Multi-objective optimization design of condenser in an organic Rankine cycle for low grade waste heat recovery using evolutionary algorithm

The optimum design of a condenser is significant in an organic Rankine cycle to achieve higher waste heat utilization efficiency. Based on the mathematical model of a condenser using plate heat exchanger (PHE), some key geometric parameters on the total heat transfer surface area and pressure drop of the condenser are examined. In order to obtain geometric parameters of a plate heat exchanger, a multi-objective optimization of the condenser in organic Rankine cycle is conducted to achieve the optimal geometry design. The total heat transfer surface area and pressure drop are selected as two objective functions to minimize both total heat transfer surface area and pressure drop under the constant heat transfer rate and LMTD conditions. The plate width, plate length and plant distance are selected as the decision variables. Non-dominated sorting generic algorithm-II (NSGA-II) which is an effective multi-objective optimization method is employed to solve this multi-objective optimization design of PHE. The results show that an increase in channel distance or plate width increases the total heat transfer surface area and decreases pressure drop in the condenser. It is noted that the plate length of PHE has a positive effect on the optimization design of PHE. By multi-objective optimization design of the PHE, a Pareto optimal point curve is obtained, which shows that a decrease in total heat transfer surface area of a condenser can increase the pressure drop through the condenser.     

Numerical study on air-side performance of an integrated fin and micro-channel heat exchanger

A new type of aluminum heat exchanger with integrated fin and micro-channel has been proposed. The air-side heat transfer and flow characteristics of the integrated fin and micro-channel heat exchanger are systematically analyzed by a 3D numerical simulation. The effect of flow depth, fin height, fin pitch and fin thickness at different Reynolds number is evaluated by calculating Colburn factor j and Fanning friction factor f. A parametric study method is used to analyze the fin designed parameters affecting the performance of the heat exchanger. The results show that the contribution ratio of the fin geometries in descending order is flow depth, fin pitch, fin height and fin thickness. The air-side performance of the integrated fin and micro-channel heat exchanger is compared with that of the multi-louver fin micro-channel heat exchanger and the wavy fin micro-channel heat exchanger.     

应用遗传算法优化设计紧凑式换热器

应用遗传算法对锯齿型板翅式换热器的结构进行了优化设计.以预定的传热量、两侧压降、体积等设计要求为适应度函数,通过对设计变量构成的种群个体进行适应度评估以及选择、交叉、变异等遗传操作,得到符合设计约束的换热器最优结构参数(如翅高、翅长、间距,以及长、宽、高等).此外,对10次计算得到的设计参数进行了统计分析.结果表明:相关结构参数的不确定度很小,应用遗传算法可以快速方便地对紧凑式换热器的几何结构参数作出符合设计要求的选择.     

A method to reduce the flow depth of a plate heat exchanger without a loss of heat transfer performance

With the aim of improving heat exchanger compactness, this study investigates how the optimum configuration of an air–liquid plate heat exchanger changes as the heat exchanger depth decreases. In this respect, optimization of an air–liquid plate heat exchanger with a given frontal area and a given depth is achieved. The optimum fin pitch and plate pitch are obtained to maximize the heat transfer rate based on heat transfer and pressure loss correlations in finned channels. Then, the focus of this study is placed on how the optimum channel configuration changes when the heat exchanger depth decreases for compactness. The results illustrate that the heat transfer performance can remain unchanged if the geometric parameters, such as the plate thickness, the plate pitch, the fin thickness, and the fin pitch, are reduced proportionally to the square root of the flow depth reduction given that the flow remains laminar. This finding is arranged into a simple scaling rule to obtain the configuration of a more compact heat exchanger from an existing configuration. In addition, the scaling arguments are extended to practical situations where the fin thickness and the plate thickness are not properly reduced following the scaling rule due to limitations on available material thicknesses.     

