Numerical investigation of fouling on cross-flow heat exchanger tubes with conjugated heat transfer approach

Although fouling on heat exchanger tubes is extensively investigated, due to the lack of energy resources, the effects of fouling on heat exchangers is still an important area of study and gaining more and more attention every day. In this study we investigated the effects of fouling on heat transfer and flow structures numerically for cross-flow heat exchanger tube geometry. The distributions of temperature, heat transfer coefficient and heat flux at the surface of fouling were obtained for single and double layer fouling cases. In the analysis, Reynolds number and the blockage ratio were fixed to 100 and 0.1 respectively. We used ANSYS software in our analyses and compared some of our results with the literature.     

同轴径向热管换热器壳程流场数值模拟

基于多孔介质模型和分布阻力方法,引入Al-sanea和Taborek两种阻力关系式模拟同轴径向热管换热器壳程的流场。结果表明:换热器壳程静压沿烟气流动方向呈线性分布;随入口烟气速度的增加,换热器阻力损失增大、压降增大;且随入口烟气速度的增加,压降增加的速率增大。     

Second law analysis and heat transfer in a cross-flow heat exchanger with a new winglet-type vortex generator

In this paper a second law analysis of a cross-flow heat exchanger (HX) is studied in the presence of a balance between the entropy generation due to heat transfer and fluid friction. The entropy generation in a cross-flow HX with a new winglet-type convergent–divergent longitudinal vortex generator (CDLVG) is investigated. Optimization of HX channel geometry and effect of design parameters regarding the overall system performance are presented. For the HX flow lengths and CDLVGs the optimization model was developed on the basis of the entropy generation minimization (EGM). It was found that increasing the cross-flow fluid velocity enhances the heat transfer rate and reduces the heat transfer irreversibility. The test results demonstrate that the CDLVGs are potential candidate procedure to improve the disorderly mixing in channel flows of the cross-flow type HX for large values of the Reynolds number.     

Numerical studies on flow and heat transfer in membrane helical-coil heat exchanger and membrane serpentine-tube heat exchanger

The flow and heat transfer characteristics of synthesis gas (syngas) in membrane helical-coil heat exchanger and membrane serpentine-tube heat exchanger under different operating pressures, inlet velocities and pitches are investigated numerically. The three-dimensional governing equations for mass, momentum and heat transfer are solved using a control volume finite difference method. The realizable k-ε model is adopted to simulate the turbulent flow and heat transfer in heat exchangers. There flows syngas in the channels consisting of the membrane helical coils or membrane serpentine tubes, where the operating pressure varies from 0.5 to 3.0 MPa. The numerically obtained heat transfer coefficients for heat exchangers are in good agreement with experimental values. The results show that the syngas tangential flow in the channel consisting of membrane helical coils is significant to the heat transfer enhancement to lead to the higher average heat transfer coefficient of membrane helical-coil heat exchanger compared to membrane serpentine-tube heat exchanger. The syngas tangential velocity in the membrane helical-coil heat exchanger increases along the axial direction, and it is independent of the gas pressure, increasing with the axial velocity and axial pitch rise and decreasing with the radial pitch rise.     

Thermal characterization of a cross-flow heat exchanger with a new flow arrangement

The objective of the present paper is to thermally characterize a cross-flow heat exchanger featuring a new cross-flow arrangement, which may find application in contemporary refrigeration and automobile industries. The new flow arrangement is peculiar in the sense that it possesses two fluid circuits extending in the form of two tube rows, each with two tube lines. To assess the heat exchanger performance, it is compared against that for the standard two-pass counter-cross-flow arrangement. The two-part comparison is based on the thermal effectiveness and the heat exchanger efficiency for several combinations of the heat capacity rate ratio, C¹, and the number of transfer units, NTU. In addition, a third comparison is made in terms of the so-called “heat exchanger reversibility norm” (HERN) through the influence of various parameters such as the inlet temperature ratio, τ, and the heat capacity rate ratio, C¹, for several fixed NTU values. The proposed new flow arrangement delivers higher thermal effectiveness and higher heat exchanger efficiency, resulting in lesser entropy generation over a wide range of C¹ and NTU values. These metrics are quantified with respect to the arrangement widely used in refrigeration industry due to its high effectiveness, namely, the standard two-pass counter-cross-flow heat exchanger. The new flow arrangement seems to be a promising avenue in situations where cross-flow heat exchangers for single-phase fluid have to be used in refrigeration units.     

