Condensation Flow Regimes Research Articles

Comparative analysis of condensation heat transfer and pressure drop of low GWP near-Azeotropic mixture and R-290 in a plate heat exchanger

Downward condensation of a near-azeotropic low GWP refrigerant mixture of R290/R1270 (65%/35% wt.), named HC01, was investigated inside an offset strip-fin (OSF) flow-structured plate heat exchanger (PHE). This study tested the prototype heat exchanger with HC01 and compared the condensation heat transfer coefficient (CHTC) and pressure drop (CPD) with the pure R-290 for working conditions of refrigerant mass flux (G) = 20–50 kg/m2s, inlet vapor quality (xin) = 0.2–0.9, saturation temperature (Tsat) = 40–55 °C, and heat flux (q) = 4–13 kW/m2. It was observed that the condensation of HC01 occurred entirely within the forced convection condensation flow regime, despite a slight change in the CHTC at a lower xin as the G increased. The CHTC demonstrated an increase in response to higher values of G and q, but decreased with increasing Tsat. Similarly, the CPD exhibited an increase with higher G and a decrease with lower Tsat, whereas the q had minimal influence. Comparison results showed that the HC01 exhibited 14.2% higher average CHTC but 6.5% lower CPD than that of R-290 because of the differences in properties, such as liquid thermal conductivity, viscosity, surface tension, latent heat, and discrepancies in liquid and vapor phase densities. Correlations were generated to predict the values of the Nusselt number (Nu) and friction factor (FF) of HC01, with mean absolute errors of 12.1% and 12.7%, respectively.

A comprehensive investigation of non-condensable gas and condenser temperature effects on power plant ejector performance by considering condensation flow regime

Considering that the cycle of the steam power plant (Rankin) works with steam and water as working fluid in a closed cycle, the condenser will be one of the most important parts of this cycle, which has the task of changing the phase (condensation) of the turbine output steam and making it possible to transfer it to the beginning of the cycle. In addition to the working conditions of the cycle, such as the temperature and pressure of the turbine end, the performance of the condenser depends on the environmental conditions, such as the ambient temperature and the absence of non-condensable gases. In this paper, the effects of the condenser temperature and mass fraction (MF) of the air inside the condenser are investigated on the performance of the ejector (POE). It is shown that the effect of the condenser temperature on the POE is much higher than the effect of the air MF. With the increase of the condenser temperature and the air MF, the flow rate of the suction flow improves, and the entrainment ratio (ER) rises. With the temperature increment from 320 K to 340 K (without air in the condenser), ejector efficiency reduced by 61.16%, and exergy destruction enlarged by 12.13%. By incrementing the air MF from 0 to 0.35 at the temperature of 340 K, ejector efficiency, exergy destruction, and ER improve by 2.84%, 6.29%, and 3.88%, respectively.

Condensation of R1234yf in a plate heat exchanger with an offset strip fin flow structure for electric vehicle heat pumps

The energy-efficient plate heat exchanger (PHE) and refrigerant R1234yf, which has a low global warming potential (GWP), can be used to realize an energy efficient heat pump (HP) system for electric vehicles (EV), extending their driving range. Therefore, the characteristics of R1234yf in an offset-fin strip (OSF) flow-structured PHE are critical for heat-exchanger design. This study investigates the condensation heat transfer coefficient (C-HTC) and two-phase frictional pressure drop (2P-FPD) of R1234yf during condensation in an OSF flow-structured PHE under various operating conditions. First, a modified Wilson plot method was used to determine the multiplier (C) and Reynolds number exponential (n) for the coolant side as −0.426 and 0.494, respectively. When the heat flux (q), average vapor quality (xa), and mass flux (G) increased, the C-HTC increased, whereas it decreased with saturation temperature (Tsat). Despite the force-convective condensation flow regime, the C-HTC increment was minimal with G at lower xa owing to the lesser significance of the shear effect. Additionally, the 2P-FPD was unaffected by q but increased considerably with an increase in xa and G and a decrease in Tsat. Based on the current experimental database, empirical correlations for forecasting friction factor and Nusselt number were developed with a 91% predictability.

