Study Of Enhanced Heat Transfer And The Gas-liquid Two-phase Flow Vibration In Horizontal In-tube In The Utilization Process Of Liquefied Natural Gas's Cold Energy

Liquefied natural gas (LNG) must be vaporized to gas for delivery to the user, and releases a great deal of high-quality cold energy during its vaporization process. The effective utilization of the cold energy will bring enormous economic and social benefits. How to effectively improve the efficiency of LNG cold energy utilization has become a hot topic.In this paper, the thermodynamic theory of LNG cold energy utilization firstly is explored. In order to maximize the utilization of LNG cold energy, one of the most important equipments is the heat exchanger. So the second part pays attention to the theoretical and experimental study of the active enhanced heat transfer technique in the process of LNG cold energy utilization. Enhanced heat transfer by the fluid's self-excited vibration, which makes the heat transfer surface vibrate, is still a new field. There exists strong phase transition and gas-liquid two-phase flow in the process of LNG cold energy utilization, and the two-phase flow is more prone to self-excitedly vibrate than the single-phase fluid flow. The third part studies the mechanism of the two-phase self-excited vibration in horizontal in-tube. The later two parts provide a pre-theoretical support to the new enhanced heat transfer technique by the gas-liquid two-phase self-excited vibration.In the first part, the system exergy analysis, exergy balance analysis, LNG cold exergy analysis and energy levels analysis have been presented in the utilization process of LNG cold energy. This work is an overall thermodynamic analysis of LNG cold energy utilization in terms of the "quantity" and "quality" of the energy. And then the thermodynamics principles of the cascade utilization of LNG cold energy is drawn: the temperature difference between hot and cold fluids should be as small as possible; the cold energy level of the "payment"should matches the " income" level; the comprehensive utilization of LNG cold energy distribution must be reasonably arranged under different temperatures (including ambient temperature, LNG temperature, and the user's temperature); and the economy, the environment, and the equipment should be also overall considered; and so on. In addition, the arrangement criterion of the comprehensive utilization of LNG cold energy (including the direct cold energy utilization and power utilization) was proposed after the level analysis of LNG cold energy and a figure is presented to quickly ensure the comprehensive utilization scheme. .In the second part, the essence of active enhanced heat transfer is analyzed and it is considered that the enhanced heat transfer is caused by the periodic change of the boundary layer's thickness. Based on the analysis, a numerical theoretical model of periodic renewing boundary layer is built to study the properties of enhanced heat transfer and then it is verified by a test and others'references. The results show that the periodic renewing boundary layer model can reflect well the rules of active enhanced heat transfer such as by brushing surface or the surface vibrating. The conclusion is gotten that periodic renewing boundary layer affects more obviously heat transfer in laminar flow than in turbulence flow and around a typical period heat transfer is most enhanced.The third part is based on the understanding of heat transfer can be enhanced by the self-excited vibration of the fluid. The gas-liquid two-phase vibration during the process of LNG vaporization is studied by Oscillation fluid dynamics and the parameter polynomial method in horizontal in-tube (including straight, increasing, and reducing cross-section tubes). The results show that, compared to straight tube, increasing tube can strengthen the pressure oscillation from the inlet to the outlet, but the pressure drop between them decreased; on the contrary, the reducing tube can make the pressure drop increase, but the pressure oscillation decrease. Both of amplitudes of the pressure oscillation and the gas density oscillation decrease with the increase of the mass flow. And the vibration of the two-phase flow in horizontal pipe has nothing to do with the location of pressure drop curve, which is different with the vertical tube.These three parts of this paper is to improve the utilization of LNG cold energy and the research results have great signifacance to save energy. In additional, the second and third parts of the research provide a new idea for the study of the enhanced heat transfer technique and the vibration of two-phase flow.