Learning Forum

[erling.gjeset]

Hello,

I'm currently simulating what, at least in theory, should be a fairly simple heat transfer problem, and while everything seems to be running smoothly without errors, I can't get the energy equation that Fluent is giving me to add up. There's probably just a term I'm missing in my own calculations, but I'd appreciate any help from you to identify what this is.

I have two 2D heat generating solid bodies encapsulated by a flow volume with a 1 mm slot inlet/outlet (see attached DM screenshot & mesh). The cooling fluid is oil based, and the properties are set to vary with temperature. Otherwise, the simulation is static and, according to my Re calculations, laminar.

I then run the simulation and it converges nicely after some 260+ iterations, with no console errors or warnings, and the energy and mass balance within reasonable tolerances. Attached is a readout from the console after I ran my simulation.

However, this is where the problem arises: The first time I received the results, I found it curious that the temperature difference between inlet and outlet was so low, so I wanted to do a simple control volume calculation to see if the energy balance was respected.

Since the temperature difference is so low, I have assumed that the fluid properties are constant for this CV calculation.

The heat generated from both the bodies amount to 1081 W.

The mass flow rate is an input parameter for my setup, so with this, the inlet temperature, the simulated outlet temperature and the cp, we have:
Q = m cp dT ~= 858 W

Using the average enthalpy and mass flow rate given from the simulation, we have:
Q = m dh ~=844 W (which is slightly lower, but within reason)

Neither of these, however, account for the remaining approx. 230 W generated by the solids.

The most frustrating thing is that when Fluent calculates the Total heat transfer rate (as shown in the console readout), the energy balance seems to be respected without issue, but I can't for the life of me figure out how the program arrives at the 130+ kW at the inlet/outlet, since this doesn't correspond to enthalpy x mass flow rate as I would expect. I've tried looking at the documentation, but it doesn't seem to give me much more of a clue.

Could anyone here help me find out what I'm missing or doing wrong?

[Rob]

The inlet/outlet flux is against a reference, I think it's 298.15K. Also use mass weighted on flow boundaries for anything for the flow (velocity, temperature etc).