An approach for the optimum design of heat exchangers

In this paper, an approach for the optimum design of heat exchangers has been presented. Traditional design method of heat exchangers involves many trials in order to meet design specifications. This can be avoided through the present design method, which takes the minimization of annual total cost as a design objective. In alternative optimum design methods, such as Lagrange multiplier method, by changing one variable at a time and using a trial–error or a graphical method, optimum results are obtained in a long time. In the present design optimization problem, the total annual cost has been taken as the objective function and heat balance, and rate equation have been taken as equal constraint. The method using the penalty function transforms the constrained problem into a single unconstrained problem. To solve the optimal problem, the method of steepest descent has been used. Initial design variables include the tube‐inside coefficient of heat transfer, tube‐outside coefficient of heat transfer, temperature difference and outside tube area of heat transfer. The changes in variables are considered simultaneously to reach an optimum solution. The results show that the present approach is a powerful tool for optimum design of heat exchangers and is expected to be beneficial to energy industry. Copyright © 2004 John Wiley & Sons, Ltd.     

ANN modeling of compact heat exchangers

This paper focuses on the heat transfer analysis of compact heat exchangers through artificial neural network (ANN). The ANN analysis includes heat transfer coefficient, pressure drop and Nusselt number in the compact heat exchangers by using available experimental results in a case study. In this study, data sets are established in 15 different test channel configurations. A feed‐forward back‐propagation algorithm is used in the learning process and testing the network. The learning process is applied to correlate the heat transfer analysis for different ratios of rib spacing and height, various Reynolds numbers, different inlet–outlet temperatures, heat transfer areas and hydraulic diameters. Various hidden numbers of the network are trained for the best prediction of the heat transfer analysis. Heat transfer coefficient, pressure drop and Nusselt number values are predicted by the network algorithm. The results are then compared with the experimental results of the case. The trained ANN results perform well in predicting the heat transfer coefficient, pressure drop and Nusselt number with an average absolute mean relative error of less than 6% compared with the experimental results for staggered cylindrical ribbed and staggered triangular ribbed of test channels in the case study. The ANN approach is found to be a suitable method for heat transfer analysis in compact heat exchangers. Copyright © 2007 John Wiley & Sons, Ltd.     

Investigation of dimpled fins for heat transfer enhancement in compact heat exchangers

Direct and Large-Eddy simulations are conducted in a fin bank with dimples and protrusions over a Reynolds number range of Re_H = 200 to 15,000, encompassing laminar, transitional and fully turbulent regimes. Two dimple-protrusion geometries are studied in which the same imprint pattern is investigated for two different channel heights or fin pitches, Case 1 with twice the fin pitch of Case 2. The smaller fin pitch configuration (Case 2) develops flow instabilities at Re_H = 450, whereas Case 1 undergoes transition at Re_H = 900. Case 2, exhibits higher Nusselt numbers and friction coefficients in the low Reynolds number regime before Case 1 transitions to turbulence, after which, the differences between the two decreases considerably in the fully turbulent regime. Vorticity generated within the dimple cavity and at the dimple rim contribute substantially to heat transfer augmentation on the dimple side, whereas flow impingement and acceleration between protrusions contribute substantially on the protrusion side. While friction drag dominates losses in Case 1 at low Reynolds numbers, both form and friction drag contributed equally in Case 2. As the Reynolds number increases to fully turbulent flow, form drag dominates in both cases, contributing about 80% to the total losses. While both geometries are viable and competitive with other augmentation surfaces in the turbulent regime, Case 2 with larger feature sizes with respect to the fin pitch is more appropriate in the low Reynolds number regime Re_H < 2000, which makes up most of the operating range of typical compact heat exchangers.     

