Strengthening mechanism of pulse combustion drying process
    Material particles or droplets in the drying process, for example, pulse combustion drying process can be analyzed possible strengthening mechanism. Particles or droplets drying is a complex process of heat and mass transfer, drying medium to transfer energy to the material, the material temperature while water passes from the interior outward and evaporated. Driven by heat and mass transfer during drying the main driving force is the temperature gradient, moisture gradient, the material surface vapor pressure difference and surface tension difference. Obstacles in the drying process the heat and mass transfer resistance is generally believed that there are three: the first is an external resistance; second is the surface resistance, the resistance of the boundary layer, the gas and particle / droplet heat and mass transfer between When the gas-solid / liquid two-phase boundary and the boundary layer is formed between the boundary layer and preventing a hot fluid particle / droplet heat and mass transfer between, and forming a surface resistance; third is the solid particles within the resistance, and material characteristics. To increase the drying rate, enhanced heat transfer and mass transfer process, we must try to improve the mass transfer of power and reduce drag. Reducing the particle / droplet diameter, or by some change in the energy field inside the particle structure can reduce the internal resistance. For reducing the external resistance and boundary layer resistance, can be used the following methods.
    (1) increasing the particulate material and the relative speed between the air flow, while increasing the gas-solid / liquid two-phase friction, thinning of the boundary layer.
    (2) reduce the particle diameter, particle surface is conducive to reducing the thickness of the boundary layer. Reduce heat transfer resistance, increasing the effective area of ​​heat and mass transfer; additionally reduce the particle diameter of the particles can be reduced moisture migration within the distance to the surface of the particles and the migration time, which are conducive to improved heat and mass transfer and drying rate.
    (3) particle subjected to additional forces, such as inertial or centrifugal force, tends to reduce the boundary layer thickness.
     Since pulse combustion heat generated, high-speed, high-frequency pulsating flow, and the role of acoustic energy, help to reduce the three resistance, enhanced heat transfer and mass transfer process, profiling below.
    1 As pulsating combustion temperature air, the particle / liquid moisture in the material and drying medium temperature difference between great. The heat flux density and the temperature difference is proportional to the measured test pulse combustion exhaust gas generated by the right temperature up to 600 ℃ friends, and the material temperature is usually about 15 ℃; great temperature difference between the heat transfer enhancement.
     Drying rate increases with the temperature difference. Increasing the temperature gradient, the material can not only accelerate the evaporation of surface moisture, the temperature rise of the material can also accelerate the diffusion rate of the water within the material. Therefore, the heat generated in the pulse combustion drying medium, particulate material heat and mass transfer have been strengthened. For general conventional drying process, the drying medium temperature should not be too high, because when the drying medium temperature to make the material surface water evaporates faster than the rate of diffusion of the internal moisture. Will shorten the drying section of the material constant, falling rate drying period extended. In addition, at high temperatures, rapid drying of the material is easy to form a surface "sclerosis" is difficult to spread out so that the internal water; of times, using conventional drying methods such as fixed-bed drying, the drying medium and material as long contact time, a number of heat-sensitive high-temperature drying of materials should not be used. For the pulse combustor tail pipe drying, the material with the high temperature medium is very short contact time, the material together with rapid evaporation of water during the drying process, the surface temperature of the material is not too high, the pulse combustor test measured dry material after the tube temperature does not exceed 60 degrees. Therefore, pulse combustion drying, can be used to strengthen the high-temperature heat and mass transfer.
    2 due to pulsating combustion oscillation pulse effect the drying medium, the material flow in this field and sometimes oscillating acceleration sometimes slow, is always in stable motion, the particulate material and the relative velocity between the drying medium large, and thus help to improve the pass heat and mass transfer rates. 3 high-frequency pulsating combustion pressure fluctuations, so speed up the drying process. As pulsating combustion moisture content of the drying medium is low, so the water vapor partial pressure is very small, thus improving the drying rate. Further, since the oscillation frequency pressure fluctuations, it is easy to tear and thinning particle / vapor boundary surface of the droplets, the water inside the particles under the effect of the pulsating pressure, quickly reach the particle surface was quickly evaporated. Pressure airflow turbulence can also improve the synchronization of the liquid capillarity to improve the material moisture diffusion coefficient. The measured pulse combustion pressure oscillations strength of 10kPa. Because of the pressure pulsation, and strengthen the drying process and the high-temperature air heat between wet materials, mass transfer process.
    4 pulsating combustion waves can the role of the drying process of heat and mass transfer have been strengthened. According to the literature description, sonic energy can accelerate the drying process 1-2 times, therefore, some material may shorten the drying time, for example, drum drying acoustic field gluconate, drying time is only 20-30min, and the industrial production under the conditions of the drying process up to now more than 8h. Acoustic field drying mechanism is: through sound vibrations, so that the material in the part of the combination of water and material separation, while the transmitted acoustic energy is absorbed and converted into heat the material, so that the material in the evaporation of moisture migration from the materials. Under the action of acoustic energy in the gas-solid / liquid surface boundary layer is vulnerable to damage, thereby strengthening the external heat and mass transfer. Pulse combustion oscillation is a periodic combustion, the combustion process produces a strong sound and vibration, according to the design of test measured pulse combustor 25kw bare (i.e. not installed air exhaust muffler) the noise intensity is about 100dB . Therefore, the pulse combustion drying process, the acoustic energy produced by pulse combustion field of heat and mass transfer with enhanced effect. Pulsating combustion wave energy on the one hand to strengthen the drying process favorable heat and mass transfer, but noise pollution on the environment. Therefore, in the application of pulse combustion drying should take effective measures to eliminate noise.
5 the temperature gradient, pulsating flow field, pressure field and the acoustic energy synergistic effect on the drying process of heat and mass transfer from the strengthening effect.

 

 

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