Flow Heat Transfer Characteristics In A High Pressure Stirred Microalgae Hydrothermal Reactor

Biomass is a renewable resource that can be used as a substitute for coal,oil,natural gas and other primary energy sources.Zero carbon emission from biomass resources utilization is one of the powerful guarantees and effective means to achieve the goal of double carbon.It plays an important role in promoting the high-quality development of renewable energy.Microalgae is a typical biomass,which not only can achieve large-scale concentrated culture,but also has the advantages of high photosynthetic efficiency,fast cell proliferation and short culture cycle.At present,the main methods to produce bio-fuels from microalgae biomass include direct combustion,transesterification,hydrothermal liquefaction,thermochemical gasification,pyrolysis and anaerobic digestion.Compared with other conversion technologies,hydrothermal liquefaction technology has a wider source of raw materials and can realize the complete conversion of organic components of biomass.The flow and heat transfer characteristics in microalgae hydrothermal reactor have a very important effect on the hydrothermal transformation process of microalgae.Therefore,in this paper,microalgae as the research object,using the experimental and numerical simulation methods to study the flow heat transfer process in the reactor,to provide a prediction method and theoretical basis for the design and application of stirred microalgae hydrothermal reactor.Firstly,the flow process in the reactor was simulated.The flow process in the reactor was simulated by using two research methods: frozen rotor and transient state.The calculation accuracy and calculation time are considered comprehensively when the grid independence is verified.By observing the velocity curves of four monitoring points,the time required for the velocity field to enter the stable state in the kettle is obtained,which indicates the feasibility of the frozen rotor model.The reliability of the frozen rotor model was verified by comparing the transient simulation results with those of the frozen rotor after it entered the stable state.The transient numerical model is used to simulate and analyze the start-up stage of the reactor,and the velocity field distribution in the start-up stage is compared.A numerical model of frozen rotor was used to compare the eddy current intensity distribution under different speed conditions,and the influence of speed on the flow process in the kettle was investigated.The influence of thermocouple on the flow process in the kettle is also analyzed.Secondly,the flow process of microalgae slurry was analyzed.The liquid-solid mixture model and the homogeneous model were used to simulate the flow mixing process of microalgae slurry in the kettle.The mixing process of microalgae particles and water in the reactor was simulated by liquid-solid two-phase model under different accumulation densities,accumulation heights and rotation speeds,and the distribution characteristics and mixing uniformity of microalgae particles in the reactor at different times were observed.Based on the experimental study,the microalgae grout can be regarded as a typical pseudoplastic non-Newtonian fluid.According to the relationship between viscosity and shear rate obtained in the experiment,the flow characteristics of microalgae grout in the reactor were studied by using the homogeneous model,and the effects of microalgae grout concentration and reactor speed on the flow characteristics were analyzed in depth.Finally,the heat transfer process in the reactor is studied experimentally and simulated numerically.The non-isothermal flow model is used to simulate the single-phase heat transfer process in the kettle,and the distribution of temperature field at different time is analyzed,especially in the unstable stage of temperature rise.The influence of different heating surface and rotating speed on temperature field distribution in the kettle was investigated.The gas-liquid twophase temperature rise experiment in the reactor was carried out,and the gas-liquid two-phase non-isothermal flow model was used to simulate the gas-liquid two-phase heat transfer process in the reactor,and the experimental temperature rise curve was modified according to the simulation results.The influence of initial pressure on the heating process is considered in both experiment and simulation.The temperature transfer process in the kettle under different wall heating modes is also discussed by using a two-phase non-isothermal flow model.