Airside Passive Heat Transfer Enhancement, Using Multi-Scale Analysis and Shape Optimization, For Compact Heat Exchangers With Small Characteristic Lengths
by Daniel Fernandes Bacellar
The study of compact heat exchangers (HX) is a very common, although broad topic that draws interest from many engineering applications. Most technologies contain at least one HX serving as a fundamental component for the proper system functioning. The rapid worldwide population growth, increasing demand for energy resources, widespread environmental concerns, space exploration efforts and economy are all good reasons for developing smaller, lighter and more efficient HX’s. This research sheds the light on the next generation of heat exchangers, with a focus on air-to-fluid applications.
For incompressible flows and low-pressure applications, the HX’s airside thermal resistance is the major limitation to overall thermal conductance. On conventional surfaces fins are required, but bring many drawbacks. Among these include being prone to fouling/frosting, reduced heat transfer coefficient, higher friction resistance, and more material consumption. Tubes by nature provide more valuable heat transfer than do fins; there is little focus on tubes in the literature.
The first objective of this work is to discuss the fundamental aspects of primary (tubes) and secondary (fins) surfaces, with the aid of numerical analyses. The latter demonstrates how the reduction of characteristic length and novel shapes impact surface performance and compactness of finless and finned tubes. A further discussion is presented arguing that conventional fin concepts are not always beneficial.
The second objective of this work entails developing a comprehensive multi-scale analysis with topology and shape optimization methodology leveraging automated CFD simulations and approximation assisted optimization. Novel finless air-to-fluid HX concepts were developed, for single-phase and two-phase applications, and achieved more than 20% reduction in size, 20% better performance and 20% less material than state-of-the-art HX’s including microchannel HX’s. Two prototypes (one manufactured in metal 3D printing) were tested in an in-house wind tunnel. The numerical predictions agree with the experimental results in less than 5% deviation for total capacity, 10% for airside heat transfer coefficient and 20% for air pressure drop.
Finally, the last objective is to present the development of robust and computationally inexpensive tools that can accurately predict CFD simulation responses for conventional tube and fin surfaces using small diameter tubes (<5.0mm).
Doctoral Dissertation
http://hdl.handle.net/1903/19066
Steady State Modeling and Optimization for Performance and Environmental Impact of Advanced Vapor Compression Systems
by Mohamed Beshr
The use of heating, ventilation, air conditioning, and refrigeration (HVACR) systems is always increasing. This is because the HVACR systems are necessary for food production and ability to inhabit buildings that otherwise would be inhabitable. The basic vapor compression (VC) cycle which is still the main underlying HVACR technology worldwide, has already reached its limits and researchers are investigating more creative and complex cycles to improve capacity and efficiency. This motivates the development of a generalized vapor compression system simulation platform.
This thesis presents a comprehensive vapor compression system steady state solver which has several novel features compared to the existing solvers. Firstly, this solver is capable of simulating large number of different vapor compression system designs. This includes system configurations comprising more than 500 components, multiple air and refrigerant paths, and user defined refrigerants. Also, the solver uses a component-based solution scheme in which the component models are treated as black box objects. This allows a system engineer to quickly assemble and simulate a system where-in the component models and performance data comes from disparate sources. This allows different vapor compression systems design engineers, and manufacturers to use the solver without the need to expose the underlying component model complexities. We validate the solver using a residential air source heat pump system, a vapor injection heat pump system with a flash tank, and a CO2 two-stage supermarket refrigeration system with mechanical subcooler.
Moreover, designing a HVACR system while primarily considering its environmental impact requires an evaluation of the system's overall environmental impact as a function of its design parameters. The most comprehensive metric proposed for this evaluation is the system's Life Cycle Climate Performance (LCCP). Hence, this thesis presents an open-source and modular framework for LCCP based design of vapor compression systems. This framework can be used for, not only evaluation, but also LCCP-based design and optimization of vapor compression systems to minimize the environmental impact of such systems. Furthermore, the framework provides insights into various other challenges such as selection of appropriate systems for various climates and the choice of next generation lower global warming potential (GWP) refrigerants.
