Optimization of the humidification of cold stores by pressurized water atomizers based on a multiscale CFD model

Abstract

Humidification during long-term cooled storage of fruits is being introduced into practice to prevent excessive moisture loss and quality degradation. High pressure fogging is one of the few systems that can be used in ultra low oxygen (ULO) storage rooms. For system design and optimization, a CFD based multiscale model was used. At the smallest scale, the flow through stacked products in boxes was predicted using a direct model that combined discrete element (DE)-CFD modelling. At larger scale, a loaded cool room model that predicts the storage room air velocity, temperature and humidity distributions and fate of the water droplets was developed. The loaded product was considered as a porous medium, where the anisotropic loss coefficients were determined from a combined DE-CFD simulation. A Lagrangian particle tracking multiphase flow model was used. An interval humidification of 1 min on and 15 min off with a water pressure of 80 bar (3.59 L h⁻¹) for a ULO storage room at −1 °C on average gave a relative humidity of 96.3%. The amount of sprayed water deposited on the stack and room surfaces depend on the application pressure and nozzle position and direction. Good agreement was found between measured and predicted results.

Introduction

The relative humidity (RH) of storage rooms affects the quality of the stored products. Numerous previous studies reported a significant effect of RH on weight loss and firmness (Henriod, 2006, Hertog et al., 2004, Paull, 1999, Tu et al., 2000). The most important factors that affect the weight loss are the rate of respiration and the water vapour pressure difference between the surrounding air and the product surface. A low optimum storage temperature combined with a high RH of the surrounding air can reduce the weight loss. However, due to the condensation and freezing of water vapour on the cooling coils, maintaining the required high RH at subzero storage conditions is very difficult. The temperature difference between the cooler surface and the room air is the most important factor that controls the rate of condensation/freezing of water vapour on the cooling coils (Paull, 1999). To maintain the required high RH, humidifying the cooling air using steam or atomized water droplets are the commonly used methods. However, an excessive RH level may encourage microbial spoilage and thus it is necessary to maintaining the RH at an acceptable level (Tassou and Xiang, 1998). Most of the previous studies discussed the advantages of humidification on decreasing the saleable weight loss and maintaining the product quality during storage. But it is hard to find any studies on the optimal operation and design of such humidification systems for sub-zero storage conditions which are commonly used for storing products such as pear.

Since it affects the storage room temperature during the time of injection, steam humidifier is not a good alternative. Cold water atomizing humidifier can avoid this increase of the storage room temperature, but due to the low evaporation rate of the water droplets at sub-zero optimum storage temperature (for instance, −1 °C for conference pears or −2 °C for chicory root) maintaining high RH (>95% RH) with minimum deposition of the droplets on the product surface is very challenging. There are four types of atomizers that are commonly used for such humidification purpose: high pressure, ultrasonic, air-assisted and rotary atomizes. However, for ULO storage at sub-zero temperature the high pressure system is the viable option. Due to the low operating temperature and low ejection velocity of the droplets from ultrasonic atomizer (Rajan and Pandit, 2001), the clogging of the ejector due to the freezing of the droplets on the surface makes its operation for such storage condition very difficult. In case of the air-assisted nozzles, the high velocity atmospheric air that is used for the atomization disturbs the air composition of the ULO room. Rotary atomizers are quite complex (Bayvel and Orzechowski, 1993). Experimentally, the suitability of the pressurized water atomizers for humidification of storage rooms at −1 °C was investigated successfully (Delele et al., 2007a).

The use of experimental methods to set the optimum operating parameters that give such a high relative humidity without causing excessive water deposit on the product is very difficult. The use of CFD modelling is an alternative to the expensive and tedious experiments. A number of studies to model the air flow pattern and temperature distribution in cold storage rooms demonstrate the appropriateness of the method (Chourasia and Goswami, 2007, Hoang et al., 2003, Hoang et al., 2000, Nahor et al., 2005, Tassou and Xiang, 1998, Xu and Burfoot, 1999). The humidification system is usually not modelled and for calculating the pressure loss coefficients for a flow through stacked products, these previous studies commonly used the Ergun (1952) correlation that does not consider any effect of the confining and vented box walls often encountered in fruit storage. For a flow through loaded vented strawberry package, Ferrua and Singh (2008) observed a non-homogeneous and complex flow distribution; and stressed the need for geometrical and mathematical models that are capable of generating the geometry and predicting the flow through the package and product. The humidification system was modelled using a particle tracking approach (Delele et al., 2007b, Gouesbet and Berlemont, 1999).

In this paper, we present and validate a complete CFD model that predicts the storage room air velocity, temperature and humidity distributions and fate of the water droplets that were sprayed from humidifying nozzles. In this study the loss coefficients were determined using an explicit combined DE-CFD simulation that takes into account product geometrical properties, box design and the randomness of the product stacking (Delele et al., 2008).