Analysis of secondary flow in shell-side channel of trisection helix heat exchangers

The flow characteristics of shell-side fluid in the tube-and-shell heat exchangers with trisection helical baffles with 35° inclined angles are numerically analyzed. The secondary flow distribution of the fluid in the shell-side channel is focused on. The results on meridian planes indicate that in the shell-side channel, the center part of fluid has an outward tendency because of the centrifugal force, and the peripheral region fluid has an inward tendency under the centripetal force. So in a spiral cycle, the fluid is divided into the upper and lower beams of streamlines, at the same time the Dean vortices are formed near the left baffle, and then the fluid turns to centripetal flow near the right baffle. Finally the two beams of streamlines merge in the main flow. The results of a number of parallel slices between two parallel baffles with the same sector in a swirl cycle also show the existence of the secondary flow and some backward flows at the V-gaps of the adjacent baffles. The secondary flows have a positive effect on mixing fluid by promoting the momentum and mass exchange between fluid particles near the tube wall and in the main stream, and thus they will enhance the heat transfer of the helix heat exchanger.

Influence of Nanoparticle Shapes of Boehmite Alumina on the Thermal Performance of a Straight Microchannel Printed Circuit Heat Exchanger

The efficiency and irreversibility defined based on the second law of thermodynamics provide a new path for heat exchangers design and make performance analysis more straightforward and elegant.The second law of thermodynamics is applied in a Straight Microchannel Printed Circuit heat exchanger to determine the thermal performance of different shapes of Boehmite Alumina compared to Al2O3 aluminum oxide.The various forms of non-spherical Boehmite Alumina are characterized dynamically and thermodynamically through dynamic viscosity and thermal conductivity,using empirical coefficients.The non-spherical shape includes platelet,cylindrical,blades,and bricks forms.Graphical results are presented for thermal efficiency,thermal irreversibility,heat transfer rate,and nanofluid exit temperature.The non-spherical shapes of Boehmite Alumina show different thermal characteristics concerning the spherical shape when there are variations in fluid flow rates and the nanoparticles fraction.Furthermore,it was theoretically demonstrated that non-spherical particles have higher heat transfer rates than spherical particles,emphasizing platelets and cylindrical shapes for the low volume fraction of nanoparticles and bricks and blades for high volume fraction.

Comparison of heat transfer performances of helix baffled heat exchangers with different baffle configurations

Numerical simulations were performed on flow and heat transfer performances of heat exchangers having six helical baffles of different baffle shapes and assembly configurations, i.e., two trisection baffle schemes, two quadrant baffle schemes, and two continuous helical baffle schemes. The temperature contour or the pressure contour and velocity contour plots with superimposed velocity vectors on meridian, transverse and unfolded concentric hexagonal slices are presented to obtain a full angular view. For the six helix baffled heat exchangers,the different patterns of the single vortex secondary flow and the shortcut leakage flow were depicted as well as the heat transfer properties were compared. The results show that the optimum scheme among the six configurations is a circumferential overlap trisection helix baffled heat exchanger with a baffle incline angle of 20°(20°TCO) scheme with an anti-shortcut baffle structure, which exhibits the second highest pressure dropΔpo, the highest overall heat transfer coefficient K, shell-side heat transfer coefficient hoand shell-side average comprehensive index ho/Δpo.

Numerical Investigation of Zigzag Bending-Angle Channel Effects on Thermal Hydraulic Performance of Printed Circuit Heat Exchanger

This study investigated the effects of zigzag-flow channel bending angle in printed circuit heat exchangers(PCHEs) using a computational fluid dynamics method with ANSYS-FLUENT simulation.The three-dimensional model of PCHE with a 15° curved,zigzag channel was conducted for preliminary validation.The comparisons between the CFD simulation results and the experimental data showed good agreement with some discrepancies in the heat transfer and pressure drop results.In addition,different bending angle configurations(0°,3° to 30°) of zigzag channels were analyzed to obtain better thermal-hydraulic performance of the zigzag channel PCHE under different inlet mass flow rates.The criteria of heat transfer and frictional factor were applied to evaluate the thermal-hydraulic performance of the PCHE.The results showed that the 6° and 9°bending channel provided good thermal-hydraulic performance.New correlations were developed using the 6°and 9° bending channel angles in PCHE designs to predict the Nusselt number and friction factor.

