Analysis of influence of heat exchangerfouling on heat transfer performancebased on thermal fluid coupling

A study on heat transfer performance by thermal fluid coupling simulation for the fouling in a shell-tube heat exchanger used in engineering was presented. The coupling simulation was performed in a fluid and solid domains under three different fouling conditions: fouling inside the tube, fouling outside the tube, and fouling inside the shell. The flow field, temperature, and pressure distributions in the heat exchanger were solved numerically to analyze the heat transfer performance parameters, such as thermal resistance. It is found that the pressure drop of the heat exchanger and the thermal resistance of the tube wall increase by nearly 30% and 20%, respectively, when the relative fouling thickness reaches 10%. The fouling inside the tube has more impact on the heat transfer performance of the heat exchanger, and the fouling inside the shell has less impact.

Laminar Heat Transfer Modeling and Investigation of Thermal Dispersion in a Flat Plate Heat Exchanger： Estimating Heat Transfer Coefficient, Heat Flux and Nusselt Number

PHEs （plate heat exchangers） are among the most common thermal equipments in diverse industries particularly in oil and gas companies. This wide usage is obviously due to significant benefits of these heat exchangers over other types. In this article, a behavioral analysis of heat transfer in fiat plates of these heat exchangers in laminar flow situation through CFD （computational fluid dynamics） simulation using FLUENT 6.3.26 software is done. The study reveals results graphically based on fluid＇s behavior in co-current and counter current flows and discusses thermal indexes consisting of heat transfer coefficient, Nusslet and total heat flux in both conditions. Eventually, a comparison via the graphical results is presented between the two types of flow directions.

Mathe matical model and calculation for heat transfer during condensation on surfaces of corrugated plates

A mathematical model was established for condensation on surfaces of verticalcorrugated plates based on the mechanism of heat transfer enhancement to thin down the liquid filmdue to surface tension effect between corrugated plate surfaces and liquid films. The relative heattransfer coefficients of condensation on corrugation plates were calculated in contrast withequivalent vertical plane ones. The heat transfer enhancement effects for the main geometricparameters such as pitch, height, corrugation angle, tilt angle, and fillet radii of corrugationswere analyzed to guide the optimization of corrugation structure for application. A two-scalecorrugation is suggested, which can compromise both the enhanced heat transfer effect and adequatecross section area for flows, and it makes the heat transfer coefficient 1 to 2 times more than thatof an equivalent plane one.

FLOW RESISTANCE AND HEAT TRANSFER CHARACTERISTICS OF A NEW-TYPE PLATE HEAT EXCHANGER

A new-type corrugation Plate Heat Exchanger （PHE） was designed. Results from both numerical simulations and experiments showed that the flow resistance of the working fluid in this new corrugation PHE, compared with the traditional chevron-type one, was decreased by more than 50%, and corresponding heat transfer performance was decreased by about 25%. The flow field of the working fluid in the corrugation PHE was transformed and hence performance difference in both flow resistance and heat transfer was generated. Such a novel plate, consisting of longitudinal and transverse corrugations, can effectively avoid the problem of flow path blockage, which will help to extend the application of PHEs to the situation with unclean working fluids.

Numerical simulation and experimental research on heat transfer and flow resistance characteristics of asymmetric plate heat exchangers

The asymmetric plate heat exchanger(APHE)has the possibility of achieving balanced pressure drops on both hot and cold sides for situations with unbalanced flow,which may in turn enhance the heat transfer.In this paper,the single-phase water flow and heat transfer of an APHE consisted of two types of plates are numerically(400≤Re≤12000)and experimentally(400≤Re≤3400)investigated.The numerical model is verified by the experimental results.Simulations are conducted to study the effects of,an asymmetric index proposed to describe the geometry of APHEs.The correlations of the Nusselt number and friction factor in the APHEs are determined by taking and working fluids into account.It is found that an optimal exists where the pressure drops are balanced and the heat transfer area reaches the minimum.The comparison between heat transfer and flow characteristics of the APHEs and the conventional plate heat exchanger(CPHE)is made under various flow rate ratios of the hot side and the cold side and different allowable pressure drops.The situations under which APHE may perform better are identified based on a comprehensive index.

