Yueshe Wang

Performance of Forced Convection Heat Transfer in Porous Media Based on Gibson–Ashby Constitutive Model

A novel simulation model is developed for predicting the performance of forced convection heat transfer in the porous metal foam. Based on the physical geometry of the Gibson-Ashby constitutive model, the theoretical model proposed is able to predict the mechanical behaviors and thermal physical properties of porous materials simultaneously. The theoretical predictions of the overall heat transfer coefficient and pressure drop were compared with available experimental data for two different porous foam tubes. The first tube has a porous diameter of 0.6mm and porosity of 0.402, and the other tube has a diameter of 1.6mm and porosity of 0.462. The results show that the relative deviation of the flow pressure drop between the prediction and the experimental data are in the range from 5% to10% while the relative deviation of the overall heat transfer coefficient is about 20%. These deviations are acceptable for applications in engineering. So the feasibility of the Gibson-Ashby constitutive model to be used to predict the performance of flow resistance and convective heat transfer in porous foam ducts is satisfactorily validated.

Numerical Simulation on Structure Optimization of Liquid-Gas Cylindrical Cyclone Separator

With the further development of oilfield, liquid-gas separation has become an essential problem. Cylindrical cyclone separators are popular in the industrial process due to the advantage that they are simple, compact, and inexpensive to manufacture. In this paper, a three-dimensional turbulence model including Reynolds stress model was established to describe the mixture flow field in the separator. Through the numerical simulation, the separation efficiency was investigated under different parameter cases such as separator length, gas phase outlet diameter, and inlet shape. It can be indicated from the simulation results that the separation efficiency decreases with the increase of the separator length, and the separation efficiency increases firstly and then decreases with the increase of the gas phase outlet diameter as well as the liquid phase outlet. Furthermore, the rectangular inlet is more suitable than the circular inlet with the separation efficiency changing from 66.45% to 79.04%. In the end, the optimal geometrical structure was presented with separation efficiency of 86.15%.

Effect of One-Target Focus Type on Hydrodynamic Characteristics of Tower Solar Cavity Receiver

On account of one-target focus type of the heliostats in the tower solar power technology, the heat transfer was analyzed for the vapor-liquid two-phase or single-phase superheated steam in the parallel heated panel bundles of the solar cavity receiver. A nonlinear mathematical model of the hydrodynamic characteristics in the evaporation panels was developed to obtain the flow rate distribution, thermal deviation, and two-phase flow circulation reliability of the working fluid under the severe nonuniform heat flux from the one-target focus of the heliostats. The simulation results show that for the evaporation panels the flow distribution can synchronize with that of the heat flux at the low heat flux, while for the superheater sections the flow distribution decreases with the increase of heat flux. This desynchrony may give rise to stagnation or backflow of the working fluid and lead to the panels burst or erosion due to the local overheating in some extreme situation.

Effect of Step-Change Radiation Flux on Dynamic Characteristics in Tower Solar Cavity Receiver

The heat flux on the inner surface of the tower solar thermal power plant system will show the characteristics of noncontinuous step change, especially in nonnormal and transient weather conditions, which may result in a continuous dynamic variation of the characteristic parameters. Therefore, the research of dynamic characteristics plays a very important role in the operation and the control safely in solar cavity receiver system. In this paper, based on the noncontinuous step change of radiation flux, a non-linear dynamic model is constructed to obtain the effects of the non-continuous step change radiation flux and step change feed water flow on the receiver performance by sequential modular approach. The subject investigated in our study is a 1 MW solar power station constructed in Yanqing county, Beijing, China. This study has obtained the dynamic responses of the characteristic parameters in the cavity receiver such as drum pressure, drum water level, main steam flow, and main steam enthalpy under step change radiation flux. And the influence of step-change feed water flow to the dynamic characteristics has also been analyzed. The results could provide general guidance for security operation and control in solar cavity receiver system.

