Refrigerant Flow Research Articles

Making sense of data for variable refrigerant flow units

Sound data for variable refrigerant flow units are generally tested under anechoic conditions. This introduces significant error when extrapolating to different acoustic environments. Further, depending on the type of unit - ducted or non-ducted, the reported test configuration bears little resemblance to normal operating conditions. Quiet and bad data are not mutually exclusive; however, engineers must be aware of the challenges and potential pitfalls associated with manufacturer data. This paper will compare available manufacturer data tested in anechoic and reverberant chambers and their respective standards, as well as field measured data in common, lightly furnished spaces.

Unsteady heat transfer characteristics of R-407C in the subcooled flow boiling stemming from the oscillating refrigerant flow rate

The objective of this experiment is to investigate the unsteady heat transfer of R-407C subcooled flow boiling (SCFB) due to the oscillating refrigerant flow rate within a narrow tube oriented horizontally for application in energy efficiency air conditioning system. The refrigerant flow rate is input in the cycle of triangular oscillations. Influences of oscillation amplitude (ΔG/G‾) and oscillation period (tp) of the periodic flow rate on SCFB heat transfer are studied. The present results disclose that either ΔG/G‾ or tp has minor impact on time-average SCFB heat transfer performance. However, obvious changes in the temporal variations of the wall temperature on the pipe (Tw) and convective heat transfer coefficient (hr) are found, leading to three heat transfer mechanisms of single phase convection, intermittent nucleate boiling, and persistent subcooled boiling for various input heat fluxes. The Tw of persistent subcooled boiling presents the reversed tendency compared to that of single phase convection due to the variation of the triangular R-407C oscillation. In the case of intermediate heat flux, the unusual boiling phenomenon, called intermittent nucleate boiling, can be obtained depending on the R-407C mass flux. The visualized bubble images are also provided to further describe the intermittent nucleate boiling.

Ejector and Vapor Injection Enhanced Novel Compression-Absorption Cascade Refrigeration Systems: A Thermodynamic Parametric and Refrigerant Analysis

This study investigates the performance of two novel compression absorption cascade refrigeration systems, the Ejector Compression Absorption Cycle (ECAC) and Ejector Injection Compression Absorption Cycle (EICAC), in comparison to traditional system. In these cascaded systems, the absorption cycle (top cycle) is modified by adding a refrigerant hear exchanger (RHX) which provides higher mass flow of refrigerant to increase the COP. The simple vapor compression cycle (bottom cycle) performance is enhanced by incorporating the ejector and Vapor Injection technologies. A systematic analysis is accomplished to establish the optimal operating conditions for performance enhancement, taking into account of ejector parameters and the effect of different environmentally friendly refrigerants by the energy and exergy method. The findings demonstrate that both ECAC and EICAC systems can achieve near 15 % and 6 % higher COP, respectively, compared to conventional cascade system when using the R41-LiBr/H2O refrigerant pair under different working conditions. Maximum exergy efficiency is found to be achieved at around 73 °C, with ECAC and EICAC showing higher exergy efficiency of near 20 % and 10 %, respectively than the conventional system. The analysis also reveals that while the COP of all layouts augments linearly with increasing evaporator temperature, the exergy efficiency decreases at different rates, making the cascade systems more efficient for low-temperature applications. The LiBr/H2O refrigerant pair demonstrates superior COP and exergy efficiency as HTC refrigerant, while R161, R290, and R1270 perform better as low-temperature refrigerants for both ECAC and EICAC from both energetic and exergetic perspectives. The results of this detailed theoretical thermodynamic analysis provide a comprehensive understanding of the performance of ECAC and EICAC systems and offer valuable insights for further improvement and optimization.

Power consumption prediction of variable refrigerant flow system through data-physics hybrid approach: An online prediction test in office building

Variable refrigerant flow (VRF) system contains numerous sensors and has the advance for fast response, which is suitable for building demand response (DR) management. Fast and accurate power consumption prediction of VRF system is essential for DR. As traditional prediction methods, white-box models are difficult to build on operational data, while black-box models cannot make interpretable predictions. Neither of them can meet the requirements of power consumption prediction for VRF system under demand response. Therefore, a grey box model for power consumption of VRF system is proposed in this study, which has the advantage of data-driven and interpretability.The proposed model consists of four sub-models, which predict the building thermal load, compressor frequency, high pressure state and low pressure state of VRF, respectively. These predictions are finally used as inputs to the power prediction model. The proposed model is verified on both offline test and online test. The results show that the model is capable of predicting the power consumption accurately under high time resolution. During the online test, the MAE, CV-RMSE, and R2 of the model are 1296.41 W, 24.65% and 0.90, respectively. The proposed model can be used as the evaluation tool of DR management for decision making.

