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PL
Przeprowadzono gruntowny przegląd literatury na temat korelacji spadku ciśnienia, opracowano bank danych doświadczalnych obejmujący szeroki zakres zmian parametrów przepływowych oraz dokonano weryfikacji możliwości przewidywania wartości spadków ciśnienia w przepływach dwufazowych gaz-ciecz w minikanałach. Porównano wartości spadków ciśnienia obliczone zgodnie z dostępnymi w literaturze korelacjami z wartościami eksperymentalnymi. Analiza uzyskanych wyników obliczeń wykazała, że większość korelacji ma bardzo ograniczony zakres ważności i nie opisuje danych doświadczalnych z zadowalającą dokładnością. Zaproponowano równania do obliczania spadku ciśnienia w przepływie w minikanałach, i to niezależnie od rodzaju przepływu.
EN
Fundamentals and a review, with 17 refs., of pressure drop correlations and exp. pressure drop data. Performance of the correlations in correctly predicting the diverse data sets was verified. Most of the correlations developed are very restricted in terms of handling a wide variety of data sets.
2
Content available Flow structures during refrigerants condensation
EN
For several years, in the world are carried out studies on the refrigerants condensation in minichannels. These studies are aimed at understanding the condensation process and facilitate the design of mini heat exchangers. It is very important to optimize heat transfer and drive power consumption of the is the knowledge of the processes occurring during refrigerants condensation in pipe minichannels. It is important in this case to make a individual flow structures maps for the refrigerants, due to the significant effect of flow structured formed in the minichannel on the heat transfer and flow resistance. Unfortunately, in relation to the number of publications about condensation in minichannels, the number of published flow maps is relatively small. Due to the fact that the condensation process takes place differently in the minichannels and conventional channels, there is no possibility of using the flow maps for conventional channels to identification flow patterns in minichannels. One of the most popular flow maps for the condensation in minichannels is the map created by Colleman and Garimella, which was made for the R134a refrigerant. The authors conducted their own experimental studies of refrigerants R134a, R404A and R407C condensation in pipe minichannels with an internal diameter d = 3.3 - 0.31 mm. These studies results were subjected to calculation identification of flow structures using the map of Colleman and Garimella. These results are compared with the criteria published on Thome and Cavallini flow maps.
EN
This article presents the results of experimental research of R404A, R407C and R410A high-pressure refrigerants condensation in vertical pipe minichannels with an internal diameter d below 2,5 mm. The study determined the local and average heat transfer coefficient in the full range of vapor quality, x= 1-0. On the basis of experimental investigations, the dependence of heat transfer coefficient on the vapor quality x, the mass flux density G and the channel internal diameter d was obtained.
EN
Analysis of the state of-the-art in research of minichannel heat exchangers, especially on the topic of flow maldistribution in multiple channels, has been accomplished. Studies on minichannel plate heat exchanger with 51 parallel minichannels with four hydraulic diameters, i.e., 461 μm, 574 μm, 667 μm, and 750 μm have been presented. Flow at the instance of filling the microchannel with water at low flow rates has been visualized. The pressure drop characteristics for single minichannel plate have been presented along with the channels blockage, which occurred in several cases. The impact of the mass flow rate and channels’ cross-section dimensions on the flow maldistribution were illustrated.
EN
Investigations of refrigerant condensation in pipe minichannels are very challenging and complicated issue. Due to the multitude of influences very important is mathematical and computer modeling. Its allows for performing calculations for many different refrigerants under different flow conditions. A large number of experimental results published in the literature allows for experimental verification of correctness of the models. In this work is presented a mathematical model for calculation of flow resistance during condensation of refrigerants in the pipe minichannel. The model was developed in environment based on conservation equations. The results of calculations were verified by authors own experimental investigations results.
EN
Detailed studies have suggested that the critical heat flux in the form of dryout in minichannels occurs when the combined effects of entrainment, deposition, and evaporation of the film make the film flow rate go gradually and smoothly to zero. Most approaches so far used the mass balance equation for the liquid film with appropriate formulations for the rate of deposition and entrainment respectively. It must be acknowledged that any discrepancy in determination of deposition and entrainment rates, together with cross-correlations between them, leads to the loss of accuracy of model predictions. Conservation equations relating the primary parameters are established for the liquid film and vapor core. The model consists of three mass balance equations, for liquid in the film as well as two-phase core and the gas phase itself. These equations are supplemented by the corresponding momentum equations for liquid in the film and the two-phase core. Applicability of the model has been tested on some experimental data.
