Comparison of Vacuum Chamber Tested Biporous Wicks With Thermal Ground Plane Testing

2010 ◽  
Author(s):  
Sean W. Reilly ◽  
Ladan Amouzegar ◽  
Ivan Catton
Keyword(s):  
Vacuum Chamber ◽  
Heated Surface ◽  
Ground Plane ◽  
Surface Heat ◽  
Operating Conditions ◽  
Heated Wall ◽  
Testing Methods ◽  
Spacing Parameter ◽  
Heat Spreading ◽  
Wall Surface Temperature

Investigation of bi-porous wicks has yielded an effective method for increasing surface heat transfer when the heat flux is high. It was further found that addition of a mono-porous layer on the heated surface significantly reduced the heated wall surface temperature. These bi-layer wicks were designed for use in 3″×5″ heat spreading devices called Thermal Ground Planes (TGP) in order to transfer heat from a 1 cm2 source. In this work we will investigate the performance of a biporous wick with a monoporous layer in various test set-ups to show the versatility of this heat pipe-substrate. Tests were performed at UCLA and at Advanced Cooling Technologies (ACT) to investigate the wick. Experiments at UCLA were conducted in a vacuum chamber setup to isolate the performance of the wick whereas at ACT the wick lined the evaporator side of a TGP. In order to more closely simulate the operating conditions in a TGP and characterize the vapor spacing parameter, some tests at UCLA were performed with a restrictor plate above the wick similar to the space above the wick in the TGP. The data collected using both these experiments showed similar trends of performance as a function of the spacing above the wick. The motivation of this paper is then to validate that the two testing methods provide similar results while independently addressing different parameters.

Simulation of Buoyancy-Driven Flow for Various Power Levels at the NSTF

2016 ◽  
Author(s):  
Adam R. Kraus ◽  
Rui Hu ◽  
Darius D. Lisowski ◽  
Matthew Bucknor
Keyword(s):  
Natural Convection ◽  
Cooling System ◽  
Heated Surface ◽  
Turbulence Models ◽  
Operating Conditions ◽  
Heated Wall ◽  
Test Facility ◽  
Cfd Simulations ◽  
Wide Range ◽  
Convection Model

The Reactor Cavity Cooling System (RCCS) is an important passive safety system that is being incorporated in a number of high temperature reactor design concepts. The Natural convection Shutdown heat removal Test Facility (NSTF), located at Argonne National Laboratory, is an experiment with the objective of investigating the flow and thermal behavior of a particular air-cooled RCCS design. It consists of 12 ducts surrounded by a cavity with a heated wall, through which air flows via natural convection before exiting through two chimneys. The NSTF is a ½-scale facility, and is well instrumented in order to provide data for code validation, including Computational Fluid Dynamics (CFD)-grade data in a number of locations. Instrumentation includes fiber-optic Distributed Temperature Sensors (DTS) throughout one of the riser ducts and in the upper plenum. In conjunction with the experimental tests, CFD simulations were performed to support the design and optimization of these natural convection systems. The CFD simulations were performed using the “as-tested” geometry of the NSTF. All CFD simulations were steady-state. Both a full natural convection model and a smaller forced primary flow model were tested. The influence of boundary conditions, notably at the cavity walls, was tested. Initial simulations assumed adiabatic walls but these were later adapted to simulate heat losses, aided by thermal images taken of the exterior NSTF surfaces during testing. Simulations were run for tests at two different power levels. A number of turbulence models were compared to test their influence. Simulation results were compared with experimental data. Convergence was generally good for both models. It was found that the natural convection model was indeed beneficial for correctly estimating local temperatures in a number of areas, particularly near the top of the riser ducts and from DTS measurements along the flow path. Flow in the heated cavity was complex. In general, the experimental trends were predicted well by CFD, although magnitudes could be improved in some areas. The turbulence models tested had a relatively small effect on the shape of the temperature profile in the ducts and on heated surface temperatures. Results from the simulations have been of direct use in improving test procedures and choosing locations for more accurate instrumentation. In future work, full natural convection simulations of more tests will be performed. After this has been completed, best practices can be established for accurately simulating these general types of natural convection systems across a wide range of operating conditions.

scholarly journals Full Surface Heat Transfer Characteristics of Stator Ventilation Duct of a Turbine Generator

