管内填充多孔介质强化换热的数值研究

管内填充多孔介质强化换热的基本原理是构造热边界层,增大壁面附近流体的温度梯度,并且流动阻力增幅不大。本文运用数值模拟的方法,模拟填充多孔介质管内的流场和温度场,探讨填充比例φ、渗透率Da以及空隙率ε对管内对流换热的影响规律。研究表明,提高填充比例φ和减小渗透率Da都能明显提高换热效果,但也增加了管内流动阻力。空隙率ε对强化换热作用不大,但高空隙率可以明显降低管内流动阻力,在实际中应选用空隙率较大的多孔介质。     

Field synergy optimization and enhanced heat transfer by multi-longitudinal vortexes flow in tube

The field synergy equation for steady laminar convection heat transfer was derived by conditional variation calculus based on the least dissipation of heat transport potential capacity. The optimum velocity field with the best heat transfer performance and least flow resistance increase can be obtained by solving the synergy equation. The numerical simulation of laminar convection heat transfer in a straight circular tube shows that the multi-longitudinal vortex flow in the tube is the flow pattern that enhances the heat transfer enormously. Based on this result, a novel enhanced heat transfer tube, the discrete double-inclined ribs tube (DDIR-tube), is developed. The flow field of the DDIR-tube is similar to the optimal velocity field. The experimental results show that the DDIR-tube has better comprehensive heat transfer performance than the current heat transfer enhancement tubes. The present work indicates that new heat transfer enhancement techniques could be developed according to the optimum velocity field.     

平板间充填颗粒和壁面开槽时的流动与传热强化特性的实验研究

提出了在平行平板间填粒的同时,运用板面和加开微型纵槽构成复合多孔介质,以改善流体流动与传热综合性能的方案,并报告了实验结果及其分析。研究的结果表明,对于这种复合多孔结构,在适宜的颗粒直径dp,槽宽w,槽深d及板间距δ匹配下,传热比未开槽时有明显提高,而流动阻力却有所下降,因而证明是综合提高多孔介质强化传热技术经济性的有效措施。     

Heat Transfer Enhancement in Shell-and-Tube Heat Exchangers Using Porous Media

A numerical simulation has been carried out to investigate the heat transfer enhancement in a shell-and-tube heat exchanger using a porous medium inside its shell and tubes, separately. A three-dimensional geometry with k-? turbulent model is used to predict the heat transfer and pressure drop characteristics of the flow. The effects of porosity and dimensions of these media on the heat exchanger's thermal performance and pressure drop are analyzed. Inside the shell, the entire tube bundle is wrapped by the porous medium, whereas inside the tubes the porous media are located in two different ways: (1) at the center of the tubes, and (2) attached to the inner wall of the tubes. The results showed that this method can improve the heat transfer at the expense of higher pressure drop. Evaluating the method showed that using porous media inside the shell, with particular dimension and porosity can increase the heat transfer rate better than pressure drop. Using this method inside the tubes leads to two diverse results: In the first configuration, pressure loss prevails over the heat transfer augmentation and it causes energy loss, whereas in the second configuration a great performance enhancement is observed.     

Interfacial heat transfer coefficient for non-equilibrium convective transport in porous media

The literature has documented proposals for macroscopic energy equation modeling for porous media considering the local thermal equilibrium hypothesis and laminar flow. In addition, two-energy equation models have been proposed for conduction and laminar convection in packed beds. With the aim of contributing to new developments, this work treats turbulent heat transport modeling in porous media under the local thermal non-equilibrium assumption. Macroscopic time-average equations for continuity, momentum and energy are presented based on the recently established double decomposition concept (spatial deviations and temporal fluctuations of flow properties). Interfacial heat transfer coefficients are numerically determined for an infinite medium over which the fully developed flow condition prevails. The numerical technique employed for discretizing the governing equations is the control volume method. Preliminary laminar flow results for the macroscopic heat transfer coefficient, between the fluid and solid phase in a periodic cell, are presented.     

Numerical study and sensitivity analysis on convective heat transfer enhancement in a heat pipe partially filled with porous material using LTE and LTNE methods

An improved design for convective heat transfer in a heat pipe partially filled with porous medium is presented. In the present study, porous media is used to increase heat transfer in laminar flow inside the tube with a constant heat flux boundary condition. The porous segments are set in different positions in the tube, while the ratio of porous volume to total volume of tube is considered to be constant. A performance evaluation criteria (PEC), which takes account of both heat transfer and pumping power, is defined to find the enhanced mode of porous media arrangement. According to the current results, PEC increases with the number of porous segments. Moreover, the sequence of unequal segments arrangement within the tube is from the largest to the smallest part for a higher PEC. Effects of parameters including porosity, Darcy number, and ratio of effective coefficient of thermal conductivity to coefficient of thermal conductivity of fluid (TKR) are investigated for sensitivity analysis. Simulations are conducted using the local thermal equilibrium method. In addition, the local thermal nonequilibrium is also used for comparison. For TKR numbers less than 10, these models show the same results with negligible differences except for TKR more than 10.     

