Improvement of small channel heat transfer correlation using genetic algorithm for R290 refrigerant

The primary issues among the discussions on two-phase flow in small channels are the uncertainties about the contributions of nucleate boiling and forced convective towards the total two-phase heat transfer coefficient, the accuracy of the predicted two-phase heat transfer coefficient which remains unsatisfactory, measured by the mean absolute error (MAE) between the correlation and experimental data, particularly that can accommodate pre-and post-dryout regions, and the limited experimental work for alternative refrigerants for the establishment of related correlations for a specific refrigerant. This thesis presents the results obtained using an optimization approach, Multi-objective Genetic Algorithm (MOGA) to show the conflicting effect of nucleate boiling and forced convective during two-phase flow of the natural refrigerant R290 in a small channel at the saturation temperature of 10°C under optimized conditions of mass flux, heat flux, channel diameter, and vapor quality. Subsequently, Single Objective Genetic Algorithm (SOGA) was utilized to improve a selected superposition two-phase heat transfer correlation for R290. Experimental data points of R290 from reported experiments in 1.0 to 6.0 mm circular diameters were used to minimize the MAE while searching for the optimum constants and coefficients in the suppression factor (??), and convective factor (??), for the pre-and the post-dryout regions. The newly optimized correlation for R290 has MAE between 17 and 34% for all case studies which involves 40% improvement from the original correlation. Validation was done against a new data set to see the applicability and limitation of the developed correlations. The proposed method is capable of obtaining a precise empirical prediction that fits well with experimental data, as an approach to further improve any existing correlations which can reduce the number of experiments and consequently minimizes associated cost involved. The improved correlation obtained in the present study provides an improved prediction of heat transfer coefficient that in turn leads to accurate design and consequently saves material, refrigerant, and cost for compact heat exchanging devices

Thermal and Hydraulic Performance of Heat Exchangers for Low Temperature Lift Heat Pump Systems

The work presented in this dissertation focused on investigating and understanding the hydraulic and thermal design space and tradeoffs for low temperature difference high performance heat exchangers for a low temperature lift heat pump (LTLHP) system, which benefits from a small difference between the condensing and evaporating temperatures of a working fluid. The heat exchangers for the LTLHP application require a larger heat transfer area, a higher volume flow rate, and a higher temperature of heat source fluid, as compared to the typical high temperature lift heat pump system. Therefore, heat exchanger research is critical, and it needs to be balanced between the heat transfer and pressure drop performance of both fluids in the heat exchanger. A plate heat exchanger (PHX) was selected to establish a baseline of a low temperature lift heat exchanger and was investigated experimentally and numerically. The traditional PHX is designed to have the identical surface area and enhancements on both fluid sides for ease of production. However, fluid side heat transfer coefficients and heat transfer capacities can be drastically different, for example, single-phase water versus two-phase refrigerant. Moreover, the PHX needs to have a large cross sectional flow area in order to reduce the heat-source fluid-side pressure drop. In the experimental test, the PHX showed a relatively low overall heat transfer performance and a large pressure drop of the heat source fluid side under LTLHP operating conditions. The CFD simulation was carried out to further improve the potential of the PHX performance. However, there were limitations in the PHX. It was concluded that the PHX was restricted by two main factors: one was a large pressure drop on the heat source fluid-side due to corrugated shape, and the other was low overall heat transfer performance due to the low refrigerant-side mass flux and resulting low heat transfer performance. A concept of a novel low temperature lift heat exchanger (LTLHX) has been developed based on the lessons learned from the PHX performance investigation for the application to the LTLHP. Geometries were newly defined such as a channel width, channel height, channel pitch, and plate flow gap. Two design strategies were applied to the novel heat exchanger development: the flow area ratio was regulated, and plates were offset. The design parameters of the novel heat exchanger were optimized with multi scale approaches. After developing the laboratory heat exchanger test facility and the prototype of the novel LTLHX, its performance was experimentally measured. Then the thermal and hydraulic performance of the novel LTLHX was validated with experimental data. The heat transfer coefficient correlations and the pressure drop correlations of both the water-side and refrigerant-side were newly developed for the novel LTLHX. The overall heat transfer performance of the novel LTLHX was more than doubled as compared to that of the PHX. Moreover, the pressure drop of the novel heat exchanger was drastically lower than that of the PHX. Lastly, the novel heat exchangers were applied to a water source heat pump system, and its performance was investigated with parametric studies

