Regime of unsteady flow over heat sink

For my master thesis I'm simulating the cooling of an 2.5D microchip (2 dies next to each other on an interposer (slice of silicium with interconnections)). So I want to calculate the flow over the chip and the attached ducted straight fin heat sink. The fluid can only flow through the fins of the heat sink. The inlet velocity is over 2 m/s so that there is no influence of natural convection. The flow can be seen as a combination of a forward facing step and a backward facing step.

I calculated the Reynolds number between the fins and it was 1075. So the flow should be laminar (or at least that is what i think). Because of the high velocity the flow because unsteady behind the heat sink (I can only calculate the flow with the transient option in fluent). I assumed that it was an unsteady laminar flow. But when I look to the velocity in a point, it isn't periodic in time. So the flow behind the heatsink could be in the transitional or turbulent regime. But I'm not sure of that.

Is there a way to check the regime behind the heatsink?

• Do you have images of the heat sink geometry and simulation results? If I had to guess you're heat sink doesn't smoothly transition into free stream again so you end up with some separation at the tailing edge of the fins. Feb 20, 2014 at 18:45
• This seems like more of a question of fluid dynamics than electronics. Feb 20, 2014 at 18:53
• @Spehro - indeed it is, but it's certainly a valid concern for electronics design; a good answer would be a useful resource. Feb 20, 2014 at 18:59
• @BrianDrummond Indeed. I wonder if he might have a better chance of getting a good answer elsewhere though. Feb 20, 2014 at 19:51
• The heat sink is 28 by 28mm. There are 10 fins with a height of 10 mm. The cross section of the channel between the fins is 2x10mm. The entrance and exit duct has a width of 28 mm and a height of 15.24 mm. The complete flow has to go through the fins. Feb 22, 2014 at 14:32

Having done both CFD simulations and simplified closed-form calculations of similar problems, I prefer closed-form. My approach would be to use the fin geometry and flow rate to calculate an equivalent film coefficient $h$ for a flat plate the same size as the base of the heat sink. Incropera and DeWitt's intro to heat transfer book is a good source for the formulas for that. Then use that as a boundary condition for conductive heat transfer from the dies to the heat sink surface. A steady-state heat conduction problem is much easier to set up, and faster to solve, than a transient CFD problem would ever be. At a minimum, it will be a good check against the simulation.