Interaction of the transfer processes in semitransparent liquid droplets

The study presents the mathematical model of unsteady heat transfer in evaporating semitransparent droplets of non-isothermal initial state and the numerical research method, evaluating selective radiation absorption and its influence on the interaction of transfer processes. The relation of the transfer processes inside droplets and in their surroundings and the necessity of thorough research of these processes are substantiated. When modeling the combined energy transfer in water droplets, the evaluation of thermoconvective stability in evaporating semitransparent liquid droplets is presented; the influence of the droplet initial state on its heating and evaporation process is investigated. The influence of heat transfer peculiarities on the change of the evaporating droplet state is indicated. Main parameters, which decide the peculiarities of the interaction of unsteady transfer processes in droplets and their surroundings, are discussed. The results of the numerical research are compared to the known results of the experimental studies of water droplet temperature and evaporation rate.     

AE94.3A型燃气轮机进气冷却的应用可行性研究

对AE94.3A型燃气轮机燃气-蒸汽联合循环热力系统平衡进行研究进而发现,与同类型、同等级不同型号机组相比,AE94.3A型联合循环机组余热锅炉的排烟温度较高,排烟余热仍有进一步利用的空间。通过设计优化,扩大省煤器受热面,回收烟气余热加热给水,驱动热水型溴化锂制冷机制冷,用于机组满负荷调峰时的压气机进气冷却或厂房及办公区域空调供冷,对改善燃气轮机联合循环的运行性能,实现能源梯级利用,提高能源利用率和机组经济性运行起到了很大作用。     

Modeling of shell and tube heat recovery exchanger operated with nanofluid based coolants

The emergence of several challenging issues such as climate change, fuel price hike and fuel security have become hot topics around the world. Therefore, introducing highly efficient devices and heat recovery systems are necessary to overcome these challenges. It is reported that a high portion of industrial energy is wasted as flue gas from heating plants, boilers, etc. This study has focused on the application of nanofluids as working fluids in shell and tube heat recovery exchangers in a biomass heating plant. Heat exchanger specification, nanofluid properties and mathematical formulations were taken from the literature to analyze thermal and energy performance of the heat recovery system. It was observed that the convective and overall heat transfer coefficient increased with the application of nanofluids compared to ethylene glycol or water based fluids. It addition, 7.8% of the heat transfer enhancement could be achieved with the addition of 1% copper nanoparticles in ethylene glycol based fluid at a mass flow rate of 26.3 and 116.0 kg/s for flue gas and coolant, respectively.     

Interaction of transfer processes during unsteady evaporation of water droplets

The change of water droplets state is modelled numerically under various heat and mass transfer conditions during their unsteady evaporation. The modelling is performed using the method of combined analytic–numeric research of heat and mass transfer in a two-phase “droplets–gas” flow. The algorithm of an iterative research is constructed for the analytically obtained system of integral equations. Regularities of heat transfer process interaction are examined. The dependence of the droplet state change on its heating manner is determined. Unsteadiness and interaction of transfer processes, as well as selectivity of radiant absorption in water droplets are evaluated. It is indicated that cognition of the droplet state change regularities in the case of conductive heating is very important in determination of two-phase flow and in construction of an engineering research method.     

大型F级燃气蒸汽联合循环烟气余热利用探讨

探讨了大型F级联合循环机组利用烟气余热进行凝结水/给水加热、燃料气加热,进气冷却,以及利用烟气余热制冷和采暖方面的应用,分析表明,大型联合循环电厂烟气余热利用潜力较大,充分利用烟气余热,具有一定的节能意义.     

Analysis of evaporating mist flow for enhanced convective heat transfer

Enhancement of forced convective heat transport through the use of evaporating mist flow is investigated analytically and by numerical simulation. A two-phase mist, consisting of finely dispersed water droplets in an airstream, is introduced at the inlet of a longitudinally-finned heat sink. The latent heat absorbed by the evaporating droplets significantly reduces the sensible heating of the air inside the heat sink which translates into higher heat-dissipation capacities. The flow and heat transfer characteristics of mist flows are studied through a detailed numerical analysis of the mass, momentum and energy transport equations for the mist droplets and the airstream, which are treated as two separate phases. The coupling between the two phases is modeled through interaction terms in the transport equations. The effects of inlet mist droplet size and concentration on the thermal performance of the heat sink are analyzed parametrically. The results provide insight into the complex transport processes associated with mist flows. The simulations indicate that significantly higher heat transfer coefficients are obtained with mist flows as compared to air flows, highlighting the potential for the use of mist flows for enhanced thermal management applications.     

