A pre-processor was developed to assist CFD experts and non-experts in performing steady, three-dimensional Navier-Stokes analysis of a class of inlet-bleed problems involving oblique shock-wave/ boundary-layer interactions on a flat plate with bleed into a plenum through rows of circular holes. With this pre-processor, once geometry (e.g., hole dimensions and arrangement) and flow conditions (e.g., Mach number, boundary-layer thickness, incident shock location) are inputted, it will automatically generate every file needed to perform a CFD analysis from the grid system to initial and boundary conditions. This is accomplished by accessing a knowledge base established by experts who understand both CFD and the class of problems being analyzed. For experts in CFD, this tool greatly reduces the amount of time and effort needed to setup a problem for CFD analysis. It also provides experts with knobs to make changes to the setup if desired. For non-experts in CFD, this tool enables reliable and correct usage of CFD. A typical session on a workstation from data input to the generation of all files needed to perform a CFD analysis involves less than ten minutes. This pre-processor, referred to as AUTOMAT-V2, is an improved version of a code called AUTOMAT. Improvements made include: (1) multi-block structured grids can be patched in addition to being overlapped; (2) embedded grids can be introduced near bleed holes to reduce the number of grid points/cells needed by a factor of up to four; (3) grid systems generated allow up to three levels of multigrid; (4) CFL3D is supported in addition to OVERFLOW, two well-known and highly regarded Navier-Stokes solvers developed at NASA’s Langley and Ames Research Centers; (5) all files needed to run RONNIE for patched grids and MAGGIE for overlapped grids are also generated; and (6) more design parameters can be investigated including the study of micro bleed and effects of flow/hole misalignments.

CFD simulation based on steady RANS were performed to assess the usefulness of adding a pair of “downstream” vortex generators (VGs) to improve the effectiveness of film cooling a flat plate through one row of inclined holes. Each VG in the pair is a rectangular plate with span S, chord C, and thickness t that is oriented at +45 or −45 degrees with respect to a plane that passes through the center of the film-cooling hole and placed at a distance D downstream of the hole, where D is the hole diameter. The separation between the VGs in the pair is smallest at their leading edges (0.72D) so that the VGs form a V-shape. Parameters studied include: S/D = 0.0, 0.25, 0.5, 1.0; C/D = 0.0, 0.2, 0.4; and blowing ratios of BR = 0.5, 1.0, and 2.0. Results obtained show “downstream” VGs can significantly increase lateral spreading of the film-cooling jet and thereby greatly improve film-cooling effectiveness. Results obtained also show the effects of S/D, C/D, and BR on adiabatic effectiveness, pressure loss, and vortical structures formed.

For convective heat transfer involving multiple sources of different temperatures in the flow field such as in film cooling, the adiabatic-wall temperature, T ad , is used as the reference temperature to define the heat-transfer coefficient (HTC). Studies based on computational Fluid Dynamics (CFD) have always obtained T ad by requiring the cooled or heated surface to be adiabatic. Similarly, most experimental studies that measured T ad have sought to mimic adiabatic wall by using solids with very low thermal conductivity. Other experimental studies have obtained T ad by making two assumptions: (1) T ad at any given point on the cooled or heated surface is independent of the surface temperature, T s , and the surface heat flux, q s ″ , at that point and (2) the HTC, h ad , which equals q s ″ /( T s – T ad ), depends only on the ratio of q s ″ to T s – T ad . With these two assumptions, measuring q s ″ at two different T s or vice versa at a point yields T ad and h ad at that point. In this study, CFD simulations, based on steady RANS, were performed to assess the assumptions invoked by CFD and experimental studies that seek to obtain T ad and h ad . The assessment was made by studying film cooling of a flat plate with an adiabatic wall and with isothermal walls, where the temperature of the isothermal wall, T s , ranged from the lowest to the highest temperatures in the flow. Results from this study show T ad obtained by enforcing an adiabatic wall does not satisfy the requirement: where and when T s – T ad = 0, q s ″ = 0. Therefore, h ad approaches infinity where T s – T ad is either zero or nearly zero, but q s ″ ≠ 0. Also, obtaining T ad and h ad by measuring two sets of ( T s , q s ″ ) was found to yield non-unique values that depended strongly upon the pair of ( T s , q s ″ ) chosen. To overcome the shortcomings of existing methods, a new method was developed in this study to obtain T ad that does satisfy the requirement: T s – T ad = 0 where and when q s ″ = 0. Also, the method developed yields an h ad that is continuous across T s – T ad = 0. By using the new method developed, errors in T ad and h ad obtained by existing methods were assessed.

