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Experimental Investigation into the Direct Contact of Working Fluid with Phase Change Material and its Effect on the Heat Transfer Rate

Ranjbar Kermani, Javad | 2023

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 56876 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Shafii, Mohammad Behshad
  7. Abstract:
  8. The unique properties of phase change materials (PCMs), such as high latent heat and nearly constant phase change temperature, have made them a suitable option for use in thermal energy storage systems and thermal management systems. However, the low thermal conductivity of these materials has imposed limitations on their widespread use in industries. In this study, for the first time, the effect of injecting a boiling fluid (BF) into the PCM container to enhance heat transfer and accelerate the solidification process has been experimentally investigated. In this regard, paraffin wax and acetone have been selected as the PCM and the BF, and the effects of parameters such as the initial temperature of the PCM (75°C to 95°C), the flow rate of the injected BF (0.32 to 0.57 Lit/min), the height of the paraffin inside the container (15 cm to 35 cm), and the nozzle diameter (2 mm to 4 mm) on temperature distribution, solidified paraffin surface, and heat transfer have been evaluated. Furthermore, in order to more accurately evaluate the process and visualize it, a container with low thickness, known as a Hele-Shaw cell, has been used. The results indicate that due to the injection of acetone into the container and its direct contact with paraffin, acetone absorbs the heat of the paraffin and, as a result of boiling, creates significant turbulence and mixing. Consequently, the high heat transfer coefficient of boiling, formed vortices, and turbulent flow inside the container significantly improve the heat transfer and solidification time. Moreover, the obtained results reveal that the flow rate of the injected fluid is the most influential parameter in improving the freezing process, followed by parameters such as the height of the paraffin in the container, the initial temperature of the paraffin, and the nozzle diameter, respectively. For instance, when paraffin with maximum height is placed inside the container, the maximum flow rate is able to reduce the average temperature by 25°C in 10 s, while for flow rates of 0.46 Lit/min and 0.32 Lit/min, the temperature reduction is less than 10°C and 5°C, respectively, for the same period. Moreover, the full solidification time obtained in this study shows three orders of magnitude, at least 63 times, and 36 times improvement compared to the pure PCM, PCM modified with various methods (e.g., fin, nanoparticles, metal foams), and conventional direct contact methods. In this regard, it was observed that by employing the highest injection flow rate, the current method is capable of solidifying 0.7, 0.5, and 0.3 L of molten paraffin in 48, 33, and 18 s, respectively
  9. Keywords:
  10. Solidification Time ; Phase Transformation ; Phase Change Material (PCMs) ; Heat Transfer Rate ; Boiling Fluid (BF)

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