A discrete multicomponent fuel evaporation model with liquid turbulence effects

A new approach to simultaneously account for finite thermal conductivity, finite mass diffusivity and turbulence effects within atomizing multicomponent liquid fuel sprays has been developed in this study. The main contribution of this paper is to incorporate the liquid turbulence effect in modeling the boundary layer heat and mass resistance during multi-component droplet evaporation. The finite conductivity model is based on an existing two-layer film theory, where the turbulence characteristics of the droplet are used to estimate the effective thermal conductivity. The present paper extends the two-layer film theory formulation to include multi-component mass diffusivities within the droplet liquid phase. In this model four regions are considered: the interior region of the droplet, droplet-side interface, gas-side interface, and the surrounding gas phase. Approximate solutions to the quasi-steady energy and mass transfer equations were used to derive an explicit expression for the heat and mass flux from the surrounding gas to the droplet–gas interface, and within the multi-component droplet. Extension of the model to high pressures using the Peng–Robinson equation of state is also considered. The validation study was carried out for a bi-component decane/hexadecane fuel, followed by application studies of complex gasoline–ethanol blended fuels evaporating in hot gas environments.     

Energy saving analysis and thermal performance evaluation of a hydrogen-enriched natural gas-fired condensing boiler

Hydrogen-enriched natural gas (HENG) has attracted widespread attention due to its lower pollutant emissions and industrial decarbonization in the past decades. HENG combustion boosts the water content in the flue gas, which is highly favorable for condensing boilers to recover additional latent heat. The energy saving and thermal performance of a condensing boiler burning HENG were evaluated at a constant heat load of 2.8 MW in this study. The variations in combustion products and boiler efficiency were investigated based on the material balance and energy conservation. The heat transfer calculations were employed to evaluate the thermal performance of boiler heating surfaces. The energy recovery performance of the condenser was assessed via a thermal design method. Results show that H₂ enrichment enhances the radiation intensity of the flame due to the incremental triatomic gases with higher emissivity in the furnace. The heat absorption ratio increases with H₂ enrichment in the radiative heating surface, while it shows a reverse tendency in the convective heating surface. The condensing boiler efficiency based on lower heating value increases from 101.83% to 110.60%, the total heat transfer rate of the condenser increases from 2.77 × 10⁵ W to 4.61 × 10⁵ W, and the total area required decreases from 46.45 m² to 42.16 m², as the H₂ enriches from 0 to 100% under the exhaust flue gas temperature of 318 K. Although the amount of recoverable heat in the exhaust flue gas increases considerably after H₂ blending, the original condenser with natural gas as the designed fuel could meet the requirements of the heat recovery for HENG without increasing the extra heating surface. When the H₂ fraction is enriched from 0 to 100%, CO₂ emission intensity drops from 6.05 × 10⁻⁸ kg J⁻¹ to 0. This work may offer some theoretical references for the application and generalization of HENG condensing boilers.     

吸收式热泵在燃气采暖冷凝热回收中的应用

作者建立了一种利用吸收式热泵回收燃气锅炉冷凝热的系统，解决了供热回水温度高而难以回收烟气冷凝热问题，比现有的锅炉烟气冷凝热回收技术提高效率5％以上。计算分析结果表明，这一工艺的应用可产生显著的经济、节能和环保效益。     

Evaluation of retrofitting a conventional natural gas fired boiler into a condensing boiler

The exit flue gas temperature of a conventional gas fired boiler is usually high and a great amount of heat energy is lost to the environment. If both sensible heat and latent heat can be recovered by adding a condensing heat exchanger, the efficiency of the boiler can be increased by as much as 10%. In this paper, based on combustion and heat transfer calculations, the recoverable heat and the efficiency improvement potential of different heat recovery schemes at various exit flue gas temperatures are presented by performing design calculations. The payback period method has been used to analyze the feasibility of retrofitting a conventional gas fired boiler into a condensing boiler in a heating system in detail. The results show that the most economical exit flue gas temperature is 40–55 °C when a conventional natural gas fired boiler is retrofitted into a condensing boiler simply by adding a condensing heat exchanger. It is feasible to use the return water of a heating system as the cooling medium of the condensing heat exchanger because the return temperature varies with the ambient temperature and is lower than the dew point of the water vapor in the flue gas in most periods of a heating season in some regions, which has been verified by retrofitted case.     

