R134a Flow Research Articles

Investigation of two-phase flow distribution in the vertical annular distribution header of a variable refrigerant flow heat pump system

The characteristics of two-phase flow distribution from a vertically installed annular refrigerant distribution header to multiple branch ports are investigated, with a specific focus on simulating a real variable refrigerant flow heat pump (VRF HP) system. A test section is fabricated to replicate an actual distribution header for empirical investigation, utilizing an air–water mixture as the working fluid. A series of experiments are conducted to examine distribution characteristics, involving variations in the location of the main inlet port, operational mode, total mass flow rate, and void fraction. The experimental results indicate that the two-phase mixture exhibits a relatively uniform liquid phase distribution to the branch ports when the main inlet port is positioned at the bottom side. However, maldistribution to branch ports is intensified as the inlet void fraction increases and the flow rate of the two-phase mixture decreases. Furthermore, a series of computational fluid dynamics analyses is performed to investigate the distribution characteristics of R410A, the actual working fluid utilized in VRF HP systems. These analyses are conducted under the operational conditions typical of VRF HP systems. Despite identical Reynolds numbers for the two-phase R410A flow and air–water mixture flow, the former exhibits a notably uneven distribution to branch ports owing to the distinct thermophysical properties, particularly density and viscosity. The findings from this investigation enhance the comprehension of two-phase flow distribution characteristics in a vertically installed refrigerant distribution header under real VRF HP system conditions, offering valuable insights into mitigating the maldistribution of refrigerant flow to branch ports.

Predictive Modeling for Microchannel Flow Boiling Heat Transfer under the Dual Effect of Gravity and Surface Modification

This paper investigates the heat transfer performance of flow boiling in microchannels under the dual effect of gravity and surface modification through both experimental studies and mechanistic analysis. Utilizing a test bench with microchannels featuring surfaces of varying wettability levels and adjustable flow directions, multiple experiments on R134-a flow boiling heat transfer under the effects of gravity and surface modification were conducted, resulting in 1220 sets of experimental data. The mass flux ranged from 735 kg/m2s to 1271 kg/m2s, and the heating heat flux density ranged from 9 × 103 W/m2 to 46 × 103 W/m2. The experimental results revealed the differences in the influence of different gravity and surface modification conditions on heat transfer performance. It was found that the heat transfer performance of super-hydrophilic surfaces in horizontal flow is optimal and more stable heat transfer performance is observed when gravity is aligned with the flow direction. And the impact of gravity and surface modification on heat transfer has been explained through mechanistic analysis. Therefore, two new dimensionless numbers, Fa and Conew, were introduced to characterize the dual effects of gravity and surface modification on heat transfer. A new heat transfer model was developed based on these effects, and the prediction error of the heat transfer coefficient was reduced by 12–15% compared to existing models, significantly improving the prediction accuracy and expanding its application scope. The applicability and accuracy of the new model were also validated with other experimental data.

Identifying steady and pulsatile two-phase flow regimes with pressure drop signals and acoustic emissions

Identifying two-phase flow regimes is vital for understanding phase-change heat and mass transfer processes. Traditionally, flow regime identification has relied on subjective flow visualization studies, which are then converted to flow regime maps applicable to specific operating conditions. However, these conventional, largely subjective, maps fall short in predicting abrupt changes in flow patterns that often occur during regular operation of vapor compression and absorption heat pumps and other thermal systems. Conventional flow regime maps are also inadequate for addressing the transient, intermittent, or periodic flow regimes that occur in boiling and condensation enhanced using acoustic and other emerging techniques. Therefore, there is a pressing need for alternative flow regime identification techniques that can adapt and reliably track rapid and local changes in two-phase hydrodynamics. Promising candidates for dynamic flow pattern classification include high-resolution pressure drop signals and acoustic emission spectra, which can provide insights into the local hydrodynamics within a flow channel. To assess the feasibility of these measurement techniques, differential pressure and condenser microphone measurements are recorded alongside high-speed videos of a two-phase flow of saturated R134a. The total mass flux and vapor quality are varied to understand the effects of phase velocity and liquid inventory on wave propagation in the channel. Additionally, forced oscillations are introduced to a steady two-phase flow to analyze their impact on flow patterns and their corresponding pressure drop and acoustic signals. Statistical analyses, including Gaussian mixture modeling, are employed to reveal characteristic pressure drop probability associated with each flow pattern, which form the basis for developing a model capable of predicting flow regimes in both steady and oscillating flows. The resulting framework introduces a new flow regime identification technique that can adapt to dynamic operating conditions, benefiting a wide range of thermal systems and phase-change processes.

