Chunqing Tan

Rheological behaviour of nanofluids

This work aims at a more fundamental understanding of the rheological behaviour of nanofluids and the interpretation of the discrepancy in the recent literature. Both experiments and theoretical analyses are carried out with the experimental work on ethylene glycol (EG)-based nanofluids containing 0.5–8.0 wt% spherical TiO2 nanoparticles at 20–60 °C and the theoretical analyses on the high shear viscosity, shear thinning behaviour and temperature dependence. The experimental results show that the EG-based nanofluids are Newtonian under the conditions of this work with the shear viscosity as a strong function of temperature and particle concentration. The relative viscosity of the nanofluids is, however, independent of temperature. The theoretical analyses show that the high shear viscosity of nanofluids can be predicted by the Krieger–Dougherty equation if the effective nanoparticle concentration is used. For spherical nanoparticles, an aggregate size of approximately 3 times the primary nanoparticle size gives the best prediction of experimental data of both this work and those from the literature. The shear thinning behaviour of nanofluids depends on the effective particle concentration, the range of shear rate and viscosity of the base liquid. Such non-Newtonian behaviour can be characterized by a characteristic shear rate, which decreases with increasing volume fraction, increasing base liquid viscosity, or increasing aggregate size. These findings explain the reported controversy of the rheological behaviour of nanofluids in the literature. At temperatures not very far from the ambient temperature, the relative high shear viscosity is independent of temperature due to negligible Brownian diffusion in comparison to convection in high shear flows, in agreement with the experimental results. However, the characteristic shear rate can have strong temperature dependence, thus affecting the shear thinning behaviour. The theoretical analyses also lead to a classification of nanofluids into dilute, semi-dilute, semi-concentrated and concentrated nanofluids depending on particle concentration and particle structuring.

Influences of blade bowing on flowfields of turbine stator cascades

To understand the ine uences of blade positive and negative bowing on e owe elds of turbine stator cascades with low aspect ratio, six sets ofcascades with different bowed angles of the blades were designed and tested. A e ve-hole pitotmicroprobe,which waslocated upstream anddownstream ofthecascades, was used to survey thee owe elds in detail. The results were different from other published results with a different kind of turbine stator blade proe le. When a blade has positive or negative bowing, the passage vortices do not change, whereas the characteristics of the trailing vortices, such as the intensity, locations, size, and structure, do change distinctly. In the case of blade positive bowing, the local spanwise distribution of the yaw e ow angle at the cascade exit is signie cantly affected, with very little changes in the cascade overall loss.

Analysis of an Air Powered Engine System Using a Multi-Stage Radial Turbine

The performance and design criteria of air powered multistage turbines are studied thermodynamically in this paper. In-house code is developed in the C++ environment and the characteristics of four-stage turbines with inter-heating are analyzed in terms of maximum thermal efficiency, maximum exergy efficiency and maximum work output over the inlet temperature range of 293 K-793 K with inlet pressure of 70 bar. It is found that the maximum thermal efficiency, maximum exergy efficiency and maximum work output are 62.6%, 91.9%, 763.2 kJ/s, respectively. However, the thermal efficiency, exergy efficiency and work output are not equivalent for the four-stage radial turbine. It is suggested that at low working temperatures both maximum exergy efficiency and maximum work output can be used as the design objective, however, only maximum work output can be used as the design objective for the four-stage radial turbine over the working temperature range in this work.

