Browsing by Author "Windom, Bret C., committee member"

Now showing 1 - 5 of 5

Open Access
A computational examination of conjugate heat transfer during microchannel flow boiling using finite element analysis
(Colorado State University. Libraries, 2018) Burk, Bryan E., author; Bandhauer, Todd M., advisor; Windom, Bret C., committee member; Henry, Charles S., committee member
As technology advances, electronic components continue to produce more heat while at the same time growing smaller and being arranged in ever more compact packages. This has created the need for new thermal management systems able to both dissipate the large heat loads and meet the diminishing size requirements. Microchannel heat exchangers have become an integral part of such advanced cooling systems as they provide an exceedingly large surface area over which heat transfer can occur while maintaining a diminutive size. Current microchannel devices primarily use single-phase flow to dissipate the heat. As heat loads increase, so too must flow rates. Due to associated issues with extremely large pressure drops and high pumping power requirements, the practical capacity of single-phase microchannel coolers has largely been met. One particularly promising avenue forward is to utilize flow boiling with similar microchannel heat exchanger designs. The very high latent heat of vaporization associated with phase change for many fluids allows for a large amount of heat to be dissipated in flow boiling using a relatively low flow rate as compared to single-phase systems, drastically reducing the issues related to pressure drop. Additionally, two-phase heat transfer is associated with much higher heat transfer coefficients, allowing for smaller heat transfer surface areas (and thus smaller overall devices) and lower driving temperature differences for the same heat removal rates. Microchannel flow boiling studies to date have assumed 1D heat conduction through the heat exchanger material and have developed correlations to predict average heat transfer coefficients. Unfortunately, with the high heat fluxes expected in the near future, and with heat loads being applied at small, localized hotspots, the 1D assumption is no longer valid. Conjugate heat transfer must be considered, and local heat transfer coefficient correlations are necessary for the design of future thermal management systems. This thesis describes a first of its kind computational model that uses finite element analysis to analyze the conjugate heat transfer problem, complete with local heat transfer coefficients. This work serves as both proof of concept and an evaluation of the predictive capabilities of five published heat transfer correlations when applied locally to a high heat flux microchannel heat exchanger that has been previously tested. Modeling results show highly variable local heat flux profiles along the microchannel walls, confirming the need to consider conjugate heat transfer. Significant heat spreading resulted in peak local heat fluxes of roughly 0.5× that of the uniformly applied heat flux with 31.4% - 64.1% of total applied heat dissipated outside the region projected directly above the heater. As determined via local temperature comparisons, the correlation from Agostini and Bontemps provides the best overall agreement with average root mean square temperature differences of 2.6°C, though trends suggest that this difference may increase as heat flux increase further than those values tested here.
Open Access
Experimental investigations for improving the accuracy of flow measurement in irrigation canals
(Colorado State University. Libraries, 2021) Pugh, Joseph E., author; Venayagamoorthy, S. Karan, advisor; Gates, TImothy K., advisor; Windom, Bret C., committee member
Flow measurement in open-channels refers to the process by which a volume amount of water passing through a channel cross section is quantified per unit time. In the irrigation water management context, this is done to account for available water resources so that water distribution systems can be properly managed to achieve adequate and efficient allocation. Today, irrigation water use remains the largest consumptive draw on our collective water budget, while the availability of this resource is becoming increasingly scarce. Additionally, access to reliable irrigation water acts as a major factor in maintaining strong crop production and healthy rural livelihoods. Improvement in the accuracy of flow measurement methodologies is then motivated by a need for implementing more conservative practices to limit unnecessary waste and to promote effective and equitable allocation. The two principal means by which flow is quantified in open-channels are the velocity-area method and the use of stage-discharge relationships associated with hydraulic structures placed within the channels. The present study investigates improvements to the accuracy of flow measurement for each of these methodologies. The velocity-area method involves the integration of several point measurements of velocity multiplied by sub-components of the channel cross-sectional area to achieve an estimate of flow rate via the principle of continuity. Traditionally, these methods have been time-consuming and subject to inaccuracy due to the