Study On Heat Transfer Characteristics And Mechanism Of Heat Exchanger Within Honeycomb Ceramic Oxidation Bed

The emission of coal mine ventilation air methane(VAM)is more than 2.0×1011m3 every year in China,which can be utilized effectively to produce steam with thermal reverse-flow oxidation.Due to the intermittent shift of airflow direction in the oxidation bed,the temperature and velocity field of the heat extraction zone that located at both sides of the high temperature area in oxidation bed,varies periodically with air reversing,of which the heat transfer mechanism is quite complicated.It is of important scientific significance to study the transient heat transfer mechanism of the finned tube in the small space formed by the oxidation bed honycomb ceramics and split-flow honycomb ceramics,and the effect of operational parameters,which may also applys a theoretical base for the design of heat extraction system in coal mine VAM oxidation instruments.This research aims at revealing transient heat transfer mechanism of finned tube insmall space formed by honycomb ceramics,in which experimental and simulation methods are both adopted.An reverse-flow oxidation bed experimental system was built,on which temperature distribution of the oxidation bed and the temperature variation at the inlet of heat extraction zone was obtained,then a qusi-steady heat extraction experimental system and simlation model were built,on which the compound heat transfer mechanism without thermal storage and release of split-flow honycomb ceramics.Trasient heat extraction characteristics was finally studied on the reverse-flow oxidation heat extraction experimental system and with simulation model,the result of that was also compared with those obtained from qusi-steady research,so that the function of split-flow honycomb ceramics' thermal storage and release was revealed.The main results are as following:1)The thermal storage and release of honycomb ceramics forms a central-high temperature distribution under most ofthe conditions,and a heat extraction zone inlet temperature fluctuates like the sine wave(except for the VAM concentration of 1.5%.).2)The effect of inlet temperature and Re on heat transfer characteristics was researched by experimental method,and a experimental function for heat transfer of H finned heat exchanger was achieved.3)The radiation heat transfer component was discriminated from the convective component by comparing the heat transfer characteristics between silver plated tube and ordinary tube,and a radiation contribution rate was adopted to evaluate the heat transfer component.Under the conditions adopted by the experiments,the contribution rate is restraint within 0.48?0.61.and the heat transfer coefficient of the finned tube is 1.47 times of that in vacant bed.4)The larger convective heat transfer coefficient occurs at the positions,the connecting line of which to the tube center form an angle of 83°?127° with main stream direction,while the poor convective heat transfer region is formed near the recirculation zone near the lee side of the tube.The distribution of the angle coefficient caused by the shielding effect of fins and the position relationship between H finned tube and the surrounding ceramics forms the increasing distribution characteristics of the radiant heat transfer coefficient from the root to the top of the fin.The distribution of the angular factor caused by the shielding effect of fins and the position relationship between fins,central tube and the surrounding ceramics form the increasing distribution characteristics of the radiation heat transfer coefficient from the root to the top of a fin.5)The reverse of air flow causes the destruction of original velocity field and boundary layer,and the establish of new ones,this process lasts as long as 12s,which lead to a sharp increace of average heat transfer coefficient and a drop of radiation contribution rate.6)Split-flow honeycomb ceramic exhibits significant temperature hysteresis effect,which brings down the average heat transfer under unsteady condition to a level only 0.25 of that under qusi-steady conditions?Through the experimental study on the operating parameters of different methane concentration,inlet air velocity and operating cycle time,experimental correlation of the average heat transfer coefficient is obtained,which provides theoretical support and design basis for the research of heat exchanger under the periodic heat load condition.