The effect of uniform surface mass transfer on the buoyancy-induced flow in a porous medium adjacent to a horizontal heated surface with a power law variation of wall temperature, T_wαx^λ, is considered in this paper. It is found that a similarity solution exists for the problem when λ = 2. Approximate solutions based on the local nonsimilarity method are obtained for other values of λ. Numerical solutions were carried out up to the third-level of truncation and results are presented for a wide range of massflux parameter and wall temperature distributions.

W. J. Minkowycz;平 郑;F. Moalem

International Communications in Heat and Mass Transfer

1985

A Pt microheater (140×100μm²) is fabricated on a glass wafer and enclosed in a silicon microchannel of trapezoidal cross section by MEMS technology. With the aid of a high-speed CCD and data acquisition system, subcooled flow boiling phenomena and temperature response on the surface of the microheater under pulse heating are observed and recorded. Experiments are conducted for six pulse widths (50μs, 100μs, 200μs, 600μs, 1ms, and 2ms) under different mass and heat fluxes. With increasing heat flux at a fixed pulse width and different mass fluxes, four flow regimes including single phase, nucleate boiling, film boiling and dry out are identified. Since flow boiling regimes are relatively independent of mass flux, correlation equations based on experimental data for the transitional heat flux of different flow boiling regimes are obtained in terms of pulse width only. It is also found that pulse width and mass flux have little influence on boiling inception time, and the classical analytical solution for the nucleation inception time in terms of heat flux is verified experimentally.

Gang Chen;Xiaojun Quan;平 郑

International Communications in Heat and Mass Transfer

2010-8

平 郑

Letters in Heat and Mass Transfer

1976

During the past decade, a great deal of research work has been carried out on near-field photonics and nano-optics relating to photo-thermal conversion process. In this paper, recent analytical, numerical and experimental studies on radiation properties in visible and near-infraed wavelengths of electromagnetic waves incident on nanoparticles and metamaterials with applications to solar energy harvesting are reviewed. These nanoparticles and metamaterials are made of both isotropic and anisotropic materials. Mie theory is employed for analyses of absorption properties of spherical nanoparticles composed of different materials. The finite difference time domain (FDTD) method and the rigorous coupled-wave analysis (RCWA) method are used for numerical studies of absorption properties of nanoparticles with arbitrary shapes and power dissipation density distribution of metamaterials, respectively. In addition, a perfect absorber with pyramidal nanostructures made of Bi₂Te₃ is proposed, and measurements of absorption properties of some fabricated metamaterials are summarized. Potential applications of these nanostructures for solar energy harvesting enhancement are discussed. The results of these studies have provided not only improved understanding of physical mechanisms in enhanced absorption of solar energy by nanoparticles and metamaterials, but also clarified contradictory explanations on the photo-thermal conversion process in solar energy harvesting as well.

Zhaolong Wang;平 郑

International Journal of Heat and Mass Transfer

2019-9

The problem of steady natural convection film boiling about a heated isothermal vertical plate in a porous medium filled with a subcooled liquid is considered. With the boundary layer approximations, similarity solutions are obtained for the buoyancy-induced flow in the vapor and subcooled liquid layers with a distinct interface. At a given vapor Rayleigh number, the Nusselt number is found to be uniquely dependent on the vapor film's dimensionless thickness, which in turn depends on three dimensionless parameters related to the degree of superheating of the wall, the extent of the subcooling of the surrounding liquid, and a property ratio of the vapor and the liquid phases. It is found that the effect of the increase of the subcooling of the surrounding fluid tends to decrease the vapor boundary layer thickness, increase the liquid boundary layer thickness and increase the surface heat flux. On the other hand, the increase of the wall superheating tends to increase the vapor layer thickness, decrease the liquid layer thickness and increase the surface heat flux. Application to boiling heat transfer about a dike intruded into an aquifer is discussed.

平 郑;Ashok K. Verma

International Journal of Heat and Mass Transfer

1981-7

Pressure drops in a fully developed laminar incompressible reciprocating pipe flow have been investigated analytically and experimentally. An exact analytical solution for the instantaneous and cycle-averaged friction coefficients of a fully developed laminar reciprocating flow has been obtained. It was found that, although the dimensionless axial velocity profiles of a fully developed flow depend only on the kinetic Reynolds number, the friction coefficients depend not only on the kinetic Reynolds number but also the dimensionless oscillation amplitude of fluid. Experiments have been carried out to measure the pressure drops of a laminar reciprocating flow at downstream of a long pipe at various frequencies and fluid displacements. Comparisons are made for the time-resolved and the cycle-averaged friction coefficients between the analytical solution and experimental data. It is shown that the analytical solution is in good agreement with measurements.