Heat Transfer and Friction Characteristics of Printed Circuit Heat Exchangers with Different Channel Structures for S-CO_(2) Power System

Printed circuit heat exchanger(PCHE)has been widely used in supercritical carbon dioxide(S-CO_(2))power systems because of its high heat transfer efficiency and good compactness.However,due to the large variety of PCHE configurations,channel selection in practical applications lacks a basis.Therefore,this paper discussed the heat transfer and friction characteristics and the synergy of three fields in the channel under the guidance of the field synergy principle for four typical PCHE channels.Additionally,the comprehensive performance of four channels was compared.Finally,the heat transfer and friction factor correlations for S-CO_(2)in four channels were established.The findings demonstrate that the synergy of velocity and pressure fields of the straight channel PCHE is better(β≈180°),so its resistance loss is relatively small.The zigzag and sinusoidal wavy channels and the airfoil fins can reduce the angle a between the temperature gradient and velocity,thus enhancing the heat transfer.The sinusoidal wavy channel can reduce flow resistance compared to the zigzag channel due to the rounded corners.The streamlined airfoil structure can guide the flow and reduce backflow,thus reducing resistance losses.In the range of Re studied in this paper,the maximum error of the proposed heat transfer and friction factor correlations of PCHE is 7.0%,which shows good fitting accuracy.The research in this paper can provide a reference for the selection and design of PCHE with different channel configurations.

Structural Assessment of Printed Circuit Heat Exchangers in Supercritical CO_(2) Waste Heat Recovery Systems for Ship Applications

Printed circuit heat exchangers(PCHEs)are considered as the most promising heat exchangers for use of the supercritical carbon dioxide(S-CO_(2))Brayton cycle.As crucial components operating at high pressure and thermal load at the same time,PCHE structural integrity evaluations are essential.In this study,to assess the structural strength of PCHEs serving as recuperators and precoolers in the S-CO_(2) Brayton cycle as a waste heat recovery system for marine engines,the finite element method(FEM)is used and compared with a currently used method from ASME codes.The effects of temperature and pressure on the hot and cold sides are studied in terms of the temperature and pressure differences between the two sides and the main factors affecting its strength discussed.Then,detailed stress intensities of a PCHE under design conditions are investigated,and the results indicate that the highest stress appears at the middle of the semicircular arc of the channel,except for a concentration near the channel tip regions.Stresses of the PCHE are mainly caused by both pressure and temperature differences,with the minimum effect from temperature.The synthesis of the temperature and pressure fields exhibits a complicated action on the total stress under the design conditions.FEM was a more comprehensive means for structural assessment than the method from ASME codes.Further structural optimization of PCHE is conducted to ensure a maximum life span.This research work can provide theoretical guidance for structural integrity assessment of PCHE for the S-CO_(2) Brayton cycle.

Experimental and Numerical Study of an 80-kW Zigzag Printed Circuit Heat Exchanger for Supercritical CO_(2)Brayton Cycle

In this study,an experimental system was built to investigate the global performance of an 80-k W zigzag printed circuit heat exchanger(PCHE).It could meet the requirement of the pre-cooler for the supercritical carbon dioxide(S-CO_(2))Brayton power cycle and the modified effectiveness considering the pinch point is between 61.5%and 79.3%.When the outlet S-CO_(2)temperature is near the pseudo-critical point,the thermo-physical properties have more effects on heat transfer performance compared to flow characteristics.For the local performance,the mass flow rates of both sides have crucial influences on the location where the peak of S-CO_(2)Nusselt number occurs while only the S-CO_(2)flow rate affects the variation of the peak value.In addition,the influence of the radius of curvature on the secondary-flow should not be ignored.In the end,new empirical correlations were proposed considering the drastic variations of the Prandtl number.

Fluid-Thermal-Mechanical Coupled Analysis and Optimized Design of Printed Circuit Heat Exchanger with Airfoil Fins of S-CO_(2) Brayton Cycle

Printed Circuit Heat Exchanger(PCHE) with high-efficiency and compact structure has great application prospect in the supercritical carbon dioxide(S-CO_(2)) power systems for the next generation of high-temperature concentrated solar and advanced nuclear energy. However, the high operating temperature and pressure require PCHE to maintain good heat transfer performance, as well as reliable mechanical performance at the same time. It is necessary to carry out the fluid-thermal-mechanical coupled analysis of PCHE for the safe and efficient operation of the S-CO_(2) cycle. In this paper, a three-dimensional fluid-structure coupled numerical model was established to study the fluid-thermal-mechanical coupled characteristics of PCHE under different airfoil fin arrangements. The stress distribution of the single airfoil fin was studied, and a better airfoil arrangement that comprehensively considers heat transfer characteristics and stress distribution was obtained. Aiming at the high stress caused by the stress concentration at both ends of the airfoil fin, an optimized configuration combining straight channel and airfoil channel was proposed. The results show that the difference between the flow and heat transfer performance of the two optimized structures and the reference structure is only within 1.5%, but the maximum stresses of the two optimized structures are respectively reduced by 69.4% and 70.0% compared with that of the reference structure, which significantly reduces the stress intensity of PCHE. The result provides a new method to develop the airfoil PCHE with uniform stress distribution and good thermo-hydraulic performance.

