Experimental investigation into convective heat transfer of nanofluids at the entrance region under laminar flow conditions

Heat transfer of aqueous suspensions of carbon nanotubes (CNT nanofluids)

Heat transfer and flow behaviour of aqueous suspensions of TiO2 nanoparticles (nanofluids) flowing upward through a vertical pipe

Introduction to heat transfer

Heat transfer properties of nanoparticle-in-fluid dispersions (nanofluids) in laminar flow

An experimental system is built to investigate convective heat transfer and flow characteristics of the nanofluid in a tube. Both the convective heat transfer coefficient and friction factor of Cu-water nanofluid for the laminar and turbulent flow are measured. The effects of such factors as the volume fraction of suspended nanoparticles and the Reynolds number on the heat transfer and flow characteristics are discussed in detail. The experimental results show that the suspended nanoparticles remarkably increase the convective heat transfer coefficient of the base fluid and show that the friction factor of the sample nanofluid with the low volume fraction of nanoparticles is almost not changed. Compared with the base fluid, for example, the convective heat transfer coefficient is increased about 60% for the nanofluid with 2.0 vol% Cu nanoparticles at the same Reynolds number. Considering the factors affecting the convective heat transfer coefficient of the nanofluid, a new convective heat transfer correlation for nanofluid under single-phase flows in tubes is established. Comparison between the experimental data and the calculated results indicate that the correlation describes correctly the energy transport of the nanofluid.

Convective heat transfer and flow characteristics of Cu-water nanofluid

Excitation of surface resonance modes and presence of resonance-free hyperbolic modes are two common ways to enhance the near-field radiative energy transport, which can find wide applications in noncontact thermal management and energy harvesting. Here, we identify another way to achieve the super-Planckian thermal radiation via hyperbolic surface phonon polaritons (HSPhPs). Based on the fluctuation-dissipation theory, the near-field radiative heat flux between bulk hexagonal boron nitride (hBN) planes with the optical axis perpendicular to the radiative energy flow can be 120 times as large as the blackbody limit for a gap distance of 20 nm. When the film thickness is reduced to 10 nm, the radiative heat flux is found to increase by 26.3%. The underlying mechanism is attributed to the coupling of Type I HSPhPs inside the anisotropic hBN film, which improves the energy transmission coefficient over a broad wavevector space especially for waves with extremely high wavevectors. This work helps to deepen the understanding of near-field radiation between natural hyperbolic materials, and opens a new route to enhance the near-field thermal radiation.

Super-Planckian thermal radiation enabled by hyperbolic surface phonon polaritons

The spectral control is widely incorporated with the thermophotovoltaic (TPV) technology to improve the optical-electric conversion efficiency of the whole system. In order to match with GaSb photovoltaic cell, an 8-layer one-dimensional photonic crystals (PC) filter structure was designed as quarter-wave periodic structure by selecting silicon (Si) and silicon dioxide (SiO2) as candidate materials. The multilayer Si/SiO2 structure was developed for the matching filter to simultaneously realize the optimal matching with the spectral distribution of the high temperature emitter and the quantum efficiency of GaSb cell. The physical vapor deposition (PVD) method was used to fabricate the optical filter and the normal incidence optical property of the filter was measured within the spectral range from 0.7 to 3.3 μm. These experimental data were used to predict the spectral control performance in a TPV system. Finally, temperature performance experiments were carried out to establish its withstanding performance in high temperature environment.

Investigation of one-dimensional Si/SiO2 hotonic crystals for thermophotovoltaic filter

By accounting for the external and internal force acting on the suspended magnetic nanoparticles and motion characteristics of the suspended magnetic nanoparticles in the magnetic fluids, the three-dimensional microstructure of magnetic fluids is investigated by means of the molecular dynamics simulation method. The distribution of suspended magnetic nanoparticles and microstructure of the magnetic fluid are simulated in both absence and presence of an external magnetic field. The effects of the nanoparticles volume fraction, the dipole-dipole interaction potential and the particle-field interaction potential on the microstructures of the magnetic fluids are discussed. The main results obtained here are summarized as follows. The suspended magnetic nanoparticles tend to aggregate and make the irregular distribution structure in the absence of an external magnetic field. When the magnetic fluid is exposed to a magnetic field, the magnetic nanoparticles suspended in the carrier fluid tend to remain chained-alignment in the direction of the external magnetic field. The tendency of chain-alignment morphology of the suspended magnetic nanoparticles is enhanced with the nanoparticles volume fraction, the dipole-dipole interaction potential and the particle-field interaction potential.

Simulation and control scheme of microstructure in magnetic fluids

The invention discloses a laser and intermediate and far infrared compatible invisible membrane structure which is formed by ZnS materials and Ag materials in an alternated layer upon layer mode and totally has five layers. A Ag/ZnS/Ag membrane structure between top-layer ZnS and bottom-layer ZnS forms an asymmetrical F-P cavity, the aim of high absorption and low reflection of the membrane structure under 1.06 mu m laser wavelength can be achieved by means of an absorption enhancement effect of the asymmetrical F-P cavity, and an outer-layer Ag membrane enables the membrane structure to have high reflectivity in infrared waveband. The laser intermediate and far infrared compatible invisible membrane structure can be used for achieving compatible invisibility of 1.06 mu m laser and intermediate and far infrared.

Yimin Xuan

Papers

Convective heat transfer and flow characteristics of Cu-water nanofluid

Super-Planckian thermal radiation enabled by hyperbolic surface phonon polaritons

Investigation of one-dimensional Si/SiO2 hotonic crystals for thermophotovoltaic filter

Simulation and control scheme of microstructure in magnetic fluids

Laser and intermediate and far infrared compatible invisible membrane structure