Flow boiling heat transfer in a plate heat exchanger with mixed chevron angle plates

This study designed a 120-plates PHE with combination of 60 plates with 65° chevron angle and 60 plates with 35° chevron angle. The plates with 65° chevron angle were assembled in the half part of the PHE which near the inlet port to provide higher flow resistance in comparing to the other 35° plates. The 35° chevron angle plates were put away from the inlet port to reduce the flow resistance caused by the longer flow path. It is aimed to reduce the flow maldistribution in the multiplate heat exchanger and improve its heat transfer performance. Refrigerant R-410A flow boiling heat transfer performance in the mixed angle PHE and another PHE with 65° chevron angle was tested for comparison. The test results show that heat transfer coefficient in the mixed chevron angle PHE is 20% to 70% higher than that in the 65° chevron angle PHE with 25% lower pressure drops. The mixed angle plates apparently reduced the liquid refrigerant flow deficient region and provided a better heat transfer performance.

Effect of Rotational Angle of Discrete Inclined Ribs on Horizontal Flow and Heat Transfer of Supercritical R134a

This work numerically studied the heat transfer and flow characteristics of supercritical R134a in horizontal pipes equipped with DDIR, considering variations in the rotation angle of DDIR. The aim is to improve the effects of the DDIR configuration on the heat transfer of supercritical flow. After validation with experimental data, the AKN model was employed to examine the effects of four sets of rotation angles (0°, 30°, 45°, and 60°) on the axial and circumferential heat transfer characteristics of DDIR horizontal tubes under the influence of strong (q1/G1 = 0.1 kJ/kg) and medium (q2/G2 = 0.056 kJ/kg) buoyancy. Results show that variations in the rotation angle do not induce significant alterations in the flow field, thus exerting minimal influence on the axial heat transfer characteristics. Meanwhile, the rotation angle determines the relative positioning of the circumferential inner wall temperatures and heat flux distribution, although the magnitude of this effect remains inconspicuous. The rotational angle parameter can be reasonably neglected in the future design and installation of heat exchangers.

An artificial neural network-based numerical estimation of the boiling pressure drop of different refrigerants flowing in smooth and micro-fin tubes

Abstract In thermal engineering implementations, heat exchangers need to have improved thermal capabilities and be smaller to save energy. Surface adjustments on tube heat exchanger walls may improve heat transfer using new manufacturing technologies. Since quantifying enhanced tube features is quite difficult due to the intricacy of fluid flow and heat transfer processes, numerical methods are preferred to create efficient heat exchangers. Recently, machine learning algorithms have been able to analyze flow and heat transfer in improved tubes. Machine learning methods may increase heat exchanger efficiency estimates using data. In this study, the boiling pressure drop of different refrigerants in smooth and micro-fin tubes is predicted using an artificial neural network-based machine learning approach. Two different numerical models are built based on the operating conditions, geometric specifications, and dimensionless numbers employed in the two-phase flows. A dataset including 812 data points representing the flow of R12, R125, R134a, R22, R32, R32/R134a, R407c, and R410a through smooth and micro-fin pipes is used to evaluate feed-forward and backward propagation multi-layer perceptron networks. The findings demonstrate that the neural networks have an average error margin of 10 percent when predicting the pressure drop of the refrigerant flow in both smooth and micro-fin tubes. The calculated R-values for the artificial neural network’s supplementary performance factors are found above 0.99 for all models. According to the results, margins of deviations of 0.3 percent and 0.05 percent are obtained for the tested tubes in Model 1, while deviations of 0.79 percent and 0.32 percent are found for them in Model 2.

Heat transfer characteristics of R134a flow boiling in a microfin tube under typical ORC pressures based on comparison with a smooth tube

The experimental investigation on the flow boiling heat transfer of an organic working fluid under typical organic Rankine cycle (ORC) operating conditions is insufficient, which directly affects the performance of the ORC system. In this paper, the flow boiling characteristics of R134a in a horizontal microfin tube were experimentally investigated in the reduced pressure range 0.6 ∼ 0.9. The general convection heat transfer characteristics of R134a flow boiling, along with the effects of heat flux, mass flux, pressure, and rewetting conditions, were investigated in a microfin tube and compared with that in a smooth tube. Results show that the main flow regime in the microfin tube under typical ORC operating pressures is the stratified flow with dry-out occurring at the top surface. The microfin effectively enhances the heat transfer and restrains the wall temperature separation through inducing strong helical flow due to buoyancy and centrifugal forces. With the heat flux increasing, the micro-fin strengthens the nucleate boiling by enhancing the vaporization cores, leading to more pronounced heat transfer enhancement compared with that in the smooth tube. The heat transfer on the bottom surface shows almost negligible sensitivity to variations in mass flux. However, in regions of high vapor quality, an increase in mass flux results in higher heat transfer coefficients on the top surface. Due to the interaction between intensified nucleate boiling and weakened gas convection when pressure is increased, it slightly affects the heat transfer coefficient. Unlike the smooth tube, the rewetting in the microfin tube can be maintained within a larger range of vapor quality due to the spiral flow caused by the micro-fin.