Surface treatment to improve corrosion resistance of Al plate heat exchangers

The correlations between thermal and physical properties were studied through thermal conductivity measurements, hardness tests, salt spray tests (AASS) among the surface treatment samples named K20, K40 with thickness of 20, 40 μm respectively and raw sample named K00. In thermal conductivity measurements, there are little differences among the samples as K00, K20 and K40, they exhibit 153.39, 150.69 and 149.76 W/(m·K), respectively. According to hardness tests, K00, K20 and K40 exhibit 87.9, 259.7 and 344.8 in Vickers values. In the result of salt spray tests to examine the effects on corrosion resistance, K00, K20 and K40 exhibit the grade of 3?5, 2.0?9.8 and 10, respectively. The mutual relation of the above results was analyzed. It is found that the surface treatments do not affect the thermal conductivity of aluminum and result in the improvement of physical properties. As a result of the technology, the surface improvement of aluminum alloy specimen is achieved without thermal degradation. It validates the ability of the aluminum plate heat exchangers with surface treatment to enhance the corrosion resistance. Present work is performed as the first fundamental threshold in the process of aluminum plate heat exchangers development to check out its possibility, therefore the next step-experimental and numerical study of practical aluminum plate heat exchangers will be made.

Experimental investigations on the heat transfer characteristics of micro heat pipe array applied to flat plate solar collector

This paper introduces a novel fiat plate solar collector （FPC） using micro heat pipe array （MHPA） as a key element. To analyze the thermal transfer behavior of flat plate solar collector with micro heat pipe array （MHPA-FPC）, an indoor experiment for thermal transfer characteristic of MHPA applied to FPC was conducted by using an electrical heating film to simulate the solar radiation. Different cooling water flow rates, cooling water temperatures, slopes, and contact thermal resistances be- tween the condenser of MHPA and the heat exchanger were tested at different heating powers. The experimental results in- dicate that MHPA-FPC exhibits the enhanced heat transfer capability with increased cooling water flow rate and temperature. Total thermal resistance has a maximum decline of approximately 10% when the flow rate increases from 180 to 360 L h-1 and 38% when the cooling water temperature increases from 20~C to 40~C. When the inclination angle of MHPA-FPC ex- ceeds 30~, the slope change has a negligible effect on the heat transfer performance of MHPA-FPC. In addition, contact thermal resistance significantly affects the heat transfer capability of MHPA-FPC. The total thermal resistances lowers to nearly half of the original level when contact material between the condenser of MHPA and the heat exchanger changes from conductive silicone to conductive grease. These results could provide useful information for the optimal design and operation of MHPA-FPC.

Experimental Analysis of Performance of Heat Exchanger with Plate Fins and Parallel Flow of Working Fluids

Heat exchangers are devices in which heat is transferred from one fluid to another fluid as a result of temperature difference. Heat exchanger presented in the current paper in which inside the tubes flows water, but outside the tubes flows air aims to enable cooling of circulating water, which serves to cool the engine of a machine. Such exchangers find application in the automotive industry as well as heating and cooling equipment and HVAC systems etc. The surface of the heat exchanger by the air side always tends to be much larger using surface fins in order to facilitate equalization of thermal resistance for both sides of the heat exchanger, because the rate of transmission of heat from the water side is much greater. Furthermore, the paper will present analytical and experimental studies involved for determination of performance of plate-fin heat exchanger for various flows of working fluids in order to get the highest values of performances i.e.: overall heat transfer coefficient U, efficiency of heat exchanger ε, maximal and real heat transferred, pressure drop, air velocity and Reynolds number from the air side of heat exchanger etc. The present scientific paper is based on the fact that from the experimental model made for laboratory conditions, conclusions are derived that can be used during installation of such heat exchanger on certain machines in order to predict their performance.

Heat Transfer Enhancement and Vortex Flow Structure in the Spirally Fluted Tubes

The turbulence kinetic energy and heat transfer performance of air in spirally fluted tube were numerically studied at a constant wall temperature with Reynolds number(Re)between 5000 and 45000.Furthermore,the flow dynamics and heat transfer performance of spirally fluted tubes with five different geometric parameters as well as the effects of separation vortex and swirling wake flow on heat transfer and flow resistance were analyzed.According to the results,heat transfer is enhanced mainly because the fluid hit the windward side of the flute,thus generating a strong turbulence kinetic energy to further reconstruct the boundary layer.The second reason is that the formation of the recirculation zone between the flutes disturbs the boundary layer caused by the flow separation.With the increase of flute depth ratio(L_(d)/D),the separation vortex will become stronger and larger on the leeward side of flute.The separation vortex will break the boundary layer and improve the heat transfer capacity which is accompanied with the increase of fluid resistance.As the flute pitch length ratio(L_(p)/D)decreases,the spiral flow is strengthened,and meanwhile more wake flow is generated.The spiral flow causes little impact on enhancing heat transfer but inhibits the development of the separation vortex and fluid pulsation;in addition,the fluid resistance is reduced at the same time.The maximum value of the average Nusselt number appears when Re=5000,L_(d)/D=0.25 and L_(p)/D=1.00,which is 2.53 times the value of smooth tube.In view of the whole range of Reynolds number,the thermal performance enhancement factor indicates that L_(d)/D=0.15 and L_(p)/D=1.00 are the optimal geometric design parameters.

