A steady-state axisymmetric flow field of a liquid metal in a coreless induction furnace under an axisymmetric magnetic field is analyzed numerically, using a spectral finite difference method. Vorticity-stream function formulation is used in conjunction with Maxwell's equations, in a boundary-fitted coordinate system. For boundary conditions, both no-slip on the wall and no shear stress tensor on the free surface are used as dynamic conditions, and a field equivalent to the magnetic field induced by external coils is adopted as an electromagnetic field condition. Presented are streamlines, magnetic streamlines, and radial profiles of the axial velocity component at two Reynolds numbers for various parameters. It is found that the flow field varies remarkably according to the Reynolds number, the dimensionless height of the liquid metal, and the dimensionless height of external coils.
Three-dimensional vortical flow structures and velocity fluctuation near the rotor tip in an axial flow fan having two different tip clearances have been investigated by experimental analysis using a rotating hot wire probe and a numerical simulation. It is found that a tip leakage vortex is observed in the blade passage, which has a major role near the rotor tip. The tip leakage vortex formed close to the leading edge of the blade tip on suction side grows in the streamwise direction, and forms a local recirculation region resulting from a vortex breakdown inside the blade passage. The recirculation region is enlarged by increasing the tip clearance. The larger recirculation region induces the acceleration of the through flow, thus resulting in the increase of the broadband noise. High velocity fluctuation is observed at the interference region between the tip leakage vortex and the through flow in the flow field where the tip leakage vortex is tightly rolled up without its breakdown. Near the casing wall, a discrete frequency is formed between tip leakage vortex core and rotor trailing edge.
The near wake generated by a triangular cylinder and a screen, respectively, has been investigated with a view to provide information on the effect of initial conditions on turbulence structures of various scales based on the wavelet multi-resolution analysis. The velocity data were simultaneously obtained at x/h (x is the streamwise distance downstream of the wake generator and h is the height of the wake generator)=20 using a rake of eight X-wires aligned in the plane of mean shear. A vector wavelet multi-resolution technique was applied to decompose the velocity data into a number of wavelet components based on the central frequencies. The instantaneous sectional streamlines and vorticity field were used to reveal the turbulent structures at each wavelet component/central frequency. It was found that the behavior of large- and intermediate-scale structures depend on the initial conditions and the small-scale structures are independent of the initial conditions. The contributions from the wavelet components to the time-averaged Reynolds stresses and vorticity variance were estimated. Both the large-scale and intermediate longitudinal structures make the most significant contributions to Reynolds stresses in the triangular cylinder wake, but the contribution from the large-scale structures appears dominating in the screen wake. The relatively small-scale structures of the triangular cylinder wake contribute most to the total spanwise vorticity variance.
Experimental and numerical studies relating to the effects of sidewall temperatures on the flow pattern and the bifurcation to chaos of Rayleigh-Bénard cells in a rectangular enclosure with a length: width: height aspect ratio of 6: 4: 1 are presented. Experiments were carried out using air with several Rayleigh numbers (Ra) of up to 32000. At Ra=8000 there appear three steady flow patterns consisting of the following: four transversal rolls with axes parallel to the short sidewalls, two cells, and two longitudinal rolls with axes parallel to the long sidewalls. At Ra=8200, the pattern consisting of two longitudinal rolls begins firstly to exhibit time-dependent sinuous oscillatory flow among three steady flow patterns. The resulting sinuous oscillation takes the form of a standing wave. Further increase of Ra results in a series of successive subharmonic bifurcations and eventually chaos. Furthermore, it was found that the dependence on Ra of the peak oscillation frequency f1 changes at Ra=20000 due to a change of the dynamical mode in the two oscillatory rolls. Both the two-cell pattern and the two-longitudinal-roll pattern undergo a series of successive subharmonic bifurcations, but the changes in the spatial structure exhibited by each through these bifurcations are quite different.
The effects of casing shapes on the performance and the interaction between an impeller and a casing in a small-size turbo-compressor are investigated. Numerical analysis is conducted for the turbo-compressor with circular and single volute casings from the inlet to a discharge nozzle. The optimum design for each element is important to develop the small-size turbo-compressor using alternative refrigerant as a working fluid. Typically, the rotating speed of the compressor is in the range of 40000-45000rpm because of the small size of an impeller diameter. A blade of an impeller has backswept two-dimensional shape due to tip clearance and a vane diffuser has wedge type. In order to predict the flow pattern inside the entire impeller, the vaneless diffuser and the casing, calculations with multiple frames of reference method between the rotating and stationery parts of the domain are carried out. For compressible turbulent flow fields, the continuity and time-averaged three-dimensional Navier-Stokes equations are employed. To evaluate the performance of two types of casings, the static pressure recovery and loss coefficients are obtained with various flow rates. Also, static pressure distributions around casings are studied for different casing shapes, which are very important to predict the distribution of radial load. To prove the accuracy of numerical results, measurements of static pressure around the casing and pressure difference between the inlet and the outlet of the compressor are performed for the circular casing. The comparison of experimental and numerical results is conducted, and reasonable agreement is obtained.
In the present work, for both a COSMOS-HP (Counter-Stream-Mode Oscillating-Flow Heat Pipe) and a dream pipe, the optimum conditions yielding the highest effective thermal conductivity and/or the highest operating coefficient are analyzed for oscillating flows of a given amplitude S. The parameters used in the optimization are the thermophysical properties of the operating liquid, the channel size and the frequency of oscillating flow. Based on the analytical results of the optimum conditions, both the optimum liquid and the optimum oscillating flow conditions are discussed. The highest effective thermal conductivity of COSMOS-HP is compared with that of a dream pipe, and it is found that the former is much higher than the latter.
