Investigation Of Stability And Thermodynamic Performance Of Microencapsulated Phase Change Suspensions

Microencapsulated phase change suspension(MPCS) as a latent function of fluid in a heat exchange system and heat storage system plays an important role. For better understanding of the thermal properties and heat transfer characteristics of microencapsulated phase change suspensions,the paper proposed a series of work.Firstly, the stability of microencapsulated phase change suspension was theoretical and experimental studied. Results showed that the equivalent diameter of the particles, density difference between particles and base fluid, dynamic viscosity of basic liquid are important factors affecting the stability of suspensions. The stability of microencapsulated phase change suspension is better when we using smaller particles and base fluid of larger dynamaic viscosity.Based on the theoretical analysis of stability, we try to prepare the microencapsulated phase change suspension that meet the standards of following-up experiments by adjusting the density of both phase change particle and base fluid. Results showed that: for 20μm phase change particles, when density of base fluid(alcohol/water) is 0.9343g/mL the suspension can remain stable for 24 hours without stratification while for 5μm phase change particles, when density of base fluid(alcohol/water) is 0.9288g/mL,the suspension can remain stable for 24 hours without stratification.Establishment of viscosity empirical formula of high concentration for both 20μm and 5μm microencapsulated phase change suspension. Based on the traditional empirical formulas, new formulas of viscosity were derived separately by theoretical analysis and introducing new factors. Results showed that: for 20μm microencapsulated phase change suspension, it still be Newton fluid with its concentration up to 50 wt.%. It should be noticed that modified de Kruif formula and Brinkman-Roscoe formula have a high precision on calculating 20μm microencapsulated phase change suspension while for 5μm microencapsulated phase change suspension, when its concentration reached by 40 wt.%, shear-thining occurred,the rheological properties changed from Newton fluid to plastic fluid. It should be noticed that modified Brinkman-Roscoe formula has a high precision on calculating 5μm microencapsulated phase change suspension.Polymer drag reduction agent and surfactant drag reduction agent have different effects on microencapsulated phase change suspension viscosity. Results show that the polymer drag reduction agent(polyacrylamide, sodiμm carboxymethyl cellulose and polyethylene oxide) is less effective when compared to surfactant drag reduction agent(CTAC/NaSal). By changing the concentration of CTAC/NaSal as well as flow velocity of working fluid, we may draw a conclusion that friction reduction rate can reach 3.85% when the concentration of the mass flow rate of the fluid is 70.46Kg/h and the concentration of CTAC/NaSal is 0.0015%.Convective heat transfer experiments of microencapsulated phase change suspension proved that when comparing to traditional heat transfer fluids(water), it do have certain advantages. Results showed that: when in the same working condition(same inlet temperature and mass flow), the wall temperature and inner fluid temperature of microencapsulated phase change suspension stay at a certain level. However, for water, the wall temperature and inner fluid temperature increased gradually. When to the outlet, both the outlet temperature and wall temperature of water is higher than that of microencapsulated phase change suspension, 13.24℃ and 28.92℃, respectively. Heat transfer coefficient and heat transfer characteristics of microencapsulated phase change suspension which in the phase change state are larger than that of water.