内插多孔介质微通道热沉的数值模拟

以水为流动介质,在微通道内添加堆叠金属丝网多孔介质,采用局部非热平衡假设和双能量方程模型,分析了内插不同目数金属丝网的微通道在层流状况下的传热与阻力特性,并采用数值计算方法对微通道热沉进行了数值模拟。结果表明,在微通道内插入多孔介质能显著提高热沉的对流换热系数、降低加热面平均温度,但阻力增加较大,且当插入的金属丝网目数为100目时,微通道热沉的对流换热系数较大,与填充其他目数金属丝网相比阻力增加较小。     

Performance of Forced Convection Heat Transfer in Porous Media Based on Gibson–Ashby Constitutive Model

A novel simulation model is developed for predicting the performance of forced convection heat transfer in the porous metal foam. Based on the physical geometry of the Gibson-Ashby constitutive model, the theoretical model proposed is able to predict the mechanical behaviors and thermal physical properties of porous materials simultaneously. The theoretical predictions of the overall heat transfer coefficient and pressure drop were compared with available experimental data for two different porous foam tubes. The first tube has a porous diameter of 0.6mm and porosity of 0.402, and the other tube has a diameter of 1.6mm and porosity of 0.462. The results show that the relative deviation of the flow pressure drop between the prediction and the experimental data are in the range from 5% to10% while the relative deviation of the overall heat transfer coefficient is about 20%. These deviations are acceptable for applications in engineering. So the feasibility of the Gibson-Ashby constitutive model to be used to predict the performance of flow resistance and convective heat transfer in porous foam ducts is satisfactorily validated.     

等热流圆管内相变微胶囊悬浮液层流换热实验研究

用基液代替水来配置微胶囊相变悬浮液,并对实验数据的准确性进行了检验。在等热流密度环境下对管道内的该悬浮液进行加热实验,对相变微胶囊悬浮液的质量分数、St、入口过冷度、粒径和Re等因素影响强化换热的效果进行了分析。结果显示影响微胶囊相变悬浮液管内层流换热最主要的因素是微胶囊的质量分数和St。     

Entropy generation in conduits filled with porous medium totally and partially

Exergy or entropy generation analysis as a tool of applied thermodynamics is becoming standard practice for optimizing energy conversion systems and in identifying the deficiency of a component in a system. Porous media is used to enhance heat transfer in heat exchangers (HE). Also, porous layer adjusted to the inner surface simulates the fouling effect in heat exchangers. Optimizing the heat exchanger inserted with porous media and understanding the fouling effect on the rate of heat transfer and fluid flow is crucial for HE design and operations, which motivate the present work. The present work mainly investigates entropy generation due to flow in a pipe fully or partially filled with porous medium. The pipe is assumed to be isothermal. The porous layer is inserted at the core of the pipe or attached on the inner surface of the pipe. Forced, laminar flow is assumed. The effects of porous layer thickness and permeability of layer on the rate of entropy generation were investigated. Developing and fully developed flow conditions are considered in the analysis.     

Numerical investigation into the convective heat transfer of TiO2 nanofluids flowing through a straight tube under the laminar flow conditions

A numerical study has been performed by using both single phase method and combined Euler and Lagrange method on the convective heat transfer of TiO₂ nanofluids flowing through a straight tube under the laminar flow conditions. The effects of nanoparticles concentrations, Reynolds number, and various nanoparticle aggregates sizes are investigated on the flow and the convective heat transfer behaviour. The results show significant enhancement of heat transfer of nanofluids particularly in the entrance region. The numerical results are compared with the experimental data and reasonable good agreement is achieved.     