Design and multi-objective optimization of heat exchangers for refrigerators

In this study, a detailed thermodynamic model for a refrigerator based on an irreversible Carnot cycle is developed with the focus on forced-air heat-exchangers. A multi-objective optimization procedure is implemented to find optimal design values for design variables. Minimizations of energy consumption and material cost were the two objectives considered. Since these objectives are conflicting, no single design will satisfy both simultaneously. The result of this research is a set of multiple optimum solutions, which are called ‘Pareto optimal solutions’. Air and refrigerant side correlations were combined with an elemental approach to model the heat exchangers. This paper presents a detailed design model development. A limited validation is presented with experimental test-data obtained from a typical household refrigerator. Detailed simulation models are typically complex and computationally demanding. An optimization algorithm requires several evaluations of such models. Response surface based metamodels for objective functions were used to save computational effort. A genetic-algorithm based optimization tool is used for multi-criteria optimization.     

Multi-objective optimization of a multi water-to-water heat pump system using evolutionary algorithm

This paper deals with the energy recovery in the dairy industry. Thermodynamic, economic and environmental optimization of three water-to-water heat pumps has been studied in order to replace totally or partially a fuel boiler used to produce heat at different temperature levels in a cheese factory. These heat pumps have their evaporators connected to one effluents source and two of them are equipped by storage tanks at the condenser side. Multi-objective optimization permits optimal repartition of mass flow rates of effluents and optimal choice of electrical power of the compressors and volumes of storage tanks. The thermodynamic objective is based on the exergy destruction in the whole system. The economic objective is based on the investment cost and the operating cost obtained with the heat pump system. The environmental impact objective has been defined and expressed in cost terms by considering a CO₂ taxation (carbon tax) on the GHG emissions. This objective has been integrated with the economic objective. Multi-objective genetic algorithms are used for Pareto approach optimization.     

A Modified Entropy Generation Number for Heat Exchangers

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紧凑式两相换热器中管式换热元件沸腾传热实验研究

以烃类物质（丙烷和正戊烷）作为工质,进行了紧凑式换热器中带有加工配置表面的管式换热元件池沸腾实验研究。其中,单管实验温度工况为２５３Ｋ￣２９３Ｋ（饱和工质）。实验中所采用的换热元件为重入式结构加工配置表面的强化传热管和光管以及低助管。针对由４５根光管或带有加工配置表面的管子所构成的叉排管束进行了实验研究,实验工质为丙烷和正戊烷,实验温度分别为两种工质在２６３Ｋ和３０８Ｋ之间的饱和和温度。并将所得实     

换热网络全局优化的多维峰谷轮换法

在峰谷轮换法的基础上提出了利用多维峰谷轮换法对以年综合费用最少为目标函数的无分流换热网络进行优化.在换热网络变量寻优过程中,首先将目标函数极小化得到一个局部极小值点,然后沿着多个方向进行变量搜索,找出最先跳出局部极小值点的变量组合,使得跳出局部极小值的搜索效率高于单变量的搜索效率,并可寻找到更好的换热网络结构,将极小化与跳出局部极小值点过程交替进行直至求得全局最优解.通过算例验证了该方法的可行性,且可找到比其它方法的优化结果更优的换热网络结构.     

Prediction of heat transfer rates for shell-and-tube heat exchangers by artificial neural networks approach

This work used artificial neural network(ANN)to predict the heat transfer rates of shell-and-tube heatexchangers with segmental baffles or continuous helical baffles,based on limited experimental data.The BackPropagation (BP) algorithm was used in training the networks.Different network configurations were alsostudied.The deviation between the predicted results and experimental data was less than 2%.Comparison withcorrelation for prediction shows ANN superiority.It is recommended that ANN can be easily used to predict theperformances of thermal systems in engineering applications,especially to model heat exchangers for heattransfer analysis.     

The control of a refrigeration system using a compact plate heat exchanger as an evaporator

This paper will show some of the problems that may be encountered when using a compact plate exchanger as an evaporator in a refrigeration system which utilizes a thermostatic expansion valve as its controlling device.     

