1. Limits of CO 2 Cooling A clear explanation that anyone can understand (the goal) Hans Postema & Joao Noite PH-CMX-DS1

2. Introduction The Production Readiness Review for the CO2 cooling for the Pixel Phase 1 upgrade is foreseen for the 9 th of May. This presentation is aimed at the referees who do not necessarily have extensive experience in 2-phase cooling. Please let me know if you see anything in this presentation that could be explained clearer or simpler. Hans Postema & Joao Noite PH-CMX-DS2

3. Limits of CO 2 Cooling Evaporative CO 2 cooling is complex. Tube is fixed as 1.4mm ID due to space limitation inside the detector. Dry-out phenomena limits the maximum power that can successfully be extracted. Dry-out can lead to high temperatures at the detector. Hans Postema & Joao Noite PH-CMX-DS3

4. Limits of CO 2 Cooling Hans Postema & Joao Noite PH-CMX-DS BUBBLY INTERMITTENT ANNULAR DRYOUT MIST Cooling capacity disappears before all the liquid is evaporated. Mist flow: the wall is dry, mist around the center of tube 4

5. Limits of CO 2 Cooling Hans Postema & Joao Noite PH-CMX-DS B I A M 1.7g/s 3.0g/s D BUBBLY INTERMITTENT ANNULAR DRYOUT MIST Dryout line The dryout line represents the points of maximum heat transfer coefficient. 5

6. Limits of CO 2 Cooling Hans Postema & Joao Noite PH-CMX-DS At -20°C the heat transfer coefficient decreases extremely rapidly at powers above the dryout line. At 15°C dryout starts at a much lower power but the heat transfer coefficient decreases more gradually above the dryout line. 6

7. Hans Postema & Joao Noite PH-CMX-DS BPix Critical Layers @ -20°C Max Power “Old” Safety margin OK Qexit = 46% Qdryout = 55% Very low safety margin Qexit = 38% Qdryout = 41% +Z Layer #2-Z Layer #1 7

8. Hans Postema & Joao Noite PH-CMX-DS BPix Critical Layers @ 15°C Max Power “Old” (for comparison, not part of the requirements) NO safety margin Qexit = 75% Qdryout = 40% Deep inside dry-out region +Z Layer #2-Z Layer #1 NO safety margin Qexit = 63% Qdryout = 26% Deep inside dry-out region 8

9. Hans Postema & Joao Noite PH-CMX-DS BPix Critical Layers @ 15°C Standby “Old” NO safety margin Qexit = 55% Qdryout = 44% 11% inside dry-out region +Z Layer #2-Z Layer #1 NO safety margin Qexit = 37% Qdryout = 33% 4% inside dry-out region 9

10. Hans Postema & Joao Noite PH-CMX-DS BPix Critical Layers @ 15°C Standby “Old” B I A DM 1.6g/s 1.8g/s 2g/s 2.2g/s 2.4g/s 2.6g/s 2.8g/s 3g/s 3.2g/s 3.4g/s 3.6g/s Counter intuitive: More flow does not always solve the problem. 10

11. Hans Postema & Joao Noite PH-CMX-DS BPix Numbers 11

12. Conclusions Assumption: – Values for power include sufficient safety margin, a safety margin in the cooling system is therefore not needed At -20°C, max power operation is close to the dry- out line, is this safe enough? At +15°C, standby power operation is deep inside the dry-out region, is this safe enough? Increasing the flow might not provide a solution Hans Postema & Joao Noite PH-CMX-DS12

13. Afterword Building the lightest pixel detector in the history of high energy physics is an admirable quest that I am happy to participate in. Extreme performance is generally achieved using the absolute minimum safety factors. Extremely low safety factors can only be used safely when experience is at a high level and unforeseen issues are unlikely to occur. In how far does the above apply to our project? Hans Postema & Joao Noite PH-CMX-DS13