Kazuya Tatsumi

NUMERICAL ANALYSIS FOR HEAT TRANSFER CHARACTERISTICS OF AN OBLIQUE DISCRETE RIB MOUNTED IN A SQUARE DUCT

Three-dimensional numerical simulation was conducted by making use of a Reynolds Average Navier-Stokes (RANS) approach for the flow in a duct accompanying longitudinal vortices, and discussions are given for the related thermal fields. In the first half of this article, applicability of the turbulence models adopted in the present study is discussed based on the comparison with some published experimental results. The main part of the article treats the numerical results for the flow around a discrete rib attached obliquely to the flow direction onto the bottom wall of a square duct in which a fully developed turbulent flow is established at the inlet. Due to the rib inclination and the gaps existing between the rib ends and the side walls, noticeable heat transfer augmentation is obtained downstream of the rib, produced by a strong secondary flow motion.

Numerical simulation for heat and fluid characteristics of square duct with discrete rib turbulators

Attachment of rib turbulators to flow passages is one of the popular means of heat transfer enhancement. Typical use of the rib turbulators is found for example in the serpentine cooling air channel for the internal cooling of the gas turbine blades. Many studies therefore have been carried out for the heat transfer in ribbed channels and various types of ribs have been tested. A standard case of full-span ribs attached perpendicularly to the flow direction has been the first target and its friction and heat transfer characteristics have been reported in the references [1–3]. Further studies were made for other cases of attaching oblique ribs and V-shaped ribs to the channel wall. In these cases, secondary flow is incurred in the channel and enhances the fluid mixing between the near wall and core regions in the duct, eventually resulting in the enhancement of local heat transfer at the channel walls [4–6]. Discrete ribs also drew attention with which heat transfer enhancement due to the incurred secondary flow is achieved paying smaller pressure loss penalty [7– 10]. In relation with the existence of the spanwise gaps, secondary flows are generated downstream their spanwise edges. Generation of the secondary flows in addition to the flow separation and reattachment makes the phenomenon quite complicated with the discrete ribs. Detailed studies of the flow structure and characteristics of the related heat transfer are required but such studies have scarcely been made so far. In the present article, some results of the three-dimensional numerical computation conducted for flow and thermal fields over two types of array of ribs attached to a channel wall will be presented. One is of fullspan ribs and the other is of discrete ribs, where both arrays are attached in a position perpendicular to the flow direction. The discussion will be given to the effects of three-dimensionality and unsteadiness of the flow and thermal fields for twofold purposes, one to test the applicability of numerical study to the type of flow under concern and another to provide hints to experimental works to be made in future.

Single-cell cloning and expansion of human induced pluripotent stem cells by a microfluidic culture device.

The microenvironment of cells, which includes basement proteins, shear stress, and extracellular stimuli, should be taken into consideration when examining physiological cell behavior. Although microfluidic devices allow cellular responses to be analyzed with ease at the single-cell level, few have been designed to recover cells. We herein demonstrated that a newly developed microfluidic device helped to improve culture conditions and establish a clonality-validated human pluripotent stem cell line after tracing its growth at the single-cell level. The device will be a helpful tool for capturing various cell types in the human body that have not yet been established in vitro.

Turbulence characteristics and mixing performances of viscoelastic fluid flow in a serpentine microchannel

Flow velocity measurement and visualization using particle image velocimetry and fluorescent dye were carried out for a viscoelastic fluid flow in a serpentine microchannel for the purpose to quantitatively evaluate the unsteady flow characteristics that is observed even under very low Reynolds number regime due to the combined effect of the viscoelastic fluid properties and the channel shape. Sucrose water solution (Newtonian fluid) and the polyacrylamide-sucrose water solution (viscoelastic fluid) were used as working fluids. The mixing performance markedly increased when the Reynolds number exceeded a certain value in the polyacrylamide solution case. The single-point, cross-sectional and two-dimensional velocity distributions showed that low frequency fluctuation was produced in the polyacrylamide solution case. Particularly large fluctuation in the channel spanwise direction was observed in the upstream area of the serpentine channel. On the other hand, the amplitude of the fluctuation decreased in the downstream region. The fluctuation in the upstream region is believed to be generated by the flow instability at the curved part of the channel, while the fluctuations in the downstream area were attributed to the local instability and the vortices provided from the upstream region.

Measurement of electroosmotic flow velocity and electric field in microchannels by micro-particle image velocimetry

A method for measuring the distribution of electroosmotic flow velocity and electric field intensity in a microchannel by micro-particle image velocimetry (μPIV) is described. Two types of particles with differing electric surface properties were used as tracer particles in order to subtract the velocity component due to the effects of the electrophoretic force from the velocity of the particles. A calibration experiment was first carried out using a one-dimensional microchannel to obtain the correlation functions between the apparent electric field intensity and the velocity of the two particles. μPIV measurements were then carried out in the target microchannel to measure the electroosmotic flow and electric fields by using the same two tracer particles and the correlation function. To validate the present method, experiments were conducted for two types of microchannels. One was a straight channel that consisted of a material different from that used in the calibration, and the other was a corrugated channel. The results were compared with those of an experiment using fluorescent dye, as well as with numerical simulations. Good agreement was observed in both comparisons, affirming the validity of the proposed method.

