(89b) Heat Transfer and Pressure Drop Characteristics of a Phase Change Material Nanoemulsion

Abstract: Thermal energy storage (TES) systems using phase change slurries (PCS), are capable of resolving the frequent mismatch between energy supply and demand, because of their high energy densities as they use both the sensible heat and latent heat of the phase change material during the phase transition. As an application of phase change slurry, heat transfer and pressure drop characteristics of a beeswax nanoemulsion with 10.1% beeswax, 70% distilled water, and 19.9 % surfactants, have been investigated experimentally. This paper describes the preparation of the beeswax nanoemulsion and the experimental data on convection heat transfer and friction pressure drop in a heated test section. Those properties are useful for understanding how useful this type of nanoemulsion is in practical applications such as a latent heat thermal storage system and heat transport fluid. The test section is a straight 304 stainless steel pipe with an inner diameter of Di =11.3 mm, tube wall thickness of 0.71 mm and length L=1.04 m. Five type-K thermocouples are mounted on the test section wall at equal intervals to measure the tube wall temperature. The tube is thermally insulated in order to minimize the heat loss. Two other thermocouples are inserted at the entrance and exit of the test section to measure the bulk fluid temperatures. Ohmic heating of the test section is achieved by passing direct current from a DC power supply with a maximum heat flux of 10.4 kW/m2. At room temperature, the pressure drop of nanoemulsion was found to be about four times higher than that of water under the same flow rate. However heating of the nanoemulsion to a temperature above the melting point of the beeswax (62 oC) in the test section increased the pressure drop. The maximum recorded pressure drop was 17.6 kPa at 80˚C and the flow rate also dropped sharply due to the high viscosity of the nanoemulsion. Based on the pressure drop data, the nanoemulsion viscosity at 30, 60 and 70 oC were determined to be 20, 23 and 73 times greater than that of water at the same temperature. Near the melting point of the beeswax, the wall temperatures show fluctuations which are absent at below and above the melting temperature. The average convective heat transfer coefficients calculated at a heat flux of 7.15 kW/m2 and based on the recorded bulk fluid and tube wall temperatures were 226, 435 and 935 W/m2K at flow rates of 0.15, 0.45 and 1.2 GPM. They were found to be consistent with the values predicted by a Sieder-Tate correlation.