large number of measurements needed. In recent decades, the development of Acoustic Doppler Current Profilers (ADCPs) has offered a means for measuring flow using the velocity-area method with time-efficiency and less intrusiveness into the flow. However, opportunity still exists for refining the operational protocols for this device to quantify and reduce measurement uncertainty, especially within the irrigation water management context. With this motivation, a StreamPro ADCP manufactured by Teledyne RD Instruments was used to quantify flow rates in man-made irrigation canals with the aim of determining best practices for: the method of deployment of an ADCP using a moving boat, the duration of the measurement transect per unit width of canal, and the number of transects to use in computing mean steady discharge. The purpose of these refinements is to better resolve a mean representation of the fluctuating turbulent velocity flow field by lessening user-induced uncertainty and estimating the point of diminishing returns for additional data collection. Suggested protocols developed from this experimentation indicate that a reduction of 30 to 70% in the values of uncertainty metrics can be accomplished utilizing a remotely operated tagline deployment method with a minimum transect duration relative to the canal top width of 24 s/m (7.3 s/ft), equivalent to 24 vertical pings per meter of an ADCP measuring at 1 Hz; and at least six total transects taken in reciprocal directions included in measurements of mean steady discharge. These findings provide further specificity beyond current guidelines to enhance the likelihood of practical implementation by ADCP users. Refinement of ADCP measurement protocols also serve as a chief aid in the accuracy of hydraulic structure calibration. As permanent fixtures within an open-channel, these structures rep- resent a means of obtaining continuous flow measurement without the time cost of velocity-area methods. The fundamental principle upon which hydraulic structures operate is the stage-discharge relationship. This equation relates an upstream measurement of the water surface elevation to the flow rate passing through the channel. However, the theoretical derivation of stage-discharge equations require several simplifying assumptions that must be corrected for using empirical calibration. In the present study, the nature of the stage-discharge relationship for a particular hydraulic structure known as the Obermeyer pivot weir is investigated. This device was primarily designed as a control for upstream water levels, and the current effort to establish a means by which the weir can be used for flow measurement purposes represents a novel contribution to the literature. In general, research on pivot weirs remains sparse, and no consensus has been reached concerning the correct theoretical approach for establishing the stage-discharge equation for this type of structure. Here, using a set of field observations, four alternative approaches are investigated to elucidate the optimal method for the use of pivot weirs for flow measurement. Specifically, a hypothesis concerning the effect of the changing angle of a pivot weir on the flow dynamics is tested. A recommended approach is given and informed by knowledge of practical implementation limitations. Finally, preliminary investigations using a laboratory model of the Obermeyer pivot weir are discussed, which offer insight into the complex nature of the dynamics for flow over this type of structure. The results of the present study offer important refinements to the current body of knowledge found within the literature and identify promising avenues for future research.
Open Access
High efficiency air delivery system for solid oxide fuel cell power generation
(Colorado State University. Libraries, 2024) Mitchel, Lars Jared-Brian, author; Bandhauer, Todd M., advisor; Windom, Bret C., committee member; Cale, James, committee member
Distributed power generation systems can be used in the electric grid to reduce peak loads, raise power quality, and reduce/eliminate transmission losses. One distributed energy system with distinct advantages is a Solid Oxide Fuel Cell (SOFC) integrated with an Internal Combustion Engine (ICE) which has the capability to operate at electric efficiencies as high as 70%. This research aimed to produce and test a high efficiency air delivery system that supports the SOFC-ICE to generate power on the scale of 80 kW. The air balance of plant (BOP) system utilized low speed scroll-type rotating compressors and brazed plate and frame heat exchangers for efficient preheating. The scroll compressors were modeled in GT-Suite and the remaining air BOP system was modeled with thermodynamic and heat transfer equations. Then testing was done on the compressors and heat exchangers to validate the model so that the air BOP system performance could be accurately predicted within a range of conditions. Both compressors were run from a range of 20 g/s to 60 g/s with the heat through the system being swept from 100°C to 600°C which yielded compressor efficiencies over 60% and heat exchanger effectiveness over 0.90. The validated model was then used to make predictions about system performance at on and off-design conditions.
