Choked Flow Characteristics of Subcritical Refrigerant Flowing Through Converging-Diverging Nozzles

Zhenying Zhang,Lili Tian,Lirui Tong,Yanhua Chen

doi:10.3390/e16115810

Zhenying Zhang, Lili Tian + Show 2 more

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https://doi.org/10.3390/e16115810

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Abstract

This paper presents the experimental results the choked flow characteristics of a subcritical refrigerant through a converging-diverging nozzle. A test nozzle with a throat diameter of 2 mm was designed and developed. The influence of operating conditions on the choked flow characteristics, i.e., the pressure profile and mass flow rate under choked flow conditions are investigated. The results indicate that the choked flow occurs in the flow of subcritical refrigerant through nozzles under the normal working conditions of air-conditioners or heat pumps. The pressure drop near the throat is about 80% of the total pressure drop through the nozzle. The critical mass flux is about 19,800 ~ 24,000 kg/(s·m2). The critical mass flow rate increases with increasing the upstream pressure and subcooling. Furthermore, the relative errors between the model predictions and the experimental results for the critical mass flux are also presented. It is found that the deviations of the predictions for homogeneous equilibrium model and Henry-Fauske model from the experimental values are −35% ~ 5% and 15% ~ 35%, respectively

Highlights

One of the major contributors to the inefficiencies of the basic vapor compression refrigeration cycle is the throttling loss that results from the isenthalpic expansion from the condensing pressure to the evaporating pressure in the throttling valve
The flow of refrigerant through the nozzles corresponds to the critical mass flux, which is the highest flux that can be achieved by decreasing the downstream pressure under given upstream conditions
The noticeable pressure drop near the throat is caused by the mechanism that when the vapor phase appears in the liquid phase, the liquid phase hardly accelerates to match the abrupt increase of the specific volume

Summary

Introduction

One of the major contributors to the inefficiencies of the basic vapor compression refrigeration cycle is the throttling loss that results from the isenthalpic expansion from the condensing pressure to the evaporating pressure in the throttling valve To reduce this loss, two phase energy recovery expanders [1,2,3,4,5,6,7,8] or ejectors [9,10,11] replacing the throttling valve have been widely investigated. The potential energy of the incoming high-pressure subcooled or saturated liquid flow is converted to the kinetic energy of the exiting liquid-vapor flow This behavior is defined as a flashing acceleration process. A deep understanding of the choked flow characteristics of the refrigerant through a converging-diverging nozzle is very important for the optimum design of the impulse turbo expanders or ejectors

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