# Homework Help: How calculate the Heat flux requiered to evaporate sub cool water

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1. Sep 9, 2017

### Vincenzo

1. The problem statement, all variables and given/known data

I was trying to validate a calculation I didusing ANSYS fluent, it's about a flow of water that enters a tube, the tube is 16m long and is being heated with a Heat Flux of q'=1077328,47 [W/m^2], now after the simulation runs and I check the results I find out that the steam quality at the exit of the tube is around 69% and not higher, and I need to get a higher quality steam, around 80%.

the mass flow is m'=0,5509 [kg/s]
the radius of the volume is R=0,01905 m
Length L=16m
Water enters at: T=20[°C]
velocity of entrance is: V=0,484 [m/s]
Pressure: P=2000 [psi] or 1,379e+7 [Pa]
Tast= 335,472°C

I have all the thermodinamics properties of the water at 20 °C, 180 °C, 320°C, 340°C and the saturated temperature 335,472°C

Now I ask because I know I'm getting the heat flux wrong but, as far as I knew, when studing Forced Internal Convection it is a good way to get into the problem using the properties at intermedium temperature to find the heat flow and heat flux, and that's why I used the propierties of water of 180°C, but, I know that the properties between 180°C and 320°C are way to different, the especific heat is almost 1,5 times higer and between 320°C and 335,472°C the specific heat increases more than 10%.

So, how can I get a more real approximation? Obviusly I can't go and use the Cp at 335.472°C and I need one Cp that allow me to find a representative Heat flow Q' to get a more representative heat flux. What other things I need to considerate? I know that Reynold's number grows too high when I choose a viscocity of higher temperature and it causes Nusselt's number grow, and in consecuence it increases the convection factor h.

2. Relevant equations

Heat Flow: Q'=m'*Cp*ΔT
Q'=As*h*ΔT
Heat Flux: q'=h*ΔT
Reynold's number Re=V*D/μ
Nusselt's number (A big long eq that depends of Prandtl, Reynold and friction factor)
Nusselt's Number Nu= h*D/k

3. The attempt at a solution

I was thinkg in doing tables on excel and repeat the calculations to differents lenghts (everything depending on lenght), the thing is, it's a long work, I will have to find many times the different properties of water and do some average to find the heat flux and I just need and approximation to find, validate and confirm the steam quality X=100%.

Thanks for the time!

2. Sep 14, 2017

### PF_Help_Bot

Thanks for the thread! This is an automated courtesy bump. Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post? The more details the better.