The transportation of sensitive commodities in strict ambient conditions is necessary not only on the side of fulfilling regulations but also maintaining their quality to reduce the rate of losses. Temperature is the main component that affects the goods and its control is through an airflow pattern in the cargo systems. Similarly, getting to observe the pattern of airflow helps to predict the hot spots so that one takes the necessary actions to ensure minimization of their effects. This piece will aim at defining the CFD characteristics of refrigerated trailers and the need for improvement of airflow necessary for preserving these perishable foods. These characteristics will help determine the areas likely to be hot and establish strategies to recommend for cooling. The objective of the project is to develop an environment for testing that will make it possible to conduct other airflow tests for learning about refrigerated trailers and how to improve preservation of perishable goods. The paper will also provide advice and recommendations on improving airflow and ideas for the research. This study investigates three airflow models of refrigerated cargo systems by applying turbulence flow, heat and mass transfer models. The analyses of these three models reveal that significant improvement could be achieved by applying the proper arrangements of inlets on the ceiling of the trailer body.
Population growth and consumers' continued demand for fresh food and changes in eating habits have contributed to an increasing demand for food transportation and low-temperature storage. Refrigerated transport is necessary to maintain the quality and extend the shelf life of fresh, frozen and perishable products during transport (Oro et al., 2014). In the cold chain, perishable foods must be temperature controlled at all stages. Sometimes, during transitions (such as airport run-offs, container docks, etc.) or improper handling (such as unloading in a non-refrigerated environment), the products can change very quickly from one cool atmosphere to another which is not refrigerated (Badia-Melis et al., 2014). Therefore, refrigerated vehicles are an important part of the cold supply chain system. However, many traditional refrigerated cargo systems are not designed to support the homogeneity of the temperature inside cargoes. Indeed, refrigerating equipment is usually placed on one side of transportation systems as this is considered to be more practical. Such a configuration thus leads to significant temperature differences in the two distinct parts of a refrigerated cargo, which might affect the quality, safety and shelf life of perishable foods. In this study, it is highlighted that in the most commonly used traditional refrigerated trailer models, lower air velocity and higher product temperature are observed at the rear. There is also a partial product chilling risk at the front of the refrigerated trailer.
Recent developments in the refrigerated transport sector have led to air temperature tolerances in the order of 2 C in refrigerated truck containers. With such equipment, the non-uniform cooling of perishable goods can result in a considerable loss of product quality, rendering the products unfit for sale. During the transport of fruits and vegetables, the load of the product is subject to internal production of heat, moisture and chemical degradation due to respiration. If ventilation is insufficient, the temperature, moisture content and ethylene concentration increase, causing several detrimental phenomena to the quality of the products, namely: premature ripening (due to a high concentration of ethylene), microbial growth, change in color and loss of firmness (Moureh et al., 2009a).
Recent surveys have shown that little or no research on the characterization of air velocity in a truck loaded with pallets has been carried out (Moureh and Flick, 2004). This can be attributed to the complexity of directly measuring local air velocities and air flows in the thin air spaces located between pallets and crates (Moureh and Flick, 2004). Several studies have shown significant temperature and humidity heterogeneity, with non-uniform airflow in refrigeration equipment leading to deterioration in food quality and safety. Quality changes can be microbiological (growth of microorganisms), physiological (maturation, senescence and respiration), biochemical (browning reactions, lipid oxidation and pigmentary degradation) and / or physical (loss of moisture) (Laguerre et al., 2013). In the refrigeration of food products, controlling the temperature along the cold chain is essential to maintain the quality of the product. The International Institute of Refrigeration estimates that if developing countries could acquire the same level of refrigerated equipment as industrialized countries, more than 200 million tons of perishable food would be preserved, or about 14 percent of current consumption in these countries (Laguerre et al., 2013).
Refrigerated vehicles are an essential part of the cold supply chain. Providing a uniform and fast cooling environment during the postharvest handling of produce remains a challenge (Tanner and Amos, 2003). On the contrary, numerous conventional refrigerated freight systems do not support the homogeneity of the temperature inside cargoes. Refrigerating equipment is normally placed on one side of the trailer as this is viewed as more practical. Subsequently, contrasts in the critical temperature happen in two distant parts of a refrigerated cargo, which may influence the quality, safety and shelf life of perishable foods.
