Lei Chai

Stability and Thermophysical Properties of Binary Propanol–Water Mixtures-Based Microencapsulated Phase Change Material Suspensions

In order to obtain stable latent functionally thermal fluids for heat transfer and heat storage, microencapsulated phase change material (MPCM) suspensions with binary propanol-water mixtures of different proportions as base fluid were formulated. The stability study finds the binary propanol-water mixtures, after having stood for 48 hr, with a density of 941 kg/m(3) exhibit the best stability. The morphology and thermophysical properties of the 10-40 wt.% MPCM suspensions, such as diameter distribution, latent heat and heat capacity, rheology and viscosity, thermal conductivity, and thermal expansion coefficients, were studied experimentally. The influence of MPCM concentration and temperature on the thermophysical properties was analyzed as well.

Experimental Study on the Stability and Thermophysical Properties of Binary Propanol-Water Mixtures Based Phase Change Microencapsule Suspensions

Phase change microencapsules are the microsized particles made of phase change materials (paraffin wax ect.) sealed by the thin shell (polymer ect.) via the methods of microencapsulation. During last decade, due to the large amount of melting/solidifying heat, much attention have been paid on their application in environmental control, building, textiles and electronics ect. Also the novel thermal fluids by phase change microencapsules suspending in the traditional thermal fluids have shown their superior heat storage density and convective heat transfer performance, which can behave as heat storage media and heat transfer media simultaneously. However, the density difference between the phase change microencapsules and tranditional unitary fluid would lead to the unstable suspending states which seriously affect the heat storage and heat transfer performance.Binary mixtures such as alcohol-water etc have already played the important roles in the heat transfer equipments. In this paper, binary propanol-water mixtures of various proportion were formulated as the base fluids, and their stabilities were studied. The result shows that binary propanol-water mixtures with the desity of 941kg/m3 showed the best stability and no stratification was found after standing for 48 hours.The morphology and diameter distribution of the microencapsule particles were tested by the scanning electron microscope (SEM) and Malvern Nanosizer respectively, and the result show that the diameter of the particles is in the range of 10–80μm with the average value of 26.4μm. The phase change enthalpy and the effective heat capacity of phase change microencapsule suspensions with the concentration of 10–40wt% were measured by the differencial scanning calorimeter (DSC) and it was found the phase change enthalpy of the phase change microencapsule is 152.8J/g and the undercooling is only 7.3°C. The effect of concentration and temperature on the rheological behavior and viscosities of suspensions were experimentally studied by the TA DHR-G2 rheometer. The result shows that the suspensions behave as Newtonian fluids even when the concentration is as high as 40wt% and the viscosities fit well with Vand model. By the Hot Disk 2500S thermal constant analyzer (Sweden), the thermal conductivities of 0–40wt% suspensions were tested at 20–70°C and the variation was analyzed further. The concentration and expansion of MPCM particles during the phase change period were found to affect the thermal expansion coefficient of the MPCM suspensions obviously. The above experimental result and analyzation of stability and thermophysical properties will provide a complete and important data for the application in heat storage and heat transfer.Copyright

Numerical study on thermal performance characteristics of a cascaded latent heat storage unit

This study presents a numerical investigation on a cascaded shell-and-tube 3PCMs latent heat thermal storage unit filled with three types of phase-change materials namely, C24H50, C21H44, and C18H38. The mathematical model is based on the enthalpy method and considers the effect of the natural convection of liquid phase-change material during melting. The transient heat transfer and thermal storage characteristic of the 3PCMs unit are analyzed and compared with those of a 1PCM unit that utilizes C24H50. The influences of heat transfer fluid flow rate and inlet temperature on the efficiency, accumulated energy, and exergy of the latent heat thermal storage unit are studied. Results show that the 3PCMs unit can improve the heat transfer rate greatly and shorten the heat storage time effectively, as much as 58% more energy can be stored than the 1PCM unit at the same charging time. The increase of both inlet temperature and flow rate of the heat transfer fluid will lead to a shorter charging time, whereas the advantage of the 3PCMs unit on storage performance over the 1PCM unit becomes more insignificant.