1d analysis of co2 sub-cooled/supercritical ejector refrigeration cycle

losses are one of the highest thermodynamic losses. When the refrigerant pressure decreases, the
current isenthalpic process dissipates the refrigerant kinetic energy as friction heat. The isenthalpic
process generates a larger quantity of refrigerant vapor flash compared to isentropic process. As a
result, the refrigerating capacity is reduced. In order to recover part of the potential kinetic energy of
the expansion process, various researchers have attempted to use other expanders rather than the
expansion engine. An ejector constitutes an attractive alternative to the expansion valve of
refrigeration systems because of its low cost, its ability to handle two-phase flow without damage, and
there is no moving parts [2,3,4,5,6].
A 1D model of the CO 2 ejector has been developed to characterize the ejector operation. This
document presents the validation of the 1D model and the description of the CO 2 ejector operation for
different geometries, and different evaporating and gas cooler outlet temperatures.
Models are validated generally referred to experimental results or by compared to validated models.
For the ejector 1D model, a test bench has been designed to test ejectors, which have been sized using
the 1D model. The main constraint that affects the test bench components is the CO 2 high pressure.
The main objective of the test bench is the validation of the 1D model of sub-cooled/ supercritical
ejector cycle.
To reduce the costs of the test bench and to
obtain more accurate measured values, the
heat exchangers are cooled by water. The test
bench includes a cooled water loop and a CO 2
refrigeration loop (Figure 1).
The measured error is lower than 3%
compared to the different calculated values.
Taking in account the uncertainty span of the
measurement instruments, the estimated error
on the capacities is lower than 5%.
2. TEST-BENCH COMPONENTS
Figure 1: CO 2 refrigeration loop with ejector.
3. EJECTOR TEST RESULTS AND 1D MODEL ADAPTATION
The objectives of the tests are:
- Adaptation of the 1D model of the ejector.
- Characterization of the constant pressure chamber (CPC) length.
- Characterization of the constant area chamber (CAC) diameter.
A CFD study of vapor ejector has shown that the diffuser length should be larger than 9 D const area , and
the CAC length should be larger than 3 D const area . Therefore, the realized ejector bodies have a CAC
length varying between 6.3 and 8.6 D const area , and the lengths of the diffusers vary between 10.8 and
40.7 D const area (Figure 2.a).
4. TESTS
54 different ejectors can be assembled based on the different parts realized for the tests:
- 6 body cores (constant area diameter of 1.5, 2.0, 2.5, 3.0, 3.5, and 4.0 mm)
- 3 nozzles (throat diameter of 0.75, 1.0, and 1.25 mm)
- 3 disc thicknesses (8, 12, and 16 mm).
18 assembled ejectors have been sufficient to adapt the 1D model and to characterize the non-tested
ejectors. The test result interpretation is divided in two parts: nozzle characterization and ejector body
characterization.
8 th IIR Gustav Lorentzen Conference on Natural Working Fluids, Copenhagen, 2008 2