Open Access
Improving the cold temperature properties of tallow-based methyl ester mixtures using fractionation, blending, and additives
(Colorado State University. Libraries, 2017) Elwell, Caleb, author; Marchese, Anthony J., advisor; Smith, T. Gordon, committee member; Windom, Bret C., committee member
Beef tallow is a less common feedstock source for biodiesel than soy or canola oil, but it can have economic benefits in comparison to these traditional feedstocks. However, tallow methyl ester (TME) has the major disadvantage of poor cold temperature properties. Cloud point (CP) is an standard industry metric for evaluating the cold temperature performance of biodiesel and is directly related to the thermodynamic properties of the fuel's consituents. TME has a CP of 14.5°C compared with 2.3°C for soy methyl ester (SME) and -8.3°C for canola methyl ester (CME). In this study, three methods were evaluated to reduce the CP of TME: fractionation, blending with SME and CME, and using polymer additives. TME fractionation (i.e. removal of specific methyl ester constituents) was simulated by creating FAME mixtures to match the FAME profiles of fractionated TME. The fractionation yield was found to be highest at the eutectic point of methyl palmitate (MP) and methyl stearate (MS), which was empirically determined to be at a MP/(MP+MS) ratio of approximately 82%. Since unmodified TME has a MP/(MP+MS) ratio of 59%, initially only MS should be removed to produce a ratio closer to the eutectic point to reduce CP and maximize yield. Graphs relating yield (with 4:1 methyl stearate to methyl oleate carryover) to CP were produced to determine the economic viability of this approach. To evaluate the effect of blending TME with other methyl esters, SME and CME were blended with TME at blend ratios of 0 to 100%. Both the SME/TME and CME/TME blends exhibited decreased CPs with increasing levels of SME and CME. Although the CP of the SME/TME blends varied linearly with SME content, the CP of the CME/TME blends varied quadratically with CME content. To evaluate the potential of fuel additives to reduce the CP of TME, 11 different polymer additives were tested. Although all of these additives were specifically marketed to enhance the cold temperature properties of petroleum diesel or biodiesel, only two of the additives had any significant effect on TME CP. The additive formulated by Meat & Livestock Australia (MLA) outperformed Evonik's Viscoplex 10-530. The MLA additive was investigated further and its effect on CP was characterized in pure TME and in CME/TME blends. When mixed in CME/TME blends, the MLA additive had a synergistic effect and produced lower CPs than the addition of mixing MLA in TME and blending CME with TME. To evalulate the cold temperature properties of TME blended with petroleum diesel, CPs of TME/diesel blends from 0 to 100% were measured. The TME/diesel blends were treated with the MLA additives to determine the effects of the additives under these blend conditions. The MLA additive also had a synergistic effect when mixed in TME/diesel blends. Finally, all three of the TME CP reduction methods were evaluated in an economic model to determine the conditions under which each method would be economically viable. Each of the CP reduction methods were compared using a common metric based on the cost of reducing the CP of 1 gallon of finished biodiesel by 1°C (i.e. $/gal/°C). Since the cost of each method is dependent on varying commodity prices, further development of the economic model (which was developed and tested with 2012 prices) to account for stochastic variation in commodity prices is recommended.
Open Access
Transient modeling of an ambient temperature source centrifugal compressor steam generating heat pump
(Colorado State University. Libraries, 2024) Ryan, Kelly Patrick, author; Bandhauer, Todd M., advisor; Windom, Bret C., committee member; Herber, Daniel R., committee member
As the US electricity grid transitions to renewable power generation, electrifying end-uses that are currently fossil fuel fired presents a promising path towards deeper decarbonization, and next generation high temperature heat pumps are a viable solution for decarbonizing fossil fuel fired steam boilers. These next-gen systems require a higher degree of design complexity and more finely tuned control strategies than existing systems, and therefore can benefit from complex transient modeling that has not been previously implemented for these types of systems. A transient study of a novel steam-generating heat pump with steam delivery temperature of 150°C was conducted using physics-based simulation software. The model used manufacturer supplied performance data to calibrate each competent, providing reliable preliminary validation to the model. The model was set up to match the configuration of a prototype system constructed at Colorado State University. It was found that the results of the transient model agreed well with the steady state model of the heat pump at the design point. Transient conditions including cold startup to full load operation, full load operation to part load operation, and part load operation back to full load operation were modeled and the system was found to operate with stability. Compressor and expansion valve performance was investigated. Compressors were found to operate within their performance maps for both steady and transient operation. A control strategy was developed for the expansion valves to prevent liquid ingestion when transitioning to turndown operation. The system COP was predicted for both full and part load operation and in transition between them.