Experimental Research On Mechanism And Parameters Of Air-Impingement Jet Drying Of Particulate Materials

Air-impingement is a new drying technology. Since the air emitted from a nozzle impinge the surface of object directly with very high speed, which would produce a very thin boundary layer between air and object surface. The heat exchange rate of this drying method is several times higher than that other heated air drying. This dissertation has conducted a systematic study on the drying mechanism and experiments of air impinging particulate materials in order to explore the relationship between the structural and technological parameters with dry index of the materials. On the basis of analyzing the property of air-impingement technology, An experimental equipment for continuous and batch drying was designed. This equipment was also designed under the consideration of convenience to replace, adjust and control the structural and technological parameters that affecting the heat transmission. A method was put forward for solving the non-uniformity of heat transfer rate distribution of air-impingement drying. The material fluidizing state under the air-impingement was synthetically studied. The relationship between the structural (the width of impingement chamber, nozzle spacing, nozzle diameter and nozzle to target spacing) and technological factors (the air velocity at the nozzle outlet) was obtained at the moment of particulate materials starting to fluid ize under the air impingement. By introducing the concept of fluidizing extent factor, the fluidization technological parameters under different structure have a comparable factor. Under the prerequisite that particulate materials fluidized, the drying test was conducted in structure factor and technology factor two variations. The effect of different structural factors and factor levels on particulate material dry indexes was explored by such drying tests. On the basis of optimization of structural parameters, the relationship between the air impingement drying performance and technological factors was investigated by systematic experimental study. Uniform Design was used for the sake of introducing relatively uniform test conditions and obtaining precise test results. This method provided a basis for conducting test study by taking the drying parameters as variables under multi-levels and related regression equations were obtained. The main results of the experimental study are as follows: 1. The research results of average heat and mass transmission while air impinges flat surface may be used in the study of drying particulate material if the quality index did not take into account; 2. When particulate materials was in the state of normal operation, the required air impingement velocity at nozzle outlet, heat consumption and intensity of vaporization would change as the width of chamber increases, but the extent of variation was related to the stagnation region of impingement field. The smaller the diameter of nozzle, the higher the air velocity at nozzles outlet that was required to produce fully flow of materials. When nozzle diameter increases from 10 mm to 20 mm, the best drying index appeared at 15 mm nozzle diameter. The increase of nozzle spacing needed to increase the air velocity to keep materials in the fluidizing state. With the range of nozzle spacing from 60 mm to 100 mm, the best drying index was found 4, when the spacing was 82.5 mm. The height of nozzles doesn抰 have significant effect on the required air v...