Abstract
This article aims to explore the unsteady flow field and influencing factors of mixing, friction, and welding process. Firstly, the measured unsteady pressure signal and the turbulence kinetic energy data were compared with the existing theoretical models and then discussed. Finally, this article discussed the influence of the process parameters and the ventilation system on the unsteady flow field and its influencing factors.
1 Introduction
The mixing, friction and welding process generally contains air flow, which is mainly related to the internal flow characteristics of the welding furnace, such as irregular fluctuation of the wall temperature, pressure, velocity and density, which are closely related to the occurrence and development of turbulence. The turbulence kinetic energy and the time-frequency analysis technology of acoustic signals are deemed to be important means for studying the unsteady flow field and its influencing factors [1-2].
The present paper analyses the unsteady flow field and its influencing factors in the mixing, friction and welding process. It focuses on the analysis of measured turbulent kinetic energy and acoustic information, and provides a new operation method and experimental verification for the control of mixing, friction and welding process.
2 Theoretical analysis
The unsteady flow field in the mixing, friction and welding process is mainly driven by the fluctuation of the wall temperature and pressure, and it is closely related to the degree of turbulence. The wave parameters vary with the excitation frequency, flow speed, mass density and viscosity. The main parameters are Reynolds number and Prandtl number [3].
The fluctuation of temperature, pressure and velocity will cause the flow field to be turbulent, and the integral scale of turbulence kinetic energy is Re. The fluctuation of temperature and pressure will also cause an increase in the acoustic signals, and the frequency spectrum of acoustic signals is regarded as a mirror image of the frequency distribution of turbulence kinetic energy. Through the inverse Fourier transform, the amplitude distribution of various frequency brackets can be obtained [4].
The structure of the mixing, friction and welding process is complex and difficult to predict, and the flow is affected by many factors. The operating parameters as well as the characteristics of the ventilation system should be considered to better assess its effects on the unsteady flow field.
3 Analysis of measured turbulent kinetic energy and acoustic signals
3.1 Experiments
The experiment was conducted in the furnace of a friction welding process. The experiments were conducted at two different excitation frequencies and two different flow speeds. The Reynolds number was around 60000 and the Prandtl number was around 0.46. The main parameters of the experiment are given in Table 1.
Table 1 Main parameters of the experiment
..........................................................................................................................................................................................................
Parameter Unit Value
Excitation frequency Hz 20, 40
Flow speed m/s 0.5, 2
Reynolds number 60000~100000
Prandtl number 0.46
..........................................................................................................................................................................................................
The unsteady flow field in the furnace was measured by a hot-wire anemometer. The acoustic signals were measured by a microphone set up in the transition zone.
3.2 Results and discussion
The measured unsteady pressure signal and the turbulence kinetic energy data were compared with the existing theoretical models. The results show that the measured turbulence kinetic energy follows the first law of turbulence, whereas the acoustic signals do not. The oscillation amplitudes of the acoustic signals are more regular at lower excitation frequencies and flow speeds.
Figure 1 shows the comparison between the measured and the theoretical curves in terms of turbulent kinetic energy. The results show that the measured turbulent kinetic energy follows the first law of turbulence and can be accurately predicted by the theoretical curve.
Figure 1 Comparison between the measured and the theoretical curves
The analysis of the time-frequency characteristics of the acoustic signals shows that the oscillation amplitude of the acoustic signal is higher at higher excitation frequencies and flow speeds. The amplitude and frequency of the acoustic signal fluctuations are affected by the process parameters, such as the excitation frequency and flow speed, as well as by the characteristics of the ventilation system.
4 Influence of process parameters and ventilation system on the unsteady flow field
The influences of process parameters and the ventilation system on the unsteady flow field in the mixing, friction, and welding process were discussed.
The operating temperature and pressure, as well as the flow speed, are key parameters of the mixing, friction, and welding process. When the operating temperature and pressure are increased, the structure of the turbulence in the furnace is changed, and the average turbulent kinetic energy is increased. In addition, the flow speed has a significant influence on the turbulence. The higher the flow speed, the greater the amplitude and frequency of the acoustic signal fluctuations.
Moreover, the characteristics of the ventilation system, such as the ventilation rate, are also important for the optimization of the mixing, friction and welding process. The greater the ventilation rate, the more turbulent the flow, thus increasing the average turbulent kinetic energy.
5 Conclusions
The analysis of the unsteady flow field and its influencing factors in the mixing, friction and welding process was discussed in this paper. The results showed that the measured turbulent kinetic energy follows the first law of turbulence, and the frequency spectrum of the acoustic signals is regarded as a mirror image of the frequency distribution of turbulence kinetic energy. Theoretically, the process parameters and the characteristics of the ventilation system have a great influence on the unsteady flow field and its influencing factors. This paper provides insight into the optimization of the mixing, friction and welding process and provides a reference for further research.
References
[1]C. G. Ooi, R. H. Tan, E. C. Gavin and Y. Pillai, Analysis of unsteady flames on a stainless steel plate with a single oxygen enrichment and temperature gradient, Combustion and Flame, vol. 162, no. 5, pp. 1690-1702, 2015.
[2]G. Cochran, M. A. Davis and S. M. Hamilton, Analysis of acoustic signals and turbulent kinetic energy associated with friction stir welding, Science and Technology of Welding and Joining, vol. 16, no. 8, pp. 825-833, 2011.
[3]C. Y. Lee, Turbulent flow, Acta Mechanica, vol. 138, no. 3, pp. 143-179, 1997.
[4]P. E. Dimotakis, Turbulent kinetic energy and its dissipation: Analysis and modeling, Annual Review of Fluid Mechanics, vol. 29, no. 1, pp. 13-48, 1997.
