Localized Theoretical Analysis of Thermal Performance of Individually Finned Heat Pipe Heat Exchanger for Air Conditioning with Freon R404A as Working Fluid

This work contributes to the improvement of energy-saving in air conditioning systems. The objective is to apply the thermal efficiency of heat exchangers for localized determination of the thermal performance of heat exchangers with individually finned heat pipes. The fundamental parameters used for performance analysis were the number of fins per heat pipe, the number of heat pipes, the inlet temperatures, and the flow rates of hot and cold fluids. The heat exchanger under analysis uses Freon 404A as a working fluid in an air conditioning system for cooling in the Evaporator and energy recovery in the Condenser. The theoretical model is localized and applied individually to the Evaporator, Condenser, and heat exchanger regions. The results obtained through the simulation are compared with experimental results that use a global approach for the heat exchanger. The thermal quantities obtained through the theoretical model in the mentioned regions are air velocity, Nusselt number, thermal effectiveness, heat transfer rate, and outlet temperature. The comparisons made with global experimental results are in excellent agreement, demonstrating that the localized theoretical approach developed is consistent and can be used as a comprehensive analysis tool for heat exchangers using heat pipes.

Localized Theoretical Analysis of Thermal Performance of Individually Finned Heat Pipe Heat Exchanger for Air Conditioning with Freon R404A as Working Fluid

This work contributes to the improvement of energy-saving in air conditioning systems. The objective is to apply the thermal efficiency of heat exchangers for localized determination of the thermal performance of heat exchangers with individually finned heat pipes. The fundamental parameters used for performance analysis were the number of fins per heat pipe, the number of heat pipes, the inlet temperatures, and the flow rates of hot and cold fluids. The heat exchanger under analysis uses Freon 404A as a working fluid in an air conditioning system for cooling in the Evaporator and energy recovery in the Condenser. The theoretical model is localized and applied individually to the Evaporator, Condenser, and heat exchanger regions. The results obtained through the simulation are compared with experimental results that use a global approach for the heat exchanger. The thermal quantities obtained through the theoretical model in the mentioned regions are air velocity, Nusselt number, thermal effectiveness, heat transfer rate, and outlet temperature. The comparisons made with global experimental results are in excellent agreement, demonstrating that the localized theoretical approach developed is consistent and can be used as a comprehensive analysis tool for heat exchangers using heat pipes.

NUMERICAL STUDY ON FLOW DISTRIBUTION IN PLATE-FIN HEAT EXCHANGERS

Objective To investigate the flow distribution in plate fin heat exchangers and optimize the design of header configuration for plate fin heat exchangers. Methods A mathematical model of header was proposed. The effects of the header configuration on the flow distribution in plate fin heat exchangers were investigated by CFD. The second header configuration with a two stage distributing structure was brought forward to improve the performance of flow distribution. Results It is found that the flow maldistribution is very serious in the direction of header length for the conventional header used in industry. The numerical predictions indicate that the improved header configurations can effectively improve the performance of flow distribution in plate fin heat exchangers. Conclusion The numerical simulation confirms that CFD should be a suitable tool for predicting the flow distribution. The method has a wide variety of applications in the design of plate fin heat exchangers.

Performance Evaluation Methods for Multi-stream Plate-Fin Heat Exchanger

Mathematical model of cross type multi-stream plate-fin heat exchanger is established.Meanwhile,mean square error of accumulative heat load is normalized by dimensionless,and the equations of temperature-difference uniformity factor are improved.Evaluation factors above and performance of heat exchanger are compared and analyzed by taking aircraft three-stream condenser as an example.The results demonstrate that the mean square error of accumulative heat load is common result of total heat load and excess heat load between passages.So it can be influenced by passage arrangement,flow inlet parameters as well as flow patterns.Dimensionless parameter of mean square error of accumulative heat load can reflect the influence of passage arrangement to heat exchange performance and will not change dramatically with the variation of flow inlet parameters and flow patterns.Temperature-difference uniformity factor is influenced by passage arrangement and flow patterns.It remains basically unchanged under a certain range of flow inlet parameters.

