Impact of turbulence model in coupled simulation of ethylene cracking furnace

doi:10.11949/j.issn.0438-1157.20181129

Abstract

Abstract:

The ethylene cracking furnace equipped with the bottom burner and the side wall burner is more and more widely used. Different combustion modes affect the turbulent flow state in the furnace. Considering the turbulent flow in the cracking furnace and the gas injection, the combustion and heat transfer are strong. The nonlinear coupling effect, for this purpose, explores the influence of different turbulence models in the cracking furnace/reactor coupling simulation is critical for the precise design and optimization of the cracking furnace. In this paper, a coupling simulation for a 100000 t industrial ethylene cracking furnace was carried out for different turbulence models. The turbulent flow model established by the standard k-ε model, RNG k-ε and Realizable k-ε model was evaluated by CFD numerical simulation. The simulation results of the three turbulence models are compared with the industrial data. The distribution of velocity, temperature and turbulence capacity in the cracking furnace is analyzed. The results show that the Realizable k-ε model is superior to the other two models in flame stability and reaction efficiency. And based on the Realizable k-ε turbulence equation, the calculation results of the heat flux and the outer wall temperature distribution of the furnace tube are closer to the actual working conditions.

Key words: turbulent flow, CFD, numerical simulation, cracking furnace, reactor coupling

摘要：

配备底部烧嘴和侧壁烧嘴的乙烯裂解炉应用越来越广泛，不同燃烧模式影响着炉膛内湍流流动状态，考虑到裂解炉中湍流流动与燃气喷料、燃烧和传热有较强的非线性耦合作用，为此探究不同湍流模型在裂解炉/反应器耦合模拟中的影响对于裂解炉的精确设计和优化至关重要。针对不同湍流模型对某十万吨工业乙烯裂解炉进行了耦合模拟，利用CFD数值模拟对采用标准k-ε模型、RNG k-ε和Realizable k-ε模型所建立的湍流流动模型进行评估。将三种湍流模型的模拟结果与工业数据进行比较，重点分析了裂解炉内的速度、温度、湍流能力等参数的分布情况，表明Realizable k-ε模型在火焰稳定性、反应效率等方面优于其他两种模型，且基于Realizable k-ε湍流方程的反应管模型在热通量、炉管外壁温度分布计算结果更接近实际工况。

关键词: 湍流, 计算流体力学, 数值模拟, 乙烯裂解炉, 反应器耦合

CLC Number:

Chengzhen NI, Wenli DU, Guihua HU. Impact of turbulence model in coupled simulation of ethylene cracking furnace[J]. CIESC Journal, 2019, 70(2): 450-459.

倪城振, 杜文莉, 胡贵华. 乙烯裂解炉耦合模拟中湍流模型的影响分析[J]. 化工学报, 2019, 70(2): 450-459.

Figures/Tables 14

Fig.1 SL-II cracking furnace structure

Table 1 Cracking furnace structure size and operating conditions

Item	Parameters
furnace segment
length (x-direction)/m	18.94
width (y-direction)/m	3.56
height (z-direction)/m	13.707
number of floor burners	36
number of wall burners	48
firing condition
fuel gas ?ow rate in bottom/(kg/s)	1.2439
fuel gas ?ow rate in side/(kg/s)	0.2025
oxygen excess/% (vol)	2
fuel composition/%(mass)
CH₄	97.686
H₂	0.516
CO	0.897
C₂H₄	0.899
reactor coils
number of reactor tubes	24
number of passes	2
inlet tube diameter×10³/m	64
outlet tube diameter×10³/m	121
thickness of tube×10³/m	6.5
feed rate/(kg/s)	11.11
steam dilution/(kg/kg)	0.6
coil inlet temperature/K	883
coil outlet pressure/kPa	206

Fig.2 Flue gas z-velocity component along width of furnace in plane 0.525 m at different heights

Fig.3 Temperature distribution of turbulence model in plane 0.225 m

Fig.4 Turbulence intensity distribution of turbulence model in plane 0.525 m

Fig.5 Turbulent flow energy of different turbulence models along height of furnace

Fig.6 Turbulent dissipation rate of different turbulence models along height of furnace

Fig.7 Turbulent viscosity of different turbulence models along height of furnace

Fig.8 NO concentration distribution of turbulence model in plane 0.225m

Fig.9 Distribution of NO mass fraction along height of furnace in different turbulence models

Fig.10 Heat flux of different turbulence models and temperature distribution curve of outer wall of furnace tube

Fig.11 Comparison of Prandtl numbers for different turbulence models

Table 2 Comparison of simulated and industrial values between different turbulences

项目	工业值	标准 k-ε模型	Realizable k-ε模型	RNG k-ε模型
出口油气温度/K	1108	1102.8	1104.6	1099.6
最大管壁温度/K	1247.6	1251.9	1253.5	1260.4
过剩氧含量/%(vol)	2	1.81	1.90	1.90
炉管油气压降/MPa	0.0433	0.0350	0.0366	0.0355
P/E	0.68	0.72	0.68	0.0.74
乙烯收率/%(mass)	30.143	29.86	29.74.	29.46

G_k	——由平均速度梯度引起的湍流动能
U_i	——速度矢量
Y $i *$	——时间τ^*后细微尺度内组分i的质量分数
μ_t	——湍流黏性系数
ξ^*	——细微尺度长度分数
ρ	——流体密度
σ_ε	——湍流Prandtl数
τ^*	——反应时间尺度

G_k	——由平均速度梯度引起的湍流动能
U_i	——速度矢量
Y $i *$	——时间τ^*后细微尺度内组分i的质量分数
μ_t	——湍流黏性系数
ξ^*	——细微尺度长度分数
ρ	——流体密度
σ_ε	——湍流Prandtl数
τ^*	——反应时间尺度

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