高热流低流速条件下超临界CO2在小圆管内的对流传热特性

doi:10.11949/j.issn.0438-1157.20181440

摘要/Abstract

摘要：

为获取高热流、低流速条件下超临界CO₂的传热规律，开展了超临界CO₂在内径2 mm水平小圆管内对流传热试验研究，并重点探讨了变物性、浮升力和热加速等效应对传热过程的影响。试验参数范围：系统压力7.6~8.4 MPa，质量流速400~500 kg/(m²?s)，热通量0~200 kW/m²，流体温度20~60℃，Reynolds数1.2×10⁴~4.3×10⁴。分别采用Gr/Re ²和Kv作为浮升力效应和热加速效应的判别因子。结果显示，在高热流低流速工况下，浮升力效应显著（Gr/Re ² > 10^-3），同一个截面处的上壁面传热系数始终小于下壁面传热系数。浮升力效应是高热流低流速工况下传热恶化的主要诱发因素。试验中热加速因子较小（Kv < 8.5×10^-7），其效应可以忽略。将试验数据与典型的传热经验关联式作对比，结果表明Liao-Zhao关联式的计算结果与试验结果最吻合。

关键词: 超临界二氧化碳, 传热, 流动, 浮升力, 关联式

Abstract:

To obtain the heat transfer law of supercritical CO₂ under high heat flow and low flow rate, the experimental study on convective heat transfer of supercritical CO₂ in a small circular tube with an inner diameter of 2 mm was carried out, and the effects of variable properties, buoyancy and thermal acceleration were discussed. The operating parameters were as follows: system pressure p = 7.6—8.4 MPa, mass flux G = 400—500 kg/(m²?s), heat flux q = 0—200 kW/m², fluid temperature T _b = 20—62℃, and Reynolds number Re = 1.23×10⁴—4.3×10⁴. The dimensionless parameters of Gr/Re ² and Kv were adopted to represent the buoyancy effect and thermal acceleration, respectively. The results show that the buoyancy effect is significant (Gr/Re ² > 10^-3) under high heat flux and low mass flux conditions, and the heat transfer coefficients at top wall are always lower than those at bottom wall for a certain section. The buoyancy effect is the main factor that cause heat transfer deteriorate, while the thermal acceleration criterion is insignificant (Kv < 8.5×10^-7) and thus can be neglected. The experimental data were compared with typical heat transfer empirical correlations, in which the Liao-Zhao correlation fit well with the experimental data.

Key words: supercritical CO₂, heat transfer, flow, buoyancy, empirical correlation

中图分类号:

TK 124

颜建国, 朱凤岭, 郭鹏程, 罗兴锜. 高热流低流速条件下超临界CO₂在小圆管内的对流传热特性[J]. 化工学报, 2019, 70(5): 1779-1787.

Jianguo YAN, Fengling ZHU, Pengcheng GUO, Xingqi LUO. Convective heat transfer of supercritical CO₂ flowing a mini circular tube under high heat flux and low mass flux conditions[J]. CIESC Journal, 2019, 70(5): 1779-1787.

图/表 16

图1 试验系统图

Fig.1 Schematic diagram of experimental loop

图2 试验段示意图

Fig.2 Schematic diagram of test section

表1 参数的不确定度

Table 1 Uncertainties of experimental parameters

参数	不确定度/%
压力/ MPa	0.2
质量流速/(kg/(m²?s))	0.8
流体温度/℃	0.5
壁面温度/℃	0.4
热通量/( kW/m²)	4.6
传热系数/(W/(m²?K))	6.6

表2 工况

Table 2 Test conditions

序号	p/MPa	G/(kg/(m²?s))
1	7.6	400
2	8.0	400
3	8.4	400
4	8.4	450
5	8.4	500

图3 去离子水传热标定

Fig.3 Validation of experimental system using deionized water

图4 超临界CO2热物性（p = 7.6 MPa）

Fig.4 Thermo-physical properties of supercritical CO2 at p = 7.6 MPa

图5 不同热通量下的传热系数

Fig.5 Heat transfer coefficients at different heat fluxes

图6 质量流速对传热的影响

Fig.6 Effect of mass flux on heat transfer

图7 压力对传热系数的影响

Fig.7 Effect of pressure on heat transfer

图8 截面Ⅲ处的上下壁温及传热系数

Fig.8 Top and bottom wall temperatures and heat transfer coefficients at section Ⅲ

图9 浮升力因子Gr/Re 2

Fig.9 Buoyancy effect factor Gr/Re 2

图10 质量流速对浮升力的影响

Fig.10 Effect of mass flux on buoyancy force

图11 热加速因子Kv

Fig.11 Thermal acceleration factor Kv

表3 传热经验关联式

Table 3 Heat transfer empirical correlation

名称	关联式
Dittus-Boelter	$N u b = 0.023 R e b 0.8 P r b 0.4$
Gnielinski	$N u b = f / 8 R e - 1000 P r b 1 + 12.7 f / 8 P r f 2 / 3 - 1 1 + d L 2 / 3 P r b P r w 0.11$
Liao-Zhao^[26]	$N u b = 0.124 R e b 0.8 P r b 0.4 G r R e b 2 0.203 ρ w ρ b 0.842 c ˉ p c p, b 0.384$
Jackson^[27]	$N u b = 0.0183 R e b 0.82 P r b 0.5 ρ w ρ b 0.3 c ˉ p c p, b n$
Pitla et al.^[28]	$N u = N u b + N u w 2$
Li et al.^[29]	$N u b = 0.023 R e b 0.8 P r b 0.4 ρ w ρ b 0.3 c ˉ p c p, b n$
Krasnoshchekov et al.^[30]	$N u b = N u 0 ρ w ρ b 0.3 c ˉ p c p, b n$

表3 传热经验关联式

Table 3 Heat transfer empirical correlation

名称	关联式
Dittus-Boelter	$N u b = 0.023 R e b 0.8 P r b 0.4$
Gnielinski	$N u b = f / 8 R e - 1000 P r b 1 + 12.7 f / 8 P r f 2 / 3 - 1 1 + d L 2 / 3 P r b P r w 0.11$
Liao-Zhao^[26]	$N u b = 0.124 R e b 0.8 P r b 0.4 G r R e b 2 0.203 ρ w ρ b 0.842 c ˉ p c p, b 0.384$
Jackson^[27]	$N u b = 0.0183 R e b 0.82 P r b 0.5 ρ w ρ b 0.3 c ˉ p c p, b n$
Pitla et al.^[28]	$N u = N u b + N u w 2$
Li et al.^[29]	$N u b = 0.023 R e b 0.8 P r b 0.4 ρ w ρ b 0.3 c ˉ p c p, b n$
Krasnoshchekov et al.^[30]	$N u b = N u 0 ρ w ρ b 0.3 c ˉ p c p, b n$

图12 Nu试验值与计算值对比

Fig.12 Comparison between experimental and calculated Nu

图13 Liao-Zhao公式的预测误差

Fig.13 Prediction performances of Nu using Liao-Zhao correlation

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高热流低流速条件下超临界CO₂在小圆管内的对流传热特性

Convective heat transfer of supercritical CO₂ flowing a mini circular tube under high heat flux and low mass flux conditions

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