Simulation of Absorption Cycles for Integration Into Refining Processes
by Christopher Michael Somers
The oil and gas industry is an immense energy consumer. Absorption chillers can be used to recover liquid natural gas (LNG) plant waste heat to provide cooling, which is especially valuable in the oil and gas industry and would also improve energy efficiency. This thesis details the modeling procedure for single and double effect water/lithium bromide and single effect ammonia/water chillers. Comparison of these models to published modeling results and experimental data shows acceptable agreement, within 5% for the water/lithium bromide models and within 7% for the ammonia/water model. Additionally, each model was integrated with a gas turbine as a waste heat source and parametric studies were conducted for a range of part load conditions, evaporator temperatures, and ambient conditions. Finally, the best chiller design was selected among the three evaluated here, and an annual performance study was conducted to quantify the expected cooling performance and related energy savings.
http://hdl.handle.net/1903/9394
Electrostatic Gas-Liquid Separation from High Speed Streams--Application to Advanced On-Line/On- Demand Separation Techniques
by Mohamed Saeed Alshehhi
The separation of suspended droplets from gases has been one of the basic scientific and technical problems of the industrial era and this interest continues. Various industrial applications, such as refrigeration and HVAC systems, require control of fine droplets concentrations in moving gaseous mediums to maintain system functionality and efficiency. Separating of such fine droplets can be achieved using electrostatic charging as implemented in electrostatic precipitators (ESPs). They use electrostatic force to charge and collect solid particles.
The objective of the present work was to study the feasibility of using wiretube electrostatic separator on the removal of fine water and oil droplets from air stream based on corona discharge ionization process. A parametric study was conducted to find key parameters affecting the separation process. This goal was approached by simulating the charging and separation phenomena numerically, and then verifying the modeling findings through experiments.
The numerical methodology simulated the highly complex interaction between droplets suspended in the flow and electrical field. Two test rigs were constructed, one for air-water separation and the other for air-oil separation. A wiretube electrostatic separator was used as the test section for both test rigs. The separation performance was evaluated under different electric field and flow conditions. Finally, based on the results, a novel air-water separator prototype was designed, fabricated and tested.
The numerical modeling results qualitatively showed acceptable agreement with the experimental data in terms of the trend of grade efficiency based on droplets size. Both numerical modeling results and experimental data showed that with a proper separator design, high separation efficiency is achievable for water and oil droplets. Based on the experimental data, at flow velocity of 5 m/s and applied voltage of 7.0 kV, the maximum separation efficiency for water and oil was 99.999% and 96.267%, respectively. The pressure drop was as low as 100 Pa and maximum power consumption was 12.0 W.
http://hdl.handle.net/1903/1000
Prediction of Heat Transfer and Pressure Drop of Condensing Refrigerant Flow in a High Aspect Ratio Micro-Channels
by Ebrahim Saeed Abdulla Al-Hajri
This thesis presents a detailed study of parametric characterization of two-phase condensing flow of two selected refrigerants R134a and R-245fa in a single water-cooled micro-channel of 0.4 mm X 2.8 mm cross-section (0.7 mm hydraulic diameter and 7:1 aspect ratio) and 190 mm in length. To avoid flow mal-distribution associated with typical micro-channel tube banks, a single micro-channel was fabricated utilizing an innovative approach and used for the present study experiments. The study investigated the effects of variations in saturation temperature ranging from 30 oC to 70 oC, mass flux from 50 to 500 kg/m2s, and inlet super heat from 0 oC to 15 oC on the average heat transfer and overall pressure drop coefficient of the micro-channel condenser. In all cases the inlet vapor quality was kept at 100% quality (saturated vapor) and the outlet condition was always kept at 0% quality (saturated liquid). Accuracy of the fabricated channel geometry with careful design and choice of instrumentation of the test setup resulted in energy balance and average heat transfer coefficient uncertainties within +/-11% and +/-12%, respectively. It is observed that saturation temperature and mass flux have a significant effect on both heat transfer coefficient and overall pressure drop coefficient, where as the inlet super heat has little effect. This study provides further understanding of the potential micro-scale effects on the condensation phenomenon for the tube geometry and the dimensions investigated in the present study combined with flow visualization study. No previous study has addressed the unique single micro-channel geometry being investigated in the present work combined with the two-phase flow visualization of the flow regimes in the present micro-channel geometry. The letter was a major undertaking of the present work and represents one of the main contributions of the present work. The results of the present work shall prove useful in contributing to better understanding of any micro-scale effects on the condensation flow of the two selected refrigerants (one commonly used high pressure refrigerant, R134a) and the other a new low pressure refrigerant (R245fa). It is also expected that the results of this study will lead to future work in this area, realizing the fast penetration of the micro-channel technology in various compact/ultra compact heat exchangers, including refrigeration, petrochemical, electronics, transportation, and process industries.
http://hdl.handle.net/1903/9845
Development of Multi-Scale, Multi-Physics, Analysis Capability and its Application to Novel Heat Exchanger Design and Optimization
by Omar A. Abdelaziz
Heat exchanger development using enhanced heat transfer surface designs and optimization techniques is a continuing effort that is constrained by current state of the art technology. Assessment of novel geometries and concepts are currently limited to experimental and numerical investigations on discrete levels. This dissertation aims at the advancement of the heat exchanger technology through the development of multi-scale multi-physics simulation tools for conventional and novel heat exchanger designs.
