Parametric study of solidification of PCM around a cylinder for ice-bank applications

This paper presents the results of a numerical parametric study of the solidification of a phase change material (PCM) around a cylinder carrying a heat-transfer fluid (HTF) inside. A pure conduction model is used for the PCM and tube wall, the finite volume method is used together with the interface immobilization technique for treating the phase-change process. The convection problem inside the tube is solved by an energy balance with a Nusselt number value, obtained from the steady-state values for constant wall heat-flux conditions. The effects of the HTF entry temperature, the initial PCM temperature and the thermal conductivity of the tube material on the evolution of the solidification front are studied. Results for the temperature distribution during the process, phase-change interface velocity and thermal energy stored in the system are also presented.     

Soret and Dufour effects on convective instabilities in binary nanofluids for absorption application

Thermodiffusion (Soret effect) and diffusionthermo (Dufour effect) effects on convective instabilities in nanofluids have been theoretically investigated. Thermodiffusion implies that mass diffusion is induced by thermal gradient, which is so-called the Soret effect. Diffusionthermo implies that heat transfer is induced by concentration gradient, which is so-called the Dufour effect. By using the linear stability theory under one-fluid model, a characteristic dimensionless parameter was newly obtained. From the instability analysis with given conditions, it is found that the convective motion in nanofluids sets in easily as the Soret and Dufour effects and the initial concentration of nanoparticles increase.     

Exergy analysis of an ionic-liquid absorption refrigeration system utilizing waste-heat from datacenters

Ionic-liquid (IL) was introduced as an absorbent of an absorption refrigeration system designed for high power electronics cooling. IL is a salt in liquid-state, which is nonvolatile, thermally-stable, nonflammable, and environmentally-benign. It provides an alternative to the normally toxic working fluids, such as ammonia, also eliminates crystallization and metal-compatibility issues of the water/LiBr system. The performance of IL absorption refrigeration system was theoretically examined using exergy analysis. Various combinations of refrigerant and imidazolium-based ILs were chosen as working fluid pairs. The thermodynamic properties of ILs were evaluated using the correlations based on group contribution methods. A non-random two-liquid (NRTL) model was built and used to predict the solubility of the mixtures. Both the coefficient of performance (COP) and the exergetic coefficient of performance (ECOP) were evaluated. The effects of operating conditions on ECOP were explored. Also, the exergy destruction of each component was evaluated and discussed as a means to identify the critical component(s) of the system that would require optimization.     

Experimental investigation of ice slurry heat transfer in horizontal tube

Heat transfer of ice slurry flow based on ethanol–water mixture in a circular horizontal tube has been experimentally investigated. The secondary fluid was prepared by mixing ethanol and water to obtain initial alcohol concentration of 10.3% (initial freezing temperature -4.4 °C). The heat transfer tests were conducted to cover laminar and slightly turbulent flow with ice mass fraction varying from 0% to 22% depending on test performed. Measured heat transfer coefficients of ice slurry are found to be higher than those for single phase fluid, especially for laminar flow conditions and high ice mass fractions where the heat transfer is increased with a factor 2 in comparison to the single phase flow. In addition, experimentally determined heat transfer coefficients of ice slurry flow were compared to the analytical results, based on the correlation by Sieder and Tate for laminar single phase regime, by Dittus–Boelter for turbulent single phase regime and empirical correlation by Christensen and Kauffeld derived for laminar/turbulent ice slurry flow in circular horizontal tubes. It was found that the classical correlation proposed by Sieder and Tate for laminar forced convection in smooth straight circular ducts cannot be used for heat transfer prediction of ice slurry flow since it strongly underestimates measured values, while, for the turbulent flow regime the simple Dittus–Boelter relation predicts the heat transfer coefficient of ice slurry flow with high accuracy but only up to an ice mass fraction of 10% and Re_cf > 2300 regardless of imposed heat flux. For higher ice mass fractions and regardless of the flow regime, the correlation proposed by Christensen and Kauffeld gives good agreement with experimental results.