FLOW DEVELOPMENT OF CO-FLOWING STREAMS IN RECTANGULAR MICRO-CHANNELS

The diffusion and flow development characteristics of two co-flowing, laminar streams in a high aspect ratio rectangular micro-channel have been examined. A long, thin splitter plate initially separates the two streams such that fully developed flow in each of the two channels is established prior to merging. The co-flowing micro-channel has an aspect ratio of 16 with a width of 1006 μm and a height of 63 μm. Micro-Particle Image Velocimetry (μPIV) was utilized to observe the interaction between the streams for a range of flow rate ratios ranging from one to nine, for Reynolds numbers of one and ten. For flow rate ratios greater than one, a cross-stream pressure gradient exists immediately downstream of the splitter plate, which results in a strong lateral flow of the faster moving fluid into the slower moving fluid. Despite this rapid expansion, the fluids in the two streams do not mix. The two streams eventually recover a fully developed velocity profile across the entire channel. A model is presented to predict this development length based on the pressure imbalance between the two streams. The model is expressed in terms of the flow rate ratio between the streams, which is shown to be a function of channel aspect ratio. An asymptotic condition for the development length is found for high flow rate ratios and high aspect ratio channels. It is shown that existing entrance length relationships greatly underpredict this development length.     

Parametric analysis on multi-stage high pressure reducing valve for hydrogen decompression

Hydrogen fuel cell electric vehicle (FCEV) can reduce air pollution as well as achieve efficient use of hydrogen energy. Farther travel distance requires larger hydrogen storage pressure, thereby imposing more demanding working conditions on the pressure reducing system. In this paper, a multi-stage high pressure reducing valve (MSHPRV) for hydrogen decompression in FCEV is proposed, and the effects of different structural parameters on its internal flow characteristics are investigated to achieve a better hydrogen decompression process. Results show that compared with perforated plate, multi-stage perforated sleeves and valve core hold the dominant position in hydrogen throttling process. Larger multi-stage perforated sleeve diameter, perforated plate diameter and pressure ratio relate to larger hydrogen kinetic energy, turbulence vortex and energy consumption. However, with the increase of perforated plate stage and perforated plate radius, the turbulent intensity and energy consumption inside MSHPRV decreases correspondingly. This study can provide some technical supports for achieving hydrogen decompression in FCEV when facing harsh working conditions, or help with dealing energy conversion during decompression process.     

余热驱动的有机朗肯循环-复叠式制冷系统?分析

针对余热的有效利用,建立了有机朗肯循环-复叠式制冷系统的热力学模型,其中:有机朗肯循环系统分别采用R123、R1234ze、R245fa、R600a、RC318、R141b等六种工质;复叠式制冷系统分别采用R22/R23、R404/R23、R290/R744、R717/R744等四种工质对。选择系统?效率作为性能评价指标,运用热力学第二定律研究系统运行参数对系统?效率的影响,分析了系统各部件的?损失,并指出了能量利用的薄弱环节,提出了有效提高系统性能的建议,为系统的优化提供参考。结果表明,对系统?效率而言,R141b和R717/R744是最佳工质。系统主要部件按?损失大小依次为凝汽器、膨胀机、高温级冷凝器、发生器、高温级压缩机、低温级蒸发器、蒸发冷凝器。尽可能提高压缩机的等熵效率,优化设计换热器的结构,减小传热温差,才能减少不可逆损失,提高换热器的?效率。     

Heat extraction from supercritical water-cooled nuclear reactors for hydrogen production plants

Many current and future hydrogen production methods, such as steam methane reforming and thermochemical water splitting cycles, require large amounts of heat as the major energy input. Using nuclear heat is a promising option for reducing emissions of greenhouse gases and other pollutants, thereby helping achieve clean and sustainable future energy systems. Various heat transfer fluids are compared and evaluation criteria are proposed for the selection of a heat transfer fluid. It is determined that helium is a promising option due to it being inert and chemically stable and having good heat transfer properties. The intermediate heat exchanger for the heat extraction is analyzed and designed using the log mean temperature difference (LMTD) method with helium serving as the heat transfer fluid to extract heat from the supercritical water. It is found that if the heat extraction load is in the range of 100–330 MW_th, which approximately corresponds to a hydrogen production range of 40–125 tonnes per day, then a multi-tube and single-shell counter flow heat exchanger with a shell diameter of 0.7–1.3 m and length of 6.7 m encapsulating 420–1600 tubes of 0.025 m diameter would be appropriate according to the practical working conditions on the shell and tube sides. The analysis also shows that the diameter of the heat exchanger does not depend strongly on the heat transfer load if the load is smaller than 330 MW_th (125 tonnes H₂/day). This provides flexibility in case adjustments to the heat extraction load become necessary. However, if the heat load is larger than 330 MW_th, for example, 500 MW_th for 200 tonnes hydrogen per day, then a multi-tube and single-shell counter flow heat exchanger is not appropriate because the length-to-diameter ratio is outside of the recommended range.     

