1,721,012 research outputs found
Analysis of body force effects on flow boiling and condensation with finite inlet quality
Get PDFThis study explores flow boiling pressure drop of FC-72 in a rectangular channel subjected to single-side and double-sided heating for vertical upflow, vertical downflow, and horizontal flow with positive inlet quality. Analysis of temporal records of pressure transducer signals is used to assess the influences of orientation, mass velocity, inlet quality, heat flux, and single-sided versus double-sided heating on magnitude of pressure drop oscillations, while fast Fourier transforms of the same records are used to capture dominant frequencies of oscillations. Time-averaged pressure drop results are also presented, with trends focusing on the competing influences of body force and flow inertia, and particular attention paid to the impact of vapor content at the test section inlet and the rate of vapor generation within the test section on pressure drop. Several popular pressure drop correlations are evaluated against the present pressure drop database. Predictions are presented for subsets of the database corresponding to low and high ranges of inlet quality and mass velocity. The correlations are ranked based on mean absolute error, overall data trends, and data spread. While most show general success in capturing the data trends, they do so with varying degrees of accuracy. Further, this study concerns the development of a set of mechanistic criteria capable of predicting the flow conditions for which gravity independent flow condensation heat transfer can be achieved. Using FC-72 as working fluid, a control-volume based annular flow model is solved numerically to provide information regarding the magnitude of different forces acting on the liquid film and identify which forces are dominant for different flow conditions. Separating the influence of body force into two components, one parallel to flow direction and one perpendicular, conclusions drawn from the force term comparison are used to model limiting cases, which are interpreted as transition points for gravity independence. Experimental results for vertical upflow, vertical downflow, and horizontal flow condensation heat transfer coefficients are presented, and show that, for the given test section, mass velocities above 425 kg/m2s ensure gravity independent heat transfer. Parametric evaluation of the criteria using different assumed values of mass velocity, orientation, local acceleration, and exit quality show that the criteria obey physically verifiable trends in line with those exhibited by the experimental results. As an extension, the separated flow model is utilized to provide a more sophisticated approach to determining whether a given configuration will perform independent of gravity. Results from the model show good qualitative agreement with experimental results. Additionally, analysis of trends indicate use of the separated flow model captures physics missed by simpler approaches, demonstrating that use of the separated flow model with the gravity independence criteria constitute a powerful predictive tool for engineers concerned with ensuring gravity independent flow condensation heat transfer performance
High heat flux dissipation using small diameter channels
No full textIncreased heat dissipation rates from electronic chips creates the need for high heat flux cooling schemes with a special emphasis on practical considerations such as pressure drop and flow rate. Devices such as fusion reactor components and rocket nozzles have heat dissipation rates of 10,000 W cm\sp{-2} These heat fluxes are orders of magnitude greater than electronic devices, thus requiring ultra-high heat flux cooling schemes. In this thesis, a cooling technology is proposed where high heat fluxes are achieved by flow boiling in miniature heat sinks. High heat dissipation is achieved by forcing a boiling liquid through small channels that run through the heat sink. For boiling conditions, the limiting heat flux is the critical heat flux, CHF; however, flow boiling in small diameter channels offered an enhancement in CHF over most boiling configurations. Pertaining to electronic cooling applications, flow boiling of R-113 at low mass velocity in mini-channel (D = 2.54 mm) and micro-channel (D = 510 m) heat sinks is presented as a practical cooling technology. Also, complimenting the experimental study, predictive tools for optimizing heat sink design based upon channel diameter, channel spacing, CHF pressure drop, and flow rate are developed and presented as a complete package for incorporating miniature heat sink technology into electronic cooling scheme design. For ultra-high heat flux applications, miniature heat sink technology is also proposed; however, flow boiling of water at high mass velocity is the choice of coolant. In order to adapt miniature heat sink technology for ultra-high heat flux applications, an experimental investigation of CHF for water in small diameter tubes was performed and predictive CHF correlations developed for two distinct parametric regions defined as high and low pressure regions. A complete design methodology for incorporating two-phase miniature heat sink technology into cooling scheme design is presented
Design and analysis of heat exchangers for high pressure metal hydride hydrogen storage
