基于火积理论的螺旋折流板换热器的优化结构分析

为了比较平面螺旋折流板换热器和折面螺旋折流板换热器的传热和阻力性能,应用了换热器常用评价标准PEC准则和火积理论对两种换热器实验结果进行了分析,同时采用火积耗散极值原理对两种换热器的传热火积耗散率、阻力火积耗散率以及总火积耗散率进行了对比。结果表明:火积耗散理论分析换热器性能的结果与传统换热器评价标准PEC准则相符,说明了火积耗散理论的可靠性;折面螺旋折流板换热器的综合性能得到了有效的改进,火积耗散率也均优于原始结构,表明折面螺旋折流板换热器的性能得到较大改善;两种换热器传热火积耗散率值要远远大于阻力火积耗散率,约为阻力火积耗散率的一千余倍,说明传热损失为换热器的主要不可逆损失。     

Numerical simulation on the performance of trisection helical baffle heat exchangers with small baffle incline angles

ABSTRACT

Numerical simulation was conducted on oil–water heat transfer in five circumferential overlap trisection helical baffle shell–and–tube heat exchangers (cothSTHXs) with 16 tubes and incline angles of 12°, 16°, 20°, 24°, and 28° and a segmental baffle heat exchanger of the identical tube layout for comparison under laminar flow calculation conditions. The local images represent shell-side flow patterns, and heat transfer properties are presented showing the detailed “secondary vortex flow” and “shortcut leakage flow” patterns to explain the different characteristics of the six schemes. The simulation curves of the heat transfer coefficient and pressure drop are compared with those of the experimental ones, with satisfactory agreement. The average values of the shell-side heat transfer coefficient and the comprehensive index h_o/Δp_o of the 12° helical scheme are respectively 47% and 51% higher than those of the segmental baffle scheme with about the same pressure drop.     

折弯弓形折流板换热器流动换热性能研究

为了提高折流板换热器的换热性能,改变了折流板换热器的折弯夹角和折流板间距,利用ANSYS Fluent对换热器壳程流体流动与换热过程进行模拟,分析了不同折流板折弯夹角α (110°,135°,170°和180°)、折流板间距（250,300和350 mm)和雷诺数（10 000,20 000和50 000）对换热器壳程压力、速度和温度的影响。结果表明：增大雷诺数对改善流动死区有很大的作用,雷诺数为50 000时的流动死区相对于雷诺数为10 000时面积减小较大;随着夹角α的减小,折流板背流侧的流动死区面积逐渐减小、换热器的表面传热系数和进出口压降力越大,夹角α为110°时出口温度最小、进出口压降最大,夹角α为135°时PEC最大且换热器综合性能最优;折流板间距增大,压力变化梯度减小,压差变化幅度减小,壳程出口温度变化不成正比关系,间距为300 mm时出口温度最低。     

Experimental Studies on Thermal Performance and Flow Resistance of Heat Exchangers with Helical Baffles

In this study, the shell-side heat transfer performance and flow resistance of the shell-and-tube heat exchangers with third-symmetrical, quarter-symmetrical, quarter-unsymmetrical helical baffles and segmental baffles were experimentally obtained. Except for the baffles, these heat exchangers had the same geometrical configuration and number of tubes. Cold and hot water were used as working fluids in the shell and the tube side, respectively. The experiments were done with the cold water volumetric flow rate ranging between 3 and 7 m³/h and the hot water volumetric flow rate constant at 5.5 m³/h. The results show that the heat exchanger with segmental baffles has higher shell-side heat transfer performance and flow resistance than those with helical baffles. Among the three helical baffles used, the third-symmetrical helical baffle offers the highest shell-side heat transfer performance and flow resistance. The quarter-unsymmetrical helical baffle offers the lowest shell-side flow resistance. Its performance of shell-side heat transfer is also the lowest one but close to that of the quarter-symmetrical helical baffle, so the quarter-unsymmetrical helical baffle provides the best conversion efficiency in all heat exchangers mentioned. Compared with the segmental baffle, the shell-side Nusselt numbers that the third-symmetrical, the quarter-symmetrical, and the quarter-unsymmetrical helical baffle offer decrease on the average by about 26%, 37%, and 38%, respectively, and the corresponding shell-side Euler numbers they provide decrease on the average by about 33%, 49%, and 55%, respectively. Thus, the relative shell-side conversion efficiencies increase by about 9%, 25%, and 39% on the average, respectively.     