回热器孔隙率对VM循环热泵性能的影响分析

孔隙率是表征回热器结构和效率的重要参数,影响整个系统的流动和传热特性。以热驱动斯特林循环的VM(Vuilleumier,维勒米尔)循环热泵为研究对象,建立了其内部回热器的模型,研究了在不同的热源温度、系统压力、转速、容积比和工质的情况下,孔隙率对整个热泵系统性能的影响。结果表明：随着孔隙率的增加,系统性能系数先增加后减小,在0.6左右达到最佳。在相同孔隙率的情况下,系统性能系数随着热源温度、系统平均压力、容积比、转速的增加而分别增大,并且增加的幅度是越来越小的。对于工质而言,氦气和氢气的性能较优,而氮气的性能较差。综合考虑安全等因素,宜选氦气作为工质。     

Experimental investigation for optimization of design parameters in a rectangular duct with plate-fins heat exchanger by Taguchi method

This study presents the determination of optimum values of the design parameters in a heat exchanger with a rectangular duct by using Taguchi method. The heat exchanger has plate-fins containing periodically interrupted diverging and converging channel flow domains. The experimental investigation for the established heat exchanger involves short rectangular fins attached in 8 × 8 arrays to a surface having various inclination angles. The effects of the six design parameters such as the ratio of the duct channel width to height, the ratio of the winglets length to the duct channel length, inclination angles of winglets, Reynolds number, flow velocity and pressure drop are investigated. In the Taguchi experimental design method, Nusselt number and friction factor are considered as performance parameters. An L₂₅ (5⁶) orthogonal array is chosen as an experimental plan for the design parameters. The analysis of Taguchi method conducted with an optimization process to reach minimum pressure drop (friction factor) and maximum heat transfer (Nusselt number) for the designed heat exchanger. Experimental results validated the suitability of the proposed approach.     

基于Workbench的固体电蓄热装置换热通道参数优化

为了解决固体电蓄热装置储释热过程的温度分布均匀性问题,需要对蓄热体换热通道结构参数开展优化设计。根据对称性建立了表征空气与蓄热材料传热过程的传热单元,以传热单元平均温度最大和压力损失最小为优化目标,基于ANSYS Workbench的响应面优化模块对蓄热砖的几何参数(高度、宽度、厚度)和换热通道宽度开展优化设计。结果表明：恒定入口流速下,换热通道高度和宽度的增加,对传热单元平均温度的提高影响较小,但是换热通道宽度对压力损失的改变有显著影响;蓄热砖宽度或高度对于蓄热过程平均温度的改变影响明显,水平方向蓄热砖宽度由15 mm增加到40 mm平均温度降低约7.5 K,竖直方向蓄热砖高度由15 mm增加到30 mm整体蓄热温度线性降低(降温幅度7.5 K),竖直方向蓄热砖高度为20 mm、入口温度1 023 K时升温速度达到297.5 K/h;通过传热单元结构参数敏感性分析可知,设计换热通道选择的蓄热砖宽度和高度均不宜过大。     

不同螺旋角度螺旋折流板换热器传热性能与压降的研究

本文对螺旋折流板换热器和传统的弓形折流板换热器进行了壳程传热性能和压降特性的对比，同时通过实验方法对8°、12°、18°、25°、30°、40°螺旋角无搭接的螺旋折流板换热器进行了壳程传热性能和压降特性的研究，得出螺旋折流板换热器的螺旋流动强化了传热，螺旋折流板换热器的压降比弓形折流板换热器的压降小。     

