Direct numerical simulation of turbulent heat transfer with a rectangular orifice has been performed for Reτ0(=uτ0δ⁄ν) = 300, where uτ0 is the friction velocity calculated from the mean pressure gradient imposed to drive the flow, δ the channel half width and ν the kinematic viscosity. The Prandtl number is 0.71. The ratio of slit height to channel height is assumed to be β=0.3,0.4,0.5,0.6 and 0.7. For β=0.3-0.6, the mean flow becomes asymmetric in the wall-normal direction by the Coanda effect behind the orifice. In the case of β=0.7, however, the mean flow is symmetry. The Nusselt number profiles over the bottom and top walls are different significantly for the asymmetric cases. Large-scale Kelvin-Helmholtz(K-H) vortices are generated at the orifice edges. An entrainment process is observed in the temperature field around these vortices. Subsequently, these K-H vortices become deformed and break up into disordered small-scale structures in the shear layers behind the orifice. In this scenario, the turbulent transport is promoted in the temperature field. In addition, the separation, the reattachment and also the contraction effects are discussed on the profiles of the mean temperature, the temperature variance and the turbulent heat fluxes.
In previous study, the characteristic of the condensation heat transfer on the dispersed vertical surface were investigated experimentally for the application of the finned surface to the thermoelectric generator utilizing boiling and condensation as the electrodes of the thermoelectric module. A prediction model for this diapered finned surface was proposed, based on Adamek-Webb model of the condensation on a finned tube. In this study, a condensation heat transfer experiment on a vertical dispersed finned surfaces using FC5312 was carried out, in order to enhance the condensation heat transfer coefficient by optimizing the fin size on a dispersed heat transfer surface. Experimental parameters were the fin width, thickness, height and the dispersed fin length. As the results, it was found from the experiment there was a dispersed fin length corresponding to the condensation at the maximum and its value was 1.75 mm. As the characteristic, the condensation changed from slowly increasing to rapidly increasing and then decreasing at a steep grade, with decreasing the dispersed fin length. In addition, the fin height did not affect this optimum dispersed fin length and the dispersed fin length affects the dependence of the condensation on different fin thickness. Further, the prediction values have a good agreement with the experimental data except the case of short dispersed fin length.
Numerical simulation about the hydrothermal process of two inches bulk single crystal growth has been done. The autoclave is assumed to be axisymmetric. In the present research, the authors discuss the natural convection heat transfer when the shape of baffles is flat or funnel-shaped. When the funnel-shaped baffle is used, the amount of flowing between a raw material zone and a crystal growth zone increases significantly. However, temperature difference between the two zones does not become very small. Therefore the funnel-shaped baffle is effective in hydrothermal crystal growth process from the viewpoint of transport phenomena. It is found that the best angle of the baffle is about 20 degrees in our calculation. Similar results are provided when the porous model in the raw material zone is employed.
Conventional modeling including drift-flux model and two-fluid model is based on “continuous flow hypothesis”, being constructed by time-averaging, and thus both phases are defined in every spatio-temporal space. This makes it possible to apply to a variety of two-phase flow dynamics, while the intrinsic void fraction fluctuations, typically observed in slug and churn flows, are hardly simulated. In order to break through such a problem caused by time-averaging, discrete bubble model based on one-dimensional mass conservation equation, i.e. void propagation equation, has been developed. This model takes into account, as momentum effects, the wake effect induced by preceding bubbles, the local pressure fluctuation and the compressibility of gas phase together with the phase re-distribution due to geometrical constrains. Thus obtained spatio-temporal fluctuation characteristics of void fraction well simulated inherent two-phase behavior not only in a steady flow but also in an oscillatory flow.