天寿 赵;平 郑

International Journal of Heat and Fluid Flow

1996-4

Condensation in the presence of non-condensable gas on a biomimetic pillared subcooled surface with hybrid wettability (with hydrophilic top and hydrophobic side and bottom) is investigated using a newly developed 3D multi-component multiphase lattice Boltzmann model. Three preferred nucleation sites with different surface wettability contrasts are found: (i) at the corner of side and bottom hydrophobic surface; (ii) at the center of the bottom hydrophobic surface, and (iii) on the top hydrophilic surface of the pillar. Influencing factors, such as pillar geometrical parameters, subcooling degree and non-condensable gas concentration, on dropwise-to-filmwise condensation transition are examined. For dropwise condensation on a top hydrophilic pillar surface, the droplet undergoes a two-stage growth pattern from “changing contact line” to “constant contact line”. Non-condensable gas is found to aggregate near the condensing interface in the vapor phase and at corners of pillar side surface and bottom surface. Increasing pillar width or pillar height (with other geometric parameters remained unchanged), as well as decreasing degree of wall subcooling or non-condensable gas concentration, can delay transition from dropwise to filmwise condensation.

Qing Guo;平 郑

International Journal of Heat and Mass Transfer

2019-1

The multiple-relaxation-time (MRT) lattice Boltzmann method (LBM) with multi-reflection solid boundary conditions is used to study anisotropic permeabilities of a carbon paper gas diffusion layer (GDL) in a fuel cell. The carbon paper is reconstructed using the stochastic method, in which various porosities and microstructures are achieved to simulate different samples. The simulated permeability and tortuosity show anisotropic characteristics of the reconstructed carbon papers with in-plane permeability higher than through-plane, and in-plane tortuosity lower than through-plane. The calculated permeabilities are in good agreement with existing measurements. The relationship between the permeability and the porosity is fitted with empirical relations and some fitting constants are determined. Furthermore, the obtained relationship of tortuosity and porosity is used in a fractal model for permeabilities. The results indicate that the fractal model and the Kozeny-Carman equation provide similar predictions on the through-plane permeability of the carbon paper GDL.

Liang Hao;平 郑

Journal of Power Sources

2009-1-1

A facility capable of generating steady and oscillating flows was constructed and experiments were conducted to investigate the pressure-drop characteristics of regenerators packed with wire screens. Both the velocity and pressure-drop across the regenerator were measured. To accurately determine the correlation between pressure- drop and velocity, the experiments covered a wide range from very low to very high Reynolds numbers, Re^-1/2_h. The steady flow results reveal that a three-term correlation with a term proportional to Re^-1/2_h in addition to the Darcy-Forchheimer twoterm correlation will fit best to the data. This Re^-1/2_h term accounts for the boundary layer effect at intermediate Reynolds number. The results also show that the correlation for oscillating flows coincides with that for steady flows in 1 < Re_h < 2000. This suggests that the oscillating flows in the regenerators behave as quasi-steady at the frequency range of less than 4.0 Hz, which is the maximum operable oscillating flow frequency of the facility.

Chin Tsau Hsu;Huili Fu;平 郑

Journal of Fluids Engineering, Transactions of the ASME

1999-3

The phenomenon of coalescence induced droplets self-propelled jumping on superhydrophobic surfaces (SHS) is numerically simulated in this paper using 2D lattice Boltzmann method (LBM). To overcome numerical instability problems of a two-phase flows with a high liquid/vapor density ratio and a high liquid/vapor viscosity ratio, the equation of state is modified according to an existing method and the multiphase relaxation time (MRT) method is adopted. The simulated jumping velocity and jumping height of droplets with different radii are found in good agreement with experimental observations. In addition, the reason why coalescence droplets will jump on superhydrophobic surface with a sufficiently high contact angle is explained based on a qualitative analysis.

Xiuliang Liu;平 郑;Xiaojun Quan

International Journal of Heat and Mass Transfer

2014-6