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Microchannel condensers play an essential role in cryogenic two-phase heat management systems due to their efficient heat transfer characteristics. Thus, it is worth conducting an in-depth study on microscale condensation characteristics of cryogenic fluids. This paper delves into the flow condensation process of methane in microchannels. A two-dimensional transient model with high accuracy for cryogenic fluids has been developed by combining a self-defined program for the source term of the phase transition model. The model fully considers the boundary layer thickness and accurately explores the mesh accuracy. The complete condensation flow patterns are captured for various vapor quality, mass flux, and wall subcooling degrees. The injection flow is a unique flow regime for condensation in microchannels. The decrease in wall subcooling degree and increase in mass flux leads to the separation point at the neck of the injected flow moving towards the exit, while the annular flow region is expanding and the flow pattern transition is lagging. The mass flux improves the heat transfer coefficient more significantly at high vapor quality. During injection and bubble flow, the wall shear stress and local heat transfer coefficient are subject to bouncing and oscillations, which may induce fluctuations in the upstream annular flow. The prediction performance of six classical heat transfer correlations is evaluated. The results indicate that the Nie et al. correlation has the highest comprehensive prediction accuracy with MRD and MARD of -5.00 % and 15.83 %, respectively. [ABSTRACT FROM AUTHOR]

• Boiling coefficient increased with nozzle inlet pressure and specific enthalpy. • Boiling coefficient ranged from 300 to 4000 in the investigated trans-critical region. • Modified HNB showed an accuracy of 3 % in predicting the nozzle flow rates. • Proposed method for determining saturation pressure was superior to constant value. A modified homogeneous non-equilibrium boiling approach (HNB) was proposed for high-fidelity modelling of the trans-critical flashing process in two-phase R744 ejector nozzles. In the modified approach, the method of determining the metastable liquid saturation pressure was improved by integrating the metastable liquid enthalpy. Experimental tests were conducted on two different nozzles to calibrate the lumped accommodation coefficient in the modified HNB. The calibration dataset consisted of 30 trans-critical test points, with the motive inlet pressure ranging from 7.5 MPa to 11.5 MPa and temperature varying from 21 °C to 45 °C. It was demonstrated that the boiling coefficient was strongly related to nozzle inlet pressure and specific enthalpy. The coefficient was therefore fitted as a function of the operating condition. The modified HNB with the fitted function was then validated against another set of test data composed of 29 test points. The results demonstrated that the relative errors of the nozzle mass-flow rates were within 3 %. Finally, the modified HNB with the improved metastable liquid saturation pressure was compared and discussed with the existing HNB approach in the literature. [ABSTRACT FROM AUTHOR]

• The two-phase distribution in the header of a microchannel heat exchanger was quantified by a flexible capacitance sensor. • Distribution of local vapor quality showed that gases were concentrated upstream of the header, while liquids were concentrated in the middle and lower reaches. • Compared with the downstream, the upstream flow had less stability with more discrete capacitance signals. • The increase of mass flux caused that both the local vapor quality maximum point and the minimum point in the header were shifted back toward the downstream. The two-phase distribution has always been one of the research focuses of microchannel heat exchangers, because the maldistribution could seriously affect the heat transfer performance. This paper developed a capacitive method to quantify the two-phase distribution of vapor-liquid flow in the aluminum header of a microchannel heat exchanger. A flexible capacitance sensor was used to quantify the two-phase distribution with R134a. The inlet mass fluxes vary from 17.47 kg m−2 s−1 to 52.42 kg m−2 s−1 and inlet vapor qualities range from 0.2 to 0.6. The normalized time-averaged capacitance signal was processed by a Gaussian function to obtain the mathematical expectation and the standard deviation. The local vapor quality in the header was solved for using a correlation derived from the pre-experiments. The distribution of local vapor quality showed that gases were concentrated upstream of the header, while liquids were concentrated in the middle and lower reaches. The local vapor quality reached a minimum and remained constant at 0.1 in the middle and lower reaches, unaffected by changes in inlet conditions. As the mass flux increased, both the local vapor quality maximum point and the minimum point in the header were shifted back toward the downstream. Additionally, the increase of mass flux caused the fluctuation of the standard deviation increased, which meant the stability of the flow decreased. The two-phase flow measurement technique based on flexible capacitance sensors proposed in this study realized the extension of the capacitive method from the experimental stage to the industrial application stage. [ABSTRACT FROM AUTHOR]

• Ammonia condensation has been experimentally studied in 8 mm and 11 mm tubes. • A new phenomenological model has been proposed for ammonia condensation. • The lowest condensation HTCs characterise the downstream flow. The paper proposes the experimental study of ammonia condensation in aluminium tubes with 8 mm and 11 mm internal diameters. The new study aimed to compare and extend the existing ammonia in-tube condensation data. The lines were tested mainly in horizontal configuration at mass velocities of 20 to 160 kg·m−2s−1, saturation temperature of 35 to 65 °C and inlet vapour quality of 0.1 to 0.9. Shah (1979), laminar-laminar Dobson and Chato (1998), and Cavallini et al. (2001) models predict well ammonia condensation HTCs at mass velocities of 120 to 160 kg·m−2s−1. A new phenomenological model has been proposed for the mass velocities of 20 to 120 kg·m−2s−1, predicting the test data with an accuracy of ±20 %. Similar ammonia condensation HTCs have been obtained for horizontal and upstream flows at a mass velocity above 120 kg·m−2s−1. The lowest condensation HTCs at the considered boundary conditions characterise the downstream configuration of an in-tube condenser at a mass velocity below 120 kg·m−2s−1. However, inclining the ID 8 mm tube by 15° from the normal substantially increases the condensation HTCs by 10 to 30 %, depending on vapour quality. At the inclinations of 15 to 45° from the horizontal line toward the downstream flow, the condensation HTCs exceed the ones of horizontal flow by 10 to 15 % at the vapour quality of <0.5 and low mass velocity of 80 kg·m−2s−1. [ABSTRACT FROM AUTHOR]

• Single- and two-phase frictional pressure losses in stainless steel smooth tubes (5.5 and 8.5 mm) and six aluminium straight inner-grooved profiles have been experimentally studied on ammonia. • The adequacy of existing correlations and approaches has been validated. • A new correlation has been proposed for the frictional pressure losses in inner-finned tubes. The experimental study of single-phase and two-phase frictional pressure losses has been performed with ammonia flow in stainless steel smooth tubes (internal diameter of 5.5 mm and 8.5 mm) and six straight inner-grooved aluminium tubes with different nominal diameters (7.15 to 13.3 mm), length (0.45 to 2.0 mm) and thickness (0.35 to 0.8) of fins. Single-phase test boundary conditions correspond to Reynolds numbers of 1000 to 20,000. Two-phase tests have been carried out at saturation temperature of 45 to 65 °C, mass velocity of 50 to 210 kg· m −2s−1 and vapour quality of 0.1 to 0.9. Comparison of obtained test data with the existing frictional losses approaches shows that the Blasius model is appropriate to single-phase flows in smooth tubes, and Müller-Steinhagen and Heck's (1986) model is most accurate in predictions of two-phase frictional losses in smooth tubes. Existing approaches of single-phase frictional pressure loss predictions in the pipes and channels with longitudinal fins (Carnavos, 1980; Edwards and Jensen, 1994) are not adequately suited to the ammonia test points obtained in the considered straight inner-grooved tubes. A new semi-empirical model has been proposed, based on generalisation around the revealed geometric dimensionless factor, which accurately describes friction pressure losses in the considered inner-finned profiles. The simplest homogeneous model for defining two-phase friction pressure losses, using McAdams et al.'s (1942) recommendation for mixture dynamic viscosity definition and a new model of single-phase friction factor, best fit both quantitatively and qualitatively to ammonia two-phase test points in most of the considered straight inner-grooved tubes. [ABSTRACT FROM AUTHOR]

