Study On The Flow And Thermal Effects Of Pressure Oscillating Tubes

The fluid flow in pressure oscillating tubes is a type of fluid waves under periodic input The process of such flow involves both energy transfer and various patterns of wave motion, resulting in thermal effects on the wall of the tubes. Understanding the characteristics of how the internal wave-flow filed responding to external stimuli is one of the fundamental problems in studying pressure oscillating tubes. The research into the flow and thermal effects has theoretical and practical values for the development of "wave" machines.The flow and thermal effects of pressure oscillating tubes has attracted wide attention in the area of air expansion refrigeration and pressure exchange. Various pressure-oscillating-tube based equipments have been developed and effectively applied. Such equipments usually have a simple structure, can work smoothly under different conditions and are able to adapt to gas-liquid two-phase flow. However, they also have some disadvantages, for instance, containing liquid inside the tube, large sized and vibrating strongly. To date, there has been limited theoretical and experimental research to look into the flow and thermal effects inside the tubes systematically.The aim of this research is to investigate the flow and thermal effects in pressure oscillating tubes and their potential values for practical applications. This thesis presents a detailed account of the techniques developed, the platforms designed and the numerical simulation and experiments conducted to validate the results, including:(1) To simulate the complex flow process containing both shock and expansion waves in a multiple-tube system over multiple periods. Numerical models, where a sliding-mesh method was employed to describe the relative slip between rotary and static components, were established for investigating the performance of pressure oscillating tubes. The numerical analysis was applied for quantitative modeling of the unsteady-flow behaviors of shock waves, expansion waves and the separation interface. It was also used for describing the energy transfer caused by wave flow inside the tubes, thus, laying a foundation for numerical simulation and performance analysis of pressure oscillating tubes.(2) To analyze the flow and thermal effects of one-end-closed pressure oscillating tubes through experiments and numerical simulation. An experimental platform was constructed and applied for studying the flow and performance of the tubes with different sizes under different operating conditions. The parameters such as inner transient pressure and outer wall temperature were measured in real time. The effect of expansion ratio, pressure and frequency on the refrigeration performance was examined. The results indicated that the tube-wall temperature increased rapidly at the open end and then decreased gradually along the tube. The flow and the distribution of the tube-wall temperature were affected by several parameters, including the length of the tube, jet flow frequency and expansion ratio. The results also demonstrated shock waves were a major factor affecting the distribution of the tube-wall temperature.In this study, a sudden-expansion structure, where an expansion chamber was connected to pressure oscillating tubes, was proposed. It has been identified that the expansion chamber can weaken the reflected shock waves, thus, enhancing the refrigeration performance. It was also found that the chamber can weaken the wave transfer between tubes in a multiple-tube system. To enhance the efficiency, discharge liquid and disperse heat, a heat exchanger was attached to pressure oscillating tubes. The experiments indicated that attaching an expansion chamber at the close end of the tubes can improve the isentropic refrigeration efficiency by 15%. In the study, the flow in such tubes was also investigated.(3) To investigate the flow and performance of pressure oscillating tubes propelled by an integrated jet oscillator. The pressure, velocity and flux in a bistable wall-attaching jet flow unit were analyzed. The influence of operating parameters (e.g. pressure and pressure ratio) and structural parameters (e.g. potential difference and nozzle width) on the flow velocity, wall-attachment effect and jet-pressure recovery was studied. In order to understand oscillation characteristics in a sonic under-expansion wall-attaching jet oscillator, experiments were designed to test and examine the oscillating frequencies of the sonic oscillator with different control-channel lengths. The refrigerating efficiencies of Jet-Oscillation Gas Wave Refrigerators working at different oscillating frequencies were measured.(4) To investigate the flow and thermal effects of double-end-open pressure oscillating tubes. An experimental platform was constructed for testing the effect of operating and structural parameters on the performance of the tubes. Numerical simulation was conducted to provide a quantitative and graphic depiction of the flow of shock waves and the separation interface in a multiple-tube system over multiple periods. The results indicated that a backward compression wave was generated at the high-temperature discharge end. This reduced the refrigeration performance. Accordingly, a structure with a two-stage discharge chamber was proposed to decrease the temperature at the high-temperature discharge end and, as a result, reduce the influence of the backward compression wave. Numerical simulation and experiments were conducted to examine the injection loss during the process of gradually opening and closing of the high-pressure nozzles. The results indicated that the loss can be reduced to a certain degree by increasing the injection-nozzle width or adjusting the jet injection angle. The analysis on the leakage loss demonstrated that there was a larger amount of the loss due to the clearance between the tube and the high-pressure injection nozzle as compared to that caused by the clearance between the tube and the high-temperature discharge nozzle. Based on the above results, we proposed a new external-circulation dissipative gas wave refrigerator. Its refrigeration efficiencies under different operating and structural parameters were measured.