火电厂换热器火积耗散分析

基于描述热量传递能力的物理量——火积,定义了评价换热器传热性能的火积耗散原理.同时,推出了换热器火积耗散的公式,并且通过代人数据计算,得出了火电厂各级加热器的火积耗散,并进行分析,对优化换热器的性能有一定的指导作用.     

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。     

螺旋折流板管壳式换热器壳程传热性能及压降的研究

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壳程的阻力的对比,同时通过实验方法对25°、40°螺旋角的螺旋折流板和弓形折流板换热器进行了壳程传热性能和壳程阻力的研究,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壳程阻力比弓形折流板换热器的壳程阻力小。     

圆柱形内肋强化换热数值模拟与火积耗散分析

对内肋管内部流体的湍流换热过程进行了数值模拟,讨论了肋高和肋的轴向夹角对换热的影响。相比于普通圆管,内肋圆管内的传热性能明显得到提高。无量纲肋高度和角度分别为0.8°和40°时传热效果最佳,而在0.1°和40°时换热与阻力的比值(Performance Evaluation Criteria,PEC)最大,综合换热性能最佳,可用于强化地源热泵地埋管换热。此外,本研究从火积耗散与传热效率的角度分析了内肋强化传热机理,得到管壁冷却管内流体的火积传递效率计算式,为内肋管强化换热的深入分析提供了依据。     

球突翅片的传热流动特性及等效热阻数值分析

为了提高翅片管换热器的传热系数和减小压降,提出了一种球突型翅片,通过数值模拟研究其传热与流动性能,同时应用(火积)耗散理论对其传热的不可逆性进行分析。计算结果表明:与平片相比,其传热能提高26.21%～39.53%,而阻力系数仅提高16.62%～27.04%,同时综合性能增加16.54%～32.56%;这说明该翅片具有高传热系数低压降的特点,是一种性能优良的翅片。通过(火积)耗散分析可以看出:球突翅片的等效热阻减小,其传热的不可逆性减弱。     

不同折流板换热器的传热与流阻性能对比

以柴油(水)为工质,在流量范围为4m^3／h≤W0≤19m^3／h内,用两种不同壳体结构的折流板换热器进行对比试验。研究得到了两种换热器的壳侧传热膜系数和流动阻力随流量变化的关系曲线。研究结果表明：在该试验条件下,螺旋折流板换热器的壳侧传热膜系数比弓型折流板换热器高33％-136％,具有较好的强化传热效果,而壳侧流动阻力则比弓型折流板换热器低15％-35％。螺旋折流板换热器具有较好的传热与流阻特性,用于石油、化工等领域具有广阔的前景。     

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。     

折流板位置对换热器性能的影响

采用FLUENT数值模拟方法,研究了简化模型下弓形折流板和螺旋折流板换热器,对应于不同间距/螺距时,流动参量的变化对换热器整体流动与传热性能的影响,进而研究非等距换热器.结果表明,两种结构对应的壳程压力损失和换热系数均随壳程流量的增加而增大,而螺旋折流板结构单位压降下换热系数大于弓形折流板,并且其性能受折流板螺距变化的影响较小,体现了螺旋折流板结构的优越性.为进一步研究非等距型换热器提供了依据.     

螺旋折流板换热器的制造技术研究

螺旋折流板换热器是一种新型高效换热器,具有壳程流动阻力小,换热效率高,抑振和防垢性能好等优点,已经引起了越来越多研究者的关注.介绍了非连续螺旋折流板换热器和连续螺旋折流板换热器的制造技术,可以更好地促进螺旋折流板换热器的研究.     

