Cooling of GEM detector CFD _GEM 2012/03/06 E. Da RivaCFD _GEM1

1. Geometry and material properties E. Da RivaCFD _GEM2

Geometry and material properties Copper, k = 400 W m -1 K -1 PCB, FR-4, k = 0.25 W m -1 K -1 E. Da Riva3CFD _GEM A single chip is considered in the CFD simulations

Geometry and material properties Soldered = perfect thermal contact Gap-pad k = 2 W m -1 K -1 Chip, silicon, q = 1 W Water, T = 18°C All surfaces are adiabatic but the inner surface of the pipe E. Da Riva4CFD _GEM

Geometry and material properties Connector k = 0.27 W m -1 K -1 Volume including the GEM sensors is not taken into account. This surface is assumed as adiabatic. E. Da Riva5CFD _GEM

2. Schematic and estimations E. Da RivaCFD _GEM6

E. Da RivaCFD _GEM7 Schematic of the problem 18°C Water heat transfer coefficient T Cu strip TT Gap-padPCB T chip q = 1W T GEM PCB, connectors, etc. q = 0 W  The temperature of the GEM sensors will be the same as the PCB.  In order to reduce the sensor temperature, the thermal resistances must be reduced: -) higher water flow rate -> higher heat transfer coefficient -) shorter Cu strip or larger “cross section” -) Thinner gap-pad or higher thermal conductivity -) good thermal contact Cu strip/gap-pad and gap-pad/PCB

Water in the pipe E. Da RivaCFD _GEM8 Di [mm] Vel. [ms -1 ] Flow rate [kg s -1 ] Temp. rise [K] Re [-] HTC [Wm -2 K -1 ] Δp (2 m pipe) [bar] * * Heat load per VFAT chip = 1 W # VFAT chips = 30 Total heat load = 1 W * W = 34W * Erosion limit for copper pipe  Low water temperature rise is good to achieve a uniform sensor temperature.  The diameter of the pipe may be reduced if needed because of geometrical constraints.

3. Simulation results E. Da RivaCFD _GEM9

E. Da RivaCFD _GEM10 With water at 18°C, the temperature of the GEM sensor is expected to be around 60°C.

E. Da RivaCFD _GEM11 The dominant thermal resistance is due to the low PCB thermal conductivity and the small interface area between chip and PCB

E. Da RivaCFD _GEM12 The resistance due to water heat transfer coefficient is negligible (wall temperature very close to water temperature)

E. Da RivaCFD _GEM13 Schematic of the problem 2 18°C Water heat transfer coefficient T Cu strip TT Gap-padPCB T chip T GEM PCB, connectors, etc. Negligible~ 10 KDominant thermal resistance Increase heat contact area -) Increase Cu thickness -) Increase Cu width

E. Da RivaCFD _GEM14 Conclusions The dominant thermal resistance is between the chip and the PCB Direct thermal contact between the copper strip and the chip may be needed (putting the chip above the PCB instead of below?) A further step may be increasing the thickness and the width of the copper strip The diameter of the pipe may be slightly reduced if needed.