Chin Tsau Hsu

Modified Zehner-Schlunder models for stagnant thermal conductivity of porous media

Abstract Modified Zehner-Schlunder models are proposed for calculations of the stagnant thermal conductivity of two types of porous media with spatially periodic structures. The area-contact model is developed to take into consideration finite area contacts between spheres in a packed-sphere bed. It is shown that results based on the area-contact model are in better agreement with experimental data than those based on the original Zehner-Schlunder model especially at high solid/fluid thermal conductivity ratios. The phase-symmetry model is developed for a sponge-like porous medium where each phase is continuously connected and phase symmetry must be maintained in the expression of the stagnant thermal conductivity. The characteristics of these two models are illustrated.

A Lumped-Parameter Model for Stagnant Thermal Conductivity of Spatially Periodic Porous Media

Based on a lumped-parameter method, algebraic expressions for the stagnant thermal conductivity of some two-dimensional and three-dimensional spatially periodic media are obtained. The geometries under consideration include arrays of touching and non-touching in-line square and circular cylinders (two-dimensional), as well as touching and nontouching in-line cubes (three-dimensional). A comparison of results based on these algebraic expressions with existing numerical solutions and experimental data shows that they are in excellent agreement.

A closure model for transient heat conduction in porous media

Equations governing the transient heat conduction in porous materials consisting of solids and fluids of different thermal properties were derived with a volumetric average scheme under the assumption of nonthermal equilibrium. The derivation leads to a macroscopic two-equation system which requires the closure modeling of new unknown terms due to interfacial transport, namely, the tortuosity term and the interfacial heat transfer term. Closure relations were obtained from the microscopic equations for temperature fluctuation under quasi-steady assumption. The closure coefficients appeared in the closure relations then depend on the media geometry as well as thermal properties. To demonstrate these dependencies, the closure coefficient for the thermal tortuosity is evaluated based on the effective stagnant thermal conductivity model proposed by Hsu et al. (1995) for periodically packed cubes, and the coefficient for interfacial heat transfer based on a quasi-steady heat conduction of dispersed spheres immersed in fluids. The salient features as well as the applicability and limitation of the newly proposed transient heat conduction model were discussed.

On Pressure-Velocity Correlation of Steady and Oscillating Flows in Regenerators Made of Wire Screens

A facility capable of generating steady and oscillating flows was constructed and experiments were conducted to investigate the pressure-drop characteristics of regenerators packed with wire screen Both the velocity and pressure-drop across the regenerator were measured, To accurately determine the correlation between pressure-drop and velocity, the experiments covered a wide range from very low to very high Reynolds numbers, Re h . The teady flow results reveal that a three-term correlation with a term proportional to Re -1/2 h in addition to the Darcy-Forchheimer two-term correlation will fit best to the data, This Re - h 1/2 term accounts for the boundary layer effect at intermediate Reynolds number. The results also show that the correlation for oscillating flows coincides with that for steady flows in 1 < Re h < 2000. This suggests that the oscillating flows in the regenerators behave as quasi-steady at the frequency range of less than 4.0 Hz, which is the maximum operable oscillating flow frequency of the facility.

Method of phase-Doppler anemometry free from the measurement-volume effect

A novel method is developed to improve the accuracy of particle sizing in laser phase-Doppler anemometry (PDA). In this method the vector sum of refractive and reflective rays is taken into consideration in describing a dual-mechanism-scattering model caused by a nonuniformly illuminated PDA measurement volume. The constraint of the single-mechanism-scattering model in the conventional PDA is removed. As a result the error caused by the measurement-volume effect, which consists of a Gaussian-beam defect and a slit effect, can be eliminated. This new method can be easily implemented with minimal modification of the conventional PDA system. The results of simulation based on the generalized Lorenz-Mie theory show that the new method can provide a PDA system free from the measurement-volume effect.

Effective stagnant thermal conductivity of wire screens

fore, the absorption of radiation by inert particles is found to play an important role in indirectly igniting combustible gases. The change of particle number density at a fixed particle size also exerted a strong influence such that the smaller the particle number density, the longer the ignition delay. As the particle number density decreases, the radiation plays a smaller role, because the absorption coefficient decreases with a fixed particle size and the ignition delay increased.

High accuracy optical particle sizing in phase-Doppler anemometry

A novel structure for phase-Doppler anemometry (PDA) is proposed in order to eliminate the measurement-volume effect (MVE). This new PDA system consists of four receiving detectors in an optimized optical orientation. The method for the determination of the optimized optical orientation angle is introduced. In this method the vector sum of refractive and reflective rays is taken into consideration in describing a dual-mechanism-scattering model caused by a nonuniformly illuminated PDA measurement volume. The constraint of the single-mechanism-scattering model in the conventional PDA is removed. As a result the error caused by MVE, which consists of a Gaussian beam defect and a slit effect, can be eliminated. Analytical and experimental studies were conducted to investigate the accuracy of particle sizing using this new system. Both the experimental results and the simulation based on the generalized Lorenz-Mie theory show that the new method can provide a high-accuracy PDA system free of the MVE.

Minimum deviation of spatial frequency in large-particle sizing

A new method is introduced to improve the accuracy of sizing large particles in two-phase flows. This method uses a photodetector array to measure directly the spatial frequency of the light intensity scattered from a spherical particle in the measurement volume (or cross-sectional area). The effects of both the reflected and the refracted rays are considered. The phase conversion factors from the refracted and the reflected rays are used to minimize the measurement volume effect (errors) in sizing large particles. Furthermore, optimization results are compared with the results from the simulation based on the generalized Lorenz-Mie theory. A new particle-sizing system that combines the conventional and the new methods is suggested.

Optimization of optical parameters for particle sizing in multiphase flows

Abstract Optimization of laser light scatter detection technique based on the geometrical optics method for measuring the size, velocity and refractive index of spherical particles was performed. A new optical orientation for extended phase-Doppler anemometry (EPDA) had been developed to improve the accuracy of particle material recognition in multiphase flow. The effects of both the intensity and phase factor of the reflected and refracted light were examined. The inverse sign of the phase factors from the refracted light and reflected light resulted in a (2 π – φ ) effect on phase determination. This was used for minimizing and validating the errors in the refractive index measurements of large particles. Furthermore, optimization results were compared with the results of simulation based on the generalized Lorenz–Mie theory (GLMT). A newly developed signal validation scheme was also described in detail. The scheme was used to reject the inaccurate data in order to further improve the accuracy of the recognition.

THEORETICAL ANALYSES ON FLAPPING MOTION OF SUBMERGED TURBULENT PLANE JETS

Although the flapping oscillatory motion is organized, its effect on the mean flows of turbulent plane jets is similar to the effect of turbulence. Therefore, the close relation for the stress due to flapping-induced velocity components is assumed to be similar to the relation for the turbulent Reynolds stress, and the similarity theory applicable for mean flows of turbulent plane jets is extended to that of unstable flapping turbulent plane jets. The results show the effect of flapping motion on mean flows can be regarded as two folds: one is to enhance the mixing by flapping-induced Reynolds stress and the other is to modify the effective advection. The results from similarity analyses are in good agreement with the experimental data.