Heat Transfer Analysis And Optimization Design Of Submerged Combustion Vaporizer(SCV)

As a clean fuel,Natural Gas(NG) has the characteristics of high calorific value and no pollution,is widely used in chemical production,civil heating and other industries.The Liquefied Natural Gas(LNG)obtained after pressurized liquefaction is suitable for marine transportation and has a high recovery rate,which needs to be heated and revaporization before being connected to the tubeline.With the increase of the proportion of natural gas in China's energy structure,the research on corresponding vaporization equipment is helpful to ensure the stability of natural gas supply chain.The Submerged Combustion Vaporizer(SCV)is a kind of LNG vaporization equipment with high thermal efficiency and compact structure,which has been widely used in the revaporization process of LNG receiving station.1)A heat transfer calculation program based on C++ programming language was established for the heat transfer calculation of submerged combustion vaporizer,such as the large temperature difference between inlet and outlet of LNG inside tube,the drastic change of thermal properties,and the heat transfer of gas-liquid two-phase flow across tube bundle,which is applicable to the heat transfer calculation under supercritical and subcritical pressures.Methane is used instead of LNG to calculate heat transfer,and the accuracy of the calculated results is verified with the actual working conditions.2)In the engineering design of SCV,the selection of heat transfer model inside and outside tube has difference on the predicted result of heat transfer coefficient,which will affect the actual calculated total heat transfer area.Because the design process needs to control the area margin in a reasonable range,this paper analyzes the applicability of five inside-tube heat transfer models under different working conditions.Under the maximum load,the calculation area margin ranges from 10.99% to 17.78%,which meets the design margin requirements,and the Petukhov model is the most accurate in predicting the heat transfer coefficient inside tube.The pressure drop of the heat exchange tube mainly occurs in the latter half of the tube with higher fluid temperature.The method of twisted tape intercalation for the inlet tube can enhance heat transfer and keep the pressure drop from being too large.3)The factors that affect the heat transfer outside tube,such as gas holdup and flue gas tube opening ratio,are discussed.The reasons for the attenuation of heat transfer outside the tube under high load are obtained,that is,the increase of gas holdup outside tube caused by heat load would inhibit the heat transfer outside tube to some extent.In terms of agitating the water bath more fully and reducing the gas holdup inside overflow weir,the higher opening ratio of flue gas tube has better effect.4)The heat transfer characteristics of SCV under subcritical pressure are discussed.The heat transfer inside tube is dominated by single-phase convection,the tube length in the gas-liquid two-phase zone is relatively small,but the heat transfer coefficient is obviously enhanced.The heat transfer intensity in the two-phase zone is mainly dominated by nuclear boiling heat transfer,with the increase of operating load,the nuclear boiling intensity decreases first and then increases,and the threshold value appears around 0.3 times of standard load.The heat transfer performance under supercritical pressure is compared with that under supercritical pressure,the results show that the SCV unit operating under subcritical conditions can obtain better heat transfer performance by increasing the pressure to near critical pressure.