Numerical analysis of laminar flow heat transfer of nanofluids in a flattened tube

In this work, a three-dimensional analysis is used to study the heat transfer performance of nanofluid flows through a flattened tube in a laminar flow regime and constant heat flux boundary condition. CuO nanoparticles dispersed in ethylene glycol with particle volume concentrations ranging between 0 and 4 vol.% were used as working fluids for simulating the heat transfer of nanofluids. Effects of some important parameters such as nanoparticle volume concentration, particles Brownian motions, and Reynolds number on heat transfer coefficient have been determined and discussed in details. Results have shown that the heat transfer coefficient increases with increase in the volume concentration level of the nanoparticle, Brownian motion and the Reynolds number. Numerical results have been validated by comparison of simulations with those available in the literature.     

Fluid flow and heat transfer in fluidized bed vertical shell and tube type heat exchanger

Measurements were made on the effects of circulating solid particles on the characteristics of fluid flow and heat transfer in the fluidized bed vertical shell and tube type heat exchanger with counterflow. The present work showed that the flow velocity range for collision of particles to the tube wall was higher with heavier density solid particles, and the increase in heat transfer was in the order of sand, copper, steel, aluminum, and glass.     

Detection of fouling in a cross-flow heat exchanger using a neural network based technique

This paper presents a method for the detection of fouling in a cross-flow heat exchanger. A numerical model is used to generate data when the heat exchanger is clean and corresponding data when fouling occurs. In a first step, the model is used to generate a long time series by simulating a clean heat exchanger. This allows the determination of a neural network model of the heat exchanger. Then, hundred sets of data are generated by simulating a fouled heat exchanger and it is checked that the simple Cusum test can be used to detect fouling without any false alarm, whatever the reference time series is.     

Numerical analysis of heat flow in contact heat transfer

A finite difference analysis of heat conduction problem in a cylinder terminating in a frustum of a cone is presented. The constriction can be either in vacuum or in a gaseous environment. A fine mesh of 2500 × 800 was used for the construction of the grid such that very small constrictions could be analysed sufficiently accurately. Small constrictions i.e., small contact areas separated by large voids filled with a gas are typical of most practical applications involving contact heat transfer. The result of the finite difference analysis shows that gap conductance is predominant for all the gases considered. Gap-to-solid conductance ratio increases as the cone angle decreases due to the decrease of gap thickness. It also indicates that increase of conductance ratio is less significant at higher constriction angles. Finally, predicted conductance parameters are compared with the experimental results for different interfacial gases and a very good agreement is obtained.     

Thermal-hydraulic performance of novel louvered fin using flat tube cross-flow heat exchanger

Experimental studies were conducted to investigate the air-side heat transfer and pressure drop characteristics of a novel louvered fins and flat tube heat exchangers. A series of tests were conducted for 9 heat exchangers with different fin space and fin length, at a constant tube-side water flow rate of 2.8 m³/h. The air side thermal performance data were analyzed using the effectiveness-NTU method. Results were presented as plot of Colburn j factor and friction factor f against the Reynolds number in the range of 500–6500. The characteristics of the heat transfer and pressure drop of different fin space and fin length were analyzed and compared. In addition, the curves of the heat transfer coefficients vs. pumping power per unit heat transfer area were plotted. Finally, the area optimization factor was used to evaluate the thermal hydraulic performance of the louvered fins with differential geometries. The results showed that the j and f factors increase with the decrease of the fin space and fin length, and the fin space has more obvious effect on the thermal hydraulic characteristics of the novel louvered fins. __________ Translated from Journal of Shanghai Jiaotong University, 2007, 41(3): 380–383 [译自: 上海交通大学学报]     

