斯特林发动机回热器传热和流动特性研究

为优化斯特林发动机回热器的综合性能,采用多孔介质模型对不同孔隙率、丝网材质及不同结构的金属丝网回热器进行了多段式数值模拟研究。基于回热器效率、流阻损失和综合性能参数等指标对模拟结果进行了分析。结果表明:回热器的效率和流动损失随孔隙率的减小而增大;单段式小导热率填料的回热器流动阻力非常大,而采用热端大孔隙率高导热率,冷端小孔隙率低导热率材料的多段式基体结构,能进一步降低回热器流动阻力损失,提高效率,获得更好的回热器性能。     

孔隙率对斯特林发动机回热器影响的数值模拟

回热器作为斯特林发动机的核心部件,其工作条件极其恶劣,流动和传热过程更是复杂,迄今为止还没有一种准确描述其工作过程的方法.针对孔隙率对回热器传热及流动特性的影响,利用Fluent软件进行了三维数值模拟,通过模拟发现填料孔隙率的变化对回热器内压损及蓄热能力都有很大的影响.     

微型燃气轮机圆筒原表面回热器的性能试验研究

对微型燃气轮机圆筒形原表面式回热器的传热与流动性能进行了试验研究.利用计算机控制检测传热性能试验台对该回热器在①两侧流量相等、改变两侧温度及②固定两侧温度,改变系统质量流量的情况下进行了传热性能和阻力性能试验.结果表明:随着质量流量的增加,回热器的传热系数增大,传热量逐渐增加,回热器的两侧压降也增大;在等流量时,回热器两侧的压降有所不同,高压低温侧压降比低压高温侧压降大,但低压高温侧压降增加较快,因此在设计回热器时必须重视两侧压降的变化情况,根据试验结果得出了传热和阻力随工况改变的变化趋势.     

航空发动机中回热器传热和阻力性能的数值模拟

航空发动机中的回热器是以椭圆形管结构为载体,即以椭圆管共同组成回热器,这是一种新型的紧凑式高温回热器,其在航空等相关工业领域备受青睐。但是,现阶段,我国并未明确回热器的传热性能以及强化机制,而相关准则和规范也不甚完善。对此,通过FLUENT数字化模拟回热器传热和阻力性能,可以在很大程度为实现回热器结构优化奠定良好的理论基础。主要对椭圆管回热器传热与阻力性能的影响因素进行了进一步分析,并以综合性能评价指标为基础,最优化了回热器结构综合性能,在此基础上,提出了平椭圆管结构的回热器,同时对其传热和阻力性能进行了计算分析,进而实现了平椭圆管回热器结构综合性能的最优化,最后还以场协同原理为基础,深入探究了椭圆形管回热器传热的强化机制。     

不可逆变温热源斯特林热机的?生态学性能研究

基于?的生态学目标函数,使用一种新的指标——?生态指标对不可逆变温热源斯特林热机进行了分析。在热机高低温热源的热容均为有限的前提下,考虑斯特林热机的热阻、回热损失、内不可逆性以及热漏对其影响。研究了回热器的回热效率和高低温热源的热容的比值等参数对斯特林热机的功率、效率和?生态学指标的影响,并将结果与另一种生态目标函数的结果进行了比较,得到最佳功率输出功率和热效率。     

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。     

斯特林发动机瞬态模型及特性分析

为获得斯特林发动机的动态特性和优化方案,将损失机制和压力梯度耦合进控制方程中,提出一维瞬态斯特林循环分析模型及分析方法,并针对GPU-3斯特林发动机进行模型验证和特性分析。模型的指示功率相对误差平均值约为4.8%,热效率的相对误差小于1%。当氦气工质在热源温度为977 K、平均压强为2.76 MPa时,输出功率随转速的升高先增大后减小,同时流动阻力损失由0.174 kW上升至3.179 kW,最佳运行转速范围约2500~3000 r/min。最大的3项损失分别为流动阻力损失、配气活塞穿梭传热损失和有限速度压力损失。回热器压降占总压降的90%以上,瞬态值高达188 kPa,减小回热器压降损失是减小流动阻力损失的有效途径。     