A Calculation Method for Evaluating Thermal Loss of Solar Cavity Receiver

One of the most important components of solar thermal power system is the solar central receiver, which performance is in direct relation to the efficiency of the whole power generation system. The calculation of internal surface heat flux and thermal efficiency of receiver therefore plays a very important role in receiver design. A calculation method of heat loss of a cavity receiver is put forward in this paper. The first step of this method is to use the Monte Carlo method to simulate the track of solar beam and compute surface heat flux inside the receiver. The second step is to employ the correlations of flow boiling heat transfer to figure out the convective heat transfer coefficient inside the tubes and then the wall temperature of the boiling tubes laid inside the receiver. The last step is to simulate the air flow field inside the receiver to calculate convective thermal loss of the receiver. As every step can not be independently done, all steps are coupled and an iterative scheme is needed. Internal surface heat flux and thermal efficiency of the receiver can be finally gained with this method, and the result can provide theoretical guide for receiver design.

Sand particle lift-off velocity measurements and numerical simulation of mass flux distributions in a wind tunnel

Lift-off velocity of saltating sand particles in wind-blown sand located at 1.0 mm above the sand bed surface was measured using a phase Doppler particle analyzer in a wind tunnel. The results show that the probability distribution of lift-off velocity can be expressed as a lognormal function, while that of lift-off angle follows an exponential function. The probability distribution of lift-off angle conditioned for each lift-off velocity also follows an exponential function, with a slope that becomes steeper with increasing lift-off velocity. This implies that the probability distribution of lift-off velocity is strongly dependent on the lift-off angle. However, these lift-off parameters are generally treated as an independent joint probability distribution in the literature. Numerical simulations were carried out to investigate the effects of conditional versus independent joint probability distributions on the vertical sand mass flux distribution. The simulation results derived from the conditional joint probability distribution agree much better with experimental data than those from the independent ones. Thus, it is better to describe the lift-off velocity of saltating sand particles using the conditional joint probability distribution. These results improve our understanding of saltation processes in wind-blown sand.

Numerical research of dynamic characteristics in tower solar cavity receiver based on step-change radiation flux

The solar cavity receiver is an important light-energy to thermal-energy convector in the tower solar thermal power plant system. The heat flux in the inner surface of the cavity will show the characteristics of non-continuous step change especially in non-normal and transient weather conditions, which may result in a continuous dynamic variation of the characteristic parameters. Therefore, the research of dynamic characteristics of the receiver plays a very important role in the operation and the control safely in solar cavity receiver system. In this paper, based on the non-continuous step change of radiation flux, a non-linear dynamic model is put forward to obtain the effects of the non-continuous step change radiation flux and step change feed water flow on the receiver performance by sequential modular approach. The subject investigated in our study is a 1MW solar power station constructed in Yanqing County, Beijing. This study has obtained the dynamic responses of the characteristic parameters in t...

Modeling the evolution of aeolian sand transport in a wind tunnel

An Eulerian-Lagrangian numerical simulation is performed to study aeolian sand transport in a wind tunnel. The evolutions of three parameters, sand mass flux, decay rate of sand mass flux profile and sand particle mean horizontal velocity with time, are investigated. The results show that: Sand mass flux takes more time to reach the saturated state than the last two parameters. First, we prove the evolution with time in the simulation is similar to the evolution in the length direction in a wind tunnel and then the experimental data from unsaturated wind tunnel can be used in the quantitative comparison with simulated saturated results. Second, detailed comparison is carried out. The tendency of sand mass flux is consistent with the experimental data. Decay rate of sand mass flux profile and sand particle mean horizontal velocity fit the experimental data well especially for cases of small free stream wind velocity, and the deviation is getting larger with the increasing of free stream wind velocity.

Drop deformation and breakup with diffuse interface method

Two‐dimensional numerical simulation of the deformation and breakup of an isolated liquid drop suspended in immiscible viscous fluid under shear flow was performed with the diffuse interface method. The governing equations of the model were described by Navier—Stokes—Cahn—Hilliard equations. The surface tension was treated as a modified stress. In the paper, the critical Capillary number was plotted as a function of viscosity ratios with the method of approximation. Besides, From the numerical observations, the breakup of the droplets occurred by three mechanisms, namely, necking, end pinching, and capillary instability. Quantitative results for the deformation and breakup of drop are presented.