Flow process calculation and performance analysis of methane BOG ejector

doi:10.11949/0438-1157.20230237

Abstract

Abstract:

The introduction of ejectors in the boil-off gas (BOG) reliquefaction system in a liquefied natural gas (LNG) carrier can save energy and improve system operating efficiency. Due to the special storage condition of LNG, it is important to design a high-performance ejector. The structural design calculation of methane BOG ejector under giving operating condition was carried out by using the gas dynamic method. Based on the Mixture multiphase flow model and SST k-ω turbulence model, a numerical model of methane BOG ejector was established. The effect of operating conditions and structural parameters on the entrainment performance of methane BOG ejector was calculated and analyzed. The results showed that for the ejector with a fixed structure, there is an optimal motive pressure for the entrainment ratio to reach the maximum. Increasing the induced pressure can always improve the entrainment ratio. Under giving design operating conditions, keeping other structural parameters fixed, the energy losses caused by shock waves and vortices inside the ejector is the lowest and the entrainment ratio reaches a maximum when the primary nozzle outlet diameter and primary nozzle outlet position are 9.6 mm and 41 mm, respectively.

Key words: methane ejector, BOG re-liquefaction, entrainment ratio, CFD, two-phase flow

摘要：

在液化天然气（LNG）运输船中的蒸发气（BOG）再液化系统中引入喷射器可以节约能源，提高系统工作效率。由于LNG的存储工况较为严苛，设计高性能的喷射器至关重要。采用气体动力学函数法对给定工况下的甲烷BOG喷射器进行了结构设计。基于Mixture多相流模型和SST k-ω湍流模型建立了甲烷BOG喷射器数值计算模型，计算分析了运行参数和结构参数对甲烷BOG喷射器引射性能的影响规律。研究结果表明，对于固定结构的甲烷BOG喷射器，存在最佳工作压力使引射比达到最大值，提升引射压力可以持续提高引射比。在设计工况下，保持其他结构参数不变，当主喷嘴出口直径和主喷嘴出口位置分别为9.6 mm和41 mm时，喷射器内部由激波和涡流造成的能量损失最小，引射比达到最大值。

关键词: 甲烷喷射器, BOG再液化, 引射比, 计算流体力学, 两相流

CLC Number:

TK123

Chao NIU, Shengqiang SHEN, Yan YANG, Bonian PAN, Yiqiao LI. Flow process calculation and performance analysis of methane BOG ejector[J]. CIESC Journal, 2023, 74(7): 2858-2868.

牛超, 沈胜强, 杨艳, 潘泊年, 李熠桥. 甲烷BOG喷射器流动过程计算与性能分析[J]. 化工学报, 2023, 74(7): 2858-2868.

Figures/Tables 20

Fig.1 Schematic diagram of the methane BOG ejector

Table 1 Design parameters of the methane BOG ejector

参数	数值
工作流体压力/MPa	20.00
工作流体温度/K	203.15
引射流体压力/MPa	0.30
引射流体温度/K	153.15
混合流体压力/MPa	1.20
工作流体质量流量/(kg/s)	0.60
引射比	0.2932

Table 2 Structural parameters of the methane BOG ejector

参数	数值
主喷嘴喉部直径D_t/mm	3.8
主喷嘴出口直径D_o/mm	10.2
主喷嘴出口位置NXP/mm	52.0
圆柱形混合管直径D_mt/mm	16.7
圆柱形混合管长度L_mt/mm	71.0
扩压室长度L_d/mm	124.0
工作流体入口直径D_m/mm	12.8
引射流体入口直径D_s/mm	35.0
混合流体出口直径D_d/mm	32.0

Table 3 Mass and energy source terms of evaporation-condensation model

源项	蒸发过程（T>T_sat）	冷凝过程（T<T_sat）
质量源项	液相： $S m = - β e α l i q ρ l i q T s a t - T T s a t$	液相： $S m = β c α v a p ρ v a p T s a t - T T s a t$
质量源项	气相： $S m = β e α l i q ρ l i q T s a t - T T s a t$	气相： $S m = - β c α v a p ρ v a p T s a t - T T s a t$
能量源项	$S E = - β c α l i q ρ l i q T s a t - T T s a t h f g$	$S E = β c α v a p ρ v a p T s a t - T T s a t h f g$

Table 3 Mass and energy source terms of evaporation-condensation model

源项	蒸发过程（T>T_sat）	冷凝过程（T<T_sat）
质量源项	液相： $S m = - β e α l i q ρ l i q T s a t - T T s a t$	液相： $S m = β c α v a p ρ v a p T s a t - T T s a t$
质量源项	气相： $S m = β e α l i q ρ l i q T s a t - T T s a t$	气相： $S m = - β c α v a p ρ v a p T s a t - T T s a t$
能量源项	$S E = - β c α l i q ρ l i q T s a t - T T s a t h f g$	$S E = β c α v a p ρ v a p T s a t - T T s a t h f g$

Fig.2 Schematic diagram of the structural mesh and the flow domain of the methane BOG ejector

Fig.3 Mesh independence study

Table 4 Calculation results of GCI estimation

网格类型

网格数/个

安全

因子F_s

ε $a 21$ =0.82%

ε $a 23$ =7.50%

GCI $f i n e 21$ =0.45%

GCI $f i n e 32$ =1.36%

N₂=76346

N₃=31502

Table 4 Calculation results of GCI estimation

网格类型

网格数/个

安全

因子F_s

网格收敛误差ε

GCI

结构网格

N₁=127231

1.25

ε $a 21$ =0.82%

ε $a 23$ =7.50%

GCI $f i n e 21$ =0.45%

GCI $f i n e 32$ =1.36%

N₂=76346

N₃=31502

Fig.4 Comparison of entrainment ratio between the simulation and the experiment

Fig.5 Mass flow rate at inlets of methane BOG ejector variation with the motive pressure

Fig.6 Entrainment ratio of methane BOG ejector variation with the motive pressure

Fig.7 Mach number contours under various motive pressures

Fig.8 Liquid volume fraction along axis under various motive pressures

Fig.9 Mass flow rate and entrainment ratio of methane BOG ejector variation with the induced pressure

Fig.10 Mach number contours under various induced pressures

Fig.11 Entrainment ratio and mass flow rate under various primary nozzle outlet diameters

Fig.12 Mach number contours under various primary nozzle outlet diameters

Fig.13 Liquid volume fraction along axis under various primary nozzle outlet diameters

Fig.14 Entrainment ratio and mass flow rate under various primary nozzle exit positions

Fig.15 Mach number contours under various primary nozzle exit positions

Fig.16 Static pressure contours and pathlines under various primary nozzle exit positions

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