Electrohydrodynamics in convective boiling heat exchangers has been studied since the early 1990’s and has been shown to result in a large variation in the average performance enhancement of these systems. The behaviour of EHD assisted convective boiling heat exchangers, is still largely unpredictable owing to a number of conflicting parameters which are rarely kept constant in empirical studies, i.e. flow pattern and heat flux. In this thesis, it is hypothesised that by reducing the number of confounding variables in the experimental test conditions, and understanding the behaviour of EHD in convective boiling systems from a flow pattern dependent point of view, this can allow for the development of flow pattern dependent experimental correlations & numerical models to develop a methodology for performance prediction, control strategies and system integration for an EHD assisted convective boiling heat exchange device. A 30 cm long, smooth, concentric, annular test section is used to analyse the effect of EHD on convective boiling performance under constant flow pattern, constant, low heat flux, and negligible free charge conditions. Saturated boiling conditions for flow-rates between 60 kg/m2s and 180 kg/m2s and thermodynamic quality range of 0.25 - 0.55 were tested. Heat transfer enhancement ranged from 0.95 to 2.3 fold and pressure drop penalty varied from 1.4-3 fold over these test conditions. The local EHD behaviour was found to be more consistent along the axial length of the test section compared to empirical data in the literature, which uses much longer test section lengths, where flow pattern can vary. An experimental database of EHD convective boiling data for horizontal annular electrode geometries was compiled to be used for analysis purposes. The performance of the heat exchanger in both free-field and high voltage conditions could be explained by looking at the flow patterns in each case. Electrostatic modelling was used to determine electric field strength distributions and interfacial stress due to the dielectrophoretic and electrostriction forces on the liquid vapour interface, which induce liquid extraction based flow pattern re-distribution in two phase dielectric flows. A fully coupled 2D, adiabatic numerical model for the effect of the electric body force on two phase flow pattern distribution was developed. Charge was neglected in this model. Two different models for the interface migration were used and compared; a moving mesh (MM) interface tracking model and a volume of fluid (VOF) interface tracking mode. Both were verified against published experimental data. For the liquid extraction verification case, the VOF model suffers interface stretching up to 300% resulting in a 42% slower extraction time and underestimated forces. However, it is useful to use the VOF model when simulating complex flow patterns which are subject to topological changes like bubble detachment or droplet coalescence as these cannot be simulated with the moving mesh model. The moving mesh model can be used to determine the error in forces and phase velocities when using the VOF model. A methodology for generating two-phase EHD flow pattern maps was developed by incorporating the electric Froude number into each of the flow pattern transition equations. A semi-analytical model was developed to determine the maximum interfacial stress due to EHD for stratified flows to reduce the requirement of numerical modeling, and thus the flow pattern map generation methodology is fully equation based. Although transition equations developed by multiple researchers were used and compared, it is recommended that the Steiner transitions equations be used for EHD two-phase flow pattern mapping, until more fundamental experimental data can be gathered to modify the semi-empirical transition equations used in more state-of-the-art maps. EHD was found to significantly affect the “stratified-stratified wavy (SSW)” and “stratified wavy – intermittent/annular (SWIA)” transitions for concentric horizontal geometries, with minimal effect on the transition to dryout and no effect on the “intermittent dispersed bubbly (IB)” transition. The EHD flow pattern maps were generated and compared against data from the present study and a database of experimental EHD convective boiling studies. The regions where maximum enhancement were seen in the literature correlate well with those regions predicted by the maps. Performance correlations for the EHD convective boiling heat transfer and pressure drop were developed. They are based on the free-field Kandlikar correlation [1] for two-phase heat transfer and the Chisholm-Laird [2] correlations for two-phase pressure drop, respectively. The EHD flow pattern map is used to determine what the flow pattern for a given applied voltage will be, and flow pattern based enhancement linear multipliers are then used to determine the EHD performance above the free-field case. EHD is a form of active enhancement, i.e. it requires power. Thus, it would be used in systems that require performance control or regulation, in addition to some niche applications like space where it can be used instead of gravity. A method for EHD controller design was established and an EHD control algorithm was designed and implemented on the test section for the flow pattern and applied waveforms that were determined to be optimal to maximize enhancement in this geometry. System identification was performed empirically to determine the transfer function between EHD voltage and heat load to be controlled for. This resulted in a 1st order plus dead-time model to which proportional-integral controller constants were tuned. Two controllers were developed; a PID control system and a Smith model predictive control system and these were compared based on their ability to regulate the output quality of the heat exchanger when subject to dynamic heat loading. Regulation was achieved for a dynamic heat load within ±25% bound from the designed steady state load. These controllers operate on one flow pattern as the test section is 30 cm long. Flow pattern dependent controller design would be required for a full length convective boiling heat exchanger. / Thesis / Doctor of Philosophy (PhD) / Control of boiling heat transfer using electric fields is hard to predict. This thesis presents a set a design guidelines based on how the electric field enhances the flow pattern.