Metal foam and fin implementation into a triple concentric tube heat exchanger over melting evolution

It is believed that it is going to be a sizeable mismatch between supply and demand when it comes to renewable resources.Lately,researchers are on course to compensate for the unpredictabilityof such resources by the employment of phase change materials(PCMs).Having multiple advantages,PCMs generally suffer from inadequate thermal conductivity which causes prolonged transition procedures.To tackle this issue,this study is fixated on two parameterswhich are linked to fins addition and porous media incorporation in a melting process within a triple concentric tube heat exchanger(TCTHX).The results provided by multiple cases underlined the significance of natural convection in the bare system,although finned and copper-metal-foam cases outshine buoyancy forces by roughly 45%and 97%,respectively.Material is a major determent when it comes to the selection of porous media as Al_(2)O_(3) registered the weakest performance among SiC,Ni and Cu,however,it managed to speed up the process by 75%which still is much higher than the finned system,implying that porous media is of higher priority over fins.The best scenario transpiredwhile fins and copper metal foam were integrated as 26%and 97%soars in efficacy have been obtained compared to individual incorporation of porous media and fins,respectively.

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.

Optimal design of the first stage of the platefin heat exchanger for the EAST cryogenic system

The size of the heat exchanger is an important factor determining the dimensions of the cold box in helium cryogenic systems. In this paper, a counter-flow multi-stream plate-fin heat exchanger is optimized by means of a spatial interpolation method coupled with a hybrid genetic algorithm.Compared with empirical correlations, this spatial interpolation algorithm based on a kriging model can be adopted to more precisely predict the Colburn heat transfer factors and Fanning friction factors of offset-strip fins. Moreover, strict computational fluid dynamics simulations can be carried out to predict the heat transfer and friction performance in the absence of reliable experimental data. Within the constraints of heat exchange requirements, maximum allowable pressure drop, existing manufacturing techniques and structural strength, a mathematical model of an optimized design with discrete and continuous variables based on a hybrid genetic algorithm is established in order to minimize the volume. The results show that for the first-stage heat exchanger in the EAST refrigerator, the structural size could be decreased from the original2.200？×？0.600？×？0.627（m^3） to the optimized 1.854？×？0.420？×？0.340（m3）, with a large reduction in volume. The current work demonstrates that the proposed method could be a useful tool to achieve optimization in an actual engineering project during the practical design process.

Numerical simulation of heat transfer and flow of cooling air in triangular wavy fin channels

Numerical computation models of air cooling heat transfer and flow behaviors in triangular wavy fin channels(TWFC) were established with structural parameters of fins considered.The air side properties of heat transfer coefficient and pressure drop are displayed with variable structural parameters of fins and inlet velocities of cooling air.Within the range of simulation,TWFC has the best comprehensive performance when inlet velocity vin=4-10 m/s.Compared with those of straight fins,the simulation results reveal that the triangular wavy fin channels are of higher heat transfer performances especially with the fin structural parameters of fin-height Fh=9.0 mm,fin-pitch Fp=2.5-3.0 mm,fin-wavelength λ=14.0-17.5 mm and fin-wave-amplitude A=1.0-1.2 mm.The correlations of both heat transfer factor and friction factor are presented,and the deviations from the experimental measurements are within 20%.

