(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

Hi there,

I am trying to complete a thermodynamics assignment. I am doing fine until I get to the following question.

"Hx = Specific enthalpy at dryness fraction = hf + x . hfg" - I am not quite sure where Hf came from, I guess they want a value for Saturated liquid from the Enthalpy tables but which one?

2. Relevant equations

I have two Hf's I have already calculated.

Hf @ Ti 63 kJ/kg

Hf @ Tb 419.2 kJ/kg

These are just the values for saturated water at 15°C and 100°C.

Other relevant equations:

X = Dryness fraction = Mev / Mw = 0.013kg / 1.805kg = 0.007

Hfg at boiling temperature = 2256.4

Hev = Hx - Hf @ Ti

Qev = Mw * Hx

3. The attempt at a solution

I would just like to add that I have calculated all of the above, they were not given to me.

When I put the numbers I assume is supposed to be there I get.

Hx = Hf + x * Hfg = 419.2 - 0.007 * 2256.4 = 435

Hev = Hx - Hf @ Ti = 435 - 63 = 372

Qev = Mw * Hx = 1805grams * 435 = 785175J

Energy delivered by kettle

Qk = Pk * t = 2100watt * 310.4 = 651840J

Efficiency of my kettle

Nev = Qev / Qk = 785175 / 651840 = 1.20 or 120% lol

That can't be 120% so I have gone wrong somewhere and I think it is to do with "Hx = Specific enthalpy at dryness fraction = hf + x . hfg"

If anyone could shed some light that would be great.

Many Thanks.

Sorry for the long post.

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# Specific enthalpy at dryness fraction

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