A Study On Thermal Conductivity Of Packed Particles

The value of study on the thermal conductivity of packed particles is obvious in many application fields,such as:food storage,packing and combustion of coal,condensation of concrete,advanced modeling process of metal materials,heat insulation or refractory materials,design of drum dryer and packed or fluidized bed,etc.However,the majority of researches on this theme have not put efforts on accumulation data of thermal conductivity of different materials in different condition but on the study of microscopic heat transfer mechanism,which led to the shortage of data support on those application fields.To solve the problems existing in the study of thermal conductivity of packed particles,this thesis carries out the following work:Firstly,the factors influencing the thermal conductivity of the packed particles are discussed from the point of view of the mechanism.The results of some scholars are summarized.From a theoretical point of view,factors that influence the thermal conductivity of the packed particles are only include the fractal dimension of the packed particles and the thermal conductivity of the material.Secondly,on the basis of the protective heat plate method,the original experimental device has been improved to measure the thermal conductivity of the packed particles.The main improvements include:to take a variable sample thickness design,an unaffected thermal resistance of particles by surface heat resistance is obtained through rigorous mathematical derivation.a protection ring is designed and simulated to hold the particles and keep it's packing shape.a pressure plate,vacuum shield and some other devices are designed.Thirdly,a series of experiments were carried out on the samples of glass beads,and the thermal conductivity of the glass beads was calculated,the conclusions of thermal conduction of glass beads are also drawn preliminarily.These conclusions are:(1)For the glass beads,when the diameter difference between the maximum and minimum particle is less than 1.5 times,the thermal conductivity is not affected by the size of the particles..And the thermal conductivities is distributed in 0.08W/(mK)without external compression force.The thermal conductivity decreases slowly with the temperature rising.(2)When the compression forces are applied on the glass beads,the thermal conductivity of the glass beads increases firstly and then tends to be flat with the increase of the pressing force,and finally the thermal conductivity reaches 0.10W/(mK).By using the equation in the second chapter,the experimental results of influences of compression force and the calculated results are highly consistent.Therefore,it can be considered that the equation is suitable for the particle size difference of less than 1.5 times the particles.(3)Due to the smaller particles filled into the pores formed between the larger particles,thermal conductivity of glass beads tends to increase when the particles of a variety of particle sizes are mixed.Since the assumption of equation is no longer true,it is not possible to use this formula to calculate the thermal conductivity of the packed particles.