No full textThis study explores the development of a hydrogen storage system using high-pressure metal hydride, Ti1.1CrMn. When absorbing hydrogen (filling), the metal hydride releases large amounts of heat causing the hydride temperature to rise. The reaction rate depends on the metal hydride temperature, decreasing significantly if the heat is not removed quickly. To store 5 kg hydrogen needed to drive 300 miles, about 36 MJ of heat is must be released. For a five minute fill time, this translates to 120 kW of heat generation rate. This is a formidable challenge considering the enormous amount of heat, poor thermal properties of the hydride, and the stringent limits on the heat exchanger’s weight and volume, let alone a host of manufacturing requirements. Additionally, the kinetics of the material is such that the rate of reaction (hydrogen absorption) depends on the ability to quickly dissipate the heat generated. A systematic heat exchanger design methodology is presented here, starting with a 1-D metal hydride layer distance criterion and progressing through a series of engineering decisions supported by computations of fill time. A modular tube-fin design is arrived at with a goal of achieving a fill time of less than 5 min. The prototype heat exchanger comprising of an intricate network of fins surrounding the metal hydride powder occupies 29% of the storage vessel volume. Experiments were performed to study the influence of various parameters on the hydriding reaction and a lowest fill time of 4 min 40s is successfully achieved. Coiled-tube heat exchanger is designed with a primary goal of reducing the volume while still achieving practical fill time. The heat exchanger consists of only a coolant tube strategically coiled around the metal hydride powder. The coiled-tube heat exchanger reduces fill time by 75% while occupying only 7% of the storage pressure vessel volume. Dehydriding tests were performed with each design to investigate the influences of hydrogen release rate, fluid flow rate and fluid temperature on the dehydriding process. Dehydriding reaction rate was accelerated by increasing the fluid temperature and/or the rate of pressure drop. Transient two and three dimensional numerical models are constructed that simulate the process of hydriding and dehydriding in Ti1.1CrMn. Spatial distributions of hydride temperature as a function of time over the entire duration of the hydriding reaction are determined, which agree favorably with the experimental data. The models are shown to be quite accurate at predicting the spatial and temporal variations of metal hydride temperature during both the reaction. Effect of pressure vessel size and parameters like hydriding pressure, packing density and effective thermal conductivity, on the size and volume of the storage system is studied. Operating conditions and metal hydride parameters required to meet the Department of Energy (DOE) fill time targets are presented. Various components of a high-pressure metal hydride system are discussed and the storage efficiencies of an optimized storage system using Ti1.1CrMn is presented
Experimental Investigations and Theoretical/Empirical Analyses of Forced-Convective Boiling of Confined Impinging Jets and Flows Through Annuli and Channels
No full textThis study comprises experimental investigations and theoretical/empirical analyses of three forced-convective (pumped) boiling schemes: (i) confined round single jet and jet array impingement boiling, and flow boiling through conventional-sized (ii) concentric circular annuli and (iii) rectangular channels. These schemes could be utilized in the thermal management of various applications including high-heat-flux electronic devices, power devices, electric vehicle charging cables, avionics, future space vehicles, etc.The first part of this study encompasses an experimental investigation of key parameters influencing Critical Heat Flux (CHF) for confined round single jets and jet arrays impinging normally onto square heated surfaces. The experiments are performed using R-134a, a fluid widely used for thermal management of electronic and power devices, especially in aerospace applications. A comprehensive R-134a CHF database is acquired that considers the effects of various geometrical parameters and operating conditions. Careful examination of data trends reveals several strategies to augment CHF, such as increasing jet velocity and/or total mass flow rate and employing larger jet diameters for a fixed velocity or smaller diameters for a fixed flow rate. Higher CHF is also achieved by increasing saturation pressure for a fixed inlet fluid temperature (i.e., higher saturation pressure combined with higher inlet subcooling). Fluid exit qualities point to two different CHF mechanisms: subcooled CHF at high flow rates and saturated CHF at low flow rates. Underlying mechanisms are also propounded for two types of CHF transients: a sudden sharp temperature escalation at lower flow rates and a mild gradual increase at higher flow rates. Close inspection of the heating surface following CHF tests shows localized burnout patterns which provide significant insight into both the flow characteristics within the confinement region and the spatial distribution of surface temperature resulting from jet interactions. Statistical inference techniques are used in conjunction with the new understanding of fluid flow and heat transfer physics to formulate a new correlation form for CHF. The resulting correlation, which is based on a consolidated database of the present R-134a and previous FC-72 