The present study focused on the thermal hydraulic performance improvement of a CO2 gas cooler with the principal goal of reducing its volume, and consequently the necessary charge of the working fluid。 To achieve this compactness, the leading methods for supercritical CO2 cooling (including oil effects) were taken from the literature and programmed into a very detailed simulation code, together with the leading air-side flow and heat transfer methods。 The effects of tube size, volumetric flow of air, type of air-side fin, miscible oil con- centration, etc。 were addressed。 Compared to an existing design, the volume and refrigerant charge of the CO2 gas cooler were reduced by at least 14%。 The simulations with oil showed that there was up to 6% adverse influence on the size of the gas cooler and its CO2 side pressure drop increased up to 2。65 times when oil concentrations of up to 3% were considered。79892

© 2016 Elsevier Ltd and IIR。 All rights reserved。

dissipates the heat removed from the inside of the cabinet to the outside ambient air。

First of all, an extensive review of the research literature on heat transfer and fluid flow of super-critical CO2 (i。e。 very high pressure CO2 gas above its thermodynamic critical con- dition) was undertaken, including the important but usually ignored influence of lubricating oil on the process (this oil enters such refrigeration systems by lubrication of the compressor)。 Regarding the air-side, a choice of the best method(s) for heat transfer and pressure drop of finned geometries for these heat exchangers was drawn from the literature that respect the types of fins permissible in cold drink equipment (keeping in mind not only the thermal aspects, but also the operating    aspects,

e。g。 in particular the minimum spacing between fins to try to minimize dust accumulation)。 Then, these methods were all programmed into the new simulation code (simulating both the air-side and the refrigerant-side of CO2 gas coolers) and nu- merous case studies were  run。

Basically, the effects of air volumetric flow, piping diam- eter, type of fin and oil concentration on the gas cooler heat transfer and pressure drop were investigated through a para- metric study, always with focus on maximizing gas cooler thermal hydraulic performance while minimizing the working fluid charge and gas cooler size。 The results of this paramet- ric study are presented graphically to illustrate the trends and advantages/disadvantages of the various design choices。 Then, based on the best design choice(s), two new gas coolers were designed and compared in terms of overall performance and refrigerant charge with respect to a current commercial CO2 gas cooler design。

2。 Literature review

1。 Introduction

An extensive investigation on the advanced thermal design methods for supercritical CO2 flows in small pipes typically found in CO2 gas coolers of compact refrigeration systems is presented, also considering the lubricating oil effects on the thermal hydraulic performance as oil has a significant impact on a supercritical flow’s heat transfer and pressure drop。 In par- ticular, a new highly detailed CO2 gas cooler simulation code (including local oil effects) was developed, and then used to design new gas coolers and make a detailed comparison rela- tive to the thermal performance and size of a typical CO2 gas cooler found in light commercial appliances。 Specifically, the CO2 gas cooler here is the heat exchanger of an actual refrig- eration system of a cold drink vending machine used to keep beverages  cold,  and  in  particular  it  is  the  device    that

Here, the methods and literature for supercritical CO2 flow and heat transfer will be reviewed and then an overview of air- side heat transfer methods will be presented。 To begin with, several prior simulation studies on CO2 gas coolers in the lit- erature will be discussed and then the prediction methods implemented in the present study will be   presented。