Investigation On Heat Extraction Mechanism And Steam-Water Circulation Characteristics Of Heat Exchanger Tubes Embedded On Both Sides Of The High-Temperature Zone In Thermal Flow Reversal Reactor

Thermal flow reversal reactors（TFRRs）,as the efficient approach to the utilization of ventilation air methane,have significant advantages in heat recovery due to high-temperature combustion.The heat extraction process of the embedded heat transfer tubes on both sides of the high-temperature zone of the TFRR is not only accompanied by periodic changes of the temperature field outside the tubes,but also involves convection,radiation,and phase change processes inside the tubes,and the heat extraction mechanism is extremely complex.Therefore,this complex heat transfer process is investigated experimentally and numerically in this paper.The synergistic mechanism of the thermal flow field outside the tubes and the flow boiling heat transfer process inside the tube with different operating parameters were systematically analyzed,which provided the theoretical basis for the effective heat extraction and stable operation of the TFRR.A heat extraction experimental device with the embedded heat exchange tubes under the quasi-steady state was built,which was used to study the heat transfer characteristics of the embedded horizontal serpentine tubes under the heat-load flow field in a narrow space.The influence law of flue gas temperature outside the tube,mass flow rate,and dryness of the working medium inside the tube on the heat transfer characteristics of the heat exchanger tubes was analyzed.The results showed that when the working medium inside the tube was single-phase liquid,the heat extraction of the heat exchanger tube was enhanced by 4.95 times,and the effective heat transfer coefficient was improved by 8.1times with the increase of flue gas temperature（713 K～1113 K）and mass flow rate（170kg/（m²·s）～1132 kg/（m²·s））.When the two-phase flow pattern in the tube was stratified-wave flow,the local heat transfer coefficient was increased by 17%;when the two-phase flow pattern in the tube was annular flow,the local heat transfer coefficient was reduced by9 times.Based on the experimental data,Martinelli parameter X_tt was used to describe the boundary layer of forced convection heat transfer,and an improved prediction model of boiling heat transfer in the embedded serpentine heat exchanger tube under forced circulation was established.The calculated data was within the error range of±20%,and the maximum error difference was 2.44%by comparison of the experiment,which met the prediction requirements under the experimental conditions.A mathematical model of heat extraction by the embedded heat transfer tubes on both sides of the high-temperature zone of the TFRR was developed,and the heat transfer characteristics of the oxidation bed were investigated by simulation.The validation was performed with experiments in the TFRR,and the average error of the temperature profile of the oxidation bed was 7.58%.The calculation has run for 59 reversal cycles to reach the cyclic steady state（CSS）.The temperature distribution of the oxidation bed shows a significant asymmetry at the heat extraction zones on both sides,as well as the temperature in the upstream heat extraction zones,is higher than that in the downstream.The temperature distribution in the cross-section of the oxidation bed shows fluctuations,with the peak of the fluctuations occurring at the shunt ceramic and the valley at the position of the embedded heat exchanger tubes.The stable heat extraction conditions are determined based on the temperature of the oxidation bed in the CSS and the heat transfer rate of the embedded heat exchanger tubes,and the influential factors of the stable heat extraction of the embedded heat exchanger tubes on both sides of the high-temperature zone are studied.The results show that the onset of methane reaction is located at the heat extraction zone when the upstream embedded heat exchanger tubes are in the stable heat extraction state;the heat extraction stability of the downstream embedded heat exchanger tubes is dependent on the peak combustion temperature.When the inlet methane concentration is greater than 0.6 vol.%and the feed air velocity is less than 0.46 m/s,stable heat extraction can be achieved on both sides of embedded heat exchanger tubes.Under stable heat extraction conditions,the peak CO concentration generated by the methane combustion process and the heat storage efficiency of the oxidation bed are reduced simultaneously by the embedded heat exchanger tubes for heat extraction by 59.1%and 14.3%,respectively.The dynamic heat extraction characteristics of the embedded heat exchanger tubes were investigated by simulation based on the heat extraction model of both sides of the high-temperature zone of the TFRR.The results show that the fluctuations of the heat transfer rate on both sides of the heat exchanger tubes are characterized by anti-phase and different amplitudes.The heat transfer rate of the heat exchanger tubes embedded in the upstream heat extraction zones is gradually reduced,and the heat transfer rate of the downstream tubes is gradually increased,with a difference of half a reversal cycle（180°phase）between them.The heat exchanger tubes in the downstream heat extraction zones are the main heat extraction tubes,whose heat transfer rate accounts for about 74%～99%of the total heat extraction rate of the heat exchanger tubes on both sides during the steady heat extraction condition.With the increase of inlet methane concentration（0.2 vol.%to0.8 vol.%）and feed air velocity（0.15 m/s to 0.6 m/s）,the heat transfer rates of the downstream heat exchanger tubes are improved by 2.52 times and 6.11 times,respectively.The heat extraction efficiency,i.e.,the ratio of total heat extraction to oxidation heat release,of the embedded heat exchanger tubes in the operating condition can be up to61.72%at the CSS.Based on the numerical calculation results,the correlation equation for predicting the effective heat transfer coefficient of the embedded heat exchanger tube is established,and the relative error range is-9.57%～7.26%,which can effectively predict the heat extraction of the embedded heat exchanger tubes.Based on the VOF model of the Euler-Euler method with surface tension and interphase heat and mass transfer as the source terms and periodic transient heat flux as the boundary condition,a heat transfer model of flow boiling in the horizontal serpentine tube under forced periodic heat load was established to study the transient response characteristics of boiling in gas-liquid two-phase（i.e.,steam-water）flow in the tubes.Comparing the experimental results,the maximum deviation was less than 5%,which proved the reliability of the model.The two-phase flow pattern in the tubes includes bubble flow,slug flow,plug flow,and stratified flow.With the anti-phase fluctuation of heat load,the flow boiling processes in the heat exchanger tubes on both sides shows anti-phase symmetry,and the boiling onset of the working medium moves periodically in the inlet section.Under the forced periodic heat load,the flow boiling fluctuation behavior of two phases occurs in the tubes.The thermodynamic characteristics at the onset and disappearance points of the flow boiling fluctuation behavior are investigated,and it is found that super-cooled boiling accounted for a larger proportion at the onset and disappearance point moments.The marginal maps of flow instability under variable heating power are presented by the dimensionless number of sub-cooling number N_sub and phase change number N_pch to determine the range of stable operating conditions.When the thermal equilibrium quality x_e≤0,it is the flow region of the single phase;when 0<x_e≤0.043,it is the flow boiling fluctuation region of the embedded heat exchanger tubes under forced periodic transient heat load;when x_e>0.043,it is the region where the flow boiling fluctuation hysteresis behavior occurs due to the mass flow rate drift in the tubes under decreasing heating power.The transient response characteristics of anti-phase fluctuations occur in the heat exchanger branches under the heat load of forced periodic anti-phase fluctuations while weakening the fluctuation amplitude of flow boiling in the collecting tube,in which the fluctuation amplitude of mass flow rate is weakened by 84%at maximum.When the disappearance point of the flow boiling fluctuation behavior in the heat exchanger tubes embedded in the upstream heat extraction zone and the onset point in the downstream tubes intersect,a continuous steam output can be ensured in the collecting tube,which confirms the feasibility of heat extraction from both sides of the embedded heat exchanger tubes in terms of the system steam-water circulation characteristics.