[modorous]

In Fluent, I am currently working on a heat transfer case involving two phases: coolant oil and air. The simulation includes the spraying of oil onto a metal geometry using a jet nozzle. The metal geometry has a heat source of 24 W, causing it to heat up to an average temperature of 90°C. The coolant oil is injected into the domain at a temperature of 40°C, and the surrounding area is filled with air at 20°C. To replicate the lab experimental setup accurately, I have extended the model to encompass a very large surrounding area around the metal geometry. The intention behind this is to account for the effects of natural convection cooling.

For the simulation, I have set up the following boundary conditions: a velocity inlet with a volume fraction of 1 for the oil, a pressure outlet with a volume fraction of 1 for the backflow air at 20°C. all other boundaries are adiabatic.

The simulation is conducted in a steady-state manner, and I have utilized the SST k-omega turbulence model. The multiphase model is VOF Implicit with implicit body force, VOF boundary is dispersed. The mesh incorporates 10 boundary layers, and all walls have a y+ value below 4, while the walls of the metal geometry have a y+ value below 1.

Currently, when calculating the total heat transfer imbalance, which considers the contributions from the inlet, outlet, and energy source, I am obtaining a value of -11W. I am guessing this is because the cooling effect from the large surrounding air domain is not calculated here.

My question is whether my assumption is correct, and if so, how can I incorporate the natural convection heat transfer from the surrounding air into the energy transfer imbalance calculation?

[CFDKareem]

Originally Posted by modorous

In Fluent, I am currently working on a heat transfer case involving two phases: coolant oil and air. The simulation includes the spraying of oil onto a metal geometry using a jet nozzle. The metal geometry has a heat source of 24 W, causing it to heat up to an average temperature of 90°C. The coolant oil is injected into the domain at a temperature of 40°C, and the surrounding area is filled with air at 20°C. To replicate the lab experimental setup accurately, I have extended the model to encompass a very large surrounding area around the metal geometry. The intention behind this is to account for the effects of natural convection cooling.

For the simulation, I have set up the following boundary conditions: a velocity inlet with a volume fraction of 1 for the oil, a pressure outlet with a volume fraction of 1 for the backflow air at 20°C. all other boundaries are adiabatic.

The simulation is conducted in a steady-state manner, and I have utilized the SST k-omega turbulence model. The multiphase model is VOF Implicit with implicit body force, VOF boundary is dispersed. The mesh incorporates 10 boundary layers, and all walls have a y+ value below 4, while the walls of the metal geometry have a y+ value below 1.

Currently, when calculating the total heat transfer imbalance, which considers the contributions from the inlet, outlet, and energy source, I am obtaining a value of -11W. I am guessing this is because the cooling effect from the large surrounding air domain is not calculated here.

My question is whether my assumption is correct, and if so, how can I incorporate the natural convection heat transfer from the surrounding air into the energy transfer imbalance calculation?

The heat flux calculation takes into account the heat transfer from each boundary. If you imagine your domain as a simple control volume, then the amount of heat going in should equal the amount leaving (assuming steady-state). Due to numerical errors in CFD there will always be slight imbalance in the heat flux. However, it should tend to 0, not something high, like 11W.

The most common reason I have found for this is iteration error i.e. you are not letting your simulation run for enough iterations. Change your residual convergence criteria to something much lower (1e-6 at least) and let the simulation continue calculating. You should see the heat flux imbalance decrease.

[modorous]

Originally Posted by CFDKareem

The heat flux calculation takes into account the heat transfer from each boundary. If you imagine your domain as a simple control volume, then the amount of heat going in should equal the amount leaving (assuming steady-state). Due to numerical errors in CFD there will always be slight imbalance in the heat flux. However, it should tend to 0, not something high, like 11W.

The most common reason I have found for this is iteration error i.e. you are not letting your simulation run for enough iterations. Change your residual convergence criteria to something much lower (1e-6 at least) and let the simulation continue calculating. You should see the heat flux imbalance decrease.

Thanks a lot. Your advice helped. After 6000 iterations heat transfer rate was reduced. But this is too computationally expensive. So I reduced the domain size and it worked. Thanks again.