Numerical Simulation Of Convective Heat Transfer Characteristics Of Microencapsulated Phase Change Materials Suspension In Mini-pipe

With the rapid development of national economy, the consumption of resources is exacerbated and results in the lack of energy gradually. Therefore, improving the efficiency of energy utilization becomes imperative. Looking for a new energy storage/transport medium becomes more and more important. The microencapsulated phase change materials(MEPCM) suspension has broad application prospect because of its high energy storage density, large apparent specific heat, reduction for the demand of heat transfer medium and decrease of the pump power consumption.In this paper, the relevant literatures are reviewed firstly and the phase change process mathematical expression of MEPCM particle is obtained. Based on the CFD-DPM model, the MEPCM particle which is effected by gravity and saffman lift,interaction between particle and fluid, particles and wall are considered comprehensively. The turbulence two-way coupling model is employed to investigate the convective heat transfer characteristics of the MEPCM suspension in a mini-pipe.In addition, the CFD-DEM coupling framework is established to research the convective heat transfer characteristics of liquid-solid two phase flow without phase change in a mini-pipe.(1) The results of CFD-DPM simulation show that the temperature distribution of the MEPCM suspension in the mini-pipe presents three main regions: non-melting region(Region?), melting region(Region?), and melted region(Region?). Under the same conditions, the fluid outlet temperature and the wall temperature of pure water, suspension without phase change and MEPCM suspension decrease successively. Along flowing direction, the temperature distribution contour of suspension without phase change is gradually evolved from a regular parabola to a more obvious convex curve, and the temperature distribution contour of MEPCM suspension presents a leptokurtic curve.(2) With increasing of MEPCM mass fraction, the flow pressure drop of MEPCM suspension is enlarged. Besides, the correlation between the effective viscosity of suspension and the mass fraction is obtained by fitted which is consistent with the predicted value of Vand viscosity correlation. It's also found that the fluid velocity in the mainstream increases, but fluid velocity near pipe wall decreases. Thetemperature decline amplitude of fluid and wall increase, and the capacity of heat transfer are enhanced accordingly. Meanwhile, the melting region becomes longer and extends to the outlet. The MEPCM particles near the pipe wall distribute densely, but the particle concentration in the mainstream area is relatively lower, which reflect the non uniform distribution of the particle phase in the turbulent flow.(3) Under the same conditions of inlet velocity and the total mass of MEPCM particles generation, as the increase of particle diameter, the pressure drop of turbulent flow in pipe and fluid velocity in the mainstream area is increased, but fluid velocity near pipe wall is decreased. The fluid temperature and the wall temperature are lower than that of pure water, and the capacity of heat transfer is enhanced accordingly. At the same time, the melting region becomes longer and extends to the outlet.According to compare between the MEPCM particles diameter with Rosin-Rammler distribution and single particle diameter, the result presents that the change trend of fluid and wall temperature keeps good consistency along the flowing direction. It is further proved that the result of convective heat transfer of the MEPCM suspension simulated by the single particle diameter has highly reliability.(4) With increasing of heat flux, the temperature of both fluid outlet and wall increase. The starting point and ending point of MEPCM melting near the pipe wall and the mainstream area are more close to the pipe inlet, and the corresponding melting region is shorter. On the contrary, while heat flux becomes smaller, melting region relatively becomes longer.(5) CFD-DEM simulation results show that with the increase of particle mass fraction, the distribution of the particles in the pipe becomes denser. Besides, the velocity of particles in the mainstream area is faster, and the velocity of particles near wall becomes slower. Therefore, the spatial distribution of particles shows a paraboloid shape in the mini-pipe. Along the flowing direction, the pressure drop per unit length is increased, which is consistent with the CFD-DPM simulation results.The particle temperature in the pipe presents a paraboloid distribution which is similar to the fluid temperature. It is found that increasing the particle mass fraction, the same temperature distribution paraboloid will move to the outlet gradually. Due to the particle disturbance, the mutual mixture among fluids is accelerated, and which results in the temperature difference between carried fluid and particle reducesgradually. The convective heat transfer between carried fluid and particle is enhanced and this phenomenon becomes more obvious with increasing of particle mass fraction.