Weihua Cai

Study on the characteristics of turbulent drag-reducing channel flow by particle image velocimetry combining with proper orthogonal decomposition analysis

Turbulent drag reduction of 30 ppm cetyltrimethyl ammonium chloride (CTAC) solution flow in a channel was investigated with particle image velocimetry (PIV) combining with proper orthogonal decomposition (POD). Measurements were made at inlet fluid temperature of 304 K and at Reynolds number 2.5×104 (based on the channel height, bulk velocity, and solvent viscosity) for both water and CTAC solution flows with 70.0% drag reduction rate. The two-component velocity fields in the streamwise-wall normal plane were recorded by PIV. In order to study the characteristics of turbulent drag-reducing channel flow, POD was performed to identify the near-wall coherent structures based on PIV-measured data. POD is a powerful low-dimensional analysis tool that can be used to identify coherent structures embedded in the turbulent shear flow. We mainly studied a comparison between the first dominant POD eigenmodes of water and drag-reducing CTAC solution flows. Coherent structures were seen as the sum of several eigenmode...

DNS study of decaying homogeneous isotropic turbulence with polymer additives

In order to investigate the turbulent drag reduction phenomenon and understand its mechanism, direct numerical simulation (DNS) was carried out on decaying homogeneous isotropic turbulence (DHIT) with and without polymer additives. We explored the polymer effect on DHIT from the energetic viewpoint, i.e. the decay of the total turbulent kinetic energy and energy distribution at each scale in Fourier space and from the phenomenological viewpoint, i.e. the alterations of vortex structures, the enstrophy and the strain. It was obtained that in DHIT with polymer additives the decay of the turbulent kinetic energy is faster than that in the Newtonian fluid case and a modification of the turbulent kinetic energy transfer process for the Newtonian fluid flow is observed due to the release of the polymer elastic energy into flow structures at certain small scales. Besides, we deduced the transport equations of the enstrophy and the strain, respectively, for DHIT with polymer additives. Based on the analyses of these transport equations, it was found that polymer additives depress both the enstrophy and the strain in DHIT as compared to the Newtonian fluid case, indicating the inhibition effect on small-scale vortex structures and turbulence intensity by polymers.

Prediction of Mass Transfer Time Relaxation Parameter for Boiling Simulation on the Shell-Side of LNG Spiral Wound Heat Exchanger:

The objective of this present study is to propose an approach to predict mass transfer time relaxation parameter for boiling simulation on the shell-side of LNG spiral wound heat exchanger (SWHE). The numerical model for the shell-side of LNG SWHE was established. For propane and ethane, a predicted value of mass transfer time relaxation parameter was presented through the equivalent evaporation simulations and was validated by the Chisholm void fraction correlation recommended under various testing conditions. In addition, heat transfer deviations between simulations using the predicted value of mass transfer time relaxation parameter and experiments from Aunan were investigated. The boiling characteristics of SWHE shell-side were also visualized based on the simulations with VOF model. The method of predicting mass transfer time relaxation parameter may be well applicable to various phase change simulations.

Numerical Simulation of Forced Convective Condensation of Propane in a Spiral Tube

A numerical simulation of forced convective condensation of propane in an upright spiral tube is presented. In the numerical simulations, the important models are used: implicit volume of fluid (VOF) multiphase model, Reynolds stress (RS) turbulence model, Lees phase change model and Ishiis concentration model, and also the gravity and surface tension are taken into account. The mass flux and vapor quality are simulated from 150 to 350 kg·m−2·s−1 and from 0.1 to 0.9, respectively. The numerical results show that in all simulation cases, only the stratified flow, annular flow, and mist flow are observed. The heat transfer coefficient and frictional pressure drop increase with the increase of mass flux and vapor quality for all simulation cases. Under different flow patterns and mass flux, the numerical results of void fraction, heat transfer coefficient, and frictional pressure drop show good agreement with the experimental results and correlations from the existing references.

Numerical Simulation on Forced Convective Condensation of Steam Upward Flow in a Vertical Pipe

A transient three-dimensional volume of fluid (VOF) simulation on condensation of upward flow of wet steam inside a 12 mm i.d. vertical pipe is presented. The effect of gravity and surface tension are taken into account. A uniform wall temperature has been fixed as boundary conditions. The mass flux is 130~6400 kg m−2s−1 and the turbulence inside the vapor phase and liquid phase have been handled by Reynolds stress model (RSM). The vapor quality of fluid is 0~0.4. The numerical simulation results show that, in all the simulation conditions, the bubbly flow, slug flow, churn flow, wispy annular flow, and annular flow are observed; in addition, the results of flow pattern are in good agreement with the regime map from Hewitt and Roberts. The typical velocity field characteristic of each flow pattern and the effect of velocity field on heat transfer of condensation are analyzed, indicating that the slug flow and churn flow have obvious local eddy. However, no obvious eddy is observed in other flow patterns and the streamlines are almost parallel to the flow direction. The simulation results of heat transfer coefficients and frictional pressure drop show good agreement with the correlations from existing literatures.

