| As one of the clean energies,natural gas is widely used with the development of economy and society.As the majority component is methane,the natural gas can produce lots of vapor by burning in the natural gas-fired boilers and humidified gas turbine cycles.The latent and water recovery from the humid flue gas of natural gas combustion can increase the overall system efficiency,and it is of great significance for the environmental protection and economy of natural gas utilization.Due to the advantages of no limitation for the dew-point temperature,high recovery,and wide suitability,the system of open cycle absorption heat pump(OAHP)is an effective way to recover the water and latent heat simultaneously.The performance of the absorber,which is the key component of OAHP,is important to the overall system efficiency,equipment size,and investment cost of OAHP.Of the various absorber,the falling-film horizontal tube absorber is applied in the OAHP for the advantages of high heat and mass transfer coefficient,simple structure,low pressure loss and high recovery.The absorption process of falling-film over horizontal tubes comprises the coupled flow,heat transfer,and mass transfer between the absorbent solution and the humid flue gas.Furthermore,large number of non-absorbable gas exist in the humid flue gas,which has important influence on the absorption process.Therefore,it is of great significance to investigate the coupled heat and mass transfer characteristics of the absorption process in the presence of humid flue gas,and summarize the matching rules of operating parameters,structure parameters,and the absorption performance of the falling-film horizontal tube absorber,which is suitable for latent heat and water recovery from humid flue gas.Aiming at the above problems,both the numerical modes are established and the experimental research were carried out,and the main works and conclusions were as follows:(1)A three-dimensional,two-phases transient flow model for the falling-film horizontal tubes flow process in the presence of counter-current flow was developed.The experimental research for model verification was performed.The relative error between the calculated results and the experimental data was within 10%.The model could accurately reflect the complex inter-tube flow behavior and predict the inter-tube flow mode transition rules.Using this model,the effects of counter-current gas flow rate in the range of 0.2752?0.5375 m s-1 on the transitional Reynolds number and the circumferential film thickness distribution in all three basic flow modes were analyzed in detail.And the axial film thickness distribution in all three basic flow modes were analyzide.The results show that wth the counter-current gas flow rate increasing,the transitions between droplet and droplet-column,droplet-column and column,and column-sheet and sheet require a higher Reynolds number,whereas the transitions between column and column-sheet require a lower one.In the circumfence direction,in both the droplet and column flow modes,the circumferential angle in which the thinnest film is located changes from the range of 90° to 100° to the range of 70° to 80°when the counter-current gas flow is imposed,whereas in the sheet flow mode,the location which is located in the 120° has no change.In the axial direction,the maximum film thickness is more than twice that of the minimumn in the droplet flow mode.In the column flow mode,regular stable-crest-stable distribution is shown around the upper perimeter of the tube.In the sheet flow mode,rgular stable-crest-stable distributon is shown all over the tube.(2)For two traditional falling-film horizontal tube absorption process-pure vapor cooled tubes and humid flue gas adiabatic tubes,the two-dimensional,two-phases transient absorption models were established respectively based on the flow model.A humid flue gas falling-film adiabatic horizontal tube absorption experimental apparatus was designed and built for validation of the transient model.The relative error between the calculated results and the experimental data was within 9.5%.The models could effectively predict the local distribution characteristics of the liquid and gas parameters.Using the models,the variation of the liquid and gas average parameters in different flow regions in the two basic flow modes with flow time were analyzed firstly,as well as the variation of the local heat transfer rate distribution in the two basic flow modes with location thoroughly.The results show that the lower the tube located,the absorption mass increases in the pure vapor cooled absorption process,whereas decreases in the humid flue gas adiabatic tubes.The average mass transfer rate in the falling-film regions in the droplet and sheet flow mode may be almost 6 and 10 times that in the inter-tube regions.(3)Based on the aforementioned three models,both a two-dimensional and a three-dimensional transient absorption models were established for humid flue gas falling-film cooled horizontal tube absorption process.The relative error between the two-dimensional calculated results and the three-dimensional calculated results was within 5%,and the two-dimensional calculated results and the experimental data conducted by our group was within 9.5%.Using the three-dimensional model the variation of the solution and humid flue gas average parameters in different flow regions in the three basic flow modes with time were firstly analyzed,as well as the variations of the local parameters and mass transfer rate both along the tube axis direction and the flow direction in the three basic flow modes.The results show that about 3.0 s,2.0 s and 2.0 s are needed to achieve a relatively stable state for the droplet,column and sheet flow modes.After that the average parameters undergo periodic cycles with cycle time equals to 0.1 s and 0.2 s in the droplet and column flow mode,respectively,whereas tend to be constant in the sheet flow mode.In the droplet flow mode,the axial wave induced by the droplet impacting will make the solution concentration increases about 3%-9%,the solution temperature increases about 2%?6%.The solution temperature in the region of droplet formation increases about 8%,while the mass transfer rate increases about 25%?150%.The humidity ratio of the humid flue gas in the axial direction presents high-low-high-low-high distribution,and the maximum occurs in the side of the second tube.In the column flow mode,the column impact make the solution concentration increases about 6%,and the solution temperature increases about 2%?6%.The humidity ratio of the humid flue gas in the axial direction presents high-low-high distribution,and the maximum occurs in the side of the second and third tubes.(4)Using the two-dimensional transient absorption model for the humid flue gas horizontal cool tube absorption process,multi-factor orthogonal experiments about the absorption performance were conducted.The calculation method of the total mass transfer area was firstly proposed.The correlations of the absorption performance index and the operating and the structed conditions were summarized.The dimensionless characteristic number equation of heat and mass transfer are provided.The results show that the humidity ration of the humid flue gas is the most important factor affecting the interface mass transfer coefficient,the water recovery ratio,and the humidity efficiency,while the solution inlet volume flow rate is the most important factor affecting the interface heat transfer coefficient and the energy utilization efficiency.The tube diameter is the most important factor affection the total mass transfer area.Using the correlations of the absorption performance index,it can be obtained that for the natural gas-fired boilers,for which the latent heat recovery is the main purpose,will be optimized. |
PDF Full Text Request