Blade Bowing Effect on Aerodynamic Performance of a Highly Loaded Turbine Cascade

A S ONE of the major measures to improve the aerodynamic performance of modern turbines, highly loaded blades (blades with a high-turning angle) have attracted much attention in the past two decades [1–7]. However, it is found that due to the high-turning angle, the highly loaded blades cause high loss because of the serious secondary flow and centralized vortices [1–7]. Blade bowing (compound leaning or curving) has been recognized as an effective way to reorganize the vortices, reduce the secondary flow loss, and improve the efficiency of turbines [4–8], and also the fundamental for the three-dimensional optimization of turbine blades [6,9–11]. It is therefore the primary motivation of this work to find out whether the blade bowing applies to improving the performance of the highly loaded turbine cascade (turning angle 113 deg). The second motivation of this paper is to carry out a systematic studywith a series of negatively and positively bowed highly loaded blades. (When the dihedral angle between the blade pressure side and cascade endwalls is acute, the bowed angle is positive and vice versa). Clearly there is a vacuum of such a systematic study in highly loaded turbine cascades, as only few certain bowing angles were investigated individually [4,5,8].

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

Energy storage sizing for office buildings based on short-term load forecasting

This paper presents a three-layer Artificial Neural Network as the short-term load forecasting model adopting the fastest back-propagation algorithm with robustness, i.e., Levenberg-Marquardt optimization, and moreover, the momentum factor is considered during the learning process. Based on predicted data by aforementioned model, size determination of energy storage system in terms of power rating and capacity is undertaken according to the desired level of shaving peak demand. The illustrative example in reference to the weather and power load data of office building from July to August in 2011 gets the results that the average relative error -0.7% and the root-mean-square error 2.79% which show aforementioned forecasting model can work effectively with the attractive percentage, i.e. 87.5%, of error within the acceptable one 2.79%; Furthermore, size determination of energy storage system adopting battery energy storage technology, i.e. 7.03kW/36.42kWh, is carried out to meet the desired peak shaving demand.

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...

Coal partial gasification studies applied to co-production of hydrogen and electricity

To search for the optimum working condition of coal partial gasification for co-production of hydrogen and electricity, a gasifier has been simulated and studied by the use of an Aspen Plus model. Oxygen-to-coal ratio, steam-to-coal ratio and steam temperature are optimization variables of the system. The optimization variables impact on syngas yield, syngas concentration, system exergy efficiency, HER and CO2 separation rate have been analyzed. It is found that the oxygen-to-coal ratio and the steam-to-coal ratio both have large influence on performances of gasifer and system and the steam temperature has the least influence.

LBO Performance Comparison Between Two Combustors With Different Swirl Cups

Lean Blowout (LBO) performance is very important to the aero and ground gas turbine combustors. A typical liquid-fueled gas turbine combustor is the one with swirl cup dome which plays an important role to the LBO. The swirl cup dome comprises swirlers and nozzle usually. The swirlers serve to generate a toroidal flow reversal that entrains and recirculates a portion of the hot combustion products to mix with the incoming fresh air and fuel, so it makes the recirculation region the sustainable source of ignition. Swirlers in present study generally are two or three stages, and the nozzle takes different atomization styles, such as pressure-swirl atomization, prefilming and airblast atomization. Different swirlers matching various nozzles form all kinds of swirl cup domes, and each swirl cup dome of combustor would have different LBO performance and other combustion properties resulting from its structure characteristics. The flow flux arrangement and spray distribution are the two important factors to determine the combustor performance. Two combustor dome test rigs were investigated, of which one comprises with three air swirlers and a fuel prefilming nozzle (dome A), and the other is composed of two air swirlers and a fuel pressure nozzle (dome B). Tests were conducted to get the LBO fuel air ratio at atmospheric pressure. To explain the experimental results, numerical simulations were performed for cool flow fields of two combustors, also the cold flow field and spray of the two combustors’ dome downstream were measured by PDA with water instead of kerosine. The flame pictures near LBO were taken. The preliminary results indicated that the combustor with dome A had better spray uniformity than the one with dome B, but it had a little worse LBO performance. The air flow mass percentage of the inner swirler of dome A should decrease to some extent in order to establish a lower pressure region at the outlet of dome A, which would be helpful to decrease the LBO fuel air ratio and so as to improve the LBO performance. The two domes had their own advantages, and if the benefits of both were integrated, it was possible to design a better swirl cup dome.Copyright