Liang Wang

Rheology of Nanofluids: A Review

The rheological behavior of nanofluids published in recent research papers and the relevant patents are reviewed in this article. Effects of various factors such as preparation, nanoparticle and base fluid properties, concentration, temperature, surface charge, and aggregation etc. on the rheological behavior of nanofluids are discussed. Brownian motion and nanoparticle aggregation are found to be the major mechanisms for rheological properties of nanofluids compared to the micro-sized suspensions. The importance of microstructure as means of understanding the mechanisms behind the rheological and heat transfer behavior of nanofluids is also disclosed.

Stability and Thermophysical Properties of Binary Propanol–Water Mixtures-Based Microencapsulated Phase Change Material Suspensions

In order to obtain stable latent functionally thermal fluids for heat transfer and heat storage, microencapsulated phase change material (MPCM) suspensions with binary propanol-water mixtures of different proportions as base fluid were formulated. The stability study finds the binary propanol-water mixtures, after having stood for 48 hr, with a density of 941 kg/m(3) exhibit the best stability. The morphology and thermophysical properties of the 10-40 wt.% MPCM suspensions, such as diameter distribution, latent heat and heat capacity, rheology and viscosity, thermal conductivity, and thermal expansion coefficients, were studied experimentally. The influence of MPCM concentration and temperature on the thermophysical properties was analyzed as well.

Experimental Study on the Stability and Thermophysical Properties of Binary Propanol-Water Mixtures Based Phase Change Microencapsule Suspensions

Phase change microencapsules are the microsized particles made of phase change materials (paraffin wax ect.) sealed by the thin shell (polymer ect.) via the methods of microencapsulation. During last decade, due to the large amount of melting/solidifying heat, much attention have been paid on their application in environmental control, building, textiles and electronics ect. Also the novel thermal fluids by phase change microencapsules suspending in the traditional thermal fluids have shown their superior heat storage density and convective heat transfer performance, which can behave as heat storage media and heat transfer media simultaneously. However, the density difference between the phase change microencapsules and tranditional unitary fluid would lead to the unstable suspending states which seriously affect the heat storage and heat transfer performance.Binary mixtures such as alcohol-water etc have already played the important roles in the heat transfer equipments. In this paper, binary propanol-water mixtures of various proportion were formulated as the base fluids, and their stabilities were studied. The result shows that binary propanol-water mixtures with the desity of 941kg/m3 showed the best stability and no stratification was found after standing for 48 hours.The morphology and diameter distribution of the microencapsule particles were tested by the scanning electron microscope (SEM) and Malvern Nanosizer respectively, and the result show that the diameter of the particles is in the range of 10–80μm with the average value of 26.4μm. The phase change enthalpy and the effective heat capacity of phase change microencapsule suspensions with the concentration of 10–40wt% were measured by the differencial scanning calorimeter (DSC) and it was found the phase change enthalpy of the phase change microencapsule is 152.8J/g and the undercooling is only 7.3°C. The effect of concentration and temperature on the rheological behavior and viscosities of suspensions were experimentally studied by the TA DHR-G2 rheometer. The result shows that the suspensions behave as Newtonian fluids even when the concentration is as high as 40wt% and the viscosities fit well with Vand model. By the Hot Disk 2500S thermal constant analyzer (Sweden), the thermal conductivities of 0–40wt% suspensions were tested at 20–70°C and the variation was analyzed further. The concentration and expansion of MPCM particles during the phase change period were found to affect the thermal expansion coefficient of the MPCM suspensions obviously. The above experimental result and analyzation of stability and thermophysical properties will provide a complete and important data for the application in heat storage and heat transfer.Copyright

Thermohydraulic performance of microchannel heat sinks with triangular ribs on sidewalls – Part 2: Average fluid flow and heat transfer characteristics

