Refrigeration System Design and Analysis

1
Refrigeration SystemDesign and Analysis

• Brent CullimoreCR Technologies

2
Background

• Background
• Vapor compression (V/C) systems are well
  established in the Automotive industry and HVAC,
  but are still emerging in thermal management of
  electronics
• Modeling of V/C systems is only a few years old
• In Automotive, driven by the need to meet EPA
  mileage/emissions standards must use transient
  drive cycles as design criteria
• Modeling helps address perpetual concerns such as
  start-up slugs, oscillations, oil and charge
  migration upon shut down, etc.
• Purpose of this paper
• Share modeling lessons learned from Ford,
  Visteon, GM, Delphi, Danfoss, etc.

3
FundamentalLesson Learned

• Self-determination of pressure requires tracking
  refrigerant mass full thermohydraulic solution
  required!

Solve for pressures, qualities, temperatures,
flow rates, heat transfer coefficients
simultaneously
4
ExampleParametric Sweep

• System Description
• R134a working fluid
• Air-cooled condenser (100ºF environment)
• Rotary compressor (defined by performance maps)
• Capillary tube throttle (0.030 x 10 ft)
• with regenerative suction tube heat exchanger
• Vary compressor RPM from 1000 to 3000
• Requires 800W to 1500W input power

5
ResultsCompressor Exit Pressure
Constant Inlet Pressure (Different systems!)
Constant Charge Mass (Apples to apples!)
6
Cause of Difference
Constant Inlet Pressure
7 increase in charge causes increased
liquid blockage in condenserand evaporator
7
SolutionGeneralized Thermal/fluid Analyzer
SINDA/FLUINT Network Example
8
ExampleHeat Exchanger Modeling
to compressor or dryer
from expansion device
Evaporator (R134a)
Aluminum Heat Exch.
Air Side
air andcondensate out
moist air in
9
Top level The Whole Loop
10
ExampleCAD-based Condenser Model
Mix and Match Methods Condenser 1D finite
difference/volume thermohydraulics Pipe
walls 2D finite difference thermalFins
2D finite element thermalAir flow 1D finite
difference network Full parametric modeling
11
Layers of theComputational Onion

• Pseudo-steady thermaland thermohydraulics (t
  10 min.)
• Steady hydraulics,unsteady thermal (t 1 min.)
• Mass/energy storagewithout flow inertia (t 10
  sec.)
• Flow inertia w/omass/energy storage (t 1
  sec.)
• Mass/energy and inertia,homogeneous
  equilibriumtwo-phase (t 1 sec.)
• Nonhomogeneous equilibriumtwo-phase (t 1
  sec.)
• Nonhomogeneous nonquilibriumtwo-phase (t
  0.1 sec.)

12
Two-phase FlowWhat phenomena are important?

• Homogeneous Equilibrium Flow
• Phases at same temperature, same velocity
• Flow regime mapping optional
• Often adequate for VC cycles
• Equilibrium Slip Flow
• Phases at same temperature, different velocities
• Flow regime information required
• Enhances accuracy in VC cycles (better void
  fraction estimation)
• Nonequilibrium Slip Flow (two fluid)
• Phases at different temperatures and velocities
• Usually not needed except for severe
  transientsand high frequency instabilities or
  control systems

13
Comparisons with TestUsing Equilibrium Slip Flow

• From Improvements in the Modeling and
  Simulation of Refrigeration Systems Aerospace
  Tools Applied to a Domestic Refrigerator,
  Ploug-Sorensen et al. Danfoss, 1996.

Cabinet Temperature
Hi/Lo Pressures
14
Conclusions

• Charge mass must be conserved in transients,
  parametric sweeps, sensitivity studies, etc.
• Component-level approaches (effectiveness,
  average coefficients, etc.) are not
  suitablelimited to concept-level trade studies
• Finite difference/volume subdivision of condenser
  and evaporator is suitable tracks mass and
  blockage effects
• Two-phase flow is not amenable to CFD approaches
• but flow network modeling (FNM) is well suited
  for the task
• Slip flow has been shown to improve accuracy, but
  full nonequilibrium nonhomogeneous (two fluid)
  modeling is usually not necessary