Experimental study of a metal hydride vessel based on a finned spiral heat exchanger

The reaction time of hydrogen in metal hydride vessels (MHVs for short) is strongly influenced by the heat transfer from/to the hydride bed. In the present work an experimental study of the geometric and the operating parameters of a finned spiral heat exchanger has been carried out to identify their influence on the performance of the charging process of the MHV. The experimental results show that the charge time of the reactor is considerably reduced, when finned spiral heat exchanger is used. In addition, the effect of different parameters (flow mass and temperature of the cooling fluid, applied pressure, and hydrogen tank volume) has been discussed and obtained results show that a good choice of these parameters is important.     

Working fluids for low-temperature heat source

The performance of different working fluids to recover low-temperature heat source is studied. A simple Rankine cycle with subcritical configuration is considered. This work is to screen working fluids based on power production capability and component (heat exchanger and turbine) size requirements. Working fluids considered are R134a, R123, R227ea, R245fa, R290, and n-pentane. Energy balance is carried out to predict operating conditions of the process. Outputs of energy balance are used as input for exergy analysis and components (heat exchanger and turbine) design. The heat exchanger is divided into small intervals so that logarithmic mean temperature difference (LMTD) method is applicable. R227ea gives highest power for heat source temperature range of 80–160 °C and R245fa produces the highest in the range of 160–200 °C. There is optimal pressure where the heat exchanger surface area is minimum. This optimal pressure changes with heat source temperature and working fluid used. The least heat exchanger area required at constant power rating is found when the working fluid is n-pentane. At lower heat source temperature (80 °C), the maximum power output and minimum heat exchanger surface area for different working fluids is comparable.     

Optimum design of double pipe heat exchanger

Heat exchangers are used in industrial processes to recover heat between two process fluids. Although the necessary equations for heat transfer and the pressure drop in a double pipe heat exchanger are available, using these equations the optimization of the system cost is laborious. In this paper the optimal design of the exchanger has been formulated as a geometric programming with a single degree of difficulty. The solution of the problem yields the optimum values of inner pipe diameter, outer pipe diameter and utility flow rate to be used for a double pipe heat exchanger of a given length, when a specified flow rate of process stream is to be treated for a given inlet to outlet temperature.     

Improvement of thermal performance of spiral heat exchanger on hydrogen storage by adding copper fins

The reaction time of hydrogen in metal-hydride vessels (MHVs for short) is strongly influenced by the heat transfer from/to the hydride bed. In the present work an experimental study of the geometric and the operating parameters of a finned spiral heat exchanger has been carried out to identify their influence on the performance of the charging process of the MHV. The experimental results show that the charge time of the reactor is considerably reduced, when finned spiral heat exchanger is used. In addition, the effect of different parameters (flow mass and temperature of the cooling fluid, applied pressure of hydrogen in the case of absorption and desorption) has been discussed and obtained results show that a good choice of these parameters is important.     

基于微通道的太阳能集热器及其性能模拟

针对传统的平板型太阳能集热器集热效率较低的问题,本文设计了一款微通道集热器,采用数值模拟方法研究了微通道集热器的工作状况,并分析了传统平板型集热器的管中心距在稳态传热条件下对集热器的效率影响。仿真结果表明:相同条件下,平板型集热器的管间距越小,集热效率越高;微通道集热器的平均集热效率比最佳管间距的平板型集热器高9.3%,比常用的两种平板型集热器分别高20.6%、30.6%。该结果有利于优化平板型集热器的设计参数,为微通道集热器的实际应用提供了依据。     

Numerical simulation and experimental research on heat transfer and flow resistance characteristics of asymmetric plate heat exchangers