PL
W artykule zestawiono wartości lokalnych współczynników przejmowania ciepła uzyskane z badań wymiany ciepła podczas przepływu czynnika chłodniczego FC-72 i wody destylowanej przez pionowy minikanał podczas zmiany przepływu. Minikanał posiadał wymiary: głębokość - 1,7 mm, szerokość - 16 mm, długość - 180 mm oraz był ogrzewany asymetrycznie. Elementem grzejnym była płyta grzejna z super stopu Haynes - 230 o grubości 0,45 mm. Lokalne współczynniki przejmowania ciepła wyznaczono na styku ciecz wrząca - powierzchnia grzejna z warunku brzegowego trzeciego rodzaju, przy wykorzystaniu metody jednowymiarowej. Temperatura powierzchni na zewnętrznej powierzchni grzejnej stykającej się bezpośrednio z otoczeniem rejestrowana była za pomocą termowizji (IRT). Płyta szklana stanowiąca drugą ściankę kanału pozwoliła na jednoczesną obserwację struktur przepływu dwufazowego. W pracy omówiono i pokazano stanowisko pomiarowe z jego obiegami i systemami. Przedstawiono przegląd literatury dotyczący zastosowania różnych czynników roboczych w badaniach innych naukowców. Zestawiono i przeanalizowano uzyskane zależności współczynnika przejmowania ciepła w funkcji odległości od wlotu do wylotu minikanału podczas zwiększania strumienia ciepła i skonstruowano krzywe wrzenia, otrzymane na podstawie badań z wykorzystaniem dwóch czynników roboczych: FC-72 i wody destylowanej, przy uzmiennianiu natężenia przepływu. Porównano uzyskane wyniki oraz zarejestrowane struktury wrzenia.
EN
The present paper compares local heat transfer coefficients obtained from the study of flow boiling heat transfer with FC-72 and distilled water in a vertical, asymmetrically heated minichannel at variate flow rate. The minichannel dimensions were as follows: depth – 1.7 mm, width - 16 mm, length - 180 mm. A 0.45 mm thick Haynes-230 plate was the heating element. The local heat transfer coefficients were determined by the one-dimensional method from the third type boundary condition at the interface between the boiling liquid and the heated surface. The temperature at the external face of the heated surface in direct contact with the ambient was registered with infrared thermography (IRT). One of the minichannel walls was made of a glass pane, thus allowing observations of two-phase flow structures. The measurement setup with its circuits and systems was described in detail. The literature review focused on the use of different working fluids in the studies conducted by other researchers. The dependencies of the heat transfer coefficient as a function of the distance from the inlet to the outlet of the minichannel at increasing heat flux were compared and analysed. Boiling curves obtained based on the studies with FC-72 and distilled water were plotted for variated flow rate. The results and recorded boiling structures were compared.
PL
Przedstawiono efektywność wymiany ciepła podczas wrzenia płynu chłodniczego przepływającego przez pionowy minikanał, asymetrycznie ogrzewany powierzchnią rozwiniętą teksturowaniem laserowo-wibracyjnym. Analizy efektywności wymiany ciepła dokonano na podstawie lokalnych współczynników przejmowania ciepła na styku powierzchnia grzejna–płyn. Zastosowanie powierzchni grzejnej rozwiniętej teksturowaniem laserowo-wibracyjnym wspomogło osiągnięcie efektywniejszej wymiany ciepła podczas wrzenia nasyconego w porównaniu z powierzchnią gładką, gdyż uzyskano wyższe wartości współczynników przejmowania ciepła.
EN
This chapter discusses the efficiency of flow boiling heat transfer for a fluid flowing through a vertical minichannel heated asymmetrically with a surface enhanced by vibrationassisted laser texturing. The study of the heat transfer efficiency involved analysing the local values of the heat transfer coefficients at the interface between the heated surface and the fluid. The use of an enhanced surface produced by vibration-assisted laser texturing contributed to more efficient saturated flow boiling heat transfer than that observed for a smooth surface because higher values of the heat transfer coefficients were obtained.
