Performance of a CO2 System with Four Parallel Pipes and Variable Heat Loads

Phase-1 Cooling Meeting, October 29th, 2012

L. Feld, K. Klein, F. Scholz, M. WlochalRWTH Aachen University

History

2

• We started to install a recirculating CO2 cooling system in 2009

• First measurements shown by Jennifer Merz in February 2010

• Measurement problems triggered continuous improvement of the set up

• e.g. installation of evacuated box, more powerful chiller, …

• Jennifer left for industry at the end of 2011

• Spring 2012: Bachelor work by Franziska Scholz; behaviour during parallel operation of four pipes

Katja Klein CO2 Studies with 4 Pipes

Motivation

3

• In the detector, the load on parallel pipes can be different

• by construction

• due to problems, e.g. when power on one part has to be switched off

• This leads to reduction of the flow resistance on pipes with lower load

• Flow will prefer to go through pipes with lower flow resistance

• Pipes with higher heat load will see lower flow danger of dryout

• Can be fixed by adding large flow resistances (capillaries) to each pipe differences in flow resistance insignificant

• We wanted to observe the effect in the lab (without capillaries)

Katja Klein CO2 Studies with 4 Pipes

The CO2 Set-up at Aachen

4Katja Klein CO2 Studies with 4 Pipes

Burst disk

Heat exchanger 1Heat exchanger 2

Flow meter Vacuum box

Detector pipes

CO

2 bottle

Vacuumpump

Expansion vessel

Chiller 1

Chiller 2

pump

Fill level sensor

Drain

The CO2 Set-up at Aachen

5Katja Klein CO2 Studies with 4 Pipes

The CO2 Set-up at Aachen

6Katja Klein CO2 Studies with 4 Pipes

Original version, before insulation• 2 x Unistat 815 from Huber• Gather CO2 pump• Expansion vessel from Swagelok• …

The CO2 Set-up at Aachen

7Katja Klein CO2 Studies with 4 Pipes

• Four stainless steel pipes of 5.25m length

• Inner / outer diameter of 1.7mm / 2.0mm

• 14 termistors on pipes 1 & 2 4 termistors on pipes 3 & 4

• Variabel heat load applied by sending currents through the pipes (up to 400W per pipe)

Vacuum box with 4 pipes

The CO2 Set-up at Aachen

8Katja Klein CO2 Studies with 4 Pipes

Dryout Measurement Method

9Katja Klein CO2 Studies with 4 PipesReduction of flow

Temperatur of thermistors vs. time Temp. of last thermistor vs. flow

150W180W200W220W

• At what vapour fraction x does dryout occur?• The last thermistor on the pipe will see it first

Apply heat load Reduce flow Measure temp. increase on last thermistor Extract Dryout from linear fit

Tem

pera

ture

[°C

]

Tem

pera

ture

[°C

]

Flow [g/min]Measurement point

P = 220W

Dryout on one Pipe

10Katja Klein CO2 Studies with 4 Pipes

76.01

1

pHH

PxDryout

Dryout

WppP tEnvironmen 7.71

0

Extract xDryout (and heat from environment) from linear fit:

D

ryou

t [g/

min

]

Effective heat load [W]

Effective heat load:

Flow on the four Pipes

11Katja Klein CO2 Studies with 4 Pipes

• Differences in flow observed between the 4 pipes (due to manifold?)

• This has to be kept in mind for the following

[g/m

in]

Measurement point

Flow in single pipes for 2800 rev./min Pipe 1Pipe 2Pipe 3Pipe 4

4-Pipe Measurement with x = 0.26

12Katja Klein CO2 Studies with 4 Pipes

• Choose start flow so that x = 0.26 no dryout expected (Note: this is the mean x!)

• Apply 60W on each pipe• Remove consecutively heat load

on pipes 4 3 1• Observations:

• Flow increases due to reduction of flow resistance

• Pressure drop over pipes decreases since pressure built up in pump decreases

• T on pipe decreases when its load is removed, due to T = P/(A) betw. pipe and CO2

• T on other pipes decreases, since p on outlet decreases

• No dryout

Total flow dp over pipes

T on pipe 1 T on pipe 2

T on pipe 3 T on pipe 4

Time

4-Pipe Measurement with x = 0.39

13Katja Klein CO2 Studies with 4 Pipes

• Choose start flow so that mean x close to dryout (determined experimentally)

• Apply 60W on each pipe• Remove consecutively heat load

on pipes 4 3 2• Remember: flow 4 < 3 < 2 < 1• Observations:

• Flow and pressure as before• At the start, signs of dryout on

pipe 4 (increased T fluctuations)• Removing heat loads shifts dryout

signs to pipe with next lower flow • Pipe 4 stabilizes

Total flow dp over pipes

T on pipe 1 T on pipe 2

T on pipe 3 T on pipe 4

Time

4-Pipe Measurement with x = 0.39

14Katja Klein CO2 Studies with 4 Pipes

• Effects depend on the order!• Apply 60W on each pipe• Remove consecutively heat load

on pipes 1 2 3• Remember: flow 4 < 3 < 2 < 1• Observations:

• When heat is removed from pipe 1, all pipes show signs of starting dryout

• Clear dryout on pipe 4 after heat is removed from pipe 2

Total flow dp over pipes

T on pipe 1 T on pipe 2

T on pipe 3 T on pipe 4

Time

4-Pipe Measurement with x = 0.31

15Katja Klein CO2 Studies with 4 Pipes

• In the end mean x was determined where there is just no sign of dryout: xfour = 0.31

• Much lower than in single pipe measurement (xsingle = 0.76)

• Remove consecutively heat load on pipes 4 3 2

Total flow dp over pipes

T on pipe 1 T on pipe 2

T on pipe 3 T on pipe 4

Time

Conclusions

16Katja Klein CO2 Studies with 4 Pipes

• Effect is clearly observed: reduction of load on one pipe can bring others, that were before stable, into dryout

• Dryout occurs at much lower vapour fraction than with a single pipe

• Would have been interesting to repeat measurement with capillaries, but Franziska‘s Bachelor thesis had to be finished before

• Now we concentrate on development of cooling blocks for the Phase-2 outer tracker module, and on DC-DC converter cooling bridges for Phase-1 pixels

Back-up slides

CO2 Phase Diagram

18Katja Klein CO2 Studies with 4 Pipes

Pressure Drops in the Set-up

19Katja Klein CO2 Studies with 4 Pipes

• Pressure is built up by pump, partly dropped by filter

• For a fixed flow resistance, increasing the revolution number increases the flow and also the pressure drop in the pump

• If flow resistance is reduced at a fixed revolution number, the flow increases, and the pressure drop in the pump decreases

• Pressure drop over pump and filter must equal pressure drop over pipes

Dependencies in the System

20Katja Klein CO2 Studies with 4 Pipes