Heat transfer enhancement of serpentine channels with twisted tape insert by computational fluid dynamics and artificial intelligence

Improving the thermal–hydraulic performance of heat exchangers is a critical challenge. In this study, we investigated the impact of twisted tape insert geometrical variables, namely pitch length (P) and diameter (W), on the Nusselt number (Nu) and friction factor (f) within serpentine channels. To accomplish this, we employed computational fluid dynamics (CFD) as our simulation technique, with water as the working fluid. The validity of the CFD data was confirmed, leading to the development of artificial intelligence (AI) subsets: artificial neural network (ANN) and genetic algorithm (GA). In the prediction models, the following input variables were considered: Reynolds number (Re), (P/D_h), and (W/D_h). The ANN and GA models achieved a mean relative error (MRE) of 0.125% and 1.326% for Nu, and 0.139% and 4.104% for f, respectively. These results demonstrate the high accuracy of the correlations, with the ANN model showcasing superior performance.     

结构化金属填充床传递特性的数值模拟

运用计算流体力学（CFD）与计算传热（CHT）方法,对结构化金属填充床内的流体动力学和传热特性进行了详细的模拟,以预测其流场和温度场.分析了床层结构参数和物性的变化对结构化金属填充床传热性能的影响,发现在Re较小且结构化材料和床层空隙率相同的情况下,气固相之间换热的比表面积越大,传热效果越好.进一步将模拟结果与传统颗粒填充床的压降与传热特性进行对照,从而推断结构化金属填充床具有很好的传递性能.     

Comparing velocities and pressures in redox flow batteries with interdigitated and serpentine channels

Serpentine channels adjacent to a thin, porous medium are a potentially attractive alternative to a conventional thick flow-through electrode for redox flow batteries. The hydrodynamics of serpentine flow fields were investigated with computational fluid dynamics, a two-dimensional model of the porous electrode based on Darcy's law, and a resistance network model at the scale of the active area. Predictions from the three models were used to map the available design space. The optimal electrode thickness, in terms of minimizing nonuniformity, was identified and compared to the result for an interdigitated flow field. Serpentine favors thicker electrodes and higher flows than interdigitated, in qualitative agreement with experimental findings. Furthermore, interdigitated designs deliver more uniform intraelectrode velocities and lower overall pressure drops than serpentine flow fields.     

Flow distribution of hydrocarbon fuel in parallel minichannels heat exchanger

In this article, the flow distribution of the Chinese No. 3 jet fuel in parallel minichannels heat exchanger under high temperature condition was investigated. The models of PFR and choked flow were established based on the real fluid model. The formation mechanism of flow maldistribution of the fuel in the freely distributed channels was studied. It was found that: under low heat flux, the slight flow rate deviation will be spontaneously eliminated; under high heat flux, the slight deviation of flow rate and heat flux will be enlarged and result in the channel with smaller flow rate entering the coking region. The feasibility and influence factors of the control method of flow distribution based on choked flow were discussed. The experimental results indicated that the minichannels fuel‐cooled plate with choked flow could maintain uniform flow distribution when the total fuel outlet temperature reached 1035 K. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2781–2791, 2018     

电动汽车电池冷却器冷却液侧传热与流动性能仿真

针对双回路液冷电池热管理系统关键部件电池冷却器进行仿真研究,将提取出的冷却液侧流道作为研究对象,分析换热器中波纹板结构、冷却液质量流量与入口温度对于流道内流动及换热的影响。研究发现,波纹及上下板间的触点结构会在流道中产生的二次流,在低Reynolds数（Re=739）下即可达到湍流,增强了换热效果。拟合了板片Nusselt数与Reynolds数的关系式,发现板片的平均传热系数随着质量流量的提高而增加,增幅可达374%,但功耗也随之迅速增加,因而,需要合理选择质量流量以平衡传热与功耗。冷却液入口温度主要通过热物性影响传热系数及压降,但整体影响幅度较小,因而在实际使用中可不考虑季节与运行因素对电池冷却器性能的影响。     

