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Number of results: 15
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Abstract

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.

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Authors and Affiliations

Dariusz Mikielewicz
Jan Wajs
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Abstract

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.

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Authors and Affiliations

Małgorzata Sikora
Tadeusz Bohdal
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Abstract

In the paper presented is the analysis of the results of calculations using a model to predict flow boiling of refrigerants such as R134a, R600a and R290. The latter two fluids were not used in the development of the model semiempirical correction. For that reason the model was verified with present experimental data. The experimental research was conducted for a full range of quality variation and a relatively wide range of mass velocity. The aim of the present study was also to test the sensitivity of developed model to a selection of the model of two-phase flow multiplier and the nonadiabatic effects. For that purpose two models have been analysed namely the one due to Müller-Steinhagen and Heck, and Friedel. In addition, the work shows the importance of taking surface tension into account in the calculation of the flow structure.
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Authors and Affiliations

Dariusz Mikielewicz
Blanka Jakubowska
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Abstract

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.
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Authors and Affiliations

Tadeusz Bohdal
Małgorzata Sikora
Katarzyna Widomska
Andrii M. Radchenko
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Abstract

Flow boiling and flow condensation are often regarded as two opposite or symmetrical phenomena. Their description however with a single correlation has yet to be suggested. In the case of flow boiling in minichannels there is mostly encountered the annular flow structure, where the bubble generation is not present. Similar picture holds for the case of inside tube condensation, where annular flow structure predominates. In such case the heat transfer coefficient is primarily dependent on the convective mechanism. In the paper a method developed earlier by the first author is applied to calculations of heat transfer coefficient for inside tube condensation. The method has been verified using experimental data from literature on several fluids in different microchannels and compared to three well established correlations for calculations of heat transfer coefficient in flow condensation. It clearly stems from the results presented here that the flow condensation can be modeled in terms of appropriately devised pressure drop.
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Authors and Affiliations

Dariusz Mikielewicz
Rafał Andrzejczyk
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Abstract

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.

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Authors and Affiliations

Małgorzata Sikora
Tadeusz Bohdal
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Abstract

The numerical simulation of the heat transfer in the flow channels of the minichannel heat exchanger was carried out. The applied model was validated on the experimental stand of an air heat pump. The influence of louver heights was investigated in the range from 0 mm (plain fin) to 7 mm (maximum height). The set of simulations was prepared in Ansys CFX. The research was carried out in a range of air inlet velocities from 1 to 5 m/s. The values of the Reynolds number achieved in the experimental tests ranged from 93 to 486. The dimensionless factors, the Colburn factor and friction factor, were calculated to evaluate heat transfer and pressure loss, respectively. The effectiveness of each louver height was evaluated using the parameter that relates to the heat transfer and the pressure drop in the airflow. The highest value of effectiveness (1.53) was achieved by the louver height of 7 mm for the Reynolds number of around 290.
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Authors and Affiliations

Artur Romaniak
1
Michał Jan Kowalczyk
1
Marcin Łęcki
1
Artur Gutkowski
1
Grzegorz Górecki
1

  1. Lodz University of Technology, Zeromskiego 116, 90-924 Łódz, Poland
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Abstract

The results of experimental investigations of heat transfer and a friction factor in an air channel of the minichannel heat exchanger are presented. The main aim of the analysis was to examine an influence of geometrical parameters of the fin shape with two geometries on heat transfer and flow characteristics of the air channel. The test rig was designed to monitor the parameters of the airflow during cooling by the minichannel heat exchanger. The analysis was conducted with the airflow in the range of 1–5 m/s. The temperature of the evaporation in a refrigeration system was set at 288.15 K. The energy balance of the refrigeration system was carried out. A numerical model describes the airflow through a part of the heat exchanger. Numerical simulations were validated with the experimental data. Numerical methods were used to evaluate the performance of the system and possibilities to improve the fin geometry. The characteristics of the friction factor (a measure of the pressure loss in the airflow) and the Colburn j-factor (heat transfer performance) were calculated. For the maximal velocity of the airflow, the Colburn factor was equal to 0.048 and the evaporator capacity equaled 1914 W.
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Bibliography