This paper investigates the two-phase leakage of the flank gap in rotary compressors by using the coupled models of compressible Reynolds equation and refrigerant-oil pair solubility. The models are verified by both the classical analytical model and the published results in the static and dynamic situations. Moreover, results also firstly show how the hydrodynamic pressure, cavitation, and leakage mass flow rate can be influenced by the motion of the roller in the rotary compressor. Compared to the static situation, the pressurized zone in the high-pressure side due to the hydrodynamic effect can potentially increase the leakage mass flow rate of the mixture. [ABSTRACT FROM AUTHOR]

• A new eco-friendly alternative mixed working fluid to replace R245fa/R134a is proposed. • Boiling heat transfer characteristics of R1234yf/R1336mzz(z) were experimentally evaluated. • Sun & Mishima correlation and Kandlikar correlation has been appropriately modified. Due to the inclusion of 18 HFCs, including R134a and R245fa, in the Kigali Amendment as controlled working fluids, the search for excellent alternatives to R134a and R245fa is urgent. R1234yf and R1336mzz(Z) are potential alternative working fluids to R134a and R245fa, but most studies have focused on the comparison of thermodynamic performance and boiling heat transfer capability of these pure working fluids. There is limited research on the boiling heat transfer performance of their mixed working fluids. Therefore, this study focuses on the flow boiling heat transfer characteristics of the mixed working fluid R134a/R245fa under non-azeotropic conditions. Experimental methods were employed to investigate the boiling heat transfer performance of the alternative working fluids R1234yf/R1336mzz(Z) (including R245fa/R1234yf and R1234yf/R1336mzz(Z) mixed working fluids) and evaluate the feasibility of R1234yf and R1336mzz(Z) as substitutes for R134a and R245fa. The results indicate that among the three mixed refrigerants, R134a/R245fa exhibits the highest heat transfer coefficient, while R1234yf/R1336mzz(Z) shows the lowest heat transfer coefficient. However, the differences in heat transfer coefficients among these three are relatively small, and their overall trends under different operating conditions are similar. It is evident that R134a/R245fa, with its good thermodynamic performance and stability, still holds potential as a transitional working fluid. R1234yf/R1336mzz(Z), with similar physical and thermodynamic properties, is an excellent alternative to R134a/R245fa. [ABSTRACT FROM AUTHOR]

• Multi-stream radial-tunnel header (MSRH) proposed to improve maldistribution. • Numerical model of MSRH and conventional cylinder header built and verified. • MSRH with orifice plate and center perforation to reduce phase separation. • Standard deviation of liquid refrigerant flow rate reduced to 9.4% and 1.61%. • Design method of length and critical diameter of center perforation presented. The Micro-channel heat exchangers (MCHXs) suffer from the non-uniform distribution of two-phase refrigerant due to the combined effects of inlet phase separation and differences in flow resistance, which becomes accentuated when faced with a large number of microchannel flat tubes. A multi-stream radial-tunnel header (MSRH) is presented to divide the refrigerant into several streams by radial-distributed tunnels and reduce the number of microchannels per stream, improving the distribution uniformity. The numerical models of conventional cylinder header (CCH) and MSRH are established and validated experimentally. Results show that the two-phase flow variation along the flow path is reduced by the radial tunnels with relatively equal flow resistance in MSRH. However, the two-phase separation and deflection will occur in the inlet bend due to the large gas–liquid density difference. Then, the MSRH with two rectifying structures is further proposed to mitigate the inlet phase separation. The average standard deviation of liquid refrigerant flow rate is reduced from 23.75 % to 9.4 % and 1.61 %, respectively. Furthermore, the design method of key parameters is also presented to provide guidance for various working conditions in actual application. [ABSTRACT FROM AUTHOR]

• The two-phase flow characteristic for R134a in both horizontal and vertical upward flow in 27 mm metal concentric tubes was revealed based on the capacitive signals. • Different flow patterns and buoyancy effect made a significantly difference on data trend between horizontal flow and vertical flow. • The amplitude features of the capacitive signals reflected the stability of the flow. • The oscillation levels could be explained by the flow pattern transition. The two-phase flow characteristics for the concentric tube would be different to a plain tube. But the existing research focused on the two-phase flow in circular tubes, without concentric structures. The purpose of this paper was to reveal the two-phase flow characteristics for R134a in both horizontal and vertical upward flow between two concentric tubes by capacitance sensors. The capacitive signals corresponding to different vapor qualities were measured with the mass fluxes from 18.37 kg m −2 s −1 to 64.31 kg m −2 s −1. The experimental data showed that the value of the time-averaged capacitive signal was mainly determined by the vapor quality. The measured capacitance was independent of the mass flux and electrode width, but greatly influenced by the flow direction. The result in horizontal flow was flatter when the fluid was closer to the single-phase and steeper at moderate vapor quality (from 0.1 to 0.9), while it was close to a straight line in vertical upward flow. Moreover, the capacitive signals had oscillation characteristics. By the probability density function, it was found that the amplitude features and oscillation levels were related to the flow patterns. The amplitude degree reflected the stability of the two-phase flow. The oscillation levels could be explained by the flow pattern transition. The corresponding relationship between flow patterns and oscillation characteristics was discovered, which could provide more information for the identification of flow patterns. [ABSTRACT FROM AUTHOR]

The refrigerant distributor helps resolve non-uniform flow distribution issues in multi-path evaporators. Using the CFD method, this paper investigates the distribution mechanism of R410A in a centrifugal distributor under air-conditioning conditions and explores the impact of structural factors on its distribution performance. The Taguchi method is employed to identify the optimal combination of structural parameters and optimization direction for achieving distribution uniformity. Furthermore, the flow distribution characteristics of the optimized distributor are investigated under different conditions. The results show that, under the working conditions of an inlet mass flow rate of 50 kg· h −1 and an inlet quality of 0.1, the non-uniformity of the mass flow rate in the outlet branches of the optimized distributor is only 1.07 %, which represents a decrease of 7.64 % compared to the original distributor. Additionally, the non-uniformity of the outlet branch quality also decreased by 9.41 %. These findings indicate that the optimized distributor exhibits superior distribution performance. Nonetheless, the degradation of the optimized centrifugal distributor's uniform distribution performance with increasing refrigerant inlet mass flow rate and quality suggests that it is more suitable for low mass flow rate and quality conditions. [ABSTRACT FROM AUTHOR]

The ejector, as one of the core components of the CO 2 trans-critical ejection refrigeration system, plays an important role in improving refrigeration capacity and reducing compressor power consumption. Although the ejector can be optimized by experiment and Computational Fluid Dynamics (CFD) simulation to improve its performance, these methods are time-consuming and complicated. This study aims to propose a method to improve the performance of the CO 2 trans-critical two-phase ejector assisted by using CFD, artificial neural network (ANN), and genetic algorithm (GA). Firstly, the influence of geometric parameters on ejector efficiency was analyzed by using CFD technology, and the database was generated. Next, the complex and time-consuming CFD model was replaced by the ANN surrogate model established to predict the ejector performance. Finally, the GA method was used to optimize the ejector to maximize the ejector efficiency. The results show that the efficiency of the optimized ejector is 35.39%. The optimized ejector increases the secondary flow velocity and eliminates the vortex in ejector, and the efficiency of the optimized ejector is raised by more than 8% on average than that of initial ejector under different primary and secondary flow conditions. The research shows that the combination of CFD, ANN, and GA has higher reliability and better performance in the optimization design of CO 2 two-phase ejector. [ABSTRACT FROM AUTHOR]