Modelling variable refrigerant flow system for control purpose

Variable Refrigerant Flow (VRF) systems are gradually gaining popularity in small and medium-sized commercial and residential buildings owing to their high part-load performance, flexible control, and ease of installation and maintenance. Developing models of VRF systems to predict their performance are important for model-based control, fault diagnostic and detection. There are VRF models published in existing literatures, however those models were developed and validated in different datasets. As a result, the model accuracy cannot be directly compared. To fill this gap, this paper presents a comprehensive review of the existing VRF models, and summarizes the input/output parameters and mathematical formulas of 16 VRF models from literature (referred to as physics-based model). Next, we validate and compare the model accuracy of existing models using the same dataset. Additionally, we develop data-driven models using the state-of-art machine learning algorithms, and compare the model accuracy between existing physics-based models with data-driven models. We find the model proposed by Hu et al. in 2019, which regresses the VRF cooling capacity and COP as a linear combination of indoor and outdoor temperatures times a cubed polynomial function of compressor frequency, is the most accurate physics-based model, with a prediction error of 22.19% in the training dataset and 22.44% in the validation dataset. XGBoost is the most accurate data-driven model, with a prediction error of 19.29% in the training dataset and 22.02% in the validation dataset. The data-driven model is more accurate while the physics-based model is more generalizable. The findings of this study can help researchers to select the proper VRF model for building energy prediction, model-based optimization, and fault diagnostic and detection.

Annual Effect of the VRF Control Algorithm in Response to the TOU Rate Plan

Many countries adopt a time-of-use (TOU) rate system, in which electricity rates vary by season and time of day, to reduce power usage during peak power consumption hours. South Korea offers a TOU rate plan that depends on the electricity usage of a building and its contracted power; in this plan, the electricity rate reaches up to 300% depending on the time of day. Hence, electrically powered variable refrigerant flow (VRF) systems are increasingly being installed in small- and medium-sized buildings requiring individual cooling and heating operations. This study aims to develop a new control algorithm to reduce electricity consumption and electricity rates for cooling and heating by VRF systems in university buildings adopting the TOU rate plan and apply it to actual buildings to verify the reduction effect. The proposed control algorithm primarily consists of a module that controls the refrigerant evaporation temperature (cooling) and high pressure (heating) according to the indoor heat load and a module that controls the indoor set temperature based on the hourly electricity rate. The developed algorithm was installed in the controller of a VRF system installed in an actual university building and the annual effect was verified using the method proposed by the International Performance Measurement and Verification Protocol. As a result, power consumption was reduced by 17.8% for heating and 4.0% for cooling due to the application of the control algorithm, and the electricity rates reduced by 19.2% and 7.3%, respectively.

Experimental research on the influence of a novel passive cooling system on the data center temperature distribution

As artificial intelligence has been leaping forward, data centers (DCs) are only capable of meeting to the daily needs of people through uninterrupted operations. A power outage or cooling system failure can result in high temperatures, trigger data center shutdowns, and even damage IT equipment. In terms of several special DCs, a prolonged cooling system interruption may lead to catastrophic and incalculable consequences. Accordingly, a novel passive cooling system was proposed in this study to address the long-term interruption of air conditioning systems. Liquid nitrogen (LN2) was driven by the pressure of the cryogenic liquid container and flowed into finned tube heat exchangers (FTHEs) in the DC. LN2, i.e., a refrigerant, can absorb heat through vaporization and give FTHEs ultra-low temperature surfaces to drop the indoor temperature. The effects of refrigerant flow rate, temperature, and extreme indoor initial temperature on the indoor temperature distribution were examined. As indicated by the results, increasing the flow rate and decreasing the refrigerant temperature increased the room cooling rate while exacerbating the degree of temperature stratification (TSD). Compared with 35 kg/h, the cooling effect was enhanced by 1.5 times for a flow rate of 50 kg/h, whereas the TSD value near FTHEs was nearly 14 °C/m, approximately twice as high as that of 35 kg/h. Slightly raising the initial ambient temperature led to the rise of the cooling effect, whereas this method slightly affected the temperature stratification. This study provides a novel idea for solving long-term DC cooling system failure while laying a basis for the effect of ultra-low temperature surfaces on ambient temperature distribution.

