Enhancement on steam dropwise condensation heat transfer with superhydrophobic surfaces of PVDF hollow fiber heat exchange tubes

doi:10.11949/j.issn.0438-1157.20181053

Abstract

Abstract:

To improve the heat transfer performance of the plastic heat exchange tube, a composite plastic heat exchange tube with a superhydrophobic surface was prepared by a two-step coating method. Firstly, the porous polyvinylidene fluoride (PVDF) hollow fiber membrane was used as the supporting layer, and polydimethylsiloxane (PDMS) was filled with thermal conductive material nano-ZnO as the skin layer to prepare the composite plastic heat exchange tube with a dense outer skin layer. Secondly, the PVDF composite plastic heat exchange tube with superhydrophobic surface was prepared by investigating the influence of teraethoxysilane (TEOS) and ammonia content, which enhanced the dropwise condensation heat transfer performance of steam. The results showed that the outer surface contact angle of the prepared heat exchange tube can reach 154°. Compared with the heat exchange tubes prepared by the melting method and the NIPS method, the total heat transfer coefficient could be increased by 85.3%—147.3%.

Key words: polyvinylidene fluoride, plastic heat exchange tube, superhydrophobic surface, dropwise condensation, heat transfer coefficient

摘要：

为了提高塑料换热管的传热性能，通过两步涂覆法制备了具有超疏水表面的复合塑料换热管。首先采用多孔PVDF中空纤维膜为支撑层，以导热材料纳米ZnO填充聚二甲基硅氧烷（PDMS）为皮层，制备了具有致密外表皮层的复合塑料换热管。其次为了强化蒸气的滴状冷凝传热，通过考察正硅酸乙酯含量，氨水含量等条件的影响，制备出了具有超疏水表面的PVDF复合塑料换热管。结果表明，所制备的换热管表面接触角可达154°，与熔融法及NIPS法制备的换热管相比，总传热系数可提高85.3%～147.3%。

关键词: 聚偏氟乙烯, 塑料换热管, 超疏水表面, 滴状冷凝, 传热系数

CLC Number:

TQ 028.8

Yexia CHAI, Huayan CHEN, Yue JIA, Dandan LI, Chunrui WU, Xiaolong LYU. Enhancement on steam dropwise condensation heat transfer with superhydrophobic surfaces of PVDF hollow fiber heat exchange tubes[J]. CIESC Journal, 2019, 70(4): 1331-1339.

柴叶霞, 陈华艳, 贾悦, 李丹丹, 武春瑞, 吕晓龙. PVDF中空纤维换热管超疏水表面强化蒸气滴状冷凝传热[J]. 化工学报, 2019, 70(4): 1331-1339.

Figures/Tables 14

Table 1 PVDF ultrafiltration membrane and membrane module parameters

PVDF ultrafiltration membrane

PVDF membrane module

Bore

size/μm

Wall thickness/μm

Module inner

diameter /mm

Membrane filament

length /mm

Loading density/%

Heat exchange

tube number/root

Effective membrane

area/m²

Fig.1 SEM images of outer surface of original membrane and heat exchange tubes

Table 2 Comparison of compactness between PVDF original membrane and heat exchange tubes

Item

PVDF original membrane

Heat

exchange tube M₁

Heat exchange tube M₂

pure water flux/(kg·m^-2·h^-1)

76.49

percentage of pressure drop /%

Fig.2 Cross-sectional morphology and characteristic X-ray energy spectrum line sweep of PVDF original film and PVDF heat exchange tube

Table 3 Comparison of mechanical properties and porosity of PVDF original membrane and heat exchange tubes

Item	PVDF original membrane	Heat exchange tube M₁	Heat exchange tube M₂
breaking strength/cN	218	223.4	257
porosity/%	60.26	55.16	52.73

Fig.3 Effect of running time on heat transfer performance of heat exchange tube

Table 4 Stability of PVDF heat exchange tube skin layer

Ultrasonic time/h	Gel layer
1	not falling off
2	not falling off
3	not falling off
4	not falling off
5	not falling off
6	not falling off

Fig.4 Influence of TEOS content on silicon particle size and surface roughness

Fig.5 Influence of TEOS content on contact angle and total heat transfer coefficient

Fig.6 Influence of TEOS content on condensation heat transfer coefficient

Fig.7 Influence of ammonia content on silicon particle size and surface roughness

Fig.8 Influence of ammonia content on contact angle and total heat transfer coefficient

Fig.9 Influence of ammonia content on condensation heat transfer coefficient

Fig.10 Comparison of heat transfer performance with other heat exchange tubes

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