2020 ◽  
Author(s):  
Shinyoung Jeon ◽  
Changmin Son ◽  
Jangsik Yang
Keyword(s):  
Heat Transfer ◽  
Cooling System ◽  
Cross Flow ◽  
Surface Heat ◽  
Operating Conditions ◽  
Scale Model ◽  
Turbine Generator ◽  
Surface Heat Transfer ◽  
Ventilation Duct ◽  
Flow Case

Turbine generator operates with complex cooling system due to the challenge in controlling the peak temperature of the stator bar caused by ohm loss, which is unavoidable. Therefore, it is important to characterise and quantifies the thermal performance of the cooling system. The focus of the present research is to investigate the heat transfer and pressure loss characteristics of typical cooling system, so-called stator ventilation duct. A real scale model was built at its operating conditions for the present study. The direction of cooling air is varied to consider its operation condition, so that there are (1) outward flow and (2) inward flow cases. In addition, the effect of (3) cross flow (inward with cross flow case) is also studied. The transient heat transfer method using thermochromic liquid crystals is implemented to measure full surface heat transfer distribution. A series of Computational Fluid Dynamics analysis is also conducted to support the observation from the experiment. For the inward flow case, the results suggest that the average Nusselt number of the 2nd duct is about 30% higher than the 3rd duct. The trend is similar with the effect of cross flow. The CFD results are in good agreement with the experimental data.

Incipient Flow Boiling in a Vertical Channel With a Wavy Wall

2010 ◽  
Author(s):  
T. Netz ◽  
R. Shalem ◽  
J. Aharon ◽  
G. Ziskind ◽  
R. Letan
Keyword(s):  
Heat Transfer ◽  
Temperature Field ◽  
High Speed ◽  
Flow Boiling ◽  
Heated Surface ◽  
Vertical Channel ◽  
Infrared Camera ◽  
Heat Fluxes ◽  
Heated Wall ◽  
Boiling Incipience

In the present study, incipient flow boiling of water is studied experimentally in a square-cross-section vertical channel. Water, preheated to 60–80 degrees Celsius, flows upwards. The channel has an electrically heated wall, where the heat fluxes can be as high as above one megawatt per square meter. The experiment is repeated for different water flow rates, and the maximum Reynolds number reached in the present study is 27,300. Boiling is observed and recorded using a high-speed digital video camera. The temperature field on the heated surface is measured with an infrared camera and a software is used to obtain quantitative temperature data. Thus, the recorded boiling images are analyzed in conjunction with the detailed temperature field. The dependence of incipient boiling on the flow and heat transfer parameters is established. For a flat wall, the results for various velocities and subcooling conditions agree well with the existing literature. Furthermore, three different wavy heated surfaces are explored, having the same pitch of 4mm but different amplitudes of 0.25mm, 0.5mm and 0.75mm. The effect of surface waviness on single-phase heat transfer and boiling incipience is shown. The differences in boiling incipience on various surfaces are elucidated, and the effect of wave amplitude on the results is discussed.

Experimental Measurements and Computational Solutions for Aerodynamic Forces on an Ahmed Body at Various Ground Clearances

2003 ◽  
Author(s):  
Ilhan Bayraktar ◽  
Drew Landman ◽  
Tuba Bayraktar
Keyword(s):  
Large Scale ◽  
Bluff Body ◽  
Ground Plane ◽  
Computational Simulation ◽  
Simulation Method ◽  
Full Scale ◽  
Operating Conditions ◽  
Navier Stokes ◽  
Ground Clearance ◽  
Ahmed Body

Reliable computer solutions to external aerodynamic flow fields on road vehicles are extremely desirable to road vehicle designers. In a previous publication a study was performed to validate a Reynolds-averaged unsteady Navier-stokes solution for the aerodynamic characterization of a large-scale bluff body. In the present study, the external aerodynamics of this body as a function of ground clearance are explored. Experimental force measurements are obtained in a full-scale wind tunnel using an Ahmed body model and test conditions representative of full-scale operating conditions. A Reynolds averaged Navier-Stokes solver is employed for computational simulation of the external flowfield at the same conditions. Experimental and computational force coefficients versus vehicle ground clearance are presented for fixed ground, moving ground, and suction slot road simulations. Experimental results using boundary layer suction are compared to computational results with a moving ground plane in order to better understand the effect of a road simulation method.

