Analysis of Dynamic Stability of Ejector Expansion Refrigeration System

The stable operation of a system is restricted by the intrinsic characteristics, namely harmonious matching among and between structure parameters and operation parameters. In this paper, we put forward an analysis method of the dynamic stability of the refrigeration system based on the First Approximation Theory of Lyapunov Stability Theorem and the evaluation of stability margin. It is carried out by linearizing the governing equations and analyzing the eigenvalue form of coefficient matrix. And the minimum logarithmic decrement is calculated to represent the stability margin. Analyzing the stability of gas cooler confirms the consistency between the mathematical stability and the actual dynamic one. The proposed method is performed in a transcritical CO2 ejector expansion refrigeration system (EERS) to analyze the system dynamic stability. The present results show that, even each component of the system is in the stable state, it cannot guarantee the dynamic stability of the whole system. Moreover, the effect of state parameter on system dynamic stability is investigated. The work supplies a guiding principle on system control and may be extended in more general thermodynamic cycle

A Visualization Investigation on the Influence of the Operating Conditions on the Phase Change in the Primary Convergent-divergent Nozzle of a Transcritical CO2 Ejector

Complex flow processes exist in the primary convergent-divergent nozzle of a transcritical CO2 ejector because of the rapid expansion of the supercritical CO2 flow, which have a significant influence on the performance of a transcritical CO2 ejector expansion refrigeration system. A visualization experiment with the direct photography method was carried out to investigate the phase change phenomena in the primary convergent-divergent nozzle of a transcritical CO2 ejector. The visualization transcritical CO2 ejector was designed as a rectangular cross section to minimize the optical distortion. In order to better interpret the phase change phenomena of CO2 flow, four pressure measurement points were lumped in the convergent-divergent nozzle to get the pressure distribution along the convergent-divergent nozzle for various operating conditions. The results revealed that the phase change position in the convergent-divergent nozzle was closely related to the primary flow inlet conditions and the suction flow inlet pressure. .The results showed that the phase change could start after or before the nozzle throat, and the phase change position moved upstream by decreasing the primary flow inlet pressure and temperature simultaneously. As keeping the primary flow inlet pressure constant, the phase change position also moved upward by decreasing the suction flow inlet pressure. In addition, the measured pressure results indicated that the pressure differences in the convergent section of the primary convergent-divergent nozzle increased as the CO2 suction flow inlet pressure decreased because of more adequate expansion of the primary flow

Visual Investigation on Effect of Structural Parameters and Operation Condition of Two-phase Ejector

As an important component in transcritical CO2 refrigeration cycle, complex flow in ejector have not been clearly elucidated. In this paper, CO2 flow in two-phase rectangle ejector was investigated experimentally by visualization measurement. The phase transition and the relaxation phenomena in the ejector were observed. By analyze the picture and the data collected from this experiment, we study the relationship between efficiency of ejector and the phase transition position in the ejector. Firstly, the microstructure of the flow pattern in the ejector was captured by a high speed digital video camera with a microscope to analyze the mixing process in mixing chamber. It was found that there were two stages with different characteristics in mixing process ,which were named fluid mixing section and fluid equilibrium section. When fluid get through mixing channel, the ejector realize the majority functions of entrainment in the first stage, and the ejector also homogenize the velocity of primary fluid and secondary flow by the way of flow core expand to almost all the channel in the second stage. Secondly, based on the comparison of pictures collected from different ejectors under different operating conditions, we found that phase transition position and the form of phase transition was mainly depended on the entrance condition of motive nozzle. For an ejector that keeps the suction nozzle under the same operation condition, when the phase transition point trend to exit of motive nozzle, in mixing channel ,motive flow will occupy more space meanwhile the relaxation phenomena occurred in longer region. It was worth mentioning that the phase transition point will change with different operation condition. But there exist only one best position where the ejector contributes to best efficiency. So, it is of great significance to treat phase transition point as an important sign which was easy to be recognized. Visualization research of ejector will be an important reference for theoretical study of flow pattern in the ejector. It also can provide some date to validate the results from the numerical calculation. The visualization study of ejector will also be the basis of further learn of shock waves and delayed phase transition in the ejector

Improved Decoding of Staircase Codes: The Soft-aided Bit-marking (SABM) Algorithm

Staircase codes (SCCs) are typically decoded using iterative bounded-distance decoding (BDD) and hard decisions. In this paper, a novel decoding algorithm is proposed, which partially uses soft information from the channel. The proposed algorithm is based on marking certain number of highly reliable and highly unreliable bits. These marked bits are used to improve the miscorrection-detection capability of the SCC decoder and the error-correcting capability of BDD. For SCCs with $2$-error-correcting Bose-Chaudhuri-Hocquenghem component codes, our algorithm improves upon standard SCC decoding by up to $0.30$~dB at a bit-error rate (BER) of $10^{-7}$. The proposed algorithm is shown to achieve almost half of the gain achievable by an idealized decoder with this structure. A complexity analysis based on the number of additional calls to the component BDD decoder shows that the relative complexity increase is only around $4\%$ at a BER of $10^{-4}$. This additional complexity is shown to decrease as the channel quality improves. Our algorithm is also extended (with minor modifications) to product codes. The simulation results show that in this case, the algorithm offers gains of up to $0.44$~dB at a BER of $10^{-8}$.Comment: 10 pages, 12 figure

Optimization of Combined Casing Treatment Structure Applied in a Transonic Axial Compressor Based on Surrogate Model

For modern high load compressors, an excellent stability-enhancing capability by casing treatment (CT) is desirable. However, it is very time consuming to accomplish effective CT design. In this study, a new combined CT structure composed of axial skewed slots and end-wall injection, was proposed to be installed in transonic axial compressors to improve the overall performance. Considering the high computation cost for CFD simulation of the flow field in transonic compressor, a Gaussian Process Regression (GPR) surrogate model combined with Latin hypercube sampling, was utilized to predict compressor performance. For optimization process, a multi-objective evolutionary algorithm (NSGA-Ⅱ) was adopted to obtain the Pareto-optimal front. The main geometric parameters of the slot and the mass-flow rate of injection were selected as design parameters, with the peak efficiency and pressure ratio being two objectives. The results indicated that the surrogate model works well in capturing the key features of the concerning target and accelerating the optimization process. The optimal scheme of the combined CT was found able to increase stall margin (SM) by 19.5% with low efficiency penalty, showing a better performance than the reference combined casing treatment (CCT) scheme. What’s more, the analysis results of entropy generation showed that the superior effect of optimized scheme (OPT) can be attributed to the improvement of exchange flow in slots and the decreased loss in the whole passage