[1] Islam S., Islam M.S., Abedin M.Z.: Review on heat transfer enhancement by louvered fin. Int. J. Eng. Mater. Manufact. 6(2021), 60–80.
[2] Muszynski T., Kozieł S.M.: Parametric study of fluid flow and heat transfer over louvered fins of air heat pump evaporator. Arch. Thermodyn. 37(2016), 3, 45–62.
[3] Dodiya K., Bhatt N., Lai F.: Louvered fin compact heat exchanger: a comprehensive review. Int. J. Amb. Energ. (2020).
[4] Wan R., Wang Y., Kavtaradze R., Ji H., He X.: Research on the air-side thermal hydraulic performance of louvered fin and flat tube heat exchangers under low-pressure environment. Exp. Heat Transfer 33(2020), 1, 81–99.
[5] Gunnasegaran P., Shuaib N.H., Abdul Jalal M.F.: The effect of geometrical parameters on heat transfer characteristics of compact heat exchanger with louvered fins. ISRN Thermodyn. (2012), 1–10.
[6] Djamal H.D., Woon Q.Y., Suzairin M.S., Hisham Amirnordin S.: Effects of geometrical parameters to the performance of louvered fin heat exchangers. Appl. Mech. Mater. 773-774(2015), 398–402.
[7] Amirnordin S.H., Didane H.D., Norani Mansor M., Khalid A, Suzairin M.S., Raghavan V.R.: Pressure drop and heat transfer characteristics of louvered fin heat exchangers. Appl. Mech. Mater. 465-466(2014), 500–504.
[8] Chan Kang H., Jun G.W.: Heat transfer and flow resistance characteristics of louver fin geometry for automobile applications. J. Heat Transfer. 133(2011), 1–6.
[9] Okbaz A., Olcay A.B., Cellek M.S., Pinarbasi A.: Computational investigation of heat transfer and pressure drop in a typical louver fin-and-tube heat exchanger for various louver angles and fin pitches. EPJ Web Conf. 143(2017), 02084.
[10] Park J.S., Kim J., Lee K.S.: Thermal and drainage performance of a louvered fin heat exchanger according to heat exchanger inclination angle under frosting and defrosting conditions. Int. J. Heat Mass Transf. 108(2017), 1335–1339.
[11] Liu X., Chen H., Wang X., and Kefayati G.: Study on surface condensate water removal and heat transfer performance of a minichannel heat exchanger. Energies 13(2020), 5, 1065
[12] Saleem A., Kim M.H.: CFD analysis on the air-side thermal-hydraulic performance of multi-louvered fin heat exchangers at low Reynolds numbers. Energies 10(2017), 6, 1–24.
[13] Bohdal T., Charun H., Sikora M.: Heat transfer during condensation of refrigerants in tubular minichannels. Arch. Thermodyn. 33(2012), 2, 3–22.
[14] ASHRAE: ANSI/ASHRAE Standard 41.2-1987: Standard Methods for Air Velocity and Airflow Measurement (2018).
[15] Manual Ansys-CFX, Release 2020 R2. http://www.ansys.com (accessed 15 July 2020).
[16] Jasinski P.B., Kowalczyk M.J., Romaniak A., Warwas B., Obidowski D., Gutkowski A.: Investigation of thermal-flow characteristics of pipes with helical micro-fins of variable height. Energies 14(2021), 8, 2048.
[17] Kang, Hie-Chan & Jun, Gil.: Heat transfer and flow resistance characteristics of louver fin geometry for automobile applications. J. Heat Transf. 133 (2011), 101802.
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Authors and Affiliations

Michał Jan Kowalczyk
1
Marcin Łęcki
1
Artur Romaniak
1
Bartosz Warwas
1
Artur Gutkowski
1

  1. Lodz University of Technology, Institute of Turbomachinery, Wólczanska 217/221, 93-005 Łódz, Poland
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Abstract

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.

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Authors and Affiliations

Henryk Bieliński
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Abstract

This paper present the results of experimental investigations of condensation of R134a refrigerant in pipe minichannels with internal diameters 0.64, 0.90, 1.40, 1.44, 1.92 and 3.30 mm subject to periodic pressure instabilities. It was established that as in conventional channels, the displacement velocity of the pressure instabilities distinctly depends on the frequency of their hydrodynamic generation. The void fraction distinctly influences the velocity of the pressure instabilities. The form of this relationship depends on the internal diameter of the minichannels and on the method of calculating the void fraction.

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Authors and Affiliations

Waldemar Kuczyński
Henryk Charun
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Abstract

The present paper describes the results of experimental investigations of heat transfer during condensation of R134a, R404A and R407C in pipe minichannels with internal diameters 0.31-3.30 mm. The results concern investigations of the local heat transfer coefficient. The results were compared with the correlations proposed by other authors. Within the range of examined parameters of the condensation process in minichannels made of stainless steel, it was established that the values of the heat transfer coefficient may be described with Akers et al., Mikielewicz and Shah correlations within a limited range of the mass flux density of the refrigerant and the minichannel diameter. On the basis of experimental investigations, the authors proposed their own correlation for the calculation of local heat transfer coefficient.
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Authors and Affiliations

Tadeusz Bohdal
Henryk Charun
Małgorzata Sikora
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Abstract

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.

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Authors and Affiliations

Tadeusz Bohdal
Henryk Charun
Małgorzata Sikora
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Abstract

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.

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Authors and Affiliations

Dariusz Mikielewicz
Blanka Jakubowska
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Abstract

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%.

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Authors and Affiliations

Tadeusz Bohdal
Katarzyna Widomska
Małgorzata Sikora
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Abstract

The present paper describes results of experimental investigations of pressure drop during the condensation of R134a, R404a and R407C refrigerants in pipe minichannels with internal diameter 0.31-3.30 mm. The results concern investigations of the mean and local pressure drop in single minichannels. The results of experimental investigations were compared with the calculations according to the correlations proposed by other authors. A pressure drop during the condensation of refrigerants is described in a satisfactory manner with Friedel and Garimella correlations. On the basis of the experimental investigations, the authors proposed their own correlation for calculation of local pressure drop during condensation in single minichannels.
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Authors and Affiliations

Tadeusz Bohdal
Henryk Charun
Małgorzata Sikora

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