Periodically Fully-Developed Flow and Heat Transfer Over Flat and Oval Tubes Using a Control Volume Finite-Element Method

A Control Volume Finite-Element Method in conjunction with imposed periodically fully-developed flow conditions was used to perform a two-dimensional, laminar, steady-flow numerical study comparing the performance of a flat tube and an oval tube to that of a round tube in a simulated heat exchanger device for the case of specified heat flux along the tube walls. The Reynolds number range for the study was 50 to 350. Fluids of Prandtl number 0.7 and 7.0 were considered. For the cases studied, the heat transfer enhancement ratio was less than one indicating that the round tube outperformed both the flat tube and the oval tube based on heat transfer considerations alone. However, for all cases studied, the heat transfer performance ratio was greater than one indicating that if both heat transfer performance and required pumping power are considered, both the flat tube and oval tube outperformed the round tube.     

Numerical investigation of porous rib arrangement on heat transfer and entropy generation of nanofluid flow in an annulus using a two-phase mixture model

The effect of porous rib arrays on the heat transfer and entropy generation of laminar nanofluid flow inside annuli is studied numerically, using a two-phase mixture model for nanofluid flow simulation. Porous media, nanoparticles, and vortex formation are simultaneously affecting the characteristics of the system. Results showed that the permeability and height of porous ribs have significant effects on the thermal performance of system. Vortex zones also affect the trend of variation of entropy and performance numbers, and local optimums exist for these two parameters. The role of nanofluid in heat transfer enhancement in recirculating zones is more significant for higher volume fractions.     

Numerical simulation of fluid flow and convection heat transfer in sintered porous plate channels

Fluid flow and convective heat transfer of water in sintered bronze porous plate channels was investigated numerically. The numerical simulations assumed a simple cubic structure formed by uniformly sized particles with small contact areas and a finite-thickness wall subject to a constant heat flux at the surface which mirrors the experimental setup. The permeability and inertia coefficient were calculated numerically according to the modified Darcy’s model. The numerical calculation results are in agreement with well-known correlation results. The calculated local heat transfer coefficients on the plate channel surface, which agreed well with the experimental data, increased with mass flow rate and decreased slightly along the axial direction. The convection heat transfer coefficients between the solid particles and the fluid and the volumetric heat transfer coefficients in the porous media predicted by the numerical results increase with mass flow rate and decrease with increasing particle diameter. The numerical results also illustrate the temperature difference between the solid particles and the fluid which indicates the local thermal non-equilibrium in porous media.     

Numerical modeling of high-temperature shell-and-tube heat exchanger and chemical decomposer for hydrogen production

Numerical simulations of shell-and-tube heat exchanger and chemical decomposer with straight tube configuration and porous media were performed using FLUENT6.2.16 to examine the percentage decomposition of sulfur trioxide. The decomposition process can be a part of sulfur–iodine (S–I) thermochemical water splitting cycle, which is one of the most studied cycles for hydrogen production. A steady-state, laminar, two-dimensional axisymmetric shell-and-tube model with counter flow and parallel flow arrangements and simple uniform cubical packing was developed using porous medium approach to investigate the fluid flow, heat transfer and chemical reactions in the decomposer. As per the investigation, the decomposition percentage of sulfur trioxide for counter flow arrangement was found to be 93% and that of parallel flow was 92%. Also, a high pressure drop was observed in counter flow arrangement compared to parallel flow. The effects of inlet velocity, temperature and the porous medium properties on the pressure drop across the porous medium were studied. The influence of geometric parameters mainly the diameter of the tube, diameter of the shell and the length of the porous zone on the percentage decomposition of sulfur trioxide in the tube was investigated as well. A preliminary parametric study of the mentioned configuration is conducted to explore effects of varying parameters on the decomposition of sulfur trioxide. From the performed calculations, it was found that the Reynolds number played a significant role in affecting the sulfur trioxide decomposition. The percentage decomposition decreases with an increase in Reynolds number. Surface-to-volume area ratio and activation energy were also the important parameters that influenced the decomposition percentage.     

波纹管内流动与传热规律的数值计算

采用三维层流及低雷诺数湍流模型对波纹管内流动与传热性能进行了数值模拟,模拟结果与试验结果吻合良好.通过数值计算拓宽了波纹管流动与传热关联式的参数范围,发现在较大雷诺数(RP)范围内波纹管阻力系数随Re的变化趋势表现为指数规律.考察了不同波纹高度、波纹间距对流动与传热的影响,并对模型参数进行了综合性能评价,结果表明:波纹高度对波纹管内流动与传热的影响较波纹间距更显著;波纹管结构的强化传热性能只有在高Re条件下才得以体现,Re越大,波纹管综合性能因子也越高.通过数值计算得到了波纹管流动与传热的最优结构参数及最佳传热雷诺数范围.     

