直接接触式环流换热器传热研究

首次开发了直接接触式环流换热器 ( DCRE)。分散相液滴不断吸热气化 ,导致气升管轴向上连续相和分散相的温度变化 ,从而产生不同的传热推动力 ,形成体积传热系数分布。在实验基础上 ,重点讨论工质的进料量、连续相和分散相间的传热温差对体积传热系数的影响 ,并从单个液滴气化传热的机理出发 ,推导出体积传热系数分布的表达式及平均体积传热系数的准数关联式。     

直接接触式沸腾换热过程气泡混合均匀度评价

针对直接接触式沸腾换热过程中气泡群扰动的多相流体混合性能评价问题,提出一种基于图像处理技术与像素偏差理论量化混合复杂流体流型均匀性的精确检测与度量新方法。通过气泡群RGB图像的灰度矩阵进行偏差计算,精确地定量描绘了直接接触式换热过程中的气-液混合状态,其值大小反映了气泡群的分布状况,而不同工况下混合均匀时间的差异反映了不同多相体系的均匀性差异;对偏差序列进行正态性检验,发现其近似符合正态分布;运用混沌检测方法对序列进行混沌特征分析,发现整个混合过程是混沌的。通过一个简单的指标实现了复杂条件下气泡群的量化和表征,为多相流流型识别提供了新的分析思路并指导换热过程的混沌强化。     

直接接触式沸腾换热过程气泡混合均匀度评价

针对直接接触式沸腾换热过程中气泡群扰动的多相流体混合性能评价问题,提出一种基于图像处理技术与像素偏差理论量化混合复杂流体流型均匀性的精确检测与度量新方法。通过气泡群RGB图像的灰度矩阵进行偏差计算,精确地定量描绘了直接接触式换热过程中的气-液混合状态,其值大小反映了气泡群的分布状况,而不同工况下混合均匀时间的差异反映了不同多相体系的均匀性差异;对偏差序列进行正态性检验,发现其近似符合正态分布;运用混沌检测方法对序列进行混沌特征分析,发现整个混合过程是混沌的。通过一个简单的指标实现了复杂条件下气泡群的量化和表征,为多相流流型识别提供了新的分析思路并指导换热过程的混沌强化。     

气泡塔生物反应器中传热特性的研究

用热探针测定了气泡塔中不同工况的给热系数;推导了一个既可用于牛顿流体;又可用于非牛顿流体的计算气泡塔中给热系数的一般理论模型,其计算值与本文的实验数据、理论分析取得了一致的结果。本工作对分析生物反应器的传热特性有重要意义。     

单气泡对层流自然对流传热影响的数值研究

在工业传热过程中,温度边界层的增厚严重降低了传热效率。研究发现在液相中加入气泡能提高相间的传热效率。采用VOF(流体体积法)研究单气泡注入对垂直管道内层流自然对流传热的影响,利用Boussinesq假设考虑自然对流的浮力项,分别研究相同气泡形状不同气泡尺寸以及相同气泡尺寸不同气泡形状对近壁流体自然对流传热的影响。结果表明：气泡直径越大,变形稳定后的上升速度和横向尺寸越大,对周围流场造成的扰动越剧烈,传热强化效果和传热强化影响区域增大;气泡直径相同时,随着Eo数的增大,气泡横向尺寸显著变大,传热增强效果变得明显。气泡注入影响传热增强的主要因素与气泡的横向尺寸和上升速度有关,横向尺寸和上升速度越大传热强化效果越好。     

黏性流体中单气泡的运动特性

采用高速CCD成像技术实验测定了单个气泡在水及不同黏度甘油水溶液中的形变特性及上升终速度.结果表明,随着液相黏度的增加,小气泡均能呈良好球形,而大气泡则由类似椭球形的不规则形状逐渐向球帽形转变;气泡的周期性振荡趋于平缓,气泡上升终速度降低.提出了计算气泡上升终速度的关联方程,其预测结果与实验值吻合良好.     

