Dynamic Characteristics of a Free-Flow-Channel Heat Exchanger

The performance of sewage heat exchangers plays an important role in the sewage source heat pump systems when the sewage does not enter the heat pump unit directly, especially when untreated sewage is used as heat or cold source. In this article, a free-flow channel heat exchanger is proposed to be used in an untreated sewage source heat pump system. First the article designs the sizes of heat exchanger according to the real system requirements, and then it models and analyzes dynamic characteristics on the simulink platform. The transfer functions that are suitable for the heat exchanger are deduced. The analysis shows that the wall temperature is 9.1°C when the untreated sewage and water temperatures at the inlet are 15 and 7°C, respectively. The result is obtained when considering the temperature at the outlet is affected by the temperature at the inlet. The variable quantity of water temperature at the outlet is affected greatly by the variable quantity of water temperature at the inlet and the variable quantity of sewage temperature at the outlet by the variable quantity of sewage temperature at the inlet. Besides, the measured values of water temperatures at the inlet and outlet are also observed from the real system where the free-flow channel heat exchanger is adopted. The variable quantities of water temperature at the outlet are calculated according to the measured values, and are compared with the model results deduced by the transfer functions. The comparative result shows that the differences between the measured and model results of water temperature variable quantities at the outlet range from ?1 to 1°C, thus getting the water temperatures at the outlet according to the model results. The differences of the water temperature at the outlet between the measured values and the deduced results are from ?0.5 to 0.5°C. It illustrates the temperature at the outlet can be estimated by modeling the temperature variable quantity at the outlet.     

对单相换热器集总参数模型动态初始负偏移的机理分析

以单相换热器的传热方程和能量平衡方程为基础,对不同类型集总参数模型的动态响应的差异,尤其是出口温度在动态过程中的初始负偏移进行了机理分析。指出采用进、出口加权平衡的集总参数模型在全工况范围内很有可能会使出口温度出现负偏移;而在用出口参数作为集总参数时,可以保证不会出现负偏移,但应采用合理的分段建模,使模型有足够的传热温差。     

余热锅炉管口区传热的数值分析

应用高温换热设备管口传热分析系统针对某天然气化工厂二次转化气余热锅炉管口区传热问题建立了物理、数学模型,进行了相应的数值分析。数值计算结果表明,新型双瓷管保护结构改善了管口区的传热性能。降低了余热锅炉管口区金属材料的温度,减缓了高温腐蚀反应速度;新型双瓷管保护结构同时改善了管口区域内保护套管的温度“畸变”,因而减轻管口区材料的热应力破坏。     

Numerical investigation on latent heat thermal energy storage in a phase change material using a heat exchanger

The use of a heat exchanger using phase change material (PCM) is an example of latent heat thermal energy storage (LHTES). In this study, the charging of PCM (RT50) is studied in a double pipe heat exchanger. The designing of the heat exchanger needs to be optimized for operating and boundary conditions to store latent heat efficiently. The size of the equipment and the amount of PCM are also important to calculate the latent heat storage capacity of the LHTES device. In this study, the amount of PCM taken is quite high to avoid sensible heat transfer and to maximize the heat content of PCM. The charging process of PCM is numerically simulated using an enthalpy-porosity model. The study includes the effect of inlet temperature and flow rate of high-temperature-fluid (HTF) and concludes that both play an important role in determining the charging time. The continuous increase in inlet temperature of HTF can decrease the charging time of PCM in the heat exchanger. However, the continuous increase in the HTF flow rate cannot show the same effect. The charging time can only be minimized with a specified flow rate regime for a specific inlet temperature of HTF. These factors consequently affect the efficiency of the heat exchanger.     

