Dynamic Simulations of CO₂
Cooling Systems
Viren Bhanot
Scope
• Component Models
• Dynamic two-phase flow modelling and experimental analysis
• Predict dynamic behaviour: Startup, shutdown, load change, set-point
change, warm operation
• Evaluate control strategies
• Operator training
• Virtual commissioning
…all models are wrong, but some are useful
2PACL
Long Micro and
Mini Channels
2P Flow
Distributors
Detector
Accumulator
Expansion Valve /
Capillary Tubes
Cooling coil
Long Transfer Lines
Large Volume
Internal Heat
Exchanger
Heater
Dry Out
Single Port
Pump
Compact PHX
Condenser
Radiator
Compressor
CORA
•
•
•
•
Exclusively for me
Low temperatures (< -35°C)
2 kW
P, T sensors @ each
inlet/outlet
• Large and small flow lines,
dummy load line
Plate Heat Exchanger
LDC Pump
Modelling Paradigms
Causal/Procedural – Simulink
• Cause-effect determined by user
• Easier debugging
• Code reuse not easy
Acausal/Object-Oriented –
EcosimPro/Dymola
• Solver determines cause-effect
• Debugging not as easy
• Easy code reuse
Object-Oriented Modelling
EcosimPro CO₂ Library
Superheated
Vapor
Lvap
Capacitance Models
• Moving Boundary Method or Finite Volume
Method
• Tank-Tube Modelling (Capacitive Resistive)
• Upwind scheme (handles reverse flows)
• Momentum equation: Full form, static form,
or pressure drop neglected (uniform
pressure)
ṁ
P, h
P, h
P, h
L2ph
Subcooled
Liquid
Lliq
L
Superheated
Vapor
1
Two Phase
Lc
i
L
ṁ
ṁ
Two Phase
Subcooled
Liquid
Ncells
Two-Phase Flow
• Evaporator, Condenser, IHX and Accumulator
• Large volumes = Large refrigerant charge difference
• Not just to get HTC and dP
• Different governing equations – therefore, component behaviour
• Dry-out
• Homogeneous Equilibrium assumption: OK for mist and bubbly flow
• Inhomogeneous Equilibrium Model (no interfacial exchange),
Separated Flow model
Two-Phase Flow
• Same boundary conditions
• HEM
• S=1
• hFlow = hDensity
• InHEM model
• Zivi slip-ratio model
• S = (ρf/ρg)(1/3)
• Enthalpy correction applied
Fluid Properties
• Flops matter
• Interpolation
polynomials or lookup
tables
• Large density changes
near saturation line
• Partial Derivatives of
density wrt P and h
• Mass conservation
issues
Accumulator
Internal Heat Exchanger
Basic Startup Cycle
With Suction Line HX
Outlook
• Interested: CUT, University of Manchester, Cryogenics
(CERN)
• Important: Reproducibility, code review,
documentation
• GitLab – Version Control
[https://gitlab.cern.ch/vbhanot/CO2L]
• JIRA – Bug and Issue tracking
Outlook
2016 (December)
• CORA refurbishment
• Refrigerant Properties
• Initial Two-Phase flow modelling
2017 (1st Half)
• Initial Data Collection
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•
•
•
Pump characterization
Accumulator set-point change
Chiller performance
Startup Cycle
• Initial Validation
• Component level
• CORA bench validation
2017 (2nd Half)
• Two-phase microchannel
modelling
• Micro and Mini channel
experimental runs
• Validation of dynamic
microchannel models
2018
• More advanced two-phase
experimental analysis and
modelling
• Controls
2019 (…)
Questions