Enhancement of heat transfer: Combined convection and radiation in the entrance region of circular ducts with porous inserts

Using porous ceramic inserts in high temperature equipment has been proven to be an effective means to enhance combined convective–radiative heat transfer. The porous ceramic insert was referred to as a convection-to-radiation converter (CRC) by previous investigators. We consider a novel application of CRC cores in a partial by pass flow system for heat transfer enhancement. Both hydrodynamically and thermally developing laminar flow is considered in the entrance region of a circular pipe with a porous insert located at the center. The momentum and Darcy–Brinkman equations are applied to the flow field in the annular gas layer and central porous layer respectively. The energy equation is coupled with the radiative transfer equation by the radiation source term. The radiative transfer is simulated by the newly developed integral equations [X.L. Chen, W. Sutton, Radiative transfer in finite cylindrical media using transformed integral equations, J. Quant. Spectrosc. Radiat. Transfer 77 (3) (2003) 233–271; W. Sutton, X.L. Chen, A general integration method for radiative transfer in 3-D non-homogeneous cylindrical media with anisotropic scattering, J. Quant. Spectrosc. Radiat. Transfer 84 (2004) 65–103] to avoid singularity problem and give high accuracy. The working fluid and porous medium are both considered as participating media. Finally, this highly non-linear system of equations is solved by a mixed iteration method. The results are compared between the cases with and without the porous insert. The porous insert enhances both convective and radiative transfer by about 35% and 105% respectively at the most. The effects of important parameters on this enhancement are discussed in detail.     

Analysis of coiled-tube heat exchangers to improve heat transfer rate with spirally corrugated wall

Steady heat transfer enhancement has been studied in helically coiled-tube heat exchangers. The outer side of the wall of the heat exchanger contains a helical corrugation which makes a helical rib on the inner side of the tube wall to induce additional swirling motion of fluid particles. Numerical calculations have been carried out to examine different geometrical parameters and the impact of flow and thermal boundary conditions for the heat transfer rate in laminar and transitional flow regimes. Calculated results have been compared to existing empirical formulas and experimental tests to investigate the validity of the numerical results in case of common helical tube heat exchanger and additionally results of the numerical computation of corrugated straight tubes for laminar and transition flow have been validated with experimental tests available in the literature. Comparison of the flow and temperature fields in case of common helical tube and the coil with spirally corrugated wall configuration are discussed. Heat exchanger coils with helically corrugated wall configuration show 80–100% increase for the inner side heat transfer rate due to the additionally developed swirling motion while the relative pressure drop is 10–600% larger compared to the common helically coiled heat exchangers. New empirical correlation has been proposed for the fully developed inner side heat transfer prediction in case of helically corrugated wall configuration.     

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In this study, fully developed laminar flow and convective heat transfer in an internally finned tube heat exchanger are investigated numerically. The flow is assumed to be both hydrodynamically and thermally developed with uniform outside wall temperature. Parameters of the thickness, length, and number of fins and thermal conductivity ratio between fin and working fluid are varied to obtain the friction factor as well as Nusselt number. The results show that the heat transfer improves significantly if more fins are used; however, the pressure drop turns out to be large in this heat exchanger. In addition, it is found that the emergence of closed-loop isotherms between the areas of two neighboring fins leads to heat transfer enhancement in the internally finned tube. When the fin number is smaller than 14, there appears a maximum Nusselt number at about 0.8 of the dimensionless fin length. Finally, an experiment is conducted to verify the numerical results.     

Numerical prediction of flow and heat transfer in a channel in the presence of a built-in circular tube with and without an integral wake splitter

A numerical investigation was carried out to study the heat transfer behavior of a circular tube in cross-flow configuration with a longitudinal fin attached at the rear of the tube. The investigated configuration is intended to model either an element of a cross-flow heat exchanger or an element of the array of pin fins. The longitudinal finning of a circular tube is assumed to be in a configuration where the fin is attached at the back of the circular tube. The longitudinal fins, built-in with the tubes, are called integral splitter plates. The splitter plate creates a streamlined extension of the circular tube. It brings about enhancement of heat transfer from the tube surface. A reduction in the size of the wake zone in comparison with the wake of a circular tube is observed. Narrowing of the wake zone reduced convective heat transfer from the tube surface but the splitter plate itself generated an extra fin area for conduction. Overall, there is an improvement in heat transfer past the circular tube with an integral splitter plate compared with the case of flow past a circular tube without a splitter plate. Flow and heat transfer results are presented for three different chord lengths of the splitter plate and three different values of the Reynolds numbers (500, 1000 and 1500). The heat transfer enhancement obtained by finning was compared with that obtained by increasing the diameter of the unfinned tubes.