Heat Transfer Modeling And Testing For Latent Heat Insulating Packages

Insulating packages are wildly used in the transportation of temperature sensitive production. The improvement of refrigerant, research of heat transfer and phase change process are the key technology of insulating packages used in cold chain. However, the heat transfer mechanisms and design method of insulating packages are scanty for presented studies. In this paper, the thermal conductivity modification of refrigerant used in insulating package was discussed. Furthermore, a new semi-infinite mathematical model was developed for the heat transfer process in insulating package. Based on the semi-infinite model, a prediction model of insulating package shelf life was proposed, and solution boundaries of insulating packages were presented. Furthermore, a process for designing insulating packages was established by using the proposed solution boundaries. Finally, a computer software used for design and certification of insulation packages was developed based on VB system.The contents are as followings.(1) Thermal conductivity improvment and modeling of refrigerantThe thermal conductivity of liquid refrigerant was modified by mixing aluminum powder, copper powder and graphite powder, and the thermal conductivity of phase change microcapsules(micro-PCM) refrigerant was modified by mixing heat transfer fluid. The results indicate that the effective thermal conductivity of liquid refrigerant increases with the mixing of powder when the mass fractions of solid particles are less than the critical fractions. However, the effective thermal conductivity of liquid refrigerants begin to sharply decrease at a relatively high mass fractions of solid particles. Similarly, the effective thermal conductivity of micro-PCM refrigerant significantly increases with the increase of mixed heat transfer fluid while the mass fraction of micro-PCM is less than the critical fraction, and this trend of increase is suddenly decrease when the mass fraction of micro-PCM exceeds the critical fraction.Furthermore, the freezing and melting speed of refrigerant modified by graphite powder was experimentally discussed. The results show that the freezing speed of refrigerant is significantly increased by mixing graphite powder, while the melting time is almost constant under the normal using condition(the refrigerant is usually used in an insulating container).The mechanism of the thermal conductivity change with the mixing powder was developed. Besides, an analysis model(FP model) based on fractal theory and percolation theory was also proposed and compared to the classic model. The results indicate that the presented model has a good agreement with the experimental results.Finally, a unified model based on FP model for calculating the effective thermal conductivity of tripartite disperse systems was presented, and effective thermal conductivity maps of ternary disperse systems were also established.(2) One-dimensional phase change model of insulating packageA one-dimensional phase change model of insulating package was presented that involves a hybrid of a heat transfer model and an iterative algorithm. The surface heat transfer coefficient was determined through applying the iteration process, and the temperature fields in energy storage systems were simultaneously calculated through the presented model.The effects of wk, lk, y, b, L and variational environment temperature on the process of heat transfer with phase change were seriously discussed. The results show that the surface heat transfer coefficient is significantly affected by wk, y and b. Furthermore, the moving speed of solid-liquid interface is change with wk, y, b and L. The heat transfer process under change environment temperature can be approximately treated as a linear superposition of several constant temperature processes.The proposed one-dimensional phase change model was compared to two classic models. The results show that the degradation form of the presented model has a good agreement with the analysis model of Neuman. Furthermore, the presented model also has a good agreement with Mehling’s model.Finally, the validity of the proposed model was experimentally verified through a home-made experiment equipment and showed consistent agreement(3) Prediction model of insulating package shelf lifeBased on the proposed one-dimensional phase change model, the empirical model of the surface heat transfer coefficient and system thermal resistance were mathematically presented, and a compact prediction model of insulating package shelf life was also developed for the application of engineering. The test results indicate that the proposed prediction model can accurately calculate the shelf life of insulating package.(4) Solution boundary and design method of insulating packageSolution boundary of insulating package was presented through the previous prediction model, and a design method including five steps was proposed based on the solution boundaries. Finally, a design software of insulating package was jointly developed with another team member.