An experiment has been initiated to study oscillatory heat transfer in a periodically reversing pipe flow. A multivariate statistical analysis was employed to obtain a correlation equation for the cycle-averaged Nusselt number in terms of the three similarity parameters: the Reynolds number Re, the dynamic Reynolds number Re_ω, and the dimensionless fluid displacement A_ω.

Tang Xiaoguo;平 郑

International Communications in Heat and Mass Transfer

1993

Microgravity effects on pool boiling heat transfer from a horizontal hydrophilic surface under constant wall temperature are simulated numerically based on an improved liquid-vapor phase-change lattice Boltzmann method with the imposition of a conjugate thermal boundary condition at solid/liquid interface. Effects of microgravity on bubble departure diameter and bubble departure frequency are investigated. Both the bubble departure diameter and the duration of one bubble cycle are found to be increasing with decreasing gravity levels. Boiling curves under normal gravity and microgravity are numerically obtained and compared. It is shown that gravity has significant effects on pool boiling curves from nucleate boiling to critical heat flux, and from transition boiling to film boiling heat transfer. Moreover, the critical heat flux is lower and it occurs at a lower wall superheat under microgravity conditions than under normal gravity. Effects of heater size in pool boiling under both of microgravity and normal gravity conditions are investigated as well.

Xiaojing Ma;平 郑;Shuai Gong;Xiaojun Quan

International Journal of Heat and Mass Transfer

2017

Sessile droplet oscillations in electrowetting on dielectric with a coplanar-electrode configuration are studied experimentally under the actuation of ac voltage with different frequencies. It was found that the experimental resonance frequencies and the number of lobes at different resonance modes agree reasonably well with a previous linear analysis. Oscillations of contact width and droplet height are in-phase at resonance modes P _2n+2while out-of-phase at P _2nwith n = 2, 4, 6,... At certain critical frequencies, the droplet oscillations are very weak and switch from in-phase (out-of-phase) to out-of-phase (in-phase). For the oscillations after resonance frequency but before critical frequency, at low frequency, the large amplitude oscillation of the contact line deforms it from a circle to having lobes; the number of lobes increases with the frequency and their position alternates in the azimuthal direction, through periodical droplet spreading and receding. For the oscillations after critical frequency but before resonance frequency, the droplet oscillation demonstrates droplet waggling with an obvious contact line at normal and abnormal stops, due to the contact line pinning at low frequency, or the transportation of lobes on the droplet surface from one end to the other at high frequency. These asymmetric oscillations will produce a more chaotic fluid flow inside the droplets than symmetric oscillations and could be used purposely to enhance mixing in droplet-based micro-fluidics.

芳军 洪;D. D. Jiang;平 郑

Journal of Micromechanics and Microengineering

2012-8

Direct numerical simulations of pool boiling heat transfer are performed by the pseudopotential liquid-vapor phase change lattice Boltzmann model. Boiling curves from natural convection regime to film boiling regime and thermal responses inside the heater are numerically obtained. It is found that the simulated critical heat flux (CHF) agrees reasonably well with existing analytical models. In addition, simulated boiling heat fluxes in film boiling regime are also in good agreement with classical solutions. The model is thus quantitatively validated. Simulation results demonstrate that temperature distribution at the top of heater surface is nonuniform, especially in CHF point and transition boiling regime.

Shuai Gong;Chao Yang Zhang;平 郑

Kung Cheng Je Wu Li Hsueh Pao/Journal of Engineering Thermophysics

2019-1-1

This paper analyzes the forces, including the Stokes drag force induced by the electrothermal flow, imposed on the particles in conventional dielectrophoresis in microchannels with interdigitated electrodes through a two-dimensional mathematical model. The conditions for the positive and negative dielectrophoresis are revealed. The electrical field, the magnitude of the negative dielectrophoretic force and Stokes drag force are presented. The levitation heights for particles at different horizontal positions with and without considering the Stokes drag force are compared, and the effect of the particle radius for the levitation height is also analyzed.

Jun Cao;平 郑;芳军 洪

Journal of Electrostatics

2008-11

In this paper, new simulation results of a saturated liquid droplet impact dynamics on hydrophilic (θ = 80°) and hydrophobic (θ = 170°) solid surfaces surrounded by a saturated vapor are obtained based on a 3D lattice Boltzmann method (LBM). A novel piecewise relaxation time is proposed to overcome numerical instability at high liquid/vapor density ratios and low liquid droplet viscosities. Simulations are carried out for low Ohnesorge (Oh) numbers in the range from 0.01 to 0.05, corresponding to a water droplet diameter in the range of 1 µm to 130 µm. The impinging droplet deformation process including breakup phenomena is illustrated on hydrophilic and hydrophobic surfaces at different Weber (We) numbers. Droplet spread factors obtained from simulations match well with existing experimental data and theoretical values, validating the accuracy of the present 3D LBM. It is found that a water droplet, after reaching its maximum spread factor, breaks up into a toroid shape with a vapor cavity or a liquid film left at its center at large We numbers on a hydrophilic and a hydrophobic surface, respectively. A map in terms of We number versus Oh number is obtained for predicting the breakup occurrence. The critical We number is monotonically increasing with the increase of the Oh number, meaning that the smaller diameter water droplet requires a larger We number for breakup. Droplet breakup is more likely to occur on a hydrophobic surface than on a hydrophilic surface at larger Oh numbers, while the behavior is opposite at lower Oh numbers.

