1. Multi-Microhannel Cooling Model Silicon Micro-Cooling Element to be applied on a pixel detector of CERN (ALICE) / Parametric Study Footprint area: (6.0 x 1.5) cm 2 Heat flux: 0.5 W/cm 2 August, 2012 Laboratory of Heat and Mass Transfer Faculty of Engineering Science Ecole Polytechnique Fédérale de Lausanne Lausanne, Switzerland

2. Actual design Cooling Element Material: Silicon Fin height: 200  m Fin width: 300  m Channel width: 200  m Base thickness: 0.180 mm 1 inlet and 1 outlet)

3. Inputs: Working fluid: HFO1234ze and C4F10 Saturation temp.: 20 o C Inlet subcooling: 0 o C Mass flux: 50 to 200 kg/m 2 s (steps of 50) Cooling Element Material: Silicon Fin height: 75 to 150  m (steps of 25) Fin width: 75  m Channel width: 75 to 150  m (steps of 25) Base thickness: 0.180 mm 1 inlet and 1 outlet) Footprint area Width: 15 mm Length: 60 mm uniform heat flux 0.5 W/cm 2 Simulation inputs The heat source is assumed to be a plane Assumptions for simulation: 1)Micro-cooling element of silicon with base thickness of 0.170 mm 2)Electrical element of silicon with thickness of 0.01 mm

4. 1 st Case: Only one side The actual simulation code considers 2D conduction along the channel. I.e. it is still not possible to evaluate effects of lateral conduction. An upgrade is in progress and soon we can do it too. 1 st case: Simulations will consider a footprint area of 60 mm per 2.5 mm

5. 1 st Case (fin height and channel width effect) HFO1234zeFin width: 75  mMass flux: 50 kg/m 2 s Smaller fin height and channel width showed a lower mean junction temperature Smaller channel width and higher fin height showed a lower peak junction temperature

6. 1 st Case HFO1234zeFin width: 75  mMass flux: 50 kg/m 2 s Higher fin height and channel width showed a lower outlet vapor quality Higher fin height and channel width showed a higher base critical heat flux

7. 1 st Case HFO1234zeFin width: 75  mMass flux: 50 kg/m 2 s Higher fin height and channel width showed a lower pressure drop (minimum of 0.07bar) The best configuration could be with a channel width of 100  m and a fin height of 120  m Pressure drop [bar]0.15 Outlet vapor quality [-]0.53 Critical vapor quality [-]> 1 Total heat flux[W/cm 2 ]0.5 Critical heat flux[W/cm 2 ]2.0 Maximum Junction Temp [ o C]21.9

8. 1 st Case (mass flux effect) HFO1234zeFin width and height: 75 and 120  m Channel width: 100  m The best mass flux could be 100 kg/m 2 s (critical heat flux about 6 times higher than the actual heat flux) Mass flux [kg/m 2 s]50100150200 Pressure drop [bar]0.150.210.240.27 Outlet vapor quality [-]0.530.270.190.15 Critical vapor quality [-]> 1 Total heat flux[W/cm 2 ]0.5 Critical heat flux[W/cm 2 ]2.03.34.45.4 Maximum Junction Temp [ o C]21.921.521.3

9. 1 st Case (HFO1234ze vs. C4F10) Fin width and height: 75 and 120  m Channel width: 100  m Mass flux: 100 kg/m 2 s HFO1234ze shows better thermo- hydrodynamic behaviour Working fluidHFO1234zeC4F10 Pressure drop [bar]0.210.31 Outlet vapor quality [-]0.270.54 Critical vapor quality [-]>1 Total heat flux[W/cm 2 ]0.5 Critical heat flux[W/cm 2 ]3.31.8 Maximum Junction Temp [ o C]21.522.2

10. 1 st Case (actual vs. new design) Working fluid: HFO1234ze Fin width and height: 75 and 120  m (new design) vs. 300 and 200  m (actual) Channel width: 100  m (new design) vs. 200  m (actual) Mass flux: 100 kg/m 2 s New design showed a lower maximum junction temperature (safer) and is much more compact DesignNewActual Pressure drop [bar]0.210.05 Outlet vapor quality [-]0.270.22 Critical vapor quality [-]>1 Total heat flux[W/cm 2 ]0.5 Critical heat flux[W/cm 2 ]3.33.4 Maximum Junction Temp [ o C]21.522.5 Simulation code considered 5 and 14 channels respectively for actual and new designs

11. 2 nd Case (full of fins) Working fluid: HFO1234ze Evaporating temperature: 20 o C Fin width and height: 75 and 120  m (new design) vs. 300 and 200  m (actual) 86 channels for new design and 30 channels for “actual” Channel width: 100  m (new design) vs. 200  m (actual) Mass flux: 100 kg/m 2 s Simulations will consider nonuniform heat flux. I.e. 0.5 W/cm 2 on the borders (2.5 mm width ) and 0.1 W/cm 2 in the middle (10 mm width) 2 nd case: Entire footprint area (60 mm per 15 mm) fully populated of fins.

12. 2 nd Case (full of fins) It was simulated only half of the micro-cooling element (assumed symmetry) New design zone 1: 2.5mm width zone 2: 5 mm width) Mass flux zone 1: 27 kg/m 2 s Mass flux zone 2: 139 kg/m 2 s

13. 2 nd Case (full of fins) “Actual” design zone 1: 2.5mm width zone 2: 5 mm width) Mass flux zone 1: 25 kg/m 2 s Mass flux zone 2: 150 kg/m 2 s The maximum junction temperature is higher than that of the new design 24.1 o C against 22.4 o C

14. General Comments New simulations still can be done to better adjust the micro-cooling element in the reality of the CERN’s pixel detector (ALICE). I.e. the results here presented had the context of mainly show in what stage the actual design of CERN is when compared with a new potential design, and also the potentiality of LTCM code.