Wyniki wyszukiwania - BazTech

1

An investigation of the heat transfer coefficient during refrigerant evaporation

Dutkowski Krzysztof, Kruzel Marcin, Bohdal Tadeusz

Instal

|

2023

|

nr 10

15--20

EN

This study presents the experimental research data on the local heat transfer coefficient during refrigerant expansion evaporation in horizontal pipe minichannel. Heat exchange took place between the heated channel wall and the working fluid flowing inside (R134a and R404A). AISI 316 stainless steel pipe minichannels with an internal diameter in the range of di = 0.64 - 2.30 mm were used. The so-called minichannels are widely used to build miniature heat exchangers. Tests carried out in the range of mass flux density G = 350 - 1400 kg/(m2s) and heat flux density reaching q = 90 kW/m2 allowed to observe the occurrence of the flashing phenomenon, not observed in conventional channels. It has been shown that in the zone covered by the flashing phenomenon, the heat exchange conditions deteriorate, and the value of the local heat transfer coefficient in this zone may drop by up to 50%.

PL

W pracy przedstawiono wyniki badań eksperymentalnych lokalnego współczynnika przejmowania ciepła podczas odparowywania czynnika chłodniczego w minikanale poziomym. Wymiana ciepła odbywała się pomiędzy ogrzewaną ścianką kanału a przepływającym wewnątrz czynnikiem roboczym (R134a i R404A). Zastosowano minikanały rurowe ze stali nierdzewnej AISI 316 o średnicy wewnętrznej w zakresie di = 0,64 - 2,30 mm. Tak zwane minikanały są szeroko stosowane do budowy miniaturowych wymienników ciepła. Badania przeprowadzone w zakresie gęstości strumienia masy G = 350 - 1400 kg/(m2s) i gęstości strumienia ciepła dochodzącej do q = 90 kW/m2 pozwoliły zaobserwować występowanie zjawiska flashingu, nie obserwowanego w konwencjonalnych kanałach. Wykazano, że w strefie objętej zjawiskiem flashingu pogarszają się warunki wymiany ciepła, a wartość lokalnego współczynnika przejmowania ciepła w tej strefie może spaść nawet o 50%.

2

The use of PIV methods in the study of two-phase flows in small diameter channels

Sikora Małgorzata, Ligus Grzegorz

Journal of Mechanical and Energy Engineering

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2022

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Vol. 6, No 1

85--89

EN

The PIV (Particle Image Velocimetry) method is one of the optical, non-invasive measurement methods for measuring fluid velocity and it can be used in the study of two-phase gas-liquid flows to determine velocity fields. The velocity distribution of the liquid and gas phases influences the formation of two-phase flow structures and, consequently, the mechanisms of energy and moment exchange in the two-phase flow. The article concerns the application of the PIV method in the assessment of hydrodynamic phenomena occurring during two-phase flow realized in pipe minichannels with internal diameters d > 2 mm. Fluorescent marker particles with a density close to that of water were used in the research. The preliminary tests were carried out on the adiabatic water-air mixture. The research aimed to check the applicability of PIV methods also in non-adiabatic flows. As a result of preliminary studies, the velocity maps of the liquid phase, histograms and velocity profiles in the vertical section of the minichannel tested were obtained.

3

Research methods in the study of heat transfer coefficient during flow in minichannels

Tychanicz-Kwiecień Maria, Grosicki Sebastian

Journal of Mechanical and Energy Engineering

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2021

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Vol. 5, No 1

59--67

EN

The paper presents the specification of research methods commonly encountered in the studies of heat transfer processes in minichannels. In particular the following methods have been emphasized: electrochemical limiting current method as well as the thermal balance method. In thermal balance method the mean heat transfer coefficient is determined by the set of experimental thermal measurements of the investigated heat exchanger. In turn, limiting current method is based on heat and mass transfer analogy. The discussed research methods have been implemented on two specially designed and constructed test facilities with compact minichannel heat exchangers, which have been presented and described in details. In order to validate the designed setup, the preliminary experimental measurements of two minichannel heat exchangers with hydraulic diameter of 2 mm and rectangular cross sections during single-phase liquid flows have been carried out. In further perspective it is planned to extend the experimental studies of minichannel heat exchangers and to compare the results obtained by both methods described.

