Experimental Validation of a CO2 Prototype Ejector with Integrated High-Side Pressure Control

Stefan Elbel (elbel@uiuc.edu), Pega Hrnjak (pega@uiuc.edu)

University of Illinois at Urbana-Champaign

Saalfelden, February 14-15, 2007

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Elbel & Hrnjak, UIUC Transcritical CO2 Ejector SystemsVDA Winter Meeting 2007

Presentation Outline

Continuation of work presented at the VDA Winter Meeting 2006• Demonstrated that CO2 is better than R134a for expansion work recovery

• Introduced numerical tools used to design CO2 ejector prototypes

• Showed initial experimental ejector data

So what’s new this year?• Ejector with high-side pressure control: Successfully tested in widespread test matrix

• Control: Derivation of practical strategies for efficient ejector operation

• Ejector efficiency: Performance study at different operating conditions including off-design

• Ejector design: Two-phase shock wave visualization and static pressure distributions

• New generation of ejectors: Improved packaging and performance comparison

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Elbel & Hrnjak, UIUC Transcritical CO2 Ejector SystemsVDA Winter Meeting 2007

How Does the Ejector Work?• Ejector works like a pump without moving parts

• High-energetic motive stream is accelerated in motive nozzle (A); static pressure low, kinetic energy very high

• Suction flow is pre-accelerated in suction nozzle to reduce mixing losses caused by shearing (B)

• Lower-energetic suction stream is entrained and accelerated by momentum transfer from the motive to the suction stream; mixing causes two velocities to equalize, pressure rise in mixing chamber (C) (possibility for shocks)

• Subsonic diffuser converts remainder of kinetic energy into static pressure (D)

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Elbel & Hrnjak, UIUC Transcritical CO2 Ejector SystemsVDA Winter Meeting 2007

Ejector for Expansion Work Recovery• Reduced throttling loss: approaching isentropic expansion• Expansion work pre-compresses evaporator flow• Two effects increase COP: +Q = -W• Works best at high ambient temperatures• Secondary benefits

• Higher compressor efficiency• Reduced evaporator pressure drop• Improved evaporator distribution

. .

N.H. Gay: US 1,836,318 (1931)

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Elbel & Hrnjak, UIUC Transcritical CO2 Ejector SystemsVDA Winter Meeting 2007

Modular Prototype Design Based on Simulation Results

Motive nozzle

Mixing section

Shim thickness determines size of suction nozzle

Four suction flow portsOne motive flow port

Diffuser

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Elbel & Hrnjak, UIUC Transcritical CO2 Ejector SystemsVDA Winter Meeting 2007

Improved Ejector Design with Integrated High-side Pressure Control

Mixing section

Diffuser

Adjustable suction nozzle area

Motive flow (2x)

Suction flow (4x)

Needle allows variation of

motive nozzle throat area

After initial proof-of-concept was established, the ejector prototype was modified to incorporate high-side pressure control

From: Kranakis (1982)

Giffard’s ejector with valve to control motive stream (1864)

Ejector invented by Henri Giffard in 1859 as feed water pump for steam locomotives

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Elbel & Hrnjak, UIUC Transcritical CO2 Ejector SystemsVDA Winter Meeting 2007

Experimental Setup

Target system: CO2 US Army ECU

Compressor and evaporator wind tunnel CO2 breadboard test facility

Evaporator loop

Gas cooler loop

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Elbel & Hrnjak, UIUC Transcritical CO2 Ejector SystemsVDA Winter Meeting 2007

Experimental Breadboard Facility• Can be easily switched between expansion valve and ejector system• Air flow rates, temperatures and humidities adjustable over wide range of

operating conditions• IHX effectiveness adjustable via bypass• Cooling capacity determined by two independent energy balances

• Air-side: Flow nozzle, temperature, pressure, and humidity measurements• Refrigerant-side: Mass flow meter, temperature, and pressure measurements • COP: Watt transducer to determine compressor power

• Balances typically agree within ± 5%

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Elbel & Hrnjak, UIUC Transcritical CO2 Ejector SystemsVDA Winter Meeting 2007

Comparison to Expansion Valve: 8% more Cooling Capacity with Ejector System

Condition:HD1, Ejector y5.35, HA2.5 OS VLS, 60Hz, x0.9

εIHX 60%

εIHX 80%

Ejector system

Expansion valve system

As predicted by model: High-side pressure can be optimized for transcritical

CO2 ejector

For same cooling capacity, ejector system can have lower εIHX

Tdischarge too high, can’t run all

conditions with expansion valve

Limit: Tdischarge = 150oC

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Elbel & Hrnjak, UIUC Transcritical CO2 Ejector SystemsVDA Winter Meeting 2007

Simultaneously, the Ejector System COP Increased by 7%

Condition:HD1, Ejector y5.35, HA2.5 OS VLS, 60Hz, x0.9

εIHX 60%

εIHX 80%

Ejector system

Expansion valve system

Ejector high-side pressure curve looks like that of

conventional expansion

valve system

0.80

0.85

0.90

0.95

1.00

1.05

1.10

Expansion valve Ejector

CO

P [-]

Expansion valve system with matched ejector system

capacity

~ 70Hz 60Hz

COP ~ +18%

Results extrapolated, because Tdischarge limit reached at 66Hz with compressor in expansion

valve system

Qmatched = 4.8kW.