Modeling of condensation heat transfer coefficients and flow regimes in flattened channels

In this paper, an adaptive neuro-fuzzy inference system (ANFIS) with fuzzy C-means clustering (FCM) structure identification is proposed to model condensation heat transfer and flow regimes in flattened smooth tubes with different aspect ratios. The FCM-ANFIS model was trained by using experimental data points for six effective chosen parameters of saturation temperature, heat flux, mass flux, aspect ratio and hydraulic diameter of the flattened tube, and vapor quality. Three flow regimes of annular flow, stratified, and intermittent flow were linked to the effective parameters based on the experimental data. Two models were proposed to predict the condensation heat transfer coefficient and the flow regime of R134a and R410a in flattened smooth tubes. Three statistical criteria were used to ascertain the accurateness of the models compared to the experimental results. It is found that while among benchmarked cases, the proposed model for the condensation heat transfer coefficient performs well, the best result with the lowest error (MAE = 0.029, RMSE = 0.036 and MRE = 2.83%) is when Tsat = 45 °C, q″ = 10 kW/m2, G = 100 kg/m2. s, β = 6 and Dh = 2.3 mm. On the other hand, in the worst-performing case when the errors are MAE = 0.239, RMSE = 0.239 and MRE = 13.36%, the predicted results are still in the uncertainty range of the experimental result when Tsat = 45 °C, q″ = 5 kW/m2, G = 200 kg/m2. s, β = 6 and Dh = 2.3 mm.

Experimental Investigation and Visualization Study of the Condensation Characteristics in a Propylene Loop Heat Pipe

At present, the structural design of a loop heat pipe (LHP) condenser mostly relies on experience. To analyze the condensation flow pattern in an LHP and to find a more accurate empirical correlation that can better guide condenser design in existing empirical correlations, a test system was designed and fabricated to observe the condensation phenomenon in the condenser and to evaluate the LHP’s heat transfer performance. Since propylene has a wide temperature range to different temperature control requirements, it was chosen as the working fluid. In this paper, the condensation process in the condenser was observed, and the operating parameters of a propylene LHP were measured at 283 K. The condensation flow patterns in the 2.5 mm ×2.5 mm channel mainly included stratified flow, wavy flow and intermittent flow. By comparing the experimental results with four typical condensation flow regime maps, it was found that the Breber and Cavallini flow regime maps can successfully predict most of the condensation flow patterns of the LHP even with ultra-small mass flux. In addition, four condensation heat transfer coefficient correlations were selected to compare the predicted values of heat transfer coefficient with the experimental results. The study showed that the predicted values from the correlation by Cavallini et al. matched 90% of the experimental values well with an error of less than 20%, and this correlation is recommended for highly accurate LHP design.

Experimental and Numerical Analysis of Condensation Heat Transfer and Pressure Drop of Refrigerant R22 in Minichannels of a Printed Circuit Heat Exchanger

A Printed Circuit Heat Exchanger (PCHE) is a type of highly complete and efficient heat exchanger that consists of numerous mini/micro-channels and has been successfully applied to the Liquefied Natural Gas (LNG) regasification project. During the research presented in this paper, the condensation flow and heat transfer performance of the R22 in PCHE hot side minichannels are analyzed via experiments and numerical simulations, respectively. A liquid nitrogen–R22 experimental loop is established to examine the pressure difference and heat transfer coefficient of R22 in the minichannels of the PCHE hot side. The inlet pressures of the R22 range from 0.5 MPa to 0.65 MPa, the mass flux values are changed from 10.52 kg m−2s−1 to 109.42 kg m−2s−1, and the inlet temperatures vary from 273 K to 289 K. The differences between experiments and simulations are analyzed by comparing the experimental values of the Nusselt number (Nu) and the friction pressure gradient with the numerical ones. Furthermore, the influences of pressure and mass flux on the Nu, as well as the friction pressure gradient, are analyzed in depth through condensation flow regimes to explore the underlying mechanism giving the results.

