Xiaoze Du

On the Specific Heat Capacity of CuO Nanofluid

This paper reviews briefly the definition of heat capacity and clarifies the defined specific heat capacity and volumetric heat capacity. The specific heat capacity and volumetric heat capacity, with our measured experimental data for CuO nanofluids, are discussed as an illustrating example. The result indicates that the specific heat capacity of CuO nanofluid decreases gradually with increasing volume concentration of nanoparticles. The measurement and the prediction from the thermal equilibrium model exhibit good agreement. The other simple mixing model fails to predict the specific heat capacity of CuO nanofluid. The nanoparticle size effect and solid-liquid interface effect on the specific heat capacity of nanofluid are discussed.

Instability of interfaces of gas bubbles in liquids under acoustic excitation with dual frequency

Instability of interfaces of gas bubbles in liquids under acoustic excitation with dual frequency is theoretically investigated. The critical bubble radii dividing stable and unstable regions of bubbles under dual-frequency acoustic excitation are strongly affected by the amplitudes of dual-frequency acoustic excitation rather than the frequencies of dual-frequency excitation. The limitation of the proposed model is also discussed with demonstrating examples.

Acoustic wave propagation in bubbly flow with gas, vapor or their mixtures

Presence of bubbles in liquids could significantly alter the acoustic waves in terms of wave speed and attenuation. In the present paper, acoustic wave propagation in bubbly flows with gas, vapor and gas/vapor mixtures is theoretically investigated in a wide range of parameters (including frequency, bubble radius, void fraction, and vapor mass fraction). Our finding reveals two types of wave propagation behavior depending on the vapor mass fraction. Furthermore, the minimum wave speed (required for the closure of cavitation modelling in the sonochemical reactor design) is analyzed and the influences of paramount parameters on it are quantitatively discussed.

Influences of non-uniform pressure field outside bubbles on the propagation of acoustic waves in dilute bubbly liquids.

Predictions of the propagation of the acoustic waves in bubbly liquids is of great importance for bubble dynamics and related applications (e.g. sonochemistry, sonochemical reactor design, biomedical engineering). In the present paper, an approach for modeling the propagation of the acoustic waves in dilute bubbly liquids is proposed through considering the non-uniform pressure field outside the bubbles. This approach is validated through comparing with available experimental data in the literature. Comparing with the previous models, our approach mainly improves the predictions of the attenuation of acoustic waves in the regions with large kR0 (k is the wave number and R0 is the equilibrium bubble radius). Stability of the oscillating bubbles under acoustic excitation are also quantitatively discussed based on the analytical solution.

Effects of mass transfer on damping mechanisms of vapor bubbles oscillating in liquids

The damping mechanisms play an important role in the behavior of vapor bubbles. In the present paper, effects of mass transfer on the damping mechanisms of oscillating vapor bubbles in liquids are investigated within a wide range of parameter zone (e.g. in terms of frequency and bubble Péclet number). Results of the vapor bubbles are also compared with those of the gas bubbles. Our findings reveal that the damping mechanisms of vapor bubbles are significantly affected by the mass transfer especially in the regions with small and medium bubble Péclet number. Comparing with the gas bubbles, the contributions of the mass-transfer damping mechanism for the vapor bubble case are quite significant, being the dominant damping mechanism in a wide region.

Investigation on the Feasibility of Oscillating-Flow Heat Pipe Applied in the Solar Collector

Oscillating-flow heat pipe (OFHP) is a new type of heat-transfer device with many novel properties, having an attractive prospect for application in solar thermal utilization. Experiments on the heat-transfer performance of the OFHP with common size and the OFHP components designed for solar collectors were carried out. The results showed that the heat-transfer performance of OFHP of common dimensions varied slightly when the filling ratio varied from 40% to 60% and confirmed that the OFHP with an appropriate filling ratio has an excellent heat-transfer performance at most of inclination angles. The experiments also confirmed the feasibility for the OFHP application in solar collectors.