The aim of this Diploma thesis is a description of gas heat pump (GHP) technology and the possibility of their usage in real life. Also units from the major manufactures of GHP units in the Czech Republic and abroad are evaluated from technical and economical point of view. The thesis as well includes a part where is decided if it is better to further develop GHP units with higher or lower performance. In the last part of thesis are cooling circuits designed for both lower and higher performance, than performance of existing GHP unit Polo 100. As a result, the units with higher performance are more perspective. Therefore heat plate exchangers, which serve as a condenser and an evaporator, are designed for unit with higher performance.

Diploma thesis deals with direct cooling systems in food processing plants. The work focuses on the description of the components of the refrigerant cycle, with emphasis on the choice of refrigerant. The result is a design of direct cooling system in the food factory in the variants with refrigerant R404A and R717. The work describes the advantages and disadvantages of various options. The work includes an experimental section which deals with the measure-ment and processing of the actual values of the refrigerant cycle and the comparison of two cold sources.

V súčasnosti sa kladie veľký dôraz na znižovanie produkcie emisií, čistotu a kvalitu životného prostredia. Tieto trendy sa týkajú všetkých odvetví, tak isto aj stavebného priemyslu a vykurovania budov. Doposiaľ bolo najrozšírenejším typom vykurovania spaľovanie fosílnych palív. Do popredia sa však dostávajú tepelné čerpadlá a začínajú zaberať výrazný podiel trhu. Kvôli dopytu po tepelných čerpadlách vzrastá počet výrobcov a konkurencia. Cieľom výrobcov je zvyšovanie účinnosti tepelných čerpadiel a zároveň znižovanie výrobných nákladov na dosiahnutie lepšej pozície na trhu. Predložená práca sa zaoberá procesmi prebiehajúcimi v chladivovom okruhu tepelného čerpadla. Skúma správanie chladiva počas kondenzácie a možnosti zvýšenia účinnosti tepelného čerpadla. Zvýšenie účinnosti je dosiahnuté podchladením chladiva počas kondenzácie na zvýšenie tepelných ziskov. To je dosiahnuté zaradením druhého elektronického expanzného ventilu za kondenzátorom. Táto technológia je však finančne náročná. Práca sa zaoberá výpočtom kapiláry na základe praktických meraní, ktorá bude zadržiavať chladivo v kondenzátore a bude plniť podobnú úlohu ako expanzný ventil. Následne sú porovnané výsledky jednotlivých meraní. Výsledky sú porovnané voči jednoduchému systému, kde nedochádza k podchladeniu chladiva. V závere práce je analýza výsledkov jednotlivých systémov.

Apresenta um estudo teórico-experimental da ebulição convectiva do fluido refrigerante R-134a no interior de tubos lisos. Os parâmetros físicos disponíveis para medida foram: pressão, temperatura, vazão de refrigerante e potência de aquecimento, os quais, juntamente com o registro fotográfico, foram utilizados para caracterizar os padrões de escoamento e as transições, investigando-se os efeitos do diâmetro do tubo, da velocidade mássica e do fluxo de calor sobre a perda de pressão e a transferência de calor. Os principais padrões de escoamento visualizados foram: o intermitente, o anular e o estratificado, nos quais constatou-se que, as transições são governadas, principalmente, pelos efeitos da velocidade mássica e do diâmetro do tubo. Dentre estes padrões de escoamento, o anular e o estratificado foram modelados analiticamente. O modelo para o escoamento anular foi utilizado na obtenção de correlações para o fator de atrito interfacial e para espessura do filme de líquido. O modelo para o escoamento estratificado foi dividido em duas partes, uma destinada a obter a configuração da interface, a qual se mostrou côncava e a outra destinada à determinação dos fatores de atrito líquido-parede e interfacial os quais foram correlacionados / The research reports a theoretical and experimental study of convective boiling of refrigerant R-134a in smooth tubes. Tests have been carried out to measure the following physical parameters at the test section: mass flow rate, pressure and pressure drop, refrigerant and surface temperatures and the electrical power. In addition to these parameters, a photographic study has been carried out from pictures taken at the test section exit in order to determine the flow regimes that intervene under the imposed operating conditions. Effects over the pressure drop and heat transfer of the mass flow rate, heat flux, quality, and tube diameter have been investigated. Three flow regimes have been found: the intermitent, the stratified and the annular. Flow regime transitions are apparently governed by the mass velocity and tube diameter. The annular and the stratified flow regimes have been semi-empirically modeled using a mechanistic approach. The annular flow model has been applied to develop correlations for two important physical parameters: the interfacial friction factor and the film thickness. Through the stratified model, the shape of the interface has been evaluated along with correlations for the liquid to wall and interface friction factors

