Convective heat transfer coefficient

Carlo09

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!

Mech_Engineer

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.

Carlo09

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

Mech_Engineer

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.

Mech_Engineer

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.

Attached Files

Mathcad - Gasoline flowing in a tube.pdf (18.4 KB, 32 views)