Determination Of Convective Heat And Mass Transfer Coefficient Of Grain Bulk Dryeration Process And Design Of Measuring Device

Grain temperature and moisture content are the most important factors affecting safe grain storage.Research on variation of temperature and humidity during grain dryeration process has great practical value for grain safe storage and optimization of ventilation.Experimental study on bulk grain stored in large scale warehouse needs long experiment period,massive human,material and financial resources.Also the experiment accuracy is limited by experimental method and test technology.Therefore,in this paper,numerical or physical simulation method was used to simulate the distribution and variation of temperature and humidity field during grain dryeration in order to provide guidance for the process of grain dryeration.Convective heat transfer and convective mass transfer are two main transmission of grain dryeration.During the process of grain dryeration,convective heat and mass transfer coefficient between internal pore and grain is one of the important parameters to characterize the intensity of heat and mass exchange.It is also a thermal physical parameter,which must be obtained for grain ventilation and drying equipment design,parameter calculation and optimization,and numerical simulation process.This parameter is related to the gas passing through strength,physical and chemical properties of gas and grain,as well as the pore geometry structure and scale,and many other factors.For this reason,the theoretical calculation and experimental measurement are both difficult.Against this background,this paper seeks to find a new method for measuring convective heat and mass transfer coefficients.The main research contents were as follows:1.By analysis on mechanism of heat and moisture transfer in bulk grain,the main way of internal heat and moisture transfer ways during dryeration process was confirmed--heat convection and mass convection.2.Based on the continuum assumption,local heat balance principle and the idea of Whitaker volume average method,we proposed a new way which is called lumped heat capacity method to determine grain convective heat transfer coefficient and mass transfer coefficient according to the lumped parameter method and the transient convective heat transfer experiments.The mathematical model describing heat and moisture transfer during dryeration was built.3.A set of testing system was designed and developed based on lumped heat capacity method.The analysis of system function demand was completed first.And then we built a measuring device using suitable parts to determine the convective heat and mass transfer coefficient of grain bulk.Basic structure of the apparatus includes a small barn for testing,the main loop temperature control,the main loop moisture control,and auxiliary loop air supply and other modules.4.Basic parameters to describe character of porous media and relevant physical parameters were determined.5.With wheat,corn and paddy taken as experimental materials,we got the convective heat transfer coefficients of these three grains after processing data got from the test bench.At last,the results obtained are compared with the literature or reference results.Meanwhile,according to the similarity between heat convection and mass convection,the convective mass transfer coefficients of these three grains were obtained.