Experimental Study of a Closed-Loop Thermosyphon Operating With Slurries of a Microencapsulated Phase-Change Material

An experimental investigation of steady, laminar, fluid flow and heat transfer in a vertical closed-loop thermosyphon operating with slurries of a microencapsulated phase-change material (MCPCM) suspended in distilled water is presented. The MCPCM particles consisted of a solid-liquid phase-change material (PCM) encapsulated in a thin polymer resin shell. Their effective diameter was in the range 0.5 to 12.5 micrometers, and had a mean value of 2.5 micrometers. The melting and freezing characteristics and the latent heat of fusion of the PCM were determined using a differential scanning calorimeter. The effective density of the MCPCM was measured, and the effective thermal conductivity of the slurries was determined using a published correlation. In the range of parameters considered, it was determined that the slurries exhibit non-Newtonian behavior. The closed-loop thermosyphon consisted of two vertical straight pipes, joined together by two vertical semi-circular 180-degree bends made of the same pipe. An essentially constant heat flux was imposed on a portion of one of the vertical pipes. The wall temperature of a portion of the other vertical pipe was maintained at a constant value. The outer surfaces of the entire thermosyphon were very well insulated. Calibrated thermocouples were used to measure the outer-wall-surface temperature at numerous points over the heated portion and the bulk temperature of the slurry at four different locations. A special procedure was formulated, benchmarked, and used to deduce the mass flow rate of the slurries in the thermosyphon. The investigation was conducted with slurries of MCPCM mass concentration 0% (pure distilled water), 7.471%, 9.997%, 12.49%, 14.95%, and 17.5%. The results are presented and discussed.