Refrigerant Flow Characteristics Research Articles

Mass flow characteristics prediction of refrigerants through electronic expansion valve based on XGBoost

In an effort to enhance the prediction accuracy of the mass flow characteristics of refrigerants through electronic expansion valves (EEVs), this study develops a mass flow model using the XGBoost machine learning algorithm. Utilizing experimental data from open literature for refrigerants R1233zd(E) and R245fa, the model aims to accurately predict the mass flow coefficient in EEVs, a crucial aspect for improving the performance and efficiency of refrigeration systems. The model's performance is evaluated using the coefficient of determination (R2) on the training and test datasets, revealing a minimal performance gap and no overfitting issue. Remarkably, our model's predictions for R1233zd(E) and R245fa datasets align consistently with experimental data, with 96.4 % and 90.6 % of the predicted data deviating from the experimental ones within ±5 %, respectively. Further, the root-mean-square error (RMSE) and coefficient of variation of the root mean square error (CV-RMSE) values are quite low, with 2.2 % and 2.33 % for R1233zd(E), and 4.23 % and 4.47 % for R245fa, indicating high prediction accuracy. Compared to the original models, the paper approach with the XGBoost algorithm significantly improves prediction accuracy. This advancement provides a promising direction for developing more efficient and reliable control strategies for refrigeration systems in various applications.

Mass flow characteristics and correlations of R454C flow through an electrical expansion valve in a heat pump split system

The use of electronic expansion valves (EEVs) in heat pump systems has seen a rapid increase in recent years due to their advantages such as fast response, accurate control, and ability to improve seasonal performance. Modeling of the refrigerant flow characteristics through EEVs is needed during design for optimizing system performance and operation. Previous studies have not led to any generalized correlations for EEVs that accurately predict performance for different refrigerants. When working with a new refrigerant and/or EEV, it is necessary to perform mass flow measurements for the EEV over a wide range of operating conditions that are then used to develop and validate a refrigerant and EEV specific correlation. R454C, a low-GWP refrigerant mixture, is gaining attention as a potential replacement for R404A and R410A systems. However, there is currently no available EEV experimental data or correlation specifically for R454C in the literature. The objective of this paper is to present experimental results and characterization of R454C flowing through an EEV in a heat pump split system across a wide range of operational conditions. The impacts of valve opening, subcooling degree and inlet pressure on mass flow rate though the EEV were investigated. Two semi-empirical correlations were developed that map the experimental data based on the Buckingham π theorem and a polynomial function. Both mapping methods predicted mass flow rate for test data within 10%. The Buckingham π correlation was slightly more accurate. Both correlations were developed to enable mass flow rate predictions for R454C passing through EEVs with different valve sizes. Finally, a relationship between refrigerant molar mass and mass flow coefficient was observed that could be useful in developing a general EEV correlation that can be applied for different refrigerants. However, further investigation is needed to validate this relationship.

Adiabatic frictional pressure gradient during flow boiling of pure refrigerant R1233zd and non-azeotropic mixtures R448A, R452A and R455A

The research on two-phase flow characteristics of refrigerants is of primary importance in several fields, such as air conditioning and refrigeration systems. Therefore, the determination of the pressure drop during flow boiling is important for the correct design of evaporators and heat spreaders systems. This paper presents a collection of experiments on flow boiling pressure drop using pure refrigerant R1233zd and new low-GWP refrigerant mixtures R448A, R452A and R455A. All tests were performed in adiabatic conditions, in a smooth horizontal stainless-steel tube having an internal diameter of 6.0 mm and a thickness of 1.0 mm. The effect of operating parameters, such as (bubble) saturation temperature (from 25 to 65 °C) and mass flux (from 150 to 600 kg/m2s) is investigated and discussed, and the performance of the chosen fluids is also compared. Finally, an assessment of existing prediction methods is carried-out to find the most suitable correlations for two-phase pressure drop evaluation.

