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11.
An experimental study was performed to investigate the effect of surface coating on the critical heat flux for downward facing boiling on the outer surface of a hemispherical vessel. Steady-state boiling experiments were conducted in the subscale boundary layer boiling (SBLB) facility using test vessels with metallic microporous coatings to obtain the local boiling curves and the local critical heat flux (CHF) limits. Similar heat transfer performance was observed for microporous aluminum and microporous copper coatings. When compared to the corresponding data without coatings, the boiling curves for the coated vessels were found to shift upward and to the right. This meant that the CHF limit was higher with surface coating and that the minimum film boiling temperatures were located at higher wall superheats. In particular, the microporous coatings were found to enhance the local CHF values appreciably at all angular locations explored in the experiments. Results of the present study showed that the microporous aluminum coating was very durable. Even after many cycles of steady state boiling, the vessel coating remained rather intact, with no apparent changes in color or structure. Although similar heat transfer performance was observed for microporous copper coatings, the latter were found to be much less durable and tended to degrade after several cycles of boiling. 相似文献
12.
Flow boiling heat transfer and pressure drop of R-134a in a mini tube: an experimental investigation 总被引:1,自引:0,他引:1
Jacqueline B. Copetti Mario H. MacagnanFlávia Zinani Nicole L.F. Kunsler 《Experimental Thermal and Fluid Science》2011,35(4):636-644
This paper presents the results of an experimental study carried out with R-134a during flow boiling in a horizontal tube of 2.6 mm ID. The experimental tests included (i) heat fluxes in the range from 10 to 100 kW/m2, (ii) the refrigerant mass velocities set to the discrete values in the range of 240-930 kg/(m2 s) and (iii) saturation temperature of 12 and 22 °C. The study analyzed the heat transfer, through the local heat transfer coefficient along of flow, and pressure drop, under the variation of these different parameters. It was possible to observe the significant influence of heat flux in the heat transfer coefficient and mass velocity in the pressure drop, besides the effects of saturation temperature. In the low quality region, it was possible to observe a significant influence of heat flux on the heat transfer coefficient. In the high vapor quality region, for high mass velocities, this influence tended to vanish, and the coefficient decreased. The influence of mass velocity in the heat transfer coefficient was detected in most tests for a threshold value of vapor quality, which was higher as the heat flux increased. For higher heat flux the heat transfer coefficient was nearly independent of mass velocity. The frictional pressure drop increased with the increase in vapor quality and mass velocity. Predictive models for heat transfer coefficient in mini channels were evaluated and the calculated coefficient agreed well with measured data within a range 35% for saturation temperature of 22 °C. These results extend the ranges of heat fluxes and mass velocities beyond values available in literature, and add a substantial contribution to the comprehension of boiling heat transfer phenomena inside mini channels. 相似文献
13.
Effect of surfactant additives on nucleate pool boiling heat transfer of refrigerant-based nanofluid
Effect of surfactant additives on nucleate pool boiling heat transfer of refrigerant-based nanofluid was investigated experimentally. Three types of surfactants including Sodium Dodecyl Sulfate (SDS), Cetyltrimethyl Ammonium Bromide (CTAB) and Sorbitan Monooleate (Span-80) were used in the experiments. The refrigerant-based nanofluid was formed from Cu nanoparticles and refrigerant R113. The test surface is horizontal with the average roughness of 1.6 μm. Test conditions include a saturation pressure of 101.3 kPa, heat fluxes from 10 to 80 kW m−2, surfactant concentrations from 0 to 5000 ppm (parts per million by weight), and nanoparticle concentrations from 0 to 1.0 wt.%. The experimental results indicate that the presence of surfactant enhances the nucleate pool boiling heat transfer of refrigerant-based nanofluid on most conditions, but deteriorates the nucleate pool boiling heat transfer at high surfactant concentrations. The ratio of nucleate pool boiling heat transfer coefficient of refrigerant-based nanofluid with surfactant to that without surfactant (defined as surfactant enhancement ratio, SER) are in the ranges of 1.12-1.67, 0.94-1.39, and 0.85-1.29 for SDS, CTAB and Span-80, respectively, and the values of SER are in the order of SDS > CTAB > Span-80, which is opposite to the order of surfactant density values. The SER increases with the increase of surfactant concentration and then decreases, presenting the maximum values at 2000, 500 and 1000 ppm for SDS, CTAB and Span-80, respectively. At a fixed surfactant concentration, the SER increases with the decrease of nanoparticle concentration. A nucleate pool boiling heat transfer correlation for refrigerant-based nanofluid with surfactant is proposed, and it agrees with 92% of the experimental data within a deviation of ±25%. 相似文献
14.