双壳程管壳式换热器的传热和阻力特性

针对弓形折流板管壳式换热器流动死区大,压降高的不足,提出外螺旋折流板内斜百叶折流板的双壳程管壳式换热器结构,外螺旋角为15°～40°,内斜百叶折流板倾角为45°。通过三维数值模拟,研究其传热和阻力特性,获得其局部流场,并与传统弓形折流板换热器进行了对比,同时分析了外壳程螺旋折流板不同倾角对其性能的影响。结果表明:双壳程管壳式换热器的壳侧流场分布均匀,流动死区减小,综合性能高于相同壳径和管束布置的弓形折流板管壳式换热器,外螺旋角为30°时,单位压降下的传热系数平均提高了24. 4%。当外螺旋角为20°时,该换热器具有最好的综合性能。     

Experimental exergy and entransy analyses on designed and fabricated crossflow heat exchanger

A crossflow heat exchanger (CFHEx) is designed and fabricated in a workshop. For designing this heat exchanger (HEx), the number of passes, frontal areas, HEx volumes, heat transfer areas, free-flow areas, ratios of minimum free-flow area to frontal area, densities, mass flow rates of flowing fluids, maximum/minimum heat capacities, heat capacity ratio, outlet temperatures of hot/cold fluids, average temperatures, mass velocities, Reynolds numbers, and convective heat transfer coefficients are evaluated by considering Colburn/friction factors. After fabrication of the HEx, effectiveness, exergy destruction, entransy dissipation, entransy dissipation-based thermal resistance, entransy dissipation number, and entransy effectiveness for hot/cold fluids sides are found at different flow rates and inlet temperatures of fluids. By experimental results, optimum operating conditions are found, which gives maximum effectiveness and entransy effectiveness but minimum rates of exergy destruction, entransy dissipation, entransy dissipation-based thermal resistance, and entransy dissipation number for the fabricated CFHEx. This study is concluded as follows: minimum exergy destruction and entransy dissipation rates (ie, 3.061 kJ/s·K and 1125.44 kJ·K/s, respectively) are found during experiment 2. Maximum entransy effectiveness of hot/cold fluids (ie, 0.689/0.21) is achieved in experiment 1. Moderate values of entransy dissipation number (ie, 4.689), entransy dissipation-based thermal resistance (ie, 0.04 s·K/J), exergy destruction (ie, 3.845 kJ/s·K), and entransy dissipation (ie, 1374.04 kJ·K/s) rates are found during experiment 1. Maximum effectiveness (ie, 0.4) for the fabricated HEx is also obtained through experiment 1. After comparative analyses, it is found that experiment 1 provides optimum results, which shows the best performance of the fabricated HEx.     

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.     

A new approach to analysis and optimization of evaporative cooling system II: Applications

After introducing the concepts of moisture entransy, moisture entransy dissipation and thermal resistance based on moisture entransy dissipation (TRMED) in part I of this study, we further analyze several direct/indirect evaporative cooling processes based on the above concepts in this part. The nature of moisture entransy, moisture entransy dissipation and TRMED during evaporative cooling processes was reexamined. The results demonstrate that it is the moisture entransy, not the enthalpy, that represents the endothermic ability of a moist air, and reducing the entransy dissipation by both enlarging the thermal conductance of heat and mass transfer, and decreasing the temperature potential of the moist air, i.e. the difference between the dry-bulb temperature of moist air over its dew-point temperature, will result in a smaller system TRMED, and consequently a better evaporative cooling performance. Then, a minimum thermal resistance law for optimizing evaporative cooling systems is developed. For given mass flow rates of both moist air and water, with prescribed moist air and water conditions, minimizing the TRMED will actually lead to the most efficient evaporative cooling performance. Finally, the thermal conductance allocation for an indirect evaporative cooling system is optimized to illustrate the application of the proposed minimum thermal resistance law.     

Optimization for a heat exchanger couple based on the minimum thermal resistance principle

Following the brief introduction to the concept of a physical quantity, entransy, the equivalent thermal resistance of a heat exchanger couple is defined based on the entransy dissipation. The minimum thermal resistance principle is applied to obtain the optimal heat capacity rate of the medium fluid and the optimal allocation of heat exchangers thermal conductance, which correspond to the maximum heat transfer rate in the heat exchanger couple. In addition, analytical expression for the optimal heat capacity rate of the medium fluid is derived, whose reciprocal equals the sum of the reciprocal of the individual heat capacity rate of the hot and cold fluids, just like the case of two electrical capacitors in series. Numerical results in the variation of the thermal resistance and the heat transfer rate with the medium fluid heat capacity rate or the thermal conductance allocation agree with the theoretical analyses. Finally, for comparison, the entropy generation rate is also calculated to obtain its relation with the thermal performance of the heat exchanger couple. The results show that there is no one-to-one correspondence of the minimum entropy generation rate and the maximum heat transfer rate. This indicates that the minimum entropy generation principle cannot be used for optimizing the heat exchanger couple.     