CFD Simulation of PWR Subchannel Void Distribution Benchmark

2010 ◽  
Author(s):  
Wang-Kee In ◽  
Chang-Hwan Shin ◽  
Tae-Hyun Chun
Keyword(s):  
Void Fraction ◽  
Cfd Simulation ◽  
Fluid Model ◽  
Small Diameter ◽  
Operating Conditions ◽  
Heated Wall ◽  
Bubble Generation ◽  
Void Distribution ◽  
Wide Range ◽  
Two Fluid Model

A CFD study was performed to simulate the steady-state void distribution benchmark based on the NUPEC PWR Subchannel and Bundle Tests (PSBT). The void distribution benchmark provides measured void fraction data over a wide range of geometrical and operating conditions in a single subchannel and fuel bundle. This CFD study simulated the boiling flow in a single subchannel. A CFD code was used to predict the void distribution inside the single subchannel. The multiphase flow model used in this CFD analysis was a two-fluid model in which liquid (water) and vapor (steam) were considered as continuous and dispersed fluids, respectively. A wall boiling model was also employed to simulate bubble generation on a heated wall surface. The CFD prediction with a small diameter of vapor bubble shows a higher void fraction near the heated wall and a migration of void in the subchannel gap region. A measured CT image of void distribution indicated a locally higher void fraction near the heated wall for the test conditions of a subchannel averaged void fraction of less than about 20%. The CFD simulation predicted a subchannel averaged void fraction and fluid density which agree well with the measured ones for a low void condition.

Approach to Heat Transfer Mechanism Beneath Boiling Bubble With MEMS Sensor

2009 ◽  
Author(s):  
Tomohide Yabuki ◽  
Osamu Nakabeppu
Keyword(s):  
Thin Film ◽  
Heat Transfer ◽  
Heat Flux ◽  
Heated Surface ◽  
Bubble Nucleation ◽  
Temperature Data ◽  
Heated Wall ◽  
Measured Temperature ◽  
Back Side ◽  
Mems Sensor

Temperature variation beneath isolated bubble during saturated boiling of water was measured with a MEMS (Micro-Electro-Mechanical Systems) sensor having high temporal and spatial resolution. Then, local heat transfer from the heated surface was evaluated by a transient heat conduction analysis of the wall with measured temperature data as a boundary condition. The MEMS sensor on a 20 × 20 mm2 silicon substrate includes an electrolysis trigger and eight thin film thermocouples on the top side, and two thin film heaters on the back side. The thin film thermocouple was calibrated with a thermal scan method using two alloy samples with different melting point. The condition of the sensor was smoothly controlled with the heater. The bubble is initiated with electrolysis at a gap of the trigger electrode, where slight hydrogen gasses are supplied as bubble nuclei. Then, local and fast temperature variations in wide region are measured with the thermocouples with cutoff frequency of 100 kHz arranged in a line at 40 – 2000 μm far from the trigger gap. Measured temperature data presents formation of microlayer and expansion of dryout area in bubble growth process and rewetting in bubble departure process. The numerical analysis showed that average heat flux beneath the bubble indicated the maximum value of 19 W/cm2 during the microlayer evaporation, and then after hitting a bottom slightly lower than a heat flux at the bubble nucleation, recovers to the nucleation level. The contribution of the heat transfer from the heated wall was evaluated to approximately one-fourth of latent heat in the bubble at departure.

scholarly journals Flow Distortion Into the Core Engine for an Installed Variable Pitch Fan in Reverse Thrust Mode

2020 ◽  
Author(s):  
David John Rajendran ◽  
Vassilios Pachidis
Keyword(s):  
Flow Field ◽  
Total Pressure ◽  
Reverse Flow ◽  
Ground Plane ◽  
Operating Conditions ◽  
Flow Distortion ◽  
Engine Operation ◽  
Variable Pitch ◽  
The Core ◽  
Reverse Thrust

