A heat-transfer laboratory experiment with shell-and-tube condenser

An in-house-designed undergraduate laboratory experiment for hands-on experiential learning of heat transfer in shell-and-tube heat exchangers is described. The experiment allows the student to identify and evaluate the factors that affect the rate of heat transfer in a shell-and-tube heat exchanger. The design affords convective heat transfer inside the tubes and condensation heat transfer outside the tubes. Experimental measurements are used to estimate the overall heat transfer coefficient at varying flow rates of the cold fluid, which is compared with that obtained from calculations using correlations available in the literature. A student survey was also conducted to evaluate their learning experience and to identity opportunities for improving the experimental program and the overall experience of the students. This feedback indicated that the vast majority of the students had an overall positive experience of the laboratory course, as it provided them with the opportunity of teamwork, leadership role, the spirit of industrial experience, and the ability to develop skills related to heat exchanger calculations.     

蒸发式冷凝器管外水膜流动实验研究

研究蒸发式冷凝器传热传质性能时,涉及到管外喷淋水在管表面的分布形式和喷淋水与空气间的对流传热传质等问题。文内建立了蒸发式冷凝器管外水膜流动的可视化测试实验平台,采用高速摄像仪分析了喷嘴对管外水膜分布的影响,同时测试了时间、风量、水量、高度等参数对水膜温度分布的影响。结果表明,喷嘴B能获得更好的水膜分布效果,蒸发式冷凝器有一个最佳喷淋水量,其换热过程主要是由循环冷却水的显热换热和蒸发换热交替占主导作用构成的,在稳定操作条件下运行一段时间后水盘中的水温能保持较好的恒定。     

薄膜蒸发器传热蒸发性能的实验研究

在自行研制的0.4m2薄膜蒸发器实验测试装置基础上,对介质水及烧碱溶液进行了蒸发实验,得到了在不同工艺操作参数下薄膜蒸发器内各料液总传热系数K和蒸发强度EI的变化曲线。实验结果表明,转子转速及进料量对设备总传热系数K及蒸发强度EI影响显著,各料液均存在最佳进料量。粘度增加,总传热系数K相应减少,蒸发强度EI相应增加,实验与数值模拟结果变化规律一致。研究结果表明,从蒸发强度而言,薄膜蒸发器更适合于高粘度料液的蒸发浓缩。     

Dynamics of convection heat exchangers

A mathematical model is developed for heat exchangers with condensing vapors on one side which includes the effects of variations in the local film heat transfer coefficients and allows for energy accumulation and conduction in the tube wall. A numerical integration scheme for the flow forced case is presented. The model is tested against experimental steady state and transient data for a steam-water exchanger. It was necessary to modify the standard correlations of heat transfer coefficients to get a match of the overall steady state heat flux actually obtained in the apparatus. With these modified heat transfer coefficients, the model proved capable of predicting flow forced transients. These results are contrasted with transients predicted by a simple lumped parameters model employing empirically fit overall heat transfer coefficients. The lumped parameters model is shown quite adequate for predicting transient behavior under the specific conditions studied. The reasons for and limitations on the success of this very simple model are explored.     

明胶软胶囊干燥特性的研究

在对流干燥实验台上进行了明胶软胶囊干燥特性的实验,研究了加热空气温度、风速、湿度对干燥过程的影响规律。实验结果表明,提高加热空气的温度,虽然提高了传热速率,但在第2降速阶段干燥速率下降,导致最终含水质量分数反而偏高;随着加热空气风速的提高,传热速率增加,干燥速率在第1降速阶段呈现加快趋势,软胶囊的最终含水质量分数稍有降低;而加热空气湿度的变化,对传热过程影响很小,但随着加热空气湿度的降低,干燥速率提高,使得最终含水质量分数明显降低。通过研究软胶囊的干燥特性,为制药工业中软胶囊的生产提供合理指导,对优化干燥过程、降低能耗、提高产品质量和经济效益具有重要意义。     

An investigation into the effects of gravity level on rate of heat release and time to ignition