data, shows good prediction accuracy, evidenced by a mean absolute error (MAE) of 16.66% for both fluids and over broad ranges of geometrical parameters and operating conditions.The second part of this study is on flow boiling in concentric circular annuli with an emphasis towards its application in the thermal management of electric vehicle charging cables.Transportation industry is presently in fast track to transition from Internal Combustion Engine Vehicles (ICEVs) to Electrical Vehicles (EVs). One main inhibitor to transitioning to EVs is the very slow charging at the networks of charging stations available worldwide. Despite many recent so-called ‘ultra-fast’ charging methods, which capitalize on a variety of single-phase liquid schemes to cool the charging cable, thermal constraints still limit the electrical current carrying capacity of the fastest commercial chargers to about 500 A. Achieving the faster charging time required for the anticipated proliferation of EVs will require increasing this current capacity to at least 2000 A, which poses formidable thermal challenges in the design of the charging cable
Single- and two-phase heat transfer enhancement in a curved, rectangular channel subjected to concave heating
No full textAn experimental program was undertaken to examine single- and two-phase heat transfer enhancement provided by streamwise curvature. Curved and straight rectangular flow channels were fabricated with 5.0 x 2.5-mm cross-sections and heated lengths of 101.6 mm in which the heat was applied to only one wall--the concave wall (32.3 mm radius) in the curved channel and a side wall in the straight. Reynolds number ranged from 9,000 to 130,000 and fluid subcooling from 3 to 29\sp\circC. The centripetal acceleration for curved flow reached 315 times earth\u27s gravitational acceleration. Single-phase convection coefficients were enhanced in the curved channel for all conditions tested. Critical heat flux was augmented due to flow curvature at all conditions with the enhancement decreasing as subcooling increased. Flow visualization tests conducted in transparent channels identified important vapor characteristics and provided insight into two-phase enhancement. A model based on the interfacial lift-off criterion was developed to predict critical heat flux from straight and curved surfaces. The model provided excellent predictions for near-saturated conditions but was less accurate for subcooled flow
Air-to-fuel heat exchanger for high mach flow turbine engines
No full textIn pursuit of gas turbine engines that are capable of meeting the requirements of high Mach propulsion applications, cooling the engine cooling air with engine fuel is an important area of technology development. Several heat exchanger designs have been proposed to extract the heat from the cooling air with varying degrees of success. This study focuses on the testing and analysis of a new multi-pass modular heat exchanger, where the modules are arranged in an annular configuration. In the process of analyzing the heat exchanger, a theoretical model was developed that could be applied to heat exchangers of various sizes and other design envelopes. To confirm the accuracy of the theoretical heat exchanger model, the newly designed heat exchanger was represented by a single module that was tested experimentally and analyzed theoretically. First, an experimental facility was designed and constructed to test the heat exchanger module under simulated engine conditions. Air and water were used as working fluids through a 1.52 cm (0.6 in) wide and 6.6 cm (2.6 in) long module that incorporated both micro-channel liquid passages and short airside fins. Simultaneously, the theoretical model was constructed to predict the heat transfer and temperature drop across the heat exchanger module for each fluid stream. With these tools in place, comparisons were made between experiment and theory, which showed good overall agreement in heat transfer rate and exit temperatures. This study shows the theoretical model is a reliable foundation for predicting the performance of heat exchanger modules under actual fuel and air turbine engine conditions
Effects of flow orientation on condensation in tubes
Get PDFThis study explores the interfacial and heat transfer characteristics of annular condensation of FC-72 in vertical downflow, vertical upflow and horizontal flow as well as the effects of channel orientation on flow condensation. Two separate condensation test modules are employed, one for high-speed video imaging of the film interface and the second for heat transfer measurements. Condensation in both test modules is achieved by rejecting the heat to a counterflow of cooling water. The heat transfer measurements are obtained along the inner wall of an 11.89-mm i.d. and 1,259.84-mm long stainless steel tube. For vertical downflow, the film at very low FC-72 flow rates is observed to be both smooth and laminar. The film turns turbulent with a very wavy interface as the flow rate of FC-72 is increased, especially for exit film Reynolds numbers above 1,800. The heat transfer coefficient decreases axially because of a gradual thickening of the liquid film. However, the data show a downstream minimum before the heat transfer coefficient increases again towards the outlet as the film transitions to turbulent flow, enhanced by the more intense downstream waves. A control-volume-based model is proposed, which incorporates an eddy diffusivity