EN
The paper presents the results of experimental heat transfer and pressure drop during condensation of the single component refrigerant R134a and zeotropic mixtures R404A, R407C, and R410A in tube minichannels of internal diameter from the range 0.31–3.30 mm. The local values and the average of heat transfer coefficient and pressure drop in the whole range of the change in mass quality were measured. On the basis of the obtained test results there was illustrated the influence of the change of mass vapor quality, the mass flux density, and the inner diameter of channel on the studied parameters. These results were compared with the calculation results based on the relations postulated by other authors. The discrepancy range was ± 50%. On the basis of given test results own correlation was developed to calculate the heat transfer coefficient and pressure drop of tested refrigerants which presents the obtained results in a range of discrepancy of ±25%.
EN
In the paper presented are the results of calculations using authors own model to predict heat transfer coefficient during flow boiling of carbon dioxide. The experimental data from various researches were collected. Calculations were conducted for a full range of quality variation and a wide range of mass velocity. The aim of the study was to test the sensitivity of the in-house model. The results show the importance of taking into account the surface tension as the parameter exhibiting its importance in case of the flow in minichannels as well as the influence of reduced pressure. The calculations were accomplished to test the sensitivity of the heat transfer model with respect to selection of the appropriate two-phase flow multiplier, which is one of the elements of the heat transfer model. For that purpose correlations due to Müller-Steinhagen and Heck as well as the one due to Friedel were considered. Obtained results show a good consistency with experimental results, however the selection of two-phase flow multiplier does not significantly influence the consistency of calculations.
EN
The paper presents four 1-dimensional models of thermal resistance of walls in a heat exchanger with rectangular minichannels. The first model is the simplest one, with a single wall separating two fluids. The second model of the so called equivalent wall takes into account total volume of intermediate walls between layers of minichannels and of side walls of minichannels. The next two more complicated models take separately into account thermal resistance of these walls. In these two models side walls are treated as fins. The results of models comparison are presented. It is shown that thermal resistance may be neglected for metal walls but it should be taken into account for the walls made of plastics. For the case of non-neglected wall thermal resistance the optimum wall thickness was derived. Minichannel heat exchangers made of plastic are larger than those built of metal, but are significantly cheaper. It makes possible to use of such exchangers in inexpensive microscale ORC installations.
12
Content available remote Gaz-ciecz w minikanale z elementami zaburzającymi
EN
Two-phase gas-liquid flow is simply used in chemical, petrochemical or process engineering. This flow is very important from substance mixing part of view. It determines very important physical phenomena as heat and mass transfer. To intensify those kind of phenomena are used mini- and micro- reactors. Because of this facts it was decided to carry out the research of two-phase gas-liquid flow in minichannel with disturbing element.
PL
W artykule omówiona została ocena możliwości wykorzystania istniejących metod obliczeniowych wyznaczania wartości współczynnika wnikania ciepła, które zostały opracowane dla syntetycznych czynników chłodniczych, w kontekście możliwości ich wykorzystania dla wrzenia propanu w kanałach poziomych. Analizie zostały poddane wybrane, często cytowane w literaturze, metody obliczeniowe w porównaniu z dostępnymi w literaturze wynikami badań eksperymentalnych dla różnych warunków cieplno-przepływowych wrzenia propanu.
EN
Paper discuss the assessment of possibility of using existing calculation methods, which has been developed for synthetic refrigerants, to predict heat transfer coefficient of propane boiling in horizontal channels. The analysis was made for selected calculation methods compared with experimental results taken from literature in various thermal-flow conditions for boiling propane.
EN
The experimental research of environmentally friendly refrigerant HFE-7100 condensation in pipe minichannels was conducted. During the investigations of HFE-7100 condensation in a minichannel with internal diameter 2 mm together with visualization of flow patterns was made. Visualization results were compared with existing flow structure maps. The identification of the range of flow patterns occurrence during the condensation process of low-pressure refrigerant HFE-7100 was made. The tests were performed throughout the whole range of condensation process.