As predicted by model: Similar COP maximizing high-side pressures for

identical εIHX

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Elbel & Hrnjak, UIUC Transcritical CO2 Ejector SystemsVDA Winter Meeting 2007

Gas cooler exit pressure [MPa]

CO

P [-]

High-side Pressure Control Equation for Transcritical CO2 Ejector System

• High-side pressure can be used to maximize performance of transcritical CO2 ejector systems

• Run different high-side pressures at different ambient temperatures

• Connect COP peaks by linearly relating the COP maximizing high-side pressure to the refrigerant temperature at the gas cooler exit

Toutdoor = low

high

Pgas cooler out = f( Tgas cooler out)

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Elbel & Hrnjak, UIUC Transcritical CO2 Ejector SystemsVDA Winter Meeting 2007

Ejector Performance in Terms of Entrainment and Pressure Ratios

• Instead of defining an overall ejector efficiency, performance is often given in pairs of

• mass entrainment ratio

• suction pressure ratio

• Trade-off: a given amount of kinetic energy contained in driving flow can pump a large suction flow across a small pressure difference or vice versa

motive

suctionm m

m=Φ

out,evaporator

out,diffusers P

P=Π

Evaporator exit Φm [-] Πs [-] Quality 72% 0.67 1.064 Quality 94% 0.52 1.086

Superheat 1oC 0.49 1.092

Superheat 7oC 0.44 1.097

Higher evaporator exit quality results in larger pressure lifts; ejector can

entrain two-phase flow w/o problems

Evaporator metering valve downstream of vapor-liquid

separator can be used to balance pressure lift and

entrainment

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Elbel & Hrnjak, UIUC Transcritical CO2 Ejector SystemsVDA Winter Meeting 2007

Ejector Performance Relatively Independent of Ambient Temperature

• Entrainment and pressure ratios stay within certain bands

• Ejector performance does not change significantly with ambient temperature

•0.0

0.2

0.4

0.6

0.8

1.0

1.2

9.5 10.0 10.5 11.0 11.5 12.0

Refrigerant pressure at gas cooler exit [MPa]

Mas

s en

trai

nmen

t rat

io Φ

m [-

]Su

ctio

n pr

essu

re ra

tio Π

s [-]

Toutdoor = 40oC 45oC 50oC

Entrainment ratio Φm

Pressure ratio Πs

1x

1 out diffuser,

m −=Φ

Highest pressure lift at high Tambient & low Phigh

Highest entrainment at low Tambient & high Phigh

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Elbel & Hrnjak, UIUC Transcritical CO2 Ejector SystemsVDA Winter Meeting 2007

Ejector Performance Investigated at Off-design Conditions

0.0

0.2

0.4

0.6

0.8

1.0

1.2

800 1000 1200 1400 1600 1800 2000Compressor speed [min-1]

Mas

s en

trai

nmen

t rat

io Φ

m [-

]Su

ctio

n pr

essu

re ra

tio Π

s [-]

Different compressor speeds at• Constant evaporator exit quality

xout = 0.95• Constant high-side pressure

P = 10MPa

Entrainment ratio Φm

Pressure ratio Πs

Entrainment ratio increases at lower speeds, while

pressure ratio decreases

By allowing variable evaporator xout, metering valve downstream of vapor-liquid separator could be used to keep Φm and Πs constant

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Elbel & Hrnjak, UIUC Transcritical CO2 Ejector SystemsVDA Winter Meeting 2007

From: Bartosiewicz et al. (2005)

A Closer Look at Different Geometries Reveals Different Two-phase Shock Patterns

Certain configurations show more than one sharp pressure increase: indication of ‘shock train’

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Elbel & Hrnjak, UIUC Transcritical CO2 Ejector SystemsVDA Winter Meeting 2007

Transparent Mixing Chamber to Study Shock Wave Formation & Location

High-Speed Camera Phantom V4.3

Resolution: 128 x 512

Frames/second: 7300

Exposure time: 2ms

250W Tungsten Halogen Light

Transparent mixing section

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Elbel & Hrnjak, UIUC Transcritical CO2 Ejector SystemsVDA Winter Meeting 2007

First High-Speed Flow Images Reveal Highly Turbulent Mixing

Still image shows finely dispersed froth flow High-speed visualization

(played at 5fps)

Flow

CO2

From: Butrymowicz et al. (2001)

Liquid jet surrounded by

gas annulus

Air-water

Two-phase shock

Flow

(u ~ 150m/s)

CO2

Flow

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Elbel & Hrnjak, UIUC Transcritical CO2 Ejector SystemsVDA Winter Meeting 2007

New Generation of Ejectors with Greatly Improved Packaging

Ejector A – Modular Design

25mmEjector B – Integrated Design

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Elbel & Hrnjak, UIUC Transcritical CO2 Ejector SystemsVDA Winter Meeting 2007

Conclusions• Ejector improved COP by up to 18% over system with expansion valve

• Ejector system can have less effective IHX (smaller / lighter / cheaper) without compromising performance

• Control strategies for ejector systems• High-side pressure control similar to that of conventional system with expansion valve• Pgas cooler out = f(Tgas cooler out)• Low cost ejector could have fixed nozzle size or spring-loaded bypass• Evaporator metering valve can be used to adjust trade-off between pressure lift and

mass entrainment to desired values (higher Q vs. higher COP)• Fixed orifice could be used instead of evaporator metering valve

• Ejector performance relatively independent of ambient conditions

• Two-phase shock waves can significantly reduce ejector efficiency

• New generation of integrated ejectors with greatly improved packaging have been successfully tested

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Elbel & Hrnjak, UIUC Transcritical CO2 Ejector SystemsVDA Winter Meeting 2007

Thank you for your attention!

AcknowledgementsWe would like to acknowledge the support provided by the following

companies and organizations for making this project possible.