| 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.
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