4

Heat transfer and pressure drop analysis of automotive HVAC condensers with two phase flows in minichannels

Skiepko Teodor

Engineering Management in Production and Services

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2021

|

Vol. 13, no. 4

174--188

EN

The air cooled automotive condensers under study are of brazed aluminium tube and center, consisting of one row array of horizontal parallel multi-port flat tubes with louver fins on air side. Each tube is with a number of smooth parallel minichannels for internal flow of refrigerant. The analysis uses decomposition of the condenser along refrigerant flow path into specific different zones as follows: two single phase zones, namely: superheated and subcooled, and a few zones of two phase flow that can appear along some specific condensation paths to be: annular/intermittent/bubble or annular/annular-wavy/intermittent/bubble or annular/wavy/stratified. The approach presented is based on experimental correlations for heat transfer and pressure drop. The heat transfer prediction is performed using ε – NTUo methodology. The results of the analysis refer to overall heat transfer rate, heat transfer in particular zones, pressure gradients heat transfer coefficients, vapour quality, condensation paths.

5

Wykorzystanie metod PIV w badaniach przepływów dwufazowych w minikanałach

Sikora Małgorzata, Ligus Grzegorz

Chłodnictwo : organ Naczelnej Organizacji Technicznej

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2021

|

R. 56, nr 6

2--7

PL

Metoda PIV (Particle Image Velocimetry) jest jedną z optycznych, nieinwazyjnych metod pomiarowych służących pomiarowi prędkości płynów i może być wykorzystywana w badaniach przepływów dwufazowych gaz-ciecz w celu wyznaczania pól prędkości. Rozkład prędkości fazy ciekłej i gazowej wpływa na formowanie się struktur przepływu dwufazowego, a w konsekwencji na mechanizmy wymiany energii i momentu w przepływie dwufazowym. Artykuł dotyczy zastosowania metody PIV do oceny zjawisk hydrodynamicznych zachodzących podczas przepływu dwufazowego realizowanego w minikanałach rurowych o średnicy wewnętrznej d > 2 mm. W badaniach zastosowano fluorescencyjne cząstki znacznikowe o gęstości zbliżonej do gęstości wody. Badania wstępne wykonano na mieszaninie adiabatycznej typu woda-powietrze. Celem badań było sprawdzenie możliwości zastosowania metod PIV również w przepływach nieadiabatycznych. W wyniku badań wstępnych otrzymano mapy prędkości fazy ciekłej, histogramy i profile prędkości w przekroju pionowym badanego minikanału.

EN

The PIV (Particle Image Velocimetry) method is one of the optical, non-invasive measurement methods for measuring fluid velocity and can be used in the study of two-phase gas-liquid flows to determine velocity fields. The velocity distribution of the liquid and gas phases influences the formation of two-phase flow structures and, consequently, the mechanisms of energy and moment exchange in the two-phase flow. The article concerns the application of the PIV method to the assessment of hydrodynamic phenomena occurring during two-phase flow realized in pipe minichannels with an internal diameter d > 2 mm. Fluorescent marker particles with a density close to that of water were used in the research. The preliminary tests were carried out on the adiabatic water-air mixture. The research aimed to check the applicability of PIV methods also in non-adiabatic flows. As a result of preliminary studies, velocity maps of the liquid phase, histograms and velocity profiles in the vertical section of the tested minichannel were obtained.

6

Wymiana ciepła podczas procesu skraplania czynnika HFE7000 w pojedynczych minikanałach rurowych