Two experiments were performed on board an aircraft flying repeated parabolic trajectories to generate free-fal conditions. The first experiment investigates the way in which rate of heat release (RHR) varies with gravity for a candle flame under an imposed low-speed flow. In line with previous studies of flame spread rate it has been shown that rate of heat release drops significantly in imcrogravity. The heat loss due to radiation decreases by a larger proportion than that due to non-radiative processes indicating a lower flame temperature. The RHR from a microgravity flame is flow rate dependent, increasing for increased flow rate at air speeds under 0.03 ms^?1. For the geometry used in this experiment hypergravity caused only a small increase in RHR. The second experiment studied the ignitability of thermoplastics under an imposed radiant heat flux. The ignition test apparatus consists of a conical spiral heating element positioned horizontally above the sample, a continuous spark ignition source and a removable heat shield. Experiments were conducted in a sealed pressure chamber on samples of either PMMA or POM, 1.5 mm thick, with a ceramic backing. There is some indication that gravity influences the time to ignition for some materials.     

A study of composite foams for diving suits subjected to high hydrostatic pressure

In order to obtain foams possessing flexibility and at the same time heat insulation under high hydrostatic pressure, composite foams with spherical rigid foams filled in flexible rubber foam at certain intervals were prepared and their thermal conductivity and flexural rigidity were studied. The following points were found: (1) With a unit model having a spherical rigid foam in the middle, the thermal conduction of a composite foam was analyzed under the conditions of steady one-dimensional heat flow. Theoretical equations giving overall coefficients of heat transmission under atmospheric and hydrostatic pressures were obtained, and the adequacy of these theoretical equations was confirmed by the measurement of overall coefficients of heat transmission of composite foams in an apparatus so constructed as to allow heat conduction experiments under pressures ranging from atmospheric to the hydrostatic pressure corresponding to 100-m depth in water. (2) The effect of the filled spherical rigid foams on heat insulation is notable under hydrostatic pressures corresponding to a 20-m depth or more in water. Under the hydrostatic pressure corresponding to a 100-m depth in water, the coefficient of heat insulation of the most closely filled composite foam used in the experiment was approximately 35% larger than that of the unfilled foam, while the theoretical most closely filled composite foam gives an approximately 110% increase. (3) Under the hydrostatic pressure corresponding to a 100-m depth in water, the flexural rigidity of the most closely filled composite foam used in the experiment was approximately one half that of an unfilled foam of the same heat insulating property.     

Investigation of flow boiling in circulating three-phase fluidised bed: Part I: Experiments and results

In flow boiling apparatus, fouling is frequently a problem. Mechanical methods to mitigate fouling include the impact of solids on the deposit to remove it. Solid particles fluidised by the two-phase boiling mixture may accomplish sufficient deposit removal to keep boiling surfaces clean. This results in a self-cleaning fluidised bed boiling heat exchanger. The particles additionally enhance the heat transfer. In this paper, a comprehensive investigation of three-phase fluidised flow boiling in a circulating system is presented. A test apparatus which is a three-phase circulating fluidised bed was built with glass construction to visualise boiling phenomena in a water system. It could also be operated as a two-phase system and this was investigated to provide the basis for comparison and also to verify reproducibility and confirm well-established flow boiling results. For the three-phase system (where steel particles were added), a range of liquid flowrates and heat fluxes was used and three different particle sizes were investigated. Three-phase results were compared with two-phase results. This work is primarily a study of boiling heat transfer enhancement as a result of addition of particles, but the effect of the particles creating a self-cleaning heat exchanger is a significant operational advantage for industrial application.As expected, heat transfer coefficients were higher overall for the three-phase system than for the two-phase system. The onset of nucleate boiling was independent of heat flux and the heat transfer coefficient initially increased with increasing Reynolds number whereafter there was some deterioration of heat transfer. Visualisation of boiling heat transfer phenomena for the three-phase system is also provided which allows mechanistic explanations of the measured phenomena.In Part II of this paper, a correlation is developed, based on boiling heat transfer modelling, to describe heat transfer during boiling in a three-phase circulating fluidised bed. A flow-dependent function is added as is a boiling heat transfer enhancement factor. This correlation is validated against this experimental data and found to show agreement within about 20% and better agreement with the data than an existing correlation.     