喷嘴释放单气泡的声发射特性

利用声发射技术在单气泡发生实验装置中研究了气液两相流中单气泡的动力学特性,使用自行开发的采集处理程序进行气泡声信号的参数提取,采用统计分析、小波变换和快速傅里叶变换对声信号进行时域和频域范围的分析。分析结果表明,声发射技术可以检测到管内气泡的声信号,具有较高的信噪比,且声信号随着喷嘴尺寸的增大而增大,随着液相表面张力的减小而减小。比较不同喷嘴直径下气泡的频率谱,发现喷嘴释放气泡发出的声信号频率为150~200 k Hz,且随着喷嘴直径的增大,峰值频率相应增大,提出了声信号峰值频率与气泡尺寸之间的关联式。同时得到了气泡上升过程中的连续形态变化,分析了气泡产生声音的机理。研究表明,声发射技术是一种灵敏度高、测量手段方便的方法,可用于气液两相流气泡运动特性的检测。     

喷嘴释放单气泡的声发射特性

利用声发射技术在单气泡发生实验装置中研究了气液两相流中单气泡的动力学特性,使用自行开发的采集处理程序进行气泡声信号的参数提取,采用统计分析、小波变换和快速傅里叶变换对声信号进行时域和频域范围的分析。分析结果表明,声发射技术可以检测到管内气泡的声信号,具有较高的信噪比,且声信号随着喷嘴尺寸的增大而增大,随着液相表面张力的减小而减小。比较不同喷嘴直径下气泡的频率谱,发现喷嘴释放气泡发出的声信号频率为150～200 kHz,且随着喷嘴直径的增大,峰值频率相应增大,提出了声信号峰值频率与气泡尺寸之间的关联式。同时得到了气泡上升过程中的连续形态变化,分析了气泡产生声音的机理。研究表明,声发射技术是一种灵敏度高、测量手段方便的方法,可用于气液两相流气泡运动特性的检测。     

超声波场中蒸汽气泡凝结过程及传热特性

利用高速摄像仪记录有、无超声波时注入过冷水中蒸汽气泡的凝结过程,以分析超声波对蒸汽气泡凝结过程及传热特性的影响。结果表明:在超声波场中,蒸汽气泡表面会形成晶格状毛细波,有效增加气泡表面积,并加强气泡周围流体热边界层扰动,从而导致凝结换热的强化及气泡凝结速度加快。基于15～60 K过冷度下,有、无超声波时较大蒸汽气泡凝结的实验数据,拟合得出有、无超声波时的气泡凝结换热经验关联式,预测误差在±30%以内。     

直接接触冷凝器中流动及传热特性

利用SV型和SK型静态混合器,并以粉末烧结的微孔钛曝气器为气体分布器,开发出一种新型直接接触冷凝器。在空气-水和水蒸气-水体系中测试并比较2种新型直接接触冷凝器的流体力学性能及冷凝传热性能。结果表明:装有SV型静态混合器的新型直接接触冷凝器较装有SK型静态混合器的新型冷凝器对水蒸气的分布更加均匀,具有更好的吸收效果,钛曝气器的利用可有效提高换热效率,并降低水蒸气冷凝所引起的振动与噪声。     

Heat transfer of bio-oil in a direct contact heat exchanger during condensation

Rapid quenching of volatiles in fast pyrolysis is important for achieving high yield and quality of the bio-oil product, but few studies have examined the condensation of volatiles and their related heat exchangers. Accordingly, we have studied the condensation characteristics of volatiles by varying heat transfer conditions in a direct contact heat exchanger. As the mass flow rate ratio of quenching oil to pyrolysis gas increased, the heat transfer rate and yield of bio-oil increased. The heat transfer rate and yield of bio-oil reached a maximum value at an intermediate air-to-quenching oil mass flow rate ratio. Additionally, the heat transfer rate and yield of bio-oil decreased as the temperature of the quenching oil increased. Experiments were also conducted to derive an empirical relationship for the volumetric heat transfer coefficient for direct contact heat exchangers.     

Heat transfer in a continuous bubble column

The effective thermal conductivity k_e was measured for a continuous bubble column operating within the bubbly flow regime. k_e was found to be independent of liquid flow rate but strongly dependent on gas flow rate and physical properties of the liquid phase over the ranges 0.1>u_G>4 cm/s, 0>u_L>3.4 cm/s, 0.00096> μ >0.0028 kg/m μ s and 0.047 >σ>0.072 N/m.     

Survey of heat transfer mechanisms in a slurry bubble column

Heat transfer mechanisms in the bulk and distributor regions of a slurry bubble column are investigated based on the measurements of local heat transfer in a 0.28 m diameter Plexiglas column. The gas, liquid and solid phases used are oil‐free compressed air, tap water and 35 μm glass beads. The slurry concentration and superficial gas velocity are varied from 0 to 40 vol% and 0.05 to 0.30 m/s respectively. Measurements have been made with a fast response heat flux probe which provided local instantaneous heat transfer coefficients. The time‐averaged heat transfer coefficients in the bulk region were on average about 50% higher than the distributor region of the column. The wall region heat transfer coefficients are well predicted by the correlation of Deckwer et al. (1980). Heat transfer mechanism in column centre can be adequately described by the consecutive film and surface renewal model.     

