【作者】 崔永章；

【导师】 田茂诚；

【作者基本信息】 山东大学 ， 热能工程， 2011， 博士

【摘要】 在当今“节能减排”大环境下如何高效利用天然气的热能引起行业重视,烟气冷凝技术就是国内外研究热点之一。考虑工程背景和烟气冷凝技术特点,采用管内插入物来强化圆管内烟气对流凝结换热是备受关注的强化传热技术。本文以数值模拟、理论分析和实验研究等方法,对圆管内置折边扭带单相对流和对流凝结换热的传热与流动性能进行了较为深入的研究,获得的主要成果如下：提出了适合耐腐蚀的新型不锈钢折边扭带作为圆管内插入物,相比常规螺旋扭带具有成型容易、结构稳定、扭带表面无裂纹、满足抗腐蚀性要求等优点,解决扭带的稳固性和持久性。该折边扭带在高扭曲比、低雷诺数下,对管内混合气体的单相对流换热和对流凝结换热均具有显著强化作用。建立了低雷诺数下内置扭带管单相对流传热和流动三维数值模型,模型中采用三维隐式求解器,控制方程采用有限体积法进行离散,压力速度耦合采用SIMPLE法。湍流模型采用三种k-ε湍流模型和RSM模型进行对比,采用壁面函数法来考虑近壁面流动和低雷诺数影响。绘制不同网格节点数,进行了模型有效性和网格独立性分析,适合低旋流、低雷诺数流动的Realizable湍流模型和增强壁面函数与实验结果良好符合.经对螺旋扭带和折边扭带数值模拟对比,低雷诺数下扭带对传热强化显著,且折边扭带传热性能高于螺旋扭带,折边扭带阻力系数高于螺旋扭带阻力系数,且随扭曲比变化较大。高扭曲比下,折边扭带传热和流动性能比优于螺旋扭带。与螺旋扭带相比,折边扭带具有非对称的速度和切向速度分布,在折边长度内断面速度和切向速度呈现先增速后减速或相反的现象,加强了主流气体的混合,强化了换热。影响单相对流传热与流动性能的折边扭带主要结构参数是折边角度、折边长度和扭带与管壁间隙等。高雷诺数下不同折边角度扭带传热和流动性能趋向一致,低雷诺数下性能随折边角度增大而增大。折边角度引起传热强化的原因是切向速度和湍流强度增强。折边长度对传热影响不大,但对阻力系数影响较大,应选择折边长度大于扭带宽度的扭带以降低压力降。在相同进口质量流量条件下,增加扭带与管壁间隙后传热和阻力呈现先减少后增加,但传热与流动性能比随间隙增加而降低。其原因是间隙增大主流气流速度减少而引起传热和阻力降低,但随着间隙增大穿越扭带间隙气流速度增大,强化了扭带端部传热增强和阻力增加。具有不同折边长度和角度而扭曲比相同的扭带,传热和流动性能基本一致。建立了内置扭带管对流换热实验台,进行了以空气为介质、低雷诺数下对流传热与流动性能实验,实验与数值模拟的传热性能与阻力系数偏差均在±5%之内,验证了所建单相对流换热数值模型的可靠性,实验得到不同壁面温度、空气流量下内置折边扭带管对流换热系数和阻力系数关联式。通过对含少量不凝气体和不凝气体占多数的水蒸汽凝结换热对比研究分析,本研究水蒸汽含量在15-25%的混合气体对流凝结换热中对流换热和凝结换热相当,需考虑液膜状态和气体侧对流换热对凝结换热的影响。建立了简化的层流膜状凝结模型及水平内置扭带管内混合气体对流凝结换热的物理模型。根据实验观察,不同扭带结构参数下液膜的状态对对流凝结影响主要表现在：液膜的积存影响气流的流动,特别是扭带端部的液膜,阻碍气流穿越扭带端部。扭曲比越小液膜积聚越多,液膜滞留时间越长,特别是与扭带旋转方向与重力方向相对侧管壁；在低雷诺数下液膜主要靠重力控制,在高雷诺数下主要受气液界面切应力控制。液膜引起阻力增加主要是管内壁液膜所引起,因此采用凝结状态管表面阻力系数与单相流管表面阻力系数比来表征液膜粗糙度对传热和流动影响。在简化液膜模型的基础上,分别采用衰减因子法、扩散层理论模型和传质导率模型分析凝结换热机理,并根据内置扭带管内混合气体对流凝结特点提出各自准则方程。衰减因子法中提出以水蒸汽含量、气体流动和扭带扭曲比的关联式。扩散层模型中利用传热与传质类比得到混合气体侧有效传热系数,并引入新准则数Ln来表示混合气体露点、进口温度和壁面温度对凝结换热的影响。传质导率模型中主要考虑了液膜粗糙度和水蒸汽凝结引起的抽吸作用,以及扭带扭曲比、扭带间隙的影响。实验结果表明：传质导率模型误差较小,能较全面反映混合气体在扭带管内的对流凝结换热特性。建立了混合气体在内置扭带管内对流凝结换热试验测试系统,试验研究了水蒸汽含量、壁面温度、混合气体流速、进口温度等对对流凝结换热和流动性能影响。水蒸汽含量和壁面温度对凝结换热量影响显著,但对阻力系数影响不大。提高流速有利于对流凝结换热,但降低了单位气体的水蒸汽凝结量。一定范围内增加进口温度,也促进对流凝结换热。同时还实验研究了扭曲比和扭带与管壁间隙对混合气体对流凝结换热的影响,其主要是影响冷凝液的状态。低扭曲比下旋流增强,强化单相对流换热,对流凝结换热时引起液膜增厚、滞留时间长,导致凝结换热降低和阻力增加。考虑冷凝液的排泄,应选择高扭曲比扭带。扭带与管壁间隙除了影响气流穿越扭带端部强化扭带端部传热外,间隙大则冷凝液膜不易积存,穿越气流也有利于冷凝液排泄。高雷诺数下应选择较大间隙,低雷诺数下可选择较小扭带间隙。以空气和水蒸汽二元混合气体作为介质,建立混合气体在内置扭带管内对流凝结的三维数值模拟模型。混合物中物性是组分和温度的函数。并采用扩散层层流底层的传质理论建立水蒸汽在壁面凝结模型,编制水蒸汽凝结引起的质量源项、能量源项和组分源项,模拟分析了混合气体的组分传递和凝结特性。分析对比传热传质模型和凝结模型下对流传热、凝结和阻力性能,差别原因是沿管长水蒸汽凝结引起流速和物性变化造成的。在恒定进口速度条件下,模拟分析扭带与管壁间隙对对流凝结换热影响。间隙增大,对流换热先增加后减低,凝结换热先增加后基本恒定,阻力系数先增加后减少。实验和模拟结果表明,间隙为lmm扭带传热和流动性能最佳。也模拟分析了壁面温度、水蒸汽含量和进口温度三个重要运行因素对混合气体对流凝结换热的影响。总之,通过本文对内置折边扭带管内混合气体单相对流和对流凝结换热的规律进行了研究,分析扭带结构参数和混合气体运行参数对对流凝结传热与流动的影响,对该类型烟气冷凝换热器设计优化具有深广的应用价值,为进一步数值模拟和试验研究提供理论基础。更多还原

【Abstract】 Natural gas flue gas condensing technology was brought to the forefront to lower energy consumption and to reduce pollutants discharge. Considering project background, circular tube with tape inserts was used to enhance convection-condensation heat transfer for flue gas. Numerical simulation, mechanism analysis and experiment research was used to study convection and convection-condensation thermal-hydraulic performance of mixture with vapor in circular tube with edgefold-twisted-tape (ETT) inserts. The study laid the foundations of deep research to flue gas condensing heat exchanger and brought out theories in supporting of designing and applying this heat exchanger. The major works