Development of Single Phase Heat Transfer Correlations for Water ＆ R134a in Rectangular Channel with Smooth Wavy Fin

Single phase heat transfer analysis of water and R134a refrigerant （liquid phase） has been carried out using CFD （Computational fluid dynamics） approach for rectangular channel with smooth wavy fin. Colburnj factor and Fanning friction factorf are predicted for wavy fin. The correlations are developed at Reynolds number range of 100-15,000. The effect of fin geometry （fin spacing, fin height, wave height and wave length） on the enhanced heat transfer and pressure drops are investigated. Results show that there is no significant variation off factor for water and liquid R134a at constant Reynolds number. However variations inj factor were observed at constant Reynolds number. Colburnj factor and Fanning friction factorfcorrelations are proposed in terms of Re and geometry parameters （h/s, a/s, L/a） for water and liquid refrigerant R134a in the present study. Two separate equations are proposed for the low and high Re regions i.e. between Re of 100-1,000 and Re of 1,000-15,000.

Development of Heat Transfer Coefficient and Friction Factor Correlations for Serrated Fins in Water Medium using CFD

Abstract： The most popularly used fin types in compact heat exchangers are the serrated fins, wavy fins, louvered fins and plain fins. Amongst these fin types the serrated fins assume lot of importance due to its enhanced thermo-hydraulic performance. Thermo-hydraulic design of CHEs （Compact heat exchangers） is strongly dependent upon the predicted/measured dimensionless performance （Colburnj factor and Fanning friction vs. Reynolds number） of heat transfer surfaces. This paper describes the numerical analysis to study the heat transfer coefficient and friction factor of Serrated fins in water medium. CFD （Computational fluid dynamics） methodology has been used to develop the single phase water heat transfer coefficient and friction factor correlations for serrated fins using ANSYS Fluent 14.5. The results are compared with previous air-cooled models and experimental results of water. The water cooled CFD analysis results shows that the Prandtl number has a large effect on the Nusselt number of the serrated fin geometry. Finally, the generalized correlations are developed for serrated fins taking all geometrical parameters into account. This numerical estimation can reduce the number of tests/experiments to a minimum for similar applications.

换热翅片几何量数字化测量技术研究及在智能制造中的应用

为解决换热翅片开窗角度的数字化测量问题,研究了面向换热器智能制造产线的翅片几何量参数测量装置。该装置采用激光轮廓传感器进行非接触测量,对获取的多组原始轮廓数据进行数据剔除和取均值预处理,将处理后的数据逐点求导以识别兴趣测量区域,在各兴趣测量区域采用定长逐点步进的方式进行最小二乘拟合运算,选取最优拟合线段进行夹角计算,可自动实现对翅片8个开窗角度及3个长度参数的测量,设计了基于JSON的数据交互格式实现与制造执行系统的数据对接。实验结果表明:装置角度测量最大允许误差(MPE)为±0.5°,具备数字化测量及数据交互能力,测量精度满足企业质控需求。

保温时间对钛合金板翅式换热器真空钎焊过程温度场及残余应力的影响

钛合金板翅式换热器成品率较低、残余应力较大,该文针对钛合金板翅结构真空钎焊过程开展热-固耦合建模与仿真研究,阐述了钎焊过程温度均匀性及残余应力分布特征,探明保温时间对钛合金板翅式换热器真空钎焊过程温度场及残余应力的影响机理.结果表明,板翅结构两侧温度较高,中部温度较低,延长保温时间可有效改善板翅结构的温度均匀性.残余应力均主要集中于翅片上表面与隔板背侧中部,在钎缝及夹持点处存在明显的应力集中现象,保温时间越长翅片钎缝的残余应力越小.模拟得到钛合金板翅式换热器真空钎焊中残余应力与试验结果相吻合,相对误差为5.3%,验证了该模型的准确性.

