Study On Temperature Regulation And Optimization Of Refrigerated Transport For Perishable Foods

As one of the most important conveyances of connecting each step of the cold chain logistics, refrigerated transportation equipment undertakes an important business of guarantying food quality, controlling transportation costs and reducing energy consumption. Therefore, it is particularly important to study precisely the control of temperature and its fluctuation range of the refrigerated transportation equipment. The dissertation focuses on the aspects of the impact of temperature and environment to the quality of perishable foods during refrigerated transportation process, temperature uniformity inside the refrigerated compartment, the thermal stability of the typical refrigerated compartment, temperature change characteristics of refrigerated compartment, the factors affecting those characteristics, and temperature fluctuations inside refrigerated compartment as well as its control measures. Besides, optimization in the design of refrigerated compartment is covered in the research. Specific contents are as follows:(1) Temperature uniformity and temperature fluctuationsCompartment inside temperature uniformity and its fluctuations will affect the quality of perishable foods during refrigerated transport process. Airpak fluid simulation software was introduced to numerically simulate the factors affecting the temperature uniformity inside the compartment. The results show that the most reasonable thickness of compartment polyurethane insulation materials between100and120mm; the best temperature uniformity could be reached when the outlet is located at the bottom position of the right front compartment; the different refrigerated transport temperature decides different wind speed inside the compartment, the higher of the air supply temperature, the faster of the reasonable wind speed, and it corresponds to a preferred wind speed. Based on the frequency domain analysis, refrigerated compartment temperature disturbance mathematical model and temperature response mathematical model are established to analyze the perturbation amplitude and phase angle of the four kinds of temperatures. It is found that the inside compartment temperature fluctuations could be reduced under certain conditions, which include increasing specific heat capacity of the compartment, insulation materials density of the compartment and reducing convective heat transfer coefficient of the inside and outside surface of compartment; their two perturbations are attenuated when refrigeration frequency and defrost frequency are within attenuation domain range; temperature amplitude reaches its minimum when refrigeration and defrost temperature perturbation are at the same phase angle; the inside compartment temperature fluctuations are reduced when the compartment is equipped with better airtightness, lighter color surface, and the vehicle change its speed with a frequent variation.(2) Thermal stability of inside compartmentBased on thermal theoretical knowledge, the attenuation and delay effect as well as the inside heat stability impact of four typical insulated compartments are investigated. It is founded that different insulating materials decide different outside sol-air temperature time lags and the attenuation coefficients, to be more specific, the bigger of the compartment thermal inertia index or resistance, the longer of the corresponding time lag and the bigger of attenuation coefficient; different traveling directions or insulating materials also decide different inner wall surface temperatures of compartment; thermal stability mainly depends on the thermal resistance and thermal inertia index of compartment insulation material, to put it another way, the higher of the thermal resistance and thermal inertia index value of the insulating material, the better of thermal stability of compartment.(3) Temperature change characteristics of compartmentBased on the dynamic thermal equilibrium theory, the inner compartment temperature changes mathematical model, which covers the temperature changes of single-temperature, multi-temperature refrigerated vehicle and door-opening process, is constructed. It is shown that the inner temperature exponentially decrease during the cooling process and increase during the door-opening process; Longer hours are required when the compartment thickness is reduced or the thermal conductivity increased, the solar radiation absorption coefficient of outside surface of compartment increased, or speed increased, compartment leak multiples increased, goods respiratory heat increased and cooling capacity is decreased. For double-temperature refrigerated compartments, longer cooling hours is consumed in the two zones when cooling capacity is smaller, goods respiratory heat increased, the speed increased or compartments thermal conductivity increased; Longer cooling hours in the freezing compartment is taken, but less cooling hours for the refrigerated compartment, when wind speed of the electric fan and the outlet area increases between the two zones; Longer cooling hours in the freezing compartment is taken, but almost the same cooling hours for the refrigerated compartment, when the total volume of the compartment is unchanged and frozen zone volume increased. Door-opening process of refrigerated trucks, turn on or off the refrigeration system, blowing wind speed in the compartment and the insulation ability of compartment have little effect on temperature changes in the refrigerated compartment; the inside temperature is increased faster if there is a greater temperature difference in and out of the compartment, the door opened broader and the compartment volume decreased. Whether there is wind or no or even wind direction outside the compartment due to the different rates of elevated temperature during the door-opening process.(4) Compartment optimization designThe smallest heat transfer coefficient and the maximum internal space of compartment as the objective function, analysis of the variation of compartment interior space and compartment heat transfer coefficient, and the best thickness of compartment insulation material under different parameters. The results showed that different speeds, different speeds, different thickness of the insulation material, different thermal conductivity of insulation materials, the smallest heat transfer coefficient and the maximum internal space of compartment are not the same; when the optimal compartment heat transfer coefficient and the optimal internal space of compartment conditions at the simultaneously time meet, thickness increases of compartment insulation material along with the optimal compartment heat transfer coefficient increases. (5) The relationship between refrigerated transport and the quality of perishable foodThe best temperature and humidity are different among different perishable foods whole logistics process, simulation pork and lychee logistics process under different temperature conditions. The experiment showed that different temperature and humidity conditions of transport and sale, lychee undergoes different changes in its brown change index, the anthocyanin-optical density difference, peel color values, pulp PH value, weight loss rate and in other indicators; he temperature and humidity conditions affect the quality of pork during the process of transport, distribution and sale; quality of the pork and its shelf life largely depend on the temperature control during the transport and sales process.