Improvement of part-load performance of gas turbine by adjusting compressor inlet air temperature and IGV opening

A novel adjusting method for improving gas turbine (GT) efficiency and surge margin (SM) under part-load conditions is proposed. This method adopts the inlet air heating technology, which uses the waste heat of low-grade heat source and the inlet guide vane (IGV) opening adjustment. Moreover, the regulation rules of the compressor inlet air temperature and the IGV opening are studied comprehensively to optimize GT performance. A model and calculation method for an equilibrium running line is adopted based on the characteristic curves of the compressor and turbine. The equilibrium running lines calculated through the calculation method involve three part-load conditions and three IGV openings with different inlet air temperatures. The results show that there is an optimal matching relationship between IGV opening and inlet air temperature. For the best GT performance of a given load, the IGV could be adjusted according to inlet air temperature. In addition, inlet air heating has a considerable potential for the improvement of part-load performance of GT due to the increase in compressor efficiency, combustion efficiency, and turbine efficiency as well as turbine inlet temperature, when inlet air temperature is lower than the optimal value with different IGV openings. Further, when the IGV is in a full opening state and an optimal inlet air temperature is achieved by using the inlet air heating technology, GT efficiency and SM can be obviously higher than other IGV openings. The IGV can be left unadjusted, even when the load is as low as 50%. These findings indicate that inlet air heating has a great potential to replace the IGV to regulate load because GT efficiency and SM can be remarkably improved, which is different from the traditional viewpoints.     

Simulation of methane steam reforming in a catalytic micro-reactor using a combined analytical approach and response surface methodology

In this study, a steady-state analytical model for heat and mass transfer in a 2D micro-reactor coated with a Nickel-based catalyst is developed to investigate microscale hydrogen production. Appropriate correlations for each species’ net rate of production or consumption, mass diffusivity, and the heat of reactions are developed using a detailed reaction mechanism of methane steam reforming. The energy and species conservation equations are then solved for the reactive mixture coupled with the wall energy equation. Finally, the response surface methodology (RSM) is employed to study the effects of channel height, inlet velocity and temperature, wall thickness and conductivity, and external heat flux on CH4 conversion. It is found that the inlet gas temperature, among different parameters, has the most influence on the overall performance of the microchannel hydrogen production. Also, the maximum necessary heat of reforming reaction increases by 84% and 26% if the CH₄ conversion changes from 50% to 60% and 60% to 70%, respectively. The developed analytical simulation can be a useful tool for designing experiments in micro-scale hydrogen production.     

Dynamic response of a unitized regenerative fuel cell under various ways of mode switching

Mode switching is an important process in unitized regenerative fuel cells. The complex interactions of heat and mass transfer during the operation of mode switching have a significant effect on cell performance. Twelve different ways of mode switching were proposed by controlling inlet boundary conditions of supplies and operating voltage. Numerical simulations were applied to analyze the dynamic response of heat and mass transfer as well as electrochemical signals under the different ways of mode switching. Current density increased with mass fraction of reactants. Cell heat source had an instant response to current density, but the temperature was slow to respond to the heat source. Hydrogen‐side inlet velocity had minimal impact on mode switching. The time for cell reaching stability increased with the increase of voltage change time, and the time for current density, mass transfer, and temperature reaching stable values increased in order. Unitized regenerative fuel cell had similar dynamic response in the 2 period: cell temperature increased in the fuel cell mode and decreased in the water electrolysis mode after mode switching.     

Thermal performance of a solar-aided latent heat store used for space heating by heat pump

In this study, the cylindrical phase change storage tank linked to a solar powered heat pump system is investigated experimentally and theoretically. A simulation model defining the transient behaviour of the phase change unit was used. In the tank, the phase change material (PCM) is inside cylindrical tubes and the heat transfer fluid (HTF) flows parallel to it. The heat transfer problem of the model (treated as two-dimensional) was solved numerically by an enthalpy-based finite differences method and validated against experimental data. The experiments were performed from November to May in the heating seasons of 1992–1993 and 1993–1994 to measure both the mean temperature of water within the tank and the inlet and outlet water temperature of the tank. The experimentally obtained inlet water temperatures are also taken as inlet water temperature of the simulated model. Thus, theoretical temperature and stored heat energy distribution within the tank have been determined. Solar radiation and space heating loads for the heating seasons mentioned above are also presented.     

