Working state map of hydrocarbon fuels for regenerative cooling

Regenerative cooling by endothermic hydrocarbon fuel(EHF)is one of the most promising techniques for thermal management of supersonic or hypersonic aircraft.How to maintain the fuel working in proper states is an important issue to maximize the cooling potential of EHT.This work proposes a novel working state map,including risking zone(RZ),thermal cracking zone(TCZ),supercritical zone(SupZ)and subcritical zone(SubZ),to differentiate possible working states of an EHF during regenerative cooling.Using n-decane flowing in a circular tube as an example,the boundaries of four zones are determined by numerical simulation covering different heat fluxes(0.2-4.0 MW·m^(-2))and mass flow rates(0.5-10.5 g·s^(-1))under two operating pressures(3.45 and 5.00 MPa).Empirical correlations for three boundary lines are obtained and the maximum cooling capacity is identified,as well as the identification of the pressure effect.The revelation of such new perspective of regenerative cooling is of great implication to the design and optimization of cooling system for future thermal management.

Experimental investigation of a latent heat thermal energy storage unit encapsulated with molten salt/metal foam composite seeded with nanoparticles

Molten salt has been widely used in latent heat thermal energy storage(LHTES)system,which can be incorporated into hybrid photovoltaic/thermal solar system to accommodate the built environment.Solar salt(60 wt.%NaNO 3 and 40 wt.%KNO 3)was employed as the phase change materials(PCMs)in this study,and both aluminum oxide(Al_(2)O_(3))nanopowder and metal foam were used to improve the properties of pure solar salt.The synthesis of the salt/metal foam composites seeded with Al_(2)O_(3)nanopowder were performed with the two-step and impregnation methods,and the composite PCMs were characterized morphologically and thermally.Then pure solar salt,the salt/2 wt.%Al_(2)O_(3)nanopowder and salt/copper foam composite seeded with 2 wt.%Al_(2)O_(3)nanopowder were encapsulated in a pilot test rig,respectively,where a heater of 380.0 W was located in the center of the LHTES unit.The charging and discharging processes of the LHTES unit were conducted extensively,whereas the heating temperatures were controlled at 240℃,260℃and 280℃respectively.Temperature evolutions at radial,angular and axial positions were recorded,and the time-durations and volumetric mean powers during the charging and discharging processes were obtained and calculated subsequently.The results show that physical bonding between Al_(2)O_(3)nanopowder and nitrate molecule has been formed from the morphological pictures together with XRD and FTIR curves.Slight changes are found between the melting/freezing phase change temperatures of the salt/metal foam composites seeded with Al_(2)O_(3)nanopowder and those of pure solar salt,and the specific heats of the salt/Al_(2)O_(3)nanopowder composite slightly increase with the addition of Al_(2)O_(3)nanopowder.The time-duration of the charging process for the salt/copper foam composite seeded with Al_(2)O_(3)nanopowder at the heating temperature of 240℃can be reduced by about 74.0%,compared to that of pure solar salt,indicating that the heat transfer characteristics of the LHTES unit encapsulated with the salt/copper foam composite seeded with Al_(2)O_(3)nanopowder can be enhanced significantly.Consequently,the mean volumetric powers of the charging process were distinctly enhanced,e.g.,the volumetric mean power of heat storage can reach 110.76 kW/m 3,compared to 31.94 kW/m 3 of pure solar salt.However,the additive has little effect on the volumetric mean power of heat retrieval because of the domination of natural air cooling.

Review of nanofluids for heat transfer applications

Research on nanofluids has progressed rapidly since its enhanced thermal conductivity was first reported about a decade ago, though much controversy and inconsistency have been reported, and insufficient understanding of the formulation and mechanism of nanofluids further limits their applications. This work presents a critical review of research on heat transfer applications of nanofluids with the aim of identifying the limiting factors so as to push forward their further development.