• An experimental rig is developed to measure the oil pumping in refrigeration compressors. • The oil flow rate is investigated in the presence of atmospheric air and refrigerant fluid. • The effects of evaporating temperature, compressor speed and oil temperature are assessed. • The presence of refrigerant significantly reduces the oil flow rate compared to experiments conducted with atmospheric air. • Visualization of the outgassing process shows that sudden depressurization can lead to intermittent oil supply. The oil pumping plays a key role in the reliability and efficiency of compressors adopted in refrigeration systems. However, the investigations available in the literature on this subject do not consider the presence of the refrigerant and its effects on the oil supply. This paper reports the development of a novel experimental rig to measure the flow rate provided by the oil pump of a hermetic variable capacity reciprocating compressor considering the presence of the refrigerant in contact with the lubricating oil. A visualization system was also developed to investigate the transient outgassing phenomenon that takes place in the oil supply system when a sudden depressurization occurs, such as during the compressor startup. Measurements were carried out with the AB ISO VG 5 oil and the R-600a as the refrigerant fluid under different pairs of evaporating and condensing temperatures and compressor speed. Considering that the AB ISO VG 5 oil and air are immiscible, tests were also performed with air to isolate the effect of the refrigerant fluid on the oil supply. The results show that the oil flow rate is decreased due to the presence of refrigerant, with this effect becoming slightly stronger as the evaporating temperature is increased. The visualization system revealed that the outgassing brought about by the solubility between oil and refrigerant fluid has the potential to temporarily interrupt the oil supply. [ABSTRACT FROM AUTHOR]

• ANNMLP provides the highest accuracy in predicting heat transfer coefficient. • SVR has the highest generalization capability to predict heat transfer coefficient. • XGBoost shows the highest accuracy and generalization capability for pressure drop. • Significant features for heat transfer coefficient: mass velocity and vapor quality. • Significant features for pressure drop: inclination angle and mass velocity. Machine learning techniques have great potential to predict two-phase flow characteristics instead of classic empirical correlations. In the present study, four different machine learning models, including multi-layer perceptron artificial neural network (ANNMLP), support vector regression (SVR), K nearest neighbors (KNN), and extreme gradient boosting (XGBoost), are employed to predict the heat transfer coefficient (HTC) and the frictional pressure drop (FPD) of R134a and R1234yf condensation flow in horizontal and inclined tubes. The dataset includes 348 points from previous works and the current research. To extend the data, an experimental study is also performed on the condensation of R134a in a horizontal tube for different mass velocities and vapor qualities. The results show that, in the best model, HTC can be estimated by ANNMLP with the mean absolute percentage error (MAPE) of 7.01%. The best prediction of FPD is achieved using XGBoost machine with MAPE of 10.87% on test data. Also, the feature importance procedure is implemented to recognize the most useful features. Based on the results, the mass velocity and the inclination angle are identified as the most influencing parameters on the prediction of HTC and FPD, respectively. [ABSTRACT FROM AUTHOR]

• We analyzed the non-uniform flow in the cross-corrugated passage of the shell-and-plate heat exchanger. • We summarize three typical gas and liquid flow paths under different working conditions. • We simulated three flow patterns and established the relationship between mean void fraction and flow patterns. • We proposed a correlation of gas-liquid two-phase flow in shell-and-plate heat exchanger. The flow path of the fluid in the passage of the shell-and-plate heat exchanger shows the characteristics of first dispersing and then gathering, which significantly affects the efficiency of the heat exchanger. Compared with the traditional rectangular plate heat exchanger (PHE), an in-depth understanding of the flow characteristics in SPHE is insufficient. The properties of the flows between the adjacent plates were numerically studied in the present study. The single-phase fluid passes rapidly between the staggered contact points, and there is noticeable pressure fluctuation at the entrance and exit. In addition, the flow diffusion and convergence cause noticeable differences in local velocity and pressure variation. For two-phase flows, three flow patterns are defined, and a subregional discussion on the flow pattern in the passage and their pressure gradient and void fraction are also provided. The void fraction is also introduced to determine the flow patterns, i.e., the dominant flow pattern is bubbly flow when α < 0.4 , 0.4 ≤ α ≤ 0.7 is for slug flow, and the primary type is film flow when α > 0.7. By analyzing the pressure drop, an empirical equation of pressure drop prediction is proposed. [ABSTRACT FROM AUTHOR]

• Refrigerant maldistribution of the novel heat exchangers is analyzed. • The decentered orifice plate creates nested loop flow to improve distribution. • The parallel multi-chamber forms separate chambers to improve distribution. • Significant reduction of refrigerant maldistribution after optimization design. 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. [ABSTRACT FROM AUTHOR]

• Ammonia void fraction in smooth tubes of 8.5 and 15.0 mm has been measured at different orientations and boundary conditions. • No existing single correlation predicts void fraction well for every orientation of the tube. • New single correlation has been proposed for slug and annular two-phase patterns at any orientation. Ammonia is an environment-friendly natural refrigerant and working fluid with negligible global warming and ozone depletion. Nowadays, ammonia is widely used in industrial refrigeration systems, heat pumps and thermal control systems, including heat pipes, loop heat pipes and two-phase mechanically pumped loops. In perspective, "green" ammonia is considered an energy carrier and carbon-free fuel. In two-phase systems, the void fraction is one of the critical factors determining the frictional and local pressure losses, boiling and condensation heat transfer and phase separation. The experimental study of ammonia void fraction in tubes with inner diameters of 8.5 mm and 15 mm is proposed in this paper. The quick-closing valve method has been chosen. Three flow configurations are considered: vertical downward, vertical upward and horizontal. The tests were provided for the two-phase ammonia at a mass velocity of 50...500 kg· m −2s−1, saturation pressure of 15...30 bar, and vapour quality of 0.05...0.30. One hundred and five points have been tested. No existing single correlation predicts the ammonia void fraction well at the considered boundary conditions for at least two orientations in the gravity field. A new single correlation has been proposed based on the Rouhani drift model with the tube inclination angle-dependent distribution parameter allowing the adequate prediction of void fraction in a tube with ammonia flow at horizontal, upward and downward orientations. [ABSTRACT FROM AUTHOR]

• Separation of two-phase ammonia has been studied in horizontal (H-H) T-junctions. • Separation in T-junction depends only on relative flow rate in certain conditions. • Simple flow pattern independent correlations are proposed for phase separation. Phase separation of a two-phase working fluid accompanies the thermal-hydraulic process in every system and device, with at least one T-junction used in a two-phase line. Adequate qualitative and quantitative prediction of phase separation in T-junction is critical for successful designing, e.g., refrigeration, thermal control, and fluid transport systems. Three simple T-junctions with different inlet and side diameters were tested in the horizontal position on two-phase ammonia at a saturation level of 40÷60 °C and mass velocities of 25÷150 kg· m −2s−1. Based on test data, it has been revealed that at the relative side branch mass flow rate above 0.6, the phase separation behaviour does not depend on inlet mass velocity, vapour quality, void fraction, flow patterns (stratified, slug and annular) and relative side branch diameter. A simplified empirical correlation has been proposed describing the ammonia test points with an error margin of ±15%. [ABSTRACT FROM AUTHOR]