The asymmetric plate heat exchanger (APHE) has the possibility of achieving balanced pressure drops on both hot and cold sides for situations with unbalanced flow, which may in turn enhance the heat transfer. In this paper, the single-phase water flow and heat transfer of an APHE consisted of two types of plates are numerically (400≤Re≤12000) and experimentally (400≤Re≤ 3400) investigated. The numerical model is verified by the experimental results. Simulations are conducted to study the effects of N, an asymmetric index proposed to describe the geometry of APHEs. The correlations of the Nusselt number and friction factor in the APHEs are determined by taking N and working fluids into account. It is found that an optimal N exists where the pressure drops are balanced and the heat transfer area reaches the minimum. The comparison between heat transfer and flow characteristics of the APHEs and the conventional plate heat exchanger (CPHE) is made under various flow rate ratios of the hot side and the cold side and different allowable pressure drops. The situations under which APHE may perform better are identified based on a comprehensive index Nu/f^1/3.     

Thermodynamic analysis of a new double-pressure condensation power generation system recovering LNG cold energy for hydrogen production

Cold energy during the LNG regasification process is usually applied for power generation, but the electricity demand varies with the time. Therefore, a thought that transforming electrical energy into hydrogen energy by PEM electrolyzer is put forward to adjust the adaptability of power output to electricity demand. This paper proposes a new double-pressure condensation Rankine cycle integrated with PEM electrolyzer for hydrogen production. In this system, seawater is used as the heat source, and binary mixed working fluids are applied. Meanwhile, multi-stream heat exchanger is introduced to improve the irreversibility of heat transfer between LNG and working fluid. The key system parameters, including seawater temperature, the first-stage condensation temperature, the second-stage condensation temperature, and outlet temperature of LNG, are studied to clarify their effects on net power generation, hydrogen production rate and energy efficiency. Furthermore, the hydrogen production rate is as the objective function, these parameters are optimized by genetic algorithm. Results show that seawater temperature has positive impact on the net power output and hydrogen production rate. The first-stage condensation temperature, the second-stage condensation temperature, and outlet temperature of LNG have diverse effects on the system performance. Under the optimal working conditions, when the LNG regasification pressure are 600, 2500, 3000 and 7000 kPa, the increasing rate for optimized net power output, hydrogen production rate and energy efficiency are more than 11.68%, 11.67% and 8.88%, respectively. The cost of hydrogen production with the proposed system varies from 1.93 $/kg H2 to 2.88 $/kg H2 when LNG regasification pressure changes from 600 kPa to 7000 kPa.     

Reducing the exergy destruction in the cryogenic heat exchangers of hydrogen liquefaction processes

A present key barrier for implementing large-scale hydrogen liquefaction plants is their high power consumption. The cryogenic heat exchangers are responsible for a significant part of the exergy destruction in these plants and we evaluate in this work strategies to increase their efficiency. A detailed model of a plate-fin heat exchanger is presented that incorporates the geometry of the heat exchanger, nonequilibrium ortho-para conversion and correlations to account for the pressure drop and heat transfer coefficients due to possible boiling/condensation of the refrigerant at the lowest temperatures. Based on available experimental data, a correlation for the ortho-para conversion kinetics is developed, which reproduces available experimental data with an average deviation of 2.2%. In a plate-fin heat exchanger that is used to cool the hydrogen from 47.8 K to 29.3 K with hydrogen as refrigerant, we find that the two main sources of exergy destruction are thermal gradients and ortho-para hydrogen conversion, being responsible for 69% and 29% of the exergy destruction respectively. A route to reduce the exergy destruction from the ortho-para hydrogen conversion is to use a more efficient catalyst, where we find that a doubling of the catalytic activity in comparison to ferric-oxide, as demonstrated by nickel oxide-silica catalyst, reduces the exergy destruction by 9%. A possible route to reduce the exergy destruction from thermal gradients is to employ an evaporating mixture of helium and neon at the cold-side of the heat exchanger, which reduces the exergy destruction by 7%. We find that a combination of hydrogen and helium-neon as refrigerants at high and low temperatures respectively, enables a reduction of the exergy destruction by 35%. A combination of both improved catalyst and the use of hydrogen and helium-neon as refrigerants gives the possibility to reduce the exergy destruction in the cryogenic heat exchangers by 43%. The limited efficiency of the ortho-para catalyst represents a barrier for further improvement of the efficiency.