三套管式加肋相变蓄热单元的强化传热特性

相变蓄热技术在利用工业余热、太阳能等解决热能与用户之间的供需不平衡问题方面起着重要的作用,然而受相变材料自身物性的限制,蓄热熔化时仍存在蓄热速度低、熔化不均匀等问题。本文以三套管式相变蓄热单元为基本结构,利用FLUENT软件模拟研究了在考虑自然对流情况下添加纵向肋片的不同参数(肋片数量、长度、厚度、偏心距及肋片排布方式)对三套管式相变蓄热单元蓄热性能的影响。结果表明,三套管式蓄热单元内外双壁面共同加热,与普通套管式蓄热单元相比,换热面积增大,相变材料全部熔化需要的时间缩短;加装纵向直肋片的三套管蓄热单元的熔化速度进一步加快,相同数量肋片下,肋片的长度和厚度是影响蓄热单元蓄热量与平均蓄热速率的主要因素,肋片长度占相变材料区域径向长度的50%时,蓄热单元的熔化速度加快幅度明显且蓄热量与平均蓄热速率较高。通过优化设计,发现内管向下偏移距离e对相变材料的熔化过程作用显著,偏心距离较短时相变材料完全熔化所需时间最少,相比光滑管蓄热单元,相变材料的熔化速率提升了48.5%,偏移距离过长则会延长蓄热时间;通过加密蓄热单元底部外层肋片的排布,蓄热单元的熔化进程有不同程度的加快,总体蓄热速率较原肋片结构有所提高。

低阻叉流板式换热器传热性能与热阻靶向调控研究

用于烟气余热回收的板式换热器存在温度分布不均的问题。为改善烟气温度分布均匀性,有效遏制换热器腐蚀,本文通过实验测试对比碳钢与铸铁材质低阻叉流板式换热器性能差异,运用数值仿真预测局部流动传热特性并开展热阻分析,采用局部热阻调控方法设计了非均匀翅片结构。实验结果表明,在450℃高温时碳钢与铸铁材质的叉流板式换热器总传热系数分别达到32.5 W/(m^(2)·K)与25.1 W/(m^(2)·K)。通过数值仿真研究发现,叉流传热方式导致烟气侧沿程截面温度分布极不均匀,在热侧出口与冷侧入口交叉区、热侧出口与冷侧出口交叉区的热阻大,极易导致局部烟气温度低于露点温度。基于热阻分析法开发非均匀翅片结构,实施局部热阻靶向调控,最终实现维持阻力几乎不变情况下,使出口截面平均温度方差由6.9℃降至1.1℃,换热器内流体温度分布均匀性得到显著提升。

空调器冷凝器结构参数对其制冷剂积存量及系统性能的影响

在保持R290空调器系统性能的前提下,为降低冷凝器中制冷剂积存量,建立冷凝器性能综合评价模型,对冷凝器不同支路数流程的结构参数进行优化研究。结果表明:管径减小使得制冷剂积存量下降、COP下降;管间距和翅片厚度增大以及翅片间距减小均使得制冷剂积存量下降、COP升高;对冷凝器的制冷剂积存量和系统COP进行多目标优化,得到最优设计结构参数为四支路流程,管径5.74 mm,管间距19.67mm,翅片间距1.43mm,翅片厚度0.092mm。本研究可为R290房间空调器减少制冷剂充注量提供参考。

翼型结构对印刷电路板换热器流动与换热特性影响

印刷电路板换热器(PCHE)作为超临界二氧化碳(S-CO_(2))布雷顿循环中的重要设备,其性能会影响系统的效率。为了提高翼型PCHE性能,探究翼型翅片结构对其流动与换热性能的影响。本工作针对S-CO_(2)布雷顿循环系统中的高温回热器,采用数值模拟的方法,选用S-CO_(2)为传热工质。通过对3种不同结构的NACA0020的翼型翅片通道的流动与换热性能进行比较,研究翼型翅片最大厚度在弦长上的位置参数对换热器的性能影响,并从热工水力性能、综合性指标、整体流动性、局部性能等方面分析了变化规律。结果表明,在翼型翅片结构中保持相对厚度不变的情况下,随着翼型翅片最大厚度在弦长上越接近翼型前缘的位置,其换热性能也会随之提高。其中NACA0020-20的翼型翅片通道在选定的工况条件下,j因子和Nu比其他两种翼型翅片通道大2.7%~8.8%和2.7%~8%,并且该翼型翅片结构可以有效地减小边界层所带来的影响,提高了其PCHE的换热性能,其综合性能更是优于其他两种翼型翅片。研究结果对于翼型PCHE的结构设计和性能优化提供了一定的依据。

