Research On Transfer Properties Of [EMIM][DEP]/H₂O And Ionic Liquid-based Nanofluids

Absorption heat pump (AHP) driven with industrial waste heat is a kind of energy-saving device. Conventional working pairs are "H2O-LiBr" and "NH3-H2O". However, "H2O-LiBr" crystallizes easily in some conditions and has corrosive tendency to iron steel devices. "NH3-H2O" has the toxicity and explosiveness. Consequently, many researches about new working fluids have been carried out to overcome these shortcomings in recent years. Ionic liquids (ILs) receive attention because of their characteristics such as wide liquid temperature range, low vapor pressure, less volatile and good solubility for most organic and inorganic solvents. It is suggested that some ILs such as 1-ethyl-3-methylimidazolium diethylphosphate ([EMIM][DEP]) have the possibilities to be new working fluids in AHP. So, it is essential to research their transfer properties systematically by experimental method and molecular dynamics (MD) simulation.Ionic liquid [EMIM][DEP] is investigated here based on the experimental researches by our group. In the part of experiments, [EMIM][DEP] was firstly prepared and then nano-silver particles and single-walled carbon nanotubes (SWCNT) were dispersed into [EMIM][DEP] (INF) and its aqueous solutions (SNF). Thermal conductivities of INF and SNF with various mass fractions of nano-silver particles were measured by transient hot-wire method at 303.15-353.15 K. The enhancement ratio is only about 1.03 when the mass fraction is 8%. Moreover, thermal conductivities of INF and SNF with various mass fractions of SWCNT were also measured. The enhancement on the thermal conductivity is obvious and the enhancement ratio with the mass fraction of 2% is 1.30 approximately.In the part of simulation, density, thermal conductivity and viscosity of ionic liquid [EMIM] [DEP] and its aqueous solution at various mole fractions of IL in the temperature range from 303.15 K to 353.15 K were investigated by MD simulation. The enhancement on the thermal conductivity of nanofluids compared with the base fluid was also researched by MD simulation. The minimum-energy geometry of [EMIM][DEP] was determined by performing geometry optimization at the B3LYP/6-311++G(d, p) level based on density functional theory using Gaussian 09 software. CHELPG method was applied to optimize the atomic charges. Other parameters of cation were obtained from Amber force field, while the parameters of anion were obtained from OPLS force field. Results indicate that the relative deviations of simulated densities compared with our previous experimental values are less than 3%. Simulations of thermal conductivity and viscosity were conducted based on the equilibrium molecular dynamics Green-Kubo method. The average relative deviation of simulated thermal conductivities of IL compared with our previous experimental results is 13.48%, but for aqueous solutions the relative deviations are about 20%. The simulated results of nanofluids show that the enhancement on the thermal conductivity of nanofluids compared with the base fluid is obvious. In addition, the contributions of kinetic energy, potential energy, virial and partial enthalpy terms to the thermal conductivity were calculated respectively. The effects of nanoparticles on these terms were researched too. Results indicate that virial term's contribution to thermal conductivity is the maximum.In conclusion, the adopted force field parameters are reasonable for simulating the transfer properties of [EMIM][DEP] and the contents provide the new working fluid in AHP with the experimental and simulated data.