Stability mechanisms of oscillating vapor bubbles in acoustic fields

Vapor bubble instability could enhance the sonochemical activities and accelerate the reaction rate. In the present paper, vapor bubble instability in acoustic fields is investigated through combining both the spherical and stiffness stabilities within a wide range of parameter zone (consisting of bubble radius, acoustic frequency and pressure amplitude) in order to determine the stability states of vapor bubbles. The status of bubble oscillations are divided into four zones in terms of their stability characteristics. Influences of several paramount parameters on the bubble stability are demonstrated in detail. Different orders of spherical instability are quantitatively given together with cases in high-frequency and low-frequency limits. The practical applications of the present work are twofold: identification of the parameter zones with rapid sonochemical reactions; validity of the spherical bubble assumption for simplification of the numerical studies.

Catalytic Cracking of Biomass High-Temperature Pyrolysis Tar Using NiO/AC Catalysts

Nickel oxide supported on activated carbon (NiO/AC) catalysts were prepared and used for catalytic cracking of tars from inert high-temperature pyrolysis of biomass. Rice husk was selected as the feedstock, and pyrolyzed at 800°C using a lab-scale pyrolysis set. The pyrolysis vapors were subjected to either thermal cracking or catalytic cracking under 500–700°C. The results indicated the thermal cracking was not very efficient to convert the tars, while the catalytic cracking reduced both the light and heavy tars significantly. The yield of the light and heavy tars were decreased from 4.4 wt% and 1.9 wt% in the non-catalytic process to 0.2 wt% and 0.1 wt% in the catalytic process by the NiO/AC (4.3 wt%) at 700°C. The catalytic cracking also increased the gas product remarkably, mainly the H2 and CO2. The catalytic performance of the NiO/AC catalyst could enhance at elevated catalytic temperatures and increased NiO content of the catalyst.

Investigation on working fluids selection for organic rankine cycles with low-temperature heat sources

ABSTRACT The Influence of mixed and pure working fluids on the performance of organic Rankine cycles (ORCs) is discussed. Specifically, the performance of mixed and pure working fluids is analyzed based on certain characteristics of low-temperature heat source and heat sink. A method of selecting binary zeotropic mixed working fluids that match with different heat sinks is introduced. Thermodynamic processes of ORCs for various heat sources are simulated in Matlab. The performance characteristics of pure and mixed working fluids are compared under different inlet temperatures and temperature gradients of sensible heat sources. The results demonstrate that when the initial temperature of a heat source is lower and its temperature gradient is higher, and the temperature gradient of the heat sink is higher, mixed working fluids have better performance than pure working fluids. However, for the opposite heat source and heat sink situations, pure working fluids perform better. Mixtures with low critical temperature components exhibit the best performance among all working fluids when the temperature gradient of the heat source is large. The analysis also shows that introduction of a recuperator may reduce the cycle efficiency when the heat source temperature is low and the temperature gradient of the heat source is large.

Thermal conductivity of epoxy composites filled by thermally reduced graphite oxide with different reduction degree

A series of selectively reduced graphite oxide was prepared by thermal reduction of graphite oxide at different annealing temperatures and used as fillers to enhance thermal conductivity of epoxy composites. The reduction degree of selectively reduced graphite oxide increases with annealing temperature changing from 600℃ to 1000℃. The out-of-plane thermal conductivity (Κo) of selectively reduced graphite oxide/epoxy composites is remarkably higher than that of graphite oxide/epoxy. For the selectively reduced graphite oxide obtained at 1000℃, Κo reaches 0.674 W/m·K when filler content is 5.4 wt%, which is 450% of pure epoxy. The enhanced Κo can be attributed to the better dispersion of selectively reduced graphite oxide in epoxy and their edges overlap to form effective thermal conductive paths in epoxy matrix. However, the achieved thermal conductivity enhancement is still comparatively lower than that of selectively reduced graphite oxide with higher reduction degree, since the interfacial bonding strength between selectively reduced graphite oxide and epoxy decreases when reduction degree of selectively reduced graphite oxide flakes becomes higher.