• A 3D numerical model is built for R134a two-phase distribution in a T-junction. • Mechanisms of two-phase distribution are analyzed under different flow patterns. • The fraction of gas flowing through the branch decreases as inlet quality increases. • An increase in pressure difference at the junction diverts more liquid to the branch. The maldistribution of gas-liquid two-phase flow will decline the efficiency of the refrigeration system as the heat transfer performance of the heat exchangers decreases. The distribution characteristics of gas-liquid two-phase flow of refrigerant mixtures R134a in the vertically-upward T-junction were investigated in this work by 3D numerical simulation using the Eulerian model. The simulations were conducted for inlet quality from 0.005 to 0.8 and inlet mass flux from 100 to 1000 kg·m−2s−1 at a saturated temperature of 272.0 K, respectively. The effect of inlet quality on the two-phase flow distribution was studied when the inlet flow patterns were intermittent flow, annular flow, stratified-wave flow, and mist flow, respectively. The fractions of the gas phase and liquid phase flowing to the branch were used to represent the gas and liquid phase distribution characteristics in the T-junction. Numerical results showed that the fraction of the gas phase flowing through the branch decreased with an increase in inlet quality within the scope of this study. The distribution uniformity of the liquid phase in the vertically-upward T-junction was the best for mist flow and the worst for stratified-wave flow at the inlet. Calculation results indicated that a decrease in the difference in the inertia forces acting on the two phases would promote the uniformity of the gas phase distribution in the vertically-upward T-junction and an increase in pressure difference at the intersection zone would lead to a larger amount of liquid phase diverting to the branch. [ABSTRACT FROM AUTHOR]

• The experimental investigation on frictional pressure drop of new refrigerant inside microfin tube. • A new general correlation of frictional pressure drop of two-phase flow inside microfin tubes has been developed. • The new correlation could broadly be applied to the diameters of microfin tubes from 2.5 mm to 9.52 mm. • The proposed correlation can be applied to a wide range of mass velocities and many kinds of refrigerants. The adiabatic pressure drop in a microfin tube with an outside diameter of 3.5 mm was experimentally studied herein. The experiments were conducted using R1234yf as a working fluid at saturation temperatures of 20 °C and 30 °C with varying mass velocities from 50 to 300 kg m−2s−1 and vapor qualities ranging from 0.1 to 0.9. The effects of experimental conditions were analyzed and discussed with present and previous data. A new two-phase flow frictional pressure drop correlation has been developed using the data with tube diameter of 2.5–9.52 mm, mass velocity of 50–1000 kg m−2s−1, and many kinds of refrigerants. The proposed correlation predicts the experimental data of other researchers with mean deviation of 17.6%. [ABSTRACT FROM AUTHOR]

This study concerns to model the flow boiling heat transfer coefficient (HTC) in smooth helically coiled tubes. A dataset including 1035 samples was collected from 13 independent studies, enveloping a broad range of geometrical and operating conditions. The predictive capability of the earlier models was assessed for straight and coiled tubes by the analyzed database that they were not precise enough. Accordingly, a new empirical model based on the least square fitting method (LSFM) was constructed using seven input effective dimensionless factors. It was found that LSFM was not able to describe the complex and nonlinear nature of HTC in smooth helically coiled tubes. Furthermore, the intelligent method of genetic programming (GP) was utilized to obtain more accurate explicit correlation for HTC, which produced an acceptable average absolute relative error (AARE) of 17.35%. Finally, the machine learning approaches of multilayer perceptron (MLP), Gaussian process regression (GPR), radial basis function (RBF) was also implemented to model HTC in smooth coiled tubes. Although all intelligent based models provided excellent results, the GPR model outperformed the others with an average absolute relative error (AARE) of 5.93% for the tested dataset. In addition to the proposed models' performance, the most influential factors in controlling the boiling HTC in coiled tubes were also detected. [ABSTRACT FROM AUTHOR]

• In run pipe, "descend-ascend" of static and total pressure happens under high F. • The local pressure drops of R1234yf, R600a and R245fa increase in turn. • An ANN predicting model of local pressure drop was established. • Sensitivity analysis of flow parameters on K 12J and K 13J was conducted. The two-phase flow behaviors in T-junction are quite complex in energy transport systems. In this paper, the two-phase flow pressure drop of refrigerants in a horizontal branching T-junction was analyzed numerically and predicted using artificial neural network. Firstly, the distribution of static and total pressure was obtained based on Eulerian method, and the parametric studies on the local pressure drop were conducted. It is observed that the vortexes in the entrance of branch pipe lead to the pressure fluctuation and irreversible pressure losses, and the "descend-ascend" of static and total pressure happens under high mass flow split ratio in run pipe. Then, the ANN predicting model of local pressure drop coefficients was established. It shows that GA-BPNN and PSO-BPNN has the best predicting ability for K 12J and K 13J respectively, and the relative errors are within 10% for most cases. Finally, the sensitivity analysis was conducted, indicating that the effect of mass flow split ratio (F) and inlet quality (x 1) is the most significant for K 12J and K 13J respectively. [ABSTRACT FROM AUTHOR]

• A new general model for FPD during saturated boiling was proposed using GP. • 6021 experimental data samples from 42 published sources were analyzed. • Present model showed favor outcomes for different flow regimes and channel sizes. • The accuracy of previous models was considerably lower than the present one. • The new model presented favor physical trends at different operating conditions. This study presents a general explicit model for estimating the saturated flow boiling frictional pressure drop (FPD) in conventional (macro) and mini/micro channels heat exchangers. An extensive database including 6021 experimental data samples has been gathered from 42 published sources, covering a broad range of fluids, channel diameters and operating parameters. The new model is based on the separated model suggested by Lockhart and Martinelli (1949) for two-phase flow. Thus, the two-phase multiplier, ϕ l o 2 has been estimated using the intelligent approach of genetic programming (GP). The presented model predicts the mentioned database with a reasonable value of average absolute relative deviation (AARD) of 21.34%. Moreover, 74.85% of predicted data have an error of lower than 30% of the experimental values. The entire database is compared with ten well-known two-phase pressure drop correlations for the evaluation of previous models. But all of them showed a total AARD of more than 27%. The GP model shows good accuracy for both conventional and mini/micro channels and different flow regimes, including low and high Reynolds numbers. In addition, it is applicable for estimating the boiling FPD in different operating conditions. Based on 752 additional data from 4 independent sources, the new model provides the best predictions for estimating the FPD in conventional and mini/micro channels. [ABSTRACT FROM AUTHOR]

• Experiments on Two-phase loop thermosyphon (TPLT) studying influencing parameters. • Self-regulation compensates changes in heat load, height difference, mass flow rate. • Identification of optimal charge avoiding instabilities and critical temperatures. • Low-GWP refrigerants R1233zd(E) and R1224yd(Z) are suitable for thermosyphons. Two-phase loop thermosyphon (TPLT) is a promising technology looking at highly effective electronics cooling. Due to strong coupling between the internal and external parameters, in this study experimental tests in steady-state are carried out using R1233zd(E) and R1224yd(Z) as a working fluid to investigate the respective influences and resulting design requirements. The relationship between the governing thermal and flow equations is presented to facilitate the interpretation of the test results. The study shows a stable flow and cooling performance over a wide range of heat loads and recooling temperatures. The refrigerant charge is identified as one of the main influencing factors, with an optimum being between excessive subcooling and beginning dry-out. Both tested refrigerants lead to basically similar results, showing minor differences regarding thermal performance and system stability. [ABSTRACT FROM AUTHOR]