冷却水参数对钠钾合金热管传热性能影响

冷却水参数对钠钾合金热管传热性能有重要影响,通过改变不同冷却水流量和冷却水温度,研究了冷却水参数对钠钾合金热管传热性能的影响规律。实验结果表明,钠钾合金热管运行于较低冷却水流量（4~18 ml·s^-1）的冷却条件时,流量对热管冷凝段外壁面的温度影响很大,而当热管运行于较高冷却水流量的冷却条件时,冷却水流量对热管外壁面温度影响较小。整体而言,增大冷却水流量可以有效地提高钠钾合金热管的传热量及其传热性能。当热管运行于较大冷却水流量的冷却条件时,冷却水温度的变化对热管传热性能影响较小。     

采用填料强化的鼓泡塔直接接触换热性能研究

以正戊烷-水为物系,进行了采用填料强化的鼓泡塔直接接触蒸发换热实验。实验采用顺流操作,考察了分散相流量、温差以及分布器孔径对体积换热系数和汽化高度的影响。实验得出：在戊烷流量为23.868 L/h,分布器孔径为2.5 mm时加填料的鼓泡塔的体积换热系数约为未加填料的2倍;在一定的操作条件下,加填料的鼓泡塔中汽化高度随分散相流量和分布器孔径的增大而增加,随温差的增大而减小;加填料的鼓泡塔中体积换热系数随分散相流量的增加而增加,随分布器孔径的增大而减小,与温差成负幂指数关系。     

液滴-蒸汽直接接触换热的研究

针对液滴-蒸汽直接接触换热过程提出了计算换热速率的模型,并对模型进行了实验验证。实验使用水滴-水蒸气的体系和实心圆锥形喷头,在4个不同的喷头压力下进行。将实验测得的结果与理论模型计算的结果进行对比,吻合良好,研究了换热过程的总换热系数随喷头的压力的变化规律。     

超临界压力水在垂直上升内螺纹管中的传热特性

在压力22.5～30 MPa,质量流速430～1200 kg·m^-2·s^-1,内壁热负荷284～719 kW·m^-2范围内,对超临界压力水在均匀加热垂直上升内螺纹管内的传热特性进行了实验研究,得到了内螺纹管内超临界压力水的传热特性,分析了压力、热负荷和质量流速变化对内螺纹管壁温及传热系数的影响,探讨了拟临界区的传热机理,并给出了能用于工程实际的传热实验关联式。实验结果表明：垂直上升内螺纹管中超临界水具有良好的传热特性。在低焓值区内螺纹管壁温随焓增平缓增加,而在高焓值区壁温随焓增的升高明显。由于热物性的剧烈变化,超临界水在拟临界焓值区发生了明显的传热强化。压力与热负荷的增大以及质量流速的减小均会导致内螺纹管壁温的升高和传热系数的减小,使得传热强化现象削弱,甚至出现传热恶化。     

惰性粒子对水沸腾传热强化的实验研究

通过加入不同种类和体积分率的惰性粒子,在垂直管中进行了水的流动沸腾传热实验,研究了三相流沸腾传热特性。实验中预先对水加热,采用了沸点进料。实验发现,传热膜系数随热通量、液体流量的增加及粒子体积分率的增大而增加。对于不同粒子,这种变化趋势比较一致,但不同粒子对沸腾传热的强化效果不同。实验结果表明:由于固体粒子的存在,强化了沸腾传热,三相流沸腾传热系数是相同条件下汽液二相流沸腾传热系数的1.3—1.7倍。     

环隙内流动沸腾传热特性的研究

本文以水和粘性液体为工质,对环隙宽度为2mm的环隙内流动沸腾传热特性进行了研究.实验结果表明,在一定的热通量范围,环隙内流动沸腾传热系数较空管的平均提高80%,总传热系数平均提高60%.当以水为工质时,总传热系数的实验值与计算值吻合较好.     

超重条件下泡沫石墨-石蜡相变传热实验研究

基于高热导率多孔材料骨架强化传热的相变蓄冷器具有热容量大、换热效率高、控温稳定、环境适应性强等优点,在航空航天领域具有广泛的应用前景。多孔介质内固-液相变换热过程中,由于相变换热产生的相变工质密度差在加速度的驱动下造成的液相浮升力变化,将引发更为复杂的固-液相变流动与换热不稳定性特征。设计搭建了离心环境条件下的多孔介质内固-液相变换热实验研究装置,实验获得了不同加速度大小和方向下多孔介质内固-液相变换热特性。结果表明,高导热泡沫石墨-石蜡相变蓄冷能够实现界面较低升温速率温控,同时加速度方向对于相变蓄冷装置整体换热效率及换热均温性存在显著影响。当加速度方向与热流方向反向或垂直时,加速度驱动液相储能工质局部自然对流将促进整体换热效率,而加速度方向与热流方向同向时换热效率降低,并且上述差异将随着加速度的提升而进一步扩大。     