Bohdal Tadeusz, Sikora Małgorzata, Formela Karolina

Chłodnictwo : organ Naczelnej Organizacji Technicznej

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2020

|

R. 55, nr 6

21--26

PL

Przejmowanie ciepła w momencie przepływu czynnika chłodniczego przez minikanał jest złożonym procesem, który warunkuje efektywność energetyczną dwufazowej przemiany skraplania. Istotnie, małe średnice kanałów determinują duże wartości współczynników przejmowania ciepła. Niewątpliwie, współczynnik przejmowania ciepła jest również zależny od gęstości strumienia ciepła, czyli parametru określającego intensywność odprowadzenia ciepła do środowiska zewnętrznego [2, 4, 8]. W związku z tym, w artykule przedstawiono wyniki badań eksperymentalnych transportu ciepła podczas dwufazowego przepływu substanThe heat transfer during the refrigerant flows through the minichannel is a complex proces that conditions the energy efficiency of the two-phase condensation. Indeed, the small diameters of the channels determines the high values of the heat transfer coefficients. Undoubtedly, the heat transfer coefficient is also dependent on the heat flux density, in other words the parameter that determines the intensity of the heat dissipation to the external environment [2, 4, 8]. Therefore, the investigation results of heat transfer during the two-phase flow of HFE 7000 refrigerant in minichannels in the article were presented. The influence of the vapour quality and the mass flux density on the energy efficiency of heat transfer was illustrated. Under averaged condition, the characteristic of the heat transfer coefficient as a function of the single minichannel internal diameter was shown.cji HFE 7000 w minikanałach. Zilustrowano oddziaływanie średniego stopnia suchości pary substancji oraz gęstości strumienia masy na efektywność energetyczną transportu ciepła. W warunkach uśrednionych pokazano również charakterystykę współczynnika przejmowania ciepła w funkcji średnicy wewnętrznej pojedynczego minikanału.

EN

The heat transfer during the refrigerant flows through the minichannel is a complex proces that conditions the energy efficiency of the two-phase condensation. Indeed, the small diameters of the channels determines the high values of the heat transfer coefficients. Undoubtedly, the heat transfer coefficient is also dependent on the heat flux density, in other words the parameter that determines the intensity of the heat dissipation to the external environment [2, 4, 8]. Therefore, the investigation results of heat transfer during the two-phase flow of HFE 7000 refrigerant in minichannels in the article were presented. The influence of the vapour quality and the mass flux density on the energy efficiency of heat transfer was illustrated. Under averaged condition, the characteristic of the heat transfer coefficient as a function of the single minichannel internal diameter was shown.

7

Badanie charakterystyk cieplno-przepływowych skraplania chłodniczych mieszanin zeotropowych w minikanałach

Bohdal Tadeusz, Widomska Katarzyna

Chłodnictwo : organ Naczelnej Organizacji Technicznej

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2019

|

R. 54, nr 1

28--34

PL

W pracy przedstawiono wyniki badań eksperymentalnych wymiany ciepła oraz spadku ciśnienia podczas skraplania mieszanin zeotropowych R407C i R410A w minikanałach rurowych o średnicy wewnętrznej d = 0,31 ÷ 3,30 mm. Wyznaczono wartości lokalne i średnie współczynnika przejmowania ciepła α i oporów przepływu (Δp/L) w całym zakresie zmiany stopnia suchości (x = 1 ÷ 0). Na podstawie uzyskanych wyników badań zilustrowano wpływ zmiany stopnia suchości pary czynnika x, gęstości strumienia masy G oraz średnicy wewnętrznej kanału d na wartość badanych parametrów. Wskazano na wyraźny wpływ właściwości czynnika chłodniczego na charakterystyki cieplno- przepływowe skraplania w minikanałach rurowych. Wyniki badań porównano z wynikami obliczeń według zależności innych autorów. Zastosowany przedział rozbieżności wynosił ±50%. Na podstawie uzyskanych wyników badań opracowano własną korelację do obliczenia współczynnika przejmowania ciepła i oporów przepływu badanych czynników, uzyskując zgodność z wynikami badań w zakresie ±25%.

EN

The paper presents the results of experimental investigations of heat transfer and pressure drop during the condensation of the zeotropic mixture R407C and R410A in pipe minichannels of an internal diameter d = 0,31 ÷ 3,30 mm. The local and average values of heat transfer coefficient α and pressure drop (Δp/L) in the whole range of the change in vapour quality (x = 1 ÷ 0) were calculated. On the basis of the obtained test results there was illustrated the influence of the vapour quality x, the mass flux density G and the inner diameter of channel d changes on the studied parameters. It was shown that there was a marked influence of the properties of the refrigerant on the thermal characteristics of the condensation in pipe minichannels. These results were compared with the calculation results based on the correlations of other authors. The discrepancy range was ± 50%. On the basis of given test results, own correlation was prepared to calculate the heat transfer coefficient and pressure drop of tested refrigerants which brought compatible test results in a range of ± 25%.