Performance investigation and design optimization of a vapor injection rotary compressor without check valve in injection path

• The reason for the backflow of injection compressor is analyzed. • The optimal start and end of injection rotation angles are proposed. • The optimal design of the injection port increases the heating capacity by 22–45%. • The optimal design of the injection port increases the COP by 3.06–5.15%. • The operating strategy is optimized based on the CFD results. In recent years, low-temperature air source heat pumps using vapor refrigerant injection technique, which can effectively improve the performance and heating capacity of the rotary compressor heat pump systems in cold regions, have become more and more widely used. The injection rotary compressor without a check valve in the injection path has a great manufacturing advantage compared to that with a check valve. Improper position of the injection port will cause serious backflow during compression, which will greatly affect the performance of the heat pump. In order to study and correct the backflow in the injection process of the compressor, this paper analyzes the refrigerant flow characteristics of the compressor with a circular injection port without a check valve based on experiments and computational fluid dynamics. Through the analysis of the cause of backflow, the optimal position of the injection port is proposed, which has effectively solved the problem of refrigerant backflow caused by the injection. In addition, the operating strategy is optimized based on the results of computational fluid dynamics. The simulation results indicate that at low ambient temperatures, the optimal design of the injection port structure has increased the heating capacity 21.75%–45.3% over the traditional compressor without injection, and the coefficient of performance for heating also has been increased by 3.06%–5.15%.

Experimental Investigation on Heating Performance of Newly Designed Air Source Heat Pump System for Electric Vehicles

Abstract Utilizing the heat from air source with heat pump system in electric vehicles shows a significant advantage from thermoelectric heat source for heat supply in cold climate. It could improve the driving range of electric vehicles considerably in winter and replace the positive temperature coefficient (PTC) heater with an acceptable cost and reliability. In this work, a newly designed heat pump system was first introduced with less components and cost. Second, experiments were conducted to investigate its cooling performance, and subsequent heating performance from −10 to 10 °C. The typical heat transfer and flow characteristics of refrigerant were recorded, and the behavior of each component including compressor, evaporator, condenser, and outside heat exchanger were analyzed and interpreted. The results showed that the heating and cooling performance of the new heat pump system could almost remain the same with traditional air-conditioning system in automobile and surely satisfy with the heat requirement of electric vehicles. In the heating mode, the maximum heating capacity increases by 13% at 400 m3/h air volume from 300 m3/h at the ambient temperature −10 °C, while the outlet air temperature decreases by 4–6%. In addition, using a heat pump system showed an increase in the driving range of electric vehicles by 25–31% as compared to PTC heaters.

Evaluation of the constituent correlations for predicting the refrigerant flow characteristics in adiabatic helically coiled capillary tubes

Capillary tubes with straight, spiral, and helically coiled shapes have been widely used as expansion devices in small refrigeration systems. Many physical models are required to evaluate the refrigerant flow rate through a helically coiled capillary tube because of the complicated flow behavior in such tube. In this study, a numerical model was developed to evaluate refrigerant flow through a helically coiled capillary tube. The mass flow rates predicted using various combinations of empirical correlations were compared with the experimental data available in the open literature, including R22, R134a, R407C, R410A and LPG. On the basis of these comparisons, a recommended set of correlations was selected to calculate the mass flow rates through a helically coiled capillary tube. The present numerical model exhibited good agreement with the experimental data.

Numerical Simulation for Falling Film Flow Characteristics of Refrigerant on the Smooth and Structured Surfaces

Flow characteristics of a liquid film flowing over a smooth surface and structured surface with the Reynolds number range from 10 to 1121 are studied. The mixture of R21 and R114 refrigerants is used as the test liquid. The 3D transient simulations are taken to capture the liquid film’s dynamic characteristics and spatial distribution. Effects of the inlet dimension, inlet flow rates, surface tension, and surface structuring on the wettability, average velocity, and film thickness are studied systematically. The obtained results show that surface tension is essential for an accurate simulation, while inlet width has no effect on the liquid film parameters in the steady-state flow regime. For low flow rates, wetting area and film thickness both are small, and a suggested range of Reynolds number is chosen to simulate further heat transfer in order to balance the film thickness and dry spots generation. It is shown that a ripple surface structure hinders the liquid film movement, reflected in a lower velocity and a larger film thickness compared to the smooth surface. Lateral movement of a liquid film can also be observed at the structured surface.