Numerical Analysis of Baffle Cut on Shell Side Heat Exchanger Performance with Inclined Baffles

In this work, an attempt has been made to decrease the pressure drop and to increase the heat transfer rate in a shell and tube heat exchanger (STHX) by tilting the baffle angle and by varying the baffle cut. The process of solving the simulation includes modeling, meshing, and analyzing the geometry of the STHX by using Pro-E, hypermesh, and computational fluid dynamics package of ANSYS Fluent, respectively. The objective of this study is to find a suitable baffle inclination and baffle cut for the efficient performance of the STHX. The baffle inclinations of 25°, 30°, 35°, and 40° were considered for three different baffle cuts of 25%, 30%, and 35% of shell inside diameter and the results were compared with segmental baffle of inclination angle 0°. The shell side flow with different inclination angles and baffle cuts results in a significant variation in heat transfer rate and pressure drop in the STHX. The results provide a clear idea that the heat transfer rate is maximum in inclined baffle heat exchanger compared to that of segmental baffle heat exchanger. Further it is found that the STHX with the configuration of 35º baffle inclination angle and baffle cut of 30% of shell inside diameter provides higher heat transfer rate with minimum pressure drop compared to all other configurations.     

Numerical investigation on the multi-objective optimization of a shell-and-tube heat exchanger with helical baffles

An improved method combining numerical simulation with multi-objective genetic algorithm (MOGA) was applied to study the flow and heat transfer characteristics of shell-and-tube heat exchanger with helical baffles (STHXsHB). It overcomes the dependence on empirical correlations. The helix angle and overlapped degree of helical baffles were chosen as optimization parameters, while the overall heat transfer coefficient K and pressure drop ΔP of STHXsHB were optimized by MOGA. The results showed that both overall heat transfer coefficient K and pressure drop ΔP varied adversely with the helix angles. The pressure drop ΔP was favorably affected by the overlapped degrees. The overall heat transfer coefficient K did not vary significantly with the overlapped degree. Three optimum configurations were obtained by the MOGA to maximize the overall heat transfer coefficient K and minimize the shell-side pressure drop ΔP. Compared with the original heat exchanger, the overall heat transfer coefficient K increased averagely by 28.3%, while the average pressure drop reduced averagely by 19.37%.     

超临界CO2倾斜螺旋槽管内冷却换热特性研究

为了提高气体冷却器内换热效率,对不同倾斜角下(-90°,-45°,0°,45°,90°)螺旋槽管内超临界CO2冷却对流换热特性进行了数值模拟,分析了各槽管内的湍动能和速度分布随倾斜角的变化趋势,并研究了不同螺旋角下倾斜角对换热特性的影响。结果表明:浮升力沿流动方向分量和垂直于流动方向分量对流动特性的影响并不相同;在类气区,流体速度对流动特性起主要作用,且换热系数随倾斜角的减小而增大;在类液区,流动特性的主要影响因素是速度梯度,此时换热系数随倾斜角的变化与类气区相反;螺旋角越大即螺旋程度越小,当流体倾斜向上流动时浮升力效应越为显著;当螺旋角为 0.70 rad时,最优倾斜角度为-45°,当螺旋角为0.94 rad时,最优倾斜角为45°。     

3D numerical simulation on shell-and-tube heat exchangers with middle-overlapped helical baffles and continuous baffles – Part II: Simulation results of periodic model and comparison between continuous and noncontinuous helical baffles

In this paper, based on the simplified periodic model the performance predictions for heat exchanger with middle-overlapped helical baffles are carried out by 3D simulation for three different helix angles (30°, 40° and 50°), and the commercial codes of GAMBIT 2.3 and FLEUNT 6.3 are adopted in the simulation. It is found that the model average heat transfer coefficient per unit pressure drop of the 40° angle case is the largest, which is in qualitative agreement with the existing literature. The predicted average intersection angle of this case is the smallest, being consistent with the field synergy principle. The performance of periodic model with continuous helical baffle is also compared with that of the noncontinuous middle-overlapped helical baffles. It is found that the heat transfer coefficient per unit pressure drop of the noncontinuous middle-overlapped helical baffles is appreciably larger than that of the continuous helical baffle, indicating that the heat exchanger with noncontinuous middle-overlapped helical baffles has its advantage over the one with continuous helical baffle.     