Removal of antimony from antimony mine flotation wastewater by electrocoagulation with aluminum electrodes

Antimony （Sb） has received increasing environmental concerns due to its potential toxic and carcinogenic properties. In the present work, the electrocoagulation technique was used to treat the flotation wastewater from a heavy antimony polluted area, and the mechanism of removing Sb was also investigated. The study focused on the effect of operation parameters such as current density, initial pH and standing time on the Sb removal efficiency. Antimony concentration of below 1 mg/L in the treated wastewater was achieved, which meets the emission standards established by State Department of Environmental Protection and State Administration of China for Quality Supervision and Inspection and Quarantine of China.

Competing effects of surface catalysis and ablation in hypersonic reentry aerothermodynamic environment

Under hypersonic flow conditions, the complicated gas-graphene interactions including surface catalysis and surface ablation would occur concurrently and intervene together with the thermodynamic response induced by spacecraft reentry. In this work, the competing effects of surface heterogeneous catalytic recombination and ablation characteristics at elevated temperatures are investigated using the Reactive Molecular Dynamics(RMD) simulation method. A GasSurface Interaction(GSI) model is established to simulate the collisions of hyper-enthalpy atomic oxygen on graphene films in the temperature range of 500–2500 K. A critical temperature Tcaround900 K is identified to distinguish the graphene responses into two parts: at T < T_(c), the heterogeneous surface catalysis dominates, while the surface ablation plays a leading role at T > T_(c). Contradicting to the traditional Arrhenius expression that the recombination coefficient increases with the increase of surface temperature, the value is found to be relatively uniform at T < T_(c) but declines sharply as the surface temperature increases further due to the competing ablation effect. The occurrence of surface ablation decreases the amounts of active sites on the graphene surface for oxygen adsorption, leading to reduced recombination coefficient from both Langmuir-Hinshelwood(L-H) and Eley-Rideal(E-R) mechanisms. It suggests that the traditional Computational Fluid Dynamics(CFD) simulation method, which relies on the Arrhenius-type catalysis model, would result in large discrepancies in predicting aerodynamic heat for carbon-based materials during reentry into strong aerodynamic thermal environment.

Particle-based hybrid and multiscale methods for nonequilibrium gas flows

Over the past half century,a variety of computational fluid dynamics(CFD)methods and the direct simulation Monte Carlo(DSMC)method have been widely and successfully applied to the simulation of gas flows for the continuum and rarefied regime,respectively.However,they both encounter difficulties when dealing with multiscale gas flows in modern engineering problems,where the whole system is on the macroscopic scale but the nonequilibrium effects play an important role.In this paper,we review two particle-based strategies developed for the simulation of multiscale nonequilibrium gas flows,i.e.,DSMC-CFD hybrid methods and multiscale particle methods.The principles,advantages,disadvantages,and applications for each method are described.The latest progress in the modelling of multiscale gas flows including the unified multiscale particle method proposed by the authors is presented.

Discrete particle modeling of granular temperature distribution in a bubbling fluidized bed

The discrete hard sphere particle model （DPM） is applied in this work to study numerically the distribu- tions of particle and bubble granular temperatures in a bubbling fluidized bed. The dimensions of the bed and other parameters are set to correspond to those of Miuller et al（2008）. Various drag models and oper- ational parameters are investigated to find their influence on particle and bubble granular temperatures. Various inlet superficial gas velocities are used in this work to obtain their effect on flow characteristics. It is found that the superficial gas velocity has the most important effect on granular temperatures including bubble granular temperature, particle translational granular temperature and particle rotational granular temperature. The drag force model affects more seriously the large scale variables such as the bubble gran- ular temperature. Restitution coefficient influences all granular temperatures to some degree. Simulation results are compared with experimental results by Muller et al. （2008） showing reasonable agreement.

