Abstract:Liquid metal MHD (Magneto-Hydro-Dynamic) systems can be employed to produce electricity from a wide range of heat resources. In such a system, a low-boiling organic fluid and a high-temperature liquid metal fluid mix. The former evaporates, and carries the latter to flow through an MHD channel, where the electricity is generated. The mixing process and the gas-liquid flow characteristics will have a significant effect on the power generating efficiency. In the present work, trifluorotrichloroethane (R113) was chosen as the organic fluid, and gallium (Ga) as the liquid metal, respectively. Numerical study was subsequently carried out on the gas-liquid flow and heat transfer in a self-designed spherical mixer. The effects of the main factors, including the inlet velocities and inlet temperatures of Ga and R113, were separately determined, with suggested values or ranges discussed in detail.
In order
to improve the characteristics of the air–oil two-phase flow
and heat transfer in the scavenge pipe of an aero-engine bearing chamber,
this paper presents several scavenge pipes with different cross-sectional
geometries, by numerically investigating the processes of the air–oil
two-phase flow and heat transfer, in comparison to a circular pipe.
The findings indicate that the tripetal cross-section shows the best
heat-transfer effect, while the four-petal cross-section has the lowest
flow resistance. Under the same working condition and the equal wetted
perimeter, the tripetal cross-section has an 8.8% higher heat-transfer
effect than the circular section, while the four-petal cross-section
has a 28.6% lower flow resistance than the circular; under the equal
cross-sectional area, the tripetal cross-section has a 9.1% higher
heat-transfer effect than the circular section, while the four-petal
cross-section has a 23.6% lower flow resistance than the circular;
under the equal hydraulic diameter, the tripetal cross-section has
a 9.2% higher heat-transfer effect than the circular section, while
the four-petal cross-section has a 21.9% lower flow resistance than
the circular. Taking both the heat transfer and flow resistance into
consideration, the four-petal cross-section exhibits the best comprehensive
performance, with the comprehensive performance coefficient decreasing
with the increase of oil inlet velocity and rising with the increase
of air inlet velocity.
The development of aeroengines toward a lighter and compact structure has put forward a stringent requirement on the lubrication systems, especially on the bearing chambers which contain complex air–oil two-phase fluids. Understanding the flow characteristics is of considerable significance to ensure the cooling and lubrication effect and to improve the working reliability of the aeroengine. Hilbert–Huang transform (HHT) was used to analyze the spectral characteristics of the pressure signals in the bearing chamber, in order to establish the correlation between the energy indicator k and the flow regime. The influences of the operating conditions and lubricant physical properties on the flow regime were discussed. The findings indicate that this approach can distinguish two typical flow regimes in the bearing chamber. With the increase of oil inflow and the decrease of draft speed, the flow regime changes from homogeneous flow to stratified flow, and k shifts from high-frequency band to low-frequency band. At the same lubricant mass inflow, k shifts from low-frequency band to high-frequency band with the increase of lubricant density, and the oil film on the wall becomes thinner. As the viscosity increases, the flow resistance grows and more oil accumulates on the wall. The flow regime converts from homogeneous flow to stratified flow, with k shifting from high-frequency band to low-frequency band. In addition, higher surface tension will cause more oil agglomeration, which results in less stable two-phase flow with irregular and uneven distribution of the oil on the wall.
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