Research on solidification characteristics of particles formed by dropping molten polyethylene glycol into oil pool

doi:10.11949/0438-1157.20250429

Abstract

Abstract:

In direct-contact latent heat storage systems, phase change material (PCM) directly contacts the heat transfer fluid (HTF), enabling higher storage density and rate. However, forming convective channels during melting takes time, and particle shape and size affect channel structure and heat storage performance. This study investigates the solidification of molten polyethylene glycol droplets falling into an oil pool through experiments and simulations. The effects of drop height, oil pool depth, and droplet diameter were analyzed using a high-speed camera and electron microscope. The results show that exceeding a critical falling height causes droplet splashing and collisions. Oil depth and droplet size significantly affect solidification before bottom contact, influencing particle formation. A semi-analytical and empirical model suggests optimal shaping occurs when the solid layer thickness is about 7.67% of the droplet radius. A surface tension correction model was proposed, and when σ = 0.1, simulation results matched experimental data well. Simulations also reveal non-uniform heat transfer: the top cools fastest due to backflow disturbance, while side heat transfer is weaker. Additionally, droplet diameter and fluid temperature exhibit nonlinear effects on the critical oil depth.

Key words: direct contact, phase change, droplet, condensation, shaping, numerical simulation

摘要：

直接接触式潜热储热通过相变材料与传热流体的直接接触，实现更高的储热密度与速率，但熔化过程需要一定时间形成对流换热通道。相变材料颗粒的形状和大小会影响通道的结构，从而影响储热性能。通过实验与数值模拟，研究了熔融聚乙二醇液滴滴入油池的颗粒凝固成形特性。利用高速相机和电子显微镜，分析了滴落高度、油池深度及液滴直径的影响。实验结果表明，超过临界滴落高度会导致液滴抛射并发生碰撞；油池深度与液滴直径显著影响液滴触底前的凝固程度，从而影响颗粒成形质量。通过半解析-半经验的计算方法，认为液滴触底时的凝固层厚度约为半径的7.67%时成形效果较好。提出用表面张力系数σ修正液滴形状的数值模型，当σ=0.1时，模拟结果与实验数据吻合良好。数值模拟揭示了液滴凝固过程中的非均匀性传热特征：顶部区域因回流扰动而冷却速率最快，侧部传热较弱。此外，液滴直径与传热流体温度对临界油池深度的影响呈非线性。

关键词: 直接接触, 相变, 液滴, 凝结, 成形, 数值模拟

CLC Number:

TK 02

Zhangjing ZHENG, Qingyun YANG, Shixing YAN, Yuchen SHI, Yang XU. Research on solidification characteristics of particles formed by dropping molten polyethylene glycol into oil pool[J]. CIESC Journal, 2025, 76(12): 6302-6313.

郑章靖, 杨清云, 闫室兴, 施宇辰, 徐阳. 熔融聚乙二醇滴入油池的颗粒凝固成形特性研究[J]. 化工学报, 2025, 76(12): 6302-6313.

Figures/Tables 19

Table 1 The thermophysical properties of PCM and HTF

物性参数	导热油	聚乙二醇4000
密度/(kg·m^-3)	857	1125
运动黏度/(mm²·s^-1)	270 (0℃)，29 (40℃)	8~11
热导率/(W·(m·K)^-1)	0.135	0.085
比热容/(kJ·(kg·K)^-1)	2.049	2.048
熔化温度/℃	—	46.93~62.75
凝固温度/℃	-12	34.21~42.8
潜热/(kJ·kg^-1)	—	117.32

Fig.1 Schematic diagram of the experimental apparatus

Fig.2 Geometric model

Fig.3 Schematic diagram of grid division for some areas

Fig.4 The time graph of polyethylene glycol droplets hitting the surface of the oil pool (z=10 cm)

Fig.5 The time graph of polyethylene glycol droplets hitting the surface of the oil pool (z=100 cm)

Fig.6 Particle diameter measurement

Fig.7 Solidified polyethylene glycol particle morphologies under different G values (side view)

Fig.8 The influence of the drop height z on the sphericity degree G and the particle diameter d

Fig.9 The shape change process of polyethylene glycol particles in contact with the bottom of the oil pool (side view)

Fig.10 Solidified particle morphologies formed under different oil pool depths and elbow diameters

Fig.11 The sphericity degree G of polyethylene glycol particles of different diameters varies with the depth of the oil pool

Table 2 Calculation results of the thickness ratio of the critical solidification layer of the droplet

液滴直径/mm

油池深度/cm

触底时间/

平均速度/(m

/

Table 2 Calculation results of the thickness ratio of the critical solidification layer of the droplet

液滴直径/mm

油池深度/cm

触底时间/

平均速度/(m

/

临界凝固层厚度比

3.9

4.95

0.0424

0.0827

4.2

5.49

0.0455

0.0802

4.9

5.30

0.0528

0.0672

Fig.12 Comparison of droplet shapes with different surface tension coefficients

Fig.13 Temperature contour maps of PCM droplet at various steps

Fig.14 Solidification degree contours of PCM droplet at various times

Fig.15 Velocity field distribution during droplet sedimentation

Fig.16 The relationship between the critical oil pool depth and the droplet diameter

Fig.17 The relationship between the critical oil pool depth and the HTF temperature

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