CFD simulations based on the ensemble-averaged compressible Navier-Stokes equations, closed by two different turbulence models — the standard k-ε model with wall functions and the two-layer model of Chen and Patel — were performed to guide the design of a two-passage experimental setup, which models a linear cascade by using only three airfoils. Results generated show that the angle of the tailboards can be adjusted to give the same flow rates through the two passages. Results also show that once the flow rates are nearly the same, the pressure distributions become nearly periodic, indicating that the two-passage experimental setup does indeed mimic a linear cascade. The discrepancies between the two-passage experimental setup and a truly periodic linear cascade include the location of the stagnation zone about the airfoil’s leading edge and the shape of the pressure distribution in the trailing-edge region.

For film-cooling of a flat plate with cooling jets issuing from round holes, turbulent mixing has been shown to scale with the velocity ratio (VR). In this paper, large-eddy simulations (LES) were performed to investigate the effect of varying blowing ratio (BR = 0.5 – 1.3), density ratio (DR = 1.1 – 2.1), and momentum-flux ratio (MR = 0.2–0.8) on adiabatic effectiveness and turbulence, while keeping the VR fixed at 0.46 and 0.63. Simulations based on Reynolds-Averaged Navier-Stokes (RANS) equations with the realizable k-ϵ and shear-stress transport k-ω models were also performed. The LES results show that separation and spreading of the film-cooling jet increase as BR, DR, and MR increase while VR remains constant. For a given VR, the LES predicts an absolute difference between the minimum adiabatic effectiveness of the lowest and highest MRs to be 2 to 5 times greater than those predicted by RANS. This is because RANS with either model cannot respond appropriately to changes in MR. However, RANS can correctly predict that adiabatic effectiveness decreases as VR increases. The LES results show the turbulent kinetic energy and Reynolds stresses near the film-cooling hole to change considerably with MRs at a constant VR, while turbulent heat flux changes negligibly. This suggests that while improved turbulence models for heat flux can improve RANS prediction of spreading, capturing trends, however, requires improved modeling of the Reynolds stresses.

Time-accurate 3-D CFD simulations based on the SST-SAS turbulence model were performed to study the effects of heat load on the unsteady flow and heat transfer in a cooling duct with a staggered array of short pin fins. For this duct, the static pressure at its exit is maintained at 25 bars, and the cooling air that enters has a temperature of 673 K with a flow rate that produces a Reynolds number of 25,000. To examine the effects of heat load, the following isothermal wall temperatures were studied: 678 K, 873 K, 1073 K, and 1,273 K, which give rises to heat loads that range from 15 kW/m 2 to 1.5 MW/m 2 . Results obtained show high heat loads to cause considerable changes in the temperature of the cooling flow along the duct, which causes significant changes in density and velocity as well as viscosity and thermal conductivity. These changes along the duct were found to affect the locations where unsteady flow separation take place around the pin fins, the magnitude of the vorticity shed in the wakes, and the shedding Strouhal number. These unsteady flow mechanisms in turn strongly affect the nature of the surface heat transfer. A correlation formula for the heat transfer, which accounts for the effects of heat loads, was developed.