Doctoral Dissertation
http://hdl.handle.net/1903/19053
Modeling and Optimization of Microgrid Energy System for Ship Applications
by Tao Cao
Microgrid energy systems are widely used in remote communities and off-grid sites, where primary energy supplies are dominated by fuels. Limited attentions have been paid to ship applications, which require thorough and in-depth research to address their unique challenges and increasing pressure on reducing fuel consumptions. This dissertation presents comprehensive microgrid system studies for ship applications in four aspects: component modeling and study, dynamic system modeling on novel designs, novel optimization based system design framework development and investigations on two enhancement options: integrating with separate sensible and latent cooling systems, maximizing heat recovery through pinch analysis.
Comprehensive component studies consist of new component models addressing unique features of ship applications. Desiccant wheels with new materials were investigated experimentally, especially under high humidity conditions for ship applications. Dynamic system modeling was conducted on several novel solar energy and waste heat powered systems, with a focus on their capabilities to reduce fuel consumptions and CO2 emissions. Results were validated against experimental data. Payload and economic studies were conducted to evaluate feasibilities of applying the designs to ship applications. A novel optimization based design framework was then developed. The framework is capable of conducting both system configuration and control strategy optimization under transient weather and load profiles, differentiating itself with current control strategy focused energy system optimization studies (Jradi and Riffat, 2014). It also extends Buoro et al. (2012)’s study on system configuration optimization to complete design from scratch with comprehensive equipment selections and integrating options. The design framework was demonstrated through a case study on container ships. Optimized systems and control strategies were found from three different scenarios: single-objective optimization, bi-objective optimization and optimization under uncertainty. Finally, two previously listed options were investigated to enhance microgrid system performance regarding thermal comfort and fuel savings.
This research fills current research gaps on microgrid energy system for ship applications. It also serves as the basis for advanced microgrid system analysis framework for any applications. The dynamic system modeling platform, optimization based design framework and enhancement methods can help engineers develop and evaluate ultra-high efficiency designs, aiming to reduce energy consumptions and CO2 emissions.
Doctoral Dissertation
http://hdl.handle.net/1903/19061
Air Side Heat Transfer Enhancement in Heat Exchangers Utilizing Innovative Designs and the Additive Manufacturing Technique
by Martinus Adrian Arie
Over the last decade, rapid development of additive manufacturing techniques has allowed the fabrication of innovative and complex designs. One field that can benefit from such technology is heat exchanger fabrication, as heat exchanger design has become more and more complex due to the demand for higher performance particularly on the air side of the heat exchanger. By employing the additive manufacturing, a heat exchanger design was successfully realized, which otherwise would have been very difficult to fabricate using conventional fabrication technologies. In this dissertation, additive manufacturing technique was implemented to fabricate an advanced design which focused on a combination of heat transfer surface and fluid distribution system. Although the application selected in this dissertation is focused on power plant dry cooling applications, the results of this study can directly and indirectly benefit other sectors as well, as the air-side is often the limiting side for in liquid or single phase cooling applications.
Two heat exchanger designs were studied. One was an advanced metallic heat exchanger based on manifold-microchannel technology and the other was a polymer heat exchanger based on utilization of prime surface technology. Polymer heat exchangers offer several advantages over metals such as antifouling, anticorrosion, lightweight and often less expensive than comparable metallic heat exchangers. A numerical modeling and optimization were performed to calculate a design that yield an optimum performance. The optimization results show that significant performance enhancement is noted compared to the conventional heat exchangers like wavy fins and plain plate fins. Thereafter, both heat exchangers were scaled down and fabricated using additive manufacturing and experimentally tested. The manifold-micro channel design demonstrated that despite some fabrication inaccuracies, compared to a conventional wavy-fin surface, 15% - 50% increase in heat transfer coefficient was possible for the same pressure drop value. In addition, if the fabrication inaccuracy can be eliminated, an even larger performance enhancement is predicted. Since metal based additive manufacturing is still in the developmental stage, it is anticipated that with further refinement of the manufacturing process in future designs, the fabrication accuracy can be improved. For the polymer heat exchanger, by fabricating a very thin wall heat exchanger (150μm), the wall thermal resistance, which usually becomes the limiting side for polymer heat exchanger, was calculated to account for only up to 3% of the total thermal resistance. A comparison of air-side heat transfer coefficient of the polymer heat exchanger with some of the commercially available plain plate fin surface heat exchangers show that polymer heat exchanger performance is equal or superior to plain plate fin surfaces. This shows the promising potential for polymer heat exchangers to compete with conventional metallic heat exchangers when an additive manufacturing-enabled fabrication is utilized.