Experimental study on rheological and thermophysical properties of seawater with surfactant additive—part I: rheological properties

The rheological properties of seawater with the addition of surfactant additive (cetyltrimethyl ammonium chloride (CTAC)/sodium salicylate (NaSal)) are measured at different temperatures, including shear viscosity and first normal stress difference (N1). The effects of the temperature, the salts, and CTAC/NaSal concentration on the rheological properties of test solutions are investigated, and the corresponding influence mechanisms are analyzed. It shows that the addition of salt can decrease the shear viscosities of the solutions, and also decrease N1 and even eliminate the sharp augment of N1 above a certain shear rate. The growing elasticity can be characterized by the increase of the initial shear rate for shear-thickening inception. High temperature can also remove the sharp increase of N1 with salt. Nevertheless, the increase of CTAC/NaSal concentration can withstand the elimination of the sharp augment of N1.

Numerical Simulation of Condensation of Upward Flow in a Vertical Pipe

A transient three-dimensional volume of fluid (VOF) simulation on condensation of upward flow of wet steam inside a 12 mm i.d. vertical pipe is presented. The effect of gravity and surface tension are taken into account. A uniform wall heat flux have been fixed as boundary conditions. The mass flux is m=130∼6000 kg m−2 s−1 and the turbulence inside the vapor phase and liquid phase have been handled by Reynolds Stress (RS) model. The vapor quality of fluid x=0∼0.4. The numerical simulation results show that in all the simulated conditions only the bubbly flow, slug flow, churn flow and annular flow are observed, in addition the results of flow pattern are in good agreement with the regime map from Hewitt and Roberts. The typical velocity field characteristic of each flow pattern and the effect of velocity field on heat transfer of condensation are analyzed. It can be found that only slug flow has an obvious local eddy around the slug gas in all simulated flow patterns. The trend of heat transfer coefficients rises throughout with the increase of vapor quality for all simulated conditions, which is good agreement with the correlation from Boyko and Kruzhilin.Copyright

The Vortex Structures of Elastic Turbulence in 3D Kolmogorov Flow with Polymer Additives

To get more insight into the physical mechanism of elastic turbulence, 3D, time‐dependent direct numerical simulations (DNS) based on finitely extensible nonlinear extension Perlin (FENE‐P) constitutive model were carried out for 3D Kolmogorov flow with polymer additives. In the present paper, the characteristics of the vortex structures, including its genertation and evolution in the elastic turbulent flow, were investigated through visualization and analyses of the enstrophy and strain transport equations.

POD Analysis and Low-Dimensional Model Based on POD-Galerkin for Two-Dimensional Rayleigh-Bénard Convection

Direct numerical simulation based on OpenFOAM is carried out for two-dimensional Rayleigh-Benard (RB) convection in a square domain at high Rayleigh number of 10 7 and Pr =0.71. Proper orthogonal decomposition (POD) is used to analyze the flow and temperature characteristics from POD energy spectrum and eigenmodes. The results show that the energy spectrum converges fast and the scale of vortex structures captured by eigenmodes becomes smaller as the eigenmode order increases. Meanwhile, a low-dimensional model (LDM) for RB convection is derived based on POD eigenmodes used as a basis of Galerkin project of Navier-Stokes-Boussinesq equations. LDM is built based on different number of eigenmodes and through the analysis of phase portraits, streamline and isothermal predicted by LDM, it is suggested that the error between LDM and DNS is still large.

Effect of polymer additives on heat transport and large-scale circulation in turbulent Rayleigh-Bénard convection

The present paper presents direct numerical simulations of Rayleigh-Bénard convection (RBC) in an enclosed cell filled with the polymer solution in order to investigate the viscoelastic effect on the characteristics of heat transport and large-scale circulation (LSC) of RBC. To overcome the difficulties in numerically solving a high Weissenberg number (Wi) problem of viscoelastic fluid flow with strong elastic effect, the log-conformation reformulation method was implemented. Numerical results showed that the addition of polymers reduced the heat flux and the amount of heat transfer reduction (HTR) behaves nonmonotonically, which firstly increases but then decreases with Wi. The maximum HTR reaches around 8.7% at the critical Wi. The nonmonotonic behavior of HTR as a function of Wi was then corroborated with the modifications of the period of LSC and turbulent energy as well as viscous boundary layer thickness. Finally, a standard turbulent kinetic energy (TKE) budget analysis was done for the whole domain, the boundary layer region, and the bulk region. It showed that the role change of elastic stress contributions to TKE is mainly responsible for this nonmonotonic behavior of HTR.