Vacuum and low pressures are needed in many applications, and the liquid-ring vacuum pump, which does not have any solid-solid contacts between moving and stationary parts, is widely used because of its low operational cost and long service life. Though progress has been made in advancing this pump, industry still has aggressive goals on improving its efficiency and performance. In this study, a reduced-order model was developed to predict the ability of liquid-ring pumps to ingest air and thereby create lower pressure as a function of pump design and operating parameters. The model developed is semi-empirical — constructed by first analyzing available experimental data to extract features and trends and then encapsulating them into a model through appropriate dimensionless parameters. This model by being in closed form shows the functional relationship between the pump’s design and operating parameters and its ability to ingest air and create a vacuum. To make predictions, this model only requires the following inputs: suction pressure, impeller’s rotational speed, and a few dimensions of the pump. The model developed was assessed by using it to predict the ability of the pump to ingest air for a wide range of suction pressures (cavitation pressure to 760 torr), rotor speeds (up to 1,750 rpm), and dimensions of the pump (radius and span of the impeller blade, hub radius) and then comparing predictions with experimental data not used in the creation of the model. The model developed was found to be accurate within 11% of the experimental data.

Large-eddy and RANS simulations were performed to examine the details of the heat-transfer mechanisms in a U-duct with a high-aspect ratio trapezoidal cross section at a Reynolds number of 20,000. ANSYS-Fluent was used to perform the simulations. For the large-eddy simulations (LES), the WALE subgrid-scale model was employed, and its inflow boundary condition was provided by a concurrent LES of incompressible fully-developed flow in a straight duct with the same cross section and flow conditions as the U-duct. The grid resolution required to obtain meaningful LES solutions were obtained via a grid sensitivity study of incompressible fully-developed turbulent flow in a straight duct of square cross section, where data from direct numerical simulation (DNS) and experiments are available to validate and guide the simulation. In addition, the grid used satisfies Celik’s criterion, and resolves the Kolmogorov’s −5/3 law. Results were also obtained for the U-duct by using RANS, and three widely used turbulence models were examined — realizable k-ε with the two-layer model in the near-wall region, shear-stress transport (SST) model, and stress-omega full Reynolds stress model (RSM). Results obtained from LES showed unsteady flow separation to occur immediately after the turn region, which none of the RANS models could predict. By being able to capture this unsteady flow mechanism, LES was able to predict the measured heat-transfer downstream of the U-duct. The maximum relative error in the predicted local heat-transfer coefficient was less than 10% in the LES results, but up to 80% in the RANS results.

Large eddy simulations (LES) were performed to investigate film cooling of a flat plate, where the cooling jets issued from a plenum through one row of circular holes of diameter D and length 4.7D that are inclined at 35° relative to the plate. The focus is on understanding the turbulent structure of the film-cooling jet and the film-cooling effectiveness. Parameters studied include blowing ratio (BR = 0.5 and 1.0) and density ratio (DR = 1.1 and 1.6). Also, two different boundary layers (BL) upstream of the film-cooling hole were investigated — one in which a laminar BL was tripped to become turbulent from near the leading edge of the flat plate, and another in which a mean turbulent BL is prescribed directly. The wall-resolved LES solutions generated were validated by comparing its time-averaged values with data from PIV and thermal measurements. Results obtained show that having an upstream BL that does not have turbulent fluctuations enhances the cooling effectiveness significantly at low velocity ratios (VR) when compared to an upstream BL that resolved the turbulent fluctuations. However, these differences diminish at higher VRs. Instantaneous flow reveals a bifurcation in the jet vorticity as it exits the hole at low VRs, one branch forming the shear-layer vortex, while the other forms the counter-rotating vortex pair. At higher VRs, the shear layer vorticity is found to reverse direction, changing the nature of the turbulence and the heat transfer. Results obtained also show the strength and structure of the turbulence in the film-cooling jet to be strongly correlated to VR.