Doctoral Dissertation
http://hdl.handle.net/1903/18369
Battery Powered Portable Vapor Compression Cycle System With PCM Condenser
by Yilin Du
A battery powered air-conditioning device was developed to provide an improved thermal comfort level for individuals in inadequately cooled environments. This device is a battery powered air-conditioning system with the phase change material (PCM) for heat storage. The condenser heat is stored in the PCM during the cooling operation and is discharged while the battery is charged by using the vapor compression cycle as a thermosiphon loop. The main focus of the current research was on the development of the cooling system. The cooling capacity of the vapor compression cycle measured was 165.6 W with system COP at 2.85. It was able to provide 2 hours cooling without discharging heat to the ambient. The PCM was recharged in nearly 8 hours under thermosiphon mode.
Master's Thesis
http://hdl.handle.net/1903/18378
Evaluation of an Extended Duct Air Delivery System for Spaces Conditioned by Rooftop Units
by Ryan Kennett
Traditional air delivery to high-bay buildings involves ceiling level supply and return ducts that create an almost-uniform temperature in the space. Problems with this system include potential recirculation of supply air and higher-than-necessary return air temperatures. A new air delivery strategy was investigated that involves changing the height of conventional supply and return ducts to have control over thermal stratification in the space. A full-scale experiment using ten vertical temperature profiles was conducted in a manufacturing facility over one year. The experimental data was utilized to validated CFD and EnergyPlus models. CFD simulation results show that supplying air directly to the occupied zone increases stratification while holding thermal comfort constant during the cooling operation. The building energy simulation identified how return air temperature offset, set point offset, and stratification influence the building’s energy consumption. A utility bill analysis for cooling shows 28.8% HVAC energy savings while the building energy simulation shows 19.3 – 37.4% HVAC energy savings.
Master's Thesis
http://hdl.handle.net/1903/18379
Embedded Two-Phase Cooling of High Flux Electronics Via Micro-Enabled Surfaces and Fluid Delivery System (FEEDS)
by Raphael Kahat Mandel
A novel cooler utilizing thin Film Evaporation on micro-Enabled surfaces and fluid Delivery System (FEEDS) is embedded into silicon die with the goal of achieving the metrics proposed by the Defense Advanced Research Project Agency’s (DARPA) ICECool fundamentals program: a heat flux of 1 kW/cm2 at superheats below 30 K, vapor qualities above 90%, pressure drops below 10% of absolute pressure, and heat densities of 1 kW/cm3. Preliminary models were used to investigate the various physical phenomena affecting two-phase flow in manifold-microchannels, including nucleate boiling, flow regime, annular film evaporation, void fraction, single-phase fully developed and developing forced convection, intra-microchannel flow distribution, and fin conduction. The various physical phenomena were then combined into a novel “2.5-D” microchannel model, which uses boundary layer assumptions and simplifications to model the 3-D domain with a 2-D mesh. The custom-coded microchannel model was first validated by comparing single-phase thermal and hydrodynamic performance to a 3-D laminar flow simulation performed in ANSYS Fluent with errors of less than 5% as long as the flow remains two-dimensional. Two-phase validation was conducted by comparing past experimental data to model predictions, and found to provide heat transfer predictions that were qualitatively accurate and correct in order of magnitude, and pressure drop predictions accurate to within 30%. A parametric study was then performed in order to arrive at a baseline geometry for meeting the ICECool metrics.