Air injection as a stabilization method is evaluated for flow boiling in a micro tube. Pyrex glass tube coated by ITO film is employed as a test tube for flow visualization with water as a working fluid. Air bubble and liquid slug lengths are controlled by changing air and liquid mass velocities. Wall temperatures and inlet/outlet pressures show very large fluctuations during flow boiling without air injection. Severe reverse flow is also observed from flow visualization. On the other hand, wall temperature and inlet/outlet pressures as well as visualized flow patterns become very stable with air injection. In addition, much higher heat transfer coefficients are obtained for air injected cases. It is observed from the flow visualization that the flow becomes much stable and shows regular patterns. 相似文献
15.
Ulrich Soupremanien Stephane Le PersonMichel Favre-Marinet Yann Bultel 《Experimental Thermal and Fluid Science》2011,35(5):797-809
This article presents experiments conducted with two single rectangular mini-channels of same hydraulic diameter (1.4 mm) and different aspect ratios for conditions of horizontal boiling flow. The Forane® 365 HX used was subcooled (ΔTsub = 15 °C) for all the boiling curves presented in the paper. Local heat transfer coefficients were measured for heat flux ranging from 25 to 62 kW m−2 and mass flux from 200 kg m−2 s−1 to 400 kg m−2 s−1. The boiling flows were observed with two different cameras (depending on the flow velocity) through a visualization window. The flow patterns in the two channels were compared for similar conditions. The results show that the boiling heat transfer coefficient and the pressure drop values are different for the two single mini-channels. For low heat flux condition, the channel with lowest aspect ratio (H/W = 0.143) has a higher heat transfer coefficient. On the other hand, for high heat flux condition, the opposite situation occurs, namely the heat transfer coefficient becomes higher for the channel with highest aspect ratio (H/W = 0.43). This is probably due to the earlier onset of dryout in the channel with lowest aspect ratio. For the two cases of heating, the pressure drop for the two-phase flow remains lower for the channel with lowest aspect ratio. These results show that the aspect ratio plays a substantial role for boiling flows in rectangular channels. As for single-phase flows, the heat transfer characteristics are significantly influenced (even though the hydraulic diameter remains the same) by this parameter. 相似文献
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18.
自然循环过冷沸腾流动不稳定性的实验研究 总被引:2,自引:0,他引:2
本文以氟里昂作工质,对自然循环过冷沸腾流动不稳定性进行了实验研究.实验过程中发现自然循环系统内可能发生高频脉动和低频脉动二种类型的过冷沸腾流动不稳定性.通过实验研究揭示了这二种类型流动不稳定性的发生机理,证实高频脉动属于声波型脉动,低频脉动属于密度波型脉动.通过实验得出了判断系统稳定性的界限,并使用积分方程无因次分析方法得出了预测密度波型流动不稳定性的经验公式. 相似文献
19.
沸点(BP)是有机分子液体的基本物理化学量, 也是化学工业生产中的重要参数. 有机分子的沸点由分子结构决定, 呈现复杂的结构-沸点关系, 函数法(Function Method)、基团贡献法(Group Contribution Method)等传统方法无法应对复杂多样有机分子结构的预测, 应用范围狭窄, 预测精度低. 本研究中, 我们利用基于人工神经网络(ANN)和支持向量机(SVM)的多组件学习器实现有机分子沸点的精准预测. 我们构建了基于可解释性描述符的ANN、基于相关性描述符的ANN及基于复合分子指纹的SVM三个异质模型, 并通过包含4550个各种类别的有机分子沸点的数据集进行训练得到了三个异质性学习器, 最后集成三个学习器对有机分子沸点进行预测. 相比于传统方法和此前的定量结构性质关系(QSPR)模型, 多组件模型结合了三种模型的优点, 展现出很好的预测精度和泛化能力以及低的过拟合, 实现了对多种类型有机分子的沸点的有效预测. 相似文献
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