| As the core component of air separation plant,the energy consumption of the purification system accounts for about 11% of the total energy consumption.The coldblown polluted nitrogen evacuated in the regeneration stage has the characteristics of large flow rate,intermittence,high humidity,and large temperature fluctuation.If the waste heat in the polluted nitrogen can be recycled,the overall energy consumption of the air purification system can be greatly reduced.The latent heat storage(LHS)technology has the advantages of high energy storage density and approximately isothermal heat storage/release,which can effectively overcome the intermittent and high humidity of the cold-blown polluted nitrogen.However,most of LHS devices used in engineering belong to shell-and-tube or plate structures,which cannot meet the high heat transfer rate and high thermal efficiency requirements of purification system.Therefore,this study uses a combination of theory and experiment to explore the mechanism of compound heat transfer enhancement with high efficiency,economy,and reliability for LHS technology.The main contents are as follows.Firstly,the mechanism of gravity promoting the melting process of PCM was investigated.A general coordinate of heat source input direction and gravity direction was established.The influence of the included angle ? under the constant wall temperature and constant heat flow boundaries on the melting process of PCM in the square cavity was systematically studied.For the constant wall temperature boundary,the melting time is the longest when the included angle ? is 0o,and as the included angle ? increases,the melting time first greatly decreases and then slightly increases.For the constant heat flow boundary,the melting time increases first and then decreases with the increase of included angle ?.The optimal included angle ? under the two boundary conditions is both between 90o and 180o,and is closer to 90o.Therefore,in order to give full play to the role of natural convection in promoting melting,the lowest point on the heat source side should be lower than the that on the PCM side for the regular LHS unit.Secondly,the heat transfer characteristics of the combined structures were analyzed to obtain the combination principle of heat transfer elements.Using heat pipe(HP),fin(Fin),and foam-metal(CF)as basic components,three combinations of HP-Fin,HPCF,and HP-Fin-CF were proposed,and their characteristics of melting and solidification,conductivity and convection were obtained.The results indicate that the HP-Fin combination has better melting performance,whereas the HP-CF combination leads to better solidification.The reason is that the former has less restriction on natural convection,while the latter has stronger spatial thermal conductivity.The HP-Fin-CF combination shows the best comprehensive performance because it has both characteristics.Based on the combination characteristics,the relative geometric relationship between the heat source end and the PCM was deduced,namely expansion,parallel,and contraction.By combining the numerical model of HP-Fin-CF and response surface analysis(RSM),four response models including melting time,solidification time,exergy storage capacity,and exergy release capacity were established.Based on the multi-objective differential evolution(DE)algorithm,the effects of the heat transfer rate and heat storage density with competitive relationship were analyzed.Thirdly,under the premise of considering the PCM mass in the sub-stage,a thermodynamics analysis of the multi-stage LHS system was carried out.For the steady cold/heat resources with small temperature difference,the temperature-enthalpy diagram in the pinch point analysis was used to graphically analyze and optimize the multi-stage LHS system.For the complex unsteady heat source,a dynamic heat transfer model including time term was established,and the differential evolution(DE)algorithm was used for calculation and analysis.The discharged exergy is more appropriate for optimizing the multi-stage LHS system that exports less high-grade thermal energy,while the discharged energy or discharged entransy is more suitable for optimizing the multi-stage LHS system that directly provides more low-grade thermal energy.At the same time,for the steady heat source,the proportion represented by sensible heat in the evaluation index increases with the increase of temperature.For the unsteady hot HTF that includes a drop in temperature,the switching operation can effectively improve the multi-stage LHS system performance,that is,when the temperature of hot HTF is lower than that of PCM in the current sub-stage,it will be directly connected to the next sub-stage.Also,as the discharging time increases,both the charged exergy and the discharged exergy increase significantly,so the discharging time should be longer than the charging time.Finally,a two-stage LHS unit was designed and tested in the purification system of air separation plant.The actual operation data of an purification system were collected as the initial design parameters.The optimal melting temperature and mass of PCMs were calculated,and the general expression of PCM temperature suitable for the heat source of the purification system was deduced.For the single-stage LHS unit,the optimal melting temperature is 59.67 °C,and for the double-stage LHS unit,the optimal melting temperature is 73.68 °C and 46.04 °C,respectively.The test results show that when the engineering standard of 40 °C is used as the baseline,the waste heat recovery rate is 52.7%.When the regeneration gas temperature of 20 °C is selected as the baseline,the waste heat recovery rate is 35.4%.As a result,the power consumption of the electric heater can be reduced by about 17.59%.Also,the sensible heat of components such as pipes and valves and the heat leakage effect of the overall system can not be ignored.In conclusion,the application of LHS technology in the purification system of air separation plant is faced with gas-solid heat transfer,PCM low thermal conductivity,and unsteady heat source difficulties,this study has conducted research from following aspects.In terms of heat transfer rate,the action law of gravity promoting PCM melting,the combination principle of heat transfer enhancement elements and the relative geometric relationship between heat source and PCM were explored.In terms of heat transfer efficiency,a calculation method for the key parameters of the multi-stage LHS system considering the PCM mass in the sub-stage was proposed.Lastly,the two terms were combined to form a highly efficient composite heat transfer enhancement method,and the relevant results were successfully applied in the purification system and certified by a third-party institution. |
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