低温烟气凝结换热及对天然气热效率的影响实验研究

为研究烟气露点附近及以下的低温烟气对流凝结换热规律与烟气换热对天然气利用热效率的影响,建立了烟气在翅片管换热器内对流凝结换热实验系统,研究了不同烟气温度、水蒸气含量对烟气凝结换热的影响,得出了烟气凝结换热实验准则关联式,分析对比了天然气利用热效率实验值与理论值.实验结果表明:当被加热水温度为23℃,烟气出口温度为73℃...     

Thermodynamic analysis of an LNG fuelled combined cycle power plant with waste heat recovery and utilization system

This paper has proposed an improved liquefied natural gas (LNG) fuelled combined cycle power plant with a waste heat recovery and utilization system. The proposed combined cycle, which provides power outputs and thermal energy, consists of the gas/steam combined cycle, the subsystem utilizing the latent heat of spent steam from the steam turbine to vaporize LNG, the subsystem that recovers both the sensible heat and the latent heat of water vapour in the exhaust gas from the heat recovery steam generator (HRSG) by installing a condensing heat exchanger, and the HRSG waste heat utilization subsystem. The conventional combined cycle and the proposed combined cycle are modelled, considering mass, energy and exergy balances for every component and both energy and exergy analyses are conducted. Parametric analyses are performed for the proposed combined cycle to evaluate the effects of several factors, such as the gas turbine inlet temperature (TIT), the condenser pressure, the pinch point temperature difference of the condensing heat exchanger and the fuel gas heating temperature on the performance of the proposed combined cycle through simulation calculations. The results show that the net electrical efficiency and the exergy efficiency of the proposed combined cycle can be increased by 1.6 and 2.84% than those of the conventional combined cycle, respectively. The heat recovery per kg of flue gas is equal to 86.27 kJ s^?1. One MW of electric power for operating sea water pumps can be saved. The net electrical efficiency and the heat recovery ratio increase as the condenser pressure decreases. The higher heat recovery from the HRSG exit flue gas is achieved at higher gas TIT and at lower pinch point temperature of the condensing heat exchanger. Copyright © 2006 John Wiley & Sons, Ltd.     

300MW循环流化床锅炉屏式受热面传热系数计算及其变化规律

根据300MW循环流化床（CFB）锅炉现场测试数据并结合以往CFB锅炉传热系数的研究成果,建立了屏式受热面烟气侧的传热模型,包括辐射传热模型和对流传热半经验公式．利用该模型对某300MWCFB锅炉在94％锅炉最大连续蒸发量（BMCR）工况下炉膛内屏式受热面的传热系数进行了计算,分析了屏式受热面管间节距、炉膛温度、工质温度、壁面黑度及烟气速度等因素对传热系数的影响．结果表明：烟气速度、炉膛温度和壁面黑度对传热系数的影响较大,所建立的传热模型能够合理地反映主要因素对CFB锅炉屏式受热面传热的影响．     

蒸发液滴内部非稳态流动的数值计算

在气液两相流VOF(volume of fluid,VOF)模型的基础上耦合CSF(continuum surface force,CSF)表面张力模型,建立了高温平板上的铺展液滴与高温空气中悬浮液滴蒸发过程中内部非稳态流动模型,对液滴蒸发过程中内部非稳态流动进行了研究。基于相变理论,采用用户自定义函数将流体相变模型加入非稳态流动模型中进行耦合计算,获得了高温平板上的铺展液滴与高温空气中悬浮液滴蒸发过程中的内部流动及变化过程。液滴蒸发过程中非稳态内部流动由液滴表面的温度梯度引发,Marangoni流动在液滴内部形成的时间非常短,流体从液滴表面高温区域流向低温区域。计算结果表明:高温平板上随着液滴蒸发的进行,液滴内部一直保持两个对称的涡流,Marangoni流动比较稳定;高温空气环境中随着液滴蒸发的进行,液滴内部四个涡流逐渐转变成两个对称的涡流;液滴内部温度分布因Marangoni流动加强传热而变得均匀,同时由于温度分布变得均匀,Marangoni流动被削弱。     