湿热地区波纹翅片管式表冷器的运行性能和调节能力研究

表冷器作为空调系统中空气的热湿处理设备在湿热地区应用广泛。然而,随着户式中央空调的发展,它与中小型冷水机的匹配及运行优化仍缺少指南。建立了某9排管空气-水逆流式波纹翅片管式表冷器的传热除湿数学模型。模拟结果分析表明:室外气象参数的影响显著,气温越高且露点温度越高,则其供冷和除湿能力显著提高;在广州和杭州六月份平均气象条件下,调节表冷器的供水温度、供水流速或管外风速,其供冷能力可分别在名义供冷能力的32%~95%以及45%~120%内调节,除湿能力也可相应随之得到调节;入口水温愈低,则变水量或变风量对表冷器供冷及除湿能力的调节作用范围越广;风速愈大且水流速度愈高,则变水温的调节作用范围越广;入口水温越低、风速越大或水流速度越大,则联合调节风量水量、水温水量或水温风量对其供冷和除湿能力的调节作用范围也越广。     

Artificial neural network modelling of the thermal performance of a compact heat exchanger

This paper reports the use of artificial neural network models to simulate the thermal performance of a compact, fin-tube heat exchanger with air and water/ethylene glycol anti-freeze mixtures as the working fluids. The model predictions were compared with experimental data over a range of flow rates and inlet temperatures and with various ethylene glycol concentrations. In addition, the inlet air flow was distorted by obstructing part of the inlet ducting near the front face of the exchanger. The artificial neural networks were able to predict the overall rate of heat transfer in the exchanger with a high degree of accuracy and in this respect were found to be superior over conventional non-linear regression models in capturing the underlying non-linearity in the data. Moreover the detailed spatial variations in outlet air temperature were also adequately predicted. The results indicate that appropriately trained neural networks can simulate both the overall and “local” characteristics of the compact heat exchanger. In addition the paper demonstrates how an alternative type of neural network, the so-called Self-Organising-Map (SOM), can be employed for heat exchanger condition monitoring by identifying and classifying the deterioration in exchanger performance which, in this case, was associated with different levels of inlet obstruction.     

基于前馈-自抗扰算法的换热器动态试验温度控制研究

为了使换热器试验测控系统满足动态换热试验中对温度控制的要求,分析了试验系统中被控温度对象动态模型,设计了前馈-自抗扰温度控制算法。动态换热试验对象为管壳式换热器,试验过程管程流体为强迫对流换热,壳程流体自然对流换热,同时管程流体循环利用,要求控制管程入口温度稳定。控制算法全面利用模型信息,将壳程温度作为管程温度控制中的干扰,为其设计前馈补偿,同时设计自抗扰控制算法,处理模型偏差问题。利用AMESim软件搭建系统模型,在Simulink中设计控制算法,进行了AMESim/Simulink联合仿真,通过对比多种控制算法,验证了在壳程温度变化干扰的换热过程中,使用前馈-自抗扰控制算法能够使管程入口温度波动更小,更快达到稳定。     

Unit-response function for ground heat exchanger with parallel,series or mixed borehole arrangement

A novel approach is presented that allows to predict fluid temperatures entering a Ground Heat Exchanger (GHE) for parallel, series and mixed arrangements of boreholes. The method determines at each time step the heat transfer rates occurring at each borehole so as to reproduce the fluid temperature at the GHE inlet for a specific borehole arrangement. The analytical finite line source model is used to compute the borehole wall temperatures, whereas the fluid temperatures are assumed to vary linearly along the pipes. The method requires to solve a linear system of equations at a small number of time steps. The different systems of equations for each arrangement are determined. A comprehensive 3D finite element numerical model shows good agreement with the computed fluid temperatures. The proposed approach is computationally very efficient. The fluid temperature unit response function can be convolved with any desired heat load to estimate fluid temperatures at the GHE inlet for a wide variety of scenarios.     

