Pipe Minichannels Research Articles

High Pressure Refrigerants Condensation Modeling in Horizontal Pipe Minichannels

The study of refrigerant condensation in minichannels is a very wide and complicated issue. An important aspect due to the range of the two-phase changes is mathematical and computer modeling. It allows providing calculation of heat transfer coefficient and pressure drops for many different refrigerants under different flow conditions. A mathematical model for the calculation of heat transfer coefficient and pressure drop during condensation of refrigerants in pipe minichannels is presented in this work. The model was obtained in MATLAB software based on conservation equations. The results of the calculations were verified by the authors’ own experimental investigations data and experimental data from the literature. This article concerns the results of R404A, R410A and R407C refrigerants condensation modeling.

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

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.

Regression Model of Dynamic Pulse Instabilities during Condensation of Zeotropic and Azeotropic Refrigerant Mixtures R404A, R448A and R507A in Minichannels

This paper presents experimental research and mathematical modeling data concerning the impact of unit dynamic instabilities on the phase-transition condensation processes of the zeotropic mixtures R404A and R448A and azeotropic R507A refrigerants in pipe minichannels. The R507 refrigerant is currently used as a temporary substitute for R404A, whereas R448A is a sustainable prospective substitute for R404A. The study presents experimental testing data for the condensation processes of these refrigerants in pipe minichannels and a proposal for the use of dimensional analysis, including the Π-Buckingham theorem, to determine the regression relationship explaining the propagation of unit dynamic instabilities. Based on the experimental studies performed, regression computational models were developed and showed satisfactory agreement in the range of 20% to 25%. They give the possibility to identify, in a utilitarian, way the speed of propagation of temperature and pressure instabilities during the liquefaction of refrigerants. The study was carried out on pipe minichannels with an internal diameter of di = 3.3, 2.3, 1.92, 1.44 and 1.40 mm.

Experimental Study of HFE 7000 Refrigerant Condensation in Horizontal Pipe Minichannels

This article presents the results obtained from our own experimental investigations on heat exchange and pressure drop during the condensation flow of the HFE 7000 refrigerant in pipe minichannels with an internal diameter of di = 1.2–2.5 mm. The influence of vapor quality x and the mass flux density G on the two-phase flow pressure drops and heat transfer is presented. The tests were performed for the mass flux density range of G = 110–4700 kg/m2s, saturation inlet temperature of Ts = 36–43 °C and heat flux density of q = 1 ÷ 20 kW/m2. The pressure drop characteristics and heat transfer coefficient as a function of the internal diameter of minichannels are illustrated. The results of experimental research on the heat transfer coefficient and two-phase pressure drop are compared with correlations developed by other authors. The best accuracy has a comparison of experimental study with correlation of Rahman-Kariya-Miyara et al. and Mikielewicz et al.

External Condensation of HFE 7000 and HFE 7100 Refrigerants in Shell and Tube Heat Exchangers.

The paper describes the results of experimental studies of media as an intermediary in heat exchange taking place in low volume conditions. Their properties predestine them both as a future-proof for transporting and storing heat materials. The paper concerns the current topic related to the miniaturization of cooling heat exchangers. There are many studies in the literature on the phase transition of refrigerants in the flow in pipe minichannels. However, there is a lack of studies devoted to the condensation process in a small volume on the surface of pipe minichannels. The authors proposed a design of a small heat exchanger with a shell-and-tube structure, where the refrigerant condenses on the outer surface of the pipe minichannels cooled from the inside with water. It is a response to the global trend of building highly efficient, miniaturized structures for cooling and air conditioning heat exchangers. Two future-proof, ecological replacements of the CFC refrigerants still present in the installations were used for the experimental research. These are low-pressure fluids HFE 7000 and HFE 7100. The tests were carried out in a wide range of changes in thermal-flow parameters: G = 20–700 kg·m−2s−1, q = 3000–60,000 W·m−2, ts = 40–80 °C.

