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Gas Turbine Systems Theory
Published in Tony Giampaolo, Gas Turbine Handbook: Principles and Practice, 2020
The velocity of the air leaving the compressor is decreased before it enters the combustor in order to reduce the burner pressure loss and the air velocity in the burner. According to Bernoulli’s Law a decrease in velocity, and its resultant increase in static pressure, is achieved in the diffuser. [Bernoulli’s Law states that at any point in a passageway through which a gas (or liquid) is flowing, the sum of the pressure energy, potential energy, and kinetic energy is a constant.] As the (subsonic) velocity of the air decreases with the expanding shape of the divergent duct, its static pressure increases, although the total pressure remains the same. (Note that total pressure is the sum of the static pressure and the velocity pressure; where velocity pressure is the pressure created by the movement of the air.)
Force-System Resultants and Equilibrium
Published in Richard C. Dorf, The Engineering Handbook, 2018
of the Reynolds number based on chord length, Re=ρVoc/μ, and the Mach number, Ma =Vo/a. For modern commercial aircraft, the Reynolds number is typically on the order of millions 106. Mach numbers range from less than 1 (subsonic flight) to greater than 1 (supersonic flight).
Measurement Systems
Published in Patrick F. Dunn, Fundamentals of Sensors for Engineering and Science, 2019
Volumetric flow rate can be determined by integrating measured velocity fields of the flow cross-section. Many different velocity sensors can be used for this purpose. In subsonic gas flows, the Pitot-static tube can be used to determine flow velocity. This strictly is not a sensor but rather two concentric tubes forming a conduit between the flow and a pressure sensor. The center tube is open at its end and aligned axially into the flow. This is the total pressure port. The second tube is sealed on its end. A short distance (typically, 3 to 8 tube diameters) from its leading edge, there are four holes at 90° intervals around the circumference whose axes are normal to the flow. These holes comprise the static pressure port.
Heat transfer performance investigation of the spherical dimple heat sink and inclined teardrop dimple heat sink
Published in Numerical Heat Transfer, Part A: Applications, 2019
In the present investigation, the numerical simulation has been conducted to obtain the flow structure in inclined teardrop dimple heat sink and spherical dimple heat sink for steady-state conditions. Depending upon the range of Reynolds number, X-momentum, Y-momentum, Z-momentum, and energy equation are determined for the laminar and turbulent flow conditions. Here flow conditions are subsonic flow because the velocity is in the range of 1.4 m/s to 4.9 m/s. The RNG k–turbulence models are preferred over the k–turbulence model as RNG k–turbulence model is modified version of the k–turbulence and is efficient in solving complex vortex flow which is generally associated with the dimple wall. The grid data are imported into the Ansys fluent software 14.0 and pressure based, the steady-state double precision solver is selected to carry out the present simulation. Unfavorable pressure gradient and flow detachment in boundary layer flow are present in the dimple heat sink cannot be determined correctly with the laminar model due to poor convergence at a lower flow rate. Hence, the turbulence model and laminar model are entirely employed in the simulations of the flat, spherical and incline teardrop dimple heat sink depending upon the Reynolds number. SIMPLEC technique is used for the pressure-velocity coupling. The residual is 10−4 for continuity, x–y–z momentum, k–and 10−8 for the energy equation. The meshing of a heat sink has been shown in Figure 6.
Direct numerical simulation of a fully developed compressible wall turbulence over a wavy wall
Published in Journal of Turbulence, 2018
Zhensheng Sun, Yujie Zhu, Yu Hu, Shiying Zhang
Mean shear stress at the wavy wall normalised by ρmU2m is shown in Figure 12. In our simulations, the amplitude of the wavy wall is high enough to make the separation bubbles appear. For the instantaneous flow field, the region near separation is characterised by large velocity oscillations. Therefore, the separation and reattachment points change from time to time. However, for the mean flow field, the locations where the mean shear stress equals to zero can be identified as the separation and reattachment points to measure the mean separation bubbles. For the case Ma033, the separation and reattachment points are located at 0.14λ and 0.58λ from the preceding wave crest in our simulation, as shown in Figure 12, which is consistent with Cherukat's DNS results [9]. (In Cherukat's simulation, the separation and reattachment points are located at 0.14λ and 0.59λ.) The simulation results are somehow different with Hudson's experimental measurements [1], where the separation and reattachment points were observed at x/λ = 0.22 and 0.58. As noted by Cherukat et al. [9], this may be mainly due to the difficulties in determination of an exact separation location in the laboratory. With the increasing of the Mach number, the separation point is almost unchanged while the reattachment point moves downstream as shown in Figure 13. (The reattachment points locate at x/λ = 0.65, x/λ = 0.68 and x/λ = 0.70 for the case Ma08, Ma12 and Ma15, respectively.) This phenomenon can also be seen in Figure 14 where the streamlines near the wavy surface are shown. The locations of the vortex cores for different Mach numbers are also shown in Figures 13 and 14. These results clearly show the Mach number effects on the flow separation. When the flow is subsonic, the separation is mainly caused by the adverse pressure gradients due to the expansion of the wavy wall. When the flow is supersonic, blockage caused by the windward side of the wavy wall is the main reason of separation, which makes the separation bubble move downstream and become more flat.