High frequency pulse heating of heaters in microactuators utilizing the rapid expansion of boiling bubbles causes a gradual increase in the temperature of the heater and the substrate material in the immediate vicinity of the heater, resulting in a loss of reproducibility of the boiling phenomena. In the present report, the maximum frequency at which the time-averaged increase in the heater temperature can be maintained within allowable limits (allowable frequency) is obtained from numerical simulations of heat conduction from the heater to adjacent materials using a model in an axisymmetrical system. Instead of direct calculation of the entire repetition process, calculations for two different heating conditions, single pulse heating and steady heating at a time-averaged power during repetition of pulses, are performed. The effects of heater size, pulse width and substrate properties on the allowable frequency are examined and approximate correlations for the allowable frequency are derived in dimensionless form based on an analytical examination. The results show that the allowable frequency increases significantly with a decrease in heater size for prescribed conditions of pulse width and temperature increases during both pulsed heating and steady heating at a time-averaged power. The simulated results are verified by comparison with those from experiments both with and without boiling.
We have investigated the premixed flame structure in highly turbulent flow with a cyclone-jet combustor. Based on the turbulent properties determined by Slot-Correlation method, the condition of Um<15m/s belongs to the flamelet regime, and that of Um>20m/s belongs to the distributed reaction zone regime on the combustion diagram. Also, we have quantitatively estimated the reaction zone thickness, using the probability of reaction zone existing. Results show that the dependence of reaction zone thickness on equivalence ratio is very similar to those of the experimental values by Yamaoka and Tsuji or the Zeldovich thickness. When the exit velocity is increased, the reaction zone thickness is almost constant for Ka>1. Hence, the persistence of laminar flamelet structure is observed, even when the Kolmogorov scale is smaller than reaction zone thickness. It could be concluded that the reaction region remains undisturbed with thin reaction zone under highly turbulent conditions. These results are useful for modeling turbulent combustion.
The effects of flame stretch on an outwardly propagating flame are considered experimentally for both laminar and turbulent flames. Although the thermo-diffusive mechanism caused by the flame/stretch interaction is well known for laminar flames or for weakly stretched flames, recent studies have shown that it influences the local and global flame structures of turbulent premixed flames which consist of strongly stretched flamelets. In this study, outwardly propagating flames of methane and propane mixtures, which are representative of light and heavy hydrocarbons, respectively, over a wide range of equivalence ratio, are used in the application to engine combustion. The primary objective of the present study is to investigate both laminar and turbulent burning velocities experimentally and to discuss the underlying mechanisms in consideration of interaction between flame stretch and the Lewis number.
The turbulent mixing flow characteristics of an intermittent diesel spray were investigated. A 5-hole diesel nozzle (dn=0.32mm) with a 2-spring nozzle holder, which is widely used in heavy-duty diesel engines, was tested. Time-resolved analysis of the turbulent mixing flow characteristics of the spray, injected intermittently into the still ambient air, was made under room temperature by using a 2-D PDPA system. The mean and the fluctuation velocities of the spray were measured. The axial velocity distribution shows similar to that of the free air jets at the downstream of the spray, and the distribution well coincides with the result proposed by Hinze at R/b<1.5. The turbulent intensity of the axial velocity component is high near the spray axis, and it decreases gradually with the increase in the radial distance. The turbulent shear stress increases with proceeding to the trailing edge as well as the downstream of the spray. The maximum value of the turbulent shear stress is observed near R/b≈1.0, regardless of the evolution time. The turbulent shear stress in the central parts of the spray is lower than that of the continuous free air jets, whereas that in the trailing edge is considerably higher.
This paper describes an experimental study of impingement heat transfer of reciprocating jet-array with piston cooling application for marine heavy-duty diesel engine. A selection of heat transfer measurements illustrates the manner by which the individual and interactive influences of reciprocating force and buoyancy on heat transfer for the impinging jet-array. It is demonstrated that the reciprocating force coupled with buoyancy interaction causes considerable heat transfer modifications from the static results. The isolated reciprocating force effect could initially reduce heat transfer to a level about 0.45 of static level with weak reciprocation but recovers when the reciprocating force increases. Heat transfer improvement and impediment could be aided by the location-dependent buoyancy effect in addition to the reciprocating force effect. An empirical heat transfer correlation, which is physically consistent, has been developed to permit the evaluation of the individual and synergistic effects of reciprocating force and buoyancy interaction on local heat transfer of the impinging jet-array.
In previous work, an RSM optimization was performed to demonstrate the emission reduction capability of the combined effects of high injection pressure, boost pressure, and cooled EGR on a HSDI diesel engine equipped with a common rail injection system. The RSM optimization led optimum operating parameters to low-temperature and premixed combustion characteristics, i. e., the MK combustion region, resulting in simultaneous reductions in NOx and PM emissions without sacrificing BSFC. However, further retardation of injection timing and increase of EGR rate from the optimum point resulted in significant deterioration of the engine stability with misfire. In the present work, further RSM optimization was conducted to investigate the effect of intake boost pressure on MK combustion. It was found that the increase of intake boost pressure shortened ignition delay, which was not favorable for MK combustion. However it allowed the use of heavier EGR and later injection timings, which intensified the characteristics of MK combustion, i. e., lower-temperature and more thoroughly premixed combustion characteristics. Compared to the previous optimum point, the NOx emission level of the new optimum point was improved by 48%, and it was possible to improve PM emissions further by 71%, while retaining BSFC at the same level. At the new optimum point NOx and PM emissions were 0.54 and 0.092g/kW-hr, respectively, which met the EPA Tier II 2004 automotive diesel mandates.