Stefan Elbel Pega Hrnjak University of Illinois at Urbana-Champaign

Transcription

1 Experimental Validation of a CO 2 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

2 Presentation Outline Continuation of work presented at the VDA Winter Meeting 2006 Demonstrated that CO 2 is better than R134a for expansion work recovery Introduced numerical tools used to design CO 2 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 2

3 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) 3

4 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) 4

5 Modular Prototype Design Based on Simulation Results Four suction flow ports One motive flow port Motive nozzle Mixing section Diffuser Shim thickness determines size of suction nozzle 5

6 Improved Ejector Design with Integrated High-side Pressure Control After initial proof-of-concept was established, the ejector prototype was modified to incorporate highside pressure control Needle allows variation of motive nozzle throat area Motive flow (2x) Diffuser Giffard s ejector with valve to control motive stream (1864) Suction flow (4x) Adjustable suction nozzle area Mixing section Ejector invented by Henri Giffard in 1859 as feed water pump for steam locomotives From: Kranakis (1982) 6

7 Experimental Setup Gas cooler loop Evaporator loop Target system: CO 2 US Army ECU Compressor and evaporator wind tunnel CO 2 breadboard test facility 7

8 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% 8

9 Comparison to Expansion Valve: 8% more Cooling Capacity with Ejector System Limit: T discharge = 150 o C ε IHX 80% Ejector system As predicted by model: High-side pressure can be optimized for transcritical CO 2 ejector Expansion valve system ε IHX 60% T discharge too high, can t run all conditions with expansion valve For same cooling capacity, ejector system can have lower ε IHX Condition: HD1, Ejector y5.35, HA2.5 OS VLS, 60Hz, x0.9 9

10 Simultaneously, the Ejector System COP Increased by 7% As predicted by model: Similar COP maximizing high-side pressures for identical ε IHX ε IHX 80% Ejector system Ejector highside pressure curve looks like that of conventional expansion valve system Expansion valve system with matched ejector system capacity COP [-] Q matched = 4.8kW COP ~ +18% ~ 70Hz 60Hz Expansion valve Ejector Expansion valve system ε IHX 60% Results extrapolated, because T discharge limit reached at 66Hz with compressor in expansion valve system Condition: HD1, Ejector y5.35, HA2.5 OS VLS, 60Hz, x0.9 10

11 High-side Pressure Control Equation for Ejector System High-side pressure can be used to maximize performance of transcritical CO 2 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 COP [-] T outdoor = low Gas cooler exit pressure [MPa] high P gas cooler out = f( T gas cooler out ) 11

12 Ejector Performance in Terms of Entrainment and Pressure Ratios Instead of defining an overall ejector efficiency, performance is often given in pairs of m suction mass entrainment ratio Φ = m m suction pressure ratio Π s P = P 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 diffuser,out evaporator,out Evaporator metering valve downstream of vapor-liquid separator can be used to balance pressure lift and entrainment Evaporator exit Φ m [-] Π s [-] Quality 72% Quality 94% Superheat 1 o C Superheat 7 o C Higher evaporator exit quality results in larger pressure lifts; ejector can entrain two-phase flow w/o problems 12

13 Ejector Performance Relatively Independent of Ambient Temperature Highest pressure lift at high T ambient & low P high Entrainment and pressure ratios stay within certain bands Ejector performance does not change significantly with ambient temperature Φ m = x 1 diffuser, out 1 Mass entrainment ratio Φm [-] Suction pressure ratio Πs [-] T outdoor = 40 o C 45 o C 50 o C Refrigerant pressure at gas cooler exit [MPa] Pressure ratio Π s Entrainment ratio Φ m Highest entrainment at low T ambient & high P high 13

14 Ejector Performance Investigated at Off-design Conditions Different compressor speeds at 1.2 Pressure ratio Π s Constant evaporator exit quality x out = Constant high-side pressure P = 10MPa Entrainment ratio increases at lower speeds, while pressure ratio decreases Mass entrainment ratio Φm [-] Suction pressure ratio Πs [-] Entrainment ratio Φ m Compressor speed [min -1 ] By allowing variable evaporator x out, metering valve downstream of vapor-liquid separator could be used to keep Φ m and Π s constant 14

15 A Closer Look at Different Geometries Reveals Different Two-phase Shock Patterns From: Bartosiewicz et al. (2005) Certain configurations show more than one sharp pressure increase: indication of shock train 15

16 Transparent Mixing Chamber to Study Shock Wave Formation & Location 250W Tungsten Halogen Light Transparent mixing section High-Speed Camera Phantom V4.3 Resolution: 128 x 512 Frames/second: 7300 Exposure time: 2ms 16

17 First High-Speed Flow Images Reveal Highly Turbulent Mixing CO 2 Air-water CO 2 Two-phase shock (u ~ 150m/s) Flow From: Butrymowicz et al. (2001) Liquid jet surrounded by gas annulus Flow Still image shows finely dispersed froth flow Flow High-speed visualization (played at 5fps) 17

18 New Generation of Ejectors with Greatly Improved Packaging Ejector A Modular Design 25mm Ejector B Integrated Design 18

19 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 P gas cooler out = f(t gas 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 19

20 Thank you for your attention! Acknowledgements We would like to acknowledge the support provided by the following companies and organizations for making this project possible. 20