Effect of hydrophilic and hydrophobic metal foams on condensation characteristics of refrigerant flow inside annular tubes: Experimental study

Effect of hydrophilic and hydrophobic metal foams on condensation characteristics of refrigerant flow inside annular tubes: Experimental study

A novel building heat pump system semi-supervised fault detection and diagnosis method under small and imbalanced data

Faulty heat pump system operating can cause excessive energy consumption and an uncomfortable indoor environment. Data-driven method is one of the most widely studied Fault Detection and Diagnosis (FDD) methods in building heat pump systems. However, the fault data is difficult to collect and label, which makes it a significant challenge to apply a data-driven method. Therefore, a novel semi-supervised FDD method that could utilize small labelled and unlabelled fault data was proposed, considering unknown fault detection in unlabelled data. The fundamental model consisted of four parts: encoder, double discriminators, generator, and bagging classifiers, and all networks were integrated to train simultaneously. Additionally, batch nuclear-norm maximization and pseudo-label were utilized to improve the model. The proposed method could realize data augmentation, pseudo-label utilization, and FDD in one end-to-end model. Normal and fault data were collected from a variable refrigerant flow heat pump system to verify this method. The proposed method was compared with several methods in previous studies. The performance indexes showed that the proposed method had averaged 65.56%, 75.50%, 82.25%, 83.30%, and 86.26% overall accuracy under each training labelled group, respectively. The unlabelled data could improve the model effectively, and the averaged detection rate of unknown fault could reach 98.00%, 98.82%, 98.88%, 98.66%, 98.94%, and 98.82%, respectively, under different numbers of unlabelled data. Additionally, the analysis of unknown fault diagnosis results indicated that faults with different severities had similar operating characteristics.

Modeling of heat-hydrodynamic processes in evaporators of low-temperature systems with intrachannel boiling of refrigerants

The introduction of new types of heat exchangers with phase transitions and the solution of problems of optimizing the design and operational characteristics are a priority within the framework of the energy saving program. Known methods for calculating the heat-hydrodynamic parameters of the flow of refrigerants often do not take into account the specifics of boiling processes at low temperatures as well as in channels with a small flow area. This paper presents the results of modeling heat transfer during the boiling of refrigerants in the channels of evaporators of heat and cold energy complexes, taking into account the true flow parameters. The proposed mathematical model of the boiling of the working substance in channels of various shapes is based on the true flow parameters which imply knowledge of the channel cross-sectional areas occupied by each of the phases. The value of the true volumetric steam content provides the most correct modeling of two-phase flows in a wide range of regime and geometric parameters. The paper uses the equations of material and heat balance in combination with the equation of heat transfer from the environment to the boiling refrigerant. The map of flow regimes is used as an empirical component. A program has been developed for calculating the proposed system of equations which is solved iteratively at each time step using the finite volume method. Comparison of calculation results with experimental data on models of round and rectangular channels with intracanal boiling of refrigerants at positive and negative saturation temperatures is performed. It is shown that the calculation error does not exceed 10 % for a round and 20 % for a rectangular flow section. The verification results showed the possibility of using the model in the framework of engineering calculations. The proposed mathematical model can be used as the basis for the calculation programs for existing evaporators and for the creation of new types of heat exchangers with in-tube boiling of the working substance. The proposed method allows optimizing both geometric and thermal-hydrodynamic parameters.

