| With the continuous improvement of energy consumption level and the steady advancement of carbon peaking and carbon neutrality policies,The Ambient Air Vaporizer(AAV)that was used in the gasification process of Liquefied Natural Gasis(LNG)critical to improving energy efficiency.However,as the lack of scientific theoretical basis for practical maintenance and optimization seriously reduces the AAV heat transfer efficiency.In order to improve the heat transfer performance of the gasifier and reduce the cost of gasification,in this paper,the fluid solid coupled heat transfer of ambient air vaporizer was modeled and investigated,and the optimization design direction and theoretical basis for operation and maintenance of the gasifier were explored.Firstly,the single finned tube and tube bundle of forced convection in finite space was established,the effects of different flow patterns on the vaporization capacity of LNG were analyzed based on the variation of heat transfer coefficients inside the tube.The distributions of both temperatures and vaporization rates in the finned tube were also studied.The results show that there are four flow patterns of bubbly flow,slug flow,agitated flow and annular flow during the flow and boiling process of LNG in the finned tube.The local heat transfer coefficients corresponding to different flow patterns were different,among which the number of sliding bubbles near the wall has a strong promotion effect on the heat transfer in the tube;During the gasification process,the local heat transfer coefficient and the gas phase volume fraction have an inverted U-shaped relationship.Secondly,a heat and mass transfer model of finned tube surface was established,and then the fluid solid coupled heat transfer model of single finned tube inside and outside was established based on the research results of flow boiling process in the tube.The effects of the temperature and humidity and velocity of the supply air outside the tube,the mass flow of LNG on the fluid outlet temperature,heat transfer coefficient and gas phase partition in the tube were analyzed.The results showed that: with the increase of supply air temperature,the thickness of frost layer increased and the outlet temperature decreased.When the supply air temperature is 273 K,the operation time of the vaporizer lasts from 15 000 s to 20 000 s,the frost thickness increases by 2mm,and the outlet temperature decreased by 15 K.The length of the gas phase zone in the pipe reduced with the decrease of the supply air temperature.And the proportion of the total tube length reduced from 75% to 35% while the when the ambient temperature changed from 303 K to 273 K.With the increase of the inlet velocity,the heat transfer coefficient of the fluid in the tube increased and reached the peak in advance,when the inlet velocity reached 0.8m/s,the heat transfer in the tube deteriorated and the fluid gasification was incomplete.Finally,this paper takes 3×3 star finned tube bundle as an example to simulate tube bundle with forced convection in a limited space.The influence of air supply mode and air supply velocity on heat transfer of single finned tube was simulated.On this basis,the change of gasification capacity of AAV tube bundle(9 tubes)under forced air supply at different air supply angles was explored.The results show that the increase of wind speed can enhance the heat transfer performance of the finned tube in a reasonable range.In the four air supply modes,compared with the direct air supply,the side air supply has a greater improvement on the heat transfer coefficient,and the side up return has the most significant improvement effect.In the air supply of finned tube bundle,the effect of eliminating cold fog is more obvious when the air supply Angle is-60°,and the heat transfer performance of finned tube bundle is optimal when the air supply Angle is 0°. |
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