Biofouling Control with Ultrasound

The flow field in shell-and-tube heat exchangers with helical baffles was measured using laser Doppler anemometry (LDA). The influence on the velocity distribution, impulsive velocity by helix inclination angle, and flow rate was investigated. The influence on heat exchanging capability and flow resistance on velocity distribution was also investigated. The dimensions of the heat exchanger shell used in these experiments were 200 2 6 2 3,000 mm (inner diameter 2 wall thickness 2 length). The heat exchanger was made of organic glass and the tube bundle consisted of 52 tubes with external diameter of 15 mm. Six different inclination angles were designed in double-helix style: 30°, 35°, 40°, 42°, 45°, and 50°. The working flow medium under normal temperature was service water. Generally, the linear velocity and impulsive velocity will increase with decreasing helix inclination angle, which promotes the heat exchanging capability. With flow volume increasing, the velocity distribution along the diameter increases on average. The pressure drop increases with decreasing helix inclination angle. For all of the helix inclination angles tested, the minimum pressure loss took place at a certain Reynolds number; and at different helix inclination angles, the Reynolds number at which the minimum pressure loss occurs is different. In general, it was concluded that the optimum helix inclination angle depends on the Reynolds number of the working fluid on the shell side of heat exchanger.     

Performance Study of a Circumferential Overlap Trisection Helical Baffle Heat Exchanger at the Shell Side

Using the three-dimensional (3D) modeling software Gambit, a mathematical model of a circumferential overlap trisection helical baffle shell-and-tube heat exchanger (cothSTHX) was developed with 34 tubes and three pulling rods with an equilateral triangle tube layout and a baffle incline angle of 20°. The numerical simulation of flow and thermal performances was performed with the analysis software Fluent. The temperature, pressure, and velocity nephograms are shown for different slices, including spiral, concentric hexagon longitudinal, meridian, eccentric longitudinal, and transverse slices. The nephograms of temperature, pressure, and velocity with superimposed velocity vectors vividly display the important parameters of the cothSTHX. The “Dean vortex secondary flow” is a key mechanism to enhance the heat transfer in heat exchangers, which is clearly depicted to show that the spiral fluid flows outward under the centrifugal force, then flows back to the axis under the radial differential pressure, forming a single vortex in each helix cycle. The structure of circumferential overlap baffles restricts the shortcut leakage flow, and the flow pattern in the cothSTHX is very close to “plug flow” on the unfolded hexagon slices.     

Application of Entransy Optimization to One-Stream Series-Wound and Parallel Heat Exchanger Networks

Entransy theory has developed in recent years to describe heat transfer behavior from another point of view. Entransy dissipation is proposed to evaluate the irreversibility of heat transfer and is used to optimize heat transfer processes. We apply this theory to the one-stream series-wound and parallel heat exchanger (HE) networks. The heat transfer optimization for the HE networks includes the distribution of the heat transfer area or heat load. The distribution of the cold fluid flow rate in parallel HE networks can also be optimized. For these problems, physical and mathematical models are set up, analyzed, and discussed with the entransy theory. It is found that the optimization objectives of these problems and the optimization directions of the extremum entransy dissipation principle are consistent. The optimized results for the distribution optimization problem with given heat load are in agreement with the uniformity principle of temperature difference field. Examples of simple one-stream HE networks are given; the distributions of the heat transfer area or heat load are optimized by the extremum entransy dissipation method.     

基于火积耗散热阻的新型板式省煤器传热分析

建立了新型板式省煤器的传热模型,计算了新型板式省煤器的火积耗散热阻以及空气侧压降,分析了新型板式省煤器结构参数及空气流速变化时,火积耗散热阻及空气侧压降的变化情况。研究结果表明:增大长轴可以减小火积耗散热阻,有利于提高板式省煤器的传热性能,并且空气侧压降变化幅度不大;增大短轴可以减小火积耗散热阻,有利于提高板式省煤器的传热性能,但空气侧压降增大;减小板束间距可以减小火积耗散热阻,有利于提高板式省煤器的传热性能,但空气侧压降增大,尤其是在板束间距小于20 mm时,继续减小板束间距会造成空气侧压降急剧增大;增大空气进口流速可以减小火积耗散热阻,有利于提高板式省煤器的传热性能,但空气侧压降增大,对换热器的磨损也会增加。     

