Takamoto Saito

Mechanism of absorption enhancement by surfactant

Abstract In absorption refrigeration technology, it is well known that the water vapor absorption into the lithium bromide (LiBr) aqueous solution is enhanced by the addition of surfactant. The Marangoni effect plays a role but its mechanism is not clearly understood yet. In the present study, the existence of instability due to Marangoni effect was investigated using the linear stability analysis, based on the salting out effect. Then the vapor absorption augmentation was estimated by the numerical simulation of cellular convection. Both theoretical results qualitatively agree well with the experimental ones.

Effect of surfactant on falling film absorption

Abstract Absorption of vapour into a falling film of absorbent solution is frequently used for absorption heat pumps and chemical processes. For an improvement in absorption rate, a small amount of surfactant is added to the solution. In the present research, processes of heat and mass transfer are experimentally investigated for the case of water vapour absorption into aqueous solutions of lithium bromide flowing on a flat plate. Variables considered are flow rate of inlet solution, concentration of additive and inclination angle of the plate. The use of 2-ethyl-1-hexanol as an additive results in a four- to five-fold enhancement in absorption rate. The occurrence of surface disturbances does not have a direct relation to the solubility limit of the additive. A qualitative discussion of additives in the role of inducing surface disturbances is presented.

Boiling heat transfer of a ternary refrigerant mixture inside a horizontal smooth tube

Abstract A theoretical model for predicting the forced convective boiling heat transfer coefficients of ternary mixtures is presented. The effect of mass diffusion near the gas-liquid interface is considered in both nucleate boiling and forced convection vaporization contributions. The predicted heat transfer coefficients are compared with experimental data, which are obtained for an HFC32: 125: 134a (23: 25: 52 wt%) mixture inside a horizontal smooth tube 6.0 mm i.d. and 4.0 m long, and in the ranges of heat flux 10–20 kW m−2 and of mass flux 150–400 kg m−2 s−1. The present predictive method shows reasonable agreement with the experimental data within an accuracy of ±30%.

Turbulence model of fire-induced air flow in a ventilated tunnel

The fire-induced air flow in a ventilated tunnel with a local gas burner is investigated experimentally and numerically. To simulate the transient three-dimensional turbulent buoyancy flow, calculations employ the turbulence k-e model which is modified to take account of the effects of the mean streamline curvature (MSC). General curvilinear coordinates are generated for the computation. The prediction is compared with the experimental data and shown to be in reasonable agreement except the region close to the heat source. The present model can also be used to predict the location of the leading edge of the heated air flow.

Study on Vapor Explosion Mechanism Related to Vapor Condensation.

In order to clear the mechanism of the vapor explosion, two kinds of experiments were performed. In the experiment 1, an upper side-open transparent vessel filled with molten metal (solder, tin, lead and zine) was immersed in the water tank. It was found that vapor cavities were generated in the metal and the vapor bubble connected with a cavity collapsed just before the occurrence of a vapor explosion. In the experiment 2, the apparatus consists of a water tank and a heated vessel with a cavity. Subcooled water in the water tank flows into the vapor cavity, immediately after the pressure in cavity is abruptly reduced because of the vapor condensation. Then the pressure rises rapidly by the vaporization of water in the cavity. A chain of phenomena, the generation of vapor cavities, the pressure descent due to vapor collapse, the water inflow into the cavity, and the rapid pressure rise, can be a part of the process of vapor explosion. The relation between the metal temperature just before the occurrence of a vapor burst and the minimum film boiling temperature criterion of Dhir & Purohit was investigated.

Temperature Stratification of Heated Air Flow in a Fire Tunnel.

The temperature stratification of heated air flow in a fire tunnel is investigated by the two- and three-dimensional experiments and numerical models described in previous papers. It is found that the heated air flow can appear to be either a stratified or a mixed flow pattern in accordance with different ventilation velocities, heat release rates, and downstream positions. To deal with tunnel fire circumstances, the mixed convection is evaluated by a macro nondimensional variable Gr*/ReH5/2, and a quantitative relationship between Gr*/ReH5/2 and a parameter which identifies the varying degrees of the stratification at each tunnel crossection has been established. By comparing the results from two- and three-dimensional analysis, the effects of secondary flow on temperature stratification are discussed. The conclusions obtained are directly relevant to the safety considerations in tunnel fires.

The Effect of Reynolds Numbers and the Direction of Interfacial Shear Stress on Turbulence below an Air-Water Interface.

Momentum transfer through an air-water interface was studied in a horizontal rectangular channel through experimental and numerical analysis. The liquid flow Reynolds numbers, based on hydraulic diameters, were 11500 and 17000. The air flow Reynolds number varied from 0 to 3260. In experiments, velocity distribution was measured by hot-film and hot-wire probes. The effect of the liquid flow Reynolds number and the direction of interfacial shear stress was investigated by performing the cocurrent and countercurrent experiments. The modified k-e model was able to reproduce these experimental results qualitatively. The range of applicability of the modified k-e model was also discussed.