Jinjia Wei

Rheological Characterization of Drag-Reducing Cationic Surfactant Solution: Shear and Elongational Viscosities of Dilute Solutions

Shear viscosities of one kind of turbulent drag-reducing cationic surfactant solution, Cetyltrimethyl ammonium chloride (CTAC)/Sodium salicylate (NaSal) aqueous solution, were experimentally investigated by use of a stress controlled Reometer. The concentration and temperature of the solution ranged from 50 to 200 ppm and 20 to 40°C, respectively. The Giesekus model was found to fit the measured shear viscosities well for different temperatures and concentrations of the surfactant solution and the model parameter values obtained by fitting were correlated with temperature at certain solution concentrations. The temperature was found to have a large influence on the viscoelasticity of the surfactant solution. By using the model parameters obtained from the correlations, 70 percent of the measured shear viscosities agreed with the prediction by the Giesekus model within ±27.3 percent. The elongational viscosities of CTAC/NaSal surfactant solution were also experimentally investigated by use of an oppositing jet Reometer. The measured data were lower than the prediction by the Giesekus model.Copyright

Rheological Characteristics and Turbulent Friction Drag and Heat Transfer Reductions of a Very Dilute Cationic Surfactant Solution

Turbulent friction drag and heat transfer reductions and rheological characteristics of a very dilute cationic surfactant solution, cetyltrimethyl ammonium chloride (CTAC)/ sodium salicylate (NaSal) aqueous solution, were experimentally investigated at various temperatures. It was found that there existed a critical temperature above which drag and heat transfer reductions disappeared and shear viscosities rapidly dropped to that of water. It was surmised that drag and heat transfer reductions had a certain relationship with rheological characteristics and a rheological characterization of CTAC/NaSal surfactant solutions was performed to clarify this relationship. The effects of Reynolds number and fluid temperature and concentration on drag and heat transfer reductions were qualitatively explained by analyzing the measured shear viscosity data at different shear rates and solution temperatures and concentrations. The Giesekus model was found to fit the measured shear viscosities reasonably well for different temperatures and concentrations of the surfactant solution and the model parameter values obtained by fitting were correlated with temperature at certain solution concentrations. From the correlation results, the temperature effect on viscoelasticity of surfactant solutions was analyzed to relate the rheological characteristics with drag and heat transfer reduction phenomena.

SIMULTANEOUS NATURAL-CONVECTION HEAT TRANSFER ABOVE AND BELOW AN ISOTHERMAL HORIZONTAL THIN PLATE

Numerical studies were conducted to investigate the simultaneous natural-convection heat transfer above and below an isothermal thin plate in an infinite space. The full two-dimensional conservation equations for laminar flows were solved. The numerical approach was based on the finite-volume technique with a nonstaggered grid arrangement. The SIMPLE algorithm was used for handling the pressure-velocity coupling. Plate width and temperature were used to vary the Rayleigh number over the range 1.0 2 10 5 to 1.7 2 10 7 . The expressions for numerical local and average Nusselt number were correlated, and horizontal velocity and temperature profiles were provided. It was found that the upper-surface Nusselt numbers are smaller than the corresponding lower-surface Nusselt numbers. Numerical results for upward- and downward-facing isothermal horizontal plates (single-sided heat transfer) were also shown for comparison. For these cases, Nusselt numbers on the upward-facing surface are larger than those on the downward-facing surface.

Heat transfer mechanisms in vapor mushroom region of saturated nucleate pool boiling

Abstract To reveal the highly efficient heat transfer mechanisms in the vapor mushroom region of saturated nucleate pool boiling, a new model was developed in which three heat transfer mechanisms were considered: the conduction and evaporation in the microlayer region underneath vapor stem, the conduction and evaporation in the macrolayer region between the vapor mushroom and the heater surface and Marangoni convection in the macrolayer region. Two-dimensional conservation equations for laminar flow in the macrolayer region and conduction in the heater were solved. The SIMPLE-algorithm was used for handling the pressure–velocity coupling. Calculated boiling curve agrees well with the published experimental data and numerical results showed that about 60% heat flux is transferred by evaporation in the microlayer region, and about 40% heat flux is evaporated at the vapor–liquid interfaces of the macrolayer due to both the conduction and Marangoni convection heat transfer in the macrolayer region. Further investigations were also carried out for the cases in which only one or two heat transfer mechanisms were considered to assess the relative influences of the microlayer evaporation and Marangoni convection.