Negative buoyant plume model for solar domestic hot water tank systems incorporating a vertical inlet

Thermal stratification in solar energy storage tanks plays an important role in enhancing the performance of solar domestic hot water systems. The mixing that occurs when hot fluid from the solar collector enters the top of the tank is detrimental to the stratification. Mathematical models that are used for system analysis must therefore be able to capture the effects of this inlet jet mixing in order to accurately predict system performance. This paper presents a computational study of the heat transfer and fluid flow in a thermal storage tank of a solar domestic hot water system with a vertical inlet under negative buoyant plume conditions. The effects of parameters such as the fluid inlet velocity and temperature as well as inlet pipe diameter on the thermal mixing were considered. The work culminated in the development of a one-dimensional empirical model capable of predicting the transient axial temperature distribution inside the thermal storage tank. Predictions from the new model were in good agreement with both experimental data and detailed computational fluid dynamics predictions.     

Thermal response of a packed bed of spheres containing a phase-change material

A computational model of the transient thermal response of a packed bed of spheres containing a phase-change material (PCM) is presented. A one-dimensional separate phases formulation is used to develop a numerical analysis of the dynamic response of the bed which is subject to the flow of a heat transfer fluid, for arbitrary initial conditions and inlet fluid temperature temporal variations. Phase-change models are developed for both isothermal and nonisothermal melting behaviours. Axial thermal dispersion effects are modelled, including intraparticle conduction (Biot number) effects. Regenerative thermal storage applications involve flow reversals to recover the stored energy; this aspect of operation is included in the present model. Results from the model for a commercial sized thermal storage bed for both the energy storage and recovery periods are presented. Experimental measurements of transient temperature distributions in a randomly packed bed of uniform spheres containing a PCM for a step-change in inlet air temperature are reported for a range of Reynolds number.     

Heat flux analysis of a cylindrical steam reformer by a modified Nusselt number

A numerical method is used to investigate a steam reformer. The reactor is assumed as a porous medium, because it is filled with catalysts of a packed-bed type, and a pseudo-homogeneous model is incorporated for a chemical reaction model. The steam reforming (SR) reaction, water–gas shift (WGS) reaction, and direct steam reforming (DSR) reaction are assumed to be dominant reactions in the steam reformer. The difference in temperature between the inside and outside of the reactor is a driving force in heat transfer, and is affected by the amount of heat adsorption by an endothermic reaction. A modified Nusselt number (Nu_M) can represent the heat transfer rate of the endothermic reactor, and thus Nu_M can be used to describe the performance of the steam reformer. The SR reaction rate is sufficiently activated when Nu_M around the inlet region is greater than 10, and fuel conversion exceeds 0.9 when the difference in Nu_M value between the inlet area and outlet area is greater than 5. The correlation between fuel conversion and operating conditions has also been studied by using Nu_M.     

Heat Transfer Performance of an Edge-Shaped Finned Tube

The third-generation heat transfer technologies, such as three-dimensional fin and dimple, are still important means of improving energy efficiency and will continue to be challenging issues. This paper presents condensation heat transfer performance of an edge-shaped finned tube fabricated by a ploughing–extruding process. The edge-shaped finned tube integrates more than one heat transfer enhancement technology and can enhance the heat transfer capacity greatly. It is seen that the overall heat transfer coefficient and heat flux increase with inlet velocity of cold water increasing, and decrease with inlet temperature of cold water increasing, whereas the shell-side heat transfer coefficient decreases with inlet velocity of cold water increasing and increases with inlet temperature of cold water increasing. At the same inlet velocity, the shell-side heat transfer coefficient for the edge-shaped finned tube is improved by 5–7 times compared to that of a smooth tube. At the same temperature difference between wall and vapor, the shell-side heat transfer coefficient is also higher than what had been reported in the literature. The shell-side heat transfer coefficient of the edge-shaped finned tube decreases with the increase of fabrication parameter feed at the same inlet velocity or inlet temperature of cold water.     