Research on Two-stage Humidification-dehumidification Heat Pump Desalination System

For the humidification-dehumidification desalination unit to perform more efficiently, a two-stage humidification-dehumidification heat pump desalination system was proposed. So as to improve the heat and mass transfer effect between seawater and moist air, the two-stage humidifier was employed to recover the waste heat of seawater discharged from the first stage humidifier. The models of a one-stage and two-stage humidification heat pump seawater desalination system were built using Aspen Plus simulation software. Multi-condition simulation studies were carried out for different inlet seawater temperatures, liquid-gas mass ratios, and refrigerant flow rates of the heat pump. According to the comparative examination of the simulation’s outcomes, it is concluded that: with the increase in inlet seawater temperature and liquid-gas flow ratio, the two-stage humidifier system’s recovery rate and gain output ratio are superior to those of the first-stage humidifier system, and the inlet seawater temperature and circulating airflow needed for the two-stage humidifier system to perform at its best are less demanding than those for the first-stage humidifier system., indicating that the two-stage humidifier dehumidification desalination technology of the heat pump has great performance improvement and a broad application prospect.

Investigations on nonlinear processes of a Gifford-McMahon type orifice pulse tube refrigerator

The target of this investigation is to illustrate the nonlinear fluid flow, heat transfer, and vortex formation phenomena in an orifice pulse tube refrigerator by numerical simulation. Numerical results confirm that jet streaming is generated at the low-temperature end of the pulse tube in between the junction of U-shaped tube and pulse tube. The primary vortex, which is generated due to the minor loss, continuously grows because of velocity change and eventually leads to the formation of secondary eddy. Consequently, flow straighteners have been adopted at the hot side of the regenerator, hot heat exchanger, and both sides of U-shaped tube. It is noticed that, jet streaming has been suppressed at the cold end of the pulse tube. Nevertheless, the streaming effect generated due to the oscillating flow of the gas parcels within the pulse tube, and third type of streaming have not been suppressed with the flow-straightener. Additionally, the numerical simulation shows different gas flow behaviors within the pulse tube that occurs due to the oscillating pressure pulse. An experimental investigation is conducted to compare experimental results with the numerical results and a valid agreement is observed between them.

The impact of the wall insulation material and variable refrigerant flow system on building energy consumption and cost

The use of VRF system and insulation in building envelope are both considered as promising option to reduce energy consumption of a building. This study analysis energy saving potential of VRF system and building insulation using modelling and simulation of a typical office building. The model office building is simulated with the weather of Hyderabad, Sindh, Pakistan using EnergyPlus building simulation software. The simulation cases include conventional and VRF air conditioning systems with and without insulation to evaluate and compare the annual cooling and energy savings and payback period. Results showed that by replacing conventional air conditioner with VRF AC electrical power can be reduced by 42-45%. It is also noted that Cellulose, expanded polystyrene, extruded polystyrene and polyurethane insulations can save around 49.5, 51.4, 51.6, and 54.54% of electricity, respectively. In the case VRF air conditioner used with Cellulose, expanded polystyrene, extruded polystyrene and polyurethane insulation may reduce electricity consumption by 66.5, 67.4, 67.5 and 68.9 %, respectively. The payback period varies from 7 to 15 months. However, cellulose with VRF air conditioner has the least payback period of around 7 months. The longest payback period of around 15 months was noted for the un-insulated office building with VRF air conditioner. Despite having longer payback period, the combination of polyurethane insulation with VRF air conditioning system is the most efficient combination.

Evaluation of correlations for mass flow rate of refrigerant through electronic expansion valve in air—water heat pump system using R32

To measure mass flow rate of R32 through EEVs under real operating conditions, a commercially available air-water heat pump (AWHP) system was operated in as wide a range as possible. Measurements showed that the effect of oil circulation on the refrigerant mass flow rate of the test system was negligible. To examine whether they were able to predict refrigerant mass flow rate of R32 in a real operating heat pump systems, previous empirical correlations available in the open literature were compared against these measured data. Previous empirical correlations, except for those of Park et al. (2007), showed poor agreement with the present experimental data, with mean absolute percentage error (MAPE) from 9.1% to 334.5% for EEV#1 (D: 2.40 mm) and from 24.1% to 122.2% for EEV#2 (D: 1.65 mm). These large differences were caused by differences in internal geometry of flow passage and operating conditions due to different refrigerants. A new empirical correlation was developed considering these aspects. The valve opening ratio was considered in terms of the valve flow coefficient variation. The thermodynamic state of refrigerant at the EEV inlet was represented by the enthalpy difference, so as to avoid discontinuity of the mathematical expression at the saturated liquid point. The new correlation showed good agreement with mass flow rate measured through both EEVs, with MAPE values of 9.3% and 14.5% for EEV#1 (D: 2.40 mm) and EEV#2 (D: 1.65 mm), respectively.