• Pressure drop of evaporating R134a flow at various flow conditions is measured. • Experiments are conducted on horizontal micro-fin tubes. • Machine learning based pipelines utilizing dimensionless features are developed. • Feature selection and algorithm optimization are utilized to optimize the pipelines. • The achieved accuracy is compared with that of state-of-the-art physical models. The present study is focused on proposing, implementing, and optimizing machine learning based pipelines for estimating the pressure drop in evaporating R134a flow passing through micro-fin horizontal tubes. Accordingly, an experimental activity is first conducted, in which the pressure drop of the flow at various operating conditions is measured. Physical models that are available in the literature are then implemented and the corresponding accuracy, while being applied to the obtained dataset, is determined. Machine learning based pipelines, with dimensionless parameters provided as features and two-phase flow multipliers as targets, are then developed. In the next step, a feature selection procedure is performed and an optimization process is then conducted to find the algorithms and the corresponding hyper-parameters, using which results in the highest possible accuracy. The optimal pipeline is demonstrated to be the one in which the liquid only two-phase multiplier is chosen as the target and is provided with only 5 dimensionless parameters as selected input features. Employing the latter pipeline leads to a mean absolute relative deviation (MARD) of 6.27 % on the validation set and 6.41 % on the test set, which is notably lower than the one achieved using the most promising physical model (MARD of 18.74 % on the validation set and 18.08 % on the test set). Furthermore, as the dataset and the obtained optimal pipeline will be made publicly accessible, the proposed methodology also offers higher ease of use and reproducibility. [ABSTRACT FROM AUTHOR]

• Two-phase flow visualization is achieved by a clear window on the inlet header. • Flow regime in the inlet header is periodic and one cycle has two or three stages. • Flow regime in the inlet header significantly affects the two-phase distribution. • Effects of mass flux, inlet vapor quality, and the number of plates are examined. This paper presents an experimental study of two-phase R134a flow in the inlet header of brazed plate heat exchangers and its effect on flow distribution among the channels. Visualization of the two-phase flow is accomplished through a 3-D printed transparent window on the inlet header of the heat exchangers. The two-phase flow distribution among channels is quantified based on infrared images of the heat exchanger sidewalls. At the test conditions, the observed flow regimes in the inlet header are periodic and two or three stages are identified in one cycle: top corner vapor flow, vapor jet flow, and (conditional) liquid blockage flow. Among them, the top corner vapor flow affects the distribution most, in which the vapor refrigerant is mainly present at the top corner of the header and branches out through the first several channels, leaving the liquid refrigerant to occupy the rest flow area of the inlet header and present a single-phase like distribution profile. When the inlet vapor quality increases, the distribution of the liquid refrigerant is improved since the vapor refrigerant can reach more downstream channels to help balance the total pressure drop. With the mass flux increases, the maldistribution caused by the header induced pressure drop is more significant, which compromises the benefit brought by the higher vapor momentum. When the number of plates is increased, the liquid refrigerant distribution is worse due to an increased pressure drop in the headers. [ABSTRACT FROM AUTHOR]

• Indication that two-phase flow in U-pipe causes vapor bypass in absorption heat pump. • Observation of vapor bypass in U-Pipe through experiment under vacuum condition. • Evaluation of effects of vapor bypass in AHP through field tests. • New design method to avoid vapor bypass and improve performance by over 6 %. A U-pipe is the key pressure separation device in a lithium bromide absorption heat pump (AHP) connecting the high-pressure condenser and the low-pressure evaporator. A recent study reported that two-phase flow occurs in the upward side of the U-pipe, which reduces the overall density of the fluid and impairs the pressure separation ability of the U-pipe. The liquid seal function of the U-pipe fails, and the vapor can flow directly from the high-pressure condenser to the low-pressure evaporator, causing a vapor bypass problem. This study experimentally investigates the U-pipe vapor bypass mass flow rate under vacuum conditions. The mass flow rate ratio of the bypassed vapor to the main liquid fluid in the U-Pipe can be as high as 10 %. The effect of this bypassed vapor on the AHP performance is investigated through simulations and field tests. The cooling coefficient of performance (COP) drops by 0.05 to 0.1 when vapor bypass occurs, providing evidence for the existence of U-pipe vapor bypass in AHPs. A practical method is suggested to solve this problem and to improve the cooling COP of the AHP by more than 6%. [ABSTRACT FROM AUTHOR]

The two-phase frictional pressure drop characteristics of flow boiling in zigzag channels of the printed circuit heat exchangers (PCHEs) under different conditions were investigated experimentally. The research results show that, the friction factor f of zigzag channel is increased by 9%~17% than that of the straight channel; as the vapor quality increases, the frictional pressure drop initially increases and then decreases, reprensting a maximum value at vapor quality of 0.9; the frictional pressure drop always decreases with the increasing saturated pressure when the void fraction is higher than 0.90, and it increases with the increasing heat flux due to the larger average vapor quality and the smaller average pressure. A new correlation for the frictional pressure drop of flow boiling in PCHEs was developed within a deviation of ±25%. [ABSTRACT FROM AUTHOR]

• CFD simulation is conducted in an intermediate header for flow separation in a microchannel condenser. • The 3-D CFD model is validated by experiments and compared to a 1-D model. • The liquid separation efficiency decreases with higher vapor separation efficiency. • The inlet tube intrusions make the other half of the header the "free-flow" region. • Void fraction and pressure profiles are also presented in the header. The separation circuitry has been proven in the past to improve the performance of microchannel condensers. In the vertical second header of the condenser, liquid separates from vapor mainly due to gravity. However, separation is usually not perfect, expressed through the separation efficiency. This study presents the phase separation result in the second header calculated by the Euler-Euler method of Computational Fluid Dynamics (CFD). Simulations are conducted for two-phase refrigerant R134a flow in the second header with 21 microchannel tubes in the 1st pass. The inlet mass flux to the second header (through the microchannels of the 1st pass) in the simulation is 166 kg m −2 s −1, 207 kg m −2 s −1, and 311 kg m −2 s −1. The inlet quality is 0.13 to 0.21. The results agree well with the experimental results with flow visualization and the results of a simpler 1-D numerical model. Results show that the liquid separation efficiency decreases as the vapor separation efficiency increases, following a linear trend in the experimental range. The void fraction result shows liquid mainly flows in the half of the header without microchannel tube intrusions. The velocity profile in the header is presented and reverse flow is identified on the exit planes of the inlet section connecting to the 2nd-upper pass and the 2nd-lower pass. The pressure profile in the header is also revealed and it indicates that the 1-D pressure assumption may still apply to two-phase flow in a header. [ABSTRACT FROM AUTHOR]

• Multiple indoor units are connected to a refrigerant distributor of a VRF system. • Distribution of two-phase flow was experimentally and numerically investigated. • Maldistribution of two-phase flow is significantly influenced by void fraction. • CFD simulation was performed to estimate distribution of R410A. • Maldistribution of R410A appeared to be less than maldistribution of air-water. In a variable refrigerant flow (VRF) system, the refrigerant is supplied from one outdoor unit to multiple indoor units through a header or refrigerant distributor. When the two-phase refrigerant flows into the distributor, uneven distribution occurs and deteriorate system performance. An experimental apparatus simulating the refrigerant distributor of a VRF system was constructed and air-water two-phase flow distribution characteristics were investigated. The air-water two-phase flow was more mal-distributed as the void fraction increased. A change in void fraction affected the distribution more than a change in mass flow rate in the experimental range. The flow distribution in the header was little affected by variation of the downstream flow resistance. Previous empirical correlations were investigated to predict the air-water two-phase flow distribution measured in this experiment, but performance was poor. To estimate the refrigerant two-phase flow distribution, computational fluid dynamics (CFD) simulations were performed using R410A as a working fluid. The mal-distribution of R410A appeared to be mitigated compared with air-water flow due to the large vapor phase density of R410A. [ABSTRACT FROM AUTHOR]