Abstract Triangular ribs mounted in the microchannel heat sink generally result in higher heat transfer coefficient, but are usually accompanied by higher pressure drop per unit length. In order to obtain some insight into the effect of geometry parameters of triangular ribs on laminar flow and heat transfer characteristics, three-dimensional conjugated heat transfer models taking account of the entrance effect, viscous heating and temperature-dependent thermophysical properties are conducted, and four non-dimensional variables related to the width, height, converging-diverging ratio and spacing of the triangular rib for both aligned and offset arrangements are designed. Effects of the geometry and arrangement of triangular ribs on thermohydraulic performance are examined by the variations of average friction factor and Nusselt number for Reynolds number (Re) ranging from 187 to 715. The studied microchannels have the same width (Wc) of 0.1u202fmm and same depth (Hc) of 0.2u202fmm in the constant cross-section region. The geometric parameters of aligned or offset triangular ribs are ranged in 0.025–0.4u202fmm for width (Wr), 0.005–0.025u202fmm for height (Hr), 0.2–5u202fmm for spacing (Sr) and 0–1 for the width ratio of converging region to a single rib (Wcon/Wr). Based on the total 660 computational cases of the microchannel heat sinks with triangular ribs, the correlations of average friction factor and Nusselt number are proposed, respectively for aligned and offset arrangements. For the studied Reynolds number range and geometry parameters of flow passage, the microchannel heat sinks with aligned triangular ribs present 1.03–2.01 times higher of average Nusselt number and 1.06–9.09 times larger of average friction factor, and those with offset triangular ribs show 1.01–2.16 times higher of average Nusselt number and 1.04–7.43 times larger of average friction factor, compared with the reference straight microchannel heat sink. Proposed heat transfer and friction factor correlations show good agreements with the computational results for the microchannel heat sinks within the parameter ranges of 187u202f≤u202fReu202f≤u202f715, 0.25u202f≤u202fWr/Wcu202f≤u202f4, 0.05u202f≤u202fHr/Wcu202f≤u202f0.25, 0u202f≤u202fWcon/Wru202f≤u202f1, and 2u202f≤u202fSr/Wcu202f≤u202f50.

Thermal Storage Characteristics of the Vertical Cylindrical Water Tank

AbstractThis study conducts experimental and numerical investigations on the thermal storage characteristics of the vertical cylindrical water tank in the heat preservation process. A new experimen...

Experimental Investigation on the Thermal-Energy Storage Characteristics of the Subcritical Water

AbstractThis paper first proposes the concept of using subcritical water as a thermal energy storage (TES) material. Subcritical water has a maximum TES temperature of 330°C and an internal energy ...

Numerical study on thermal performance characteristics of a cascaded latent heat storage unit

This study presents a numerical investigation on a cascaded shell-and-tube 3PCMs latent heat thermal storage unit filled with three types of phase-change materials namely, C24H50, C21H44, and C18H38. The mathematical model is based on the enthalpy method and considers the effect of the natural convection of liquid phase-change material during melting. The transient heat transfer and thermal storage characteristic of the 3PCMs unit are analyzed and compared with those of a 1PCM unit that utilizes C24H50. The influences of heat transfer fluid flow rate and inlet temperature on the efficiency, accumulated energy, and exergy of the latent heat thermal storage unit are studied. Results show that the 3PCMs unit can improve the heat transfer rate greatly and shorten the heat storage time effectively, as much as 58% more energy can be stored than the 1PCM unit at the same charging time. The increase of both inlet temperature and flow rate of the heat transfer fluid will lead to a shorter charging time, whereas the advantage of the 3PCMs unit on storage performance over the 1PCM unit becomes more insignificant.

Natural Convective Heat Transfer Characteristics of the Bundle Heat Exchanger in the Latent Heat Microcapsulated Phase Change Material Slurry

As a novel latent functionally thermal fluid, microcapsulated phase change material slurry (MPCMS) has many potential applications in the fields of energy storage, air-conditioning, refrigeration and heat exchanger, etc. In order to investigate the heat storage and heat transfer performance of MPCMS, natural convection in a rectangular enclosure heated by bundle heat exchanger has been studied numerically in this paper. The effects of mass concentration (Cm) of MPCMS, the vertical spaces of bundle heat exchanger on the natural convective heat transfer are investigated. The results indicate that, MPCMS with Cm=30% shows the best natural convectionperformance, and a lower position of bundle heat exchanger can strengthen the natural convection.