Experimental study of the impacts of cold mass fraction on internal parameters of a vortex tubeÉtude expérimentale des impacts de la fraction de masse froide sur les paramètres internes d'un tube vortex

https://doi.org/10.1016/j.ijrefrig.2019.05.002Get rights and content

Highlights

  • A five-hole probe and several thermocouples were used to obtain the internal parameters of a large vortex tube.

  • Four different cold mass fraction conditions (0.2, 0.4, 0.6 and 0.8) were chosen.

  • The 3-D velocities were presented, and the tangential velocity was considered to be the steady Burgers vortex form.

  • The pressure and temperature inside the vortex tube were presented and discussed.

Abstract

In order to get the internal parameters of a vortex tube, a large-scale vortex tube was designed and an experimental device was built. A five-hole probe and thermocouples were used to obtain the three-dimensional velocities, the static pressure, static temperature and total temperature distributions inside the vortex tube. Four different cold mass fraction conditions (0.2, 0.4, 0.6 and 0.8) were chosen and the impacts on the internal parameters of the vortex tube were discussed. Different from the traditional view, the tangential velocity was considered to be the steady Burgers vortex form. A reverse flow boundary was found, and the location of which was changed at different operation conditions and axial positions. Further, it was found that the lowest static temperature existed near the nozzle outlet, and a new static temperature difference distribution law was firstly proposed experimentally.

Introduction

Vortex tube is a simple temperature separation device which has convenience of use and environmental protection of working fluids. It has been widely used and has great application potential in the refrigeration, cryogenic organisms, precision instruments, aviation and many other fields as summarized by Zhang and Guo (2018).

For better application, many experimental and numerical simulation researches have been carried out on the effect of structural parameters to optimize the separation performance of a vortex tube. These research objects contain inlet nozzle numbers, diameter and length to diameter ratio of the vortex tube, tube materials. Besides external factors such as cold mass fraction, inlet pressure and inlet temperature were also considered (Im and Yu, 2012; Thakare et al., 2015; Rafiee and Sadeghiazad, 2017; Kaya et al., 2018; Hamdan et al., 2018, Kirmaci et al., 2018, Kirmaci and Kaya, 2018). The cold mass fraction is one of very important factors affecting the separation effect. A recognized cognition is that the maximum cooling effect occurs when the cold mass fraction reaches around 0.3, and the maximum heating effect is obtained at a cold mass fraction is about 0.7 as presented by Li et al. (2015). However, at present, there is still a lack of reliable mechanism explanation for the reasons of the above-mentioned influence law, little theory can be learned from the research only focus on the separation performance of a vortex tube, and there are few studies involved the impact of the cold mass fraction on inner parameters of a vortex tube. Actually, the theoretical research on the vortex tube has been seriously lagging behind, the published mechanism explanations also contradict each other (Hilsch, 1947; Fulton, 1950; Scheper,1951; Kurosaka, 1982; Ahlborn and Groves,1997; Xue et al., 2013; Guo and Zhang, 2018).

Understanding the flow pattern inside a vortex tube has an important guiding significance for the cognizance of the temperature separation mechanism of a vortex tube. Some researchers used transparent vortex tubes to get a glimpse of the internal field through visualization research. Based on the introduction of smoke and a single wool tuft stretched across the tube, Harnett and Eckert (1957) found that the radical velocity near the wall is negligible and the velocity was mainly in tangential direction. Piralishvili and Polyaev (1996) injected kerosene into the air at the mass flow rate of 1:30 and successfully observed the vortex cores in a double circuit pyrex type vortex tube. The method of color Hilbert visualization was employed by Arbuzov et al. (1997) and a large-scale vortex structure in the form of a double helix was obtained for the first time. Aydin and Baki (2006) also got clear swirl streamlines. Xue et al. (2011) revealed the existence of multiple circulation regions within the vortex tube and described a new hypothesis of the separation mechanism through the visualization with air bubbles and small plastic particles. As a whole, these researches helped a preliminary understanding of the main flow structure within the tube, but this flow-visualization technique can only give a qualitative description.

To get a further quantitative flow field, later, some experimental explorations have been carried out. One approach is to insert the thermocouples and Pitot tubes into the interior of the vortex tube for direct temperature and pressure measurement. On the basis of temperature measurement, Scheper (1951) found that the core of the tube had a higher static temperature than the surrounding outer helix, thus he put forward heat transfer theory and believed that thermal conduction occurred from the vortex core outward. Sheller and Brown (1957) obtained a similar law. While Harnett and Eckert (1957) considered that the static temperature variation across the tube was quite small (12.2°C) and was not sufficient to the heat conduction. These disputes lead to the heat transfer mode is still an enigma. Takahama (1965) conducted a series of explorations using a 3 mm diameter Pitot probe with 2 holes mounted at an angle of 81°, he assumed a forced vortex flow in the core region and a uniform tangential velocity in the outer region. Bruun (1969) also got the three-dimensional velocity distributions inside the tube using a 3 mm diameter Pitot probe with 3 holes. Ahlborn and Groves (1997) developed a smaller Pitot pipe (diameter 1.6 mm with a 0.3 mm hole) and detected in the middle of the vortex tube a secondary circulation pattern which may exist or not. The non-zero tangential speeds at the center of the tube, which might be due to instabilities with respect to the vortex core motion itself were discovered by Gao et al. (2005). Contrary to most research work that tangential velocity structure was similar to Rankine vortex, Xue et al. (2013) summarized that the swirl flow changed from a forced vortex to an irrotational vortex model. Although this contact measurement method causes a disturbance of the flow field more or less, it is still an effective technique to explore the internal flow parameters of the vortex tube. The experimental researches on internal fields of vortex tubes using Pitot tubes were summarized in Table 1. Obviously, the flow fields inside the vortex tube currently obtained are discrepant, also resulting in the divergence on the cognizance of energy transfer process inside the vortex tube, therefore, a more systematic internal field experiment is needed to compare and verify the experimental results mentioned above. Besides, the form of the flow field inside the vortex tube has a great relationship with working conditions, but unfortunately most studies as shown in Table 1 only focus on one cold mass fraction, thus the variation of internal parameters under different cold mass fraction conditions needs further exploration.