Este trabalho tem como objetivo analisar a tecnologia do ciclo refrigerante misto para obtenção de gás natural liquefeito (GNL). Nessa tecnologia, o GNL é obtido por meio do seu resfriamento através de um ciclo de refrigeração, cujo fluido refrigerante é formado por uma mistura de diversos componentes. O ciclo de refrigeração é usado para resfriar a corrente de gás natural até as condições criogênicas por meio de um trocador de calor. A determinação da composição ótima dessa mistura de refrigerantes é de suma importância para a correta e eficiente operação da planta. O modelo termodinâmico para o cálculo de equilíbrio de fases dos componentes da mistura refrigerante é o baseado na Lei de Raoult, válido para misturas e soluções ideais. Inicialmente, revisam-se os conceitos de refrigerantes mistos, curvas compostas e o ponto de pinça (pinch point), utilizados na implementação da solução computacional. A aplicação dos modelos de gás e solução ideal influencia nos resultados, mas, não obstante, produz bons resultados como os obtidos no presente trabalho. A operação eficiente do ciclo depende, sobretudo, de três parâmetros principais, quais sejam: vazão da mistura refrigerante, razão de pressões alta e baixa do ciclo de refrigeração e composição da mistura refrigerante. Da análise dos resultados obtidos conclui-se que a alteração nas proporções da composição do refrigerante muda significativamente a forma das curvas composta quente e composta fria, quando comparados à alteração dos níveis de pressão e da vazão do ciclo refrigerante. Entretanto, a operacionalização do ciclo somente ocorre se um dado conjunto de valores daqueles parâmetros satisfaça uma determinada diferença mínima de temperatura, ou ponto de pinça, entre as curvas composta quente e composta fria dentro do trocador de calor. Assim, a operação eficiente do ciclo de refrigeração requer a otimização daqueles três parâmetros operacionais. / This work has the objective of analyzing the technology of mixed refrigerant cycle for obtaining liquefied natural gas (LNG). In that technology, the liquefied natural gas is obtained by means of cooling through a refrigeration cycle, whose fluid refrigerant is formed by a mixture of various components. The refrigeration cycle is used to cool the natural gas stream to cryogenic condition with the use of a heat exchanger. The determination of the optimal composition of this refrigerant mixture is very important for the correct and efficient operation of the plant. The thermodynamic model for the equilibrium phase calculation of the refrigerant mixture is based on the Law of Raoult, which is valid for ideal mixtures and solutions. Initially, the concepts of refrigerant mixture, composite curves and pinch point used in the implementation of the numerical solution were reviewed. The application of ideal-gas and ideal-solution models has influence on the results. Nevertheless, it produces good results as those obtained in the present work. The efficient operation of the cycle depends essentially of three key parameters, which are: refrigerant flow rate, the ratio of high to low pressures of the refrigerant cycle and the mixed refrigerant composition. The results indicated that the composition variation of the refrigerant changes significantly the shape of hot and cold composite curves in comparison to the modification in the pressure levels and the refrigerant flow rate of the refrigerant cycle. However, the process will operate only if a given set of values of those parameters satisfies a minimum temperature difference, or pinch point, between the hot and cold composite curves within the heat exchanger. Thus, the efficient operation of the refrigerant cycle requires the optimization of those three operational parameters.