Abstract The flow distortion at core engine entry for a Variable Pitch Fan (VPF) in reverse thrust mode is described from a realistic flow field obtained using an integrated airframe-engine model. The model includes the VPF, core entry splitter, complete bypass nozzle flow path wrapped in a nacelle and installed to an airframe in landing configuration through a pylon. A moving ground plane to mimic the rolling runway is included. 3D RANS solutions are generated at two combinations of VPF stagger angle and rotational speed settings for the entire aircraft landing run from 140 to 20 knots. The internal reverse thrust flow field is characterized by bypass nozzle lip separation, pylon wake and recirculation of flow turned back from the VPF. A portion of the reverse stream flow turns 180° with separation at the splitter leading edge to feed the core engine. The core engine feed flow exhibits circumferential and radial non-uniformities that depend on the reverse flow development at different landing speeds. The temporal dependence of the distorted flow features is also explored by an URANS analysis. Total pressure and swirl angle distortion descriptors, as defined by the Society of Automotive Engineers (SAE) S-16 committee, and, total pressure loss into the core engine are described for the core feed flow at different operating conditions and landing speeds. It is observed that the radial intensity of total pressure distortion is critical to core engine operation, while the circumferential intensity is within acceptable limits. Therefore, the baseline sharp splitter edge is replaced by two larger rounded splitter edges of radii, ∼0.1x and ∼0.2x times the core duct height. This was found to reduce the radial intensity of total pressure distortion to acceptable levels. The description of the installed core feed flow distortion, as described in this study, is necessary to ascertain stable core engine operation, which powers the VPF in reverse thrust mode.

AlN Passivation Over AlGaN/GaN HFETs for Surface Heat Spreading

2010 ◽  
Vol 57 (5) ◽  
pp. 980-985 ◽  
Author(s):  
Naohiro Tsurumi ◽  
Hiroaki Ueno ◽  
Tomohiro Murata ◽  
Hidetoshi Ishida ◽  
Yasuhiro Uemoto ◽  
...  
Keyword(s):  
Surface Heat ◽  
Heat Spreading

Spatial Resolution in Scanning Electron Microscopy and Scanning Transmission Electron Microscopy Without a Specimen Vacuum Chamber

2016 ◽  
Vol 22 (4) ◽  
pp. 754-767 ◽  
Author(s):  
Kayla X. Nguyen ◽  
Megan E. Holtz ◽  
Justin Richmond-Decker ◽  
David A. Muller
Keyword(s):  
Electron Microscopy ◽  
Scanning Electron Microscopy ◽  
Transmission Electron Microscopy ◽  
Scanning Transmission Electron Microscopy ◽  
Vacuum Chamber ◽  
Operating Conditions ◽  
Scanning Transmission Electron ◽  
Transmission Electron ◽  
Scanning Transmission ◽  
Scanning Electron

AbstractA long-standing goal of electron microscopy has been the high-resolution characterization of specimens in their native environment. However, electron optics require high vacuum to maintain an unscattered and focused probe, a challenge for specimens requiring atmospheric or liquid environments. Here, we use an electron-transparent window at the base of a scanning electron microscope’s objective lens to separate column vacuum from the specimen, enabling imaging under ambient conditions, without a specimen vacuum chamber. We demonstrate in-air imaging of specimens at nanoscale resolution using backscattered scanning electron microscopy (airSEM) and scanning transmission electron microscopy. We explore resolution and contrast using Monte Carlo simulations and analytical models. We find that nanometer-scale resolution can be obtained at gas path lengths up to 400 μm, although contrast drops with increasing gas path length. As the electron-transparent window scatters considerably more than gas at our operating conditions, we observe that the densities and thicknesses of the electron-transparent window are the dominant limiting factors for image contrast at lower operating voltages. By enabling a variety of detector configurations, the airSEM is applicable to a wide range of environmental experiments including the imaging of hydrated biological specimens and in situ chemical and electrochemical processes.

Experimental Investigation of Heat Transfer From a Discretely Heated Protruding Pedestal to a Single Round Impinging Air Jet

2004 ◽  
Author(s):  
Amy S. Fleischer ◽  
Sharareh R. Nejad
Keyword(s):  
Heat Transfer ◽  
Experimental Investigation ◽  
Heated Surface ◽  
Operating Conditions ◽  
Surface Geometry ◽  
Stagnation Region ◽  
Exit Surface ◽  
Air Jet ◽  
Air Stream ◽  
Impingement Surface

An experimental investigation to understand the influence of the impingement surface geometry on the heat transfer from a discretely heated surface to a single round impinging jet is conducted. In this study, heat transfer at the stagnation region of a discretely heated pedestal protruding into an air stream is compared to the heat transfer on a discretely heated flat plate to determine the influence of impingement surface geometry on heat transfer for various Reynolds numbers, jet diameters and jet exit-surface spacings. The round jet issues from a tube of diameter 3.5 mm, 9.5 mm or 21 mm at jet exit-to-surface distances of 2–5 diameters with Re = 10,000–30,000. Under all operating conditions, the presence of a protruding pedestal is found to increase heat transfer.

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