| The cooling of hydrocarbon-rich gas can cause serious fouling in industrial processes,which can impedes heat recovery and increase maintenance costs.The most representative examples is the waste heat recovery of biomass and coal gasification process.Replacing the expensive and low efficiency wet scrubbers with anti-fouling heat exchangers can enable waste heat recovery and meet the requirements of pyrolysis gas purification.In the process of waste heat recovery of pyrolysis gas,it is a common phenomenon of water and heavy hydrocarbon vapor condensate simultaneously.The heat transfer resistance of pure water vapor mainly due to the condensate film,during the process of condensation heat transfer of binary mixed vapor of the immiscible liquids,the condensate distribution,flow pattern and mass transfer of mixed vapor will affect the condensation heat transfer coefficient.However,the condensation heat transfer characteristics of binary mixed steam of immiscible liquids and the liquid film dynamics characteristics of the falling film flow lack of the related research.Thus,this paper conducts a systematic study on the falling film flow and condensation heat transfer characteristic to provide a theoretical basis for waste heat recovery of hydrocarbon-rich gas.First of all,the immiscible liquids falling film flow characteristics was studied by using experimental methods,the falling film flow of silicone oil was tested as control.The mixed fluid flow rate and water content on the falling film flow characteristics were obtained.The characteristics of falling flow pattern of immiscible liquids were analyzed.Then,combined the VOF model with liquid film spreading mechanism,the falling film flow of immiscible liquids was numerically simulated.The variation of film thickness of falling film flow was analyzed.The influence of the liquid-liquid interface force on the falling flow pattern was explored.Finally,the binary mixed vapor of immiscible liquids condensation heat transfer experiment was carried out.The commonness and differences of mixed vapor condensate film,pure water vapor condensate film and cold immiscible liquids falling film were compared,the effect of binary mixed concentration and cooling water flow on condensate form,droplets growth and shedding were analyzed.The condensation heat transfer law of binary mixed vapor of immiscible liquids was revealed.The mainly research conclusions were as follow:Combining with infrared thermal imaging camera and high speed camera,the liquid film spreading characteristics of immiscible liquid on the vertical plate was systematics studied by compared with the high viscosity,low surface tension silicone oil falling film flow characteristics.The flow and spreading process of the liquid film was captured,and the influence of the flow rate and concentration ratio of mixed liquids on the film width and wet area were discussed.The results showed that the average film width gradually increased with flow rate increased,but decreased with water content increased.With the variation of the flow rate and concentration ratio of the immiscible liquid,the droplet-film,lens-film and rivulet would appear during the falling film flow process.Based on the VOF model,the falling film of high viscosity,low surface tension silicone oil and immiscible liquid on the vertical plate were numerically simulated.The effect of different flow rates on the flow morphology of liquid film was studied,and the distribution of local liquid-liquid interface shear stress was obtained.The effect of different two phase flow velocity on the interface shear stress was analyzed.The simulation results showed that the immiscible liquids were displaced forward during falling film flow process,and the phenomenon of finger-like appeared along the flow direction of falling film plate.The water droplets were squeezed by the phase interface and presented irregular shapes,and the internal pressure of water droplet was high.The surface of organic liquid film occurred greatly fluctuated.The ratio of water phase on the organic liquid film increased with water flow rate increased,the average film thickness of the organic liquid was thin,and the shear stress near the liquid-liquid interface changed severely.The condensation heat transfer characteristics of binary mixed vapor was studied.The effects of different steam concentration and cooling water flow rate on the condensate distribution was obtained by comparing with pure water vapor condensate.The variation of droplets growth and shedding was explored,and the condensation heat transfer law was obtained.The result showed that the liquid which has high surface wettability would spread on the wall,while the liquid with low surface wettability will form droplets.The droplets would go through the process of nucleation,growth,merging and shedding.There was a clear interface between the immiscible condensate.The Marangoni flow appeared on the water droplets surface due to the temperature gradient,and the Marangoni flow became more violent as the cooling water flow rate increased.Compared with the condensation heat transfer characteristics of pure water vapor,it was found that the condensation heat transfer coefficient gradually increased with the concentration of cyclohexane increased,and the heat transfer coefficient of binary mixed vapor was lower than the pure water vapor condensation heat transfer coefficient,it was indicated that the presence of cyclohexane formed a concentration boundary layer,which increased the heat transfer resistance.In the experiment,the condensation heat transfer coefficient increased significantly with the concentration of cyclohexane increased. |