Thermal-hydraulic performance of novel louvered fin using flat tube cross-flow heat exchanger

Experimental studies were conducted to investigate the air-side heat transfer and pressure drop characteristics of a novel louvered fins and flat tube heat exchangers. A series of tests were conducted for 9 heat exchangers with different fin space and fin length, at a constant tube-side water flow rate of 2.8 m³/h. The air side thermal performance data were analyzed using the effectiveness-NTU method. Results were presented as plot of Colburn j factor and friction factor f against the Reynolds number in the range of 500–6500. The characteristics of the heat transfer and pressure drop of different fin space and fin length were analyzed and compared. In addition, the curves of the heat transfer coefficients vs. pumping power per unit heat transfer area were plotted. Finally, the area optimization factor was used to evaluate the thermal hydraulic performance of the louvered fins with differential geometries. The results showed that the j and f factors increase with the decrease of the fin space and fin length, and the fin space has more obvious effect on the thermal hydraulic characteristics of the novel louvered fins.     

氦气冲刷螺旋管束传热的数值研究

采用CFD软件对氦气冲刷螺旋管束的传热特性进行了数值模拟。计算时采用了轴对称简化模型;湍流模拟采用低Re k-ε模型。通过与实验数据对比,发现低Re模型比壁面函数法更适合计算冲刷管束类型的流动。计算结果表明,顺排管束前几层平均Nu高于叉排管束,而深层管平均Nu低于叉排管束;管列距离较大时排列方式对深层管的传热影响很小;管束与边界距离约为管束中心部分氦气流道宽度的一半时,各列传热管传热和氦气出口温度都较为均匀;管束横向位置发生偏移将导致管束内流动、传热出现不均匀。结果对于螺旋管蒸汽发生器设计具有参考意义。     

错列翅片紧凑式换热器湍流流动及换热性能的数值研究

采用高雷诺数κ-ε湍流模型，对中高雷诺数下紧凑式错列翅片换热器的表面换热及流动特性进行数值模拟。结果表明，该种型式的换热器具有良好的流动和换热性能，拓宽了其空调领域的应用。     

Numerical investigation of fluid flow and heat transfer over louvered fins in compact heat exchanger

Numerical investigation of fluid flow and heat transfer characteristics over louvered fins and flat tube in compact heat exchangers is presented in this study. Three-dimensional simulations of single and double row tubes with louvered fins have been conducted. Simulations are performed for different geometries with varying louver pitch, louver angle, fin pitch and tube pitch and for different Reynolds number. Conjugate heat transfer and conduction through the fins are considered. The air-side performance of heat exchanger is evaluated by calculating Stanton number and friction factor. The results are compared with experiment and a good agreement is observed. The local Nusselt number variation along the top surface of the louver is calculated and effects of geometrical parameters on the average heat transfer coefficient is computed. Design curves are obtained which can used to predict the heat transfer and the pressure drop for a given louver geometry.     

Improved milk fouling simulation in a helical triple tube heat exchanger

Performance of a dairy heat exchanger declines as milk fouling deposits on the heating surface. It causes an increased resistance to heat flow thereby the milk outlet temperature decreases with increasing fouling thicknesses. Various models have been suggested for the prediction of fouling thickness and milk outlet temperature in a heat exchanger. The present paper describes an improved simulation model for the accurate estimation of fouling thickness and milk outlet temperature. Local fouling factor in terms of the Biot number is used in the work undertaken. Fouling thickness and milk outlet temperature are predicted as a function of time and over the entire length of the heat exchanger. Right from the beginning fouling occurred to a greater extent towards the outlet and with the progress of time the rate of increase of the fouling thickness decreased. The milk outlet temperature decreased with the time as the fouling increased.     