15
Content available Generator minipęcherzy w minikanale
PL
W pracy przeprowadzono analizę porównawczą rozwiązań konstrukcyjnych mieszalników fazy ciekłej z fazą gazową. Pokazano, że w zaproponowanych rozwiązaniach trudno jest sterować ilością pęcherzy w podłączonym do nich minikanale, a tym samym niemożliwe jest uzyskanie wszystkich rodzajów przepływu dwufazowego. W pracy przedstawiono własną koncepcję generatora mini i mikropęcherzy, który pozwala na formowanie oczekiwanej struktury przepływu dwufazowego w minikanale. Generator wykorzystano do wytworzenia minipęcherzy w minikanale. Urządzenie testowano dla czterech różnych przewodów o średnicach wewnętrznych: 1, 2, 4, 5 mm. We wszystkich przypadkach generator tworzył pęcherze o różnych średnicach.
EN
The gas-liquid mixers construction were compared in this paper. New conception of mini and micro bubbles generator is presented. The generator is used to form of mini bubbles in mini channel. The testing was conducted in tubes with the internal diameters of 1, 2, 4 and 5 mm. The different diameters of bubbles were formed for each generator constructions.
PL
W artykule zaprezentowano wstępną koncepcję stanowiska eksperymentalnego do badań wymiany masy/ciepła w elementach mini-wymienników ciepła. Proponowana metoda badawcza bazuje na technice elektrolitycznej, w której do wyznaczenia współczynników wymiany masy wykorzystuje się zależności opisujące zjawiska elektrolizy. Pomiary powstałych prądów granicznych, powierzchni katody, przy której następuje wymiana roboczych jonów, oraz stężenia jonów w elektrolicie, dają możliwość określenia intensywności wymiany masy. Otrzymane wielkości, wykorzystując analogię pomiędzy procesami przenoszenia masy i ciepła, przelicza się na współczynniki wymiany ciepła. Proponowana metoda może być również wykorzystywana w mini-skali. Wiąże się to jednak z dodatkowymi warunkami, które muszą być spełnione, aby zminimalizować błędy wynikające ze skali procesu, a związane m.in. z chropowatością powierzchni elektrod, zmianą stężenia na długości katody, czy też grubością średniej warstwy dyfuzyjnej. Zaprojektowane stanowisko będzie umożliwiało badania nad intensywnością wymiany masy/ciepła w mini-wymienniku ciepła z kanałami o przekroju prostokątnym.
EN
The paper presents the preliminary conception of the experimental stand for measurements of mass and heat transfer coefficients in mini heat exchangers. The proposed method based on the electrolytic technique in which to determine the mass transfer coefficients the equations describing the electrolysis phenomenon are used. Measurements of limiting streams established, the cathode surface at which the ion exchange exists, and the concentration of ions in the electrolyte make it possible to determine the intensity of mass exchange. The proposed method may be used in small scale applications. However, this is connected with the additional conditions that must be met in order to minimize errors resulted from the scale of the process, e.g. the surface roughness of the electrodes, the concentration change of the length of the cathode, or the average thickness of the diffusion layer. Designed test stand will allow to study the intensity of mass and heat ex-change in mini exchanger with channels of rectangular cross-section.
EN
Currently smaller and smaller channels are used to construct many devices such as mini heat exchangers or mini reactors. Therefore, predicting of two-phase flow pressure drops along minichannels is very important for proper and safely operation of these appliances. Because of this fact, it was decided to carried out introductory research of flow patterns influence on pressure drops in the two-phase flow mixture.
PL
W artykule przedstawiono wyniki badań eksperymentalnych lokalnego współczynnika przejmowania ciepła. Wymiana ciepła odbywała się między ogrzewaną ścianką kanału a przepływającym wewnątrz czynnikiem chłodniczym (R134a i R404A). Wykorzystano 12, wykonanych ze stali nierdzewnej, rurek o średnicy wewnętrznej w zakresie dw = 0,64 ÷ 2,30 mm. Rurki tej średnicy, tzw. minikanały wykorzystuje się do budowy miniaturowych wymienników ciepła. Badania przeprowadzone w zakresie gęstości strumienia masy (wp) = 350 ÷ 1400 kg/(m2s) i gęstości strumienia ciepła dochodzącej do q = 90 kW/m2 pozwoliły zaobserwować występowanie, nie obserwowanego w kanałach konwencjonalnych, zjawiska flashingu. Wykazano, że w strefie objętej występowaniem zjawiska flashingu warunki wymiany ciepła pogarszają się, a wartość lokalnego współczynnika przejmowania ciepła w tej strefie może spaść nawet o 50%.
