Characteristic analysis of humidity control in a fresh-keeping container using CFD model

Highlights

• The effects of parameters on humidification were investigated by CFD methods.

• Introduce a device used to measure the ventilation resistance characteristics.

• Humidifying time and humidity difference were affected seriously by atomizer number.

• The structure of humidifier had no significant difference on humidification rate.

Abstract

Humidity control, which is affected by the performance of humidifying device and structure of the container, is very important for delaying water loss of fresh products. Humidifying rate and humidity distribution uniformity in fresh-keeping container were investigated to evaluate the characteristic of humidity control by Computational Fluid Dynamics (CFD) models. A pressure gauge was adopted to measure the ventilation resistance of products, by which the inertial resistance and viscous resistance values were obtained. The results of humidity performance were evaluated by entropy method. The results showed that the number of ultrasonic atomizers and sensor location had a significant effect on the humidifying rate. The effects of spray height, deflector angle and products quantity on humidifying rate are not significant, but they showed significant difference in the humidity distribution uniformity on products. The results of this study provide a better understanding of humidity control, which will help for the environment control in a fresh-keeping container.

Introduction

Water loss is one of the problems for most of fruits and vegetables after being harvested (Kelly et al., 2018, Anastasia et al., 2019), and it will accelerate quality deterioration (Rodov et al., 2010). A great number of researchers have shown that relative humidity (RH) of storage environment has significant effects on the quality of the products (Henriod, 2006, Delele et al., 2009a, Jalali et al., 2019). The high water vapour pressure deficit (WVPD) caused by cooling air was considered as the primary reason that promoted the evaporation of fresh product during cooling storage (Paull, 1999), which urged the further development of packaging design for fresh produce (Pinela and Ferreira, 2017, Forero et al., 2018). Meanwhile, most agricultural products are stored under high RH and low temperature environment (Hung et al., 2011). However, it is more difficult to control RH compared to temperature in a storage environment since the RH is affected by a number of factors, such as the temperature fluctuation in the storage environment and transpiration from products. Additionally, the ventilation way, evaporator of the cooling system, indoor temperature and temperature difference between the evaporator and the air also affect humidity control (Paull, 1999). Therefore, decreasing the moisture loss and keeping the RH balanced remains problematic in a storage environment.

Mukama et al. (2019) indicated that humidity control can help to maintain the colour, firmness, and chemical quality of fruit as well as reducing weight loss. Three kinds of traditional humidification devices have been widely studied in previous researches before the extensive use of ultrasonic humidification, including high pressure, air-assisted and rotary atomizers. Delele et al. (2007) found that high pressure atomizers were more suitable for humidification in −1℃ storage rooms. With regards to the air-assisted atomizer, its high velocity of air-mist flow disturbs the flow pattern in the storage room (Delele et al., 2009a). As for the rotary atomizer, it is too complex to operate due to its redundant structure (Bayvel and Orzechowski, 2019). As one of the humidifying methods, ultrasonic humidification plays an important role in retaining the moisture of fruits and vegetables during cold storage (Fabbri et al., 2018). The diameter of water mist generated by an ultrasonic humidifier (by grouping several ultrasonic atomizers) is less than 100 nm, and it had no conspicuous heat exchange, while the mist diffuses into the air different from the traditional humidification method (Brown et al., 2007). Meanwhile, traditional humidification may promote microbial growth in the environment due to its large-sized water mist, which deposits on the surface of products, and it ultimately accelerates the deterioration of fruits and vegetables (Delele et al., 2009b, Saenmuang et al., 2012). The advantages of ultrasonic humidification are attributed to its high evaporativity of water mist (Hung et al., 2011). Thus, an ultrasonic humidifier was applied as a better humidification method in containers with controlled atmosphere for fruits and vegetables (Guo et al., 2013). However, how the parameters of ultrasonic humidifier affect the humidifying efficiency in container is unclear. Therefore, it’s important to study the effects of the parameters of ultrasonic humidifier on humidity control to improve humidifying rate and humidity distribution uniformity.

Experimental investigation is an effective way to evaluate the humidifying effects in a container. However, experiments often cost amounts of time and labour, and the effect of sensor precision on the results is highly significant. CFD methods use visual design approach to enhance 3-D visualization, which obtain a considerable amount of airflow information. The results of these methods are presented through colourful contour maps that provide excellent insights into the airflow transport process. Numerous studies have shown great interest in modelling the pattern of airflow in storage with FLUENT program (Ambaw et al., 2017, Defraeye, 2013, Delele et al., 2013, Zhao et al., 2016). Ambaw et al. (2007) developed a CFD model of a spraying process to simulate the airflow and predict the position of deposited water mist. Defraeye et al. (2013) simulated the airflow and heat exchange of spherical food products during the cooling process using the model and validated the CFD model by a series of experiments, which reveals that product is one of the factors that has a great effect on the indoor airflow. Therefore, products might also affect the humidifying rate and humidity distribution uniformity.

Model simplification is one of the essential steps in solving CFD case, which helps to reduce time cost. The porous medium model, which is used to represent the complicated characteristic of products that set as porous medium, has been proven as a reliable method to study the effects of real products on ambient air (Getahun et al., 2018, Partl et al., 2019). Hoang et al. (2015) compared the porous medium block from the CFD model with the real solid block of apple bins and found a similar temperature distribution in both cases. With the help of CFD methods and porous medium model, the parameters optimization in cold chains has been widely investigated and the results from previous studies have provide a lot of advices for improving humidity control. Guo et al. (2015) tried to find better parameters for a pipeline humidifying device by both experimental study and numerical simulation, in which the effects of velocity of the air return duct, diameters of pipes and the number of spray exits on pipes were investigated. Delele et al. (2009b, 2013) studied the effects of the vent area, shape, number and position on the airflow and heat exchange in a cold room, and they found that the distribution of humidity was mainly affected by the vent number, location and shape. It can be inferred from previous studies that the position of monitor sensor might have effects on humidifying rate and humidity distribution uniformity.

This paper presented and validated a complete CFD model that predicts the humidifying rate and the uniformity of the humidity distribution on the products surface under different conditions, including different the number of ultrasonic atomizers, spray height, deflector angle, location of monitor sensor, and stacking height of products. The results from which could help to improve the humidifying performance of ultrasonic humidification methods in a container.