CFD Analysis of Nanorefrigerant through Adiabatic Capillary Tube of Vapour Compression Refrigeration System

Over time attempts have been made to understand the flow characteristics of refrigerants through capillary tubes as well as to seek more thermally efficient working fluids for refrigeration systems. This study investigated the flow of nanorefrigerants through adiabatic capillary tubes of vapour compression refrigeration systems; and afterwards creates numerical models that will account for solution of refrigerant side pressure drop and mass flow rate. Also in this study, a CFD flow analysis was carried out using a CFD simulation/solver such that the results of the simulations obtained were discussed so as to establish a distinction between the conventional and nano-refrigerants. Upon comparison of the CFD results of nanorefrigerants (CuR134a, CuR600a) and the conventional refrigerants (R134a, R600a), the conventional refrigerants were noticed to have more isothermal regions implying that heat was not being transferred quickly enough to raise the temperature of the adjoining region thus proving that the addition of nanoparticles improves the thermophysical properties of the base fluid. Also, based on the results of the study of the flow patterns of both working fluids, the density of pressure contours in the conventional refrigerants was far larger than that of the nanorefrigerant implying that more compressor work and ultimately greater power will be required. The findings from this study were validated with experimental results showing that a CFD analysis tool/method can be employed to understudy the phenomenal changes that take place in nano-refrigerant movement through capillary tubes without recourse to experimentation.

A Review on Flow Characteristics of Refrigerant and Oil Mixture in a Heat Pump System

This paper presents a comprehensive review of oil retention and performance of oil separator in a heat pump system. Liquid volume fraction model, viscous film model and analytical model from continuity and momentum equations for predicting the oil retention amount in a heat pump system are reviewed. In addition, weighing a test section, using an oil injection and extraction device and optical analysis for measuring the oil retention amount are introduced and experimental results are analyzed. Numerical studies which used commercial tools for computational fluid dynamics or suggested a new model by simplifying the flow in cyclone separator are reviewed. Effects of droplet size and angular velocity of gas phase flow in cyclone separator on the performance are discussed. Experimental methods for measuring the efficiency of oil separator are introduced, and theoretical analysis on the efficiency and pressure drop of cyclone separator is suggested.

Flow characteristics of refrigerant and oil mixture in an oil separator

The efficiency and pressure drop in the oil separator of a multi heat pump system were experimentally investigated. R410A and polyvinylether (PVE) oil were used as the refrigerant and lubricant of the system, respectively. The refrigerant mass flow rate and the liquid circulation ratio were varied between 30 and 150 g s−1 and between 1.5% and 4.5%, respectively. Five different oil separators were considered, and the measured efficiencies and pressure drops were analyzed with respect to the height and diameter of the oil separators. The analytical results were used to develop empirical equations for predicting the efficiency and pressure drop of the oil separator. The mean absolute percentage errors between the predicted and measured efficiencies and pressure drops for the oil separators were 0.4% and 11.7%, respectively.

Investigation of Flow and Heat Transfer Characteristics in Two-Phase Flow Through Evaporator of Diffusion Absorption Refrigerator (DAR)

The application of the cooling system becomes very important for a comfortable and almost all public buildings in Indonesia using the cooling system. The cooling system is commonly requires considerable electrical power to drive the compressor used for the vapor compression cooling system in operation. Different with the diffusion absorption refrigeration system, all cycles in the system are occurred by natural circulation where the refrigerant is pressurized (partial) lower in the evaporator is obtained with the aid of an inert gas that can be either hydrogen or helium, which is mixed with the refrigerant gas, but not also react. The study was conducted by flowing the refrigerant at the evaporator copper tube passes that are part of a diffusion absorption refrigeration machine. Long dimension of passes evaporator is 2.38875 m. The method used in this study is the experimental and numerical approach, which is using Multiphase Flow Fluent CFD 6.3. simulation to validate the experimental observations of flow.Through this study, the fenonena happened to be validated by experimental observations numerically. Also can be seen flow characteristics of refrigerant flowing along the evaporator and heat transfer phenomena that occur.

A study on the flow characteristics of refrigerant and oil mixture in compressor suction line

Abstract This paper discusses the experimental results of oil retention amount and pressure drop in the gas line of refrigerant. R410A and polyvinylether oil were used as refrigerant and lubricant, respectively. The experiment was conducted under various pipe inner diameters of 14.1, 17.3 and 26.0 mm, and refrigerant mass flux and oil circulation ratio (OCR) were varied from 40 to 250 kg m −2 s −1 and 0.5–4.0%, respectively. The oil retention amount tends to increase as refrigerant mass flux decreases and OCR increases. The oil retention amount in vertical pipe was higher than that in horizontal pipe. The empirical expressions for predicting the oil retention amount and pressure drop were suggested. The mean absolute percentage errors of each expression for predicting the oil retention amount in horizontal and vertical pipes were 18.1% and 14.1%, respectively, and those for predicting the pressure drop in horizontal and vertical pipes were 22.2% and 20.1%, respectively.

