Numerical investigation of an air-conditioning unit to manage inside greenhouse air temperature and relative humidity.

摘要

This study focuses on the numerical investigation of a prototype air conditioning unit developed to control the air temperature and humidity inside crop growth chambers. The device is fed with air by a fan and consists of two ducts. In the first, the air flow is not saturated. In the second, air is saturated after having crossed wet corrugated pads. An electric resistance provides heat just before the pads to give identical temperatures at the end of the ducts. A door could be moved to control the proportions of saturated and non saturated air at the outlet. Numerical techniques based on Computational Fluid Dynamics (CFD) were implemented to analyze the flow characteristics of the mixed air produced by the device. A three dimensional steady state model using the k- epsilon closure and the Boussinesq assumption was chosen. The flow inside the pads, considered as porous media, was governed by the Darcy Forchheimer equation and the Ergun law. The boundary conditions (inlet velocity, heat source and sink, and water mass fraction) were inferred from the available experimental data. Simulations were carried out for different configurations of the aperture opening from 10 to 90% and for air inlet speeds from 1.5 to 8 m s-1. A virtual tracer gas technique was used to assess the mixing process. Simulations disclosed a non linear relationship between the air flow rate and the aperture opening. The distribution in percentage of flow rates however, remained the same whatever the inlet air speed. The mixing just downstream of the moving door was not perfect but could be improved by the addition of baffles. The water source and heat loss due to the water phase change were included in the model, leading to saturated air downstream after the pads. The promising results obtained from CFD will be used to improve the automation of the device for potential application in closed greenhouses..