• R134a two-phase flow regimes in horizontal and vertical tube are detected by capacitive sensors. • All major flow regimes that appear within the tested mass flux range are characterized statistically. • Sensors with three different axial lengths are built and evaluated. • Sensors with shorter axial lengths have the same capability to characterize flow regimes. This paper presents the identification of slug/stratified-wavy, stratified-wavy and annular regimes for horizontal flow (in the range of mass fluxes 40 – 150 kg m−2 s−1), and slug, churn, and annular flow regimes for vertical upward flow (in the range 65 – 115 kg m−2 s−1) for R134a flow through 7 mm ID tube. Flow regimes are characterized based on time plot of normalized capacitive signals, kernel density estimation (KDE), power spectral density (PSD), and visualization results from a high-speed camera. Sensors with different axial lengths (D, 2D/3, and D/2) are also tested to study the adequacy of shorter sensors for the characterization of flow regimes. Results show that all three sensors have a similar capability of characterizing flow regimes, justifying the use of the shorter sensors in many applications with limited space. [ABSTRACT FROM AUTHOR]

• Two-phase flow patterns before and inside a distributor are visualized. • The distributor performance is evaluated in terms of quality, flow rate and capacity. • The effect of distributor orientation/gravity is studied. • Two strategies for improvement are compared with the baseline test. This paper presents an evaluation of the performance of a refrigerant distributor with visualization of the two-phase flow. The distributor is a typical conical design w/o the orifice for about 4 kW R134a system. Three factors influencing two-phase flow distribution are varied: mass flow rate, distributor inlet quality, and orientation. We looked at the uniformity of mass flow rates, evaporator inlet qualities, and capacities of each circuit. The authors built a transparent distributor following the exact geometry of an original distributor to visualize two-phase flow regimes exiting the expansion device and entering the distributor. We related flow regimes with distributor performance. A high-speed camera is used to capture two-phase flow regimes. After acquiring results for a baseline situation, the authors applied two approaches in an attempt to improve flow distribution: The first is manual adjustment of the resistance of each circuit individually to achieve uniform mass flow rates, and the second is the homogenization of the flow regime before division by adding an orifice in the distributor before separation to four channels. The average deviations of capacity for each branch are 9% for the baseline case, 3.3% for manual adjusting, and only 1.6% for the homogenization approach, stressing the importance of that simple strategy. [ABSTRACT FROM AUTHOR]

Operational instability, or hunting, is widely observed in vapor compression systems (VCSs), negatively affecting the operational safety and efficiency. As one of the most significant challenges present in VCS operation and control, operational instability has been extensively studied for decades. This paper reviews the current research state on the operational instability of VCSs, including the mechanisms for triggering hunting, the measures to mitigate hunting and the related unsolved issues. Two different views, i.e. the inherent characteristics of two-phase evaporating flow and dynamic characteristics of expansion (EV)-evaporator control loop, were classified for explaining the causes of hunting. As the typical characteristics of two-phase evaporating flow, the present of slug flow upstream in an evaporator, the sudden variation of heat transfer mechanism or two-phase flow instability would trigger the unexpected change of refrigerant temperature at evaporator exit. Consequently, operational instability will be resulted in. The dynamic behaviours of the EV-evaporator control loop in terms of its nonlinearities were considered as another essential factor that would cause system instability. Superheat setpoint adjustment and adaptive superheat control were suggested as two effective measures for mitigating hunting. With the increasing applications of vapor compression cycle for electronic cooling and the multi-evaporator VCSs, more effects should be done to investigate their operational instability with the consideration of two-phase flow instability and the coupling influences of each EV-evaporator control loop. [ABSTRACT FROM AUTHOR]

• Thorough investigations on the propane flow through a capillary tube. • Investigations include the hysteresis effect on the flow of the refrigerant. • Proposition of a modelling approach to capture and predict the hysteresis effect. • Comparison between the experimental and numerical data show good results. In this paper the two-phase propane flow through a copper capillary tube with an inner diameter of 1.1799 mm is thoroughly investigated according to metastable flow and hysteresis effects of the flow. The hysteresis effect causes ambiguity in experimental data of the two-phase pressure drop or mass flow. It is assumed that the hysteresis effect occurs when metastable flow is developed. As cause for the development of the metastable flow, the wetting ratio of the capillary tube is derived. The wetting ratio is considered to be the ratio of the length of the capillary that is covered by liquid flow only and the total capillary length. The findings are transferred into a mathematical description and supplemented into an existing homogeneous flow model. The extended model is able to simulate the general course of observed hysteresis courses and shows improvement in predicting the overall data base of previous work by being able to predict ambiguous data. [ABSTRACT FROM AUTHOR]

• Two-phase frictional pressure drop inside small diameter smooth and microfin tubes is measured experimentally. • The effects of mass velocity, vapor quality, tube diameter and refrigerant properties on the pressure drop are clarified. • A new frictional pressure drop correlational for small diameter tube is proposed. This study investigated the pressure drop of adiabatic vapor-liquid two-phase flow inside horizontal smooth and microfin tubes with 2.5 mm outside diameter. R134a and R1234yf were used as working fluid and the experiments were carried out at saturation temperatures of 20 and 30°C, mass velocities ranging from 50 to 200 kg m−2s−1 and vapor qualities ranging from 0.1 to 0.9. The effects of mass velocity, vapor quality, refrigerant properties and tube diameter on the pressure drop were analyzed. Some typical previous pressure drop correlations were used to predict the experimental data. A new two-phase pressure drop correlation for small diameter tubes was proposed based on the experimental data. The new correlation agrees with the available other researchers' data and it can be broadly applied to the small diameter smooth and microfin tubes, wide range of mass velocity and many kinds of refrigerants. [ABSTRACT FROM AUTHOR]

• Evaluated previous flow condensation heat transfer coefficient correlations. • Proposed a new correlation for condensation heat transfer in mini and macro channels. • Verified the model with 6521 data points from 40 published papers. • Discussed the effects of the input parameters on the heat transfer coefficient. A new general explicit correlation is proposed to predict the heat transfer coefficient of fluids condensing in conventional and mini channels. The expression has been developed by correlating the Nu mix number with Re mix , Pr mix , phase density ratio, P res , We GT , and Fr L using genetic programming for the two-phase flow. The model has been validated with a big dataset consisting of 6521 data samples, covering a wide range of fluids used in refrigeration and heat pump industries, cross-sectional geometries (different diameters), mass fluxes, and saturation temperatures. The new generalized correlation fits the wide range of data points used with an average relative error of 17.82 %. The same database has been used to compare predictions of eight correlations available in the literature, but they failed to give a reasonable estimation of the present experimental results. [ABSTRACT FROM AUTHOR]