倾斜内螺纹管中亚临界及超临界水传热特性研究

为探究超（超）临界锅炉机组内螺纹管水冷壁的传热特性,在倾角5°~90°的?35 mm×7.75 mm倾斜上升内螺纹管中进行了实验研究,实验参数为压力15~28 MPa,质量流速600~1000 kg?m^-2?s^-1,热通量300~500 kW?m^-2。实验对比了内螺纹管与?25 mm×3 mm光管的壁温特性;讨论了倾斜角度、质量流速和压力的变化对内螺纹管中亚临界、近临界和超临界水传热的影响;拟合了不同倾斜角度下超临界水的传热关联式。结果表明：所用内螺纹管有明显的推迟传热恶化、强化传热的性能;不同倾斜角度的内螺纹管传热特性存在差异;亚临界压力下质量流速对气液两相区传热系数影响很小,近临界和超临界压力下,随质量流速的增加,整体上传热系数增大;压力为15 MPa时的传热系数最大。超临界压力下,随压力的增大,大比热容区的传热系数减小。     

微小通道内低Reynolds数液-液两相流动与换热特性实验研究

实验研究了圆形微小通道内液-液两相流的流动和换热特性。选用去离子水为分散相,高黏度二甲基硅油为连续相。通过处理高速摄像所拍摄的可视化图像,总结了液-液两相流流型和液滴的长度/形状特征。并在此基础上考察了低Reynolds数下液-液弹状流对微小通道的换热作用。结果表明,平均Nusselt数随着Reynolds数的增加而增加,且油水比越大传热系数增加幅度越明显。Nu随着含水率的增加而降低。虽然含水率增加会使两相平均热容量提高,但在低Reynolds数下,这种提高被其长液滴内较弱的循环强度所抵消。选用三种不同形式接头在相同混合速度和含水率的情况下生成不同长度的液滴,发现短液滴更有利于换热。相同工况下,液滴长度的优化可以使整体传热系数提高近26%。     

A novel laboratory experiment for demonstrating boiling heat transfer

An undergraduate laboratory experiment for hands-on experiential learning of boiling heat transfer is described. The experiment allows the students to visualize and qualify different modes of boiling heat transfer. Experimental measurements are used for estimating the boiling heat transfer coefficient at varying heat flux and in the presence or absence of stirring. In addition, the results are compared with those obtained from theoretical calculations using correlations available in the literature. Overall, the experiment is well received by the students as it provides them with the opportunity of team work, leadership role, and the spirit of industrial experience. The experiment provides them with the ability to plan and execute a relatively complex but important heat transfer operation. The experiment additionally enhances student understanding of the boiling heat transfer topic covered in classroom lectures.     

振荡热管内不同形态纳米颗粒流动及传热特性

主要针对不同形态的纳米颗粒在振荡热管内的流动及热传输特性进行了实验研究。在相同的压力、相同的热管倾角、不同的充液率条件下,对振荡热管内工质分别为Cu-水纳米流体以及Cu-水纳米流体中Cu纳米颗粒沉积后溶液的流动以及热传输特性进行了实验研究,并与工质为蒸馏水时进行了对比实验分析,以此来研究振荡热管内气、液以及纳米颗粒多相流动存在时,对热管传输特性的影响。实验表明：当振荡热管内存在气、液以及不同形态的纳米颗粒多相流动时,对其传热特性会产生很大的影响,在一定条件下会起到强化传热的作用。     

波纹管强化管内沸腾传热的试验研究

分别以煤油和水为工质,对不同流速情况下波纹管和光管的管内流动沸腾传热特性进行了试验研究。试验中采用套管式换热器,依靠管外的高温热水对管内工质加热使之沸腾。在不同流速下,根据试验测量的流量和温度等参数计算管内流动沸腾传热系数。结果表明:随着气相雷诺数的提高,传热系数随之提高;相对光管,波纹管对上升流动管内沸腾传热有明显的强化作用。根据试验结果给出了波纹管管内流动沸腾传热系数关联式。