Prediction of Equivalent Thermal Conduction Resistance of Printed Circuit Heat Exchangers

Printed circuit heat exchangers(PCHEs) have great potential to be employed in the advanced nuclear reactor systems. In this work, the equivalent thermal conduction resistance of PCHE is studied. The influences of thermal convection resistance are analyzed. The results indicate that the equivalent thermal conduction resistance of PCHEs with unequal numbers of hot plates and cold plates are sensitive to the thermal convection resistance of hot side and cold side. Specifically, for case C which has unequal number of hot and cold channels, the maximum value of equivalent thermal conduction resistance can be 1.7-2.4 times the minimum value. The equivalent thermal conduction resistance is underestimated under the isothermal boundary. In addition, the non-uniformity of the lengths of all the heat flux lines determines the influence degree of thermal convection resistance on the equivalent thermal conduction resistance. For further investigation, Latin hypercube sampling method is adopted to generate a large number of design points for each PCHE configuration. Based on the sample data, mathematical correlations and artificial neural network(ANN) for prediction of equivalent thermal conduction resistance for each case are developed. The proposed correlations of equivalent thermal conduction resistance for each case have acceptable accuracy of prediction with a wide range covering general engineering applications. The ANN model can achieve much better prediction accuracy than the proposed correlations thus it is recommended in the cases that the prediction accuracy is considered as the priority need.

Comparative study on flow and heat transfer characteristics of sinusoidal and zigzag channel printed circuit heat exchangers

Printed circuit heat exchangers(PCHEs)are promising candidates for recuperators in supercritical CO2 Brayton cycles.A comparative study is given in this paper on the flow and heat transfer characteristics of PCHEs with sinusoidal and zigzag channels.With mass flow rates of 0.6–1.8 kg/h and the bend angles of 15°–30°,the thermal-hydraulic performance of the PCHEs is discussed.Results show that the sinusoidal channel is superior to the zigzag channel in its comprehensive performance.Larger bend angles result in greater reductions in pressure drop if sinusoidal channels are used instead of zigzag channels and a maximum of 48.4%reduction can be obtained in the considered working conditions.Meanwhile,the inlet sections should be carefully optimized since these sections account for up to 31%and 17%of the total pressure drop in the sinusoidal and zigzag channels,respectively.The corner shape of the zigzag channel can be specially designed to further reduce the pressure drop.The nonuniform density and heat flux distributions in both channels are found to be related to the periodic changes of flow directions and the centrifugal forces should not be ignored when optimizing the sinusoidal and zigzag channels.

Thermal-Hydraulic-Structural Analysis and Design Optimization for Micron-Sized Printed Circuit Heat Exchanger

The Printed Circuit Heat Exchanger(PCHE) is one of the most promising heat exchangers for Synergetic Air-breathing and Rocket Engine(SABRE). To reduce pressure drop and improve compactness, the micron-sized PCHE made up of rectangular channels of tens of microns in size, is used in SABRE. In present work, we focus on thermal-hydraulic-structural characteristics of micron-sized PCHE by conducting three-dimensional(3-D) numerical simulation. Helium and hydrogen are employed as the working fluids and the Stainless Steel 316(SS316) as the solid substrate. The thermal-hydraulic performance of the micron-sized PCHE is discussed by using the commercial Computational Fluid Dynamics(CFD) software of Fluent. ANSYSMechanical is also employed to simulate stress field of representative PCHE channels. The mechanical stress induced by pressure loading and the thermal stress induced by temperature gradient are found to be equally important sources of stress. To improve comprehensive performances of micron-sized PCHE, two types of channel arrangements and different channel aspect ratios are studied. The double banking is of higher thermal-hydraulic performance compared to the single banking while the stress performance is identical for the two modes. Meanwhile, the effect of channel aspect ratio is investigated by comparing thermal-hydraulic characteristics and structural stress of the model. The rectangular channel with w/h=2 achieves the most balanced stress characteristic and higher thermal-hydraulic performance.