CFD simulation of jet behaviors in a binary gas-solid fluidized bed:comparisons with experiments

Based on the experimental observation of the fluidization characteristics of solid mixtures(resin and rapeseed)with different densities and sizes,the jet behaviours of the binary system are simulated in a twodimensional jetting fluidized bed 0.30m in width and 2.00m in height.A simple mathematical model,by introducing two additional force terms in both gas and particle phase momentum equations of Gidaspow’s inviscid two-fluid model,is used to explore the effects of jet gas velocity and mixture combination on the jet penetration depth in the fluidized bed with a binary system.Experimental results show that there is a fluidization velocity interval(u_(if)-u_(ff))for the resin-on-rapeseed(flotsam-on-jetsam)segregated bed.The simulated jet penetration depth increases with the increase of jet gas velocity and the volume fraction of the flotsam(resin),which is in fair agreement with experimental data.The above findings show that the hydrodynamic model of Brandani and Zhang(2006),by introducing the average physical properties from Goossens et al.(1971),can be used to predict the jet behaviors of a well-mixing binary system.

CFD simulation of a gas-solid fluidized bed with two vertical jets

A computational fluid dynamics （CFD） model is used to investigate the hydrodynamics of a gas-solid fluidized bed with two vertical jets. Sand particles with a density of 2660 kg/m3 and a diameter of 5.0 × 10^-4 m are employed as the solid phase. Numerical computation is carried out in a 0.57 m ×1.00 m two-dimensional bed using a commercial CFD code, CFX 4.4, together with user-defined Fortran subroutines. The applicability of the CFD model is validated by predicting the bed pressure drop in a bubbling fluidized bed, and the jet detachment time and equivalent bubble diameter in a fluidized bed with a single jet. Subsequently, the model is used to explore the hydrodynamics of two vertical jets in a fluidized bed. The computational results reveal three flow patterns, isolated, merged and transitional jets, depending on the nozzle separation distance and jet gas velocity and influencing significantly the solid circulation pattern. The jet penetration depth is found to increase with increasing jet gas velocity, and can be predicted reasonably well by the correlations of Hong et al. （2003） for isolated jets and of Yang and Keairns （1979） for interacting jets.

Pore-scale simulation of water/oil displacement in a water-wet channel

Water/oil flow characteristics in a water-wet capillary were simulated at the pore scale to increase our understanding on immiscible flow and enhanced oil recovery.Volume of fluid method was used to capture the interface between oil and water and a pore-throat connecting structure was established to investigate the effects of viscosity,interfacial tension(IFT)and capillary number(Ca).The results show that during a water displacement process,an initial continuous oil phase can be snapped off in the water-wet pore due to the capillary effect.By altering the viscosity of the displacing fluid and the IFT between the wetting and non-wetting phases,the snapped-off phenomenon can be eliminated or reduced during the displacement.A stable displacement can be obtained under high Ca number conditions.Different displacement effects can be obtained at the same Ca number due to its significant influence on the flow state,i.e.,snapped-off flow,transient flow and stable flow,and ultralow IFT alone would not ensure a very high recovery rate due to the fingering flow occurrence.A flow chart relating flow states and the corresponding oil recovery factor is established.

Comparative analysis of CFD models for jetting fluidized beds:Effect of particle-phase viscosity

Under the Eulerian-Eulerian framework of simulating gas-solid two-phase flow, the accuracy of the hydrodynamic prediction is strongly affected by the selection of rheology of the particulate phase, for which a detailed assessment is still absent. Using a jetting fluidized bed as an example, this work investi- gates the influence of solid rheology on the hydrodynamic behavior by employing different particle-phase viscosity models. Both constant particle-phase viscosity model （CVM） with different viscosity values and a simple two-fluid model without particle-phase viscosity （NVM） are incorporated into the classical two- fluid model and compared with the experimental measurements. Qualitative and quantitative results show that the jet penetration depth, jet frequency and averaged bed pressure drop are not a strong func- tion of the particle-phase viscosity. Compared to CVM, the NVM exhibits better predictions on the jet behaviors, which is more suitable for investigating the hydrodynamics of gas-solid fluidized bed with a central jet.