房间空调器小管径室外换热器流路优化

随着空调器的快速发展和国际铜价的持续升高,制冷空调行业采用的换热器在向着小体积、低成本发展的过程中往往面临着性能衰减的技术瓶颈。为突破此瓶颈,以一款冷暖型房间空调器翅片管式室外换热器为研究对象,将其换热管外径由7 mm减小至5 mm并在此基础上优化其流路,最终实现Φ5 mm换热器性能达到初始样机水平并有效降低换热器成本。首先,分别在5种标准工况下测试初始样机性能;然后,利用CoilDesigner软件来分析小管径化对换热器性能的影响,仿真结果表明,在不改变流路布置的情况下,Φ5 mm换热器的换热能力略有提升,但制冷剂侧压降增大4~7倍;进一步地,有针对性地设计了5种流路布置方案来探究分路数、分路汇合位置等因素对换热器传热及压降特性的影响规律,总结了小管径翅片管式换热器流路优化的思路,并设计了3种Φ5 mm管径室外换热器流路优化方案。仿真评估3种方案的可行性后制作样机进行性能测试,实验结果表明:方案(3)性能与原换热器相当,同时其材料成本可降低34.6%。

扁平纵向翅片管换热器的传热特性分析及结构优化

为进一步优化传统纵向翅片管换热器的传热性能,提出了一种扁平纵向翅片管换热器。基于传热流动基础理论,建立扁平纵向翅片管的物理模型,给出相关传热计算公式,优化扁平管和翅片的结构参数。进一步将扁平翅片管和圆管翅片管以及组成换热器的换热性能进行比较分析,结果表明,等截面同翅片结构参数下单位体积内气侧换热面积增加1.5倍,同质热负荷下换热器的体积减少50%以上,结构紧凑,具有较好的综合优势,为该换热器的推广应用提供参考。

水平微肋管内R513A冷凝换热特性研究

采用实验方法研究了R513A在光管和不同齿密度微肋管内的冷凝换热性能,并通过对比冷凝换热系数实验值与关联式的预测值,探究了现有关联式对R513A的适用性,实验在冷凝温度33、35、38℃,质量流速50~150 kg/(m^(2)·s)的工况范围内进行。实验结果表明:冷凝换热系数随质量流速的增大而增大,在低质量流速区域,微肋管的冷凝换热系数增长幅度更大;随着齿密度的增加,管内换热面积增大,但管内表面张力作用增强,相应的流动阻力增强,强化传热作用被削弱;与R134a相比,微肋管对R513A的强化传热效果更优;对于光管,Bashar关联式的预测效果最佳,Cavallini关联式与Dobson关联式在低质量流速区域的预测误差偏大;对于微肋管,Yu关联式的预测效果最理想,Cavallini关联式与Kedzierski关联式均低估了管内冷凝换热特性。以Kedzierski关联式为基础,拟合了新的修正关联式,修正关联式的预测值与实验值的一致性较好,其平均绝对误差与标准差分别为7.8%、8.5%。

鱼鳍型涡发生器强化管翅式换热器传热

提出了一种鱼鳍型涡发生器,并将其应用在管翅式换热器中强化传热。采用数值模拟方法研究了鱼鳍型涡发生器安装方式对换热器中流体流动和传热的影响,分析了流体流经涡发生器前后的压力变化。结果表明:涡发生器以common-flow-up方式安装在圆管下游时传热j因子提高了46.6%,摩擦因子提高了24.4%,与以common-flow-down方式安装于圆管上游相比,圆管后方的负压区更小,流动阻力更小,冷热流体的混合效果更好,因而具有更好的强化传热效果。