Study On Fluid Uniform Optimization For The Parallel Piping In Natural Gas Liquefaction Cold Box

Uneven flow distribution in parallel pipes has always been a difficulty in the development of heat exchanger, and for multiphase flow, the flow mechanism is more complex, with which uniform distribution control in the heat exchanger is more difficult. At present, with the high-speed increase of energy demand and for the urgent request of protecting ecological environment, the demand for natural gas as a kind of clean energy promotes quickly. Cold box, as the core equipment of liquefied natural gas (LNG), is facing the development trend of large-scale, high parameter. Fluid uniform distribution in cold box has important influence on internal heat transfer liquefaction performance parameters.With the natural gas liquefaction production background, this article studied the fluid distribution characteristics in piping system of large-scale plate-fin heat exchangers, and discussed the effect of different flow conditions, the structure of the piping and two-phase flow physical properties to its fluid distribution and flow resistance. It also analyzed the relationship between flow condition, fluid dynamics characteristics,structure of the plate-fin heat exchanger piping system and flow maldistribution, put forward the corresponding plate-fin heat exchanger piping system structure optimization design scheme, which played great guiding significance for improving the product performance of large cold box and the actual cold box design, processing and manufacturing. Main research content of this article is as follows:For typical parallel running of plate-fin heat exchangers, this article constructed two-dimensional physical model for inlet piping and studied flows under different gas and liquid ratio, droplet size and gas liquid relative speed by using the MIXTURE method, getting the pressure field, velocity field, gas liquid phase volume fraction and other related data. This paper also analyzed the corresponding relationship between the flow distribution and different gas and liquid ratio, droplet size,as well as gas liquid relative speed. The result was that when the liquid phase droplet size increase, which account less in the two-phase flow, the flow become more uniform; The smaller relative sliding velocity between gas and liquid, which means speed of the gas and liquid become close, could make better fluid distribution; Outlet flow distribution in each branch takes minimum value when the velocity of gas and liquid become equal.Fluid distribution characteristics of two piping (type A and type B piping) with different structure was analyzed. This paper compared fluid distribution under different flow condition and analyzed flow field of the two pipes, founding that type B piping structure is better than type A piping in terms of flow distribution. Based on type B piping structure, we analyzed fluid uniformity under different pipe diameter ratio by changing the inlet and outlet pipe diameter. The results showed that the smaller the diameter ratio, the more uniform of tube fluid distribution. What’s more, under the same diameter ratio, the fluid uniformity lessened when the branch pipe diameter decrease with the other parameters keeping constant.On the basis of 2d numerical simulation, the 3d physical model of piping was established. This paper also studied the influence of angle between the pipe inlet tube and the branch pipe to fluid distribution under the three dimensional piping mode, analyzed the flow uniformity and the pressure field, velocity field in each cross section with the result that when the angle between inlet pipe and branch pipe is -90°, fluid distribution is best.In order to optimize the two phase flow uniform distribution of piping system, medium uniform distribution measure of filling element in branch pipe joint with porous was put forward, building a piping numerical calculation model based on porous media. Simulation calculation showed the corresponding pressure and flow distribution of the inlet pipe, the header and manifold. Compared to the traditional piping way, the effect of the new measure is remarkable.