Effect of wall resistance on the total thermal resistance of a stacked microchannel heat sink

This paper reports on the different modeling approach of the total thermal resistance in a microchannel heat sink (MCHS); with wall resistance and the frequently used fin model, in comparison with experimental results. For a single stack MCHS, the wall model caused more than 10% difference but it can be extended to a stacked MCHS while the fin model could not, due to the adiabatic top condition. The wall resistance model is idealized, assuming a 100% efficient convective heat transfer while in the fin model 70% was the maximum. Meanwhile, stacking showed that at a constant flow rate, the thermal resistance could be reduced by 3% for a double stack, while increasing beyond that will decrease the thermal performance of the MCHS. The study showed the limits of models used and possible stacking of a MCHS for improved heat removal capability

NUMERICAL MODELING AND OPTIMIZATION OF SINGLE PHASE MANIFOLD-MICROCHANNEL PLATE HEAT EXCHANGER

In recent years manifold-microchannel technology has received considerable attention from the research community as it has demonstrated clear advantage over state of the art heat exchangers. It has the potential to improve heat transfer performance by an order of magnitude while reducing pressure drop penalty equally impressive, when compared to state of the art heat exchangers for selected applications. However, design of heat exchangers based on this technology requires selection of several critical geometrical and flow parameters. This research focuses on the numerical modeling and an optimization algorithm to determine such design parameters and optimize the performance of manifold-microchannels for a plate heat exchanger geometry. A hybrid method was developed to calculate the total pumping power and heat transfer of this type of heat exchangers. The results from the hybrid method were successfully verified with the results obtained from a full CFD model and experimental work. Based on the hybrid method, a multi-objective optimization of the heat exchanger was conducted utilizing an approximation-based optimization technique. The optimized manifold-microchannel flat plate heat exchanger showed superior performance over a Chevron plate heat exchanger which is a wildly used option for diverse applications

Optimised self-calibrating microfluidic systems towards design optimisation

Clean water is a finite resource, and the quality of such is best monitored by colorimetric in-situ sensors, which allow frequent, non-labour intensive sampling, and are low-cost and simple to manufacture. There are multiple types of sensors that exist in the literature, however, many are cost-prohibitive for wide deployment, or the literature does not not fully elaborate on their operation. The aim of this research was to extend the lifetime and improve the performance of a colorimetric in-situ sensor, Aquamonitrix colorimetric sensor, that was produced by T.E. Laboratories, in addition to characterising sensor behaviour. Its operation was focused on the Griess reaction, in which a vivid azo dye is produced in the presence of nitrite, that can be linearly calibrated to the absorbance by the dye from a photodectector placed at the opposite end of a microfluidic detector channel to a monochromatic light source. Using multiobjective optimisation on a numerical model of a Y-junction micromixer, it was found that both sensitivity could be increased and reagent could be conserved, by limiting the proportion of reagent used during testing to 5% to 7.5% of testing solution, as opposed to the 50% originally used by the system. The conservation of the reagent allowed for an increased sensor deployment lifetime of up to tenfold. To better understand how the parameters of analyte concentration, reagent proportion of test solution, and mean flow velocity of the solution affect sensor output, both mechanistic and data-based modelling of the continuous and stopped flow stages of the sensor were undertaken. Third-order and second-order models were identified for the continuous and stopped flow data respectively. The second-order model is analagous to the two-step Griess reaction, of which there is a first, faster step. Further characterisation of the zeroes, poles and transfer function coefficients of the third order models showed that parameterisation was possible and, using principal component analysis, reduction of parameters. Other testing on the effects of order of cycles, turbidity and heavy metals was also conducted to measure their impact on sensor output. Carryover between sensor cycles was found to be the most interfering factor on sensor output, due to the microfluidic connector components, which was eliminated after eight cycles. Overall, the performance and efficiency of the existing sensor was improved, iii and the methodologies in this dissertation can be used for other continuous-flow colorimetric sensors and reactions, or even other microreactor applications, such as in green chemistry

