Akira Takimoto

HEAT TRANSFER FROM TUBES IN MIST FLOWS

The present work has been undertaken to pursue the high performance of mist-cooled heat exchangers, Hrrt, mist-cooling experiments are conducted for staggered tubes having a fine surface structure in a vertical crossflow, and mist-cooling heat transfer is discussed including the effects of mutual interferences among the tubes. Next, mist-cooling heat transfer is analyzed for an arbitrary configuration of tubes, and design optimization for mist-cooled heat exchangers is developed upon applying the performance-evaluation criteria method of Bergles.

Numerical Analysis on Heat Transfer Characteristics of Looped Minichannel Using Phase-Change VOF Method

This study develops a numerical simulation of liquid-vapor two phase flows accompanied with the phase change. In the developed simulation, the VOF (Volume of Fluid) method is adopted for tracking the liquid-vapor interface and the temperature recovery method for estimating mass transfer due to the phase change. Firstly, the numerical simulation is performed for film-boiling and film-condensation problems for validating the evaporation (boiling) and condensation calculation. The numerical result for the film-boiling problem is in good agreement with the Berenson’s semi-empirical correlation in terms of the heat transfer coefficient. Moreover, the numerical results for the film-condensation problem are also in good agreement with the theoretical analysis by Nusselt in terms of the film growth rate and the heat transfer coefficient. These have confirmed the reliability of the developed simulation. Then, the developed simulation is applied to investigate the heat transfer characteristics of the heat transport device with a pump-driven looped minichannel. The result reveals a new interesting feature of the minichannel heat device.Copyright

Experimental investigation of the mode of phase dissociation in superheated liquid jets

Abstract The present paper gives an exclusive experimental coverage of the flashing process of pure and gas-associated solutions of water at low absolute pressures. The analyses include the phenomenon of bubble nucleation, growth and departure from the source, and the consequential properties of the flashing and disintegration of the liquid jet. The influence of dissolvable substances in the liquid on the bubble nucleation and altered liquid jet physical properties is also made clear. The outcome of the results also leads to the conclusion that, at high concentrations of the solute gas, dissociative bubble nucleation dominates the process and is associated with simultaneous bubble nucleation at the solid body-liquid interface and in the bulk of the liquid, especially at low liquid temperatures.

Heat Transfer Performance of Finless Flat Tube Heat Exchanger With Vortex Generator

As the longitudinal vortex is known to be effective in enhancing heat transfer, a three-dimensional unsteady numerical analysis has been made especially for the flow and thermal fields in a unit element of flat tube heat exchanger with vortex generator (VG) in so-called middle Reynolds number range. Both staggered and in-lined arrangements of the tubes with VG are considered and results obtained from the case with VG are compared with those without VG. The study was aimed at the influence of Reynolds number and some geometrical parameters on the heat transfer and the pressure drop. Moreover, as little research has been considered the interaction between transverse vortices and longitudinal vortices in the literatures, the effect is also investigated. It is found that the longitudinal vortex plays an important role in enhancing the local heat transfer by exchanging the fluid from the tube surface region to the fresh fluid of the main flow region and lasts over long distances. Moreover, the longitudinal vortices restrain unsteady transverse vortex shedding. As a result, heat transfer rate for the flat tube with VG case is larger compared to that without VG case. From the view point of pressure drop, increase in pressure drop for the case with VG is not so much larger due to the restraint of transverse vortex shedding. Finally, heat transfer performance becomes higher for the flat tube with VG case compared to that without VG case for the same pumping power.Copyright

Effects of Ultrasonic Irradiation on Ice Formation in Biological Tissue

A method to actively controlling crystallization is one of promising technique for cryopreservation. The object of this paper is to study the effect of ultrasonic irradiation on ice formation during freezing of biological tissue with supercooling. In the experiments, agar gel was frozen under irradiation of ultrasound at frequency of 28kHz. The measurements of temperature and the microscopic observation of ice crystals using fluorescent indicator were carried out. Firstly, it was found that the supercooled state was released by applying ultrasound at high intensity level, since ultrasonic cavitation effects were augmented. On the other hand, a weakly delay in the nucleation temperature was induced by applying ultrasound at low intensity level. Secondly, ice structure size decreased with increasing ultrasonic power under given supercooling degree at nucleation stage. The effect of ultrasound was enhanced by using large supercooling condition. Thirdly, paying attention to the thermal effect of ultrasound as changing temperature profile, sound and heat conduction in agar gel immersed in low temperature solution were numerically calculated. It was suggested that ultrasound at high frequency is effective to enlarge supercooled region.Copyright

Prediction of Condensation Heat Transfer of Binary Vapors of Immiscible Liquids on Horizontal Tube Banks. Condensation Model.

This work was undertaken experimentally and analytically to acquire a better understanding of condensation heat transfer of binary vapors of immiscible liquids on horizontal tube banks. The characteristics of condensation heat transfer on lower tubes, which received condensates inundated at the upper tube banks, were clarified through detailed experiments using an azeotropic mixture of carbon tetrachloride and water as binary vapors. Due to the combination of the organic immutable behavior and the extreme behavior of water droplets, standing droplets and continuously detaching droplets on the surface, a model of condensation was developed taking into account the sweeping effect and the inundation effect. The predictions obtained using by this model agree well with the experimental results.

Condensation Heat Transfer of Binary Vapors of Immiscible Liquids on Horizontal Tubes.

In order to enhance the condensation heat transfer of binary vapors of immiscible liquids, detailed experiments have been conducted with regard to the condensation of an azeotropic mixiture of carbon tetrachloride and water on horizontal tubes with enhanced fins. The condensation mechanism of this kind of vapor on horizontal tubes is clarified through the measurement of droplet departure frequency and heat transfer rates. The experimental results indicate that finned tubes are more effective in the enhancement of condensation heat transfer of binary vapors of immiscible liquids, and that an optimum dimension of fins exists.

Study on Condensation Heat Transfer of Binary Vapors of Immiscible Liquids (Nonazeotropic Mixtures).

In order to clarify the condensation heat transfer mechanism of binary vapors of immiscible liquids, experimental and theoretical studies have been performed on condensation of a nonazeotropic mixture of R113 and water on a cooled vertical flat plate. The condensation rate and the dynamic characteristics of droplets of water, i. e., the critical diameter and frequency of departure droplets, were measured for several concentrations of mixture. Considering the diffusion layer developing along the liquid layer and the sweeping effect by falling drops, we proposed a heat transfer model. The predictions of this model agree well with the experimental results for the mixed condensation heat transfer phenomena consisting of the water adherent drop and the organic liquid film.

Development of wet type electrostatic precipitator using a mist formation.

A newly wet type electrostatic precipitator method is developed so as to collect small particles with sub-micron radius in high efficiency. In this method, heterogeneous mist formation around particles, which results in enlargement of electric force due to increase of radius, is carried out in the forced convective and corona-electric fields between parallel cooled plates. Collecting efficiency is clarified on thee basis of heat and mass transfer characteristics.