• The presence of oil increases the frictional pressure drop by about 0–60%. • The frictional pressure drop increases with the increase of oil concentration. • The increased mass flux hides the improved impact of oil on pressure drop. • The modified correlation shows an acceptable deviation within −20% - +15%. The current work demonstrates an empirical investigation on the two-phase flow frictional pressure drops attributes of the R600a/3GS oil within an inner diameter horizontal tube, sized 8 mm. Furthermore, experiments were carried out at the nominal oil concentrations between 0% and 4% at the saturation temperature of 20 °C, having the heat flux amounting to 10.0 kW·m−2, together with the mass flux that ranged between 150 and 300 kg·m−2·s−1. Moreover, the impact of the nominal oil concentration, together with the mass flux and quality on the frictional pressure drops were not only investigated but also debated. As the findings indicate, the existence of oil augments the two-phase frictional pressure drop approximately between 0 and 60% with the present test conditions. Meanwhile, the comparison of the experimental data of the two-phase frictional pressure drops was carried out with some of the relationships present in literatures. The result showed the fact that the experimental data reasonably agree with the correlation of Schlager, with 95% of the empirical points being in an error bandwidth amounting to ±25%. Besides that, a new correlation for predicting the frictional pressure drop of R600a/oil mixture was modified on the basis of the local attributes of the oil. In accordance with the forecasted values from the modified correlation, there is an acceptable agreement with the empirical data. Moreover, the deviations are located within –20%–+15%. Accordingly, it could be put to use for predicting the frictional pressure drops of R600a/oil mixture in evaporator while designing a refrigeration system. [ABSTRACT FROM AUTHOR]

An ice slurry, which is a mixture of ice particles and aqueous solution, has many advantages in storing thermal energy because the ice particles have a high latent heat of fusion and large heat exchange area. In this study, we experimentally investigated the flow characteristics of an ice slurry in a T-junction under turbulent flow. The ice slurry was produced from 5 mass% ethanol solution. We varied the inlet ice-packing factor (IPF), Reynolds number of the inlet flow, flow-rate ratio, and branch direction and measured the pressure drop and IPF of the slurry before and after the T-junction. The branch pressure drop of the slurry in turbulent flow was similar to that of single-phase flow. The IPF for the branch tube was lower than the inlet IPF when the flow-rate ratio was low, although the IPF for the main tube was almost constant. Moreover, the branch pressure drop and IPF were used to determine the branch loss coefficient of both the branch and main tubes to evaluate the energy loss of the branch. [ABSTRACT FROM AUTHOR]

An ice slurry is a mixture of fine ice particles and an aqueous solution. Because of the latent heat of fusion of the ice particles and the wide heat exchange areas, ice slurries have many advantages as a thermal energy storage medium. The flow and heat transfer characteristics of ice slurries have been intensively studied. In the present study, the flow characteristics of an ice slurry were assessed experimentally, focusing on the distribution of the ice packing factor (IPF) and the energy losses on passing through a T-junction under laminar flow conditions. The ice slurry was produced from on a 5 mass% ethanol solution, and the inlet IPF, the Reynolds number for the inlet flow, and the flow rate ratio were all varied while monitoring the pressure drop and IPF before and after the junction. The results show that the distribution of IPF values is affected by the inlet IPF, and that the pressure drop before and after the T-junction increases along with the inlet IPF and the Reynolds number. This study also determined the branch loss coefficients for both the branch and main tubes as a means of evaluating the energy losses associated with branching. [ABSTRACT FROM AUTHOR]

• Darker continuous phase in gas-liquid two-phase image is distinguished efficiently • The Betti numbers is well curve-fitted by a four-parameter logistics model. • The topological evolution of discrete and continuous phases are synergistic. • An index for characterizing synergy is correlated with heat transfer coefficient. • New metric of flow regime control based on two-phase flow evolution is obtained. The marker-controlled multiple watershed segmentation are achieved to distinguish darker continuous phase in gas-liquid two-phase flow patterns efficiently. Each plot of the Betti numbers β i is curve-fitted using a four-parameter logistics model, for characterizing mixing effects. Similarity between adjacent pixels can be quantified by the distance. The β 1 of continuous phase decreases linearly at distance ≥ 2, which can be used to determine the threshold for segmentation. Repeated tests with different pixels and methods are conducted to ensure the repeatability and effectiveness of this model. More interestingly, we find that the rapid increase of β 1 of bubbles swarm coincides with the evolution of β 1 of continuous phase, and the median of difference of β 1 between the two phases in the visible window, as a novel metric of flow regime control is obtained and correlated with average volumetric heat transfer coefficient. This dynamic image analysis method, equipped with computational homology, provides the ability of evaluation of the spatial flow structure from a two-dimensional image and can be used to control the process. [ABSTRACT FROM AUTHOR]

• ANN is implemented for the prediction of two-phase refrigerant distribution. • The possibility for substantially improved prediction accuracy is demonstrated. • The optimised ANN model mostly achieves a ± 5% deviation without overfitting. • Reverse ANN could define optimal values of design parameters to achieve a target take-off-ratio. Due to the intrinsic complexity of two-phase flow distribution and the limited mathematical flexibility of conventional formulations of the phenomenon, previous attempts generally fall short in the accuracy and applicability of their prediction. To address these issues, this study focuses on methods with higher mathematical flexibility. Specifically, the construction and training of Artificial Neural Network (ANN) is presented for the identification of this complex phenomenon. The interaction of the numerous physical phenomena, occurring at different scales, is thus represented by the network structure, offering a formulation capable of achieving higher accuracy. Experimental data from a full-scale heat exchanger of an air-conditioning system operating over a wide range of conditions are used to train and test the ANN. The network optimisation with Bayesian regularisation against experimental data leads to a structure featuring 4 inputs, 3 hidden layers, and 3 neurons for each layer, which demonstrates deviations on the single output mostly lower than ± 10% and a correlation index higher than 98%, when the whole data set is used for training the ANN. The analysis of the network optimisation for different shares of data used for the network testing, shows higher training and testing accuracy as the number of training data increases, along with no apparent overfitting. [ABSTRACT FROM AUTHOR]

• Tested R410A-nanolubricants' heat transfer coefficients (HTCs) in evaporator tubes. • Nanoparticle deposition on the tube walls was observed. • Nanoparticle concentration had significant effects on flow boiling heat transfer. • Al 2 O 3 nanolubricant were more stable and had higher HTCs than ZnO nanolubricant. • Long-term flow boiling of R410A-nanolubricants had a gradual increase in HTCs. Lubricant in heat exchangers acts as a contaminant and it affects the heat transfer and pressure losses. Nanolubricants, that is, nanoparticles dispersed in lubricant oils, have shown potential to augment heat transfer rates in refrigerant direct-expansion evaporators. However, the mechanisms of such heat transfer enhancements are still unclear. Experiments were conducted to study the saturated two-phase flow boiling heat transfer phenomena of refrigerant R410A with two nanolubricants in a 9.5 mm I.D. smooth copper tube. The nanolubricants had non-spherical ZnO nanoparticles and spherical γ-Al 2 O 3 nanoparticles dispersed in Polyolester (POE) lubricant. Al 2 O 3 nanolubricant shared similar thermal conductivity in the wet state as that of ZnO nanolubricant. However, Al 2 O 3 nanolubricant had about 15% higher heat transfer coefficient that ZnO nanolubricant. The heat transfer coefficients of R410A-nanolubricant mixtures degraded by about 20% with respect to that of the R410A, but improved at higher vapor qualities. The experiments showed that long-term flow boiling testing of R410A and nanolubricant mixtures resulted in a continuous gradual increase of the heat transfer coefficient. In smooth copper tubes, nanoparticle deposition on the tube inner wall, which was experimentally observed, and the near wall interactions of nanoparticles promoted additional nucleate boiling and led to such increase. [ABSTRACT FROM AUTHOR]