Development of high-aspect-ratio microchannel heat exchanger based on multi-tool milling process

A high-aspect-ratio microchannel heat exchanger based on multi-tool milling process was developed. Several slotting cutters were stacked together for simultaneously machining several high-aspect-ratio microchannels with manifold structures. On the basis of multi-tool milling process, the structural design of the manifold side height, microchannel length, width, number, and interval were analyzed. The heat transfer performances of high-aspect-ratio microchannel heat exchangers with two different manifolds were investigated by experiments, and the influencing factors were analyzed. The results indicate that the magnitude of heat transfer area per unit volume dominates the heat transfer performances of plate-type micro heat exchanger, while the velocity distribution between microchannels has little effects on the heat transfer performances.

考虑物性变化及壳体传热的新型板壳式换热器板程流动传热数值模拟

板壳式换热器以优越的换热性能和耐温耐压性在化工生产等领域有着广阔的应用前景。本文采用可实现k⁃ε湍流模型结合增强壁面函数,对一种新型板壳式换热器板程流动与传热特性进行数值模拟研究,讨论了介质物性变化及壳体传热的影响,并与已有实验进行验证。重点分析了不同波纹高度在不同入口流速下的流动传热特性,基于场协同理论揭示了速度场与温度场及压强场协同分布规律。结果表明:流动传热计算不能忽略流体介质物性变化及壳体传热的影响;增大波纹高度流态由曲折流向十字交叉流转变,板间流体分布趋于均匀,换热性能增大;沿沟槽形成连续的涡结构,在触点周围形成具有周期性和中心对称的高剪切涡量集中区,垂直于流动方向场协同呈现周期性变化,波纹高度越高周期性越明显;在波纹核心流域内因流动与热流相似,其协同程度变差。

吸气预冷发动机预冷换热芯体仿生结构设计发展趋势与展望

现有超高声速飞行器预冷组合循环发动机空气预冷器内部一般由多组微细管束组成,工作过程中存在流致振动大、加工要求高等问题,因此,有必要对预冷器换热芯体进行改型设计,并对其开展进一步的流动换热研究。本文首先分析了基于仿生分形的预冷器换热芯体改型需求;然后梳理了仿蜂巢分形、树状分形与生物表面混合结构在新型预冷芯体强化换热上的应用,以及仿生物体凹凸表面结构、疏水结构在降低预冷芯体内通道表面流阻、抑制结霜上的应用,给出了3种可适用于空天领域的“筒状”仿生预冷换热芯体设计案例;最后总结并展望了仿生微通道换热结构与预冷器换热芯体设计结合的发展前景。

不锈钢板式热沉传热特性的实验与模拟研究

目的 研究流量、胀高及焊点排布方式对不锈钢板式热沉传热特性的影响。方法 搭建实验平台,加工出结构参数不同的板式热沉实验件,同时采用静力隐式算法建立相应的几何模型,通过实验与数值模拟相结合的方法,研究流量、胀高和焊点排布方式的变化对板式热沉热阻的影响规律,并分析热沉流道内部流场状态。结果 首先计算了6个工况下的热阻,发现实际换热量为540 W时,数值模拟与实验结果的最大相对误差不大于25%。然后分析了误差来源,最后得到了热沉全流道速度场与温度场的分布情况。结论 实验与模拟结果有较好的符合程度,证明了数值模拟方法的可靠性。在换热量相同时,板式热沉的热阻随流量与胀高的增大而减小,焊点为菱形排布的热沉传热性能相对更好。板式热沉内部的流场具有周期性,焊点使流体扰动增强,并形成射流和旋涡,起到了强化传热的效果。