Experimental research on double-hole steam jets dcc heat transfer characteristics under stable condensation pattern

The multi-hole spargers are widely used to discharge the high temperature and high pressure steam for depressurization in most engineering applications due to its high efficiency in heat and mass transfer. However, the steam condensation microscopic mechanisms are very complicated, especially for the multi-hole conditions. In this study, the double-hole spraying steam direct contact condensation (DCC) mechanism and heat transfer characteristics are researched based on the scaled IRWST & ADS experimental facility. The double-hole condensation flow regimes are distinguished on the condensation flow patterns diagram, and the steam condensation analytical models are then developed with characteristic parameters including L, Rex, Rx, δ etc. to quantitatively describe the double-hole condensation features. New empirical correlation for predicting the double-hole steam penetration length L is proposed. Moreover, special temperature distributions along both the steam plumes centerline and middle line reflect the interactions between the adjacent steam plumes, especially for the ellipsoidal plume pattern. In essential, the specific temperature variations reflect the equilibrium and transformation between the thermal dynamic energy and kinetic energy. Finally, the heat transfer coefficients for the double-hole steam condensation are calculated based on the steam plume analytical models. The analysis indicates that the double-hole steam condensation heat transfer coefficients are in same order of magnitude with the single-hole stable condensation case, whereas the lower boundary of heat transfer coefficients deceases to some extent. It provides experimental references for the operation and design of the actual industrial steam condensation applications.

Numerical study of condensation flow regimes in presence of non-condensable gas in minichannels

A numerical research on flow regime of R134a in horizontal minichannels was conducted. The model was on the basis of the volume of fluid approach, and inserted user-defined routines which cover condensation mass and heat transfer. The observed annular flow, injection flow and intermittent flow of pure steam condensation, and annular (annular-wavy) flow, injection flow, annular-intermittent flow and intermittent flow of gas mixture condensation were qualitatively compared against experimental data. Flow patterns of lower non-condensable gas mole concentration show similar with that of pure steam, while the fluctuation of gas-liquid interface in annular flow and the noticeable annular-intermittent flow of higher non-condensable gas mole concentration were noted. Besides, injection flow characters with larger head volume and longer neck, and intermittent flow characters with larger slug sizes in existence of non-condensable gas were found. Furthermore, the influence of mole concentration of non-condensable gas and operating conditions on flow pattern transition lines were analyzed after validating the flow pattern transition. The results showed that the injection flow transition point has a relative fixed point at pure vapor condensation, while various in presence of NCG. Besides, lower non-condensable gas mole concentration, smaller inlet gas mass flux and larger wall heat flux could lead to earlier flow pattern regime.

An Experimental Investigation of Flow Patterns During Condensation of HFC Refrigerants in Horizontal Micro-Fin Tubes

Condensation heat transfer coefficients and flow regimes in two different horizontal micro-fin tubes are examined during the condensation of refrigerants R-134a and R-410A. The present investigation has focused on determination and prediction of condensation heat transfer coefficients and finding the interrelation between heat transfer coefficients and the prevailing flow regimes. During flow visualization, flow regimes have been captured using borosilicate glass tube at inlet and outlet of the test condenser using high speed digital camera. Stratified, stratified wavy, wavy annular, annular, slug and plug flows have been observed at different mass fluxes and vapor qualities of the refrigerants. The observed flow regimes are compared with the existing flow regime maps proposed by Breber et al. [Prediction of horizontal tube side condensation of pure components using flow regime criteria, J. Heat Transfer 102 (1980) 471–476], Tandon et al. [A new flow regime map for condensation inside horizontal tubes, J. Heat Transfer 104 (1982) 763–768.] and Thome et al. [Condensation in horizontal tubes, part 2: New heat transfer model based on flow regimes, Int. J. Heat Mass Transfer 46 (2003) 3365–3387.] Thome et al. [Condensation in horizontal tubes, part 2: New heat transfer model based on flow regimes, Int. J. Heat Mass Transfer 46 (2003) 3365–3387.] flow regime map shows good agreement with experimental data.

