Study On Solar-Assisted Heat Pump Drying Grain System And It's Heat And Mass Transfer

It has been widly acceptted and concerned that the grain security is a global problem. World grain security is facing a variety of traditional and non-traditional challenges. Each year about five percent of the grain lost due to the improper drying, it is important way to ensure our national grain security that reduce the loss in grain drying and storage. Drying can be found widly used in the national economy, and consuming lots of energy. The pollution during drying also causes some environmental problems. To know well about the heat and mass transfer in grain drying, develop green and high performance grain drying equipment and system is very useful to improve our grain drying technology and ensure our grain security.Solar-assisted heat pump system will have a positive effect on energy consumption of grain drying, because solar energy is clean and renewable energy, and heat pumps have significant energy saving potential and it's the only heat recovery system which enables the temperature of waste heat to be raised to more useful level. Based on the similarity theory this work build a solar-assisted heat pump drying system to study its COP (Coefficient of Performance) and advantages.Water in grain is removed by evaporation, whether the grain can be dried or not is depend on the difference between the grain water's vapor pressure and the air's vapor pressure. Grain drying process actually is heat and mass transfer in porous media. According to the theory of heat and mass transfer in porous media and conservation law of momentum, and after a full analysis of the characteristics of heat and mass transfer in hygroscopic porous media, this paper build partial differential equations to describe heat and mass transfer and flow in hygroscopic porous media. CFD(Computational Fluid Dynamics) method is adopted to simulate the heat and mass transfer situation in wheat, which drying with solar-assisted heat pump system.Drying process is a coupled heat and mass transfer process, the latent heat and the variation of the air's absolute humidity caused by moisture transfer during drying must be taken into account during CFD simulation. In order to achieve the heat and mass transfer is changing by the variation of the moisture of grain, this paper gives the formula of energy source term, mass source term and the momentum source term used to achieve momentum loss when air pass through the wheat,and use UDF(User Defined Function) to join them into simulation.As the drying is directly affected by the air's temperature and absolute humidity, and solar-assisted heat pump drying system is the prototype of this paper, the temperature and absolute humidity of air variate as the weather and the system working mode. Meanwhile the drying process will be affected by the external surroundings, such as external temperature, solar radiation, use solar-air temperature to conside the combined influence of external surroundings, it also variate as the time. To make the simulation more accuracy, this paper use transient boundary conditions to discribe these parameters variate as time, and this also the peculiarity of this paper.In this paper, theoretical analysis, numerical simulation and experimental research method are used to study solar-assisted heat pump drying system and heat and mass transfer phenomenon during drying. The experimental results indicate that the wheat reach safety moisture 13.6%(dry basis) after 135h drying, the heat pump's average COP during drying is 3.05, and the average COP of solar-assisted heat pump system is 6.18, while the COP of the traditional hot air drying about 1.1, so compare with traditional hot air drying and heat pump drying, solar-assisted heat pump drying system has obvious advantages, has the lowest energy consumption. The simulation results show that the air temperature, absolute humidity of air and the solar-air temperature, which use transient boundary conditions perform well during drying, it makes this simulation more realistic and more accurate. The simulation result vividly gives the coupled heat and mass transfer phenomena, and shows that the wheat reach safety moisture 13.6%(dry basis) after 150h drying, with little difference between the experimental results, furthermore the simulation temperature agree well with the experimental temperature, all these indicates that the mathematical model which describe the heat and mass transfer in hygroscopic porous media and the source terms to achieve heat and mass transfer during simulation in this paper is reasonable. the simulation results also show that the wheat drying has stratification phenomenon and the bottom layer of the wheat is too dry, this is also be found in practice, by increasing the absolute humidity of the air or apply grain stir equipment could avoid this phenomenon. The CFD method can do well in practice,at last, use CFD method simulate heat and mass transfer during wheat drying with solar-assisted heat pump drying system in a national grain storage silo.