Gas/liquid mass transfer in a slurry bubble column

Gas holdups and volumetric mass transfer coefficients (k_La) for oxygen were measured in suspensions of kieselguhr (7 μm), aluminium oxide (8 μm) and activated carbon (5 μm) in water or 0.8 molar sodium sulfate solution. By comparison to previously reported specific interfacial areas (a) determined by the sulfite oxidation technique, the liquid side mass transfer coefficients (k_L) can be evaluated. Most data are well correlated as a decreasing function of the effective viscosity. However, higher k_L values are determined at low concentrations of solids with a high density.     

Mass transfer from a single gas bubble

Gases covering a wide solubility range were used to measure absorption rates from single bubbles rising freely in water. Absorption rates were not sensitive to any power of the diffusion coefficient and solubility alone proved to be an adequate correlating parameter. Experiments with bubbles containing inert and soluble gas mixtures yielded no evidence for a gas-film resistance, confirming published theory. Results for the absorption of CO₂ by NaOH solutions correlated well with the theory for pseudo-first-order fast reactions, provided allowance was made for a decreasing “physical” solubility of CO₂ with increasing ionic strength of the solutions.     

Mass transfer from a single rising bubble

The absorption rates of carbon dioxide, ethylene, and butene from single rising bubble of equivalent diameter 0.4–2.0 cm in water have been measured using an apparatus in which the bubble volume change with time of rise can be measured. The apparatus uses techniques previously reported but also is suitable for studies of mass transfer without and with chemical reaction at higher pressures (up to 4 atmospheres). The results have been compared with theoretical equations for rate of gas solution at the frontal surface of the bubble based upon potential flow theory and Higbie theory. The data are assessed together with those of others to show that they can be correlated well by This equation is valid for bubbles of equivalent diameter 0.6 < d < 4.0 cm and within the range 500 < N_Re < 20,000     

Heat transfer study in a pilot-plant scale bubble column

The effects of superficial gas velocity on heat transfer coefficient and its time-averaged radial profiles along the bed height have been investigated in a pilot-plant scale bubble column of 0.44 m diameter using air-water system. Notable differences were observed in heat transfer coefficients along the bed axial locations particularly between the sparger (Z/D = 0.28) and the fully developed flow (Z/D = 4.8) regions. In the fully developed flow region larger heat transfer coefficient values were obtained compared to those in the sparger region. About 14-22% increase in heat transfer coefficients measured in the fully developed flow region has been observed compared to those measured in the distributor region when the superficial gas velocity increases from 0.05 to 0.45 m/s. The heat transfer coefficients in the column center for all the conditions studied are about 9-13% larger than those near the wall region. It has been noted that in the fully developed flow region, the axial variation of the heat transfer coefficients was not significant.     

基于传热分区的直流蒸汽发生器换热性能仿真

针对直流蒸汽发生器二次侧复杂的流动与换热过程, 为准确研究其内热力参数空间分布规律, 将其分为预热段、过冷沸腾段、饱和核态沸腾段、强制对流蒸发段、缺液段、过热段, 基于分布参数法建立了直流蒸汽发生器一维均相流数学模型, 并自主开发了可实现自动分段计算的直流蒸汽发生器MATLAB仿真程序。仿真结果表明, 基于传热分区的模型能准确预测不同工况下壁温飞升的幅度、位置及各传热区域传热管的长度, 得到直流蒸汽发生器工作恶劣区域。所得结果可为预防直流蒸汽发生器超温爆管等事故提供理论支持。     

Heat transfer to an immiscible liquid mixture and between liquids in direct contact

Heat transfer to a mixture of two immiscible liquids has been studied in connection with the development of a process for the desalination of sea water. The liquid system consisted of water and refined mineral oil, produced by BP under the trade name of Energol WM-2.Heat transfer to water drops descending through the mineral oil was also investigated. The drag coefficients of the drops in motion were expressed as a function of the Reynolds number. A good correlation was obtained for the heat transfer coefficient expressing the Nusselt number in terms of the Peclet number.Heat transfer to a mixture of two immiscible liquids in co-current turbulent flow without phase change was extensively studied. Friction factor and heat transfer for the oil-in-water type mixtures were theoretically expressed in terms of the volume fraction of oil. The experimental data checked the theoretical derivation quite satisfactorily. No correlations could be obtained for the water-in-oil systems.Similar studies were made for heat transfer in the laminar and turbulent flow with phase change, using a pilot-plant evaporator. Curves were obtained relating the convective heat transfer coefficient to fluid velocity for the liquid mixtures. It was established that, the heat transfer coefficient in evaporation decreased by velocity in the laminar region, but increased in the turbulent region.