Heat and flow investigation of NOVEC649 refrigerant condensation in pipe minichannels

This paper presents the experimental research results of Novec649. The refrigerant was condensed in 4 pipe minichannels with hydraulic diameters of dh = 2.5, 2.0, 1.2 and 0.5 mm. This refrigerant is a low-pressure fluid with an ODP = 0 and a GWP = 1. The aim of the study was to determine average and local values of the heat transfer coefficient α and the pressure drop (Δp/L) for the entire range of the vapour quality (x = 1 ÷ 0) of Novec649 in the pipe minichannels. The tests were carried out on a specially designed and built laboratory test stand, which was described in this paper. The influence of the mass flux density G, the vapour quality x and the internal channel diameter dh on the studied parameters was illustrated. These experimental results were compared with the calculation results based on the Bohdal et al. [1] correlations.

Refrigerant condensation in vertical pipe minichannels under various heat flux density level

This work is a study of the condensation process taking place in the conditions so far unproven. It was devoted to selected problems accompanying the condensation phenomenon occurring in a mini scale. The work is a continuation and development of the authors' research to-date. Paper describes the flow condensation of high-pressure refrigerants through channels made of stainless steel AISI 304 and AISI 316L with various inner diameter di. A new heat transfer universal correlation was developed on the basis of own experimental data for high pressure refrigerants condensation in pipe minichannels.

High pressure refrigerants condensation in vertical pipe minichannels

The paper consists a description of thermal and flow experimental research on high-pressure refrigerants condensation process in vertical minichannels. The influence of mass flux density, heat flux density, the vapour quality and minichannel's internal diameter on the condensation process was investigated. Under the same conditions, R404A, R407C, R410A refrigerants were condensed. The study also consists a summary of data obtained during condensation process of all three investigated refrigerants. The results of experimental studies were compared with the results of theoretical modeling according to the dependencies developed by other authors. The compliance of the modeling data with the results of the experiment was considered unsatisfactory. Based on the results of experimental data, heat transfer coefficient and pressure drop prediction models for low heat fluxes (q up to 5000 W/m2) were obtained.

Condensation of Novec 649 refrigerant in pipe minichannels

The paper presents the results of experimental investigation of Novec 649 refrigerant condensation in tube minichannels. This is a low-pressure refrigerant. This investigations are basis for flow structures visualization during condensation in pipe minichannels. The local and the average values of pressure drop (Δp/L) and heat transfer coefficient α in the whole range of the changes of 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. These results were compared with the calculation results based on the dependencies of other authors.

An investigation of heat transfer coefficient during refrigerants condensation in vertical pipe minichannels

This article presents the results of experimental research of heat transfer coefficient in air cooled vertical pipe minichannels during R404A, R407C and R410A high-pressure refrigerants condensation. The study determined local and average heat transfer coefficient values in full range of vapour quality, x = 0 ÷ 1. On the basis of experimental investigations, the dependence of heat transfer on the vapour quality x, mass flux density G and minichannel's internal diameter dw was obtained.

Modeling of Pressure Drop During Refrigerant Condensation in Pipe Minichannels

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.

Modeling of temperature instabilities during condensation of R134a refirgerant in pipe minichannels

The propagation velocity of temperature instabilities is directly connected with the displacement of the so-called condensation front. This phenomenon was defined as a displacement of an area with a negligently small thickness that separates condensing vapour in a two-phase mixture from the liquid zone of the subcooled condensate. In the case of a phase change of condensation realized in established conditions, the condensation front displaces in the opposite direction to the vapour incoming to the channel. As a result of hydrodynamic or hydrostatic interactions, the displacement direction of the condensation front depends of whether we deal with the process of the development or decay of the condensation phase change. The present study contains a proposal concerning modeling of the propagation velocity of the condensation front in pipe minichannels for the R134a refrigerant in the conditions of the interactions of hydrodynamic instabilities. The diameters examined of minichannels were D=0.64; 0.90; 1.40; 1.44; 1.92; 2.30 and 3.30mm.

Investigation of flow structures during HFE-7100 refrigerant condensation

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.

Empirical study of heterogeneous refrigerant condensation in pipe minichannels

This paper concerns the experimental test results of homogeneous refrigerant R134a and zeotropic solutions R404A, R407C and R410A condensation in pipe minichannels with an internal diameter d = 0.90–3.30 mm. The paper presents experimental thermal and flow characteristics of condensation in comparable ranges. The experimental results were compared to the calculated results of other authors' correlations. It was found that the discrepancy between the results of tests and calculations is large, especially for zeotropic solutions, and in excess of the range of ±50%. The authors developed their own correlations, providing compatibility within ±25% for all of the tested refrigerants. Correlations are proposed for annular and annular-stratified two-phase flow structures.