This study deals with flow behavior and associated heat transfer in pulsating duct flow. As well as mechanical engineers, researchers in bioengineering field also focus on heat and mass transport characteristics of pulsating flow from the viewpoint of life science. In spite of more than thirty-year history of pulsating flow and heat transfer studies, flow structure with periodical flow rate change and its effect on heat transfer are still at question. Researchers showed conflicting results. Some of them showed heat transfer enhancement due to flow pulsation, the others reported heat transfer prevention or no impact of flow pulsation on heat transfer. In order to make clear the complicated unsteady flow structure with periodical velocity fluctuation and its effect on heat transfer, it must be necessary to have macroscopic observation and image processing analysis of both the flow and temperature fields. The objective of this study, therefore, is to make clear the pulsating flow structure and heat transfer characteristics based on the visualizations of flow and temperature fields. Particle Image Velocimetry (PIV) and color schlieren visualization was applied to flow and temperature fields, respectively. Results showed that flow pulsation induced complicated spatial velocity distributions. Series vortices were formed in flow direction and they moved in span-wise direction from the sidewalls to the center of the duct. Such flow behavior caused locally high and low speed regions. From these results, it is concluded that heat transfer enhancement or prevention depends on increase or decrease of local flow rate induced by flow pulsation.
Two-phase flow nozzles are used in the total flow system for geothermal power plants and in the ejector of the refrigerant cycle, etc. One of the most important functions of a two-phase flow nozzle is to convert the thermal energy to the kinetic energy of the two-phase flow. The kinetic energy of the two-phase flow exhausted from a nozzle is available for all applications of this type. There exist the shock waves or rarefaction waves at the outlet of a supersonic nozzle in the case of non-best fitting expansion conditions when the operation conditions of the nozzle are widely chosen. The purpose of the present study is to elucidate theoretically the character of the rarefaction waves at the outlet of the supersonic two-phase flow nozzle. Two-dimensional basic equations for the compressible two-phase flow are introduced considering the inter-phase momentum transfer. Sound velocities are obtained from these equations by using monochromatic wave approximation. Those depend on the relaxation time that determines the momentum transfer. The two-phase flow with large relaxation times has a frozen sound velocity, and with small one has an equilibrium sound velocity. Rarefaction waves which occurred behind the two-phase flow nozzle are calculated by the CIP method. Although the frozen Mach number, below one, controls these basic equations, the rarefaction waves appeared for small relaxation time. The Mach line behind which the expansion starts depends on the inlet velocity and the relaxation time. Those relationships are shown in this paper. The pressure expansion curves are only a function of the revolution angle around the corner of the nozzle outlet for the relaxation time less than 0.1. For the larger relaxation time, the pressure decays because of internal friction caused by inter phase momentum transfer, and the expansion curves are a function of not only the angle but also the flow direction. The calculated expansion curves are compared with the experimental ones. The calculations considered only the compressibility of the gas phase, but those resembled with the curve of the experiment in which the phase changes occurred.
A numerical simulation program for pulverized coal combustion based on LES (Large eddy simulation) has been developed. It was validated by comparing with experimental data of a pulverized coal jet flame ignited by a high temperature (1420 K) flue gas jet. The difference between predicted and measured coal burnout on the centerline of the jets was less than 5 percent, which corresponds to experimental error. The predicted ignition position was in good agreement with the experimental data. Clustering of coal particles was observed significantly when the Stokes number of the particles is nearly equal to unity.
In order to achieve high heat transport performance, a new heat transport device with phase change using two parallel tubes has been developed. The device consisted of two different diameter parallel tubes, which connected an electrically heated evaporator and a water-cooled condenser. Experiments were conducted systematically using water as working fluid. Time-dependent temperatures as well as pressures at the evaporator and the condenser were continuously measured and recorded. The heat transport rate was obtained from the temperature rise and mass flow rate of cooling water in condenser. The present study disclosed that a simple two-different-diameter tube system connecting a heat source and a heat sink achieved re-circulating flow and it gave an extremely high heat transport rate. It was also found in the present study that the orientation of the device had appreciable influence on the performance.