A study on the effects of CO-tubes insertion on the emission characteristics of a compact heat exchanger

The effects of CO-tubes insertion on NO_x (nitrogen oxides) and CO emissions from a compact heat exchanger were numerically simulated using a quasi one-dimensional premixed flame model, and the results were compared to experiments with a laboratory-scale heat exchanger and a commercial compact condensing boiler with a heat exchanger installed. The “CO-tubes” means that pre-cooling tubes were inserted between the burner and the main heat exchanger to reduce CO concentration. In the numerical results, the outlet concentration of NO increased as the distance between the burner and the CO-tubes was increased up to 3 cm. However, NO concentration was little affected by the CO-tubes when the distance was greater than 3 cm. Additionally, the concentration of CO reached a minimum at the distance of 3 cm. CO concentration increased when the distance from the burner to the CO-tubes was increased or decreased between 2 cm and 3 cm. These simulated results showed good agreement with the experimental results using a laboratory-scale heat exchanger. However, the maximum and minimum distances for NO and CO concentrations were different. When the CO-tubes were subsequently applied in a working commercial boiler, CO concentration was dramatically reduced by the CO-tubes and NO concentration was also slightly decreased.     

A computationally efficient hybrid time step methodology for simulation of ground heat exchangers

Ground heat exchanger design tools have become increasingly important for the sizing of energy-efficient heating and cooling systems. Most such heat exchanger design tools incorporate a simulation that uses both long and short timesteps (a “hybrid timestep” procedure). Current tools typically expect engineers to exercise judgment to determine the magnitude and duration of the shorter timestep. This paper proposes an accurate and efficient methodology for developing this hybrid timestep formulation, which is validated against hourly simulations for a set of three building types. Overall, the method performs well for design purposes, with the error in ground heat exchanger sizing averaging less than 1% and always less than 8%.     

Optimization of energy systems based on Evolutionary and Social metaphors

Optimization problems that arise in energy systems design often have several features that hinder the use of many optimization techniques. These optimization problems have non-continuous mixed variable definition domains, are heavily constrained, are multimodal (i.e. have many local optima) and, foremost, the functions used to define the engineering optimization problem are often computationally intensive. Three methods are tested here: (a) a Struggle Genetic Algorithm (StrGA), (b) a Particle Swarm Optimization Algorithm (PSOA), and (c) a PSOA with Struggle Selection (PSOStr). The last is a hybrid of the evolutionary StrGA and the socially inspired PSOA. They are tested in four purely mathematical and three energy systems thermoeconomic optimization problems. All of the methods solved successfully all the problems. The PSOStr, however, outperformed the other methods in terms of both solution accuracy and computational cost (i.e. function evaluations).     

Multi-objective optimization of HVAC system with an evolutionary computation algorithm

A data-mining approach for the optimization of a HVAC (heating, ventilation, and air conditioning) system is presented. A predictive model of the HVAC system is derived by data-mining algorithms, using a dataset collected from an experiment conducted at a research facility. To minimize the energy while maintaining the corresponding IAQ (indoor air quality) within a user-defined range, a multi-objective optimization model is developed. The solutions of this model are set points of the control system derived with an evolutionary computation algorithm. The controllable input variables — supply air temperature and supply air duct static pressure set points — are generated to reduce the energy use. The results produced by the evolutionary computation algorithm show that the control strategy saves energy by optimizing operations of an HVAC system.     

Second-law performance of heat exchangers for waste heat recovery

Exergy change rate in an ideal gas flow or an incompressible flow can be divided into a thermal exergy change rate and a mechanical exergy loss rate. The mechanical exergy loss rates in the two flows were generalized using a pressure-drop factor. For heat exchangers using in waste heat recovery, the consumed mechanical exergy is usually more valuable than the recovered thermal exergy. A weighing factor was proposed to modify the pressure-drop factor. An exergy recovery index (η_II) was defined and it was expressed as a function of effectiveness (?), ratio of modified heat capacity rates (C^∗), hot stream-to-dead-state temperature ratio, cold stream-to-dead-state temperature ratio and modified overall pressure-drop factor. This η_II–? relation can be used to find the η_II value of a heat exchanger with any flow arrangement. The η_II−Ntu and η_II−Ntu_h relations of cross-flow heat exchanger with both fluids unmixed were established respectively. The former provides a minimum Ntu design principle and the latter provides a minimum Ntu_h design principle. A numerical example showed that, at a fixed heat capacity rate of the hot stream, the heat exchanger size yielded by the minimum Ntu_h principle is smaller than that yielded by the minimum Ntu principle.     