Identification of condensation flow regime at different orientations using temperature and pressure measurements

While many prior works in the field relied upon direct optical access to determine condensation flow regimes, the present work outlines a new methodology utilizing temperature and pressure measurements to identify condensation flow regimes. For vertical upflow condensation, amplitude of dynamic temperature and pressure oscillations are shown to clearly indicate transition from counter-current flow regimes (i.e., falling film, oscillating film, flooding) to annular, co-current flow (climbing film flow regime). In horizontal flow condensation, standard deviation between multiple thermocouple measurements distributed around the tube circumference was calculated at all axial (stream-wise) measurement locations. High values of standard deviation are present for stratified flow (stratified flow, wavy-stratified, plug flow), while axisymmetric flow regimes (i.e., slug flow, annular flow) yield significantly lower values. Successful development of this technique represents a valuable contribution to literature as it allows condensation flow regime to be identified without the often-costly restriction of designing a test section to allow optical access. Identified flow regimes in both vertical upflow and horizontal flow orientations are compared to regime maps commonly found in the literature in pursuit of optimum performing maps.

Comprehensive condensation flow regimes in air gap membrane distillation: Visualization and energy efficiency

The thermal performance of air gap membrane distillation (AGMD) desalination is dominated by heat and mass transfer across the air gap between the membrane and the condensing surface. However, little is known about the impact of condensate flow patterns in some design variations of the air gap. In this study, air gap membrane distillation experiments were performed at various inlet temperatures, varying module inclination angle, condensing surface hydrophobicity, and gap spacer design to identify the effect of each on the permeate production rate and thermal efficiency of the system. Additionally, this study is one of the first with enhanced visualization of flow patterns within the air gap itself, by using a transparent, high thermal conductivity sapphire plate as the condenser surface. System-level numerical modeling is used to further understand the impact of these flow regimes on overall energy efficiency, including flux and GOR. A brief review of membrane distillation condensation regimes is provided as well. For tilting the AGMD flat-plate module, permeate flux was barely influenced except at extreme positive angles (> 80°), and moderate negative angles (< −30°), where condensate fell onto the membrane surface. The surface with the hydrophobic coating (for dropwise condensation) was shown to have better droplet shedding (with very small nearly spherical droplets) and fewer droplets bridging the gap. Superhydrophobic surfaces (for jumping droplet condensation) were similar, with much smaller droplet sizes. Such surfaces could improve AGMD efficiency while avoiding flooding, as better performance (2x higher GOR) is reached with much smaller gap sizes (<0.2 mm) than currently attainable gaps with filmwise condensation (>1 mm). Meanwhile, the hydrophilic surface for small gap sizes (< 3 mm) often had pinned regions of water around the hydrophilic surface and plastic spacer. Overall, the various results imply that the common assumption of a laminar condensate film poorly describes the flow patterns in real systems for all tilt angles and most spacer designs. Real system performance is likely to be between that of pure AGMD and permeate gap membrane distillation (PGMD) variants, and modeling shows that enhanced condensing in air gaps may improve system energy efficiency significantly, with strong relative advantages at high salinity.

Theoretical model of linear instability for condensation flow in circular channels

The paper presents a theoretical model for liquid-vapor interface instabilities for R134a condensation flow in tubes. The model is based on linear Kelvin-Helmholtz instability and capillary instability theories that neglect the effect of gravity and take into account the surface tension and shear stress effects. The normal mode method was used to analyze the system instability reactions to various perturbation wave lengths and physical conditions. Three modes were obtained with one giving the greatest instability wave length for each condition. The cooling temperature had little effect on the greatest instability. The greatest instability wave length increased with decreasing R134a mass flux and quality, with jumps between two instability states. This instability state transition can be used to predict condensation flow regime transitions for intermittent flow. The dimensionless intermittent flow criterion is in good agreement with available experimental data in the literature.