Numerical investigation of departure from nucleate boiling for subcooled flow boiling in a rolling tube

A crucial issue in floating nuclear plants is the critical heat flux (CHF) under oscillatory conditions by ocean waves. CHF must be predicted considering the dynamic motion of the floating platform. We present numerical simulations for predicting the DNB (departure-from-nucleate-boiling)-type CHF of subcooled R134a flow in a rolling tube and provide physical insight into the dynamic flow behavior regarding the DNB, considering rolling amplitudes of 15°, 30°, and 40° and a rolling period of 6 s. The local wall temperature and void fraction depended mainly on the dynamic changes in the gravitational direction. Gravity had the effect of lowering DNB. The Euler force created a secondary liquid flow that somewhat reduced the liquid temperature at the heating wall during the rolling period, thereby enhancing the convective and quenching heat fluxes. Furthermore, the secondary flow rapidly increased the liquid area fraction at the overheated wall and decreased the temporarily increased wall temperature before DNB occurred. The secondary flow had the effect of increasing DNB. In this study, the effect of increasing the DNB value is greater than the effect of reducing the DNB value. The occurrence of DNB depended on sufficient cooling of the downward-facing wall during the rolling period.

Effect of surface engineering methods on refrigerant evaporating flow characteristics in annular tubes: Experimental approach

Techniques such as mechanical, electrochemical, and chemical etching are recognized as functional strategies for modifying surface roughness, which in turn manipulates surface energy. This research aims to delve into the influence of these factors on the heat transfer and pressure drop characteristics of two-phase R134a refrigerant flow during boiling inside horizontal annular concentric tubes with internal heat flux. The study scrutinizes the heat transfer coefficient and frictional pressure drop for R134a evaporating flow in an annular tube featuring an inner diameter of 28.6 mm and an outer diameter of 42 mm. The exploration spans a broad range of parameters, which include vapor quality from 0.15 to 0.8, mass flux from 6.72 to 20.16 kg/m2s, internal heat flux from 0.608 to 2.432 kW/m2, and varying surface characteristics. The results show that escalating surface roughness via mechanical techniques introduces greater resistance to flow shear stress in comparison to other methods. Moreover, an increase in surface roughness contributes to a reduction in contact angle and an expansion of the heating area, thereby enhancing both heat transfer coefficient and pressure drop. However, the implementation of low-energy coating holds an insignificant influence on R134a heat transfer characteristics, potentially due to its diminutive surface tension. This results in minor variations in the refrigerant contact angle in relation to the coatings. Performance evaluation criteria have been employed to assess the optimal thermal performance of the surface engineering method, with peak performance achieved at a mass flux of 20.16 Kg/m2s and an internal heat flux of 2.432 kW/m2. Ultimately, the study corroborates the experimental procedure by comparing the observed outcomes with established empirical correlations for R134a flow boiling.

Investigation of Flow Boiling in Micro-Channels: Heat Transfer, Pressure Drop and Evaluation of Existing Correlations

In this study, the flow boiling heat transfer and pressure drop characteristics of refrigerant R134a in micro-channels were experimentally investigated. The tests were performed in circular horizontal micro-channels with inner diameters of 0.5 mm and 1 mm and a heating length of 300 mm. The mass velocities varied from 500 kg/m2s to 2500 kg/m2s, and the heat fluxes varied from 15 kW/m2 to 147 kW/m2. The heat transfer coefficient (HTC) and frictional pressure drop (FPD) were measured and discussed in detail. According to the results, HTC was significantly affected by heat flux, whereas it was independent of mass velocity. Nucleate boiling was the dominant heat transfer mechanism for R134a flow boiling in the micro-channels. In comparison to the 1 mm channel, the 0.5 mm channel shows better performance in heat transfer, with a maximum increase of approximately 22 %. In addition, FPD increased with increasing mass velocity and decreasing channel diameter. Finally, several existing correlations for HTC and FPD were evaluated by comparing them with the experimental values. Tran’s correlation (1996) presented a better agreement in terms of the average HTC, while for the FPD, the model of Kim and Mudawar (2013b) showed good prediction accuracy.

