6。05 m, 14。4 Pa and 0。03  bar。

3。2。Effect of internal tube diameter

Simulations were done considering four different tube inter- nal diameters (3, 4, 5 and 6 mm) at a volumetric flow rate of 400 m3 h−1。 The other geometrical and thermodynamic param- eters were considered the same as in the previous section, which were presented in Table  1。

Fig。 7 shows the CO2 local heat transfer coefficient for dif- ferent values of pipe diameter。 The highest thermal performance is obtained for the smallest diameter, which is the result of the higher Reynolds number and flow turbulence。 Concern- ing the air side, higher heat transfer coefficients were also observed when reducing the piping diameter, i。e。 56。3, 58。5, 64。2 and 75。1 W m−2 K−1 respectively in descending order of piping diameter。  In  this  case,  since  the  volumetric  flow  rate is

Fig。 4 – (a) Local CO2 side heat transfer coefficient for different air volumetric flow rates。 (b) CO2 side heat transfer coefficient versus  CO2  temperature。

constant and the free flow frontal area is reduced when in- creasing the pipe diameter, a higher air velocity between parallel pipes in the same row (higher Reynolds and Nusselt numbers) is observed。 Thus, one could erroneously say that the heat trans- fer coefficient would be increased。 However, a larger diameter tube can increase the ineffective area behind the pipes (delay of the separation point in the boundary layer – the point where the velocity gradient at the piping surface is equal to zero), which means a lower heat transfer coefficient (mathemati- cally speaking, it has an inversely proportional relationship with the diameter)。

Fig。 8 depicts the effect of internal diameter on the local heat transfer per unit of tube length。 Clearly, it can be seen that a much smaller piping length is necessary when the diam- eter is reduced, which is a consequence of the higher local heat

transfer (higher air and CO2 heat transfer coefficients as pre- viously explained)。 The piping lengths were 5。32, 6。05, 6。63 and

7。12 m, respectively for piping diameters of 3, 4, 5 and 6 mm。 Regarding pressure drop, Fig。 9 shows the values obtained for the CO2 side as a function of the piping diameter。 A con- siderable increase of pressure drop gradient was observed when decreasing the piping diameter from 4 to 3 mm。 The overall pressure drops were 0。1, 0。03, 0。01 and 0。005 bar respectively for 3, 4, 5 and 6 mm of piping diameter。 Concerning the air side, as an increase in velocity is obtained when increasing the piping diameter (reduction in air flow free area), so the pressure drop is also increased, and such a characteristic must be consid- ered when selecting the gas cooler’s fan。 The following values were obtained when considering an ascending order of piping

diameter: 12。52, 14。36, 16。53 and 19。81  Pa。

本研究侧重于CO2的热力性能,改进气体冷却器,其主要目标是减少其体积，实现这种紧凑性，超临界的机器的主要方法有：从文献资料中获取资讯以及进行数字编程，得到非常详细的耗油效果模拟代码，以及通过领先的空气侧流动和热传递方法，计算管尺寸，空气体积流量，空气侧翅片类型，混溶油的中心化影响等。与现有设计相比，本课题设计的CO2气体冷却器体积大大减小，制冷剂耗能减少到至少14％。 耗油的模拟显示，对CO2  气体冷却器尺寸的不利影响降低到6％。当油浓度高达3％时的情况下，压降增加到2。65倍。

关键词：翅片管气体冷却器,CO2,PAG油,超临界流量,电荷减少

1。 导言

在对于先进的散热设计进行了广泛的调查后，本文对于在小管中流动的超临界CO2的方面进行了研究。提出了CO2紧凑型制冷系统的气体冷却器这一研究方向，考虑了润滑油对于冷却器效果的影响，也考虑了关于超临界流的传热，和压降方面对于热液压性能的重要影响。特别是，一个新型的气体冷却器精细模拟（包括局部油脂效应），然后用于设计新的气体冷却器。并对轻型商用电器进行调查，对现有的典型二氧化碳气体冷却器的热性能和尺寸进行详细比较。该设计的具体意义在于，这里的CO2气体冷却器是应用于保持饮料冷藏的冷饮自动售货机上，以及制冷系统的热交换器。特别是它设备散热方面，即热空气从售货机内部到外部环境的排放。论文网