Experimental investigation of the effects of mass fraction and temperature on the viscosity of microencapsulated PCM slurry

The use of microencapsulated phase change materials (mPCM) is one of the most efficient ways of storing thermal energy. Microencapsulated phase change slurry (mPCM slurry) is formed when the microencapsulated phase change material is dispersed into the carrier fluid. The mPCM slurry can be used as a heat transfer medium. This paper details an experimental study that was performed to investigate rheological properties of microencapsulated phase change slurry (Micronal (R) DS 5039 X - water). Six samples of mPCM slurry were prepared with different mass ratios of mPCM to water, namely: 10:90, 30:70, 50:50, 70:30, 90:10, 100:0 (pure Micronal (R) DS 5039 X). The dynamic viscosity-shear rate curves were obtained for spindle speeds from 0.01 to 100 rpm (shear rate 0.0132-132.00 s(-1) respectively). The steady state measurement of viscosity was carried out when the slurry reached constant temperatures, namely: 10.0; 15.0, 17.5, 20.0, 22.5, 25.0, 27.5, 30.0, 40.0 and 50.0 degrees C. The dynamic viscosity of slurries increases with the mPCM concentration in dispersion rises. Only the sample of 10% mPCM may be considered as a Newtonian fluid within the test range (shear rate 0.0132-132.00 s(-1)). Increasing the shear rate ultimately causes viscosity to decrease down to the Newtonian plateau, where it seems to be constant. The higher the temperature of the slurry, the lower the shear rate value, after which the viscosity characteristic becomes linear or constant. The same principle applies to mass ratio. In vicinity of the melting point (about 25 degrees C) the phase change process of mPCM slurry does not influence the viscosity-shear rate characteristic behavior when steady state conditions are preserved. The non-steady state condition was also examined, more specifically, the temperature of the sample was increased continuously at a steady pace from 16 degrees C to 29 degrees C. This stage took 17 min with each data point collected at 15 s intervals. During a non-steady state temperature increase, near melting point, the viscosity of the slurry clearly departed from those values observed in steady states. It can therefore be concluded that around the melting point temperature, the phase change process of mPCM slurry influences the viscosity. (C) 2018 Elsevier Ltd. All rights reserved.