Wei Xiong;平 郑

Computers and Fluids

2018-5-30

Anode flooding is frequently encountered in proton exchange membrane fuel cells, which can induce local hydrogen starvation and subsequently result in carbon corrosion in catalyst layers. In this work, we develop a two-dimensional computational model and focus our attention on the mechanisms of hydrogen transport within the diffusion medium when part of the anode channel is clogged. In-plane convection is demonstrated to be the primary mechanism for hydrogen transport at the fringe of the flooded area. As the reactant flows deep inside, however, the dominating mechanism switches to diffusion due to nitrogen accumulation in the anode. Moreover, if hydrogen outside the flooded region is diluted with nitrogen, the length scale for hydrogen to deplete completely is reduced substantially, leading to an extension of the area suffering from hydrogen starvation and carbon corrosion. Meanwhile, the spatial distribution of carbon corrosion rate is emphasized in this work, and the effect of in-plane proton conduction is highlighted. This paper also sheds light on the maximum rate of carbon corrosion, which is demonstrated to be solely determined by oxygen crossover rate at high cell voltages, but is influenced by the kinetics of oxygen reduction reaction at low voltages.

Xiao Guang Yang;Qiang Ye;平 郑

International Journal of Heat and Mass Transfer

2012-8

Measurements of micro bubble nucleation temperature in single-stranded DNA (ssDNA) solution with four different concentrations (0.4,1, 6.4 and 10 μg μI^-1) and under different heat fluxes are presented in this paper. A micro thermal bubble was generated periodically by a micro bubble actuator under pulse-heating conditions. The bubble nucleation temperature was measured by employing a micro Pt heater as a self-sensing resistive temperature sensor, and the voltage signal was recorded by a 100 MHz high-speed digitizer. By examining the temporal temperature variation of the micro heater during the pulse-heating period, one can observe a 'V' shape temperature change for the cases when bubble nucleation occurred, which was speculated to be the beginning of bubble nucleation. It is found that (1) the onset bubble nucleation temperature increases with the DNA concentration and (2) at a fixed concentration, the bubble nucleation temperature increases with the heat flux from the micro heater. Based on a transient 3D heat conduction model, it is found that the size of the superheated region in the fluid just before bubble nucleation is comparable to the feature size of the micro heater, and relatively independent of the ssDNA concentration and heat flux.

佩刚 邓;Yi Kuen Lee;平 郑

Journal of Micromechanics and Microengineering

2005-3

A triple phase-change lattice Boltzmann model, based on a coupling of existing vapor-liquid phase-change model and enthalpy-based liquid-solid phase-change model, is proposed in this paper. Based on this novel model, 3D simulations for transient condensation of a dry saturated vapor and subsequent freezing at a circular cryogenic spot of an inclined hydrophobic surface are carried out. The dynamic morphologies of the droplet and ice are presented. Simulated results show that ice adjacent to the cryogenic spot is formed inside a droplet, and the droplet subsequently moves down along the vertical surface under action of gravity. Effects of droplet's dynamic behaviors on the growth of ice and averaged heat flux are presented. Temperature distributions in the ice as well as in the water and steam on the middle cross section and local heat flux at the wall are illustrated. Simulations of the triple phase-change phenomena under different tilt angles show that the ice is elongated in the direction of gravity. The wettability of the surface is shown to have important influence on the morphology of droplet but its effect on the growth of ice is small.

Xiaoping Li;Jianan Zhao;平 郑

International Journal of Heat and Mass Transfer

2017

The effects of preheating conditions (including the critical energy content, the firing rate, and the equivalence ratio) on the initiation and extinction of a super-adiabatic combustion (SAC) in a steady flow of propane/air mixture through a stack of stainless steel screens (an inert porous medium) are investigated. It is found that there are critical conditions for the initiation and extinction of a SAC in the steady flow through the inert porous column. The critical preheating energy required to initiate a SAC is found to decrease with the increase of the preheating equivalence ratio, the preheating firing rate, and the operational equivalence ratio. After the initiation of a SAC, two combustion modes (i.e., stable combustion mode and push-forward combustion mode) have been observed. These combustion modes are determined by the operational conditions.

Y. Huang;C. Y.H. Chao;平 郑

International Journal of Heat and Mass Transfer

2002-10