A system level model was created to select the working fluid, and a manifold model was created to evaluate manifold flow configuration. A novel flow configuration capable of providing an even inter-microchannel flow distribution in two-phase mode was proposed, and a working manifold designed for the baseline geometry. Experiments with a press-fit FEEDS chip were then conducted, obtaining heat fluxes in excess of 1 kW/cm2 at 45% vapor quality. The volume of the FEEDS assembly was then reduced by bonding a chip directly to a FEEDS manifold. A bonding apparatus capable of providing a uniform and conformal clamping force was designed, fabricated, and used to hermetically bond the manifold to the chip. Three bonded chips were then tested, obtaining a maximum heat flux of 700 W/cm2 at vapor qualities approaching 30% and a heat density of 220 W/cm3.
Doctoral Dissertation
http://hdl.handle.net/1903/18854
Performance of Plate Type Heat Exchanger as Ammonia Condenser
by Andrew Rivera
In this study, I experimentally analyzed the performance of a commercial semi-welded plate type heat exchanger (PHE) for use with ammonia systems. I determined performance parameters such as overall heat transfer coefficient, capacity, and pressure drop of the semi-welded PHE. This was analyzed by varying different parameters which demonstrated changes in overall heat transfer coefficient, capacity, and pressure drop. Both water and ammonia flow rates to the semi-welded PHE were varied independently, and analyzed in order to understand how changes in flow rates affected performance. Inlet water temperature was also varied, in order to understand how raising condenser water inlet temperature would affect performance. Finally, pressure drop was monitored to better understand the performance limitations of the semi-welded PHE. Testing of the semi-welded will give insight as to the performance of the semi-welded PHE in a potential ocean thermal energy conversion system, and whether the semi-welded PHE is a viable choice for use as an ammonia condenser.
Master's Thesis
http://hdl.handle.net/1903/18629
Harmonization of Life Cycle Climate Performance and Its Improvements for Heat Pump Applications
by Sarah Virginia Troch
Life Cycle Climate Performance (LCCP) is an evaluation method by which heating, ventilation, air conditioning and refrigeration systems can be evaluated for their global warming impact over the course of their complete life cycle. LCCP is more inclusive than previous metrics such as Total Equivalent Warming Impact. It is calculated as the sum of direct and indirect emissions generated over the lifetime of the system “from cradle to grave”. Direct emissions include all effects from the release of refrigerants into the atmosphere during the lifetime of the system. This includes annual leakage and losses during the disposal of the unit. The indirect emissions include emissions from the energy consumption during manufacturing process, lifetime operation, and disposal of the system. This thesis proposes a standardized approach to the use of LCCP and traceable data sources for all aspects of the calculation. An equation is proposed that unifies the efforts of previous researchers. Data sources are recommended for average values for all LCCP inputs. A residential heat pump sample problem is presented illustrating the methodology. The heat pump is evaluated at five U.S. locations in different climate zones. An excel tool was developed for residential heat pumps using the proposed method. The primary factor in the LCCP calculation is the energy consumption of the system. The effects of advanced vapor compression cycles are then investigated for heat pump applications. Advanced cycle options attempt to reduce the energy consumption in various ways. There are three categories of advanced cycle options: subcooling cycles, expansion loss recovery cycles and multi-stage cycles. The cycles selected for research are the suction line heat exchanger cycle, the expander cycle, the ejector cycle, and the vapor injection cycle. The cycles are modeled using Engineering Equation Solver and the results are applied to the LCCP methodology. The expander cycle, ejector cycle and vapor injection cycle are effective in reducing LCCP of a residential heat pump by 5.6%, 8.2% and 10.5%, respectively in Phoenix, AZ. The advanced cycles are evaluated with the use of low GWP refrigerants and are capable of reducing the LCCP of a residential heat by 13.7%, 16.3% and 18.6% using a refrigerant with a GWP of 10. To meet the U.S. Department of Energy’s goal of reducing residential energy use by 40% by 2025 with a proportional reduction in all other categories of residential energy consumption, a reduction in the energy consumption of a residential heat pump of 34.8% with a refrigerant GWP of 10 for Phoenix, AZ is necessary. A combination of advanced cycle, control options and low GWP refrigerants are necessary to meet this goal.
Master's Thesis
http://hdl.handle.net/1903/18386
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