Visualization of R134a cavitation flow in a rectangle Venturi tube and determination of thermal effects with cavity length and thickness

In order to study R134a cavitation behavior in a rectangle Venturi tube, high-speed photography combined with pressure, flow rate, and temperature measurements are performed to visualize the cavitation process and determine the cavity thickness and length through the tube in a closed-loop test system. The experimental results indicated that incipient cavitation of R134a is observed at the entrance of the tube throat when the cavitation number is below 2.50, regardless of the inlet temperature and pressure. The normalized cavity length is correlated with the cavitation number, and a critical cavitation number of 1.766 is defined to connect two linear functions. The throat and tail cavity thickness can be fitted as a cubic polynomial function with the cavitation number, and a transition cavitation number of 2.00 divides the growth rate of the thickness with the decrease of the cavitation number into fast and slow-growing parts. The temperature drop measured at the throat of the Venturi tube can be fitted to a cubic polynomial function about the cavitation number. Under the same cavitation number, the temperature drop decreases with the increase of the flow temperature. The B-factor can be fitted as a cavitation number quadratic polynomial function and a linear function of the tail cavity thickness. The transition cavitation number is defined by the transition point of the growth rate of the above function. The empirical correlation of B-factor is modified based on the throat and tail cavity thickness, and the cavitation thermal effect of R134a is accurately predicted. Combined with the inception cavitation number, the transition cavitation number, and the critical cavitation number, the shape of the cavity is divided into four regions: inception cavitation, attached cavitation, supercavitation, and fusion cavitation. The fast Fourier transformation analysis shows that the cavitation flow of R134a gradually changes from periodic flow to fully developed unsteady flow with the decrease of cavitation number.

Measurement and modeling of void fraction for developing two-phase flow of R134a in a horizontal tube after an expansion valve

This paper applies an image analysis technique to quantify the two-phase flow videos after an expansion valve. This method first finds the liquid-vapor interface and then estimates the void fraction and velocity of each phase. This paper also proposes a one-dimensional developing two-phase flow model based on the force-momentum balance. The void fraction of the developing flow predicted by the model shows a similar trend to the quantified experiment data from image analysis: void fraction will decrease and then stabilize as flow develops along the tube. In essence, the change in void fraction and flow velocity is a result of unbalanced force in liquid and vapor phase. This model is also capable of estimating the distance to achieve fully developed flow after an expansion valve. The ability to predict two-phase flow behavior is not only important to the theoretical study of two-phase flow, but also critical to two-phase flow system design to optimize heat transfer and pressure drop.

Investigation of phase change characteristics for refrigerant R-134a flow in a double pipe heat exchanger with the use of thermal non-equilibrium model

Investigation of phase change characteristics for refrigerant R-134a flow in a double pipe heat exchanger with the use of thermal non-equilibrium model

Effect of discrete inclined ribs on flow and heat transfer of supercritical R134a in a horizontal tube

In this work, discrete double inclined ribs (DDIR) are introduced into the horizontal flow of supercritical R134a to improve the thermal performance of vapor generators in the trans-critical Organic Ranke Cycle. Effects of operating and geometric parameters are investigated after model validation against experimental data. Results show that DDIR are adaptable to changes in operating conditions because spiral flow maintains the dominant position of forced convection heat transfer. DDIR enhance the PEC of horizontal ST by at least 70% and reduce circumferential thermal non-uniformity by up to 90%. Rib pitch has the most significant effect on heat transfer, followed by rib height and inclination angle. The optimal values for the three geometric parameters are 20 mm, 0.8 mm, and 60°. Finally, the mechanism of rib-induced enhancement is analyzed. Average heat transfer is improved by the strengthened mixing between bulk and near-wall fluid instead of suppressing the buoyancy effect. DDIR-induced enhancement is superior to subcritical cases owing to the integral effect of sharp variations in specific heat at supercritical conditions.

Prediction of heat transfer coefficient and pressure drop of R1234yf and R134a flow condensation in horizontal and inclined tubes using machine learning techniques

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.