A Study on the Refrigerant Flow Characteristics Flowing into the Interior of the Electromagnetic Control Valve

A Study on the Refrigerant Flow Characteristics Flowing into the Interior of the Electromagnetic Control Valve

Thermal and hydraulic analysis on the flow around the motor in semi-hermetic twin screw refrigeration compressors

A comprehensive model was developed to study the flow and thermal behavior of the motor cooled by suction refrigerant in semi-hermetic twin screw refrigeration compressors. The model is sufficiently complex to consider the special motor features, including: refrigerant flow characteristics and thermal heating caused by electrical losses; heating effect of oil from bearings and gas-oil mixture from the bypass port. The model was experimentally verified under various operating conditions. The stator winding temperatures and refrigerant thermal property variations were measured and compared with the calculated ones. The results show that our model can accurately predict the temperature distribution inside the motor. Further, this validated model was used to investigate the effects of cooling passage arrangement on temperature distribution inside the motor. It was found that the dimension and shape of the stator cooling duct has a large effect on the temperature inside the motor while the effect of the inner diameter of rotor cooling holes is negligible.

Numerical simulation of refrigerants flowing through short-tube orifices during flashing process

A numerical simulation for calculating the flashing flow characteristics of refrigerants inside short-tube orifices is presented. The refrigerant flow behaviors along short-tube orifices are separated into three sections: subcooled liquid region, metastable liquid region, and two-phase region. Annular two-phase flow pattern is developed from the mass, energy, and momentum equation. The present simulation is corroborated by comparison with the experimental data. The selection charts are also presented to estimate the refrigerant mass flow rate along short-tube orifices in practical applications.

F221 多分岐管における気液二相分配に関する研究(OS-6:次世代ヒートポンプ・冷凍システム(3))

An experimental study was conducted on gas-liquid flow distributions in a multi-pass channel with horizontal headers and upward-flow branches. Branches of the channel were heated to simulate a real operating condition of the evaporator. The refrigerant flow was visualized under the heated condition and it was compared with the adiabatic flow. Moreover, temperature distributions of branches were measured to estimate liquid distributions to branches, and they were compared with gas-liquid distribution ratios measured in an isothermal air-water flow. The refrigerant flow characteristics under the heated condition agreed qualitatively with those observed under the adiabatic condition, and the temperature distributions of branch surfaces were consistent with the water distributions. These results mean that gas-liquid distributions measured in the isothermal air-water flow can well reproduce the refrigerant flow distributions in an evaporator under operation.

THEORETICAL INVESTIGATION OF AN ADIABATIC CAPILLARY TUBE WORKING WITH PROPANE / n-BUTANE / iso-BUTANE BLENDS

In this paper, a theoretical model is developed to predict refrigerant flow characteristics in adiabatic capillary tubes using propane/n-butane/iso-butane mixtures as working fluids in a domestic refigerator. This model is based on mass, energy and momentum conservation equations for a homogeneous refrigerant flow under different inlet conditions such as subcooled, saturated and two-phase flow. Effects of inlet pressure (8-16bar), inlet vapor quality (0.001-15%), inlet subcooling degree (1-15°C), mass flow rate (1-5 kg/h), propane mass fraction (0.5-0.7), capillary tube inner diameter (0.6-1.0mm) and tube surface roughness on the capillary tube length are predicted. Results showed that the present model predicts data that are very close to the available experimental data in literature with an average error of 2.65%. Pressure of hydrocabon mixture (HCM) decreases while its vapor quality, specific volume and Mach number increase along the capillary tube. Also, the results indicated that capillary tube length is largely dependent on capillary tube diameter. Other parameters such as mass flow rate, inlet pressure, sub-cooling degree (or quality) and relative roughness influence the capillary tube length in that order. Capillay tube length as function of the significant parameters is presented in equation form. Also, capillary tube selection charts either to predict mass flow rates of propane/n-butane/iso-butane mixtures through adiabatic capillary tubes or to select the capillry tube size according to the required applications are developed. The comparison betweenn R12, R134a and the hydrocarbon mixture (HCM) of propane/n-butane/iso-butane indicated that for a given mass flow rate, the pressure drop per unit length is about 4.13, 5.0 and 12.0 bar/m for R12, R134a and HCM, respectively. The ratios of average mass flow rate of HCM with propane mass fraction of 0.6 to those of R12 and R134a are about 0.62 and 0.67, respectively. Average capillary tube length for HCM with propane mass fraction of 0.6 is longer than those of R134a and R12 by about 30% and 48%, respectively.

