Design and experiment of air-impingement jet combined with normal temperature ventilation dryer for alfalfa

Abstract: In order to solve the problems of alfalfa drying, such as smaller drying capacity, high energy consumption, and high leaf loss rate, an alfalfa dryer based on air-impingement jet and normal temperature ventilation was designed in current work. The alfalfa drying process was divided into 2 parts: high-temperature drying in air-impingement jet belt dryer based on the slit type air-impingement tube to improve the drying speed, and then drying at room temperature in the normal temperature ventilation apparatus to save energy. The slit type air-impingement tube is the main structure of air-impingement jet belt dryer. Flow field in slit type air-impingement tube was simulated using Fluent 14.0 CFD (computational fluid dynamics) software. The spoiler was added in this tube to improve the airflow uniformity of slit type nozzle exit. The location and height of spoiler are important influence factors of airflow uniformity. The structure of slit type air-impingement tube was optimized with the method of numerical simulation for flow field in which the spoiler had different parameters. The optimal parameter of spoiler was obtained, which then offered an optimum model that the variance coefficient of flow velocity was 7.7% through the 0.1-1.0 m section of nozzle exit. In order to collect information and realize automatic control, a single chip microcomputer system was designed for the room temperature ventilation apparatus. The PT100 and SHT11 sensors were used in the automatic control system to monitor the temperature and relative humidity of environment and grass. PIC16F1947 was used as the main control chip to control and dispose the information and control fan operation so as to solve the problem of moisture reabsorbtion of alfalfa in low temperature and high relative humidity environment. Alfalfa was adopted to test the performance of the normal temperature ventilation equipment combined with air-impingement jet. One group of experiment was that air-impingement jet belt dryer was used to dry alfalfa in different temperature and material thickness to the moisture content of about 45%-50%, and after that the material was dried at room temperature in the ventilation apparatus to the final moisture content of about 15%. It was observed that the moisture content of alfalfa with the thickness of 6 cm was reduced from 78.3% to 45% after 5 min drying. It can be concluded that air-impingement jet belt dryer has the ability of rapid drying for alfalfa even in high thickness. Additionally, the temperature of alfalfa was below 65 ℃ in the process of drying, which was beneficial to save thermal sensitive nutritions of alfalfa. The room temperature ventilation apparatus based on automation control could extensively reduce energy consumption of alfalfa drying. It can be concluded that the normal temperature ventilation dyer combined with hot air-impingement jet has the advantages of low-energy consumption and low leaf loss rate, as the leaf loss rate is less than 1.5%, and the energy consumption per kilogram moisture removal is 3408 kJ which is reduced by 53% compared with hot air drying. This paper exhibits a designed new equipment for alfalfa drying, the normal temperature ventilation dryer combined with air-impingement jet, which is very important to solve the problems of high energy consumption and high leaf loss rate of the current alfalfa drying equipment.