R410A and R32 condensation heat transfer and flow patterns inside horizontal micro-fin and 3-D enhanced tubes

Micro-fin tube, 3-D enhanced tube and smooth tube with an inner diameter of 9.52 mm were used as test tubes to study the condensation heat transfer performance with R410A and R32 as the working fluids at different mass flow rates (150–400 kg/m2s) and vapor qualities (0.2–0.8). For R410A and R32, the heat transfer coefficient of the micro-fin tube is 2.0–2.2 times and 1.5–2.0 times that of the smooth tube, and the heat transfer coefficient of the 3-D enhanced tube is 1.4–1.5 times and 1.5–1.6 times that of the smooth tube, respectively. The micro-fin tube is effective in thinning the condensate thickness and reducing the thermal resistance. The 3-D enhanced tube promotes the generation of turbulence and droplet entrainment, which improves heat transfer of enhanced tubes. The heat transfer coefficient of R32 is greater than that of R410A due to its higher thermal conductivity, latent heat and specific heat capacity. The frictional pressure drop increases monotonically with the mass flow rate. Considering the increment in surface area and the additional pressure drop penalty, the performance evaluation factor of the enhanced tubes ranges from 0.9 to 1.4. The study presents flow pattern maps for smooth and enhanced tubes. Enhanced tubes promote the appearance of intermittent and annular flow.

Performance analysis of micro-fin tubes compared to smooth tubes as a heat transfer enhancement technique for flow condensation

• Entropy generation of R134a flow condensation inside micro-fin and straight tubes is studied and compared. • Geometrical parameters impact on entropy generation is investigated. • Flow conditions impact on entropy generation is analyzed. • Favorable geometrical and flow conditions at which the micro-fin tube has superior performance are identified. Heat transfer enhancement techniques are accompanied by pressure drop amplification, detrimentally affecting their performance; entropy generation analysis is an effective approach to assess heat transfer enhancement along with resulting pressure drop. Current study investigates and compares the performance of micro-fin (as a passive enhancement technique) and smooth tubes during flow condensation (for R134a refrigerant) through conducting entropy generation analysis. First, the impact of geometrical and operating variables on pressure losses and heat transfer contributions to entropy generation and total generated entropy inside both types of tubes is examined. Then, the conditions at which the application of micro-fin tubes in lieu of smooth ones is justifiable and of superior performance are identified utilizing entropy generation number. The simulation results indicate that entropy generation enhances in the micro-fin tubes as tube diameter, mass velocity, vapor quality, and wall heat flux rise, and saturation temperature declines. The same is observed in the smooth tube except for the mass velocity; an increase in this parameter leads to a decreasing-increasing trend in entropy generation. Moreover, the entropy generation number results indicate that applying micro-fin tubes rather than smooth ones is justifiable, i.e., has better performance, at lower mass velocities and vapor qualities, but higher saturation temperatures and wall heat fluxes.

Numerical study of gas-liquid two-phase flow distribution of refrigerant mixtures in a vertically-upward T-junction

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.

Comparison of Cooling Performance in a Cylindrical Battery with Single-Phase Direct Contact Cooling under Various Operating Conditions

This study compares the performance according to a working fluid, the number of battery cooling block ports, and header width required for cooling according to the application of the direct contact single-phase battery cooling method in a 1S16P battery module and examines the battery cooling performance according to the flow rate under the standard and summer conditions based on an optimized model. The analysis result verified that R134a showed low-pressure drop and high cooling performance as the working fluid of the direct contact single-phase cooling system in the 1S16P battery module, and R134a showed the best cooling and stability when applied with three ports and a 5 mm header. In addition, under 25 °C outdoor conditions, the maximum temperature of the battery and the temperature difference between the batteries at 3 and 5 lpm excluding 1 lpm are 30.5 °C, 4.91 °C, and 28.7 °C, 3.28 °C, indicating that the flow rate of refrigerant was appropriate for battery safety. In contrast, in the summer condition of 35 °C, the maximum temperature of the battery and temperature difference between the batteries were 38.8 °C and 3.27 °C at the R134a flow rate of 5 lpm or more, which was verified as a stable flow condition for battery safety.

Numerical determination of condensation pressure drop of various refrigerants in smooth and micro-fin tubes via ANN method

Abstract In the current work, the pressure drop of the refrigerant flow in smooth and micro-fin pipes has been modeled with artificial neural networks as one of the powerful machine learning algorithms. Experimental analyses have been evaluated in two groups for the numerical model such as operation parameters/physical properties and dimensionless numbers used in two-phase flows. Feed forward back propagation multi-layer perceptron networks have been developed evaluating the practically obtained dataset having 673 data points covering the flow of R22, R134a, R410a, R502, R507a, R32 and R125 in four different pipes. The outputs acquired from the artificial neural network have been evaluated with the target ones, and the performance factors have been estimated and the prediction accuracy of the network models has been resourced comprehensively. The results revealed that the neural networks could predict the pressure drop of the refrigerant flow in smooth and micro-fin pipes between 10% deviation bands.