Flow and heat transfer characteristics of particles flowing along the plate with different mixing elements

doi:10.11949/0438-1157.20221011

Abstract

Abstract:

The shell-and-plate heat exchanger plays a significant role in the concentrating solar power systems. For improving the heat transfer performance of the shell-and-plate heat exchanger, the effects of plane, trapezoidal mixing element, elliptical mixing element, trapezoidal-elliptical mixing element and trigonal mixing element on particle flow and heat transfer were studied by discrete element method. The research shows that the blending rate of the plane is almost zero, and the blending rate of the trapezoidal blending unit is the highest. When particles flow around the trapezoidal mixing element, elliptical mixing element, trapezoidal-elliptical mixing element and trigonal mixing element, the temperature boundary layer is destroyed and redeveloped in the downstream region of the mixing elements. In the upstream region of the mixing elements, the mixing elements have an impediment to particle movement. The elliptical mixing element is the most impeded to its upstream particles. The feature velocity of particles is maximum in the downstream of the trapezoidal mixing element. When the particles move along the plane, there is no mixing phenomenon and the mixing efficiency of the plane is approximately zero. The average mixing efficiency of trapezoidal, elliptical, trapezoidal-elliptical and trigonal mixing elements are 3.8, 2.6, 3.2 and 2.5, respectively. The mixing efficiency of trapezoidal mixing element is the highest. In the downstream region of mixing elements, the heat transfer coefficients of trapezoidal, elliptical, trapezoidal-elliptical and trigonal mixing elements are significantly larger than that of the plane (average increased by 41.5%, 31.5%, 28.9% and 25.3%). Compared with other mixing elements, the flow and heat transfer characteristics of trapezoidal mixing element is the best.

Key words: granular flow, moving bed, heat transfer, mixing elements, discrete element method

摘要：

为了提高板壳式换热器的换热性能，通过离散元法研究了平面、梯形、椭圆形、梯形+椭圆形和三角形掺混单元对颗粒流动和换热的影响。研究表明：平面的掺混率几乎为零，梯形掺混单元的掺混率最高。颗粒在绕过除平面外的掺混单元时，温度边界层被破坏，并在掺混单元下游区域重新发展。在掺混单元上游区域，掺混单元对颗粒运动有阻碍作用，阻碍作用越大接触热阻越小。颗粒在梯形掺混单元下游的特征速度最大，入口平均温度最高。梯形掺混单元的掺混效率最高。在掺混单元下游区域，梯形、椭圆形、梯形+椭圆形和三角形掺混单元的传热系数显著大于平面(平均增加41.5%、31.5%、28.9%和25.3%)。相比其他掺混单元，颗粒外掠梯形掺混单元的流动换热特性最好。

关键词: 颗粒流, 移动床, 传热, 掺混单元, 离散单元法

CLC Number:

TK 513.5

Xing TIAN, Jiayue ZHANG, Zhigang GUO, Jian YANG, Qiuwang WANG. Flow and heat transfer characteristics of particles flowing along the plate with different mixing elements[J]. CIESC Journal, 2022, 73(11): 4884-4892.

田兴, 张家悦, 郭志罡, 杨剑, 王秋旺. 颗粒外掠含不同掺混单元平板的流动换热特性[J]. 化工学报, 2022, 73(11): 4884-4892.

Figures/Tables 14

Fig.1 Particle heat transfer path and thermal resistance model

Fig.2 Geometry of different mixing elements

Table 1 Geometric and particle parameters in simulation

变量	值
L/mm	16
W/mm	5
H/mm	66
T_inlet/K	800
T_w/K	300
ρ_p/(kg/m³)	2680
c_p_,p/(J/(kg·K))	730
k_p/(W/(m·K))	1.3
v_outlet/(mm/s)	1.0~10.0
d_p/mm	1.0

Fig.3 Verification of heat transfer model in DEM: total thermal resistance of plate at different particle velocity

Fig.4 Schematic diagram of particle flowing around different mixing elements with different color marks

Fig.5 Variations of the mixing ratio in different areas marked by different colors of zone 3

Fig.6 Variations of the average contact time (tc) at zone 1 outlet and the feature velocity (vfe) in zone 1 with outlet velocity

Fig.7 Variations of the average contact time (tc) at zone 3 outlet and the feature velocity (vfe) in zone 3 with outlet velocity

Fig.8 Variations of the mixing efficiency (P) of different mixing elements

Fig.9 Schematic diagram of temperature distribution of particle flowing around different mixing elements (voutlet=4 mm/s)

Fig.10 Variations of the heat transfer coefficient (h) of different mixing elements in upstream region (zone 1)

Fig.11 The contact resistance fitting of different mixing elements in upstream region (zone 1)

Fig.12 Average inlet temperature distribution of different mixing elements in downstream region

Fig.13 Variations of the heat transfer coefficient (h) of different mixing elements in downstream region (zone 3)

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