Tianshou Zhao

Non-precious Co3O4 nano-rod electrocatalyst for oxygen reduction reaction in anion-exchange membrane fuel cells

We report preparation of carbon-supported Co3O4 electrocatalysts with nano-rods and spherical structures by the solvent-mediated morphological control method. The catalytic properties of the prepared catalysts for the oxygen reduction reaction (ORR) in alkaline media are investigated. We show that the ORR catalytic activity of the prepared catalysts is sensitive to the number and activity of surface-exposed Co3+ ions that can be tailored by the morphology of cobalt oxides. In particular, we demonstrate that the non-precious Co3O4 electrocatalyst with the nano-rod structure (∼12 nm in length and ∼5.1 nm in diameter) prepared in the mixed solvent of water to dimethylformamide ratio of 1 : 1 exhibits a higher current density than a much more expensive palladium-based catalyst does at the low potential region.

A correlation of optimal heat rejection pressures in transcritical carbon dioxide cycles

Abstract In this work, a cycle simulation model has been developed to optimize the coefficient of performance (COP) of transcritical carbon dioxide air-conditioning cycles. The analysis shows that the COP of the transcritical carbon dioxide cycle varies nonmonotonically with the heat rejection pressure; a maximum COP occurs at an optimal heat rejection pressure. It is further revealed that the values of the optimal heat rejection pressure mainly depend on the outlet temperature of the gas cooler, the evaporation temperature, and the performance of the compressor. Based on the cycle simulations, correlations of the optimal heat rejection pressure in terms of appropriate parameters are obtained for specific conditions. The results are of significance for the design and control of the transcritical carbon dioxide air-conditioning and heat pump systems

Co-current air-water two-phase flow patterns in vertical triangular microchannels

Characteristics of co-current upward air–water two-phase flow patterns in vertical equilateral triangular channels with hydraulic diameters of 2.886, 1.443 and 0.866 mm were investigated experimentally. Flow regimes were identified by both visual observations using a high-speed motion analyzer and dynamic pressure-drop measurements. The experimental results show that the typical flow patterns encountered in the conventional, large-sized vertical circular tubes, such as dispersed bubbly flow, slug flow, churn flow and annular flow, were also observed in the channels having larger hydraulic diameters (dh=2.886 and 1.443 mm). However, for the smallest channel (dh=0.866 mm), dispersed bubbly flow pattern, characterized by randomly dispersed bubbles in continuous liquid phase, was not found, although the other typical flow patterns remained in the channel. Moreover, the experiments reveal that, for the channel of dh=0.866 mm, a so-called capillary bubbly flow pattern, characterized by a single train of bubbles, essentially ellipsoidal in shape and spanning almost the entire cross-section of the channel, existed at low gas flow rates. It is further found that in the slug flow regime, slug-bubbles were substantially elongated. Finally, flow regime transition boundaries for the triangular microchannels were compared with relevant flow regime transition models and correlations as well as the existing experimental data for small round tubes and square channels.

Measurements of Heat Transfer Coefficients From Supercritical Carbon Dioxide Flowing in Horizontal Mini/Micro Channels

Heat transfer from supercritical carbon dioxide flowing in horizontal mini/micro circular tubes cooled at a constant temperature has been investigated experimentally. Six stainless steel circular tubes having inside-diameters of 0.50 mm, 0.70 mm, 1.10 mm, 1.40 mm, 1.55 mm, and 2.16 mm were tested. Measurements were carried out for the pressures ranging from 74 to 120 bar, the temperatures ranging from 20 to 110°C, and the mass flow rates ranging from 0.02 to 0.2 kg/min. It is found that the buoyancy effect was still significant, although supercriticalCO2 was in forced motion through the horizontal tubes at Reynolds numbers up to 10 5 . The experimental results also indicate that the existing correlations developed in the previous studies for large tubes deviate significantly from the experimental data for the present mini/micro tubes. Based on the experimental data, a correlation was developed for the axially averaged Nusselt number in terms of appropriate dimensionless parameters for forced convection of supercritical carbon dioxide in horizontal mini/micro tubes cooled at a constant temperature. @DOI: 10.1115/1.1423906#

An experimental investigation of convection heat transfer to supercritical carbon dioxide in miniature tubes

