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Cooling techniques for high density electrical components and electronic devices have been studied heavily in recent years. The advancements in the electrical/electronic industry have required methods of high heat flux removal. Many of the current electrical components and electronic devices produce a range of heat fluxes from 20 W/cm2 – 100 W/cm2 . While parallel flow cooling systems have been used in the past, jet impingement is now more desirable for its potential to have a heat transfer coefficient 3-5 times greater than that of parallel flow at the same flow rate. Problems do arise when the jet impingement is confined and a cross flow develops that interacts with impinging jets downstream leading to a decrease in heat transfer coefficient. For long heated surfaces, such as an aircraft generator rotor, span wise fluid management is important in keeping the temperature distribution uniform along the length of the surface. A detailed simulation of the heat/mass transfer on a three-layer impingement/effusion cooling system has been conducted. The impingement jet fluid enters from the top layer into the bottom layer to impinge on the heated surface. The spent fluid is removed from the effusion holes and exits through the middle layer. Three different effusion configurations were used with effusion diameters ranging from 0.5 mm to 2 mm. Temperature uniformity, heat transfer coefficients, and pressure drops were compared for each effusion diameter arrangement, jet to target spacing (H/d), and rib configuration. A Shear Stress Transport (SST) turbulence fluid model was used within ANSYS CFX to simulate all design models. Three-layer configurations were also set in series for long, rectangular heated surfaces and compared against traditional cooling methods such as parallel internal flow and traditional jet impingement models. The results show that the three-layer design compared to a traditional impingement cooling scheme iii over an elongated heated surface can increase the average heat transfer coefficient by 75% and reduce the temperature difference on the surface by 75%. It was shown that for a three layer design under the same impingement geometry, the average heat transfer coefficient increases when H/d is small. The inclusion of ribs always provided better heat transfer and centralized the cooling areas. The heat transfer was increased by as much as 25% when ribs were used. The effusion hole arrangement showed minimal correlation to heat transfer other than a large array provides better results. The effusion holes’ greatest impact was found in the pressure drop of the cooling model. The pressure losses were minimal when the effective area of effusion holes was large. This minimizes the losses due to contraction and expansion.

The performance and stalling behavior of an axial-flow compressor subjected to several different inlet distortion patterns was investigated. The effect of inlet distortion on overall compressor performance was determined through the measurement of compressor characteristics for each inlet flow condition. Dynamic pressure transducers were employed to investigate rotating stall cell behavior during the inception of stall. Rotor blade response to distorted inflow was measured in the form of average blade pressure profiles by using a scanning valve. Results indicated a substantial reduction in total pressure rise capability for distorted operation. A 25 per cent loss in stall pressure rise was observed for the most severe distortion level. The stall cell was found to rotate in the direction of rotor motion, but at one-half the rotor speed. The cell encompassed the rotor blade tip region down to approximately midspan. During the onset of stall, the circumferential extent of the cell was observed to vary from 60 to 80 degrees. At the rotor blade tip the stall cell relative pressure fluctuations indicated zero flow through the cell. The amplitude of the stall cell was attenuated in the distorted flow region due to the lower air velocity behind the distortion screens. Rotor blade suction side pressure measurements indicated that increasing the circumferential extent of distortion above some "critical" value induced blade stall at higher flow rates. For the low speed compressor used, it appears that the critical angle phenomena is a function of compressor design and is independent of distortion level. / Master of Science

The flow field of a twin jet impinging on ground plane with and without free-stream and at low jet-height-to-diameter ratios was investigated using the Particle Image Velocimetry (PIV) technique. Detailed, time-averaged flow field data are obtained via the high-resolution and the high-sampling rate instantaneous velocity field that is made available via the PIV technique. A model of twin jet issuing from 0.245m circular plate, with 0.019m jet exit diameter, and with jet span to diameter ratio of 3.0 is placed in a water tunnel with the jets in tandem arrangement with respect to the free-stream. The recently upgraded PIV system, in the ESM department fluid mechanics laboratory at VA-Tech, allowed us to capture instantaneous velocity field images of about 0.076m x 0.076m, at 512(H)x512(V) frame resolution. Sampling rates of 1000 and 1200 fps were employed. Understanding the flow field at lower heights is of crucial significance to the VSTOL aircraft application. Huge jet thrust is required to initiate the take-off operation due to the high lift loss encountered while the airframe is in proximity to the ground. Therefore, jet-height-to-diameter ratios of 2 and 4 were employed in this study. Jet-to-free-stream velocity ratios of 0.12, 0.18 and 0.22 were employed in addition to the no-free-stream case. In the current study, only time-averaged flow field properties were considered. These properties were extracted from the available instantaneous velocity field data. In order to provide some details in the time-averaged velocity field, the data were obtained along several planes of interrogation underneath the test model in the vicinity of the twin jet impinging flow. Images were captured in a single plane normal to the free-stream and five planes parallel to the free-stream. A vortex-like flow appears between the main jet and the fountain upwash. This flow is found to experience spiral motion. The direction of such flow spirals is found to be dependent on the jet exit height above the ground, and on the jet-to-free stream, velocity ratios. The flow spirals out towards the vortex flow periphery and upon increasing the free-stream it reverses its direction to be inward spiraling towards the core of the vortex. The flow reversal at certain height of the jet above the ground depends on the free-stream velocity. In our discussion, more emphasis is given to the case of jet-height-to-diameter ratio of two. We also found that the largest turbulent kinetic energy production rate is found to be at the fountain upwash formation zone. / Master of Science