在近实际条件下斯特林发动机效率研究

正据《Энергетцка》2013年5-6月刊报道,亚美尼亚国立工程大学的学者研究了斯特林发动机近实际条件下的效率,并尝试考虑与热交换有限值有关的热量回收损失。在工质为范德华尔斯气体的条件下,得到在无回热器和有回热器情况下斯特林发动机的绝对内效率。结果表明,在考虑分子体积情况下,斯特林发动机的热效率取决于工质的克分子数并且比理想气体稍有增加。同时,考虑了斯特林发动机在热量与回收情况下的运行热损失,得到了回热度与热交换时间的相互关系。     

一次表面回热器动态特性的数值模拟与实验研究

对一次表面回热器(Primary Surface Recuperator,PSR)流量阶跃变化时的动态特性进行了数值分析和实验研究.根据能量守恒原理和一次表面回热器(PSR)的结构特点,导出回热器冷热流体和固体间壁非稳态温度变化的微分方程式,研究流体流量发生阶跃变化时PSR的响应时间.在冷热空气进口参数和换热量相同的条件下,当冷热侧流量分别增加为原来3倍的情况下,PSR的响应时间只有管壳式换热器的1/8,板翅式的1/3.数值分析结果与实验结果相符.由于PSR的固体壁面时间常数远小于板翅式和管壳式回热器,因此这种轻重量结构的先进回热器响应特性明显优于常规回热器.     

不同优化准则下斯特林发动机的性能分析

应用有限时间热力学原理.建立了一个考虑热阻、热漏和回热损失等不可逆因素的斯特林发动机模型;推导了最大输出功率、最大效率和生态学优化准则下,斯特林发动机性能的表达式;比较了三种优化准则下,热漏系数和回热器有效性对斯特林发动机性能的影响.研究表明:对热漏损失和回热损失较大的斯特林发动机,宜选用生态学优化准则.为斯特林发动机...     

Study the Heat Recovery Performance of Micro and Nano Metfoam Regenerators in Alpha Type Stirling Engine Conditions

This paper experimentally investigates the performance of micro and nano metfoam regenerators in alpha-type Stirling engine conditions. The thermal efficiency of this engine depends on performance of regenerator. Therefore, increase the heat recovery of regenerator raises the total efficiency. Accordingly, two types of regenerators from porous media are designed and simulated with different materials. Three-dimensional regenerator CFD simulation shows that randomize porous open cell metfoam made of silver as high conductivity and high heat capacity material is the best structure to fabricate metfoam regenerator. The porosity and matrix element diameter are optimized. The nano coating methodology enhances the activated surface. The regenerators are fabricated using casting polymer mold layer deposition. The nano silver particles are coated on the metfoam by sol-gel coating method. Experimental results show the improvement in regenerator percentage of heat recovery by 3.40% and 5.93% for micro metfoam and nano metfoam, respectively. The maximum improvement is achieved up to 8.65% in case of using the nano metfoam regenerator at 543 K.     

Experimental investigations of a gamma Stirling engine

The present work deals with the measurement and performance of a gamma Stirling engine of 500 W of mechanical shaft power and 600 rpm of maximal revolutions per minute. Series of measurements concerning the pressure distribution, temperature evolution, and brake power were performed. The study of the different functioning parameters such as initial charge pressure, engine velocity, cooling water flowrate, and temperature gradient (between the sources of heat) has been analyzed. The engine brake power increases with the initial charge pressure, with the cooling water flow, and with the engine revolutions per minute. The working fluid temperature measurements have been recorded in different locations symmetrically along both regenerator sides. The recorded temperature in regenerator side one is about 252 °C and about 174 °C in the opposite side (side two). It shows an asymmetric temperature distribution in the Stirling engine regenerator; consequently, heat transfer inside this porous medium is deteriorated. Copyright © 2011 John Wiley & Sons, Ltd.     

