Flow boiling of liquid nitrogen in micro-tubes: Part II – Heat transfer characteristics and critical heat flux

This paper is the second portion of a two-part study concerning the flow boiling of liquid nitrogen in the micro-tubes with the diameters of 0.531, 0.834, 1.042 and 1.931 mm. The contents include the heat transfer characteristics and critical heat flux (CHF). The local wall temperatures are measured, from which the local heat transfer coefficients are determined. The influences of heat flux, mass flux, pressure and tube diameter on the flow boiling heat transfer coefficients are investigated systematically. Two regions with different heat transfer mechanism can be classified: the nucleate boiling dominated region for low mass quality and the convection evaporation dominated region for high mass quality. For none of the existed correlations can predict the experimental data, a new correlation expressed by Co, Bo, We, K_p and X is proposed. The new correlation yields good fitting for 455 experimental data of 0.531, 0.834 and 1.042 mm micro-tubes with a mean absolute error (MAE) of 13.7%. For 1.931 mm tube, the flow boiling heat transfer characteristics are similar to those of macro-channels, and the heat transfer coefficient can be estimated by Chen correlation. Critical heat flux (CHF) is also measured for the four tubes. Both the CHF and the critical mass quality (CMQ) are higher than those for conventional channels. According to the relationship that CMQ decreases with the mass flux, the mechanism of CHF in micro-tubes is postulated to be the dryout or tear of the thin liquid film near the inner wall. It is found that CHF increases gradually with the decrease of tube diameter.     

沸腾传热的分形学分析

分形论是当代兴起的非线性科学的一个重要组成部分。简要地阐述了分形论形的特征，解释了几何学中的维和物理学中的量纲的不同涵义。把分形论分数维的概念扩展应用到沸腾传热中的非线性问题的分析中，结合沸腾传热的操作特征和基本原理，以沸腾传热中的推动力△Tsat作为一个动力学维，进行了q-△Tsat^Df的标绘；对Df的物理意义作了分析和探讨。     

沸腾对柴油机冷却系统数值模拟的影响

计算对比了不考虑沸腾和考虑沸腾2种冷却系统数值模拟计算,得出结论:考虑沸腾传热对内燃机冷却水腔内流动与压力的分布影响不明显,而对冷却水腔内传热过程的影响是很大的。若只考虑纯对流传热,计算结果可能与实际情况存在很大的差异。因此,在对强化内燃机进行流动与传热问题的研究时必须考虑沸腾传热的因素,以获得更为真实、准确的结果。     

Nucleate Pool Boiling of Pure Liquids and Binary Mixtures ：Part I—Analytical Model for Boiling Heat Transfer of Pure Liquids on Smooth Tubes

A mechanism is proposed for nucleate pool boiling heat transfer along with a general model for both pure liquids and binary mixtures. A combined physical model of bubble growth is also proposed along with a corresponding bubble growth model for pure liquids on smooth tubes. Using the general model and the bubble growth model for pure liquids, an analytical model for nucleate pool boiling heat transfer of pure liquids on smooth tubes is developed.     

新型强制循环热水锅炉的问题分析

新型强制循环热水锅炉具有很多优点，克服了原有锅炉缺陷。但投运以后，发生双面水冷壁爆管问题。文中对问题进行了分析，并提出解决措施。     

Critical Heat Flux of Boiling Heat Transfer in a Moderate Narrow Space

INTanDUCTI0NBoilingheattransferandcriticalheatflux(CHF)inaconfinednarrowspacehavebeenstudiedexperi-melltallybyanumberofinvestigatorsinthepastfewdecades.However,thereisnoanypopularlyacceptedmodelintheheattransferinnarrowspaceboiling,althoughsomepopularknowledgeabouttheboilingheattransferinthenarrowspacehavebeenacceptedbymanyresearchers.Theknowledgecanbecon-cludedasthatthenucleateboilingheattransferisenhancedatlowheatfluxregionanddeterioratedathighheatfiuxregi0nespeciallyatCHF.Theenhanceme…     