涡旋压缩机浮动式静盘的性能优化研究

针对浮动式静盘涡旋压缩机静盘背部温度过高且集中于排气管处的问题,提出带散热结构的浮动式静盘背压机构,研究了新机构的质能交换,分析了热量交换过程,给出了各腔体传热的数学模型,并对新机构进行CFD模拟计算,得到了不同外界温度下的静盘背部温度分布情况。与原浮动式静盘背压机构对比分析可得,新机构有效地改善了静盘背部中心温度过高的问题,使静盘温度分布更加均匀,可以解决因温度过高而造成的变形、摩擦磨损以及泄漏等问题,提高了压缩机的可靠性。

竖直多波纹板式换热器内R134a的流动沸腾实验研究

针对竖直多波纹板片内R134a的流动沸腾进行实验研究,采用高速相机捕捉并分析两相流型的衍变规律,分析干度对换热系数、压降的影响。结果表明:板片内部的汽液相于其所处平面沿水平方向存在交替分布特征;随着干度上升,板片左上角部分流域出现相对停滞的液膜区,其面积随着干度的增加而扩大;波纹板片内的主导换热机制由核态沸腾逐渐转变为对流沸腾。当干度x较低时(x<0.4),换热系数随干度的增加缓慢增加;随后换热系数随干度的上升速率加快(0.4<x<0.6),并于x=0.6时达到最大值;高干度时(x>0.7),板片内出现局部干涸现象,换热系数迅速下降。当质量通量相同时,板片的摩擦压降随干度的增加而增加,而饱和压力的上升会使得摩擦压降有所降低,最大降幅为20.4%。

基于土壤分层当量物性的地埋管换热器换热模型研究

文章综合考虑了土壤分层与地下水渗流等情况,结合实测数据,通过地埋管外壁温度计算得出地埋管内部流体温度与不同分层土壤的热物性参数,根据土壤当量物性进行分层,据此建立地埋管换热模型。经过对比验证实验发现,分层换热模型比常规均质模型更接近实测值,误差更小;基于土壤分层当量物性的地埋管换热器换热模型模拟出水温度与实测值平均绝对误差为0.21℃,模拟进、出口换热温差与实测值平均绝对误差为0.14℃,具有较高的准确度,可为工程设计提供依据并作为后续地埋管换热优化研究的基础。

Experimental Study on Heat Transfer and Pressure Drop Characteristics of Four Types of Plate Fin－and－TUbe Heat Exchanger Surfaces

In this paper, air side heat transfer and pressure drop characteristics of twelve three-row plate finandtube heat exchanger cores of four types of fin configurations have been experimentally investigated.The heat transfer and friction factor correlations for'the twelve cores are provided in a wide range ofReynolds number. It is found that in the range of Reynolds number tested, the Nusselt number of theslotted fin surface is the largest and that of the plain plate fin is the lowest while the Nusselt numbersof two types of wavy fins are somewhere in between.

混合板换热器板片结构对流道内触点分布及流动换热性能的影响

混合板式换热器作为余热利用系统中高效节能设备,其换热性能对系统效率及稳定性有重要影响。该研究基于RNG k-?湍流模型,对M型和H型板式换热器流道内流动及传热过程展开数值模拟,研究板片结构对板间触点分布的影响,及雷诺数Re和板片组合对横向流道内流动及换热特性的影响规律。结果表明:M型板间触点呈“方格”,H型呈“菱形”,且H型流流体速度较大、温度场分布较均匀。随Re数增大,压降△P及平均努塞尔数■均增大;Re数较低时,H型的■明显提高,且△P增幅较小;而高Re数(Re=6000)时,与M型相比,H型■的增幅仅为△P的25%左右。随波纹间距s减小,触点数明显增多,△P和■均增大;s为12~16 mm较合适。研究结果为混合板式换热器的设计及优化提供理论依据。

扭曲管同轴套管换热器强化传热和流阻特性

开发一种新型扭曲管同轴套管换热器,对其传热和流动特性进行实验测试,并拟合管程及壳程的Nusselt数(Nu)和摩擦因子(f)关联式。实验结果表明:随着流速的增大,总传热系数和流阻升高。在测试范围内(0.1~1.5 m/s),总传热系数增幅约为34%,管程流阻从37 Pa升高约120倍至6033 Pa,壳程流阻从361 Pa升高至130761 Pa。当1000<Re≤10000时,扭曲管同轴套管换热器综合评价因子(η)>1,最高为2.3;当10000<Re<30000时,η<1。在管程流速为0.1~0.5 m/s、壳程流速为0.1~0.4 m/s的范围内,扭曲管同轴套管换热器的整体综合性能最佳,说明扭曲管同轴套管换热器适用于小型制冷空调设备,适合Reynolds数较低的应用场景。