Fengzhi Li

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.

Reduction and turbulence characteristics in sub-zero temperature range of cationic and zwitterionic surfactants in EG/water solvent

The addition of small amounts of surfactant to a solvent in turbulent flow can reduce friction drag greatly. A new approach to energy savings in district cooling systems is to use surfactant solutions at subzero temperatures as the cooling fluids. In this paper, experiments were conducted to study the drag-reduction performances of a commercial cationic surfactant solution (oleylmethylbiss-hydroxyethyl ammonium chloride) and a newly synthesized zwitterionic surfactant solution (oleyl trimethylaminimide) in a two-dimensional channel. For testing the drag reduction at subzero temperatures, for which little data have been reported and none in channel flow, a 20% ethylene glycol aqueous solution (EG/W) was used as a solvent. The surfactant concentration ranged from 50 to 1000 ppm and the temperatures were –5 and 25°C. The turbulence features of the surfactant drag-reducing flow were measured by using the PIV (particle image velocimetry) technique. It was found that both surfactant solutions showed drag-reducing characteristics, which were affected by concentration and temperature. The maximum drag reduction was 83% at 25°C for the 200 ppm zwitterionic surfactant solution. Compared with EG/W turbulent flow, the addition of surfactant suppressed the turbulent intensities and vortex fluctuations and the Reynolds stresses were zero. The effects of addition of NaNO2 to the surfactant solutions were also investigated.

Turbulent Drag Reduction with Surfactant Additives — Basic Research and Application to an Air Conditioning System

It is well known that a small amount of chemicals such as water-soluble polymers or surfactants dramatically suppresses turbulence when they are added to liquid flow at large Reynolds number. In the last two decades, the application of surfactants to heat transportation systems such as district heating and cooling systems has attracted much interest among researchers. It has been revealed that 70% of the pumping power used to drive hot water in primary pipelines or district heating systems was saved by adding only a few hundred ppm of surfactant into the circulating water. The technological achievement requires a new design strategy for pipeline networks and heat exchangers to handle the drag reducing liquid flow. In the case of a Newtonian fluid such as water or air, the knowledge for designing fluid systems has been accumulated and the accuracy of numerical prediction is sufficient. On the other hand, the design system for surfactant solutions is not mature because drag-reducing flow phenomena are much more complicated than for Newtonian flow, for example, the friction factor for a surfactant solution depends not only on Reynolds number but also pipe diameter. In order to provide a design strategy for heat transportation systems using surfactant additives, we are now carrying out both experimental and numerical studies for surfactant solutions. In this lecture, experimental and numerical studies on the turbulence structure in drag reducing flow will be introduced. The result of an application study relating to the air conditioning system will be also shown.

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.

Numerical Simulation of Viscoelastic Fluid Flow in the Curvilinear Micro‐Channel

In this paper, three‐dimensional direct numerical simulation method (DNS) based on Giesekus constitutive models have been used to simulate viscoelastic fluid flows in a curvilinear micro‐channel. For comparison, we also carried out numerical simulations of Newtonian fluid flow in the same channel. The results obtained were in good agreement with experimental results given by Li et al. in the same geometrical size. Through numerical simulations we investigated the basic characteristics of viscoelastic flow in such a geometry and the influence of the curvature. Besides, the physical mechanism of the elastic instability in the curvilinear micro‐channel was also discussed.

Visualizations of Viscoelastic Fluid Flow in Microchannels

Solutions of flexible high-molecular-weight polymers or some kinds of surfactant can be viscoelastic fluids. The elastic stress is induced in such viscoelastic fluids and grow nonlinearly with the flow rate and results in many special flow phenomena, including purely elastic instability in the viscoelastic fluid flow. The elastic flow instability can even result in a special kind of turbulent motion, the so-called elastic turbulence, which is a newly discovered flow phenomenon and arises at arbitrary small Reynolds number. In this study, we experimentally investigated the peculiar flow phenomena of viscoelastic fluids in several different microchannels with curvilinear geometry by visualization technique. The viscoelastic working fluids were aqueous solutions of surfactant, CTAC/NaSal (cetyltrimethyl ammonium chloride/Sodium Salysilate). CTAC solutions with weight concentration of 200 ppm (part per million) and 1000 ppm, respectively, at room temperature were tested. For comparison, water flow in the same microchannels was also visualized. The Reynolds numbers for all the microchannel flows were quite small (for solution flows, the Reynolds numbers were smaller than 1) and the flow should be definitely laminar for Newtonian fluid. It was found that the regular laminar flow patterns for low-Reynolds number Newtonian fluid flow in different microchannels were strongly deformed in solution flows: either asymmetrical flow structures or time-dependent vortical flow motions appeared. These phenomena were considered to be induced by the viscoelasticity of the CTAC solutions.Copyright

Statistical Characteristics of Elastic Turbulence in a Free-Surface Swirling Flow

By using both conventional particle image velocimetry (PIV) and high-speed dynamic PIV, the statistical characteristics of elastic turbulence in a free-surface swirling flow were studied. Flow patterns of the elastic turbulence were observed in the PIV-measured velocity fields in both lateral and meridional planes for CTAC (cetyltrimethyl ammonium chloride) solution flows at low Reynolds numbers. It was found that the elastic turbulence in the tested swirling cylinder flows was characterized by large eddy motions, and the flow was random in time but rather smooth in space.