of this paper are as follows:Base on tape insert research, a new type twist tape was put forward. The ETT inserts produced with thinner ferrite stainless steel that can resist flue condensation corrosion. Comparing with classical spiral twisted tape (STT) inserts, main performance are easy make, structure strongly stability and durability, and no surface crack. Under higher twist ratio and transition flow conditions, ETT inserts has a strongly enhancement for gas convection and convection-condensation in tube.3D numerical simulation model was introduced to simulate air thermal-hydraulic performance in tube with ETT inserts under constant tube wall temperature and transition flow conditions. The numerical simulation was carried out using FLUENT that uses the finite volume method to solve governing equation. Implicit solver and SIMPLE pressure-velocity coupling algorithm were selected. A careful check for the grid-independence of the numerical solutions has been made to ensure the accuracy and validity of the numerical results. Comparing with Standard k-ε, RNG k-εand Realizable k-εand RSM turbulent model and three near wall treatment, simulation result of Realizable k-εand enhance wall function for lower swirl and low Re were satisfied with experiment result.Comparing STT and ETT inserts, ETT inserts thermal-hydraulic performance is superior to STT inserts under low Re and higher twist ratio conditions. The enhancement reasons are higher tangential velocity and main flow velocity profile. Tube with ETT inserts has a asymmetry velocity magnitude and tangential velocity profile, periodic change within an edgefold length, is first increases and later decreases in one side, or first velocity decreases and later increases in other side. Such velocity change intense gas mixing. It is found that main factors effecting heat transfer of ETT inserts are twist ratio and gap width between the tube and inserts. Twist ratio parameters include twist angle, edgefold length and twist width. Thermal-hydraulic performance under higher Re tends direction same, performance under lower Re increase as twist angle increase. Enhancement reasons for twist angle are higher tangential velocity and higher turbulent intensity. Edgefold length has little effect on heat transfer, but has strong effect on friction factor. From lower pressure drop, edgefold length should bigger than twist width. At the same inlet mass flux, heat transfer and pressure drop first decreases and later increases when gap between tube wall and twist increases. First main flow velocity decrease as gap increase that decrease heat transfer and pressure drop. Second, velocity between gap increases as gap increases that enhance heat transfer near twist tip. Performance of same twist ratio ETT inserts with different edgefold length and twist angle has same thermal-hydraulic performance ratio.An experimental system was set up to measure air convection heat transfer and pressure drop under constant wall temperature, experiment result are reasonable in agreement with simulation result within±5%, so numerical simulation model is valid for air convection heat transfer and pressure drop. Convective heat transfer coefficient and friction factor correlations were fitted under different wall temperature