垂直管内高沸点工质异丙苯流动沸腾换热与阻力

对垂直管内高沸点有机工质异丙苯的流动沸腾换热与摩擦压降特性进行了实验研究,获得了换热系数与摩擦压降沿流动方向的变化情况,并与前人模型的计算值进行了比较。实验过程中质量流速为382—786 kg/(m2.s),实验工况干度为0.1—0.6,压力为0.21—0.27 MPa。通过对实验数据的回归分析,获得了换热系数与摩擦压降的计算关联式。在对复杂的非线性方程进行回归分析时,提出了一种简单的迭代分析方法。研究结果丰富了高沸点工质的流动沸腾换热数据,为石化行业中换热器的设计提供了依据,并为强化凝结换热器的开发和性能测试提供了比较基准。     

Particle scale study of heat transfer in packed and bubbling fluidized beds

The approach of combined discrete particle simulation (DPS) and computational fluid dynamics (CFD), which has been increasingly applied to the modeling of particle‐fluid flow, is extended to study particle‐particle and particle‐fluid heat transfer in packed and bubbling fluidized beds at an individual particle scale. The development of this model is described first, involving three heat transfer mechanisms: fluid‐particle convection, particle‐particle conduction and particle radiation. The model is then validated by comparing the predicted results with those measured in the literature in terms of bed effective thermal conductivity and individual particle heat transfer characteristics. The contribution of each of the three heat transfer mechanisms is quantified and analyzed. The results confirm that under certain conditions, individual particle heat transfer coefficient (HTC) can be constant in a fluidized bed, independent of gas superficial velocities. However, the relationship between HTC and gas superficial velocity varies with flow conditions and material properties such as thermal conductivities. The effectiveness and possible limitation of the hot sphere approach recently used in the experimental studies of heat transfer in fluidized beds are discussed. The results show that the proposed model offers an effective method to elucidate the mechanisms governing the heat transfer in packed and bubbling fluidized beds at a particle scale. The need for further development in this area is also discussed. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

孔板结构换热器传热与阻力性能的数值模拟

基于周期性全截面模型及RNG k-ε湍流模型,运用计算流体力学软件FLUENT对不同孔板结构换热器壳程流体流动以及传热性能进行了数值模拟分析,并通过文献试验数据验证了该数值模拟方法的可行性和准确性。在此基础上,对比分析了三叶孔、四叶孔、五叶孔、大圆孔、小圆孔等5种孔板结构的传热与阻力性能,探讨了支撑板等结构参数对其传热与阻力性能的影响,进一步采用场协同原理探讨了孔板换热器的强化传热机理。研究结果表明：采用RNG k-ε湍流模型以及周期性全截面模型可较为准确地模拟孔板换热器壳程流体流动情况;5种模型中五叶孔换热器的传热特性最好但阻力最大,小圆孔的传热效果最差但阻力最小;随着支撑板间距以及开孔高度的增加,换热器壳程的传热系数和压力降均逐渐降低;在支撑板后,速度矢量与温度梯度之间的夹角波动幅度变化剧烈,起到了强化壳程传热的效果;其中五叶孔板的场协同角波动幅度最大,强化传热效果最好。     

多头螺旋管式换热器换热与压降计算

从实际工程设计出发,对多头螺旋管式换热器的设计进行了研究,提出了多头螺旋管束受热面结构的设计方法,推荐了螺旋管内外的传热系数和压降的计算关系式。并对200MW蒸汽发生器进行了实例设计计算。     

Two‐dimensional model of methane thermal decomposition reactors with radiative heat transfer and carbon particle growth

A two‐dimensional model of methane thermal decomposition reactors is developed which accounts for coupled radiative heat and polydisperse carbon particle nucleation, growth, and transport. The model uses the Navier–Stokes equations for the fluid dynamics, the radiative transfer equation for methane and particle species radiation absorption, the advection–diffusion equation for gas and particle species transport, and a sectional method for particle species nucleation, heterogenous growth, and coagulation. The model is applied to a tubular laminar flow reactor. The simulation results indicate the development of a reaction boundary layer inside the reactor, which results in significant variation of the local particle size distribution across the reactor. © 2011 American Institute of Chemical Engineers AIChE J, 58: 2545–2556, 2012     

