【作者】 司洪宇；

【导师】 梅宁；

【作者基本信息】 中国海洋大学 ， 港口、海岸及近海工程， 2011， 博士

【摘要】 液化天然气(LNG)在使用前,必须经过汽化后以气态输送给用户,汽化过程中会释放数量众多、品质很高的冷能,将这部分冷能有效利用会带来巨大的经济和社会效益。如何有效提高LNG冷能利用效率一直是人们关注的课题。本文主要研究了三个方面的内容,首先对LNG冷能利用的热力学理论基础进行了研究。为了最大程度地利用LNG冷能,换热器的换热性能至关重要,本文第二部分对LNG冷能利用过程中的有源强化换热技术进行了理论和试验分析。有源强化技术中的一种通过流体自激振动提供振源使换热壁面振动以达到强化换热目的的新技术目前仍然是一个全新的领域,在LNG冷能利用过程中存在强烈的相变和气液两相流动过程,两相流体流动比单相流体流动更容易产生自激振动,本文最后一部分对水平管内气液两相流体激振的机理进行了理论研究,为下一步将气液两相流体激振和强化换热技术相结合的新技术开发提供了前期的理论支持。本文首先对LNG冷能利用过程进行了系统分析、平衡分析、LNG冷能分析以及能级分析。从热力学的角度对LNG冷能利用进行了全面的“量”和“质”的分析,提出了LNG冷能梯级利用需要遵循的热力学原则:冷热流体间温差最小;不同温度段内冷能“支付”和“收益”双方的能级要匹配;不同的温度(包括环境温度、LNG温度、利用方式的温度)下综合利用LNG冷能方案分配要合理;以及综合考虑经济、环境、及设备各因素等。另外,通过能级分析,提出了LNG冷能综合利用方案(包括冷能直接利用和动力利用)分配的判断准则,并为此准则的工程应用总结出了快速判断准则图。本文第二部分通过分析认为有源强化换热的本质是边界层厚度的周期性变化导致的边界层导热性能的整体提高。基于此认识,本文建立了周期性边界层重构式强化换热技术的理论模型并进行了数值和试验研究。研究结果表明,周期性边界层重构式换热模型可以较好地解释类如刷壁式、振动强化式等有源强化换热过程的变化规律;周期性边界层重构方式对层流的换热强化效果尤其明显;边界层重构式强化换热技术存在一个特定重构周期,在该周期左右,强化换热效果最佳。本文第三部分是基于通过流体自激振动可以达到强化换热目的的认识,对LNG冷能释放过程中的气液两相流体激振(不稳定性)进行了研究。采用确定振荡流场的振荡流体力学原理和快速求解复杂流动的参数多项式法对水平直管、渐扩管、渐缩管内的气液两相流体激振过程进行了理论研究。研究结果表明,相对于截面直管,渐扩管可以起到强化从入口到出口的压力波动的效果,但会使得进出口压差减小,而渐缩管恰恰相反,它会使得进出口压差增大,却弱化从入口到出口的压力波动;压力波动的振幅和气相密度波动的振幅都是随着流量的增大而减小;水平管内气液两相流体激振的发生和压差——流量的性能曲线的位置无关。本文这三部分内容都是为了更好地提高LNG冷能利用率,为实现节能的目的服务,同时第二和第三部分的内容对强化换热技术的发展和气液两相流体激振的研究提供了一个新的研究思路。更多还原

【Abstract】 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.更多还原