In gas turbines, pressure variations induced by the rotor’s rotation and created by the stator and rotor-stator interactions cause hot gas to enter into the wheelspace (i.e., the region between the stator and rotor disks). Purge flow is used to prevent the hot gas from entering, and rim seals are used to minimize the amount of purge flow needed. Preventing hot-gas ingestion is essential to protect the structural integrity of the rotor and stator disks. In this study, a reduced-order model is developed to predict hot-gas ingestion for a given purge flow and for an axial and a radial rim seal and to guide the design of rim seals. The model developed is semi-empirical — utilizing available data in the literature and recognizing certain trends and universal properties in the data if proper dimensionless parameters are used. The model has the advantage of only requiring the following inputs to make predictions: rotor’s rotation speed, flow rate in the turbine’s hot-gas path, purge flow rate, and dimensions of the seal and the wheelspace. Thus, this model differs from existing models in that an experimental input on parameters such as discharge coefficient, minimum sealing flow rate, or mixing length across the rim seal are not needed. The model developed was assessed by using it to predict hot-gas ingestion for a range of purge flow rates (0 to minimum needed to prevent ingestion), rotor speeds (2,000 to 3,500 rpm), and dimensions of the seal geometries (amount of overlap in the seal and width of the flow path through the seal) and then comparing the prediction with experimental data not used in the creation of the model.

Steady RANS were performed to examine the ability of four turbulence models — realizable k-ε (k-ε), shear-stress transport (SST), Reynolds stress model with linear pressure strain (RSM-LPS), and stress-omega RSAM (RSM-τω) — to predict the turbulent flow and heat transfer in a U-duct with a trapezoidal cross section and with and without a staggered array of pin fins. Results generated for the heat-transfer coefficient (HTC) were compared with experimentally measured values. For the smooth U-duct, the maximum relative error in the averaged HTC in the up-leg is 2.5% for k-ε, SST, and RSM-τω and 9% for RSM-LPS. In the turn region, that maximum is 14.5% for RSM-τω, 29% for SST, and 50% for k-ε and RSM-LPS. In the down-leg, SST gave the best predictions and RSM-τω being a close second with maximum relative error less than 10%. The ability to predict the secondary flow in the turn region and the separated flow downstream of the turn dominated in how well the models predict the HTC. For the U-duct with pin fins, k-ε predicted the lowest and the least accurate HTCs, and SST and RSM-τω predicted the best. For k-ε, the maximum relative error in the averaged HTC is about 25%, whereas it is 15% for the SST and RSM-τω, and they occur in the turn. In the turn region, the staggered array of pin fins was found to behave like guide vanes in turning the flow. The pin fins also reduced the size of the separated region just after the turn.

Heat transfer and pressure drop characteristics of condensation for R410A inside horizontal tubes (d h = 3.78 mm) under normal and micro gravity are investigated numerically. The Volume of Fluid method is used to acquire liquid-vapor interface, while the low-Reynolds form of the Shear Stress Transport k∼ω (SST k∼ω) model is adopted to taking turbulent effect into account. The results indicate that the heat transfer coefficients decrease with increasing gravity accelerations, while the frictional pressure gradients increase with increases in gravity accelerations. The liquid film accumulates at the bottom of the tube, leading to a very thin liquid film attached to the upper part of inner tube wall. This accumulation effect decreases with decreases in gravitational accelerations. A more symmetrical liquid-vapor interface is obtained at lower gravity. The average liquid film thickness is nearly the same for different gravity accelerations at the same vapor quality (δ ave ≈56 μm at x = 0.9 and δ ave ≈230 μm at x = 0.5). The local heat transfer coefficients increase with increasing gravity at the top of the tube and decrease with increases in gravity at the bottom, while the bottom part of the tube has a limited contribution to the global heat transfer coefficient for stratified flow regime. The numerical data obtained under normal gravity agree well with well-known empirical correlations.