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...3/8 in. (9.52 mm) copper tube and louvered fins was found suitable to work with high R-744 pressures and has been studied as a gas cooler in a test rig built for testing carbon dioxide (C[O.sub.2]) equipment operating with air as a secondary fluid. The test rig consists of two closed-loop air circuits acting as heat sink and heat source for the gas cooler and evaporator, respectively. The tested refrigerating circuit consists of two tube-and-fin heat exchangers as the gas cooler and the evaporator, a back-pressure valve as the throttling device, a double-stage compound compressor equipped with an oil separator, and an intercooler. A full set of thermocouples, pressure transducers, and flowmeters allows measurement and recording of all the main parameters of the C[O.sub.2] cycle, enabling heat balance to be performed for both air side and refrigerant side.
Tests focused on two different gas coolers, with continuous and cut fins, and on two different circuit arrangements. Tests on each heat exchanger were run at three different inlet conditions, for both C[O.sub.2] and air. A simulation model was developed for this type of heat exchanger and three models (Dang and Hihara 2004; Gnielinski 1976; Pitla et al. 2002) proposed for the C[O.sub.2] supercritical cooling heat transfer coefficients were implemented and compared in the code.
The model results are compared with the experimental data for the finned coil; emphasis is given to the effect of heat conduction through fins between adjacent tube ranks on system efficiency. In the paper, the experimental results for transcritical C[O.sub.2] entering the gas cooler at 87.0[degrees]C (7.911 MPa), 97.6[degrees]C (8.599 MPa), and 107.8[degrees]C (9.102 MPa) with air inlet temperatures of 20.3[degrees]C, 21.5[degrees]C, and 23.0[degrees]C, respectively, are presented. By using a coil with fins modified to reduce the heat conduction, a 3.7% to 5.6% heat flux improvement was gained. This improvement can be clearly translated in terms of coefficient of performance (COP), since a low value of the C[O.sub.2] temperature at its outlet increases the cooling capacity. Considering a reference cycle with the same operating conditions, a 5.7% to 6.6% increase of COP can be obtained.
INTRODUCTION
Due to the necessity of decreasing the greenhouse effect, new fluids need to be investigated as refrigerants. Carbon dioxide (C[O.sub.2]) seems promising because of its environmental friendliness and some excellent thermodynamic and transport properties, such as high specific heat, high thermal conductivity, and low viscosity. The "traditional" finned coil heat exchangers still can be considered an opportunity for C[O.sub.2] transcritical cycles. The gas cooling process needs to be investigated, taking into account two points of view: the great variation of thermophysical properties and the large decrease in temperature occurring along the heat exchanger. Very poor evidence is provided in the open literature to the study of finned coil gas coolers (GCs) with "macro" tubes (i.e., the internal diameter of the pipe is larger than 3 mm). Recently, Hwang et al. (2005) proposed an experimental and numerical study of a three-row finned coil GC with 7.9 mm outside diameter.
For this research, a set of tests was conducted and experimental results were compared with numerical predictions obtained through a finite volume simulation software.
Although simulation results gained by our software have been found to be in agreement with experimental results for a large number of refrigerants and test conditions, a systematic deviation was seen using C[O.sub.2]. Therefore, some correlations to predict C[O.sub.2] heat transfer coefficient were tested, but the heat conduction along the heat exchanger, which has not been considered by the software, was deemed to be the most important cause of the disagreement in results. This kind of phenomenon is not considered by the simulation software at the moment, and very poor evidence is given to this problem by papers available in the open literature. Furthermore, it seems to be relevant only to finned coil heat exchangers, since it was shown to be unimportant in microchannel heat exchangers (Asinari et al. 2004).
TEST RIG DESCRIPTION AND UNCERTAINTY ANALYSIS
The C[O.sub.2] circuit (Figure 1) carries out a double compression with gas intercooling between the two compression stages and single throttling, and it is equipped with an internal heat exchanger. The compressor is a two-stage semi-hermetic reciprocating unit running at 1450 rpm (50 Hz). The nominal volumetric flow rate of the low-pressure stage (one cylinder) is 3.0 [m.sup.3]/h, while that of the second stage is 1.74 [m.sup.3]/h (volume ratio 1.7). The lubricant is a PAG oil 46 ISO grade.
The compressor power input is recorded with an electronic transducer (with an accuracy of [+ or -]0.5% of the reading value).
The intercooler (IC) heat flow is rejected to a water loop. The cooling...
NOTE: All illustrations and photos
have been removed from this article.
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