This thesis investigates the refrigerant (R-134a) flow in three minichannels having different geometries experimentally. During the last 40 years heat transfer in small scales has been a very attractive research area. Improvements in heat transfer in the refrigeration applications by means of usage of micro/minichannels provide significant developments in this area. Also it is known that experimental studies are very important to constitute a database which is beneficial for new developments and research. During the two-phase flow experiments conducted in the minichannels, low mass flow rates and constant wall temperature approach, which are the conditions in the evaporators of the refrigerator applications were applied because one of the purposes of this study is to determine the most ideal minichannel among the tested minichannels for usage in the evaporator section of the refrigerators. Two-phase flow experiments were made with refrigerant R134a in the three minichannels having hydraulic diameters of 1.69, 3.85 and 1.69 mm respectively. As distinct from the others, the third minichannel has a rough inner surface. Comparison of the experimental results of the three minichannels was made in terms of forced convection heat transfer coefficients and pressure drop at constant quality and mass flux values. As a result of the experiments, the most ideal minichannel among the tested minichannels was determined for the evaporator applications in the refrigerators.

Baigiamajame magistro darbe nagrinėjama tiesioginė šilumos atgavimo funkcija iš šilumos siurblio darbo ciklo ir jos pritaikomumas Lietuvos klimatinėmis sąlygomis. Apžvelgiami naudojamų šalčio mašinų tipai, technologiniai tiesioginio šilumos atgavimo inžineriniai sprendmiai. Taip pat detaliai paaiškinamas šilumos siurblio darbo ciklas ir tiesioginio šilumos atgavimo funkcijos veikimo principas. Taip pat šilumos atgavimo founkcija nagrinėjama konkrečiame jau egzistuojančiame administracinės paskirties pastate, kuriame sumontuota vandeninė šalčio mašina. Nustatomas šalčio mašinos darbo režimų grafikas skaičiuojamąjį Liepos mėnesį. Dviejomis nepriklausomomis kompiuterinėmis programomis sumodeliuojami šalčio mašinos freono konturų darbo ciklai. Pagal gautus rezultatus parenkami ir sumodeliuojami šilumos atgavimo šilumokaičiai ir randamos įmanomos atgauti šilumos galios ir šilumos kiekiai, kurie palyginami su pastato karšto vandens poreikiais ir atliekamas ekonominis šilumos atgavimo funkcijos vertinimas. Gauti rezultatai patvirtina, kad tokio tipo šilumos atgavimas didina pastato energijos vartojimo efektyvumą ir padeda taupyti pinigus. Taip pat nustatyta, kad Lietuvos klimatinėmis sąlygomis šilumos atgavimas įmanomas tik šiltuoju metų sezonu, kai pastate yra vėsos poreikis, tačiau nepaisant to, metiniai sutaupymai yra pakankamai dideli, todėl tiesioginis šilumos atgavimas yra efektyvi mikroklimato sistemų optimizavimo priemonė. / In this Master thesis a direct heat recovery option on the air cooled chiller and its availability in Lithuanian climate has been investigated. Also the review of chiller range, direct heat recovery technological issues and direct refrigerant vapor compression cycle has been explained in detail. Also the heat recovery option has been investigated in the specific administrative building with the chiller already installed. Chiller cooling chart has been calculated for the selected time period – a month of July. Chiller refrigerant circuits were modeled by using two independent computer applications. In accordance with the calcultated results, plate heat exchangers were designed. Possible amount of recovered heat was found and compared with building‘s heat demand for hot domectic water supply. The economical evaluation of heat recovery option was carried out. The results confirmed that the heat recovery option discussed in the thesis can increase energy consumption efficiency. Despite the fact that heat recovery in Lithuanina climate is only possible during the warm season, annual savings on energy consumption are sufficiently high to make the heat recovery measure an attractive option.