Experimental and theoretical investigation of surface temperature non-uniformity of spray cooling

In this study, a test system for spray cooling, in which the heating surface temperatures were simultaneously measured by thermocouples and an infrared imager, was set up. A mathematical model of spray cooling heat transfer characteristics was presented based on the fundamentals of dynamics and heat transfer. The temperature distribution on the heating surface was investigated by the experimental and theoretical methods, the surface temperature non-uniformity and its influencing factors were analyzed. The predictions by the model coincided with the experimental results well, and a comparison was demonstrated with a deviation below 10%. It can be concluded that the surface temperature non-uniformity is influenced by the spray characteristics, nozzle-to-surface distance, inlet pressure, heat flux, spray angle and the system pressure. In the case of the same heat flux, the surface temperature non-uniformity can be reduced by the small spray angle, low system pressure, low nozzle-to-surface distance, and the high inlet pressure.     

深层U型地热井仿真模拟及取热性能研究

为充分考虑地热梯度、提高模型计算效率,采用分段方法建立闭式U型井换热器的传热解析模型,并编写程序实现模型的耦合计算。利用系统实际运行数据对模型进行精度验证,并对入口温度和循环流量2个运行参数对地热井取热性能的影响进行分析。结果表明：经与以光纤传感器实测数据在沿程方向上的对比,该模型的平均相对误差为7.3%,模型精度满足工程使用要求。在保证基础负荷需求的前提下,地热井入口温度保持在5~16 ℃之间,循环流量在80 m³/h以上时,可保证地热井处于高效运行区间。在该工况区间内,降低入口温度比提高循环流量能获得更好的热经济性,合理的入口温度和循环流量的匹配调节能使地热井的供热能力提高10%。     

The second law optimal path of a four-bed SO2 converter with five heat exchangers

The entropy production rate of a common industrial SO₂ converter was minimized, simultaneously varying the heights of four catalytic beds, the temperature differences across five intermediate heat exchangers, and the distribution of a fixed area available for heat exchange. The entropy production rate had contributions from the chemical reaction, the pressure drop and the heat exchange. The total area for heat exchange, inlet temperature, inlet pressure, inlet composition, outlet temperature, and outlet composition remained constant in the minimization. A new path of operation was found for the given constraints that saved 16.7% of the entropy production rate compared to an industrial SO₂ converter. There were changes in chemical, mechanical and thermal contributions to the entropy production. The savings can be taken out as higher quality heat output, lower quality heat input, or reduction in the total area for heat transfer. The optimum operating conditions also indicate that the same product can be obtained under milder thermal conditions than now. Furthermore, the requirement for catalyst, as measured by the total bed height, decreased with the area available for heat transfer, from 2.4 m at 2000 m² to 1.7 m at 6000 m². We conclude that there is a significant potential to improve the second law efficiency of the particular industrial SO₂ converter that is studied.     

Numerical analysis of latent heat thermal energy storage using encapsulated phase change material for solar thermal power plant

Thermal energy storage improves the load stability and efficiency of solar thermal power plants by reducing fluctuations and intermittency inherent to solar radiation. This paper presents a numerical study on the transient response of packed bed latent heat thermal energy storage system in removing fluctuations in the heat transfer fluid (HTF) temperature during the charging and discharging period. The packed bed consisting of spherical shaped encapsulated phase change materials (PCMs) is integrated in an organic Rankine cycle-based solar thermal power plant for electricity generation. A comprehensive numerical model is developed using flow equations for HTF and two-temperature non-equilibrium energy equation for heat transfer, coupled with enthalpy method to account for phase change in PCM. Systematic parametric studies are performed to understand the effect of mass flow rate, inlet charging system, storage system dimension and encapsulation of the shell diameter on the dynamic behaviour of the storage system. The overall effectiveness and transient temperature difference in HTF temperature in a cycle are computed for different geometrical and operational parameters to evaluate the system performance. It is found that the ability of the latent heat thermal energy storage system to store and release energy is significantly improved by increasing mass flow rate and inlet charging temperature. The transient variation in the HTF temperature can be effectively reduced by decreasing porosity.