Flow boiling heat transfer in two-phase micro-channel heat sinks--I. Experimental investigation and assessment of correlation methods

This paper is the first of a two-part study concerning measurement and prediction of saturated flow boiling heat transfer in a water-cooled micro-channel heat sink. In this paper, new experimental results are discussed which provide new physical insight into the unique nature of flow boiling in narrow rectangular micro-channels. The micro-channel heat sink contained 21 parallel channels having a m cross-section. Tests were performed with deionized water over a mass velocity range of 135-402 kg/m² s, inlet temperatures of 30 and 60 °C, and an outlet pressure of 1.17 bar. Results indicate an abrupt transition to annular flow near the point of zero thermodynamic equilibrium quality, and reveal the dominant heat transfer mechanism is forced convective boiling corresponding to annular flow. Contrary to macro-channel trends, the heat transfer coefficient is shown to decrease with increasing thermodynamic equilibrium quality. This unique trend is attributed to appreciable droplet entrainment at the onset of annular flow regime development, and the increase in mass flow rate of the annular film by droplet deposition downstream. Eleven previous empirical correlations are assessed and deemed unable to predict the correct trend of heat transfer coefficient with quality because of the unique nature of flow boiling in micro-channels, and the operating conditions of water-cooled micro-channel heat sinks falling outside the recommended application range for most correlations. Part II of this study will introduce a new annular flow model as an alternative approach to heat transfer coefficient prediction for micro-channels.     

Experimental study of unsteady convective boiling in heated minichannels

Convective boiling in narrow channels may under specific conditions display an unsteady behavior. An experimental set-up has been elaborated to investigate heat and mass transfer and to analyze two-phase flow instabilities in rectangular microchannels with a hydraulic diameter of 889 μm. Depending on the operating conditions two types of behavior are observed: a steady state characterized by pressure drop fluctuations with low amplitudes (from 0.5 to 5 kPa/m) and no characteristic frequency; a non-stationary state of two-phase flow. The pressure signals exhibit fluctuations with high amplitudes (from 20 to 100 kPa/m) and frequencies ranging from 3.6 to 6.6 Hz. Steady and unsteady thermo-hydraulic behaviors depending on the two control parameters (heat flux and mass velocity) are analyzed and given in this paper.     

High heat flux removal using water subcooled flow boiling in a single-side heated circular channel

High heat flux removal from plasma-facing components and electronic heat sinks involves conjugate heat transfer analysis of the applicable substrate and flowing fluid. For the present case of subcooled flow boiling inside a single-side heated circular channel, the dimensional results show the significant radial, circumferential and axial variations in all thermal quantities for the present radial aspect ratio (R_o=outside radius to inside radius) of 3.0. A unified, dimensionless representation of the two-dimensional inside wall heat flux, and the dimensional inside wall heat flux (q_i(φ,z)) and temperature (T_i(φ,z)) data was found and used to collapse the data for all circumferential locations. Finally, 2-D boiling curves are presented and are among the first full set of 2-D boiling data presented for a single-side heated circular configuration.     

气相色谱-飞行时间质谱联用仪测定柴油烃类分子组成的馏程分布

利用气相色谱-飞行时间质谱联用仪（GC-TOFMS）建立了测定柴油烃类分子组成的馏程分布的方法,可以得到柴油样品中各种类型烃类在不同馏程段的碳数分布与平均相对分子质量。利用所建方法研究了柴油中各种类型烃类平均分子结构随馏程的变化情况,发现柴油不同馏程段的平均相对分子质量与其芳烃含量和烃类异构化程度有关：芳烃含量越低、烃类异构化程度越高,该馏程段的平均相对分子质量越高。考察了柴油加氢精制前后烃类分子组成的馏程分布变化情况,柴油经加氢精制后,饱和烃的馏程分布基本不变,只是含量有所增加,芳烃的馏程分布在低馏分段没有变化,在高馏分段含量下降,导致产物的平均相对分子质量在高馏分段高于原料的平均相对分子质量。