Effects of ejection-permeation ratio in fiber duct on the characteristics of air flow field in cold storage

Abstract: Fiber duct is widely used to cold store due to its characteristics of uniform air supply. However, the storage environment mainly depends on the proportion of air supply ejected from the duct orifice and that of fiber permeation, called ejection-permeation ratio. Taking a 50t apple cold store in Xi'an as the research object, this study aims to determine an optimal ejection-permeation ratio in the fiber duct of the air dispersion system for the storage environment. In the case of fiber duct, five ejection-permeation ratios (only ejection air supply, 3:1, 1:1, 1:2 and only permeation air supply) were used to simulate the velocity and temperature field in the cold store using CFD (computational fluid dynamics) technology. A 3D (three-dimensional) steady numerical model was established to simulate the air flow and heat transfer in three modes of air cooler (fiber air dispersion system)-cargo-storage air environment. The apple storage zone was assumed as porous medium. SIMPLE algorithm and two-equation model were used to solve the governing equation. Five types of grid quantity in structural grid division were selected to obtain the independent computing of grids in the cold storage model. Specifically, the calculating result cannot depend on the number of grids, when the number of grids was finally determined to be 1.96 million. In terms of the reliability and verification of the model, the axial velocity of the air supply from jets under the ejection-permeation ratio of 3:1 was selected to compare with the velocity attenuation formula of plane jet in previous theoretical analysis. Finally, it is concluded that the simulation results are similar to those calculated by the formula with an average relative error of 11.89%, indicating the reliability of the numerical model. Numerical simulation results show that: when the cooling capacity of the fan was set as a constant, the average velocity of the air areas in the cold store decreased linearly, while the velocity uniformity increased gradually with the decrease of the ejection-permeation ratio. Compared with the pure ejection condition, the air supply mode using fiber duct combined with ejection and permeation can better meet the requirements of wind speed in the cold storage. With the decrease of the ejection-permeation ratio, the average temperature of the cargo area in the cold storage first decreased, then increased, and finally became stable, whereas, the temperature uniformity decreased gradually. This indicates that the ejection can serve as the main air supply, while the permeation as the auxiliary. The optimal velocity for apple storage in the cold store was less than 0.5 m/s when the ejection-permeation ratio was 3:1. The average temperature in the cargo area was between 272.86-273.33 K, while the temperature range of (0±0.5) ℃ was required for apple refrigeration, at the ejection-permeation ratio of 3:1. Therefore, the air dispersion system can be design to use the air supply mode with ejection as the dominant and fiber permeation as the auxiliary in the cold storage of fruit and vegetable.