Transient behavior of CO₂ in the internal heat exchanger during the start-up of the transcritical refrigeration system

Abstract

Transient behavior of internal heat exchanger during start-up of transcritical refrigeration system is investigated in this article. The work is focused on the local transient analysis of CO₂ thermophysical properties, in order to see how these transient changes affect the heat transfer rate and the effectiveness of internal heat exchanger, as well as the coefficient of performance during the start-up of the system. The study is conducted through a numerical simulation using the commercial computational fluid dynamics (CFD) software Ansys Fluent. Validation of numerical results is carried out by using seven different empirical correlations applied for the Nusselt number. It is observed that the thermophysical properties of the hot CO₂ stream experience large changes during the transient period. This instability is accompanied by a decrease in the heat transfer rate. Finally, the change in the internal heat exchanger effectiveness during the start-up results in a loss of about 12% of the coefficient of performance.

Nomenclature

COP	=	coefficient of performance
Cp	=	specific heat (J kg−1 K−1)
D	=	dimension
eff	=	effectiveness
	=	mass flow rate (kg s−1)
P	=	pressure (Pa)
T	=	temperature (K)
t	=	time (s)
v	=	velocity (m s−1)
	=	compressor mechanical power (W)
x	=	direction (m)
y	=	direction (m)
z	=	direction (m)

Greek symbols

ρ	=	density (kg m−3)
μ	=	dynamic viscosity (Pa s)
λ	=	thermal conductivity (W m−1 K−1)
φ	=	angle (°)

Subscripts

i	=	coordinate index in x direction
j	=	coordinate index in y direction
k	=	coordinate index in z direction
t	=	turbulent

Funding

The authors would like to acknowledge to the University of Guanajuato for the sponsorship for this work.