8

Application of computer image analyzes in the investigation of refrigerants condensation in minichannels

Sikora Małgorzata, Bohdal Tadeusz

Archives of Thermodynamics

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2019

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Vol. 40, no 1

103--114

EN

The image analysis consists in extracting from the information which is available to the observer of the part that is important from the perspective of the investigated process. This process usually accompanies a considerable reduction in the amount of information from the image. In the field of two-phase flows, computer image analysis can be used to determine flow and geometric parameters of flow patterns. This article presents the possibilities of using this method to determine the void fraction, vapor quality, bubble velocity and the geometric dimensions of flow patterns. The use of computer image analysis methods is illustrated by the example of HFE 7100 refrigerant methoxynonafluorobutane condensation in a glass tubular minichannel. The high speed video camera was used for the study, and the films and individual frames received during the study were analyzed.

9

Wybrane zagadnienia modelowania Thermal-FSI określające przepływ płynu i wymianę ciepła na przykładzie benchmarkowego eksperymentu wymiennika ciepła z minikanałami

Badur J., Kornet S., Stajnke M., Ziółkowski P., Ziółkowski J., Jóźwik P., Bojar Z.

Energetyka

|

2018

|

nr 3

154--157

PL

W artykule przedstawiono zagadnienia dotyczące modelowania pracy wymiennika ciepła z minikanałami. Wyniki obliczeń numerycznych, a mianowicie: strumienie ciepła oraz temperatury wylotowe, porównano z wynikami benchmarkowego eksperymentu z wykorzystaniem płytowego wymiennika z minikanałami (MPHE). W artykule omówiono wyniki analiz Thermal-FSI dla różnych strumieni masy wody. MPHE składa się z płyt zawierających ponacinane minikanały o przekroju prostokątnym (szerokość – 1 mm, głębokość – 700 μm) i 40 mm długości oraz kolektorów (dolotowe i wylotowe). Przedstawione analizy należy uznać za użyteczne w aspekcie projektowania wymienników ciepła.

EN

Presented are issues concerning selected problems of the advanced thermal-FSI (“Fluid Solid Interaction”) approach to numerical modeling of a heat exchanger with minichannels. Compared are results of numerical calculations relating to heat flux and outlet temperatures with the benchmark experiment data obtained with the use of the Minichannel Plate Heat Exchanger (MPHE). Analysed was the heat transfer between separated mediums for different mass flow rates. The MPHE exchanger contained two gasketed brazed plates with channels of a rectangular cross-section (width – 1 mm, depth – 700 μm, 40 mm length) and inlet and outlet manifolds. The numerical simulation was performed via thermal-FSI procedure with water as the working fluid. Presented analysis should be considered as satisfied and promising in the process of designing new heat exchangers.

10

Zastosowanie komputerowej analizy obrazu w badaniach przepływu dwufazowego w minikanałach

Bohdal T., Sikora M.

Chłodnictwo : organ Naczelnej Organizacji Technicznej

|

2018

|

R. 53, nr 4

20--23

PL

Analiza obrazu polega na wyodrębnieniu z informacji docierających do obserwatora tej ich części, która jest dla niego istotna ze względu na badany proces lub użytkownika. Procesowi temu zazwyczaj towarzyszy znaczna redukcja ilości informacji składających się na obraz [10]. W zagadnieniach przepływów dwufazowych analiza komputerowa obrazu może być wykorzystana do określenia parametrów przepływowych oraz wielkości geometrycznych struktur przepływu. Artykuł przedstawia możliwości zastosowania tej metody do wyznaczania takich wielkości, jak: stopień zapełnienia, stopień suchości x, prędkość pęcherzyka w lub określania wymiarów geometrycznych poszczególnych struktur przepływu. Zastosowanie metod komputerowej analizy obrazu przedstawiono na przykładzie skraplania czynnika chłodniczego HFE 7100 w szklanym minikanale rurowym. Do badań wykorzystano kamerę poklatkową typu i-speed, a analizie poddano filmy i poszczególne klatki otrzymane podczas badań.

