Chongyoup Kim

Thermal/oxidative degradation and stabilization of polyethylene glycol

Thermal degradation and stabilization of polyethylene glycol (PEG) with a molecular weight of 6000 have been investigated in order to develop PEG as a thermal energy storage material. Low molecular weight esters including formic esters were produced as the main products of the thermal degradation of PEG at 80°C in air. No degradation was observed for PEG aged in a vacuum. The mechanism of thermal degradation is found to be the random chain scission of the main chain. Thermal degradation could be suppressed by adding 2,2′-methylene-bis(4-methyl-6-tert-butylphenol) (MBMTBP) as an antioxidant. In the stabilizing process, MBMTBP itself is transformed to dimers or trimers.

Particle migration in tube flow of suspensions

In this research, we investigated the migration of particles in the tube flow of suspension for a wide range of particle loading (φ0) and particle Reynolds number (Rep), using a magnetic resonance imaging (MRI) technique. The suspension consisted of nearly monodisperse polymethylmethacrylate spheres in a density matched Newtonian fluid. The volume fraction of the solid was 0.06–0.40. Both the velocity and the concentration distributions were measured under fully developed conditions. It has been found that, when φ0 was small (⩽0.1) and Rep was not small (>≈0.2), the particles moved toward the position at a distance of 0.5–0.6 R (tube radius) from the tube axis and the velocity profile was parabolic. When φ0=0.4, particles always moved toward the center of the tube and the velocity profile was blunted. The degree of blunting was larger for smaller Rep. Between these two limiting cases, the particle migration was dependent on Rep. When Rep is small the particles move toward the tube axis regardless of φ0. W...

Thermal degradation of poly(ethyleneglycol)

The thermal degradation of poly(ethyleneglycol) (PEG) with an average molecular weight of 4000 has been investigated from the point of view of developing PEG as a phase change material in thermal energy storage. The PEG was maintained at 80 °C for 1000 h in air and in vacuo. It was severely degraded in air; the melting point and heat of fusion were reduced by as much as 13 °C and 32 kJ/kg, respectively. In vacuo, however, almost no degradation was observed. Both the fresh and degraded PEG samples were examined by thermal and spectroscopic methods. The results suggest that the thermal degradation of PEG in air follows a random chain scission oxidation mechanism. This was confirmed by adding an antioxidant which successfully suppressed the degradation.

Collapse of spherical bubbles in Maxwell fluids

The collapse of a spherical cavitation bubble contained in a large body of upper convected Maxwell fluid is theoretically analyzed by using a variational principle approach in the Lagrangian frame for the K-BKZ rheological equation of state with potential functions. Based on the Rayleigh time scale for bubble collapse in ideal fluids, two parameters. Re and De, are identified. Using a finite element technique, a fully explicit numerical scheme is developed both for the pressure distribution calculation and for bubble surface tracking. The same problem is formulated also using the Galerkin-finite element method in the Lagrangian frame for the differential model of an upper convected Maxwell fluid. With the latter method, the viscoelastic stress can be determined explicitly. Even though the result is the same as far as the radius-time curve is concerned, each method has its own advantages. Highly oscillatory behaviors in bubble radius are observed for moderate Re and De. For large ReDe, the solution exhibits an asymptotic behavior. It is also observed that fluid elasticity accelerates the collapse in the early stage of collapse while in the later stages it retards the collapse. The retardation for a moderate range of Re is expected to be related to the reduced cavitation damage in viscoelastic fluids.

Degradation of polyacrylamide in dilute solution

The degradation of polyacrylamide in aqueous solution was investigated experimentally from the point of view of developing the polymer solution as a drag reducing fluid in district heating systems. Aqueous 200 wppm solutions of polyacrylamide were subjected to turbulent pipe flow to induce degradation. The Reynolds number was between 15 000-35 000 and the temperature was in the range of 15-70°C. To assess the degradation of the polymer, the friction factor vs Reynolds number was monitored. The results show that, in high shear flow, the degradation is caused not only by scission of polymer chains but also by a radical propagation reaction initiated by scission of polymer chains. This was confirmed by adding sodium sulfite, which effectively suppressed the degradation.

Experimental studies on the Taylor instability of dilute polymer solutions

Abstract In this research the stability of Taylor-Couette flow of Newtonian and drag reducing fluids was experimentally studied. The primary concern was to investigate the effect of added polymer on the critical Taylor number. The polymer chosen in this study was polyacrylamide, xanthan gum and polyacrylic acid and the solvent was 1:1 mixture of glycerine and distilled water. The concentration was in the range of 0–0.5 of the coil overlap concentration. The result showed that the qualitative characteristics of transition from the stable Couette flow to the turbulent flow was found to be similar to that of Newtonian fluid. In the cases of polyacrylamide and xanthan gum solutions, the critical Taylor number decreased as the polymer concentration increased. Some polyacrylic acid solutions showed increase in the critical Taylor number. The Couette flow was stabilized by the added polymer when the solvent viscosity was used in determining the critical Taylor number regardless of polymer type. This implies that the wall layer of turbulent pipe flow should be stabilized and hence manifest drag reduction. By using a dilute solution theory, the second normal stress difference coefficient was estimated. It was found that the ratio of second to first normal stress difference coefficients was negative and in the range of 0.0–0.2, and was closely related to the flexibility of polymer chain. It was also found that even a very weak shear rate dependency of viscosity could play an important role in determining the stability of Couette flow.

Stability of a CFSTR with two consecutive reactions by the Liapunov direct method

The stability of a continuous flow stirred tank reactor with two consecutive reactionsA → B → C is studied with the direct method of Liapunov. Krasovskii’s method with the identity matrix is used to obtain a Liapunov function in the analysis of the system with single or multiple steady states. The results show that this method is mathematically conservative as expected. From the viewpoint of practical stability, however, this method predicts the regions of stability adequately.

Dean’s flow of aqueous solution of poly(ethylene oxide)

In this research the flow of Newtonian and drag reducing fluids through a helical tube,i.e.. Dean’s flow, was experimentally studied. The primary concern was to investigate the effect of added polymer on the secondary motion caused by the centrifugal force. The poiymer chosen in this study was poly(ethylene oxide) with the molecular weights of 300,000, 900,000 and 4,000,000 and the solvent was distilled water. The concentration range was 0 to 100 wppm. The Dean number investigated was in the range of 10 to 20,000. In the case of distilled water, the experimental data were in good agreement with the literature. In the case of polymer solutions, it was found that the secondary motion was supressed as the concentration and molecular weight increased. However, if the molecular weight or concentration exceeded certain values, the effects were saturated. The results were also analyzed from the view point of drag reduction phenomena.