Modelling of micro-environment inside evaporatively and coolbot cooled stores using computational fluid dynamics models and changes in quality of stored tomatoes.
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Date
2018
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Abstract
The postharvest loss of fresh produce is a global problem, with the tomato fruit being subjected to a 30-50% loss of total production after harvest. The cost and the technicality of modern technology, including mechanical refrigeration, are not appropriate and sustainable for small-scale and middle-income fruit and vegetable farmers. Low-cost cooling technologies, such as evaporative cooling (EC), CoolBot-air-conditioning (CBAC) and a combination of the two (EC+CBAC), provide alternative solutions to minimize postharvest losses. However, there is insufficient information on the modelling of airflow, heat and mass transfer, using the computational fluid dynamics (CFD). The aims of this study were: i) to investigate the real-time airflow pattern, temperature, enthalpy, heat flux and relative humidity distribution inside the unloaded evaporative cooler and CoolBot-air-conditioner cooling systems, using CFD modelling techniques, ii) to evaluate the effect of inlet air characteristics on the airflow resistance inside the selected appropriate semi- and fully-loaded cold storage chambers, iii) to screen the best combinations of pre-storage disinfection treatments, combined with the low-cost cooling technologies, in terms of changes in quality of tomato fruit, and iv) to develop predictive models for the estimation of changes in quality and probability of marketability, using experimental data, obtained during the storage of tomatoes, after they have been subjected to different postharvest treatments and low-cost cooling technologies. The specific aim of this study was to investigate the real-time airflow pattern, temperature and heat flux inside EC, CBAC and EC+CBAC storage systems, using CFD models, to evaluate the changes in the quality of stored tomatoes and to develop predictive models for quality changes of tomatoes stored under EC and CBAC.
The experimental results showed that the indirect heat exchanger (IHE) and one evaporative cooling wet pad were sufficient to reduce the temperature of the hot ambient air from 34.1℃ to 22.82℃. However, using multi-layer evaporative cooling pads was proven to have significant importance in increasing the relative humidity of air leading to the storage chamber. The EC+CBAC combination was the best for maintaining an optimum temperature (8-15°C) and relative humidity (80-99%) of the micro-environment inside the cold storage chamber during the storage period. A computational fluid dynamics (CFD) model was used to investigate the airflow, temperature and heat flux across the IHE and psychrometric unit. The numerical results showed that the psychrometric unit was not sufficient for reducing the ambient air temperature to at least near the wet-bulb temperature of the ambient air, hence, the modification of the airflow regulating fan is required. The steady state and transient CFD modelling of the micro-environment was performed in order to visualize the inside of the unloaded EC, CBAC and EC+CBAC storage chambers, the non-uniform distribution of air velocities, temperature and heat flux was found inside the storage chambers. Visual observations of velocity vector magnitude and temperature uniformity distribution inside unloaded stores were demonstrated in decreasing order, from EC+CBAC, CBAC and EC, respectively.
The modelling of the semi- and fully-loaded tomatoes stacked in the EC+CBAC cold storage chamber, using CFD models, was found to be crucial for determining the airflow resistance, which is the fundamental parameter for the engineering analysis of heat and mass transfer inside the cold storage chamber during the re-design process. The results showed that the long-side of the tomato stack, facing the direction of the airflow, had a lower airflow resistance, when compared to the short-side stack. Therefore, it was demonstrated that there was better airflow through the long-side stack. The CFD models that were developed were validated by comparing the air velocity and the reported RMSE was found to be acceptable, compared to literature. The information generated in this study has resulted in EC and CBAC cold store systems being used, for the first time, for the analysis of airflow characterisation and heat transfer that takes place inside a storage chamber.
The integrated approach, combining the application of low-cost evaporative cooling storage and pre-storage disinfection treatments (i.e. chlorinated water and anolyte water), was found to be the best method, for extending the shelf-life of the tomato fruit, by maintaining the better quality of fresh tomatoes for 28 days of storage. A combination of the green maturity stage and both chlorinated and anolyte water maintained better quality parameters, in terms of the colour, firmness and many other quality indices, when using the low-cost cooling technology during storage period.
Logistic regression, polynomial, fractional polynomial, multivariate and covariance models were developed for low-cost cold storage systems for the first time, to predict the quality of the stored tomatoes under integrated postharvest handling and treatment. These models were developed for the tomato quality indices subjected to low-cost cooling technologies and pre-storage disinfection treatments and they were found to follow curvilinear relationships. Utilizing the logistic regression model, the marketability of tomatoes was found to be higher for tomatoes stored under an evaporative cooling environment than for those stored under ambient environment. The fractional polynomial, polynomial and multivariate models that were developed can be used by farmers to predict the changes in the quality of fresh tomatoes during storage, for the selective quality parameters such as tomato firmness, colour and many others. A study of model’s development, as well as the use of low-cost cooling technologies and pre-storage disinfection treatments, should be conducted for other fresh produce.