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Abstract
This article proposes an ejector model based on the real properties of CO2, which includes the critical mode and sub-critical mode (the critical mode means the primary and suction flows are both choked, and the sub-critical mode refers to only the primary flow choking). Moreover, a dynamic model of the transcritical CO2 ejector expansion refrigeration cycle is developed to simulate system responses at different ejector operational modes. The prediction results by the ejector model and system model are compared with available experimental data, respectively. Furthermore, the dynamic responses of the ejector expansion refrigeration cycle based on the entire ejector model are compared with those predicted upon the ejector model only with critical mode. The present results show the entrainment ratio provided by the ejector model coincides well with the experimental data, and most data lie within ±10% error. The system model predicts the gas cooler pressure and evaporator pressure with errors of 1.8% and 4.2% for the measured results, respectively. Moreover, the pressure and mass flow rates of the system based on the ejector model only with critical mode are higher than that by the entire ejector model. The proposed model is useful to predict performances accurately and conduct dynamic analysis reasonably.
Nomenclature
| A | = | area (m2) |
| a | = | speed of sound (m/s) |
| CM | = | critical mode |
| COP | = | coefficient of performance |
| CSCMs | = | critical and sub-critical modes |
| Cp | = | specific heat (kJ/ kg K) |
| D | = | diameter (m) |
| DAEs | = | differential algebraic equations |
| EERC | = | ejector expansion refrigeration cycle |
| h | = | specific enthalpy (kJ/kg) |
| k | = | velocity ratio of gas phase and liquid phase |
| L | = | length (m) |
| LED | = | length of equal diameter |
| = | mass flow rate (kg/s) | |
| NXP | = | nozzle exit position |
| P | = | pressure (MPa) |
| PDEs | = | partial differential equations |
| RTD | = | resistance temperature detector |
| s | = | specific entropy (kJ kg/K) |
| SCM | = | sub-critical mode |
| T | = | temperature (°C) |
| u | = | velocity (m/s), internal energy (J/kg) |
| v | = | velocity (m/s) |
| V | = | velocity (m/s), volume (m3) |
| VCC | = | vapor compression cycle |
| x | = | quality |
| z | = | spatial variable along tube length (m) |
Greek symbols
| α | = | void fraction, heat transfer coefficient (W/(m2 K1)) |
| γ | = | specific heat ratio |
| = | mean void fraction | |
| η | = | efficiency |
| μ | = | entrainment ratio |
| ν | = | specific volume (m3/kg) |
| ρ | = | density (kg/m3) |
Superscripts
| * | = | critical mode operation of ejector |
| ‘ | = | derivative |
Subscripts
| 1 | = | nozzle exit |
| 2 | = | constant area section |
| c | = | back pressure, compressor |
| cc | = | calculation of back pressure |
| co | = | limiting condition of operational mode |
| cs | = | cross-section |
| d | = | diffuser |
| e | = | evaporator |
| e1, e2 | = | two-phase, superheated zone in evaporator |
| f | = | saturated liquid |
| g | = | saturated vapor |
| gc | = | gas cooler |
| i | = | inner |
| in | = | inlet |
| is | = | isentropic |
| m | = | mixed flow |
| n | = | nozzle |
| o | = | outer |
| ou | = | outlet |
| p | = | primary flow |
| py | = | primary flow start to mix |
| r | = | refrigerant |
| s | = | suction flow |
| se | = | separator |
| sy | = | suction flow start to mix |
| t | = | nozzle throat |
| total | = | total length of evaporator |
| v | = | expansion valve |
| w | = | heat exchanger structure |
| wa | = | water |
| y | = | location where two streams start to mix |