Cette étude consiste en l’expérimentation de mélanges de fluides frigorigènes à base de CO2 dans les applications de pompes à chaleur domestiques. L’objectif est l’obtention de performances supérieures à une pompe à chaleur au CO2 double-service (eau chaude sanitaire et chauffage). L’ajout d’autres composés au CO2 déplace le point critique et de façon plus générale modifie les lignes d’équilibre de phases. On peut alors s’attendre à la réduction des pressions de fonctionnement et à une augmentation du rendement énergétique du cycle thermodynamique. Une pompe à chaleur mono-étagée au CO2 est utilisée comme référence, et les conditions de température externes à la boucle thermodynamique sont invariantes. Deux scénarii sont considérés : La production d’eau chaude sanitaire (ECS), où l’eau est chauffée de 10 °C à 65 °C ; la production d’eau de chauffage (CH) où l’au est chauffée de 30 °C à 35 °C. Dans les deux cas, l’eau glycolée en l’entrée de l’évaporateur est régulée à 7 °C. Afin de pouvoir analyser le comportement des cycles thermodynamiques avec mélanges, il est essentiel de connaitre la composition du fluide frigorigène en circulation. Pour ce faire, nous avons mis au point une technique de mesure statistique et non-intrusive de la composition: Des cellules optiques installées sur les tubes de la boucle permettent de recueillir les spectres d’absorption du fluide en circulation, grâce à un spectromètre proche infrarouge à transformée de Fourier. Un étalonnage méticuleux est effectué via l’acquisition de nombreux spectres d’échantillons ayant des compositions connues. Un modèle statistique est alors créé pour lier les concentrations aux données spectrales. Les compositions peuvent ensuite être estimées à partir de nouveau spectres, dont l’acquisition rapide permet l’analyse de la composition du fluide même en fonctionnement dynamique de la pompe à chaleur. Des mélanges de CO2 & propane, et CO2 & R-1234yf ont été testés, montrant des potentiels d’amélioration des performances de la pompe à chaleur pour les applications de chauffage des locaux. / The aim of this work is to experiment CO2 based mixtures as working fluids for heat pump applications in buildings, in order to enhance their performances compared to pure CO2 dual services heat pumps. Since adding other chemicals to CO2 moves the critical point and generally equilibrium lines, it is expected that lower operating pressures as well as higher global efficiencies can be reached. A simple stage pure CO2 cycle is used as reference, with fixed external conditions. Two scenarios are considered: water is heated from 10 °C to 65 °C for Domestic Hot Water (DHW) scenario and from 30 °C to 35 °C for Central Heating (CH) scenario. In both cases, water at the evaporator inlet is set at 7 °C to account for such outdoor temperature conditions. In order to understand the dynamic behaviour of thermodynamic cycles with mixtures, it is essential to measure the fluid circulating composition. To this end, we have developed a non intrusive method. Online optical flow cells allow the recording of infrared spectra by means of a Fourier Transform Infra Red spectrometer. A careful calibration is performed by measuring a statistically significant number of spectra for samples of known composition. Then, a statistical model is constructed to relate spectra to compositions. After calibration, compositions are obtained by recording the spectrum in few seconds, thus allowing for a dynamic analysis. Mixtures of CO2 & propane and CO2 & R-1234yf have been tested and showed great potential on enhancing performances of the heat pump for central heating applications.

I Kraftringens geotermianläggning cirkulerar köldmediet R134a som är en fluorerande växthusgas med relativt hög GWP (Global Warming Potential). När den nya F-gasförordningen trädde i kraft 2015 resulterade det i att priserna ökade markant på de köldmedierna med hög GWP. Kraftringen vill därför undersöka vilka konverteringsalternativ det finns till R134a, dels för att minska på kostnaderna men också för att minimera verksamhetens klimatpåverkan.   Olika köldmedier har jämförts med avseende på hur de påverkar anläggningens prestanda. Det har också undersökts vilka köldmedier som ligger inom rätt tryck med hänsyn till vad kompressorn och anläggningens rör är konstruerade för. Undersökningen har gjorts genom teoretiska beräkningar som är grundade på uppmätta värden från när värmepumparna går på maximal belastning. Vid urvalet av köldmedier har det utgåtts från samma förångningstemperatur och kondenseringstemperatur som vid mätningen.   I undersökningen fann vi två köldmedier, R152a och R450A, som skulle fungera som konverteringsalternativ i avseende att de ligger inom rätt tryck och temperatur. Båda alternativen har lägre GWP och en högre värmefaktor än nuvarande köldmediet R134a. Dock är R152a brandfarligt och har högre säkerhetsklass än de två andra medierna. / In the geothermal plant of 'Kraftringen' the refrigerant R134a, which is a fluorescent greenhouse gas with relatively high GWP (Global Warming Potential), circulates. When the new F-Gas regulation was introduced in 2015, it resulted in a significant price increase for high-pressure GWP refrigerants. 'Kraftringen' therefore wants to investigate which conversion options there are for R134a, partly to reduce costs, but also to minimize the climate impact of the business.   Different refrigerants have been compared with regard as to how they affect the plants performance. Moreover, it has been investigated which refrigerants are in the correct pressure range with regard to what the plants compressor and pipes are designed for. The survey has been done by performing theoretical calculations based on measured values from when the heat pumps operate at maximum load. In the refrigerant selection process, the same evaporation and condensation temperature as during the measuring was used.   In the study we found two refrigerants, R152a and R450A, which would serve as conversion options in terms of them being in the correct pressure and temperature range. Both options have lower GWP and a higher heat factor than the current refrigerant R134a. However, R152a is flammable and has a higher degree of safety than the other two refrigerants.