Flow boiling characteristics of surfactant solutions in microchannels
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摘要:
实验研究了溶质浓度为0～800 mg/kg的表面活性剂十二烷基硫酸钠（SDS）水溶液在水力直径0.8 mm的平直和树形微通道中的流动沸腾特性。结果表明：SDS对两种微通道的传热效果均有显著强化作用，在平直和树形微通道中，最大传热系数分别达1.5×105 W/（m2·K）和6×104 W/（m2·K），400 mg/kg和200 mg/kg的SDS水溶液使最大传热系数分别提升40%。这是由于SDS显著增加了成核点数量，大量气泡团聚且作为整体运动，工质流动的过程中与壁面摩擦产生活化气泡干扰主流运动，增强对流传热。平直微通道中流动充分发展，不稳定膜态沸腾阶段易见局部干涸点，树形微通道中气液相混合更加均匀，压降波动幅度较小。SDS使单相阶段压降显著减小，沸腾阶段对压降的影响随体积流量增大而减小，体积流量为150 mL/min时压降的变化小于6%。
Abstract:The flow boiling characteristics of aqueous solutions of surfactant Sodium Dodecyl Sulfate (SDS) with solute concentration of 0—800 mg/kg were investigated in flat and tree-shaped microchannels of 0.8 mm hydraulic diameter. The results showed that SDS significantly enhanced the heat transfer effect of both structures, the maximum heat transfer coefficients of 1.5×105 W/(m2·K) and 6×104 W/(m2·K) can be achieved in flat and tree-shaped microchannels, and the aqueous solutions of 400 mg/kg and 200 mg/kg increased the maximum heat transfer coefficients by more than 40%, respectively. As SDS significantly increased the number of nucleation points, many bubbles agglomerated and moved as a whole, and many activated bubbles interfered with the main flow movement during the process of fluid flow and friction with the wall, which enhanced the convective heat transfer. For the flat microchannel, the flow was fully developed, and local dry spots were easily observed in the unstable boiling phase, and more homogeneous mixing of the gas-liquid phase occurred in the tree-shaped microchannel, resulting in smaller fluctuation of pressure drop. The pressure drop in the single-phase flow was significantly reduced by SDS, and the effect on the pressure drop in the boiling phase decreased as the volume flow rate increased, the change in pressure drop at a volume flow rate of 150 mL/min was less than 6%.
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Key words:
- surfactant /
- microchannel /
- flow boiling /
- pressure drop /
- heat transfer coefficient
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图 4 不同热流密度下平直微通道流态
Figure 4. Flow patterns of flat microchannel at different heat fluxes
图 5 不同热流密度下树形微通道流态
Figure 5. Flow patterns of tree-shaped microchannel at different heat fluxes
图 8 不同SDS溶质浓度下传热系数随热流密度变化
Figure 8. Variation of heat transfer coefficient with heat flux at different SDS concentrations
A 换热面积（mm2） xout 出口干度 cp 比定压热容（J/（kg·K）） zsat 单相段长度比例 h 微通道壁面平均传热系数（W/（m2·K）） η 系统热效率 hlv 工质气液相变潜热（J/kg） Δp 进出口压差（kPa） I 电流（A） ΔT 温差（K） kCu 黄铜的导热系数（W/（m·K）） Δx 测温点到换热表面的垂直距离（mm） L 微通道长度（mm） 缩写 pin 入口压力（Pa） CHF 临界热流密度 pout 出口压力（Pa） ONB 起始沸腾点 Q 加热功率（W） SDS 十二烷基硫酸钠 Qeff 有效热流（W） 下标 Qtotal 总焦耳热（W） Cu 铜 qeff 有效热流密度（W/mm2） eff 有效 qm 质量流量（kg/s） in 入口 Tin 入口温度（K） lv 液-气相变 Tout 出口温度（K） out 出口 Tref 工质参考温度（K） ref 参考 Tsat 工质饱和温度（K） sat 饱和状态 $ \overline T_{\rm{tc}} $ 测温点均温（K） tc 热电偶测温 Tw 微通道壁面温度（K） total 总的 U 电压（V） w 微通道壁面 
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溶质浓度/
（mg/kg）剪切黏度/（mPa·s） 表面张力系数/（mN/m） 293.15 K 353.15 K 293.15 K 353.15 K 0 0.995 0.348 72.576 63.460 100 1.002 0.350 66.935 62.515 200 1.012 0.350 65.974 61.829 400 1.027 0.350 59.316 58.477 800 1.043 0.349 53.892 53.236
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表 2 实验工况
Table 2. Operational conditions of experiments
编号 体积流量/
（mL/min）表面活性剂溶质
浓度/（mg/kg）有效热流密度/
（W/mm2）组1 90 0 0.0435 ～0.2898 120 0 0.0435 ～0.3070 150 0 0.0435 ～0.2898 组2 90 100 0.0435 ～0.3070 120 100 0.0435 ～0.3070 150 100 0.0435 ～0.3070 组3 90 200 0.0435 ～0.3876 120 200 0.0435 ～0.3876 150 200 0.0435 ～0.3876 组4 90 400 0.0435 ～0.3876 120 400 0.0435 ～0.3876 150 400 0.0435 ～0.3876 组5 90 800 0.0435 ～0.3659 120 800 0.0435 ～0.3659 150 800 0.0435 ～0.3659
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表 3 参数的不确定度
Table 3. Uncertainty analysis of parameters
参数 不确定度/% 加热功率 ±1.06 有效热流密度 ±1.83 传热系数 ±9.23 压降 ±7.5
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