Phase Change Materials
July 20, 2007
Dr. John Kizito of North Carolina A&T State University presented his summer work on Phase Change Material (PCM) heat exchangers. The need for PCMs arises from the drastic temperature differences in space. For example, the International Space Station is +250F on one side and -250F on the other. Heat exchange is critical to keeping the temperature within a reasonable range.
Current thermal control options include flash evaporators, heat rejection systems (a function of size), and a lot of insulation (a function of cost, weight, and size). The PCM; however, stores excess heat from the vehicle to control the temperature when space radiators are not available to reject heat. Upon melting, the latent heat phase change material can be modeled as a heat sink, while upon freezing, the PCM can be modeled as a heat source.
Dr. Kizito’s goal is to model the performance of a PCM heat exchanger during freezing and thawing cycles. By using system requirement constraints and thermal boundary conditions, the model will predict the temperature exchange as well as the physical changes of the liquid solid interface. These materials can be made of normal alkanes or paraffins.
In his model, Dr. Kizito focuses a great deal on the Stefan number and the material’s property ratio, two non-dimensional numbers that drive the melting/solid processes. To make his model more realistic, he will eventually account for non-isotropic conduction and convective flows due to expansion/contraction. He will also add 3D analysis and will validate the model against experimental data.