profile for the liquid film that accounts for interfacial dampening of turbulence due to surface tension. The model shows good accuracy in predicting the average condensation heat transfer coefficient data, evidenced by a mean absolute error of 12.59%. Upflow condensation is complicated by the relative magnitude of the opposing vapor shear and gravity. This study also examined the different flow regimes for condensation of FC-72 in vertical upflow. Four regimes are identified, falling film, where the condensing film drains downwards by gravity opposite to low velocity vapor flow, oscillating film, corresponding to film flow oscillating between upwards and downwards, flooding, where film begins to be sheared upwards by the vapor core, and climbing film, where high vapor velocity causes the film to be sheared upwards. The four flow regimes are well segregated in a flow regime map based on dimensionless superficial velocities of the vapor and liquid. The condensation heat transfer coefficient is shown to decrease axially because of gradual thickening of the film, except for high mass velocities, where turbulence and intensified interfacial waviness cause downstream heat transfer enhancement. The annular flow model is modified to account for the reversed orientation of gravity, and shows fair predictions for the climbing film regime. The predictive accuracy of the model is influenced by flow oscillations occurring downstream of the climbing film region and inability of the model to account for interfacial waves. For condensation of FC-72 in horizontal tubes, dominant condensation flow regimes are identified for different combination of mass velocities of FC-72 and cooling water using high-speed video motion analysis.. Additionally, detailed heat transfer measurements are used to explore both axial and circumferential variations of the condensation heat transfer coefficient. Four different regimes are identified: stratified, stratified-wavy, wavy-annular with gravity influence, and wavy-annular without gravity influence. In the latter regime, which is achieved at high FC-72 mass velocities, annular film transport is dominated by vapor shear with negligible gravity effects. Using different types of regime maps, prior relations for transitions between regimes are assessed, and new, more accurate transition relations developed. The heat transfer coefficient is shown to be highest near the inlet, where quality is near unity and the film thinnest, and decreases gradually along the condensation length because of axial thickening of the liquid film. This study also explores the predictive capabilities of prior heat transfer correlations and a control-volume-based annular flow model. The experimental data of both the local and average condensation heat transfer coefficients show fair to good agreement with predictions of prior and popular correlations. But superior predictions in both trend and magnitude are achieved with the annular flow model. The study of orientation effects on flow condensation explores condensation of FC-72 in a circular tube at three different flow orientations including horizontal flow, vertical downflow, and vertical upflow with the aid of detailed heat transfer measurements and high-speed video motion analysis. Using the video analysis, the behavior of liquid film and influence of gravity are investigated for different combinations of mass velocity of FC-72 and cooling water for three flow orientations. Utilizing the condensation module for heat transfer measurements, axial and circumferential variations of the condensation heat transfer coefficient for different flow orientations are explored. Local and average condensation heat transfer coefficients from the three flow orientations are compared to each other to assess the influence of body force on condensation heat transfer. Flow conditions that negate the influence of body force are identified. Using the annular flow model, the magnitudes of different forces acting on the liquid film are compared to each other for different combinations of mass velocity of FC-72 and cooling water for each orientation
Critical heat flux enhancement in the presence of stream-wise curvature
No full textAn experimental study was undertaken to evaluate a novel cooling concept designed to cool high power heat sources by forced convective boiling in the presence of stream-wise curvature. The critical heat flux, CHF, determines the maximum nucleate boiling heat transfer limit beyond which permanent damage to the heat source may result. Stream-wise flow curvature developed an artificial body force which propelled vapor away from the heater surface, thereby extending the nucleate boiling regime to higher heat fluxes. A parametric study was made to investigate the effects of surface curvature, heat length, system pressure, and flow velocity on CHF. Detailed flow visualization experiments conducted in linear and curved channel heaters improved the fundamental understanding of liquid-vapor exchange near the heater surface and helped to identify the trigger mechanism responsible for initiating CHF. Greater liquid contact was observed and higher CHF was measured for the curved heater surface compared to the linear heater surface at equal flow velocities. Flow visualization observations were used in conjunction with experimental measurements to develop a mechanistically based CHF model that explicitly accounted for the effects of a body force. The CHF model was based on geometrical arguments, a hydrodynamic two-phase flow model and an interfacial instability analysis. Experimental measurements taken in the linear and curved channel compared well with theoretical predictions. The primary contributions of this study are the development of a unique system designed to enhance CHF, the identification of hydrodynamic conditions necessary for triggering CHF and the verification of the stream-wise curvature enhancement effect on CHF