Novel approach to typical air-conditioning behavior pattern extraction based on large-scale VRF system online monitoring data

The diversity of occupant air-conditioning (AC) behavior in buildings leads to significant discrepancies in building energy consumption. Thus, establishing models that reflect diverse realistic occupant AC behavior is crucial for the building performance simulations (BPS). Owing to the lack of large-scale samples, it is difficult to extract typical AC behavior patterns from existing studies. With advances in IT methods for data collection, large-scale on-site air-conditioning (AC) operation data of variable refrigerant flow (VRF) systems have become available. Therefore, based on the Big Data of 1200 indoor units (IDUs) of VRF systems, a novel method was proposed herein to establish realistic AC behavior models and extract typical patterns. The overall framework included three steps: data collection and preprocessing, large-scale occupant AC behavior model establishment, and typical behavior pattern extraction and evaluation. First, data preprocessing approaches were implemented to address data issues. Subsequently, based on Big Data, Bayesian parameter calibration was proposed to construct large-scale occupant AC behavior models. Afterwards, using the large-scale calibrated AC behavior models, the cooling demands of the population were simulated, and k-means clustering analysis was adopted to categorize the AC behavior into five groups representing different cooling demand levels and a typical model reflecting the group feature was selected. In the model validation, a hypothesis-based test was applied to confirm the validity of extracted typical patterns in reflecting the real distribution of district cooling demands. In the results, this study identified two occupancy patterns, i.e. “Daily occupied mode” (DOM) and “Night occupied mode” (NOM), and based on two occupancy types, five typical AC behavior patterns were extracted separately. From the typical patterns, it could be noticed that 90% of occupants had relatively low cooling demand less than 40 kWh/m2 during the cooling season. Moreover, comparing to the performance of fixed AC schedule model in district cooling demand evaluation, the extracted typical AC behavior patterns improved the accuracy up to 30.4% when calculating the bias between average modelled cooling demand and measured data. Additionally, the typical AC behavior models put forward in this study can be further integrated in BPS tools, which is promising for the simulation of cooling demand at building or district scale, and realizing AC technology evaluation in future engineering projects.

Development of refrigerant flow rate correlation for electronic expansion valve using valve flow coefficient

The mass flow rate of R32 through two different designs of electronic expansion valves (EEVs) was experimentally measured. A total of 173 mass flow rate data were produced with EEV1 (D1 = 2.40 mm) and EEV2 (D2 = 1.65 mm). Although there are numerous empirical correlations available for prediction of mass flow rate through an electronic expansion valve (EEV), previous empirical correlations failed to predict these data with reasonable margins of error. This is because those correlations only incorporated geometry parameters related to the EEV orifice and needle, even though entire geometry of flow passage inside EEV affects flow rate. An empirical correlation was developed based on the experimental data. The entire geometry of the flow passage inside the EEV was represented by a single parameter known as the valve flow coefficient. The new empirical correlation is in good agreement with the experimental data of EEV1 and EEV2, with average deviations of 5.84% and 5.87%, respectively, even though only valve flow coefficient was used to represent EEV geometry.

Two-phase refrigerant maldistribution and optimization design in novel alternatively-laminated-microchannel-tube (ALMT) heat exchangers with vertical headers

The novel alternatively-laminated-microchannel-tube (ALMT) heat exchanger contains two pairs of vertical headers and alternatively laminated microchannel flat tubes, which realizes high-efficient heat transfer between water and refrigerant and shows energy-saving potential in large-scale air conditioners. However, the microchannel flat tubes are upright inserted into vertical headers and are susceptible to refrigeration maldistribution. In this article, two-phase refrigerant maldistribution and optimization design in ALMT heat exchangers with vertical headers are studied experimentally and numerically. Refrigerant maldistribution is experimentally measured and its flow behaviors are explored with CFD simulations. The R410A is used as the working fluid. Results show that more liquid refrigerant is supplied to middle tubes but less to both ends under phase separation and gravity. Therefore, the decentered orifice plate (DOP) and parallel multi-chamber (PMC) are designed to improve refrigerant distribution. Both DOP and PMC promote gas-liquid phase mixing by accelerating refrigerant flow. The DOP creates the nested loop flow and PMC forms separate chambers inside vertical header to distribute refrigerant to each flat tube uniformly. Moreover, the formation conditions for nested loop flow are established. The average standard deviation of refrigerant flow rate reduces from 0.21 to 0.07 with DOP, and to 0.03 with PMC, respectively.

