Limits of CO 2 Cooling A clear explanation that an yone can understand (the goal )

1. Limits of CO2 CoolingA clear explanation thatanyonecan understand (the goal) Hans Postema & Joao Noite PH-CMX-DS

2. Introduction • The Production Readiness Review for the CO2 cooling for the Pixel Phase 1 upgrade is foreseen for the 9th 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-DS

3. Limits of CO2 Cooling • Evaporative CO2 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-DS

4. Limits of CO2 Cooling • Cooling capacity disappears before all the liquid is evaporated. • Mist flow: the wall is dry, mist around the center of tube BUBBLY INTERMITTENT ANNULAR DRYOUT MIST Hans Postema & Joao Noite PH-CMX-DS

5. Limits of CO2 Cooling BUBBLY INTERMITTENT 3.0g/s ANNULAR B A D DRYOUT MIST 1.7g/s M I Dryout line • The dryout line represents the points of maximum heat transfer coefficient. Hans Postema & Joao Noite PH-CMX-DS

6. Limits of CO2 Cooling • 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. Hans Postema & Joao Noite PH-CMX-DS

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

8. BPix Critical Layers @ 15°C Max Power “Old” • (for comparison, notpartoftherequirements) +Z Layer #2 -Z Layer #1 NO safety margin Qexit = 63% Qdryout = 26% Deep inside dry-out region NO safety margin Qexit = 75% Qdryout = 40% Deep inside dry-out region Hans Postema & Joao Noite PH-CMX-DS

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

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

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

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-DS

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-DS