Swirling Flow of a Viscoelastic Fluid With Free Surface: Part I — Experimental Analysis of Vortex Motion by PIV

The swirling flows of water and CTAC (cetyltrimethyl ammonium chloride) surfactant solutions in an open cylindrical container with a rotating disc at the bottom were experimentally investigated by use of a double-pulsed PIV (particle image velocimetry) system. The mass concentrations of CTAC solutions were in the range of 50–1000 ppm, and the Reynolds number based on angular velocity, kinematic viscosity of water and radius of rotating disc was fixed at 4.3 × 104 . The aspect ratio of the height of the liquid filled into the cylindrical vessel to the radius of the vessel was set to 1.0. The secondary flow patterns in the meridional plane and the tangential velocities were obtained. The flow pattern in the meridional plane for water at the present high Reynolds number differed greatly from that at low Reynolds numbers, and an inertia-driven vortex was pushed to the corner between the free surface and the cylindrical wall by a counter-rotating vortex caused by vortex breakdown. For the 1000-ppm surfactant solution flow, the inertia-driven vortex located at the corner between the bottom and the cylindrical wall whereas an elasticity-driven reverse vortex governed the majority of the flow field. The radial distributions of the time-averaged tangential velocities also differed for water and surfactant solutions. The rotation of the fluid caused a deformation of the free surface with a dip at the center. The dip was largest for the water case and decreased with increasing surfactant concentration.Copyright

Swirling Flow of a Viscoelastic Fluid With Free Surface: Part II — Numerical Analysis With Extended Marker-and-Cell Method

In Part I [1], we presented the experimental results for swirling flows of water and cetyltrimethyl ammonium chloride (CTAC) surfactant solution in a cylindrical vessel with a rotating disk located at the bottom for a Reynolds number of around 4.3 × 104 based on the viscosity of solvent. For the large Reynolds number, violent irregular instantaneous secondary flows at the meridional plane were observed by use of a PIV system. Because of the limitations of our computer resources, we did not carry out DNS simulation for such a large Reynolds number. The LES and turbulence model are alternative methods, but a viscoelastic LES/turbulence model has not yet been developed for the surfactant solution. In this study, therefore, we limited our simulations to a laminar flow. The Marker-and-Cell (MAC) method proposed for Newtonian flow was extended to the viscoelastic flow to track the free surface, and the effects of Weissenberg number and Froude number on the flow pattern and surface shape were studied. Although the Reynolds number is much smaller than that of the experiment, the major experimental observations such as the inhibition of primary and secondary flows and the decrease of the dip of the free surface by the elasticity of the solution, were qualitatively reproduced in the numerical simulations.Copyright

Study on a PCM Heat Storage System for Rapid Heat Supply

A thermal energy storage system employing phase change material (PCM) for rapid heat discharge was studied numerically and experimentally. In the numerical studies, paraffin wax FNP-0090 (product of Nippon Seiro Co. Ltd.) was used as the PCM and was encapsulated in four different capsules (sphere, cylinder, plate and tube) for investigating the effects of geometrical configurations. The capsules filled with PCM were placed in a rectangular tank. Water was used as the working fluid in a rectangular tank. Water was used as the working fluid in the circulation system to discharge the storage tank. A one-dimensional model was used for solving the water temperature distribution along the flow direction in the tank and a conductive one-dimensional phase change model was used for simulating the solidification process of the PCM inside the capsules. The effects of the capsule diameter and shell thickness and the void fraction on the performance of the heat storage system were also investigated. Results shown include the variation of the outlet water temperature of the storage tank with time, the temperature and heat release distributions of the PCM along the flow direction in the tank at different times, etc. A plate heat exchanger was used as a heat storage tank for experimental study. FNP-0090 was filled into the channels (the space between two plates) of one side of the heat exchanger and water flowed in the channels of the other side of the heat exchanger. The effect of flow rate and initial temperature on the outlet fluid temperature and heat release was investigated. The numerical results and the experimental data agreed within 10%.Copyright