Advanced Method of Variable Refrigerant Flow (VRF) System Design to Forecast on Site Operation—Part 3: Optimal Solutions to Minimize Sizes

Outdoor air conditioning systems (ACS) are used as autonomic systems as well as in combined outdoor and indoor ACS of the variable refrigerant flow (VRF) type, with variable speed compressors (VSC) as their advanced version. Methods for determining the optimal value of refrigeration capacity and providing the maximum rate of the summarized annual refrigeration energy generation increment, according to its needs at minimum compressor sizes and rational values, are applied to reveal the reserves for reducing the designed (installed) refrigeration capacity, thus enabling us to practically achieve maximum annual refrigeration energy generation as the primary criterion at the second stage of the general design methodology previously developed by the authors. The principle of sharing the total thermal load on the ACS between the ranges of changeable loads for outdoor air precooling, and a relatively stable load range for further processing air are used as its basis. According to this principle, the changeable thermal load range is chosen as the object for energy saving by recuperating the excessive refrigeration generated at lowered loading in order to compensate for the increased loads, thereby matching actual duties at a reduced designed refrigeration capacity. The method allows us to determine the corresponding level of regulated loads (LRL) of SRC and the load range of compressor operation to minimize sizes.

Enhanced refrigerant flow boiling heat transfer in microstructured finned surfaces

Flow boiling is used in air-conditioning and refrigeration, thermal management of electronics, distillation, chemical synthesis, desalination, power generation, and cryogenics. To enhance the flow boiling heat transfer, internally finned tubing has been widely used. In this work, we demonstrate a fabrication method to generate interal microstructures in 5.2 mm internal diameter finned aluminum tubing through crystallographic chemical etching to further increase heat transfer coefficients of low-GWP refrigerant R-515B. Enhancements in heat transfer coefficients are demonstrated across the range of vapor qualities with peak enhancements up to 172% observed at high vapor qualities with similar levels of enhancements demonstrated with R-134a. The effects of varying operating parameters such as heat flux, mass flux and saturation pressure, on performance improvement were also examined. Flow visualization studies performed through an adiabatic glass visualization section at the test-section exit allowed quantiification of film thickness and showcased lower conduction resistance for microstructured finned surfaces. The advantages of this structuring technique are further highlighted by the relatively minor increase in pressure drop (∼20%). We demostrate that etching of plain aluminum tubes exceeds the performance of finned tubing under high heat flux condtions, with enhancements being more pronounced at low mass fluxes, indicating potential replacement of current extrusion methods used for the manufacture of finned tubing. Surface characterization results after mechanical expansion of the etched finned tubing demonstrated the remenance of microstructures in fin valleys, thus proving potential compatibility with heat exchanger fabrication methods.

Optimization Position Variable Refrigerant Flow In Building Using CFD Method

Variable refrigerant flow type is a multi-split type air conditioner for multi-story buildings. The aim of this study getting obtains optimization placement of the condensing unit to inlet air temperature in a multi-story building using the CFD method. The simulation was conducted from the 11th to the 30th floor. There are three variations of condensing unit position on each floor (Variation 1), on every two floors (Variation 2), and on the roof (variation 3). The result of the first variation is 35.32 °C, with the difference between the highest and lowest is 3.9 °C. The average temperature of the 2nd variation is 34.44 °C, with the difference between the highest and lowest being 0.92 °C. The average temperature of the 3rd variation is 36.65 °C, with the difference between the highest and lowest being 8.03 °C. Among the three variations, the 2nd variation of the condensing unit input air temperature is lower. Then the difference between the highest and lowest temperatures is not too far apart, indicating a more even distribution of air. The 1st, 2nd, and 3rd variations condensing unit position do not exceed Panasonic VRF air conditioner operational temperature limit, which is 52 °C.