Numerical analysis for flow,heat and mass transfer in film flow along a vertical fluted tube

We conducted a three-dimensional numerical investigation of the flow, heat and mass transfer characteristics of the fluted evaporating tube with films flowing down on both the inside and outside tube walls. Condensation occurs along the outside wall while evaporation takes place on the free surface of the inside film. The three-dimensional transport equations for momentum and energy were solved by using the finite volume method (FVM). The free-surface shape is tracked by using the moving-grid technique that satisfies the space conservation law (SCL). Because of the secondary motion of the fluid, the film becomes thin at the crest whereas it thickens at the valley. The velocity and temperature fields were successfully predicted for various flute shapes.     

Conjugate fluid flow and kinetics modeling for heat exchanger fouling simulation

Thermal treatment of fluid foods represents a major unit operation in the food industry, to ensure the product's safety and quality features. But during the thermal treatments of such sensible fluids in common plate heat exchangers, food constituents such as proteins can be thermally damaged and precipitated to form fouling that greatly affect the treatment efficiency and alter the product's desired features.Computational Fluid Dynamics simulations can then be successfully exploited, bringing forth temperature and velocity information that yield for deposit distributions when coupled to biochemical notations for thermal denaturation of fluid constituents.The present work exploits such modeling for a single-channel heat exchanger during pasteurization of milk. The model enforces a conjugate system of differential equations to a heat exchanger's corrugated plate to combine flow, heat transfer and local transport of β-lactoglobulin. A preliminary computation has been performed that could be applied to geometry optimization (different corrugation shape and orientation) and for a variety of biochemically evolutive products.     

Second law analysis on the heat transfer of the horizontal concentric tube heat exchanger

In the present study, the theoretical and experimental results of the second law analysis on the heat transfer and flow of a horizontal concentric tube heat exchanger are presented. The experiments setup are designed and constructed for the measured data. Hot water and cold water are used as working fluids. The test runs are done at the hot and cold water mass flow rates ranging between 0.02 and 0.20 kg/s and between 0.02 and 0.20 kg/s, respectively. The inlet hot water and inlet cold water temperatures are between 40 and 50 °C, and between 15 and 20 °C, respectively. The effects of the inlet conditions of both working fluids flowing through the heat exchanger on the heat transfer characteristics, entropy generation, and exergy loss are discussed. The mathematical model based on the conservation equations of energy is developed and solved by the central finite difference method to obtain temperature distribution, entropy generation, and exergy loss. The predicted results obtained from the model are validated by comparing with the present measured data. There is reasonable agreement from the comparison between predicted results and those from the measured data.     

Comparison of heat transfer conditions in tube bundle cross-flow for different tube shapes

Detailed transient numerical simulations of fluid and heat flow were performed for a number of heat exchanger segments with cylindrical, ellipsoidal and wing-shaped tubes in a staggered arrangement. The purpose of the analysis was to get an insight of local heat transfer and fluid flow conditions in a heat exchanger and to establish widely applicable drag coefficient and Stanton number correlations for the heat exchanger integral model, based on average flow variables. The simulation results revealed much more complex flow behavior than reported in current literature. For each of the almost 100 analyzed cases, the time distributions of the Reynolds number, the drag coefficient and the Stanton number were recorded, and their average values calculated. Based on these average values, the drag coefficient and the Stanton number correlations were constructed as polynomial functions of the Reynolds number and the hydraulic diameter. The comparison of the collected results also allows more general conclusions on efficiency and stability of the heat transfer process in tube bundles.     

带短肋翅片管的强化传热机理研究

同时对普通翅片管和带有两个短肋的翅片管在均匀流场中、不同雷诺数下进行了流场和传热的数值模拟,分析了带有短肋的翅片管强化传热的机理。结果表明,由于翅片上带有的短肋和短肋后面的开孔,减少了翅片管管后流动的死滞区,提高了局部地区流体的流速,增加了扰动,从而起到了强化传热的作用。取入口雷诺数为20000时,加装短肋后可使总传热量增加5.1%,平均表面传热系数增加23.56%。随着雷诺数的增加,总换热量增加,强化传热效果也增强。