Experimental results of convection heat transfer to supercritical carbon dioxide in heated horizontal and vertical miniature tubes are reported in this paper. Stainless steel circular tubes having diameters of 0.70, 1.40, and 2.16 mm were investigated for pressures ranging from 74 to 120 bar, temperatures from 20 to 110 °C, and mass flow rates from 0.02 to 0.2 kg/min. The corresponding Reynolds numbers and Prandtl numbers ranged from 104 to 2×105 and from 0.9 to 10, respectively. It is found that the buoyancy effects were significant for all the flow orientations, although Reynolds numbers were as high as 105. The experimental results reveal that in downward flow, a significant impairment of heat transfer was discerned in the pseudocritical region, although heat transfer for both horizontal and upward flow was enhanced. The experimental results further indicate that in all the flow orientations, the Nusselt numbers decreased substantially as the tube diameter shrunk to <1.0 mm. Based on the experimental data, correlations were developed for the axially-averaged Nusselt number of convection heat transfer to supercritical carbon dioxide in both horizontal and vertical miniature heated tubes.

A LATTICE BOLTZMANN MODEL FOR CONVECTION HEAT TRANSFER IN POROUS MEDIA

ABSTRACT A lattice Boltzmann model for convection heat transfer in porous media is proposed. In this model, a new distribution function is introduced to simulate the temperature field in addition to the density distribution function for the velocity field. The macroscopic equations for convection heat transfer in porous media are recovered from the model through the Chapman-Enskog procedure. The model is validated by several benchmark problems, and it is found that the numerical results are in good agreement with the well-documented results in the literature.

Physical symmetry, spatial accuracy, and relaxation time of the lattice Boltzmann equation for microgas flows

In this paper, we study systematically the physical symmetry, spatial accuracy, and relaxation time of the lattice Boltzmann equation (LBE) for microgas flows in both the slip and transition regimes. We show that the physical symmetry and the spatial accuracy of the existing LBE models are inadequate for simulating microgas flows in the transition regime. Our analysis further indicates that for a microgas flow, the channel wall confinement exerts a nonlinear effect on the relaxation time, which should be considered in the LBE for modeling microgas flows.

Electrochemical Reactions in a DMFC under Open-Circuit Conditions

Department of Mechanical Engineering, The Hong Kong University of Science and Technology,Clear Water Bay, Kowloon, Hong Kong, ChinaGas evolution, principally consisting of carbon dioxide and hydrogen, was observed in the anode flow field of a direct methanolfuel cell ~DMFC! running under open-circuit conditions and at low oxygen flow rates. This finding is contrary to conventionalwisdom that electrochemical reactions cease as an external load is removed. The mechanism leading to this peculiar phenomenonis explained theoretically and confirmed experimentally.© 2004 The Electrochemical Society. @DOI: 10.1149/1.1836111# All rights reserved.Manuscript submitted July 17, 2004; revised manuscript received August 28, 2004. Available electronically November 29, 2004.

Theoretical analysis of film condensation heat transfer inside vertical mini triangular channels

Abstract An analytical model is presented for predicting film condensation of vapor flowing inside a vertical mini triangular channel. The concurrent liquid–vapor two-phase flow field is divided into three zones: the thin liquid film flow on the sidewall, the condensate flow in the corners, and the vapor core flow in the center. The model takes into account the effects of capillary force induced by the free liquid film curvature variation, interfacial shear stress, interfacial thermal resistance, gravity, axial pressure gradient, and saturation temperatures. The axial variation of the cross-sectional average heat transfer coefficient of steam condensing inside an equilateral triangular channel is found to be substantially higher than that inside a round tube having the same hydraulic diameter, in particular in the entry region. This enhancement is attributed to the extremely thin liquid film on the sidewall that results from the liquid flow toward the channel corners due to surface tension. The influences of the inlet vapor flow rates, the inlet subcooling, and the channel size on the heat transfer coefficients are also examined.

On capillary-driven flow and phase-change heat transfer in a porous structure heated by a finned surface: measurements and modeling

Abstract Characteristics of capillary-driven flow and phase-change heat transfer in a porous structure heated with a permeable heating source at the top were studied experimentally and theoretically in this paper. The experiments show that for small and moderate heat fluxes, the whole porous structure was fully saturated with liquid except adjacent to the horizontal heated surface where evaporation took place uniformly. For higher heat fluxes, a two-phase zone developed in the upper portion of the porous structure while the lower portion of the porous structure was saturated with subcooled liquid. When the imposed heat flux was further increased, a vapor blanket formed below the heated surface and the corresponding critical heat flux was reached. The heat transfer coefficient was modeled by simultaneously solving the problem of evaporating capillary meniscus in the pore level and the problem of fluid flow through a porous medium. The model is in good agreement with the experimental data, predicting the variations of the heat transfer coefficient with the increasing heating load.