EN

The image analysis consists in extracting from the information which are reached to the observer, the part that is for him important from the perspective of the investigated process. This process usually accompanies a considerable reduction in the amount of information from the image [10]. In the field of two-phase flows, computer image analysis can be used to determine flow and geometric parameters of flow patterns. This article presents the possibilities of using this method to determine the void fraction, the vapor quality x, the bubble velocity w and the geometric dimensions of the flow patterns. The use of computer image analysis methods is illustrated by the example of HFE 7100 refrigerant condensation in a glass tubular minichannal. The i-speed camera was used for the study, and the films and individual frames received during the study were analyzed.

11

Calculation method for flow boiling and flow condensation of R134a and R1234yf in conventional and small diameter channels

Mikielewicz D., Jakubowska B.

Polish Maritime Research

|

2017

|

S 1

141--148

EN

The paper presents the results of calculations performed using the authors’ model to predict the heat transfer coefficient during flow boiling for two refrigerants R134a and R1234yf. The experimental data from various past studies have been collected and the calculations have been conducted for the 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 flow boiling and flow condensation model. 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 the reduced pressure were analysed. The obtained numerical results show good consistency with those recorded in the experiments.

12

Modeling the instabilities of the condensation process of the R134a and 404A refrigerants in pipe minichannels in the conditions of periodic hydrodynamic disturbances

Kuczyński W., Denis A.

Journal of Mechanical and Energy Engineering

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2017

|

Vol. 1, No 2

179--187

EN

In the present paper, an attempt was made to model the periodic hydrodynamic instabilities of the condensation process of a refrigerant in pipe minichannels. For this purpose, a homogenous thermodynamic model with an equilibrium condition on both sides was used, which was based on the equations of the balance of the mass, momentum and energy. These equations were used for numerical modeling of flow disturbances with a phase change. This model takes into account the complexity of multiphase flows. The accuracy of the model calculations was verified by means of the experimental results obtained. A satisfactory compliance was found for this comparison, which confirms the usefulness of the computational model proposed for the determination of the influence of periodically generated hydrodynamic disturbances on the condensation process of R134a and R404A refrigerants in pipe minichannels.

13

Badanie charakterystyk cieplno-przepływowych skraplania chłodniczych mieszanin zeotropowych w mini kanałach

Bohdal T., Sikora M., Widomska K.

Instal

|

2017

|

nr 9

50--55

PL

W pracy przedstawiono wyniki badań eksperymentalnych wymiany ciepła oraz spadku ciśnienia podczas skraplania mieszanin zeotropowych R407C i R410A w minikanałach rurowych o średnicy wewnętrznej d = 0,31 ÷ 3,30 mm. Wyznaczono wartości lokalne i średnie współczynnika przejmowania ciepła a i oporów przepływu (Δp/L) w całym zakresie zmiany stopnia suchości (x = 1÷ 0). Na podstawie uzyskanych wyników badań zilustrowano wpływ zmiany stopnia suchości pary czynnika x, gęstości strumienia masy G oraz średnicy wewnętrznej kanału d na wartość badanych parametrów. Wskazano na wyraźny wpływ właściwości czynnika chłodniczego na charakterystyki cieplno-przepływowe skraplania w minikanałach rurowych. Wyniki badań porównano z wynikami obliczeń według zależności innych autorów. Zastosowany przedział rozbieżności wynosił ±50%. Na podstawie uzyskanych wyników badań opracowano własną korelację do obliczenia współczynnika przejmowania ciepła i oporów przepływu badanych czynników uzyskując zgodność z wynikami badań w zakresie ±25%.

EN

In the work results of experimental studies on heat transfer and pressure drop during condensation of R407C and R410A zeotropic refrigerants in pipes minichannels with internal diameter or d = 0.31 ÷ 3.30 mm were presented. Local and average values of heat transfer coefficient a and flow resistance (Δp/L) were calculated for the whole range of vapor quality change (x = 1 ÷ 0). Based on the obtained results, the influence of vapor quality changes x, the mass flux density G and the internal diameter of channel d on the values of the investigated parameters are illustrated. The effect of the refrigerant properties on the heat and flow characteristics during condensation was clearly indicated. The study results were compared with the results of calculations according to the correlations of other authors. The used range of accuracy was about ± 50%. On the basis of the obtained results, has been developed own correlation to calculate the heat transfer coefficient and the flow resistance of the tested refrigerants which describe phenomena with the accuracy of ± 25%.

