
#1
Nov1309, 04:53 PM

P: 15

Ok it's my first time here and I was hoping to get some help on some questions I have been given. I am a first year chem eng and I'm finding the work pretty hard so any help at all will be useful, thanks.
I need to find the convective heat transfer coefficient, h for petrol using this equation: hD/Lamdaf = 0.37 Re^0.6 so using information I am given: D = 3mm = 0.003m Lamdaf (thermal conductivity) = 0.145 w/m k (M)=viscosity = 0.0006 Pa s u=Velocity of petrol = 19.2 m/s P=density of petrol = 737.22 kg/m^3 Ok so to calculate Re I am using: Dup/(M) = (0.003*19.2*737.22)/0.0006 = 70773.12 Is this correct so far? Then I put this back into the equation and rearrange for h which I get to be =14533.475! w/m^2 k Is this correct because it seems very big to me! if not please can someone point me into the right direction.... thank you very much! 



#2
Nov1309, 06:02 PM

Sci Advisor
PF Gold
P: 2,234

I'm assuming this is flow through a pipe? You seem to have run the numbers correctly, and that convective coefficient doesn't seem out of the realm of possibility to me.
Your Reynold's number does seem a bit low, I caluclated 126,500 but I might have used some fuzzy numbers in there. 



#3
Nov1409, 07:52 AM

P: 15

It says the temperature of petrol is monitored by a thermocouple in the flow, so i'm guessing pipes?
How did you get your Re at that value? Have i used the wrong values to calculate it? Thank you for your help 



#4
Nov1609, 09:04 AM

Sci Advisor
PF Gold
P: 2,234

Convective heat transfer coefficient
I calculated the Reynold's number using the equation:
[tex]Re_{D}=\frac{\rho*u_{m}*D}{\mu}[/tex] where [tex]\rho=719\frac{kg}{m^{3}}[/tex] [tex]u_{m}=19.2\frac{m}{s}[/tex] [tex]D=3mm[/tex] [tex]\mu=3.3*10^{4}Pa*s[/tex] With these inputs the Reynold's number works out to 125,200. 



#5
Nov1609, 09:26 AM

Sci Advisor
PF Gold
P: 2,234

So since I was a third of the way there anyway, I went ahead and tried calculating the convective heat transfer coefficient. The equations I used are out of my heat transfer text book, "Introduction to Heat Transfer" by Incropera and DeWitt.
The answer I got was h= 30,950 W/m^2*K I attached the MathCAD sheet I used to calculate it rather than trying to type it out in Latex. 


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