玉门炼化总厂节能潜力及节能途径分析

玉门炼化总厂拥有常减压、催化裂化、柴油加氢改质、酮苯脱蜡(脱油)、催化重整等多套装置,总加工能力达300×104t/a。由于装置加工流程长,产品加工深度大等问题,虽然实施了一些节能措施,但创新性和高技术含量的节能新技术应用较少,2008年全厂综合能耗仍高达85.33kg标油/t原料,与国内先进企业相比差距较大。为此,提出改造催化装置余热锅炉、提高加热炉热效率、开展热联合、综合优化动力系统、充分利用低温余热等节能改造方案。其中,催化装置余热锅炉采用模块化新型结构,以消除露点腐蚀,减少烟气阻力,强化传热能力,消除炉体振动,确保能量回收为主要改造目的;提高加热炉热效率主要从优选加热炉燃烧器,完善烟气热能回收、完善加热炉吹灰、降低散热损失以及调整工艺流程等方面入手。同时,应实现装置间的能量集成优化,采取热电联产工艺,减少低温热能耗损失。预计上述节能改造实施后,全厂综合能耗可降低11~21kg标油/t原料。     

含湿混合气体水平管外对流冷凝换热实验研究

回收烟气中的潜热和显热在提高锅炉效率和环境保护方面都具有重要意义。主要针对含湿混合气体在水平单管管外的对流冷凝换热进行了实验研究。通过对实验数据的分析,得到了烟气进口温度、冷却水进口温度、水蒸气的质量分数以及Re的变化对含湿混合气体在水平单管管外冷凝换热的影响。     

新型复合防腐表面烟气对流凝结换热实验研究

烟气对流凝结换热强化和换热表面防腐是天然气热能动力设备烟气余热回收利用关键技术。不同防腐表面耐腐蚀性能不同,且换热性能也不同。采用CCD高速摄像仪,对烟气在新型复合防腐表面上的凝结形态和凝结过程进行了可视化观测和换热实验研究,采用对图像边缘提取法,获得凝结液的边缘曲线。研究表明,烟气在新型复合防腐表面上的凝结为珠状凝结,凝结液珠最大粒径为0.2～0.28 mm,与其他表面形成的膜状凝结相比,在实验范围内,珠状凝结换热可提高约7倍。为增强烟气对流凝结换热和开发烟气冷凝余热回收利用技术提供了参考和依据。     

火电厂节能环保供热装置的设计及应用

排烟热损失是锅炉各项热损失中最大的一项,它直接影响锅炉效率及发电煤耗。一般情况下,排烟温度每升高10℃,煤耗将增加2 g/(kW·h)左右。因此,降低排烟温度、回收烟气热量对于节能降耗、提高经济效益具有重要的实际意义。对火电厂节能环保供热装置的设计及应用进行了探讨。该设备采用烟气余热回收技术,将高温排烟热量传递给空气,再进行城市及工业区供热,实现了热量的回收,节省了燃煤量,同时也减少了污染物的排放,还提高了锅炉效率,有利于城市及工业区的可持续发展。     

有机热载体炉烟气余热回收技术浅析

长期以来,有机热载体炉排烟温度偏高,造成大量的能源浪费,严重影响了锅炉运行的经济性。为减少热能损失,对烟气余热回收技术的可行性方案、工程应用实例进行了阐述;并指出通过锅炉烟气余热利用技术回收排烟中的显热和潜热,可以大大提高有机热载体炉的热效率,实现节能降耗。     

Heat Transfer and Flow Characteristics Inside Droplet Formed on Water Surface

Abstract

Dynamics and heat transfer of a silicone oil droplet formed on a water surface are investigated. The silicone oil droplet is heated by the water in a constant temperature container. Temperature and velocity fields inside the droplet are simulated in line with the experimental conditions. The influence of the droplet volume on the flow and heat transfer characteristics is also incorporated in the analysis. The oil droplet pinning and its geometric features for different droplet volumes are examined. Temperature predictions are validated with a thermal camera data. It is found that temperature predictions agree well with the thermal camera data. The constant temperature heating of the water container wall gives rise to two counter rotating circulation cells inside the water, which in turn modifies temperature and flow fields in the water. The flow direction occurs from the droplet top region towards the droplet–water interface. The heated fluid in the region close to the droplet–water interface is carried by the flow current to the droplet sides giving rise to temperature increase in these regions. The values of the Bond number attains greater than unity. The Nusselt and the Bond numbers increase with the droplet volume.