分相式多孔壁微通道流动冷凝传热强化与减阻

微通道换热器应用广泛,强化传热和减阻是新型换热器设计的重要目标。为了同时实现这两相目标,本文提出了一种分相式多孔壁微通道冷凝器,利用微针肋阵列组成的多孔壁在冷凝传热过程中实现了汽液两相分离。采用实验研究方法对比了分相式多孔壁微通道与普通实心壁微通道的流动和传热特性,结果证明分相式微通道在冷凝传热中同时具备强化传热和减阻的作用。深入研究了通道内两相流动摩擦耗散原理并提出了相分离减阻理论,指出汽液两相流内部摩擦耗散的减小是分相流减阻的关键。另一方面,分相过程使针肋换热面侧壁直接与高温蒸汽接触,极大消减了蒸汽与换热壁面之间的传热液膜厚度。沿流动方向不断扩展的液通道截面与不断减缩的汽通道截面积适应了流动冷凝过程延工质流动方向"水渐多,汽渐少"的规律,保证沿程传热效果不会恶化。     

喷雾冲击条件下液膜流动与传热模型

针对现有喷雾冷却计算模型的不足,以质量、动量、能量守恒方程为基础,建立喷雾冷却非沸腾区的液膜流动与传热方程,并采用数量级分析的方法对方程简化,最后运用数值方法对模型进行求解。给定液滴速度及液体温度,由模型计算液膜厚度、平均热通量与液体流出温度,与实验测试结果对照。结果显示,液膜厚度的计算结果与实验结果相差6%以内;平均热通量和液体离开待冷却表面的最终温度计算结果与实验结果相差10%以内,且超过60%的计算结果偏差小于5%。计算结果与实验结果的高度匹配证明该模型可较好地反映喷雾冷却过程的流动与换热。由模型可以获取不同位置处液膜厚度与温度,从而加深对喷雾冷却传热机理的理解。     

Experimental and CFD investigation of convective heat transfer in helically coiled tube heat exchanger

Experimental studies on isothermal steady state and non-isothermal unsteady state conditions were carried out in helical coils for Newtonian as well as for non-Newtonian fluids. Water and glycerol–water mixture (10 and 20% glycerol) were used as Newtonian, and 0.5–1% (w/w) dilute aqueous polymer solutions of Sodium Carboxy Methyl Cellulose (SCMC) and Sodium Alginate (SA) as non-Newtonian fluids are used in this study. These experiments were performed for coil curvature ratios as δ = 0.0757, 0.064 and 0.055 in laminar and turbulent flow regimes (total 258 tests). The CFD analyses for laminar and turbulent flow were carried out using FLUENT 12.0.16 solver of CFD package. The CFD calculation results (Nu_i, U, T₂ and Tw_o) for laminar and turbulent flow are compared with the experimental results and the work of earlier investigators which were found to be in good agreement. For the first time, an innovative approach of correlating Nusselt number to dimensionless number, ‘M’, Prandtl number and coil curvature ratio using least-squares power law fit is presented in this paper which is not available in the literature. Several other correlations for calculation of Nusselt number for Newtonian and non-Newtonian fluids, and two correlations for friction factor in non-Newtonian fluids (based on 78 tests and 138 tests) are proposed. These developed correlations were compared with the work of earlier investigators and are found to be in good agreement.     

弯曲内肋换热管综合换热性能的数值模拟

基于纵向涡强化换热理论提出了新型的强化换热管--弯曲内肋强化换热管。运用数值计算的方法,在Re=500～40000研究了新型强化换热管结构参数肋宽a、导程p、肋深e和Re数对Nu、f及换热性能评价指标PEC的影响,并拟合出换热管Nu、f的量纲为1关联式。结果表明:由于内肋的作用,在换热管的近壁面区域能够有效诱导产生沿纵向的涡旋结构,破坏壁面边界层,实现强化换热;导程p和肋深e对综合换热性能影响较大,肋宽a对综合换热性能影响较小,在Re=500～40000范围内,新型换热管较优的结构参数为p=10mm、e=0.6mm、a=1.8mm;与普通平滑圆管相比,当Re<2300,其换热性能Nu可达到普通圆管的2.9～7.0倍,沿程阻力系数f约为普通圆管的1.4～3.6倍;当500< Re <2300,其综合换热评价指标PEC可以达到2.6～4.6;当2300 <Re <40000时,其PEC可以达到1.2～2.3。     

