Turbulent heat transfer and mixing enhancement characteristics in Ross LPD static mixer

doi:10.11949/0438-1157.20220232

Abstract

Abstract:

The performance of turbulent flow and heat transfer in Ross low pressure drop (LPD) static mixer were experimentally and numerically investigated under constant heat flux condition. The effects of Reynolds number Re and staggered angle were evaluated in the turbulent state with a range of Re from 2640 to 17600. The Nusselt number, Darcy friction coefficient, comprehensive heat transfer coefficient, and synergistic angle between velocity field and temperature/pressure gradient were used to evaluate the heat transfer enhancement performance in the mixer. The stretching characteristics of fluid element in the Ross LPD and Kenics static mixers were analyzed based on computational fluid dynamics (CFD) and Lagrangian particle tracking (LPT) method. The results show that the performance of turbulent resistance and heat transfer in Ross static mixer predicted by SST k-ω model was in good agreement with experimental results. A longitudinal vortex with the similar scale of the flow field is induced in the Ross mixer, and its vortex center shifts periodically between the centers of circular and semicircular sections. The average stretching rate in the cross section of Ross static mixer is 3.36—1.72 times that in Kenics mixer which indicates that the enhancement ability of dispersion micromixing efficiency gradually weakens with the increasing axial length. The comprehensive heat transfer performance of Ross LPD gradually becomes much better with the increase of Re. Furthermore, it is obvious larger than 1 which indicates that η is superior to that in the KSM when Re>7040. When the blade angle is 30°, the comprehensive heat transfer coefficient of performance has the maximum value. The Ross LPD internal inserts have the technical advantages of high efficiency with low flow resistance and the potential for structural improvement.

Key words: static mixer, mixing, heat transfer, PEC, field synergy

摘要：

在湍流状态Re=2640~17600下，采用恒热通量传热实验与数值模拟相结合的方法，系统研究Reynolds数Re和交错角对Ross LPD型静态混合器内湍流流动与传热性能影响，采用Nusselt数、Darcy摩擦系数、综合传热系数、速度场与温度梯度和压力梯度协同角等参数评价混合器内传热强化性能；基于CFD与LPT相耦合分析混合器内流体微元拉伸率。研究结果表明：SST k-ω模型预测Ross型静态混合器湍流阻力及传热结果与实验结果具有很好一致性；Ross混合器流场内形成与流场尺度较为接近的纵向涡，其涡心在圆形截面与半圆形截面中心间周期性迁移，横截面内湍流分散混合效率是Kenics的3.36~1.72倍；当Re>7040时，Ross LPD综合传热性能明显优于KSM；当叶片夹角为30°时，综合传热性能系数具有最大值；Ross LPD内插件具有高效低阻的技术优势和结构改进潜力。

关键词: 静态混合器, 混合, 传热, 综合传热性能系数, 场协同

CLC Number:

TK 124

Huibo MENG, Tong MENG, Yanfang YU, Zongyong WANG, Jianhua WU. Turbulent heat transfer and mixing enhancement characteristics in Ross LPD static mixer[J]. CIESC Journal, 2022, 73(8): 3541-3552.

孟辉波, 蒙彤, 禹言芳, 王宗勇, 吴剑华. Ross LPD型静态混合器内湍流传热与混合强化特性[J]. 化工学报, 2022, 73(8): 3541-3552.

Figures/Tables 13

Fig.1 Geometries of Ross LPD static mixer in different views

Table 1 Specifications of the Ross LPD static mixer

参数	数值
入口段长度 (l₁)	40 mm
元件长度 (l₂)	82.91 mm
相邻元件距离 (l₃)	60 mm
出口段长度 (l₄)	60 mm
椭圆板长轴 (a)	88.11 mm
椭圆板短轴 (b)	40 mm
管径 (D)	40 mm
叶片夹角 (α)	30°
元件个数	12
元件厚度 (δ)	2 mm

Fig.2 Schematic diagram and testing apparatus of heat transfer experiment under constant heat flux

Fig.3 Validation of experimental validity

Fig.4 Model validation test for the choice of turbulent models

Table 2 Error between predicted results of different models and experimental data

Model	Error of f/%	Error of Nu/%
SST k-ω	7.07	6.31
Standard k-ω	7.32	9.88
BSL k-ω	9.03	9.1
Realizable k-ε	14.86	191.43
Transition k-kl-ω	7.52	25.43
Standard k-ε	14.42	225.66
RNG k-ε	13.13	310.87
Transition SST	7.11	19.68
Reynolds stress	12.06	136.95

Fig.5 Grid independence tests

Table 3 Initialization parameter for grid independence verification

边界层层数(B)	网格数量/个
0	1095201
5	1562317
7	1746037
10	1999405
13	2721679
15	2813697

Fig.6 Streamlines and temperature distribution at different cross sections of Ross LPD

Fig.7 The comparison of heat transfer and flow resistance in KSM and Ross LPD

Fig.8 The volume-weighted average synergy angle comparison in KSM and Ross LPD

Fig.9 The strain rate in KSM and Ross LPD

Fig.10 Comparison of Ross LPD heat transfer capacity and flow resistance at different blade angles

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