Modeling of a Two-Phase Region Length of the Condensation of R134a and R404A Refrigerants in Pipe Minichannels With Periodic Hydrodynamic Instabilities

The heat removal process of a compact condenser in a refrigeration compressor system, constructed on the basis of pipe minichannels, occurs in three regions: the removal region of superheat, the two-phase proper condensation region, and the liquid subcooling region. The intensity of heat removal in the two-phase region depends on the heat efficiency of the condenser. The length of this region is determined by the process parameters and the external influences. This paper analyzes the influence of periodically generated hydrodynamic disturbances on the length of the condensation region for R134a and R404A refrigerants. Experimental tests were conducted for six diameters of pipe minichannels: 0.64, 0.90, 1.44, 1.92, 2.30, and 3.30 mm. A range of frequencies from 0.2 to 5 Hz of periodically generated disturbances showed an explicit and unfavorable influence on the decrease of the length of the proper condensation region. This influence reduced the efficiency of the process. A computational model was developed to calculate the length of the two-phase region under disturbance conditions. The model proposed was verified by the results of the experiment, and compliance was obtained in the range of ±15%.

Impact of Periodically Generated Hydrodynamic Disturbances on the Condensation Efficiency of R134a Refrigerant in Pipe Mini-Channels

This article discusses an experimental analysis of the influence of periodically generated external hydrodynamic disturbances on the condensation process of the R134 refrigerant in circular pipe mini-channels. The investigations were carried out for horizontal single mini-channels with internal diameters in the range of d = 0.64–3.30 mm. It was established that within the range of frequencies under investigation (f = 0.2–5 Hz), i.e., the frequencies at which disturbances are generated, the disturbances exert an influence on the structure of the characteristic condensation areas in the mini-channel. The disturbances have an unfavorable impact on the reduction of the two-phase length of specific condensation and a reduction of the heat flux in this area. The heat removal intensity in the two-phase area has an impact on the efficiency of the operation of a compact condenser. The impact of more important parameters on this efficiency is discussed.

Modeling of the propagation of a pressure wave during the condensation process of R134a refrigerant in a pipe minichannel under the periodic conditions of hydrodynamic disturbances

The present paper covers the results of theoretical and experimental investigations concerning the propagation of a pressure wave triggered by external hydrodynamic influences in the condensation process of R134a refrigerant in pipe minichannels with an internal diameter of d=0.64–3.30mm. An ensemble-averaged gas-liquid two-fluid model with slippage of the velocity of phases was used when there was no thermodynamic equilibrium between the phases. The momentum and mass conservation equations were used for the analysis. In the model, the average values of parameters were assumed for the individual phases that describe the state of the system. The influence of the friction resistances of the flow in minichannels was also taken into consideration. The heat transfer between phases in the condensation process was described with Fourier’s one-dimensional equation. With the use of the author’s self-developed computational code, the following were defined in the MATLAB 8 program: the propagation velocity of the pressure wave cTPF, the attenuation coefficient η of this wave and the dependence of these quantities on the angular frequency ω of disturbances generated and on the void fraction α. The results of the model calculations were compared with the results of the experimental investigations, and satisfactory compliance was obtained.

Heat transfer during condensation of refrigerants in tubular minichannels

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.

Pressure drop during condensation of refrigerants in pipe minichannels

Pressure drop during condensation of refrigerants in pipe minichannelsThe 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.

Thermal and flow characteristics of the condensation of R404A refrigerant in pipe minichannels

This article presents the results of experimental investigations of heat exchange and pressure drop during the condensation of R404A refrigerant in stainless steel pipe minichannels with internal diameters of 1.4–3.30mm. A review is provided of the present state of knowledge concerning the condensation of this refrigerant in conventional channels and in small-diameter channels. It is emphasized that there are few prior publications concerning this issue. The test setup is described as well as the results of the experimental tests. We discuss the dependence of the heat transfer coefficient and the pressure drop of the R404A refrigerant on both minichannel diameter and process parameters. The pressure drop during the condensation of this refrigerant is satisfactorily described by the Friedel and Garimella correlations. Based on the experimental tests, we propose a new correlation for the calculation of the local heat transfer coefficient. The values calculated from this correlation were in agreement with the experimental results to within ±20%.