Comparison of performance of heat regenerators: Relation between heat transfer efficiency and pressure drop

Heat regenerators transfer heat from one gas to another, with an intermediate storage in solids. The heat transfer surface for gas flow application should provide at the same time high surface area and low friction factor. Three geometries of heat transfer surface, monolith, stack of woven screens and bed of spheres, have been compared. Their performance was evaluated from the pressure drop of the heat regenerator working at a given heat transfer efficiency. The comparison was performed using numerical simulation and published measurements of heat transfer and flow friction characteristics. By adjusting the length and the period of the exchanger, it is possible to obtain the same heat transfer efficiency with the three geometries. Beds of spheres give very short and compact heat regenerators, working at high pressure drop. At the opposite, monoliths form long regenerators working at low pressure drop. Stacks of woven screens cover a wide range of performance: low porosity woven screens give high heat transfer efficiency and high pressure drop, while high porosity woven screens offer performance similar to that of the monoliths. Copyright © 2001 John Wiley & Sons, Ltd.     

Optimal regenerator performance in Stirling engines

The key component of a Stirling engine is its regenerative heat exchanger. This device is subject to losses due to dissipation arising from the flow through the regenerator as well as due to imperfect heat transfer between the regenerator material and the gas. The magnitudes of these losses are characterized by the Stanton number St and the Fanning friction factor f, respectively. Using available data for the ratio St/f, results are found for the Carnot efficiency and the power output of the regenerator. They depend on the conductance and on the ratio of pressures at the two sides of the regenerator. Optimum results for efficiency and power output of the regenerator are derived in the limit of zero Mach number. The results are applied to the Stirling engine. The efficiency and the power output of the engine are found for given amplitude of the compression piston. Optimization with respect to regenerator conductance and piston phase angle leads to a maximum possible value of the power output. Under optimal conditions, the Carnot efficiency just below this maximum is close to 100%. Copyright © 2009 John Wiley & Sons, Ltd.     

Unsteady thermal flow analysis in a heat regenerator with spherical particles

Heat regenerator occupied by regenerative materials improves thermal efficiency of regenerative combustion system through the recovery of sensible heat of exhaust gases. By using one-dimensional two-phase fluid dynamics model, the unsteady thermal flow of regenerator with spherical particles, were numerically analysed to evaluate the heat transfer and pressure drop and to suggest the parameter for designing heat regenerator. It takes about 7 h for the steady state in the thermal flow of regenerator, where heat absorption of regenerative particle is concurrent with heat desorption. The regenerative particle experiences small temperature fluctuation below 10 K during the reversing process. The thermal flow in heat regenerator varies with inlet velocity of exhaust gas and air, configuration of regenerator and diameter of regenerative particle. As the gas velocity increases with decreasing the cross-sectional area of the regenerator, the heat transfer between gas and particle enhances and pressure losses increase. As particle diameter decreases, the air is preheated higher and the exhaust gases are cooled lower with the increase of pressure losses. At the same exhaust gases temperature at the regenerator outlet, the regenerator length need to be linearly increased with inlet Reynolds number of exhaust gases. It is confirmed that inlet Reynolds number of exhaust gases should be introduced as a regenerator design parameter. Copyright © 2002 John Wiley & Sons, Ltd.     

Multi-objective optimization of rotary regenerator using genetic algorithm

The rotary regenerator (heat wheel) is an important heat recovery equipment, which rotates between two cold and hot streams. The pressure drop and effectiveness of rotary regenerator are important parameters in optimal design of this equipment for industrial applications. For optimal design of such a system, it was thermally modeled using -NTU method to estimate its pressure drop and effectiveness. Frontal area, ratio of hot to cold frontal heat transfer area, matrix thickness, matrix rotational speed, matrix rod diameter and porosity were considered as design parameters. Then fast and elitist non-dominated sorting genetic algorithm (NSGA-II) method was applied to find the optimum values of design parameters. In the presented optimal design approach, the effectiveness and the total pressure drop are two objective functions. The results of optimal designs were a set of multiple optimum solutions, called ‘Pareto optimal solutions’. The sensitivity analysis of change in optimum effectiveness and pressure drop with change in design parameters of the regenerator was also performed and the results are reported.     