不同螺旋角螺旋折流板换热器传热性能对比实验

对螺旋角为8°、12°、18°、30°、40°的螺旋折流板换热器进行了壳程传热性能和压力降测试,得到了相应结构下的总传热系数和压力降。然后通过对试验数据的整理分析,并进行曲线回归,得到了不同螺旋角的螺旋折流板换热器换热系数和压降经验计算公式。研究表明,实验条件下,30°螺旋角的螺旋折流板换热器的单位压降传热系数要优于8°、12°、18°、40°螺旋角的螺旋折流板换热器的传热系数。     

Comparative Study of Convective Heat Transfer Performance of Steam and Air Flow in Rib Roughened Channels

A comparative experimental study of heat transfer characteristics of steam and air flow in rectangular channels roughened with parallel ribs was conducted by using an infrared camera. Effects of Reynolds numbers and rib angles on the steam and air convective heat transfer have been obtained and compared with each other for the Reynolds number from about 4,000 to 15,000. For all the ribbed channels the rib pitch to height ratio(p/e) is 10, and the rib height to the channel hydraulic diameter ratio is 0.078, while the rib angles are varied from 90° to 45°.Based on experimental results, it can be found that, even though the heat transfer distributions of steam and air flow in the ribbed channels are similar to each other, the steam flow can obtain higher convective heat transfer enhancement capability, and the heat transfer enhancement of both the steam and air becomes greater with the rib angle deceasing from 90° to 45°. At Reynolds number of about 12,000, the area-averaged Nusselt numbers of the steam flow is about 13.9%, 14.2%, 19.9% and 23.9% higher than those of the air flow for the rib angles of 90°,75°, 60° and 45° respectively. With the experimental results the correlations for Nusselt number in terms of Reynolds number and rib angle for the steam and air flow in the ribbed channels were developed respectively.     

Experimental Investigation of Heat Transfer and Resistance Characteristics of a Finned Oval-Tube Heat Exchanger With Different Air Inlet Angles

The air inlet flow direction is not orthogonal to the heat exchanger surface in many cases. To study the performance of the heat transfer and pressure drop of a heat exchanger with different air inlet angles, this paper shows the experimental system about a finned oval-tube heat exchanger inclined toward the air incoming flow direction. The heat transfer and pressure drop characteristics of four air inlet angles (90°, 60°, 45°, and 30°) are studied separately for the Reynolds number ranging from 1300 to 13000 in this study. The experimental correlations of Nusselt number and resistance coefficient of the air side are acquired. The results show that the overall heat transfer coefficients become smaller and smaller with the decrease of the air inlet angles, while the pressure drops have significant changes. The heat transfer performances of the heat exchanger under the three inclined air inlet angles are worse than that at 90°. Among the three inclined angles, the performance at 45° is the best under identical mass flow rate criterion and at low Reynolds number under identical pressure drop criterion; that at 60° is the best at large Reynolds under identical pressure drop criterion. Finally, some conclusions are attained about the effects of the air inlet angles on the heat transfer and pressure drop performance of the finned oval-tube heat exchanger.     

Heat Transfer and Thermal Resistance Characteristics of Fin with Built-In Interrupted Delta Winglet Type

The heat transfer and fluid flow characteristics of a new type of fin with built-in interrupted delta winglets is studied in this paper by three-dimensional numerical simulation. In order to ensure reliability of numerical model, plate fin with common-flow-up delta winglets is firstly simulated. The comparison of numerical and experimental results shows a maximum deviation of 11.4% within the entire range of Reynolds number. The computational results show that heat transfer capacity and overall performance increase by 35–60% and 19–64%, respectively. The flow field visualization shows that the interrupted delta winglets can produce longitudinal vortices at the rear of delta winglets and reduce the wake zone behind the tube, so the proposed fin can enhance heat transfer accompanied by low pressure loss. The field synergy theory and entransy dissipation extremum principle are employed on analyzing the mechanism of heat transfer enhancement. The results indicate that enhancement heat transfer mechanism of interrupted delta winglets can be explained as the result of the decrease of synergy angle and reduction of the entransy dissipation.     