Flow-Based Optimization of Products or Devices

Flow-based optimization of products and devices is an immature field compared to the corresponding topology optimization based on solid mechanics. However, it is an essential part of component development with both internal and/or external flow. The aim of this book is two-fold: (i) to provide state-of-the-art examples of flow-based optimization and (ii) to present a review of topology optimization for fluid-based problems

Методологија за синтезу реактора заснована на концептима интензификације процеса и примени метода оптимизације

In this Ph.D. thesis, a new methodology for Reactor Synthesis Based on Process Intensification Concepts and Application of Optimization Methods (ReSyPIO) is presented and applied to two different cases. In Chapter 1: Introduction – Motivation and Objectives, the motive for the research is presented, and Hypotheses are formulated. The ReSyPIO methodology that rests upon these Hypotheses and consists of three consecutive stages is briefly described in this Chapter. The first stage encapsulates all present phases and phenomena inside the reactor functional building block, called module. Modules come as a direct result of a conceptual representation of the analyzed system. In the second stage, modules are further segmented if needed and interconnected, creating a reactor superstructure that is mathematically described for all desirable operating regimes. In the last stage of the ReSyPIO methodology, the optimal structure, operating conditions, and the operational regime are determined with the use of rigorous optimization. All three stages of the ReSyPIO methodology have a backflow, meaning that if analysis leads to impractical, nonfunctional or inefficient results, modifications in reactor superstructure and modules can be made. The objective is to conceptually and numerically derive the most efficient reactor structure and a set of operating conditions that would be used as a starting point in the future reactor design. Chapter 2: Literature Review is used to cover and review the most important research published in the area of Process Intensification and different Process System Engineering techniques. Different approaches and studies present in academia are highlighted and their elements compared with the presented ReSyPIO methodology with the accent on its advantages and contribution to the engineering science community.Also, in this Chapter, an array of well researched analytical and numerical approaches is presented that could be used in the future to strengthen the ReSyPIO methodology further and facilitate its easier application. In Chapter 3: Description of the ReSyPIO Methodology Reactor Synthesis based on Process Intensification and Optimization of Superstructure is explained in detail, with a graphical representation of the main building block, called Phenomenological Module. A general explanation is given on how to form a reactor superstructure and mathematically describe it with sets of material and energy balance equations that correspond to a number of present phases and components in the system. The ReSyPIO methodology is first applied to a generic case of two parallel reactions in Chapter 4, called Application of the ReSyPIO Methodology on a Generic Reaction Case. The case corresponds to two parallel reactions that could be found in the fine chemical industry. The reactions are endothermic and slow with the undesired product. After the application of the ReSyPIO methodology, an optimal reactor structure consisting of a segmented module with 17 side inlets for the reactant and heat source is obtained. It is recommended for the reactor to work in a continuous steady-state mode as the dynamic operation would not lead to a sufficient increase in reactor efficiency...У овој докторској дисертацији је представљена и примењена нова методологија за синтезу реактора заснована на концептима интензификације процеса и примени различитих оптимизационих техника (Reactor Synthesis Based on Process Intensification Concepts and Application of Optimization Methods – ReSyPIO). У поглављу Увод – Мотивација и циљеви, формиране су хипотезе на којима почива ReSyPIO методологија и дата је мотивација за истраживање. ReSyPIO методологија је укратко представљена и описана кроз три узастопне етапе. Прва етапа уоквирава све присутне фазе и феномене у реактору унутар функционалних градивних јединица, названих модули. Модули представљају резултат концептуалног приказа анализираног система. У другој етапи, модули се по потреби могу даље поделити у сегменте и међусобно повезати, креирајући суперструктуру реактора. Суперструктура је математички описана за све режиме рада реактора од интереса. У последњој етапи ReSyPIO методологије, оптимална структура, услови и режим рада реактора су одређени применом ригорозне оптимизације. Све три етапе ReSyPIO методологије имају повратни ток, што значи да уколико анализа води ка непрактичним, нефункционалним или неефикасним решењима, модификација математичког модела, суперструктуре и/или модула је могућа. Циљ примене ReSyPIO методологије је да се концептуалним и нумеричким приступом дође до оптималне препоруке за структуру реактора, оперативне услове и режим рада, која би била почетна претпоставка у будућем дизајну уређаја. Преглед литературе даје опис и приказ свих истраживања од интереса, из области Интензификације процеса и Теорије и анализе процесних система. Наглашени су различити приступи и студије присутне у истраживачкојзаједници, а њихови елементи упоређени са представљеном ReSyPIO методологијом са акцентом на предностима и научном доприносу. У овом поглављу је дат и низ добро истражених аналитичких и нумеричких приступа који би могли да буду коришћени у оквиру ReSyPIO методологије и олакшају њену примену. У поглављу Опис ReSyPIO методологије, је детаљно објашњена синтеза реактора заснована на концептима интензификације процеса и оптимизацији суперструктуре. Прво је дата процедура за графичку и концептуалну репрезентацију система, преко главних градивних јединица, феноменолошких модула. Потом је објашњено како се креира суперструктура реактора. На крају је дат уопштен поступак за математички опис суперструктуре преко скупова једначина материјалног и енергетског биланса, чији број зависи од броја присутних фаза и компонената у систему. ReSyPIO методологија је први пут примењена на случају две генеричке паралелне реакције у поглављу под називом Примена ReSyPIO методологије на случају генеричке реакције. Овај случај одговара реакцијама које се могу наћи у индустрији финих хемикалија. Реакције су ендотермне и споре, при чему је кинетички фаворизовано креирање нежељеног производа. Након примене ReSyPIO методологије, добијена је оптимална структура реактора која се састоји од сегментисаног модула са 17 улаза за извор топлоте и реактант који се дозира. Предложено је да реактор ради континуално, у стационарном режиму рада, јер би динамички режим рада резултовао недовољним повећањем ефикасности реактора..

