[en] Nonlinear characteristics of cellular counterflow diffusion flame near the radiative extinction limit at large Damköhler number are numerically investigated. Lewis number is assumed to be 0.5 and flame evolution is calculated by imposing an infinitesimal disturbance to a one-dimensional(1-D) steady state flame. The early stage of nonlinear development is very similar to that predicted in a linear stability analysis. The disturbance with the wavenumber of the fastest growing mode emerges and grows gradually. Eventual, an alternating pattern of reacting and quenching stripes is developed. The cellular flame temperature is higher than that of 1-D flame because of the gain of the total enthalpy. As the Damköhler number is further increased, the shape of the cell becomes circular to increase the surface area per unit reacting volume. The cellular flames do not extinguish but survive even above the 1-D steady state extinction condition

[en] The cooling tower has air inlets at its base, a network of pipes which distributes the air to be cooled above the packing, and valves to isolate a part of the network. It includes also a bypass circuit, provided with means to control the flow rate fraction which is by-passed

[en] The velocity of the normal fluid component was directly measured by using a laser Doppler velocimeter. It was found that the normal fluid jet entrains the surrounding superfluid to result in generating a zero relative velocity (ZRV) flow field even very close to the jet nozzle exit. The feature of superfluid turbulence was investigated by measuring the variation in the frequency and the decay of the second sound Helmholtz oscillation with the heat flux q in the case of small q. It was confirmed from the velocity measurement result that the flow in the case of small heat flux is in a weak superfluid turbulent state, different from fully developed superfluid turbulent one. The flow state transfers from the weak state to the fully developed turbulent state as the heat flux increases. The transition was found to be characterized by the increase in the turbulent fluctuation of the normal fluid velocity

No abstract available

[en] A linear stability analysis of a diffusion flame with radiation heat loss is performed to identify linearly unstable conditions for the Damkohler number and radiation intensity. We adopt a counterflow diffusion flame with unity Lewis number as a model. Near the kinetic limit extinction regime, the growth rates of disturbances always have real eigenvalues, and a neutral stability condition perfectly falls into the quasi steady extinction. However, near the radiative limit extinction regime, the eigenvalues are complex, which implies pulsating instability. A stable limit cycle occurs when the temperatures of the pulsating flame exceed the maximum temperature of the steady state flame with real positive eigenvalues. If the instantaneous temperature of the pulsating flame is below the maximum temperature, the flame cannot recover and goes to extinction. The neutral stability curve of the radiation induced instability is plotted over a broad range of radiation intensities

[en] A method for calculating the separation characteristics of the counterflow cascade with non-mixing of relative concentrations a so called R-cascade designed for gaseous mixture separation, is proposed. The details of R-cascade calculation associated with an arbitrary selection of the flow separation ratio at one of the cascade stages are established. It is found out that the interval of flow separation ratio existence in a cascade with separation stages along the channel narrows down, when the stage number increases. Therefore, in this case the flow separation ratio at the first stage should be chosen as a parameter and the cascade is calculated in the direction from dump towards outgoing sections

[en] To clarify the effect of chemical structure of Dimethyl ether(DME) on NOx formation in nonpremixed counterflow flame, DME flame was investigated numerically to compare the flame structures and NOx emissions with C₂H₆ and Mixed-fuel. Numerically, the governing equations were solved using the Oppdif code coupled with CHEMKIN package, and DME flames were calculated by Kaiser's mechanism, while the C₂H₆ flames and Mixed-fuel flames were calculated by the C₃ mechanism. These mechanisms were combined with the modified Miller-Bowman mechanism for the analysis of NOx. Numerical results of nonpremixed counterflow flames show that the EINO of DME nonpremixed flame is low as much as 50 % of the C₂H₆ nonpremixed flame. The cause of EINO reduction is attributed mainly to the characteristics of partial premixed flame due to the existence of oxygen atom in DME and partly to the O-C bond in DME, instead of C-C bond in hydrocarbon fuels

[en] The boiling heat-transfer mode in a narrow gap cooling was studied based on analyzing boiling curves obtained in the experiments on transient cooling and rewetting of hot vertical surfaces in narrow gap channels. The experiments were carried out using annular channels with a narrow gap between inner and outer walls. The inner rod of the channel was made of stainless steel and the outer wall of glass tube for visualization. The initial temperature of the wall was around 800°C, and the gap sizes were 1.0 mm and 4.0 mm. The results indicated that the heat transfer during the gap cooling was significantly limited by the counter current flow (CCF) in a narrow gap of 1.0 mm. (author)

[en] Abstract only

[en] A simple model stochastic differential equation reproduces features of the observations of Griswold, Lorenson and Tough on the transition between turbulent states TI and TII in liquid-helium counterflow. The drift component of the model is based on the universal unfolding of the pitchfork bifurcation. A linear noise term models internal fluctuations in the counterflow system. The variance of fluctuations as a function of the heat current is calculated and found to have either one or two maxima depending on the precise form of the unfolding. These results are consistent with measurements of the total power of fluctuations made by Griswold et. al., who found one or two maxima depending on the temperature at which measurement were made. The existence of new unstable steady states is predicted. A modified pitchfork model with a nonlinear noise component reproduces several observations made by the same authors on liquid helium counterflow in the presence of deliberately applied fluctuations of the heat current. This model suggests an explanation for the sharp peak in the probability density observed close to the laminar state in the experiments