New correlation to predict the heat transfer coefficient in-tube cooling of supercritical CO2 in horizontal macro-tubes

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Abstract

The heat transfer coefficient during gas cooling process of supercritical CO2 without lubricating oil in horizontal macro-tubes has experimentally investigated. Investigation has done on two stainless steel circular tubes having inside-diameter of 4.55 mm and 7.75 mm and carried out for CO2 mass fluxes of 200–600 kg/m2 s, inlet fluid pressures of 7.5–10.0 MPa, and the inlet fluid temperatures of 90–100 °C.

The experimental results indicate that the gas cooling pressure, the inner tube diameter, the mass flux and the temperature of CO2 have significant effects on the heat transfer coefficient, especially near pseudo-critical region. The heat transfer coefficient decreases as the cooling pressure increases otherwise increases as mass flux increases. At any temperatures, smaller inner tube diameter shows higher heat transfer coefficient.

In comparison between present experimental data and existing correlations from references, most of supercritical heat transfer correlations show large deviations with this present experimental data. Therefore, based on experimental data obtained in this present work, a new correlation is proposed to predict more accurate heat transfer coefficient of supercritical CO2 in horizontal macro-tubes under cooling conditions. The majority of the experimental values are within 13% of the values predicted by the new correlation.

Introduction

A number of researchers [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16] have studied for heat transfer in-tube cooling of supercritical CO2 as alternative refrigerant. CO2 (R-744) has recently been rediscovered as a possible vapor compression working fluid mainly due to its unique combination of ecological and personal safety. CO2 is a non-toxic and non-combustible natural fluid that offers full environmental safety. It is widely available in sufficient quantities and at a very reasonable cost all over the world, and there is no need to recover and recycle it. The physical properties of CO2 are quite different from those of conventional refrigerants. As a consequence, novel cycles and circuiting concepts, new control schemes and redesigned components (compressors and heat exchangers) need to be developed.

Since in some applications such as air-conditioning and heat pump systems, the heat rejection temperatures are usually above the critical temperature of CO2 (31.1 °C), cycles using CO2 as the refrigerant will have to operate in a trans-critical cycle. As such, the heat rejection takes place above the critical pressure (about 7.4–12 MPa) in a so-called gas cooler (corresponding to the condenser in the conventional subcritical systems), while the heat absorption remains below the subcritical region. Consequently, the heat transfer characteristics of CO2 in a horizontal tube are considerably different from that of fluorocarbon fluids. Moreover, the CO2 has smaller vapor and liquid density ratios than those of the conventional refrigerants, so that it experiences less mal-distribution between the liquid and vapor phases in the gas cooler.

In recent years, previous reviews on cooling heat transfer of supercritical CO2 in horizontal tubes were conducted by many researchers [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16] as shown in Table 1. Dang [12], Gao and Honda [13], Mori et al. [14], Zlingerli and Groll [15] and Yun et al. [16] studied the effect of lubricating oil on heat transfer of supercritical CO2. In studies without oil effect [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], studies by Yoon et al. [1], Dang and Hihara [2] and Kim et al. [11] are related to macro-scale tubes, and studies by Pettersen et al. [3], Kuang et al. [4], Huai et al. [5], Huai and Koyama [6] and Liao and Zhao [7] to micro-scale tubes (3 mm is used as the threshold diameter to distinct macro-and micro-tubes here [17]) for air-conditioning, heat pump and refrigeration systems. In particular, the studies by Pettersen et al. [3], Kuang et al. [4], Kuang and Ohadi [10], Huai et al. [5] and Huai and Koyama [6] are on multi-port extruded micro-tubes, which are similar to those of the most interest for automobile air-conditioning systems.

The most interesting diameter of a gas cooler used on commercial air-conditioning and heat pump systems is generally in the macro-scale tubes of 3–8 mm. From earlier literature reviews, comparatively few investigations [1], [2], [8], [9], [11] have been conducted on cooling heat transfer of supercritical CO2 in this tube range. Some researchers have proposed cooling heat transfer correlations for supercritical CO2 by modifying previous correlations to fit their own data. Comparison of experimental data to these correlations is still not satisfactory. Therefore, more careful experimental correlation for gas coolers which applies to macro-scale tubes of 3–8 mm is still needed.

In this study, the cooling heat transfer characteristics of supercritical CO2 are investigated experimentally in horizontal macro-tubes of 4.55 mm and 7.75 mm inner diameter. The present experimental results for tubes are compared with previous correlations proposed for relatively similar test conditions. Then, a suitable heat transfer correlation is presented by analyzing the experimental data for the heat transfer coefficient of supercritical CO2 cooling in macro-scale tubes of 4.55 mm and 7.75 mm.

Section snippets

Test facility

Fig. 1 shows a schematic diagram of the test facility. Basically, it is consisted of three circuits: the closed refrigerant circuit (CO2 in this study), the cooling water circuit for the test section and the cold ethylene/water mixture circuit for subcooling the refrigerant.

Since the operating pressure of the apparatus is between 7.5 MPa and 10.0 MPa, the experimental apparatus is mainly constructed by stainless steel tube (SUS 316 type). As shown in Fig. 1, the refrigerant loop is composed of a

Physical properties of CO2

Extreme variation in physical properties at near pseudo-critical region is one of important characteristics of supercritical CO2. Fig. 4 shows typical variations in the density: (a), the specific heat (b), the viscosity (c), the thermal conductivity (d) and the Prandtl number (e) of CO2 with temperatures at the pressure ranging from 8.0 MPa to 10.0 MPa. All the physical properties of CO2 were plotted based on the data from NIST Refrigerants Database REFPROP 7.0 [18].

As shown in Fig. 4a and b, the

Conclusions

The heat transfer coefficients during the gas cooling process of supercritical CO2 without oil in macro-tubes of 4.55 mm and 7.75 mm were investigated experimentally. The experimental results show that gas cooling pressure, the inner tube diameter, the mass flux and temperature of CO2 have significant effects on the heat transfer coefficient, especially near pseudo-critical region. The heat transfer coefficient is decreased when cooling pressure is increased otherwise increased when mass flux is

Acknowledgments

The work presented in this paper is part of the project ‘Development of high efficient cooling and heating system using natural refrigerant of CO2’ sponsored by Ministry of Commerce, Industry, and Energy. The support of these sponsors is gratefully acknowledged.

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