Emissivity characteristics of aluminum alloy surfaces and assessment of multispectral radiation thermometry (MRT) emissivity models
No full textExperiments were performed to examine the emissivity characteristics of aluminum alloy samples over the spectral range of 2.05 to 4.72 μm and temperatures of 600, 700 and 800 K. AL 1100, 7150, 7075 and 2024 samples with polished, 6-μm, and 14-μm surface finishes were tested. Additionally, extruded and saw-cut samples were tested to examine the effects of extreme roughness on emissivity. Overall, aluminum alloys are shown to buck the general trend of increasing emissivity with increasing temperature for metallic surfaces in the infrared range. The emissivity of aluminum samples generally decreased between 600 and 700 K and increased between 700 and 800 K. The latter increase is attributed to surface discoloration at 800 K. For the polished, 6-μm and 14-μm samples, the emissivity decreased appreciably between 2.05 and 3.5 μm, and increased slightly between 3.5 and 4.72 μm. Spectral variations were far less pronounced for the extruded and saw-cut surfaces. The experimental results were used to assess the accuracy of eighteen multispectral radiation thermometry (MRT) emissivity models for temperature measurement. It is shown that drastic changes in the shape of emissivity distribution preclude the use of a single function to accurately represent every band of the measured spectrum. Better predictions were achieved using the simplest form of MRT emissivity models and minimum number of wavelengths were required by the model. Overall, two relatively simple models provided the best overall predictions for different alloys, temperatures and surface roughnesses. This study also explored the relationship between the emissivity of aluminum alloy surfaces and surface roughness. Different theories governing this relationship were reviewed and categorized. A previous model by Agababov was found to be an effective means for both characterizing surface roughness and incorporating roughness features in emissivity models. Four emissivity models were examined based on their proven accuracy at determining both emissivity and surface temperature using the MRT technique. Those models were tested both in their basic form and modified with the Agababov roughness function for accuracy in inferring surface temperatures of AL 7075 samples. The results show that modified models yield better accuracy in temperature prediction
Low temperature hybrid micro-channel/micro-jet impingement cooling
No full textThis study proposes a new hybrid cooling scheme for high-flux thermal management of electronic and power devices. This scheme combines the cooling benefits of micro-channel flow and micro-jet impingement with those of indirect refrigeration cooling. Experiments were performed to assess single-phase cooling performance using HFE 7100 as a working fluid. Excellent numerical predictions of this performance was achieved using the standard k-&egr; model. The proposed cooling scheme is shown to involve complex interactions of impinging jets with micro-channel flow. Increasing jet velocity allows jets to penetrate the micro-channel flow toward the surface, especially in shallow micro-channels, greatly decreasing wall temperature. In addition to the numerical predictions, a superpositioning technique is introduced that partitions the heat transfer surface into zones that are each dominated by a different heat transfer mechanism, and assigning a different heat transfer coefficient value to each zone. This study also examined the two-phase cooling performance of the hybrid cooling scheme. Vapor layer development along the micro-channel is shown to be fundamentally different from that encountered in conventional micro-channels. In the hybrid scheme, subcooled jet fluid produces repeated regions of bubble growth followed by collapse, rather than the continuous growth common to conventional micro-channel flow. By reducing void fraction along the micro-channel, the hybrid scheme contributes greater wall temperature uniformity. Increasing subcooling and/or flow rate delay the onset of boiling to higher heat fluxes and higher wall temperatures, but also increase critical heat flux considerably. By dividing the test surface into a portion that is dominated by jet impingement and another by micro-channel flow, and applying the appropriate CHF correlation for each portion, the CHF data for this hybrid cooling configuration is predicted with a mean absolute error of 15.2%. This study also explores the single-phase and two-phase cooling performance of a hybrid cooling module in which a series of micro-jets deposit coolant into each channel of a micro-channel heat sink. This creates symmetrical flow in each micro-channel, and the coolant is expelled through both ends of the micro-channel. Three micro-jet patterns are examined, decreasing-jet-size (relative to center of channel), equal-jet-size and increasing-jet-size
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