The impact of heat exchanger fouling on the optimum operation and maintenance of the Stirling engine

This paper focuses on the effect of heat exchanger fouling on the performance of the Stirling engine in combined heat and power (CHP) application. Fouling results from using biomass fuels and affects the heat exchanger that transfers heat into the engine. This heat exchanger is referred to as the heater. The heat exchanger that recovers heat from the flue gases is also affected by fouling. To determine the performance of the Stirling engine, a commercial Stirling analysis tool is applied together with models that have been developed for the heat transfer in the heater, regenerator and cooler of the engine. The Stirling engine model uses constant temperatures for the heat addition and rejection, with the theory of displacement engine as a basis. The fouling in the heat exchanger is taken into account by using a fouling factor that corresponds with the degradation in the total heat transfer coefficient. The Stirling engine model together with the model for heat exchanger fouling makes it possible to estimate the effect of fouling on the performance of the Stirling engine. A cost model is developed for the engine to translate changes in performance into economy in CHP operation. In the studied application, the Stirling engine is operated by the heat demand. Together with the selected control method, performance and cost models compose a tool for the simulation and optimization of the system. The use of the models to determine the optimal cleaning interval of the heat exchanger surfaces is considered.     

Empirical estimation of regenerator efficiency for a low temperature differential Stirling engine

An experimental method of regenerator evaluation is proposed in this paper. The configuration of the experimental equipment used in the method is similar to that of an alpha-configuration Stirling engine with a phase angle of 180°. The temperature of the hot side heat exchanger is controlled by an electric heater, and the heat sink was room air. An air conditioner controlled the temperature of the room air. The temperature and pressure of the working fluid were measured during the piston motion. A #18 stainless steel mesh was used as a regenerator matrix for a low temperature differential Stirling engine (LTDSE). The regenerator efficiency can be calculated based on the measurement results. The product of the swept volume, the density of the working fluid, the specific heat and the difference in the working fluid temperatures between the hot side and the cold side is greater than the amount of the internal energy fluctuation. The reason for this is assumed to be the temperature fluctuation in the region between the two heat exchangers. The walls of the region are made of acrylic resin. The amount of the temperature fluctuation in the region is assumed to be uniform. The regenerator efficiency is calculated as a function of the temperature fluctuation in the region. The evaluation method does not require a fast-response thermocouple. The prediction of the regenerator efficiency is possible basted on some experimental results of same matrix. Polyurethane foam and #18 stainless steel mesh, layered parallel to the stream line of the working fluid, were each tested. These materials can realize a non-rectangular regenerator without the generation of waste. Non-rectangular regenerator includes regenerator that can be installed into narrow gaps. The regenerator efficiency of the stainless steel mesh layered parallel to the stream line of the working fluid was significantly less in comparison to that of the normal mesh layers. In the polyurethane foam case, a pressure loss was observed.     

Performance optimisation of solar receivers that use oil as a heat transfer fluid

This article presents the use of a wire mesh in optimising the performance of two volumetric solar receivers that use oil as a heat transfer fluid. Computational fluid dynamics models have been used to optimise the receivers. Varied parameters (including the use of a wire mesh) of the receiver models were changed in the optimisation process. Based on the models, prototype receivers were developed and tested. After that, the models were validated against experiments and the results compare well. The results indicate that the use of a wire mesh placed inside a receiver improves its performance. An optimal wire mesh porosity was found as ≈0.95 mainly because the efficiency is increased without inducing an adverse pressure drop inside the receiver.