Investigation On Mechanism Of Flow And Heat Transfer Of Cryogenic Solid-liquid Pipe Flow

Superior to normal boiling liquids,cryogenic slurries have lower temperature,higher density and higher heat capacity.Slush hydrogen is a potential aerospace propellant,while slush nitrogen is a potential coolants for high temperature superconducting cable.Since the 1960 s,many progresses on the preparation,observation,flow and heat transfer characteristics of cryogenic slurries have been made.However,the flow and heat transfer mechanism of cryogenic solid-liquid flow is very complicated,and the basic research is not yet enough to achieve the application of cryogenic slurries in the field of aerospace and high temperature superconductivity.In the present work,experimental and theoretical studies were carried out on the measurements of density,liquid level and flow rate and the pipe flow and convective heat transfer mechanism.The main research contents are as follows:1)A two-fluid numerical model based on the population balance equations was built to study the flow and heat transfer characteristics of cryogenic solid-liquid flow in horizontal pipes.In the CFD model,the apparent slush viscosity formula was introduced and the population balance equations was coupled,so as to modify the solid-liquid drag law and the interphase heat transfer equations and to calculate the particle size distrubtion caused by particle breakage.Therefore,the influence of particle size and concentration on the interphase mass,momentum and energy transfer was considered.The model was validated to have high accuracy and reliability on the simulation of flow and heat transfer of cryogenic slurry.2)An experimental setup for the flow and heat transfer of slush nitrogen pipe flow was built,and the methods and structures of the capacitance-type density meter,liquid level meter and flowmeter for cryogenic slurry measurements were optimized.The freeze-thaw method was used for the slush nitrogen preparation.The high-precision,high-resolution,high-linearity capacitance-to-digital converter was adopted to collect the capacitance signal,which improves the accuracy and dynamic response resolution of the capacitance type meters.The liquid level meter is equipped with a coaxial shielding tube to reduce mechanical disturbance and electromagnetic interference.The test results show that the optimized meters have improved accuracy,sensitivity and linearity.The densimeter has a theoretical accuracy of ±0.16%,and the liquid level meter has an improved sensitivity of 46.76 pF/m and an improved theoretical accuracy of ±0.23%.Compared with that calculated by the measured variation of liquid level,the flow velocity measurement of the flowmeter has a relative error within ±7.5%.3)Experimental and numerical studies on the flow friction characteristics of slush nitrogen in horizontal circular and corrugated pipes were carried out.The pressure drop reduction of the slush nitrogen is related to the flow rate,particle size,and pipe diameter,etc.Under present work conditions,the pressure drop of slush nitrogen increases with the increasing flow velocity and solid fraction,and is higher than that of the subcooled liquid nitrogen at the same flow velocity.For slush nitrogen flows in horizontal pipes with larger diameters,with higher flow velocity and lower particle concentration,the pressure drop reduction is more likely to occur.In the horizontal tube,for homogeneous flow in a horizontal pipe,the friction factor of slush nitrogen is very close to that of the subcooled liquid nitrogen.For the flow in the corrugated pipes,the solid fraction has little effect on the flow loss,and slush nitrogen has almost equivalant flow performance with the subcooled liquid nitrogen.The empirical correlations between the flow friction factor and the slush Reynolds number in the horizontal circular and corrugated pipes of slush nitrogen were obtained.4)Experimental and numerical studies on the convective heat transfer of slush nitrogen in horizontal circular and corrugated pipes were carried out.The heat trasnsfer deterioration of the slush nitrogen is related to the flow rate,particle size,and pipe diameter,etc.Under present work conditions,the heat transfer coefficient of slush nitrogen increases with increasing flow velocity and decreasing solid fraction,and is lower than that of the subcooled liquid nitrogen at the same flow velocity.The main reason for the heat trasnsfer deterioration of the slush nitrogen is that the accumulation of high-concentration particles inhibits the convective heat transfer of the liquid phase of slush nitrogen.In addition,the aggregation of particles with larger sizes weakens the turbulent kinetic energy of the liquid phase,and inhibits the liquid convection and the interphase heat transfer.The empirical correlation between the Nusselt number and the slush Reynolds number in the horizontal circular pipe of slush nitrogen was obtained.