IN-SITU ADDITIVE MANUFACTURING OF METALS FOR EMBEDDING PARTS COMPATIBLE WITH LIQUID METALS TO ENHANCE THERMAL PERFORMANCE OF AVIONICS FOR SPACECRAFT

With advances in micromachinery, the aggregation of sensors, and more powerful microcontroller platforms on satellites, the size of avionics for space missions are getting dramatically smaller with faster processing speeds. This has resulted in greater localized heat generation, requiring more reliable thermal management systems to enhance the thermal performance of the avionics. The emergence of advanced additive manufacturing (AM), such as selective laser melting (SLM) and engineering materials, such as low-melting eutectic liquid metal (LM) alloys and synthetics ceramics offer new opportunities for thermal cooling systems. Therefore, there has been an opportunity for adapting in-situ AM to overcome limitations of traditional manufacturing in thermal application, where improvements can be achieved through reducing thermal contract resistance of multi-layer interfaces. This dissertation investigates adapting in-situ AM technologies to embed LM compatible prefabricated components, such as ceramic tubes, inside of metals without the need for a parting surface, resulting in more intimate contact between the metal and ceramic and a reduction in the interfacial thermal resistance. A focus was placed on using more ubiquitous powder bed AM technologies, where it was determined that the morphology of the prefabricated LM compatible ceramic tubes had to be optimized to prevent collision with the apparatus of powder bed based AM. Furthermore, to enhance the wettability of the ceramic tubes during laser fusion, the surfaces were electroplated, resulting in a 1.72X improvement in heat transfer compared to cold plates packaged by conventional assembly. Additionally, multiple AM technologies synergistically complement with cross platform tools such as magnetohydrodynamic (MHD) to solve the corrosion problem in the use of low melting eutectic alloy in geometrically complex patterns as an active cooling system with no moving parts. The MHD pumping system was designed using FEA and CFD simulations to approximate Maxwell and Navier-Stokes equations, were then validated using experiments with model heat exchanger to determine the tradeoff in performance with conventional pumping systems. The MHD cooling prototype was shown to reach volumetric flow rates of up to 650 mm3/sec and generated flow pressure due to Lorentz forces of up to 230 Pa, resulting in heat transfer improvement relative to passive prototype of 1.054

CFD Modeling of Complex Chemical Processes: Multiscale and Multiphysics Challenges

Computational fluid dynamics (CFD), which uses numerical analysis to predict and model complex flow behaviors and transport processes, has become a mainstream tool in engineering process research and development. Complex chemical processes often involve coupling between dynamics at vastly different length and time scales, as well as coupling of different physical models. The multiscale and multiphysics nature of those problems calls for delicate modeling approaches. This book showcases recent contributions in this field, from the development of modeling methodology to its application in supporting the design, development, and optimization of engineering processes