Improving the Efficiency of Induction Heating in the Air Gap of the Magnetic Circuit

Improving the efficiency of induction heating of parts in the air gap of the magnetic circuit is associated with the use of surface and edge effects.Through modeling in ANSYS Electromagnetics Suite 19.2 and experimental studies identified patterns of edge effect in the heated parts.To ensure the uniformity of induction heating of small parts and reduce the soldering time,the electrical switch of soldered parts is used,which with the help of device controller forms a secondary circuit with low electrical resistance and high density of eddy currents.

Fluid flow and heat transfer characteristics of spiral coiled tube: Effects of Reynolds number and curvature ratio

Numerical analysis was performed to investigate flow and heat transfer characteristics in spiral coiled tube heat exchanger. Radius of curvature of the spiral coiled tube was gradually increased as total rotating angle reached 12n. As the varying radius of curvature became a dominant flow parameter, three-dimensional flow analysis was performed to this flow together with different Reynolds numbers while constant wall heat flux condition was set in thermal field. From the analysis, centrifugal force due to curvature effect is found to have significant role in behavior of pressure drop and heat transfer. The centrifugal force enhances pressure drop and heat transfer to have generally higher values in the spiral coiled tube than those in the straight tube. Even then, friction factor and Nusselt number are found to follow the proportionality with square root of the Dean number. Individual effect of flow parameters of Reynolds number and curvature ratio was investigated and effect of Reynolds number is found to be stronger than that of curvature effect.

Micro segment analysis of supercritical methane thermal-hydraulic performance and pseudo-boiling in a PCHE straight channel

The printed circuit heat exchanger(PCHE) is receiving wide attention as a new kind of compact heat exchanger and is considered as a promising vaporizer in the LNG process. In this paper, a PCHE straight channel in the length of 500 mm is established, with a semicircular cross section in a diameter of 1.2 mm.Numerical simulation is employed to investigate the flow and heat transfer performance of supercritical methane in the channel. The pseudo-boiling theory is adopted and the liquid-like, two-phase-like, and vapor-like regimes are divided for supercritical methane to analyze the heat transfer and flow features.The results are presented in micro segment to show the local convective heat transfer coefficient and pressure drop. It shows that the convective heat transfer coefficient in segments along the channel has a significant peak feature near the pseudo-critical point and a heat transfer deterioration when the average fluid temperature in the segment is higher than the pseudo-critical point. The reason is explained with the generation of vapor-like film near the channel wall that the peak feature related to a nucleateboiling-like state and heat transfer deterioration related to a film-boiling-like state. The effects of parameters, including mass flow rate, pressure, and wall heat flux on flow and heat transfer were analyzed.In calculating of the averaged heat transfer coefficient of the whole channel, the traditional method shows significant deviation and the micro segment weighted average method is adopted. The pressure drop can mainly be affected by the mass flux and pressure and little affected by the wall heat flux. The peak of the convective heat transfer coefficient can only form at high mass flux, low wall heat flux, and near critical pressure, in which condition the nucleate-boiling-like state is easier to appear. Moreover,heat transfer deterioration will always appear, since the supercritical flow will finally develop into a filmboiling-like state. So heat transfer deterioration should be taken seriously in the design and safe operation of vaporizer PCHE. The study of this work clarified the local heat transfer and flow feature of supercritical methane in microchannel and contributed to the deep understanding of supercritical methane flow of the vaporization process in PCHE.

套管式地埋管换热器节能特性的研究与应用

大庆油田进入开发后期存在着一定数量的废弃井,采用套管式地埋管换热器可以获取到废弃井的中深层的地热能。首先对废弃井转换为地热井的可行性进行分析,通过确定中深层地埋管换热方式,建立外入内出型传热计算模型并对废弃井结构改造。再具体分析大庆油田的地层土壤温度分布和确定边界条件,得出符合现实的理论计算公式。进一步分析套管式地埋管换热器取热评价指标与传热影响因素研究,得到流体流量和进口温度对传热影响因素,及循环流体进口温度对运行特性的影响对取热效率的影响规律。最后,通过对两种工况的废弃井现场试验,得到套管式地埋管换热器平均换热量为478.5 kWh。为利用大庆油田1700口废弃井的节能改造找到了一条切实可行的出路。

