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Self-excited oscillations that occur in the cryogenic helium tube system are seen as thermoacoustic instabilities. To increase the operation stability of the system and suppress the conversion of thermal energy into acoustic waves, one solution where an external sound source driven by a piston is applied to control oscillations in a helium tube system. In this work, a numerical study of minimizing self-excited oscillations is conducted. Oscillation frequency and amplitude of the coupling system of the helium tube system and the external sound source are acquired as functions of the piston speed and frequency. It is found that the suppression of pressure oscillations can be achieved when the driving frequency is far from the self-excited oscillation frequency of the cryogenic helium tube. Increasing the driving speed of the piston gives rise to the coupling oscillation amplitude, and the control strategy leads to 99.2% oscillation amplitude reduction. Moreover, the nonlinear dynamics behavior caused by the coupling of self-excited oscillations and the external sound source is revealed. Synchronization of frequency occurs when the system is coupled to the external sound source oscillating with different frequencies. Our findings could provide a guideline in minimizing self-excited oscillations in the cryogenic helium system. [ABSTRACT FROM AUTHOR]

In this paper, an innovative method of adopting a piping insert with an abrupt contraction or expansion structure has been proposed to eliminate undesirable thermally driven gas oscillations in cryogenic systems. In addition, the influences of the piping inserts' geometrical parameters and position on the performance of thermoacoustic oscillation in cryogenic systems have been investigated. A series of simulations has been conducted, and simulation bifurcation diagrams show that thermoacoustic oscillation can be completely stopped, which, in other words, is called the amplitude death (AD) phenomenon. When the pipeline inserts are equipped in the cold part, AD can be achieved regardless of the pipeline inserts' radius. When the pipeline inserts are located in the warm part, AD can be realized by the pipe inserts with an abrupt contraction. The identification of nonlinear characteristics of different state transitions with different insert geometries is further conducted by unfolding these original time series in an embedding space. The nonlinear change process of thermoacoustic oscillation is demonstrated in the phase space diagrams. As the insert radius increases, the system transforms from a periodic state to a quasi-periodic state and then from a quasi-periodic state to an AD state. This work will have practical impacts on the design of oscillation-free piping systems. [ABSTRACT FROM AUTHOR]

Thermoacoustic oscillations generally exist in combustion devices and cryogenic systems, causing system instability and other problems. The actual cryogenic system has complex pipe connections, however, there are few studies on the thermoacoustic oscillation of complex structures, which brings challenges to suppressing thermoacoustic instability. Therefore, this study focuses on the T-shaped helium tube existing in the actual cryogenic system and explores the oscillation characteristics of the complex cryogenic thermoacoustic system. With the change of branch position and length, mode transition phenomenon appears. We obtained the effects of the branch position and length on the amplitude and frequency of each mode and found that the self-excited oscillation in the T-shaped tube could be suppressed to some extent by choosing the proper length and position of the branch tube. Further research found that the first mode of the oscillating pressure in the main and branch tube was in-phase synchronized, and the second mode was antiphase synchronized. By linking the flow direction with the oscillation mode, it is found that the first mode corresponds to the first flow direction, and the second mode corresponds to the second or third flow direction. The finding is valuable for understanding thermoacoustic oscillations in T-tubes and provides a reference for controlling the self-excited oscillation of complex cryogenic systems. • The effects of two critical parameters on thermoacoustic oscillations of T-shaped cryogenic helium tubes are studied. • The mode transition phenomenon is found, and the mode variation law is obtained. • Choosing the proper branch position and length can suppress self-excited oscillation. • The asynchrony of oscillation pressure in the main and branch tube is reflected in the second mode. • The corresponding relationship between the oscillation mode and the flow direction is revealed. [ABSTRACT FROM AUTHOR]

• Minimizing self-excited thermoacoustic oscillations in cryogenic system is studied. • The dynamics of cryogenic thermoacoustic system subjected to acoustic forcing are investigated. • The phase relationships between self-excited and coupled waves are analyzed. • For small driving phase, the oscillation-free operation is achieved finally. Self-excited thermoacoustic oscillations usually occur in many practical systems such as rocket motors, gas turbines and cryogenic distribution systems. We study the dynamics of a self-excited cryogenic thermoacoustic system subjected to acoustic forcing. The effect of the driving phase on the amplitude of the coupled oscillation is analyzed and the results present two states. To provide a guide for the selection of driving signal, the dynamics of a self-excited cryogenic thermoacoustic system subjected to acoustic forcing are investigated. When ϕ = 30 ° , the radius of the limit cycle gradually shrinks, and self-oscillation amplitude is suppressed by 99.5%. For small driving phase, the self-oscillation can be suppressed, while for large driving phase, the phase portraits depict period-1 limit cycle oscillations. The results show that excessive driving phase has unfavorable effect on self-excited oscillation, and small drive phase can achieve oscillation suppression better. To reveal the coupling mechanism of externally forced oscillation and system self-excited oscillation, the phase relationships between self-excited oscillations and coupling pressure fluctuation waves in different periods are analyzed. Hence, the work indicates that under low-frequency disturbances, the system oscillation will have a phase delay, which will cause the hysteresis dissipation and restrain the system oscillation. [ABSTRACT FROM AUTHOR]