Additionally, some scholars have used other non-contact measurement methods to obtain the internal field of the vortex tube. Selek et al. (2011) investigated through infrared thermography (IRT) technique. Kobiela et al. (2018) applied a PIV system and demonstrated the importance of pressure gradients in the prediction of swirl-induced thermal energy separation. However, PIV approach was limited apply only on low-pressure inlet conditions in view of the strong centrifugal force caused by strong swirl, it is often difficult to obtain accurate flow traces for tracer particles. Moreover, a number of numerical simulations (Aljuwayhel et al., 2005, Dutta et al., 2011, Bej and Sinhamahapatra, 2016, Rafiee and Sadeghiazad, 2017, Guo and Zhang, 2018) were launched by CFD recent years as a supplement of the cognition on the performance of vortex tubes.

It is evident that the experimental research on the internal field of the vortex tube is far from enough to clearly describe the specific internal flow pattern, and the cause the influence of different operating factors on the temperature separation performance is still unknown. In our previous work (Li et al., 2015), the influences of inlet pressure and cold mass fraction on the performance of the vortex tube were explored. The results showed that at the cold outlet, the temperature measured at the wall was a bit lower than that in the center while at the hot outlet, the temperature measured at the wall was higher than that in the center. Furthermore, we tried to give an explanation about maximum cooling and heating effect at µc values of 0.3 and 0.8, respectively. But in our previous work only the boundary conditions were measured, further study of the internal flow pattern of the vortex tube was required to attain deeper understanding of the vortex tube. Thus, the purpose of this study is to investigate the entire flow behavior and the effect of different cold mass fraction conditions on the flow structure inside the vortex tube, discuss the impacts of inner parameters on the performance of the vortex tube, and help to understand the specific process of internal flow within the vortex tube.

Section snippets

Experimental setup

It is hard to obtain the precise experimental investigations because of the strong swirl flow pattern, the high turbulence intensity and the small dimensions of the tube. The experimental research becomes more difficult when the measurements are taken by intrusive probes which can break the flow pattern. In order to obtain accurate quantitative observations of the flow in the vortex as far as possible, a large-scale vortex tube with a length of Lvt = 600 mm and diameter of Dvt = 40 mm was

Results and discussion

During our studies, the inlet pressure was maintained at 550 kPa, and the cold mass fraction was adjusted to 0.2, 0.4, 0.6 and 0.8, respectively. The internal 3-D velocities, static pressures, static and total temperatures were measured to understand the flow behavior inside the vortex tube.

Conclusions

Although many experimental studies and numerical simulations on the internal fields of the vortex tube have been carried out, the flow fields inside the vortex tube currently obtained were discrepant, also resulting in the divergence on the cognizance of energy transfer process inside the vortex tube. Besides, most studies have only considered the variation law under a cold flow condition, thus the internal flow fields of a large vortex tube under different cold mass fraction conditions (0.2,

Acknowledgments

This work was financially supported by State Key Laboratory of Technologies in Space Cryogenic Propellants (SKLTSCP1710) and The National Natural Science Foundation of China (No. 51776187).

References (38)

Cited by (27)

  • CFD simulation and thermodynamic analysis of energy separation in vortex tube using different inert gases at different inlet pressures and cold mass fractions

    2023, Energy
    Citation Excerpt :

    Many experimental and theoretical studies were conducted on VT, which can be broadly classified into (i) visualization of VT flow field to understand the energy separation mechanism, (ii) enhancement of VT performance either by variations of geometrical and operational parameters or by incorporating some innovative geometrical configuration, (iii) innovative applications of VT. Many experimental studies were conducted by inserting probes inside the VT to measure the flow and temperature distributions [11–14], although the measurements are prone to error due to the flow distortions caused by the inserted probes. Flow structure was qualitatively observed by spiraling lines of smoke inside a transparent VT by Aydin and Baki [15].

  • Experimental evaluation of vortex tube and its application in a novel trigenerative compressed air energy storage system

    2022, Energy Conversion and Management
    Citation Excerpt :

    Meanwhile, modeling the energy separation behavior of the fluid inside the vortex tube using computational fluid dynamics (CFD) software calculations is another important way to show the performance of the vortex tube. Most of the current research on vortex tubes is focused on experimental studies [50], CFD studies [51], and performance optimization [52] of the vortex tube performance itself. Avci et al. [53] experimentally investigated the effect of nozzle aspect ratio (AR) and nozzle number on the performance of vortex tubes.

View all citing articles on Scopus
View full text