高温熔盐印刷电路板式换热器的热工水力特性研究

印刷电路板式换热器(Printed Circuit Heat Exchanger,PCHE)换热效率高,结构紧凑,可作为小型模块化熔盐堆热量传输的关键设备,对其流动换热特性的研究具有重要意义。采用数值模拟方法,以翼型通道的熔盐(FNaBe)-氦气换热器为研究对象,通过计算流体动力学(Computational Fluid Dynamics,CFD)数值模拟计算对其熔盐侧的传热特性和阻力特性进行对比分析,分别得到不同温度、翅片类型和不同节距结构下的熔盐(FNaBe)-氦气换热器的流动换热特性,并与传统直通道结构对比进行综合评估。研究结果表明,数值模拟的计算结果与实验结果对比符合较好,翼型翅片流道结构相较于直通道结构能够强化传热、降低阻力,其中节距为8 mm的NACA0025翼型翅片流道结构的流动传热特性最好。本研究建立的数值模拟方法能够用于印刷电路板式换热器的流动传热特性的预测,拟合出节距为8 mm的NACA0025翼型翅片结构的经验关联式,为后续换热器的设计提供理论基础。

竖进平出型地埋管群换热器结构与控制优化

【目的】为提高浅层地热能利用效率,亟须对影响地源热泵系统性能的地埋管换热器结构和控制开展优化设计。【方法】建立了一种竖进平出型地埋管群换热器模型,研究其换热系统性能,探究其最佳结构及运行方式。【结果】管群布置横纵比越低,单位面积换热量越大,横纵比超过0.15后对系统影响较小;换热器间距越小,土壤利用率越高,但热短路损失越大;双列双出型埋管每延米换热量相较于双列单出型结构提升7%以上;负荷较大工况下叉排布置结构的换热效果更好;整个夏季优化结构的平均出口温度下降2.28℃,换热器周围土壤温度下降1℃以上;实际运行过程中应优先考虑采用流速随负荷变化控制模式。【结论】研究结果可为地源热泵换热器的设计、制造提供理论依据。

PCHE叶轮整流效果数值模拟研究

PCHE入口处有着成千上万的微型通道,流体进入通道时存在流量分配不均匀的问题,导致PCHE整体换热性能差,本文提出了一种叶轮型整流器来提高流量的均匀性分配。通过数值模拟的方法,以流体流量不均匀系数α和压降P_(l)作为性能指标进行了参数化对比,分析了整流器结构参数(中心圆环D、叶片的数量N和叶片螺距P)对入口封头内流场的影响。研究结果表明,叶轮对中心集中流量的破坏、诱导产生的多个涡流对流场均匀发展起到关键性作用,中心圆环直径越小,螺距越小时,叶轮的整流效果更好,四叶片叶轮由于其几何对称性,引导流体更均匀的分布在流道。

印刷电路板换热器在超临界二氧化碳布雷顿循环系统中的动态特性研究

为了制定有效的系统控制策略,保证系统安全稳定运行,针对印刷电路板换热器(PCHE)在核能超临界二氧化碳布雷顿循环中作为回热器的应用进行了研究,分析了PCHE在不同扰动条件下的性能表现。采用分段设计方法,构建了一维稳态设计模型,对PCHE进行了稳态传热设计及分析。以稳态设计作为初始条件,利用Simulink软件,建立了PCHE的动态仿真模型,模拟了流体入口温度或流量发生变化时PCHE的动态响应。结果表明:回热器冷侧入口温度发生变化时,热侧出口温度变化明显,出现了约12 K波动,且热侧出口温度响应相对较快,而热侧入口温度变化对热侧出口温度影响微小,仅波动1 K左右;流量变化对回热器效能影响较大,冷侧流量减小或热侧流量增加使得回热器效能降低了6%左右,较大的回热器热惯性使得两侧出口温度对流量的响应较慢,出现了一定的延迟。研究结果可为系统调节与控制提供可靠的指导。

超临界CO_(2)印刷电路板换热器热工水力特性研究

对于超临界CO_(2)布雷顿循环中的印刷电路板换热器,其复杂结构与工作介质的特殊物理性质将导致集总参数方法出现较大的计算误差。为提升印刷电路板换热器的换热参数的计算精度,本文基于Modelica语言,采取分节点建模方法,开发了印刷电路板换热器的热工水力特性计算程序,对其稳态运行时的换热参数进行了分析。结果表明:印刷电路板换热器的通道内超临界CO_(2)的对流换热系数沿流动方向发生显著变化。与设计值相比,分节点计算程序的最大相对误差小于3%,计算精度相比于集总参数方法显著提升,因此有必要采取分节点计算方法以提高计算精度。研究结果可为超临界CO_(2)布雷顿循环系统中印刷电路板换热器的设计和仿真提供参考。