• A new model of a thermosiphon desorber under sub-atmosphere was established. • The known correlations were checked to the validities of the model. • The heat transfer and mass flow rate behaviors have been investigated in the model. The thermosiphon desorber or bubble pump is advantageous in absorption cooling devices, since the processes of refrigerant desorption and solution pumping is combined in one hermetical unit. This supersedes the necessity of a mechanical solution pump. In the application with NH 3 /H 2 O the thermosiphon desorber typically operates at a pressure of around 15 bar with a liquid vapor density ratio in the range of 102, which is in a well-known regime of two phase flow. Nevertheless, for the application of thermosiphon desorbers in H 2 O/LiBr absorption chillers, which are commonly operated at a pressure below 0.1 bar with the liquid vapor density ratio in the range of 104, there is a lack of heat transfer correlations for two-phase flow. First attempts to fill this gap of flow boiling heat transfer correlations in the thermosiphon desorber at sub-atmospheric pressures is presented in this paper. A model is formed by solving simultaneously the mass, energy, and momentum equations. The well-known correlations for predicting single-phase and two-phase flow heat transfer coefficient and frictional pressure drop have been reviewed to find a possibility for applying them to the thermosiphon desorber. The mass flow rate and heat transfer behavior with the different flow length of the riser tube and with changing the pressure has been theoretically studied in the model. For verification, the model has been compared to experimental data using pure water. This will be presented in the second part of the paper. [ABSTRACT FROM AUTHOR]

• We investigate the lubrication system for compact hermetic reciprocating compressor. • We compare mass flow rate results obtained by experimental and numerical methods. • We investigate the oil climbing time by using high-speed camera. • We illustrate the instantaneous oil isosurfaces by using Sliding Mesh Method. • Sliding Mesh Method agreed well with measurements after 3000 rpm. • Moving Reference Frame method is computationally cost-effective. Sufficient lubrication of the moving parts in a variable capacity inverter compressor is vital since it can directly affect the performance and expected lifetime. In this study, the lubrication system of a compact inverter compressor (CIC) is numerically and experimentally investigated. In the numerical modeling, a finite volume-based algorithm is used to model two-phase (air–oil) flow inside the compressor using Volume of Fluid Method (VoF) method. Transient behavior of the oil flow under laminar flow conditions is both simulated by imposing Sliding Mesh (SM) and the Moving Reference Frame (MRF) methods at various crankshaft speeds varying between 1200 and 4500 rpm. The measurements are taken using an experimental setup to compare/validate CFD results obtained from SM and MRF methods. Flow visualizations are performed with a high-speed camera to determine the oil climbing and required time for sufficient lubrication precisely. Moreover, the acceleration of the crankshaft is determined via high-speed camera and employed as a user-defined function to model the start-up period of the compressor. It is shown that with increasing crankshaft speeds, the average oil mass flow rate released from the upper part of the crankshaft is increasing almost linearly. It is also shown numerically that with increasing oil viscosity, the mass flow rate decreases. The comparison of experimental and CFD results reveals that the MRF solutions are in a better agreement with the measurements up to 2800 rpm. The SM method became favorable as a CFD method due to better agreement with measurements between 3000 and 4500 rpm. [ABSTRACT FROM AUTHOR]

In this study, the two-phase pressure drop of R1234yf refrigerant (as an alternative to R134a) in a smooth horizontal tube is investigated. Tests are carried out and under several conditions of vapor quality (0% to 100%), mass velocity (200, 300 and 400 kg/m²s), heat flux (0, 7 and 14 kW/m²) and evaporation temperatures (20 °C and 30 °C). The internal diameters tested are 3.2 mm, 4.8 mm, 6.4 mm and 8.0 mm. A comparison of pressure drop between R1234yf and R134a is also carried out. The results demonstrate that the two-phase pressure gradient of R1234yf is approximately 20% lower than that of R134a. The experimental data is compared with 19 correlations from the literature. From this comparison, it is concluded that the experimental data are in accordance with the correlation proposed by Xu and Fang with a mean absolute error of 20% and with approximately 65% of the data predicted within a ±30% error band. [ABSTRACT FROM AUTHOR]

• Experiments on void fraction of R1234yf in a horizontal smooth tube. • Effects of experimental conditions on void fraction of R1234yf is analyzed. • Experimental data of void fraction of R1234yf are compared with correlations. • The quick close valve method was used to measure the local void fraction. Void fraction is an important parameter to design and simulate thermal systems involving two-phase flows. In this research, an experimental investigation of the void fraction in adiabatic two-phase flow of R1234yf in horizontal and smooth tubes with an internal diameter of 4.8 mm was carried out. For the experimental tests, the vapor quality ranges from 0.1 to 1 while two saturation temperatures (15 and 25°C) and two mass flow rates (180 and 280 kg. m −2s−1) are investigated. The quick-closing valve(s) method is used to measure the volumetric void fraction. Tests are also undertaken with R134a used as a reference in this study. The results highlight that the void fraction of R1234yf is 5% lower than that for R134a. In addition, the experimental data of R1234yf were compared against seven correlations from the literature: the Baroczy and the Hughmark correlations were shown to provide the best prediction, with a mean absolute error of 2% and 3.2%, respectively. [ABSTRACT FROM AUTHOR]

• Identified variables to predict transition from gravity- to momentum-dominated flow. • Developed detailed model to predict path pressure drop. • Predicted liquid distribution based on path pressure drop and header flow regime. • Calculated vapor distribution by equating pressure drop across each flow path. Mini- and microchannel heat exchangers have the potential to drastically decrease the size and cost of thermal systems, but they often underperform because of flow maldistribution. Maldistribution can be particularly acute when a two-phase mixture enters a heat exchanger header, which is difficult to address because the flow phenomena in these situations are poorly understood. Experimental studies in recent years have attempted to gain a better understanding of the flow regimes and distribution in manifolds, but methods to predict distribution in multi-channel heat exchangers have been lagging. A new modeling approach, which predicts the liquid and vapor flow rates in each channel of a microchannel heat exchanger based on header flow patterns and component pressure drop, is presented here. The proposed model shows good agreement with previously reported experimental results, predicting most of the data within the experimental error. [ABSTRACT FROM AUTHOR]

• Measures two-phase flow distribution for a range of heat exchanger inlet conditions. • Defines transition criteria between gravity and momentum dominated flows. • Describes the relationship between header flow regime and distribution. • Discusses the effects of path pressure drop on distribution. • Compares distribution characteristics in different header geometries. Mini- and microchannel heat exchangers have the potential to drastically decrease the size and cost of thermal systems, but they often underperform because of flow maldistribution. Maldistribution can be particularly acute when a two-phase mixture enters a heat exchanger header, which is difficult to address because the flow phenomena in these situations are poorly understood. In the present study, flow distribution in microchannel heat exchanger manifolds is investigated. The work focuses on two-phase flow regimes and distribution characteristics of saturated R134a in heat exchanger headers. The results quantify the effects of inlet mass flux (23.9 < G in < 95.7 kg m−2 s−1), inlet quality (0.10 < x in < 0.90), header flow regime and header pressure drop on distribution. Additionally, several header geometries (rectangular, triangular, and vane) are investigated to identify pathways to mitigate maldistribution in heat exchangers. [ABSTRACT FROM AUTHOR]

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