Investigations of axial conduction effect on local heat transfer performance in PCHE

doi:10.11949/0438-1157.20190377

Abstract

Abstract:

Numerical investigations of the axial conduction effect on the local heat transfer performance of the printed circuit heat exchanger (PCHE), which has straight channels and use supercritical pressure carbon dioxide (SCO₂) as working fluid, were conducted under laminar conditions. Variations of local heat flux, heat transfer coefficient, heat effectiveness and entropy generation due to heat transfer in PCHEs were obtained under the conditions with or without axial conduction, when the wall thickness, diameter as well as the inlet temperature difference between the two sides get changed. Then, the effect of the axial heat conduction could be analyzed by comparing the results with/without the axial conduction. It was found that the axial conduction could greatly impact the temperature distributions in both solid and fluid domains and lead to significant changes to the convective heat transfer of SCO₂ in PCHEs. When the axial conduction exists, the local heat flux variations get more even along the flow direction, and smaller heat transfer coefficient of the cold side as well as larger heat transfer coefficient of the hot side in the region of T<T _pc are also observed. The axial conduction would enlarge the peak heat effectiveness slightly and bring it to a lower bulk temperature. The heat transfer entropy production mainly occurs in the high temperature region, and the axial heat conduction can effectively reduce the local heat transfer entropy. Increasing the wall thickness and diameter can increase the influence of axial heat conduction, while increasing the inlet temperature difference, the influence of axial heat conduction does not change much.

Key words: printed circuit heat exchanger, supercritical pressure carbon dioxide, heat transfer, axial conduction, laminar flow, numerical simulation

摘要：

通过改变印刷电路板式换热器（PCHE）通道壁厚、直径以及冷热侧CO₂进口温度差，数值分析了层流条件下，轴向导热对PCHE内超临界压力CO₂流动换热的影响。结果表明，考虑轴向导热时，局部热通量沿程变化更加平缓，冷侧对流传热系数在小于拟临界温度区域变小，而热侧对流传热系数在小于拟临界温区域增大；轴向导热仅对热效率峰值附近区域有所影响，其使得峰值略有增大且向低温侧移动；传热熵产主要发生在高温区域，轴向导热可有效降低局部传热熵产。增大壁厚和直径可增大轴向导热的影响，而增大进口温差，轴向导热的影响变化不大。

关键词: 印刷电路板式换热器, 超临界压力CO₂, 传热, 轴向导热, 层流, 数值模拟

CLC Number:

TK 124

Haiyan ZHANG, Jiangfeng GUO, Xiulan HUAI, Xinying CUI. Investigations of axial conduction effect on local heat transfer performance in PCHE[J]. CIESC Journal, 2019, 70(12): 4590-4598.

张海燕, 郭江峰, 淮秀兰, 崔欣莹. PCHE内轴向导热对局部换热性能的影响研究[J]. 化工学报, 2019, 70(12): 4590-4598.

Figures/Tables 9

Fig.1 Cross section of numerical model

Table 1 Detailed sizes and boundary conditions of different structures

Case No.	D/mm	t _f /D	t _p /D	入口温度/K
Case No.	D/mm	t _f /D	t _p /D	热侧	冷侧
1	0.8	0.5	0.5	390.15	295.15
2	1.2	0.5	0.5	390.15	295.15
3	1.6	0.5	0.5	390.15	295.15
4	1.6	0.5	0.5	415.15	290.15
5	1.6	0.5	0.5	440.15	285.15
6	1.6	0.5	1	390.15	295.15
7	1.6	1	0.5	390.15	295.15

Table 2 Grid independence test

序号	网格数	壁面y ⁺		出口壁温值/K		相对误差/%
序号	网格数	热侧	冷侧	热侧	冷侧	热侧	冷侧
1	287292	2.43	2.26	296.47	365.13	0.064	0.637
2	516912	1.84	1.59	296.39	366.02	0.037	0.395
3	840516	1.06	0.83	296.31	367.18	0.010	0.079
4	1150720	0.71	0.51	296.28	367.47	0	0

Table 3 Comparison between numerical results and experimental results by Ishizuka[8]

项目	温度变化值 /K		压降值 /Pa
项目	热侧	冷侧	热侧	冷侧
实验结果	169.6	140.38	24180	73220
数值结果	170.58	137.286	26152.8	76000.5
相对误差/%	0.58	2.20	8.16	3.80

Fig.2 Temperature distributions in fluid (a) and solid domains (b) with and without axial conduction

Fig.3 Local heat flux q (a) and heat effectiveness ε(b) with different wall thicknesses

Fig.4 Relations of local convective heat transfer coefficient h(a), heat transfer entropy generation number Ns 1 ,T (b) and axial wall conduction number M c (c) with bulk temperature with different wall thicknesses

Fig.5 Local heat flux q(a), convective heat transfer coefficient h (b), heat transfer entropy generation number Ns 1 ,T (c) and axial wall conduction number M c (d) with different diameters

Fig.6 Local heat flux q(a), convective heat transfer coefficient h(b), heat transfer entropy generation number Ns 1 ,T (c) and axial wall conduction number M c (d) with different inlet temperature difference between two sides

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