# Heat Transfer problem

1. Nov 23, 2007

### blitzn

Hi guys,

Hoping to see if I can get a bit of help with a heat transfer equation. Apologies if this is not in the correct forum as this is my first post.

I need to find out how long it will take a bag of 1000L of water (in a 1m3 cubic bag) to heat up to a specific temperature. The bag is placed in a metal frame.

The temperature outside the bag will stay at a constant temperature that is higher than in the bag. The reason that I'm having problems is that my heat transfer knowledge is limited to working out temperature differences for planar surfaces that only deal with conduction.

The water in the bag is not being agitated but will be moving due to natural convection.

Much appreciated if anyone can point me in the right direction i.e. as to what equations to use. Also is it true that I have to treat the vertical and horizantal parts of the bag differently?

Thanks,

2. Nov 26, 2007

### MechanicalMan

Have you studied Lumped Systems Analysis?

3. Nov 27, 2007

### blitzn

No I haven't. Would you say that is where the answer lies?

4. Nov 27, 2007

### MechanicalMan

Well, whenever I'm trying to find how long it would take some to heat up or cool down, I always think of Lumped Systems Analysis, as there is a time rate term in the equations.

Essentially, the energy balance breaks down into {Change in Internal Energy During Time Interval dt} = {rate of heat flow to or from ambient during time interval dt}.

As long as you know how to properly formulate the energy balance, you should be able to solve for your time as required.