微肋阵通道流动沸腾换热与压降特性

为探究不同截面微肋阵通道内的流动沸腾换热机理,以去离子水为工质,在质量流速为96～224 kg·m^-2·s^-1,有效热通量为10～240 W·cm^-2的范围内,对圆形、菱形、椭圆形微肋阵通道内流动沸腾换热及压降特性进行了实验研究,同时对微通道内流动沸腾的不稳定性进行了分析。通过实验发现：在低热通量下,核态沸腾占主导地位,而在中高热通量下,薄膜蒸发对流换热为主要沸腾机制;沸腾传热系数随着热通量和出口干度的增加而减小,两相压降随着热通量和出口干度的增加而增大;微肋阵肋间形成的次级通道宽度对换热和两相压降有很大的影响,次级通道越宽,气泡越容易脱离,换热效果越好,压降越大;微肋的存在抑制了气泡的反向流动,减小了沸腾不稳定性,推迟了临界热通量的发生,椭圆形微肋阵通道的流动沸腾稳定性最好,而圆形微肋阵通道的流动沸腾稳定性最差。     

刮膜式分子蒸馏器传热特性及壁面优化

使用CFD软件建立了刮膜式分子蒸馏器的三维模型,以EHP-EHS混合物为研究对象,并通过编写用户自定义函数（UDF）,研究刮膜式分子蒸馏过程传质存在时的传热特性,得到充分发展的温度场及流场,分析了蒸发壁面温度的分布及转子转速对其影响和局部Nu数的分布。结果表明：液膜表面的升温过程是周期地和波动地达到动态稳定温度的;转子转速越大,周期越小,温度波动次数越多,温度稳定性越好;局部Nu数在刮膜器刮擦的位置突增,刮膜器的刮擦作用是影响平均Nu数增大的主要因素。经过壁面优化,液体湍流程度增大,传热效果得到不同程度的增强,凸起矩形排列、三角形排列、螺旋形排列时平均Nu数分别是光滑蒸发面的1.32倍、1.23倍、1.04倍,为进一步优化刮膜式分子蒸馏器提供参考。     

折流板切口方向对管壳式换热器传热性能影响

为了研究单弓形折流板的切口方向对管壳式换热器传热与流动性能的影响,文中通过建立3个不同折流板切口方向的管壳式换热器简化实体模型,运用CFD软件Fluent对管壳式换热器壳程传热与流动状态进行了三维数值模拟。以水为壳程流体介质,在不断改变壳程进口流速,使得壳程进口雷诺数Re在10 000到70 000范围内变化时,得到了不同状态下的壳程流场与温度场。根据数值模拟结果,以总传热系数α,壳程总压降Δp以及单位压降下的传热系数α/Δp作为综合衡量标准,分析不同折流板切口方向时的管壳式换热器壳程流场与温度场。数值模拟分析结果表明:折流板为垂直切口方向时,管壳式换热器总传热系数最大,压降最小,综合性能最好,另外2种折流板切口方向的管壳式换热器综合性能差不多。     

Fixed bed catalytic reactor modelling—the radial heat transfer problem

An interpretation of the state‐of‐the‐art in modelling radial heat transfer in fixed bed catalytic reactors is presented. The persistence of the classical k_r–h_w model is discussed, problems with typical approaches to obtaining and analysing experimental heat transfer data to get k_r and h_w are explained, current correlations for k_r are evaluated and the contentious history of h_w is elaborated. A brief discussion of alternatives to the k_r–h_w approach and their pros and cons is attempted, and the review is concluded by a look at computational fluid dynamics (CFD) in fixed bed radial heat transfer modelling. © 2011 Canadian Society for Chemical Engineering