Effect of helical baffles and water-based Al2O3, CuO,and SiO2 nanoparticles in the enhancement of thermal performance for shell and tube heat exchanger

In this study, Shell and tube heat exchanger (STHX) with 22% cut segmental baffles and helical baffles with 20°, 30°, 40° inclination angles are considered for three-dimensional CFD analysis using the ANSYS FLUENT tool to investigate the performance of STHX. OHTC and comprehensive performance index are higher for 40° helical baffles when compared to segmental baffle and 20°, 30° helical baffle heat exchangers with water as working fluid. Hence, further investigations are carried out for 40° helical baffle heat exchangers. Numerical investigations are extended with nanofluids (Al₂O₃, CuO, and SiO₂) at 1%, 3%, and 5% volume concentrations for each nanofluid. Under the same mass flow rates, 40° helical baffles with Al₂O₃ nanofluid as working fluid provided better heat transfer rates when compared to the other two nanofluids and base fluid. Also, the authors noticed that the 5% volume (vol) concentration nanofluids provided better heat transfer enhancements when compared to 1%, 3% volume concentrations, and base fluid. Enhancements (10.33%–8.24%) from lower to the higher mass flow rate in 40° HB with Al₂O₃ nanofluid at 5% volume concentration are observed when compared to water as base fluid.     

Effect of helix angle of spirally corrugated tube on condensation heat transfer characteristics

Spirally corrugated tubes with different helix angles have been used in experiments on the characteristics of condensation heat transfer and flow resistance. The result of the experimentation shows that the helix angle plays a vital part in the spirally corrugated tube during the process of condensation heat transfer. This paper analyzes the mechanism of how the helix angle influences the characteristics of condensation heat transfer and flow resistance, and presents an experimental correlation formula of condensation heat transfer which takes the helix angle into account as a factor. ©2008 Wiley Periodicals, Inc. Heat Trans Asian Res, 37(5): 275–282, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20208     

A new approach to analysis and optimization of evaporative cooling system I: Theory

Using the analogy between heat and mass transfer processes, the recently developed entransy theory is extended in this paper to tackle the coupled heat and mass transfer processes so as to analyze and optimize the performance of evaporative cooling systems. We first introduce a few new concepts including the moisture entransy, moisture entransy dissipation, and the thermal resistance in terms of the moisture entransy dissipation. Thereinafter, the moisture entransy is employed to describe the endothermic ability of a moist air. The moisture entransy dissipation on the other hand is used to measure the loss of the endothermic ability, i.e. the irreversibility, in the coupled heat and mass transfer processes – this total loss is shown to consist of three parts: (1) the sensible heat entransy dissipation, (2) the latent heat entransy dissipation, and (3) the entransy dissipation induced by a temperature potential. Finally the new thermal resistance, defined as the moisture entransy dissipation rate divided by the squared refrigerating effect output rate, is recommended as an index to effectively reflect the performance of the evaporative cooling system. In the end, two typical evaporative cooling processes are analyzed to illustrate the applications of the proposed concepts.     

Effect of the Chevron Angle on Cooling Heat Transfer Characteristics of Supercritical Pressure Fluids in Plate Heat Exchangers

ABSTRACT

For the development of industrial heat pump systems supplying a high-temperature heat source over 130°C, the authors have studied on cooling heat transfer of supercritical pressure fluids flowing in chevron-type plate heat exchangers (PHEs). In this study, to examine the effect of chevron angle on cooling heat transfer of supercritical pressure refrigerants, experiments were conducted for HFC134a and HFO1234ze(E) flowing in the PHEs with the chevron angles from 30° to 65°. In the experiments, cooling heat transfer coefficients were obtained in the wide range of bulk fluid enthalpy from vapor-like high temperature to liquid-like low temperature, changing the pressure in the reduced pressure range from 1.01 to 1.2 at the mass flow rates of 7 and 11 kg/min. Especially for the enthalpy region of the pseudo critical point and its vicinity in which good heat transfer appeared, the effect of chevron angle on heat transfer of supercritical pressure fluids was clarified based on the measurements. Furthermore, the effect of chevron angle was examined for the wide angle range from 0° to 90° with estimating the heat transfer coefficient for the angles 0° and 90° from appropriate correlations. Besides, the present data were compared with some conventional heat transfer correlations.