# Air speed in car radiator -- Heat Exchange calculations

1. Aug 13, 2015

### r4m0n0

Hello, this is my first post. This question is related to my final thesis.

I'm calculating the heat exchange area of a radiator through heat exchange equations:

Q = U*A*LMTD

I have as data Q, and LMTD (this one through trivial calculations)

In order to calculate U, I need the coefficients for external and internal (both tubes and fins) convection.
Internal convection coefficient is quite easy to calculate.
My doubts come when trying to calculate the external convection coefficient. As you may know to calculate the coefficient you need the Reynolds number (amongst other things).
Reynolds depends on the fluid speed (in this case air speed) and in order to calculate it I thought of the following equation:

v=(V_dot_air)/(no.of tubes*area of tubes) --->

The thing is... this calculation doesn't take in account the fins of the radiator, thus not having an accurate value for velocity.
But if I took in account the area of the fins into the equation, the velocity of air would be very VERY slow.

Can anyone suggest a better way to calculate air speed through a car radiator?

Thanks in advance :)

(Image enclosed: a simplification of a car radiator)

2. Aug 13, 2015

### Nidum

It's forced convection not natural convection .

Mass flow and velocity of cooling air is primarily determined by the cooling fan and possibly by forward velocity of vehicle depending on ducting arrangement .

3. Aug 13, 2015

### Staff: Mentor

If you have data on Q and LMTD, then you have enough to calculate U experimentally. So why are you trying to calculate it from the flows?

Chet

4. Aug 14, 2015

### r4m0n0

@Nidum:

Yes, it is forced convection, I never said otherwise. I'm calculating the air velocity "from scratch" so in order to know how the ducting arrangement and the cooling fan type/size I must calculate before the air velocity.

@Chestermiller:

Calculate U experimentally? Can you give me more insight on that? I have no physical model, everything is theorical.

5. Aug 14, 2015

### Staff: Mentor

Oh. You didn't say that. OK, how do you know the coolant flow rate to get the heat transfer coefficient on the tube side?

There are definitely heat transfer correlations available for air flow across banks of finned tubes. But you need to do the research yourself to find them.

Chet