Convective heat transfer coefficient

**Carlo09** · Nov13-09, 05:53 PM

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** · Nov13-09, 07:02 PM

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** · Nov14-09, 08:52 AM

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** · Nov16-09, 10:04 AM

I calculated the Reynold's number using the equation:

$LaTeX Code: Re_{D}=\\frac{\\rho*u_{m}*D}{\\mu}$

where
$LaTeX Code: \\rho=719\\frac{kg}{m^{3}}$
$LaTeX Code: u_{m}=19.2\\frac{m}{s}$

$LaTeX Code: \\mu=3.3*10^{-4}Pa*s$

With these inputs the Reynold's number works out to 125,200.

**Mech_Engineer** · Nov16-09, 10:26 AM

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.