Study On Air Leakage And Heat Transfer Law Of Loose Coal Based On Thermal Balance Equation

Coal spontaneous combustion seriously threatens the safety of coal production,storage and transportation,and brings great losses to the coal industry.Coal spontaneous combustion is a complex dynamic process,and the thermal storage environment caused by unbalanced heat release and heat dissipation will have an important impact on it.In order to prevent spontaneous combustion of coal,it is necessary to start with the external conditions and clarify the role of external factors in the heat dissipation process of accumulating coal.Therefore,it is necessary to study the air flow in the coal body and the heat transfer performance of the coal body itself to reveal the influence of the external thermal storage environment on the spontaneous combustion of coal.Based on the energy balance equation,this paper deduces the heat storage equilibrium equation of loose coal body.From this equation,it can be seen that the main factors affecting the heat storage area of loose coal body are equivalent thermal conductivity coefficient lambda λ0,wind speed v and void fraction n.According to the above parameter v,the air flow model dP/dh=l/du02-k/d2u0 under uniform arrangement of ideal spheres is deduced.It is concluded that the change of pressure gradient caused by coal particle size,pressure and stacking height is an important factor affecting the wind speed.Based on the above theory,relevant experiments were carried out and observed by electron microscopy.It was found that the morphology of coal particles was different after different kinds of coal were crushed,thus affecting the porosity of loose coal.Then the voidage measurement experiments were carried out one by one.The rule is that the bulk density of loose coal increases with the deepening of metamorphism,and the voidage of the same coal increases with the decrease of particle size.Based on the mathematical model of airflow movement under uniform arrangement of ideal spheres,an experimental device for airflow simulation was designed on the basis of references.Four kinds of coal samples were selected to test the air flow law in coal voids considering particle size,wind pressure and stacking height.The following conclusions were drawn:under the influence of coal quality,the void structure has a significant effect on air flow,particle size d has a significant effect on air flow,and when the voids are large,the inertia force of air flow dominates,particle size d is proportional to wind speed v2.With the decrease of particle size,the wind speed decreases and the viscous.Gradually dominant,d2 is proportional to wind speed v;wind speed change is positively correlated with pipe diameter,the influence of pipe diameter lies in the size of inlet wind pressure,the bigger the pipe diameter,the obvious pressure drop;with the increase of coal sample height,pressure gradient decreases,turning points appear at height of 1cm and 3cm respectively,and wind speed decreases exponentially.DRE-2C thermal conductivity tester based on Transient plane heat source method was used to test the thermal physical parameters of different coal types,particle size,humidity and other conditions,and the change rule of thermal conductivity with external factors was studied.Then,the artificial heat source was introduced and an experimental device was designed to study the heat dissipation rule of loose coal body under the influence of heat source intensity(60,80,100),particle size range(2-4 mesh,4-10 mesh,10-18 mesh)and environmental humidity(0%,50%,100%).The experimental results show that the temperature difference between No.1 and No.3 is 7℃ at 60℃,and 11.5℃at 100℃.With the increase of the temperature of the heat source,the temperature gradient of each measuring point in loose coal body increases with the increase of the intensity of the heat source;the temperature rise speed difference between No.1 and No.2 measuring points is larger,the temperature rise speed difference is 0.17℃/min,and that between No.2 measuring point and No.3 measuring point increases with the increase of the temperature of the heat source.The difference of temperature rising speed between the measuring points is small,the difference is 0.06℃/min,and the temperature rising speed of coal decreases with the increase of heat source distance under different heat source intensity;the heat conduction coefficient of loose coal body is greatly affected by voidage n;the closer the location of heat storage area is to heat source,the greater the influence of voidage on temperature rising,and the weaker the influence of voidage is with the increase of distance;Considering the environmental humidity of the heat storage area,the effect of 0%,50%and 100%humidity on heat transfer in the heat storage area is tested.At different locations from the heat source and closer to the heat source,the heat transfer effect is strengthened with the increase of humidity effect,while at a relatively far distance from the heat source,the heat transfer effect is weakened with the increase of humidity effect.