Adiabatic capillary tube flow of carbon dioxide in a transcritical heat pump cycle

Abstract

SUMMARYFlow characteristics of an adiabatic capillary tube in a transcritical CO 2 heat pump system have been investigated employing the homogeneous model. The model is based on fundamental equations of mass, energy and momentum which are solved simultaneously. Two friction factor empirical correlations (Churchill, Lin et al., Int. J. Multiphase Flow 1991; 17(1):95-102) and four viscosity models (Mcadams, Cicchitti, Dukler and Lin) are comparatively used to investigate the flow characteristics. Choked condition at the outlet is also investigated for maximum mass flow rate. Subcritical and supercritical thermodynamic and transport properties of CO 2 are calculated employing a precision property code. Choice of viscosity model causes minor variation in results unlike in chlorofluorocarbons (CFCs) refrigerants.Relationships between cooling capacity with capillary tube diameter, length and maximum mass flow rate are presented. A lower evaporating temperature yields a larger cooling capacity due to the unique thermodynamic properties of CO 2 . It is also observed that an optimum cooling capacity exists for a specified capillary tube.

Performance evaluation of a non-adiabatic capillary tube in a transcritical CO2 heat pump cycle

“…Parameters are chosen in such a way that there is no choking at the exit of the capillary tube. Onset of negative entropy change is used as a criterion to detect the choked flow condition in the capillary tube flow [9].…”

“…They concluded that a system operates closer to optimum conditions with capillary tube than a pre-fixed high pressure. Recently Agrawal and Bhattacharyya [9] presented the flow characteristics of an adiabatic capillary tube flow in the CO 2 transcritical heat pump cycle. Occurrence of choking, cause and effects, were also discussed in their analysis.…”

Exergy assessment of an optimized capillary tube-based transcritical CO2heat pump system

“…However, this simplified study did not include heat transfer, fluid flow and internal surface effects of the capillary tube. Agrawal and Bhattacharyya [19] presented characteristics and first law analysis of adiabatic capillary tube flow including choked flow in a transcritical heat pump cycle based on steady flow energy balance. Exergy analysis of a capillary tubebased transcritical carbon dioxide heat pump system for simultaneous cooling and heating applications including heat transfer and fluid flow effects in the heat exchangers have not been reported yet.…”

An experimental study of a carbon dioxide-filled thermosyphon for acquisition of low-temperature waste energy

“…Agrawal and Bhattacharyya [11] investigated flow characteristics of an adiabatic capillary tube in a transcritical carbon dioxide heat pump system. Aprea and Maiorino [12] described experiments comparing a commercial available R134a refrigeration plant subjected to a cold store and a proto type carbon dioxide system working as a classical 'spilt-system' to cool air in residential applications in a transcritical cycle.…”