following gas(es) calorically perfect assumption is exact? 1. O2 2. AR 3. Ne 4. CO2

Explanation / Answer

a)

1. at triple point there are no degrees of freedom, the p vs T behaviour if just fixed to that point. we cannot change any property at this stage.

2. in subcooled region, there is only one state so degrees of freedom F=2, we can change pressure and temperature individually

3. In super critical region also F=2, only in wet region as two phases exist F=1

B)

1.The HHVis determined bybringing all the combustion products back to the pre-combustion temperature and in particular condensing any vapour produced

2.The difference between two values depend on the calorific value of the fuel. HHV = LHV + heat of vapourisation *(no of moles of water/ no of moles of fuel in)

3.For a gas turbine we use LHV

C) nozzle

D)oxygen closely obeys these laws

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