In this study, three airflow models of reduced-scale refrigerated cargo systems are investigated by applying turbulence flow, heat and mass transfer models. Each of these models has different airflow characteristics. Model 1 is the most commonly used refrigerated trailer model in the cold supply chain system. Model 2 and Model 3 have elongated inlets on top of the refrigerated trailer. Model 2 has one outlet on one side in the front part, whereas Model 3 has two outlets on the front and back sides of the refrigerated trailer. In comparison with Model 2, the placement of the additional second outlet in Model 3 aims to cope with the temperature increase at the back of the trailer.
The prevailing research reflects the increasing amount of international literature in this area based on different viewpoints. For example, studies of CFD characteristics in relation to refrigerated systems include research on a reduced-scale trailer loaded with palletized cargoes (Moureh and Flick, 2004; Moureh et al., 2002, 2009a, b, c, d; Rodriguez-Bermejo et al., 2007; Smale, 2004; Tapsoba et al., 2006, 2007a, b; Alvarez et al., 2003; Defraeye et al., 2013, 2015a, b, c). Cardinale et al. (2016a, b) conducted a numerical and experimental analysis of convective flows within an intermodal container to verify the uniformity of cold distribution. They also compared the results of the numerical analysis with the experimental data acquired during the simulation to validate the implemented model. Their research provides an opportunity to reduce the heterogeneity of the temperature distribution as well as increase the efficiency of refrigeration systems.
Defraeye et al. (2015a, b) investigated the feasibility of cooling an ambient load of citrus fruits in refrigerated containers during shipping. They explored the practice of hot loading citrus fruit into refrigerated containers for cooling during shipping as an alternative to forced air pre-cooling. A refrigerated container was theoretically able to cool the product in less than 5 days; however, they found that these cooling rates are not currently realized in practice. Indeed, such cooling is only successful for exporting sweet oranges. Katai et al. (2016) examined the hydrodynamic airflow relationships in two adjustable compartment units of a mobile refrigerated container and proposed cooling and ventilation elements.
Getahun et al. (2017a) developed and validated a CFD model for cooling produce inside a fully loaded refrigerated container based on a porous medium approach. They used wind tunnel experiments to characterize the airflow resistance of fruits stacked on a standard pallet, and pressure drop data were used to develop a porous CFD model of the airflow and heat transfers. Their simulation successfully replicated the airflow and temperature profiles inside the container, allowing them to identify areas of high and low air circulation and cooling.
Getahun et al. (2017b) applied a 3D model of a refrigerated container packed with stacks of apples to study the performance of commonly used apple packaging box designs and effect of the resistance of vertical airflow on cooling. This study revealed the inadequacy of packaging designs for cooling operating conditions characterized by a vertical airflow inside refrigerated containers. In addition, they found that pallet orientation/configuration influences airflow distribution, the fruit cooling rate and temperature uniformity in a refrigerator due to its effect on networking and the transfer of temperature.
Kan et al. (2017) studied the impact of cargo stacking modes on the temperature distribution inside marine refrigerated containers. The results showed that temperature distributions become disordered with increasing stack height; the difference in temperature rises with an increase in the length of the battery; the temperature tends to be isothermal when the stack space or the space between the stack and the surface of the sidewall increases. The results of the simulation concurred with the experimental results.
Moureh and Flick (2004) analyzed the airflow and temperature distribution in a typical refrigerated truck loaded with pallets. The experiments were carried out on a reduced-scale model (1:3.3) of a refrigerated vehicle trailer. Ventilation performance and temperature homogeneity were characterized with and without a blowing air duct system. The numerical modeling of the airflow was performed by using the CFD code and a second-moment model, namely the Reynolds stress model (RSM). The results obtained using the RSM showed good agreement with the experimental data. The numerical and experimental results clearly show the importance of air ducts in reducing temperature differences throughout the cargo. Moureh et al. (2002) also conducted a numerical and experimental study of the airflow in a typical refrigerated truck loaded with pallets. Numerical modeling was performed for CFD and two levels of turbulence modeling: the standard k-epsilon model and a second-moment model (the RSM). Only the results obtained by using the RSM agreed with the experimental data.
Moureh et al. (2009) analyzed the characteristics of air velocity inside ventilated pallets loaded in a refrigerated vehicle with and without air ducts. A scaled-down model and CFD predictions were used to study experimentally and numerically the airflow patterns in a typical refrigerated truck loaded with ventilated pallets filled with spherical objects. Again, the numerical modeling of the airflow was performed by using the CFD code and RSM. They found that a blown air duct system considerably improves the homogeneity of ventilation in a vehicle.
Han et al. (2016) established a 3D model of a refrigerated truck and used an unstable CFD-SST (shear stress transport) calculation model to simulate the temperature distribution inside a refrigerated container with and without air ducts.
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