Experimental Performance Comparison of Shell-Side Heat Transfer for Shell-and-Tube Heat Exchangers with Different Helical Baffles

In this study, experiments were carried out to study the effects of baffle overlap proportion on the shell-side flow resistance and heat transfer performance of the shell-and-tube heat exchangers with helical baffles (STHXsHB). Three STHXsHB with an overlap proportion of 10% and helix angles of 20°, 30°, and 40° were tested. Comparisons were made of the experimental data of the STHXsHB with the same helix angles but 50% overlap proportion. The theory of entransy dissipation was employed to evaluate the irreversible loss in STHXsHB with different helix angles and overlap proportions. The results indicated that both the baffle overlap proportion and the helix angle have a great effect on the shell-side flow resistance and heat transfer. For a given helix angle, the comprehensive performance of STHXsHB with small overlap proportion is always better than that with large overlap proportion at the same mass flow rate or Reynolds number on the shell side. However, for the same heat transfer area, working conditions, and helix angle, the STHXsHB with large baffle overlap proportion has less irreversibility in the heat exchange process, according to the theory of entransy dissipation. In addition, experimental results demonstrated that the configuration of the relatively large helix angle and baffle overlap proportion is the preferred alternative in STHXsHB.     

Two energy conservation principles in convective heat transfer optimization

Improving heat transfer performance is very beneficial to energy conservation because heat transfer processes widely existed in energy utilization systems. In this contribution, in order to effectively optimize convective heat transfer, such two principles as the field synergy principle and the entransy dissipation extremum principle are investigated to reveal the physical nature of the entransy dissipation and its intrinsic relationship with the field synergy degree. We first established the variational relations of the entransy dissipation and the field synergy degree with the heat transfer performance, and then derived the optimization equation of the field synergy principle and made comparison with that of the entransy dissipation extremum principle. Finally the theoretical analysis is then validated by the optimization results in both a fin-and-flat tube heat exchanger and a foursquare cavity. The results show that, for prescribed temperature boundary conditions, the above two optimization principles both aim at maximizing the total heat flow rate and their optimization equations can effectively obtain the best flow pattern. However, for given heat flux boundary conditions, only the optimization equation based on the entransy dissipation extremum principle intends to minimize the heat transfer temperature difference and could get the optimal velocity and temperature fields.     

Analysis of two‐stage waste heat recovery based on natural gas‐fired boiler

Waste heat recovery (WHR) is crucial to the efficiency improvement of natural gas‐fired boiler systems. Two‐stage WHR systems based on the natural gas‐fired boiler were analyzed from the viewpoints of thermal efficiency and heat transfer irreversibility. An overall entransy dissipation‐based thermal resistance was derived to evaluate the irreversibility of WHR, including the entransy dissipations during condensation and in absorption heat pump (AHP). Compared with the basic WHR system, the two‐stage WHR systems have higher boiler efficiency and less irreversibility. The air‐humidified system recycles both the heat and vapor in flue gas, while the unutilized latent heat in the recovered vapor causes the boiler to be less efficient than the AHP system. Investigation on heat exchanger effectiveness of two‐stage WHR systems illustrated: in the two‐stage WHR system with air humidification, the increasing effectiveness of both heat exchangers could effectively increase boiler efficiency and reduce heat transfer irreversibility. In the two‐stage WHR system with AHP, boiler efficiency has a local optimum when the dew point occurs near the outlet of the first heat exchanger; increasing the second heat exchanger effectiveness is more efficient in improving boiler efficiency. The present work may provide available references and guidance for the design and optimization of the two‐stage WHR systems.