Reaction Characteristics of Methanol Noncatalytic Partial Oxidation Stabilized by Ceramic Honeycomb

A ceramic honeycomb was employed in a 1-kW class compact tubular fuel reformer based on noncatalytic partial oxidation (POX) of methanol. POX was stabilized on the outlet surface of the ceramic honeycomb under various experimental conditions with different equivalence ratios (φ) and thermal loads (qload); the latter represents the thermal input to the reformer calculated from the lower heating value (LHV) of methanol. The best performance of the reformer was achieved at φ = 3.5, and the adiabatic flame temperature was achieved. The location where the reaction occurred was determined, and the reaction was found to stabilize at an almost identical location under a wide range of thermal load conditions. This robust feature of the reaction is a significant characteristic of the ceramic honeycomb. By inserting a secondary honeycomb downstream of the reaction region, it was possible to move the reaction region upstream. This shifting of the reaction region was a result of the energy regeneration attributed to the adiabatic features of the honeycomb, and resulted in an improvement in reformer performance.

Experimental Study on a Compact Methanol-Fueled Reformer With Heat Regeneration Using Ceramic Honeycomb

On the way to a new era of our society which will be based on hydrogen energy, it is needed to develop on-site hydrogen production systems to cover current insufficient infrastructures of hydrogen supply network systems. For this, a highly efficient compact reformer can be one of the most suitable solutions for on-site production of hydrogen which is supplied to distributed electric power-generation systems. But, the local and overall energy balance in the reformer should be precisely controlled since the reforming reaction processes of hydrocarbon fuels are very sensitive to reaction temperature in the reformer. For smaller reformers, in particular, the amount of heat loss through the outer surfaces is large enough to dominate the reactions. An appropriate way for thermal energy management, therefore, is necessary to accomplish highly efficient reformers. For these backgrounds, a compact tubular-typed fuel reformer was fabricated in this study, and was applied to produce hydrogen from methanol, focusing on the partial oxidation reaction (POR). The reformer was composed of a stainless steel pipe as the reactor exterior and ceramic honeycomb blocks inserted in two locations of the reactor. The honeycomb blocks are expected to assist the reforming reactions and transfer the thermal energy of the exhaust gas to the reaction region, acting as a heat regenerator. The upstream-side honeycomb block was aimed to perform an effective heat exchange from the reactor wall to the reactant gas. By inserting the block, the reforming reaction became stable at right after the block. The maximum hydrogen production was achieved in the condition of equivalence ratio, around 3.5. The other honeycomb block was inserted in the downstream of the reaction zone to convert the thermal energy of exhaust gas to radiation energy which can be transferred to the upstream reaction region. Comparing to the case without the downstream-side block, the temperature of the reaction region became higher. Gas temperatures in the downstream region, on the other hand, became lower. Methanol conversion ratio and hydrogen production ratio enhanced due to the higher temperature at the reaction region. These results indicate that the thermal energy possessed by the exhaust gas was regenerated in the reaction region by the downstream-side honeycomb block and contributes to enhance the efficiency of the fuel reformer.© 2010 ASME

Numerical study on fluid-flow characteristics of peristaltic pump

Flow characteristics in a peristaltic pump, which can be applied to Micro Total Analysis Systems (μ-TAS), were numerically investigated. For the present computational analysis, we assumed a simple sinusoidal wave pattern to simulate the peristaltic motion of flexible channel wall and applied the Immersed Boundary method to the treatment of the wall boundary. The effects of the parameters such as the amplitude and velocity of the peristaltic motion upon the flow characteristics and pump performance were evaluated in the present paper. Saw-toothed or trapezoidal wall patterns were also calculated to examine the potentiality of the pumping flow rate increase.

The Effect of Upstream Flow Conditions to Laminar Mixing in a Miniature Combustion Chamber

A three-dimensional unstructured finite-volume method is used to investigate laminar flow characteristics of a miniature chamber with a possible application to micro gas turbine combustor design. The chamber is cylindrical in shape and 20mm in diameter with the fuel stream entering via a single jet in the center of one end of the can. Oxidizer jets are generated by a circular baffle plate having six holes surrounding the fuel jet. Attention is given to the effect of the inlet conditions on the flow structure and mixing pattern inside the chamber. Computations are carried out with the calculation domain inlet being positioned at two different locations; (1) at the immediate entrance to the combustion chamber (2) one combustor diameter upstream of the baffle plate. Numerous inlet conditions are considered including ‘top-hat’, fully-developed, swirling, an annular backward facing step and some asymmetrically skewed profiles. The baffle plate is shown to have a significant smoothing effect on the inlet conditions for a Reynolds number of 100.Copyright

Numerical investigation on fluid flow and heat transfer characteristics in a peristaltic micropump

A two-dimensional unsteady numerical simulation using Immersed Boundary method is carried out to investigate the fluid flow and heat transfer characteristics in a channel of peristaltic micro pump. The heat transfer target wall is a portion of the immovable top wall, while the rest of the wall including the movable bottom wall with fluctuating in the form of a sinusoidally progressive wave maintains the adiabatic condition. The obtained result shows a pair of large-scale recirculation in the case of relatively larger amplitude enhances the convection heat transfer and reduces the size of reverse flow region near the target wall.