Thermo-dynamical measurements for ATLAS Inner Detector(evaporative cooling system)

Q Science (General);QC Physics;TJ Mechanical engineering and machinery;TK Electrical engineering. Electronics Nuclear engineering

Bitadze, Alexander ; Bates, Richard

University:University of Glasgow

Department:School of Physics and Astronomy

关键词: CERN, LHC, ATLAS, Inner Detector, Evaporative cooling, Hexafluoroethane (R116), C2F6,Octafluoropropane(R218),C3F8, pressure drops, thermal profile, blend mixtures, Sonar analyses, ultrasonic.;   

Others  :  http://theses.gla.ac.uk/5186/7/2013BitadzePhd.pdf

来源: University of Glasgow

【 摘 要 】

During the construction, installation and initial operation of the EvaporativeCooling System for the ATLAS Inner Detector SCT Barrel Sub-detector, someperformance characteristics were observed to be inconsistent with the original designspecifications, therefore the assumptions made in the ATLAS Inner DetectorTDR were revisited. The main concern arose because of unexpected pressuredrops in the piping system from the end of the detector structure to the distributionracks. The author of this theses made a series of measurements of thesepressure drops and the thermal behavior of SCT-Barrel cooling Stave. Tests wereperformed on the installed detector in the pit, and using a specially assembledfull scale replica in the SR1 laboratory at CERN. This test setup has been usedto perform extensive tests of the cooling performance of the system includingmeasurements of pressure drops in different parts of system, studies of the thermalprofile along the stave pipe for different running conditions / parameters andcoolant flow measurements in the system. The pressure drops in the system andthe associated temperatures in the barrel cooling loops have been studied as afunction of the system variables, for example; input liquid pressure, vapour backpressure, module power load and input liquid temperature. Measurements wereperformed with 10, 11, 12, 13 barabs inlet liquid pressure in system, 1.2, 1.6, 2.0,2.5, 3.0, 4.0, 5.0, 6.0 barabs vapour back pressure in system, and 0 W, 3 W, 6 W,9 W, 10.5W power applied per silicon module. The measurements clearly showthat the cooling system can not achieve the design evaporation temperature of -25C in every part of the detector (SCT Barrel loops) in case of 13 barabs nominal inlet liquid pressure, 1.2 barabs minimum possible back pressure and 6W nominalpower per SCT Barrel silicon module and especially at the end of the ATLAS IDoperation period when modules will work on full power of 10.5 W. This will leadto the problem of thermal run-away of the ATLAS SCT, especially near the endof the operational period after significant radiation exposure has occurred. TheLHC luminosity profile, depletion voltage and leakage current values and the totalpower dissipated from the modules were revised. Thermal runaway limits for theATLAS SCT sub-detector were also revised. Results show that coolants evaporationtemperature necessary for the sub-detector's safe operation over the fulllifetime (10 years) is -15C with a safety factor of 2. Laboratory measurementsclearly show that the cooling system can not achieve even this necessary evaporationtemperature of -15C. It is now impossible to make mechanical modificationsto the cooling system, for example; changing the diameter of the cooling pipes, orthe thermal performance of the in-system heat exchanger or reducing the vapourback pressure. It was therefore decided to investigate changes to the cooling fluid and to test mixtures of Hexafluoroethane (R116) C2F6 and Octafluoropropane(R218) C3F8 at differing ratios instead of just pure C3F8 coolant presently used.For this purpose, a new "blending" machine was assembled in the SR1 laboratory,with a new device an "on-line acoustic flow meter and fluorocarbon coolantmixture analyzer" (Sonar Analyzer) attached to it. The Machines were connectedto the already existing laboratory test station and new extensive tests were performedto investigate different proportion of C3F8/C2F6 blends to find the mixtureratio which resulted in the best operational performance as measured by: thetemperature distribution, pressure drops and flow parameters over the system,to ensure best cooling performance of SCT Barrel cooling loops for long termATLAS SCT operation. Measurements were performed with different percentageof C2F6 (1%, 2%, 3%, 5%, 10%, 20%, 25%) coolant in the C3F8/C2F6 mixture,for different power (0 W, 3 W, 6 W, 9 W, 10.5W) applied to dummy modules on the SCT cooling stave, with 13 barabs inlet liquid pressure and for different vapour back pressures (1.2, 1.6, 2.0, 2.5, 3.0 barabs) in the system.Results prove that with 25% of C2F6 in the blend mixture, it is possible tolower the evaporation temperature by ~10C in the case of nominal operationparameters of the system. The ATLAS Inner Detector Evaporative Cooling Systemcan therefore reach the necessary evaporation temperature and therefore canguarantee thermal stability of the SCT, even at the end of the operation period.