A unified heat exchanger design and optimization framework was developed. This framework integrates the multi-scale multi-physics simulation capabilities with previously developed approximation assisted optimization techniques. The optimized designs are then interpreted in order to provide design guidelines for next generation air-to-water heat exchangers. These capabilities required the development of: (a) ε — NTU solver capable of analyzing the performance under geometrical variability, (b) systematic integration approach for CFD simulation at the segment level with the ε — NTU solver at the heat exchanger level, (c) refrigerant distribution analysis tool. The developed simulation tools were verified numerically using systematic techniques adopted from literature and validated experimentally using measured data from a prototype heat exchanger. The structural integrity under conventional operating pressures of the novel heat exchanger design was analyzed using FEM for different tube materials and different wall thicknesses. Finally, existing single phase water flow in microtubes correlations were investigated numerically. The best matching correlation was selected for incorporation within the multi-scale simulation tool.
The approach described in this dissertation for the design and optimization of novel and conventional heat exchanger designs resulted in significant improvements over the current state of the art. Example performance improvements achieved in this dissertation show potential for 84 percent material savings and 61 percent volume savings for the same airside and refrigerant side pressure drop. The experimental investigations were in good agreement with the simulation results and demonstrated the superior performance of the novel design.
http://hdl.handle.net/1903/9566
Development of a Component Based Simulation Tool for the Steady State and Transient Analysis of Vapor Compression Systems
by Jonathan M. Winkler
Computer simulations have become a commonly used tool to aid engineers design and optimize vapor compression systems. Generally, the simulation of vapor compression heat pump systems falls into one of two categories; steady state and transient. Steady state simulations are typically very accurate and highly detailed, and used for design and optimization of vapor compression systems and components. Transient simulations generally utilize more assumptions to reduce complexity and computational time, and are used to design and evaluate control strategies. However, utilizing two separate simulation tools to perform steady state and transient simulations presents the challenge and burden of increased software development and maintenance effort and inconsistency in the predicted results.
This thesis presents a simulation tool that simultaneously serves the industry needs of an integrated steady state and transient vapor compression simulation tool. The tool is developed using a component-based architecture allowing for the users to incorporate in-house component models into the simulation and the component-based framework is discussed in detail. Particular emphasis is placed on transient heat exchanger simulation; resulting in an algorithm that reduces complex and detailed heat exchanger models into simplified, faster versions that still have sufficient accuracy so that transient and steady state results converge to the same performance under steady state conditions. Thus, consistency in the results between the steady state and transient simulations is preserved.
Nearly all pre-existing vapor compression system simulation tools are limited to the standard four component cycle. However, for enhanced efficiency and thermal comfort, multi-component systems are gaining in popularity. The component-based framework implemented by the simulation tool allows for the simulation of cycles with additional components.
During the development of the simulation tool, the need of a faster, more robust solution algorithm to solve a steady state vapor compression system became evident. Thus, a new solution algorithm was developed, thoroughly tested, and compared with existing solution techniques in the literature. An improvement of 50% was achieved in the solution algorithm's robustness. A more advanced method to determining initial guess values for steady simulations was developed and reduced the number of component evaluations by approximately 40%.
http://hdl.handle.net/1903/9493
Development of an Advanced Heat Exchanger Model for Steady State and Frosting Conditions
by Varun Singh
Air-to-refrigerant fin-and-tube heat exchangers are a key component in the heating, air conditioning and refrigeration industry. Considering their dominance, the industry has focused immensely on employing computer modeling in their design and development. Recently, advances in manufacturing capabilities, heat exchanger technology coupled with the move towards new environment-friendly refrigerants provide unprecedented challenges for designers and opportunities for researchers. In addition, the field of Computational Fluid Dynamics (CFD) has assumed a greater role in the design of heat exchangers.
This research presents the development of an advanced heat exchanger model and design tool which aims to provide greater accuracy, design flexibility and unparalleled capabilities compared to existing heat exchanger models. The heat exchanger model developed here achieves the following.
- Account for tube-to-tube conduction along fins, which is known to degrade the performance of heat exchangers, especially in carbon dioxide gas coolers
- Study and develop heat exchangers with arbitrary fin sheets, which meet performance as well as packaging goals with minimal consumption of resources
- Allow engineers to integrate CFD results for air flow through a heat exchanger, which the modeling tool employs to develop its air propagation sequence leading to improved accuracy over existing models which assume normal air flow propagation
- Function in a quasi-steady state mode for the purpose of simulating frost accumulation and growth on heat exchangers, and completely simulate local heat transfer degradation, as well as blockage of flow passage on air side
Additionally, the heat exchanger model was used to investigate gains that are enabled due to the presence of cut fins in carbon dioxide gas coolers and develop design guidelines for engineers. Finally, this dissertation analyzes the implications of minimum entropy generation on heat exchanger performance criteria of heat capacity and pressure drop, as well as evaluates the ability of entropy generation minimization as a design objective. This also serves as the first step toward an expert knowledge-based system for guiding engineers towards better designs, during the process of heat exchanger design.
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