Planning the optimum cleaning schedule based on simulation of heat exchangers under fouling

An investigation of variations in outlet temperatures of heat exchangers under fouling was carried out. The simulation of heat exchangers was performed by employing a linear fouling deposit function. The formation of deposits reduces heat exchangers effectiveness. There is inherently a linear nature between outlet and inlet temperatures of heat exchangers. The outlet temperatures can also be affected by up‐stream exchangers serving the same streams, and the up‐stream influence can be transferred in the heat system. The mathematical model of the cleaning cycle was outlined, based on the objective function of minimizing cost in unit operation time. According to the results, some heat exchangers can be given cleaning priority when the system is shut down, in order to maximize economic benefit. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20384     

三套管式相变蓄热器强化传热研究

基于三套管式相变蓄热器的特点,提出应用T字形翅片来强化相变蓄热器的传热性能。研究结果表明:添加翅片可有效地降低蓄热器中相变材料的凝固和融化时间,直翅片和T字形翅片的混合强化结构能使凝固过程比未强化结构节省74%的时间,使融化过程节省60%的时间。因此直翅片和T字形翅片的混合使用可以达到进一步强化传热的目的。     

A comparative analysis of different heat exchangers containing phase change materials

针对日益严重的环境问题,“煤改电”已经成为实现北方清洁供暖的有效手段。此外,我国每年在余热利用,尤其是在中低温品质热能利用上还相当不充分。在此大背景下,以相变储热供热技术为切入点,着重对目前相变储热换热器进行了比较,定性分析了板式、管壳式、热管式及其他异形（储热砖/球）换热器的优缺点。并通过数值模拟的方式,定量比较了相同换热面积及边界条件下,管壳式和板式相变换热的二维相变材料熔化模型,管壳式换热器需6 h完全熔化,板式换热器需8.5 h完全熔化,主要原因在于二者在换热管/板在排布上差异导致。但考虑到相较于管壳式换热器,板式换热器结构紧凑、加工工艺简单、拆卸方便,未来可形成通过制成储热砖的方式实现模块化运行,为后期维护提供了很大便利,因此具有巨大发展潜力。     

A finite line‐source model for boreholes in geothermal heat exchangers

An analytical solution of the transient temperature response in a semi‐infinite medium with a line source of finite length has been derived, which is a more appropriate model for boreholes in geothermal heat exchangers, especially for their long‐duration operation. The steady‐state temperature distribution has also been obtained as a limit of this solution. An erratic approach to this problem that appears in some handbooks and textbooks is indicated. Two representative steady‐state borehole wall temperatures, the middle point temperature and the integral mean temperature, are defined. Differences between them are compared, and concise expressions for both are presented for engineering applications. On this basis the influence of the annual imbalance between heating and cooling loads of the geothermal heat exchangers is discussed regarding their long‐term performance. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(7): 558–567, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10057     

Impact of scale build‐up models on energy utilization in heat exchangers

In this paper an energy utilization index is presented. It is used to investigate the impact of scale build‐up in heat exchange equipment. The investigation is carried out using several scale build‐up models. The models employed in this paper are the linear, logarithmic, and the asymptotic one. It has been demonstrated by a realistic example that the type of scale build‐up model assumed greatly affects energy utilization and maintenance scheduling for heat exchange equipment. Copyright © 1999 John Wiley & Sons, Ltd.