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<p> Determination of boiling heat transfer rate during liquid jet impingement cooling (LJIC) depends on the intensity of bubble generation that is dependent on many flow and surface conditions such as jet velocity, liquid temperature, and surface superheat. Many empirical correlations have been developed previously to determine the total wall heat flux under various LJIC flow velocity, subcooling and surface superheat. However, only few studies have been able to model the governing heat transfer mechanisms associated with LJIC. In many industrial applications, there is a need to determine the effect of any changes in processing parameters on the total rate of heat transfer. Mechanistic heat transfer models can fulfill such need and allow for efficient model modifications at minimum cost and time.</p> <p> Three models have been developed in this study that address the underlying physics associated with jet impingement heat transfer in both single phase and nucleate boiling regimes. The first model accounts for the effect of bubble generation on the overall heat transfer rate at the jet stagnation by introducing a bubble-induced diffusivity (BID). The BID is added to molecular diffusivities in the momentum and energy Equations. The BID model adopts an analogous approach to the eddy diffusivity concept used in turbulence flow modeling. The BID model has been developed to provide a cost effective simulation tool of boiling heat transfer during LJIC by considering bubble generation effect on the overall heat transfer rate while avoiding the need to simulate extremely small time and length scales associated with phase change.</p> <p> The second model is a scenario identification procedure (SIP) that has been developed to predict the bubble growth termination (BGT) scenario. Considering the effect of jet velocity, water subcooling and surface superheat, the SIP identifies whether a bubble would locally collapse or slide by identifying the most probable equilibrium condition (thermal or dynamic) that the bubble would reach first. The main objective of the SIP is to avoid any inaccurate assumption of the probable BGT scenario. In this case, such procedure could improve the predictions of a more comprehensive wall heat flux model of the areas affected by various heat transfer mechanisms.</p> <p> The third model is a mechanistic wall flux partitioning (WFP) model that has been developed to predict the local wall heat transfer rate over the distance between jet stagnation and ten times the jet width. The WFP model assumes that primary heat transfer is due to sensible heating of liquid by forced convection and transient conduction. The WFP model incorporates a unified single-phase heat transfer model that is capable of capturing the observed secondary peaks downstream of stagnation. The WFP model also incorporates a sub-model that predicts abrupt changes in the liquid film thickness due to the formation of hydraulic jumps.</p> <p> The development of these three models have been carried out using experimental data obtained from a set of experiments that has been conducted to investigate the variation of the heat transfer rate and bubble dynamics under a planar free liquid jet. Investigation of bubble dynamics has been conducted using both intrusive optical probe and non-intrusive high speed imaging of the flow filed. The experiments have been conducted at atmospheric pressure, considering jet velocity of 0.4 to 1.7 m/s, degree of water subcooling of 10 to 28 °C, degree of wall superheat of -25 to 30 °C. Such variations have been studied along a heated surface between stagnation and ten times the jet width. Experiments were conducted using a 1 mm x 8 mm planar jet impinging on a heated horizontal flat copper surface. The distance between the jet and the heated surface was 10 mm. The experimental data have been used to develop a group of sub-models for single-phase heat transfer, bubble diameter, bubble population density, bubble release frequency, and bubble growth time. These sub-models have been used in the closure of the aforementioned models.</p> <p> The three models have been validated using independent experimental data. The BID model is capable of predicting stagnation heat flux within -15% and +30%. The SIP model was able to predict the right bubble growth termination scenario of 80% of the investigated cases. The WFP model is capable of predicting the local total wall heat flux within± 30%.</p> / Thesis / Doctor of Philosophy (PhD)

Within this study, the presented material has the objective of providing insight and design characteristics for gas jet nozzles that experimentalists and researchers should consider when utilizing this experimental method. Firstly, this study introduces the developing history and necessity for gas jet experiments and its well-known drawbacks, eventually leading to recent studies and founded knowledge regarding the nozzle geometry dependence on the flow field. The simulation methodology of this study will be presented where the discretization of the computational domain, selection of the flow physics model, and overall design of the nozzle geometry is explained and justified. The flow field data from these simulations will then be presented and compared against various analytical relations taken from literature to analyze differences among the different datasets. Finally, interpretation and discussion of the results will lead to design recommendations, reasoning, and optimization of gas jet nozzles that experimentalists should consider when deciding to incorporate the gas jet nozzle within their experiments.

The noise generated by jet engines represents a significant environmental concern that still needs to be addressed. Accurate and efficient numerical predictions are a key step towards reducing jet noise. The current standard in highidelity prediction of jet noise is large eddy simulation (LES), which resolves the large turbulent scales responsible for the low and medium frequency noise and models the smallest turbulent scales that correspond to the high frequency noise. While LES requires significant computational resources to produce an accurate solution, it fails to resolve the noise in the high frequency range, which cannot be simply ignored. To circumvent this, in this dissertation the Coupled LES-Synthetic Turbulent method (CLST) was developed to model the missing frequencies that relate to un-resolved sub-grid scale fluctuations in the flow. The CLST method combines the resolved, large-scale turbulent fluctuations from very large eddy simulations (VLES) with modeled, small-scale fluctuations from a synthetic turbulence model. The noise field is predicted using a formulation of the linearized Euler equations (LEE), where the acoustic waves are generated by source terms from the combined fluctuations of the VLES and the synthetic fields. This research investigates both a Fourier mode-based stochastic turbulence model and a synthetic eddy-based turbulence model in the CLST framework. The Fourier mode-based method is computationally less expensive than the synthetic eddy method but does not account for sweeping. Sweeping and straining of the synthetic fluctuations by large flow scales from VLES are accounted for in the synthetic eddy method. The two models are tested on a Mach 0.9 jet at a moderately-high Reynolds number and at a low Reynolds number. The CLST method is an efficient and viable alternative to high resolution LES or DNS because it can resolve the high frequency range in the acoustic noise spectrum at a reasonable expense.

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