Chang-Ying Zhao

Experimental observations and lattice Boltzmann method study of the electroviscous effect for liquid flow in microchannels

The existing experimental data in the literature on hydrodynamics for liquid flow in microchannels are analyzed and the reasons causing the diversities are discussed and summarized. The present experimental data for deionized water flow in glass microtubes with diameters ranging from 50 to 530 µm show that the friction factors and transition Reynolds numbers from laminar to turbulent flow are in good agreement with the conventional theoretical predictions. However, the friction factors in stainless steel microtubes with diameters of 119 and 172 µm are much higher than the conventional theoretical predictions. This discrepancy is attributed to the large surface relative roughness or dense roughness distribution in the stainless steel tubes. Numerical simulations taking into account the electroviscous effect are carried out by using the lattice Boltzmann method. The simulation results show that the electroviscous effect does not play a significant role in the flow characteristics for channel dimensions of the order of microns and hence it can be neglected in engineering applications for moderate electrical conductivity of the liquid and conductivity of the walls. From the literature review and the present test data, it is validated that for liquid flow in smooth microchannels the conventional theoretical prediction for flow characteristics should still be applied.

Natural convections in conjugated single and double enclosures

The natural convection in single and double conjugated enclosures are numerically investigated. The single and double enclosures are formed by low conductance walls with finite thickness. The outside vertical surfaces of the conducting walls are of the third kind of boundary condition while the top and bottom outside surfaces are adiabatic. The problem studied is characterized by a dominant horizontal temperature gradient and the thermal boundary conditions at the cavity surfaces can not be specified in priori. Numerical results reveal the characteristics in such kind of enclosures and show the importance of the thermal boundary conditions on the natural convection in enclosures. It is also found that the natural convections in the conjugated double enclosures are basically the same, with a major difference in their fluid temperature levels.ZusammenfassungEs wird der Wärmetransport durch freie Konvektion in Einzel- und Doppelhohlräumen numerisch untersucht, welche von Wänden endlicher Dicke und niedriger Wärmeleitfähigkeit gebildet werden. An deren vertikalen Außenseite soll die Randbedingung 3. Art herrschen, während Boden- und Deckenseite als adiabat vorausgesetzt seien. Das untersuchte Problem ist durch einen dominanten horizontalen Temperaturgradienten charakterisiert, sowie den Umstand, daß die Randbedigungen auf den Berandungen der Hohlräume nicht von vornherein festgelegt werden können. Numerische Ergebnisse lassen das charakteristische Übertragungsverhalten solcher Hohlräume erkennen und zeigen den Einfluß der thermischen Randbedingungen auf die natürliche innenseitige Konvektion. Ferner wurde gefunden, daß freie Konvektionsvorgänge in Doppelhohlräumen grundsätzlich nicht anders als in Einzelhohlräumen ablaufen, abgesehen davon, daß diese auf unterschiedlichen Temperaturniveaus stattfinden.

Thermal analysis in phase change materials (PCMs) embedded with metal foams

The heat transfer enhancement for phase change materials (PCMs) has received increasing attention nowadays, since most of PCMs have low thermal conductivities which prolong the charging and discharging processes. Metal foams, as a sort of novel material with high thermal conductivity, are believed to be a promising solution to enhance the heat transfer performance of PCMs for thermal energy storage systems. The effects of natural convection on heat transfer enhancement for PCMs embedded with metal foams are investigated in this paper. The numerical investigation is based on the two-equation non-equilibrium heat transfer model, where the coupled heat conduction and natural convection in PCMs are considered at phase transition and liquid zones. The numerical results are validated by experimental data. In order to investigate the effect of metal foams on heat transfer, two different cases are compared in this study, which are the Case A (PCMs embedded with metal foams) and the Case B (pure PCMs). At the solid zone, heat conduction plays a dominant part because of natural convections absence, thus metal foams achieve much higher heat conduction rate than pure PCMs, and this can be attributed to the high thermal conductivity of metal foams skeleton and the heat can be quickly transferred through the foam solid structure to the whole domain of PCMs. At the two-phase zone and liquid zone, natural convection takes place and becomes the dominant heat transfer mode, but metal foam structures suppress the natural convection inside the PCMs owing to big flow resistance in metal foams. In spite of this suppression caused by metal foams, the overall heat transfer performance of Case A is still superior to the counterpart of Case B (pure PCMs), implying the enhancement of heat conduction offsets or exceeds the natural convection loss. The results show that the heat transfer enhancement due to the natural convection in PCMs embedded with metal foams is not as strong as expected, since metal foams have big flow resistance and the natural convection is suppressed. It also shows that better heat transfer performance can be achieved by using the metal foams of smaller porosity and bigger pore density. Last but not least, a series of detailed velocity and temperature profiles are given through numerical solutions, in order to present a vivid evolution of flow field and temperature profiles in the whole melting process.

Heat Transfer Enhancement of Phase Change Materials (PCMs) in Low and High Temperature Thermal Storage by Using Porous Materials

In this paper the solid/liquid phase change heat transfer in porous materials (metal foams and expanded graphite) at low and high temperatures is experimentally investigated, in an attempt to examine the feasibility of using metal foams to enhance the heat transfer capability of phase change materials for use with both the low and high temperature thermal energy storage systems. In this research, the organic commercial paraffin wax and inorganic hydrate calcium chloride hydrate salts were employed as the low-temperature materials, while the sodium nitrate is used as the high-temperature PCM in the experiment. The heat transfer characteristics of these PCMs embedded with open-cell metal foams were studied experimentally. The composites of paraffin and expanded graphite with different graphite mass ratios, namely, 3%, 6% and 9%, were also made and the heat transfer performances of these composites were tested and compared with metal foams. Overall metal foams can provide better heat transfer performance than expanded graphite due to their continuous inter-connected structures. But the porous materials can suppress the natural convection effect in liquid zone, particularly for the PCMs with low viscosities, thereby leading to the different heat transfer performance at different regimes (solid, solid/liquid and liquid regions). This implies that the porous materials don’t necessarily mean they can always enhance heat transfer in every regime.Copyright

Solid/Liquid Phase Change Heat Transfer in Latent Heat Thermal Energy Storage

Phase change materials (PCMs) have been widely used for thermal energy storage systems due to their capability of storing and releasing large amounts of energy with a small volume and a moderate temperature variation. Most PCMs suffer the common problem of low thermal conductivity, being around 0.2 and 0.5 for paraffin and inorganic salts, respectively, which prolongs the charging and discharging period. In an attempt to improve the thermal conductivity of phase change materials, the graphite or metallic matrix is often embedded within PCMs to enhance the heat transfer. This paper presents an experimental study on heat transfer characteristics of PCMs embedded with open-celled metal foams. In this study both paraffin wax and calcium chloride hexahydrate are employed as the heat storage media. The transient heat transfer behavior is measured. Compared to the results of pure PCMs samples, the investigation shows that the additions of metal foams can double the overall heat transfer rate during the melting process. The results of calcium chloride hexahydrate are also compared with those of paraffin wax.

Numerical simulation of film condensation on vertical plate embedded in metallic foams

Film condensation on vertical plate embedded in metal foams was numerically investigated based on some modification for Nusselt theory. Related numerical model was established by introducing Brinkman-Darcy model. Advection and inertial force in the condensate film were considered, from which the non-linear effect of cross-sectional temperature distribution on condensation heat transfer was also involved. Condensation characteristics were discussed and compared with those on smooth surface. Effects of key parameters on condensate film thickness were examined. Furthermore, with either the increase in porosity or the decrease in pore density, the condensate layer becomes thinner.