Numerical investigation in effects of refrigerants’ thermodynamic properties on performance of supersonic ejector

ABSTRACT

Using zeotropic mixtures with unique temperature glide characteristics as the working medium in heat-driven ejector refrigeration system will potentially reduce the irreversible losses in heat exchange process. However, the intrinsic link between the pure/mixture refrigerants’ thermophysical properties and the entrainment performance of supersonic ejector has not yet been elucidated. In this study, a novel ejector CFD model is established by coupling the real-gas thermodynamic equation and species transport equation, then the relationships between thermophysical properties of pure/mixture refrigerants and ejector’s dimensionless operational parameters were thoroughly investigated. The results indicated that the ejector expansion ratio rises as primary flow inlet temperature or refrigerant’s normal boiling point increases, while the dimensionless parameters of primary nozzle are relatively insensitive to the variation of primary flow inlet temperature and refrigerant’s boiling point, resulted in changes of supersonic jet expansion behaviour. Moreover, the specific volume and adiabatic index change characteristics resulted in minor fluctuations of primary nozzle’s dimensionless parameters. When the bubble point temperature is fixed at 30℃ in the condenser, the saturated vapor/liquid phase pressure glide characteristics of R600a/R1234ze mixture may lead to an increasement of ejector’s compression ratio.

KEYWORDS:

• Heat-driven ejector refrigeration system
• Supersonic ejector
• Thermodynamic properties
• Mixed refrigerant
• Computaional fluid dynamics

Nomenclatures

Abbreviation	=
A	=	cross-sectional area, m2
a	=	sound speed, m/s
D	=	diffusion coefficient
e	=	internal energy, kJ/kg
H	=	total enthalpy, kJ/kg
h	=	specific enthalpy, kJ/kg
J	=	diffusive flux of species, kg/(m2·s)
m	=	mass flow rate, kg/s
P	=	pressure, Pa
$S_{c_t}$	=	effective Schmidt number for the turbulent flow
T	=	temperature (℃)
t	=	time, s
u	=	velocity, m/s
x	=	axial coordinate, mm
y	=	radial coordinate, mm
Z	=	mass fraction
Abbreviations	=
AR	=	area ratio
CFD	=	computational fluid dynamics
COP	=	coefficient of performance
CR	=	compression ratio of ejector
M	=	relative molecular mass, kg/kmol
ER	=	expansion ratio
HERS	=	heat-driven ejector refrigeration system
Ma	=	Mach number
NBP	=	normal boiling point, ℃
Greek letters	=
$\mathit{\gamma }$	=	adiabatic index
$\bar{\gamma }$	=	average adiabatic index of the expanding process
$\mathit{\mu }$	=	entrainment ratio, viscosity, Pa·s
$\mathit{\rho }$	=	density, kg/m3
$\overline{\stackrel{‾}{\tau }}$	=	stress tensor, Pa
$\mathit{k}$	=	thermal conductivity, W/K
$\mathit{v}$	=	specific volume, m3/kg
$\mathrm{\Delta }\mathit{T}$	=	difference in normal boiling point
Subscripts	=
c	=	condenser, ejector outlet
e	=	secondary flow, the exit plane
eff	=	effective
g	=	primary flow
l	=	liquid phase
n	=	primary nozzle
s	=	saturated state
v	=	vapor phase

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This research is supported by National Natural Science Foundation of China [52176006], Basic and Applied Basic Research Foundation of Guangdong Province [2022A1515011496] and Guangdong High-level Talent Project [2021QN02L165].