基于燃气发电机组的冷凝换热器数值模拟及优化设计

发布时间：2018-05-28 02:02

本文选题：冷凝换热器 + 冷凝传热　；参考：《山东大学》2017年硕士论文

【摘要】：燃气发电机组是社会生活中广泛使用的设备之一,但其热效率仅有300%～40%,大部分热量都被排烟及冷却水带走,造成能量的极大浪费。余热利用的方式很多,但不管哪种方式,高效的换热设备是余热利用的基本要求。冷凝式余热换热器与一般换热器相比,烟气出口温度更低,不但回收更多显热,而且利用了潜热,能量的利用率提高。同时,烟气中的有害气体也大量溶于水,使污染物排放降低。因此冷凝式换热器不但节能而且环保,本文选择这种换热器作为回收烟气余热的装置。本文根据实际情况进行燃气发电机组的余热利用设计,主要包括冷却水及烟气回收换热器的选择及系统整体布置,得到余热利用系统图。分析烟气的冷凝换热模型,选择合适的冷凝传热系数,根据烟气冷凝换热的特性,建立数学公式,设计计算流程,利用已知条件对冷凝式余热换热器进行结构设计。然后根据结果使用计算流体力学(CFD)的前处理软件Gambit对换热器建立三维物理模型,对模型划分非结构性网格,并初步设置边界条件。在FLUENT中进行材料、操作条件及边界条件设置,选定合适的湍流模型、多相流模型,编译用户自定义程序。通过FLUENT实现对耦合传热传质过程求解,对数值模拟的结果进行处理得到烟气侧的温度、速度、压力分布图及不同初始参数(进口速度、温度等)对换热性能(换热系数、热效率)的影响。数值模拟结果表明:通过分析烟气侧的温度、压力、速度分布图可以看到烟气流动的变化趋势,发现换热器存在的一些问题并进行结构优化。不同的参数对换热性能影响有大有小,冷却水入口流量的增大可同时提高换热器的换热系数和热效率,但造成进出口压差大幅增大及出口水温大幅降低,需根据需要合理选择入口水流量。随着冷却水入口温度的升高,热效率逐渐降低,这是因为随着水侧的平均温度升高,管壁的平均温度也会升高,因此靠近壁面的烟气更难到达露点放出潜热;随着烟气速度的增加,换热器的热效率也下降,这是因为烟速增加,烟气与冷却水的换热时间就会减少,热量回收不充分,换热效率下降。另外烟速的增大使壳侧沿程阻力增大,进出口压差稍增,对内燃机影响不大。烟气中水蒸气的含量越大,混合气中水蒸气的分压上升,烟气的饱和温度升高,水蒸气在更高温度就可冷凝放热因此,潜热回收量增多,换热效率增大。本文基于燃气发电机组进行的烟气冷凝式余热换热器的设计计算和计算流程编制,对燃气内燃机及其他类似设备的烟气冷凝式余热换热器计算设计有一定的参考价值。对CFD仿真结果进行分析,经过图表分析得到的不同因素对换热器热效率及换热系数的影响规律,对之后的冷凝式余热换热器设计及改进具有一定的理论参考价值。
[Abstract]:Gas generator unit is one of the widely used equipments in social life, but its thermal efficiency is only 300% and 40%. Most of the heat is taken away by exhaust smoke and cooling water, resulting in a great waste of energy. There are many ways to use waste heat, but in either way, efficient heat exchanger is the basic requirement of waste heat utilization. Compared with the conventional heat exchangers, the condensed heat exchanger has lower exit temperature, more sensible heat recovery, and higher energy utilization rate by utilizing latent heat. At the same time, the harmful gases in the flue gas are also dissolved in water, reducing the pollutant discharge. Therefore, condensing heat exchanger is not only energy saving and environmental protection, this heat exchanger is chosen as the device for recovering waste heat from flue gas. In this paper, the waste heat utilization design of gas generating set is carried out according to the actual situation, including the selection of cooling water and flue gas recovery heat exchanger and the overall arrangement of the system, and the system diagram of waste heat utilization is obtained. This paper analyzes the condensation heat transfer model of flue gas, selects the appropriate condensation heat transfer coefficient, establishes the mathematical formula according to the characteristics of the flue gas condensation heat transfer, designs the calculation flow, and designs the structure of the condensing waste heat exchanger by using known conditions. Based on the results, a three-dimensional physical model of the heat exchanger is established by using the pre-processing software Gambit of computational fluid dynamics. The model is divided into non-structural meshes and the boundary conditions are preliminarily set up. Material, operation conditions and boundary conditions are set in FLUENT, suitable turbulence model and multiphase flow model are selected, and user-defined programs are compiled. The coupled heat and mass transfer process is solved by FLUENT, and the results of numerical simulation are processed to obtain the heat transfer performance (heat transfer coefficient) of the flue gas side such as temperature, velocity, pressure distribution and different initial parameters (inlet velocity, temperature, etc.). The effect of heat efficiency. The numerical simulation results show that the variation trend of flue gas flow can be seen by analyzing the temperature, pressure and velocity distribution of the flue gas side, and some problems existing in the heat exchanger can be found and the structure of the heat exchanger can be optimized. The influence of different parameters on the heat transfer performance is very small. The increase of the inlet flow rate of cooling water can increase the heat transfer coefficient and heat efficiency of the heat exchanger at the same time, but the pressure difference between the inlet and the outlet and the water temperature at the outlet can be greatly increased. According to the need for reasonable selection of inlet water flow. With the increase of the inlet temperature of cooling water, the thermal efficiency decreases gradually, which is because with the increase of the average temperature of the water side, the average temperature of the pipe wall will also increase, so it is more difficult for the smoke near the wall to reach the dew point to release latent heat. With the increase of flue gas velocity, the heat efficiency of heat exchanger also decreases, because the heat transfer time between flue gas and cooling water decreases with the increase of smoke speed, the heat recovery is not sufficient, and the heat transfer efficiency decreases. In addition, the increase of smoke velocity increases the side resistance of the shell and the pressure difference between the inlet and outlet, which has little effect on the internal combustion engine. The higher the content of water vapor in the flue gas, the higher the partial pressure of water vapor in the mixture, the higher the saturation temperature of the flue gas, and the higher the water vapor can be condensed and exothermic. Therefore, the amount of latent heat recovery increases and the heat transfer efficiency increases. Based on the design calculation and calculation flow of flue gas condensing waste heat exchanger carried out by gas generator set, this paper has a certain reference value for the calculation and design of flue gas condensing waste heat exchanger for gas combustion engine and other similar equipment. The simulation results of CFD are analyzed, and the influence of different factors on heat exchanger efficiency and heat transfer coefficient is analyzed, which has a certain theoretical reference value for the design and improvement of condensed waste heat exchanger.
【学位授予单位】：山东大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TK172;TM314

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