Research on refrigerant flow characteristics and performance of a multi-functional heat pump system

Multi-functional domestic heat pump systems (MDHPS) can reduce operating costs, lessen thermal pollution to the environment, and improve equipment utilization rate. Compared with the traditional heat pump system, the MDHPS has a wider range and more dramatic changes in operating conditions. This paper incorporates the refrigerant flow characteristic and performance analysis of an electronic expansion valve (EEV)-controlled MDHPS which operating in four typical modes (sanitary water heating mode, space cooling mode, space cooling combined with condensing heat recovery mode, radiant floor heating mode). The calculating results demonstrate that there are considerable differences in the refrigerant mass flow range when the MDHPS operates in the different modes. Moreover, sudden changes in working conditions due to mode switching easily result in large-scale fluctuations of system parameters. Therefore, to ensure the MDHPS to work more reliably and efficiently, a control scheme of the EEV specialized for the MDHPS is proposed and applied in the presented experimental system. The experimental results indicate that the system is able to perform efficiently and stably under various conditions by this control method. In the sanitary water heating mode, the average heating coefficient of performance (COP) reaches 2.2, 3.2 and 4.1 when the ambient temperature is 5 °C, 20 °C and 35 °C, respectively. In the condensing heat recovery mode, besides yielding free sanitary hot water, the average cooling COP can be improved 9% when the water temperature ranges from 30 °C to 55 °C. In the radiant floor heating mode, the COP is 3.3 at 6 °C ambient temperature.

Visualization and void fraction measurement of decompressed boiling flow in a capillary tube

Abstract A capillary tube is often used as a throttle for a refrigerating cycle. Subcooled refrigerant usually flows from a condenser into the capillary tube. Then, the refrigerant is decompressed along the capillary tube. When the static pressure falls below the saturation pressure for the liquid temperature, spontaneous boiling occurs. A vapor–liquid two-phase mixture is discharged from the tube. In designing a capillary tube, it is necessary to calculate the flow rate for given boundary conditions on pressure and temperature at the inlet and exit. Since total pressure loss is dominated by frictional and acceleration losses during two-phase flow, it is first necessary to specify the boiling inception point. However, there will be a delay in boiling inception during decompressed flow. This study aimed to clarify the boiling inception point and two-phase flow characteristics of refrigerant in a capillary tube. Refrigerant flows in a coiled copper capillary tube were visualized by neutron radiography. The one-dimensional distribution of volumetric average void fraction was measured from radiographs through image processing. From the void fraction distribution, the boiling inception point was determined. Moreover, a simplified CT method was successfully applied to a radiograph for cross-sectional measurements. The experimental results show the flow pattern transition from intermittent flow to annular flow that occurred at a void fraction of about 0.45.

A comparison of flow characteristics of refrigerants flowing through adiabatic straight and helical capillary tubes

This paper presents a numerical investigation of the flow characteristics of helical capillary tubes compared with straight capillary tubes. The homogenous two-phase flow model developed is based on the conservation of mass, energy, and momentum of the fluids in the capillary tube. This model is validated by comparing it with the experimental data of both straight and helical capillary tubes. Comparisons of the predicted results between the straight and helical capillary tubes are presented, together with the experimental results for straight capillary tubes obtained by previous researchers. The results show that the refrigerant flowing through the straight capillary tube provides a slightly lower pressure drop than that in the helical capillary tube, which resulted in a total tube length that was longer by about 20%. In addition, for the same tube length, the mass flow rate in the helical capillary tube with a coil diameter of 40 mm is 9% less than that in the straight tube. Finally, the results obtained from the present model show reasonable agreement with the experimental data of helical capillary tubes and can also be applied to predict the flow characteristics of straight capillary tubes by changing to straight tube friction factors, for which Churchill's equation was used in the present study.