• A solution to control thermoacoustic oscillations is proposed. • Dual temperature gradient is developed by the introduction of the adjustment tube. • The influence of parameters sensitive to the oscillation-free operation is revealed. • The phase relationship between acoustic waves is identified. Suppression of gas oscillation is essential to realize the stable operation of the cryogenic helium piping systems. In the current study, a structural solution, that is, a tube is connected to the main pipe as an adjustment accessory to control unfavourable gas oscillations. The model with dual temperature gradients is developed by introducing an adjustment tube. The effects of the parameter sensitive to the distribution of the dual temperature gradients on the thermoacoustic oscillation (TAO) characteristics are revealed. The results demonstrate that the asymmetry of the temperature distribution and a large frequency deviation makes it easier to achieve the oscillation-free operation of the system. At the initial start-up stage, the phase relationship between the acoustic waves is shown. As the cold length to warm length ratio decreases, the phase difference between sound waves increases, and the oscillation-free operation is accomplished when the cold length to warm length ratio is 0.11. Therefore, an approach of adjusting the length ratio in the adjustment tube can control thermoacoustic oscillations of cryogenic fluid. [ABSTRACT FROM AUTHOR]

Insufficient dynamic information on helium piping systems limits our ability to understand and identify thermoacoustic oscillation (TAO) phenomena in these systems. In this study, we investigate the associated oscillation behaviours and the effects of the temperature distribution steepness, S , and the inflection point position, x 0 , on the TAO characteristics through numerical simulations. The results indicate that x 0 has a greater impact on the oscillation behaviour and is a decisive factor for oscillation-free operation. It is found that oscillation-free operation can be achieved when x 0 = 0.9 m. To identify the nonlinear dynamic characteristics related to TAO, the original pressure time series are unfolded in phase portraits and Poincaré maps. In contrast to the period-1 and period-2 limit circle oscillations at other positions, the trajectory at x 0 = 0.9 m displays a transition from limit circles to a fixed point (zero amplitude), corresponding to a series of points including the point (0,0) in the Poincaré map. Therefore, an approach of adjusting the inflection point position can be used to control the state of the TAO, and the transition in the nonlinear dynamic characteristics can be identified, which can also provide guidelines for designers to mitigate unfavourable TAO. [ABSTRACT FROM AUTHOR]

Acoustic pressure amplifier tubes (APAT) have shown excellent performance in thermoacoustic engine (TAE) systems when coupled with loads. Nonetheless, the oscillatory mode transition caused by the introduction of APAT and loads can significantly influence system performance. In order to investigate the effect of APAT dimensions and load impedances on the mode transition and system performance, the TAE system coupled with APAT and load was numerically solved by CFD software. Combined with the nonlinear dynamic methods, three oscillatory states are found in the coupled system: low frequency limit cycle oscillation, quasi-periodic oscillation, and high frequency limit cycle oscillation. The parameter selection scheme favoring the excitation of high frequency modes is also given. There are the optimal APAT length, diameter, position and acoustic resistance, to allow the load to obtain the maximum acoustic power. Comparing the system performance under different oscillatory modes, it is found that the existence of high frequency modes is helpful to take full advantage of the APAT's acoustic pressure amplification effect. Furthermore, the correspondence between the oscillatory modes, the pressure distributions and the stream directions are identified. Based on this, the principle of APAT's acoustic pressure amplification under different modes is explained, and the change tendency of high frequency and low frequency with APAT length and position is predicted reasonably. We believe that this work will provide a reference for the design and optimisation of the couple between TAE and load. • Varying APAT dimensions and load impedance triggers mode transitions in TAE system. • Performance is better in high frequency mode than that in low frequency. • Show the correspondence between different physical fields. • The amplification effect of APAT is determined by the acoustic field distribution. [ABSTRACT FROM AUTHOR]

• The PID temperature control method is applied in the cryostat with consideration of heat capacity hysteresis. • The response of temperature oscillation under different PID modes is analyzed. • The proper PID coefficients are given to satisfy the temperature stability of the cryostats. • The temperature time series are unfolded in phase portraits and Poincaré maps to analyze the temperature fluctuation characteristics. A fast response active temperature control method and ultra-high temperature stability in a cryostat are essential for the low-temperature primary gas thermometry. In this paper, numerical analysis models of the Proportional-Integral-Derivative (PID) active temperature control method on a cryocooler-based cryostat's performance with different coefficients are constructed. Furthermore, the heat capacity hysteresis is also considered to reduce temperature fluctuation further. The response of temperature oscillations under different PID modes is analyzed and compared to address the difficulty in accurately regulating the temperature oscillations. The system can quickly respond to the required temperature by predicting and optimizing the control coefficients. The flange temperature fluctuations are reduced by magnitude. We unfolded the temperature time series in phase portraits and Poincaré maps to identify the temperature fluctuation characteristics with different regulation coefficients. Numerical research in this paper realizes a faster response to disturbances with better temperature stability according to the dynamic characteristics of temperature fluctuation in the sample chamber. The pre-calculation of the control coefficient makes the heating power fluctuate in a small range, preventing overshoot and effectively reducing the percussion to the experimental equipment. [ABSTRACT FROM AUTHOR]