and inlet mass flux.Under vapor volume fraction within 0.15 to 0.25, convection heat transfer has the equal order of condensation heat transfer, so effect of condensation film and mixture must be considered; a simplified laminar condensation film model was build. Through experiment, condensation film decrease heat transfer area. Lower twist ratio, higher condensation film was colleted and longer condensation film resident. Under lower Re condensation film was controlled by gravity, was controlled by shearing force under higher Re. Friction factor was induced by condensation film on tube wall, so tube wall friction factor ratio under condensation and convection condition can indicate film roughness.Degradation factor method, diffusion layer method and mass transfer conductance method were used to analyze condensation mechanism. According vapor condensation in tube with ETT inserts, Correlation with degradation factor method is function of vapor volume fraction and Reynolds and twist ratio. Bases on heat and mass transfer, mixture effective heat transfer coefficient was derived, a new dimension parameter Ln was put forward to consider effect of vapor volume fraction and inlet temperature and wall temperature. In mass transfer conductance method, film roughness and suction effect by condensation was introduced, and also effect of twist ratio and gap between tube and tape inserts.An experimental system was set up to convection-condensation heat transfer and pressure drop for mixture with vapor. Wide investigation were made to study effect of vapor fraction and wall temperature and inlet temperature, and also twist ratio and gap between tube and wall. Under convection condition, heat enhanced by intensive swirl velocity of low twist ratio inserts. Under convection-condensation conditions, film thickness becomes thicker and resident time become longer by intensive swirl flow, so condensation heat transfer was decreased and pressure drop was increased. Higher gap can enhance flow velocity through gap and condensation film can be discharged easily.Neglecting film thickness,3D numerical simulation model was introduced for convection-condensation of binary mixture of air and vapor in tube with ETT inserts. A wall condensation model was set up with diffusion layer theory, mass source and energy source and species source were programmed with UDF. Mixture Physical Properties are function of composition and temperature. Comparing heat and mass transfer model and condensation model, difference of thermal-hydraulic performance are owing to velocity decrease and physical properties change by condensation. At the same inlet velocity condition, convection heat flux and friction factor increase first and decrease after as gap increase, but condensation heat flux increase first and then retain constantly. The effects of changes to inlet vapor volume fraction, and wall temperature, and inlet mixture temperature are presented and discussed.In this paper, convection-condensation thermal-hydraulic performance are presented, especially effect factor are ETT inserts structure and mixture parameter. It has huge applied value in flue condensing heat exchanger design optimization. Basis is supplied to further numerical simulation and experiment study.更多还原