Coupled Heat And Mass Transfer Behaviors Of Typical Moist Mineral Porous Media In Microwave Field With Dielectric And Magnetic Loss

The efficient utilization of fossil fuels possesses the vital importance in alleviating energy crisis for worldwide and China.The thermal power generation and metallurgy industry are the main consumers of fossil fuels.The moisture content in the feed mineral materials has significant effect on the quality of the metallurgical products,the safe operations of power plant boiler and metallurgical blast furnace and the energy efficiency of the fossil fuels.Microwave heating has emerged as a time-saving and cost-effective technology,which inspirits the overwhelming increase in the applications of drying processing.The current research presents a comprehensive investigation on the drying behavior of typical moist mineral porous media with dielectric and magnetic loss mechanisms.A mathematic and physical model based on the linear non-equilibrium thermodynamics for the coupled heat and mass transfer behavior of typical moist mineral porous media in dielectric and magnetic drying was developed and solved numerically.The coupled heat and mass transfer mechanisms within the dielectric and magnetic typical mineral porous media were illustrated both in laboratory and theoretical studies.The results showed that addition of Na2CO3,Na2SO4 and fly ash could enhance moisture migration,reduce the apparent activation energy,increase the average sample temperature and improve the temperature distribution uniformity within the dielectric lignite thin-layer porous media.Sodium carbonate among three additives could be preferable one,followed by sodium sulfate and coal fly ash.The energy efficiencies of the dielectric and magnetic hematite thin-layer were 10 times higher than that of the dielectric lignite thin-layer.Based on the experimental and theoretical investigations,the drying performance of pellet samples with dielectric and magnetic loss mechanisms were examined in the present research.The results noted that the values of the effective moisture diffusivity varied from 6.72×10-9-5.92×10-8 m2 s-1 for the dielectric lignite sphere sample at 231-700 W.The pumping phenomenon was only detected on the surface of raw dielectric lignite sphere block in the warm-up period at power levels of 539 and 700 W.The results of the effective moisture diffusivity ranged from 1.22 × 10-8-9.25 ÷ 10-8 m2 s-1 for the dielectric and magnetic hematite pellet sample at 119-700 W.The uneven thermal and moisture expansion or contraction due to the non-uniform temperature and moisture distribution gave rise to the collapse or cracks within the pellet sample during microwave drying,which were all circumferentially distributed on sample surface.The tensile strength of the hot air pretreated dielectric and magnetic hematite pellet was 7.5 MPa,while that was 13.2-14.0 MPa for the microwave heating.The transport mechanisms of moist mineral porous media under convective hot air drying were assessed theoretically and experimentally.The results illustrated that the increase in hot air temperature and velocity would facilitate the heat and mass transfer characteristics within the lignite and hematite thin-layers.The values of the effective moisture diffusivity were from 5.10×10-9-4.06×10-8 m2 s-1 at the hot air temperature of 100-160? and velocity of 0.5-2.0 m s-1.The average surface heat transfer coefficients of the lignite and hematite thin-layers were 3.41-16.62 W m-2 K-1 in the falling rate period under the experimental conditions.The linear non-equilibrium thermodynamics microwave-multiphase transport model that applied to the typical moist mineral porous media in microwave drying with dielectric and magnetic loss mechanisms can project the coupled heat and mass transfer behaviors well with reasonable deviations.The calculation results addressed that no pumping phenomenon was detected within the thin layer samples.The regions in the pellet samples with low electric and magnetic intensities(4×102-103 V m-1,4-10 A m-1)possessed obvious pumping phenomenon.The diffusion-thermal effects of liquid water or water vapor diffusion at high microwave energy intensity regions(103-6×103 V m-1,10-20 A m-1)could weaken the heat transfer,while those at the low microwave energy intensity regions(4×101-103 V m-1,4-10 A m-1)would enhance the heat transfer at the initial drying stage and weaken that at other drying stages.The hot air assisted microwave drying could promote the heat and mass transfer behaviors within the thin layer and pellet samples,and improve the temperature distribution uniformity.In comparison with the microwave drying,the temperature distribution index would decrease to 3.6-4%for hot air assisted microwave drying.