14

Modeling of pressure drop during refrigerant condensation in pipe minichannels

Sikora M., Bohdal T.

Archives of Thermodynamics

|

2017

|

Vol. 38, no. 4

15–-28

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.

15

Determination of dryout localization using a five-equation model of annular flow for boiling in minichannels

Wajs J., Mikielewicz D.

Archives of Thermodynamics

|

2017

|

Vol. 38, no. 1

123--139

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.

16

Przegląd badań dotyczących obliczania oporów przepływu w kanałach o małej średnicy

Pietrzak M.

Inżynieria i Aparatura Chemiczna

|

2016

|

Nr 1

34--35

PL

Na podstawie aktualnego przeglądu literaturowego przedstawiono wyniki analizy metod obliczania oporów przepływu w mini- i mikrokanałach. Zwrócono uwagę na problem właściwego doboru korelacji obliczeniowych, ponieważ metody odpowiednie dla konwencjonalnych kanałów nie mogą być wprost zastosowane w przypadku mini- i mikrokanałów. Wskazano procedury obliczania oporów przepływu.

EN

Analysis results of methods used for the calculation of frictional pressure drop in mini- and microchannels based on the current state-of-the-art are presented in the paper. A selection of appropriate calculation correlations appears in case of mini- or microchannels. This difficulty stems from the fact that calculation methods developed for conventional channels cannot be directly applied to channels with a smaller internal diameter. The procedures for the calculation of frictional pressure drop are indicated.

17

Validation of the generalized model of two-phase thermosyphon loop based on experimental measurements of volumetric flow rate

Bieliński H.

Archives of Thermodynamics

|

2016

|

Vol. 37, no. 3

109--138

EN

The current paper presents the experimental validation of the generalized model of the two-phase thermosyphon loop. The generalized model is based on mass, momentum, and energy balances in the evaporators, rising tube, condensers and the falling tube. The theoretical analysis and the experimental data have been obtained for a new designed variant. The variant refers to a thermosyphon loop with both minichannels and conventional tubes. The thermosyphon loop consists of an evaporator on the lower vertical section and a condenser on the upper vertical section. The one-dimensional homogeneous and separated two-phase flow models were used in calculations. The latest minichannel heat transfer correlations available in literature were applied. A numerical analysis of the volumetric flow rate in the steady-state has been done. The experiment was conducted on a specially designed test apparatus. Ultrapure water was used as a working fluid. The results show that the theoretical predictions are in good agreement with the measured volumetric flow rate at steady-state.

18

The analysis of thermal and flow characteristics of the condensation of refrigerant zeotropic mixtures in minichannels

Bohdal T., Widomska K., Sikora M.

Archives of Thermodynamics

|

2016

|

Vol. 37, no. 2

41--69

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%.

19

Prediction of flow boiling heat transfer coefficient for carbon dioxide in minichannels and conventional channels

Mikielewicz D., Jakubowska B.

Archives of Thermodynamics

|

2016

|

Vol. 37, no. 2

89--106

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.

20

Condensation of refrigerant R407C in multiport minichannel section

Bohdal T., Charun H., Sikora M.

Archives of Thermodynamics

|

2016

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Vol. 37, no. 4

3--18

EN

Analysis of the state of-the-art in research of refrigerant condensation in miniature heat exchangers, so-called multiports, was made. Results of refrigerant R407C condensation in a mini condenser made in the form of two bundles of tubular minichannels from stainless steel with an inside diameter 0.64 mm and length 100 mm have been presented. Two exchangers consisted of four minichannels and 8 minichannels have been investigated. The values of average heat transfer coefficient and frictional pressure drops throughout the condensation process were designated. The impact of the vapor quality of refrigerant and the mass flux density on the intensity of heat transfer and flow resistance were illustrated. A comparative analysis of test results for various refrigerants in both mini heat exchangers were made.