Mechanism model of critical heat flux in narrow rectangular channel based on flow oscillations

doi:10.11949/0438-1157.20220178

Abstract

Abstract:

The maximum operating power of the device is limited by the critical heat flux (CHF), however, the flow oscillation can cause premature critical heat flux (called PM-CHF) and reduce the stable operating range. In order to study the critical heat flux in a narrow rectangular channel under flow oscillation conditions, this paper conducted experiments to visualize the boiling crisis in a narrow rectangular channel under vertical upward flow condition with deionized water as the working medium, with mass flux range of 350—2000 kg/(m²·s), narrow gap size range of 1—5 mm, and system pressure range of 1—4 MPa. The results show that CHF increases linearly with increasing mass flow rate in the narrow rectangular channel. The flow oscillation occurs when the mass flux is small, and the oscillation period is about 0.1 s. The flow oscillation leads to the early onset of boiling crisis, during which the flow pattern is slug-churn flow. Based on the flow oscillation and the bubble dynamics characteristics in the narrow rectangular channel, the theoretical analysis and derivation are carried out from the perspective of flow oscillation. In results, a PM-CHF mechanism model is established, and the error is within 30%.

Key words: narrow rectangular channel, PM-CHF, two-phase flow, flow regime, bubble

摘要：

设备最大运行功率受临界热通量(CHF)限制，而流量振荡会导致沸腾危机早发，此时的临界热通量称为PM-CHF。为了研究流量振荡条件下窄矩形通道内的临界热通量，进行单侧加热窄矩形通道内竖直向上流动条件下沸腾危机可视化实验，实验工质为去离子水，质量流速范围为350~2000 kg/(m²·s)，窄缝宽度范围为1~5 mm，系统压力范围为1~4 MPa。结果显示，在窄矩形通道中CHF随质量流速的增加而线性增加。当流速较小时会发生流量振荡，振荡周期约为0.1 s。流量振荡继而导致沸腾危机早发，其流型表现为弹状流-搅混流。此外，针对本实验观察到的流量振荡和窄矩形通道内气泡动力学特性，从流量振荡的角度进行理论分析与推导，建立窄矩形通道内由于流动失稳引起的PM-CHF机理模型，预测误差在30%以内。

关键词: 窄矩形通道, PM-CHF, 两相流, 流域, 气泡

CLC Number:

TL 334

Meiyue YAN, Jian DENG, Liangming PAN, Zaiyong MA, Xiang LI, Jiewen DENG, Qingche HE. Mechanism model of critical heat flux in narrow rectangular channel based on flow oscillations[J]. CIESC Journal, 2022, 73(7): 2962-2970.

闫美月, 邓坚, 潘良明, 马在勇, 李想, 邓杰文, 何清澈. 基于流量振荡的窄矩形通道内临界热通量机理模型[J]. 化工学报, 2022, 73(7): 2962-2970.

Figures/Tables 12

Fig.1 Schematic diagram of experimental loop

Fig.2 Schematic diagram of visualization part

Fig.3 Schematic diagram of thermocouple locations

Table 1 Range of experimental parameters

参数	工况
实验压力p/MPa	1~4
窄缝宽度ε/mm	1~5
加热长度 L/mm	600
质量流速G/(kg/(m²·s))	350~2000
入口过冷度ΔT_in,sub/K	60~120
加热方式	单面加热
加热材料	不锈钢
流向	向上流动
工质	去离子水

Fig.4 The effect of mass flux on CHF

Fig.5 The variation of flow pattern when CHF occurs under conditions of flow instability

Fig.6 The variation of mass flux and temperature when CHF occurs under condition of flow oscillation

Table2 Instability models

现有模型	具体项目
Helmholtz不稳定性^[25-26]	研究对象：下层流体密度高于上层流体密度，两流体交界面均与交界面平行，但速度不同，当两者相对速度超过临界值时，发生Helmholtz不稳定性
Helmholtz不稳定性^[25-26]	CHF机理：加热壁面上小气泡聚合形成大气泡，大气泡底部的微液层因蒸发而完全耗尽时发生沸腾危机，大气泡长度取决于Helmholtz不稳定性
Taylor不稳定性^[25]	研究对象：上层流体密度高于下层流体密度，讨论两流体受到垂直交界面的扰动时引起的不稳定现象
Taylor不稳定性^[25]	CHF机理：在池式沸腾中，临界热通量为以最危险波长为直径的气泡的蒸发热通量

Fig.7 The comparison between the prediction of sublayer dryout model and the experimental value

Fig.8 The schematic diagram of bubble behaviors and heat flux distribution when CHF occurs

Table 3 Correlations of nucleate site density

N a = 210 Δ T w 1.8

—

[32]

N a = 0.34 (1 - c o s θ) Δ T w 2.0 ?????? Δ T w, O N B < Δ T w < 15 ? K N a = 3.4 × 10 - 5 (1 - c o s θ) Δ T w 5.3 ??????? Δ T w ≥ 15 ? K

G: 124—886 kg/(m²·s)

ΔT_in,sub: 6.6—52.5 K

[33]

N a = 3.1 × 10 - 7 Δ T w 8.19595

p: 0.1—0.5 MPa

G: 400—1600 kg/(m²·s)

ΔT_in,sub: 20—60 K