EN
The paper presents the results of experimental investigations on the heat transfer coefficient. The heat was transferred between the heated channel wall and the refrigerant flowing inside the channel. Refrigerants R134a and R404A were used as a working fluid. During the experiments 12 tubes were used. The tubes were made of stainless steel and their inner diameter was: 0.45, 0.55, 0.80, 1.10, 1.15, 1.30, 1.35, 1.40, 1.60, 1.68, 1.94 and 2.30 mm. Channels with the above mentioned diameters are called minichannels and they are used to build miniaturized heat exchangers. The study was conducted in six series in which the mass flux density was changed (Table 1). During each series the heat flux density was stepwise increased and then decreased. Its maximum value was q = 90 kW/m2. This allowed obtaining the forced convection heat transfer or boiling phenomenon. In some cases, it was noted that the flashing phenomenon occurred. This phenomenon occurred when the local pressure was lower than the saturation pressure (at a given temperature). It revealed the evaporation of a liquid, not by applying heat from the wall, but by using the latent heat (heat of evaporation) [4, 5]. Reduction in the local pressure below the saturation pressure was common in minichannels. The reason is large pressure drops which occur in tubes of such small diameters. The flashing phenomenon was not observed inside conventional channels. The results of this study showed that in the zone of its occurrence the value of the local transfer coefficient decreased significantly (Figures 5 ÷ 8). During the experimental investigations the value of the local heat transfer coefficient decreased to 50%. From the conducted research it follows that the existence of the flashing phenomena in heat exchangers is harmful.
EN
Gas-liquid two-phase flow in minichannels has been the subject of increased research interest in the past few years. Evaluation, however, of today's state of the art regarding hydrodynamics of flow in minichannels shows significant differences between existing test results. In the literature there is no clear information regarding: defining the boundary between minichannels and conventional channels, labelling of flow patterns. The review of literature on the hydrodynamics of gas-liquid flow in minichannels shows that, despite the fact that many research works have been published, the problem of determining the effect of diameter of the minichannel on the hydrodynamics of the flow is still at an early stage. Therefore, the paper presents the results of research concerning determination of flow regime map for the vertical upward flow in minichannels. The research is based on a comprehensive analysis of the literature data and on the research that has been carried out. Such approach to the mentioned above problems concerning key issues of the two-phase flow in minichannels allowed to determine ranges of occurrence of flow structures with a relatively high accuracy.
PL
W artykule omówiono technikę jednoczesnego pomiaru stopnia zapełnienia przy wrzeniu w przepływie i rozkładu temperatury powierzchni grzejnej. Przedmiotem badań było rozpoznanie struktur przepływu dwufazowego, stopnia zapełnienia i stopnia suchości oraz zmiennych wraz z nimi współczynników przejmowania ciepła przy wrzeniu w przepływie płynu przez pionowy, prostokątny minikanał.
EN
Heat transfer to boiling liquid flowing through narrow minichannels has become a modern solution to heat transfer enhancement in small, mainly electronic devices. The paper discusses the method for simultaneous measurement of the flow boiling void fraction and heating surface temperature distribution, which aims at determining local values of the vapor quality and heat transfer coefficients. The studies were conducted for boiling FC-72 in a rectangular, vertical and asymmetrically heated minichannel. Employing a high-speed camera on the measurement stand constructed by the author helped to determine, for the set sections, the void and liquid fractions as a function of variable thermal and flow parameters such as channel hydraulic diameter, heat flux, pressure, mass flux and temperature of liquid subcooling at the minichannel inlet. Measurements of the heating surface temperature distribution and photographic observations of the two-phase flows, when conducted simultaneously, allow deter-mining the dependence of the recorded local void fraction on the local temperature. The liquid crystal thermography technique helped to perform the repetitious measurements of the temperature on the heating surface. One of the advantages of this technique is a possibility of observing the instantaneous temperature field of the heating surface. There were observed the following flow patterns: bubbly flow, bubbly-slug flow, slug flow, slug-wispy flow and wispy flow.
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