Numerical study of condensation flow regime transition in wavy microchannels

ABSTRACTA numerical research on flow regime transition in wavy microchannels was conducted. The model was based on the volume of fluid approach and user-defined routines including interfacial mass transfer and latent heat. The observed droplet flow, annular–wavy flow, injection flow, and slug–bubbly flow were qualitatively compared against experimental data and transition lines were established. The effects of inlet vapor velocity, wall heat flux, and microchannel geometry characteristics on the annular length, occurrence frequency of injection flow, initial slug volume, and bubble detachment frequency were investigated.

The development of a dynamic single effect, lithium bromide absorption chiller model with enhanced generator fidelity

Single effect, lithium bromide absorption chillers offer the ability to utilize low-pressure steam to produce chilled water for satisfying various comfort cooling needs. Previous attempts have been made to characterize dynamic and steady-state absorption chiller operation. Though these models perform adequately, they are based on hot water driven absorption chillers. Commercially available absorption chillers often can run on both hot water and low-pressure steam. In this paper, the mathematical framework for a dynamic single effect, lithium bromide absorption chiller model capable of using low-pressure steam is presented. The transient thermodynamic FORTRAN model is grounded on mass, energy, and species balances, and builds on prior modeling efforts. Well-known correlations for heat transfer coefficients are used to describe both tube-side and shell-side heat transfer rates in each primary chiller component. To account for the absorption chiller unit receiving steam, a heat transfer model for condensation inside horizontal tubes based on distinct internal condensation flow regimes is incorporated within the generator. This heat transfer model is used with two-phase flow pressure drop equations to establish steam temperature, quality, and pressure along the generator tube bundle. Steam consumption trends are established asa function of fluctuating external conditions. These trends reasonably align with information made available online by the manufacturer, though some deviation does occur at low chiller capacities and cooling water temperatures. Additionally, the transient response of internal and external parameters from a step increase in heat input supplied to the generator mimics results of other dynamic absorption chiller models found throughout literature.

R134a condensation flow regime and pressure drop in horizontal microchannals cooled symmetrically and asymmetrically

An experimental investigation was conducted for two-phase flow regimes and friction pressure drop during condensation of refrigerant R134a in oval parallel microchannels with a hydraulic diameter of 301.6µm, an aspect ratio of 2.46 and a length of 50mm. The effects of microchannel cooling methods, including asymmetric cooling and symmetric cooling, were studied. The flow regimes and pressure drops were recorded for qualities of 0.1–0.9 and mass flux of 60–250kg/(m2s). The inlet saturation temperature of R134a is 31.3°C. Film wavy flow, corner wavy flow, slug flow and bubbly flow were observed. The flow regimes were mapped and compared with 8 flow regime maps in the literature. Two flow regime criteria were given using vapor Weber number. The wavy-intermittent flow transition locations were measured and compared with the correlations in the literature. The transition location moves downstream with increasing refrigerant mass flux and inlet quality. The transition locations were more upstream for the condensation flow cooled symmetrically then that cooled asymmetrically. The friction pressure drop increases with increasing mass flux and quality. The pressure drop for microchannels with symmetric cooling is lower than asymmetric cooling. The pressure drop data was compared with the correlations in the literature.

Condensation heat transfer characteristics and flow regime in a horizontal rectangle channel of a multichannel cylinder dryer

ABSTRACTThe present study investigates the phenomenon of steam condensation in a rectangle horizontal channel with a mass flux range of 20–70 kg/m2/s and a vapor quality of 0.2–0.8. Mass flux and vapor quality were presented to primarily affect the heat transfer coefficient in shear-dominated flow regime, such as the mass flux is greater than 40 kg/m2/s. At the low mass flux, only slug flow, plug flow, and wave flow were observed. At the high mass flux, the wave flow and annular flow were observed even at the low-vapor quality, and the slug and plug flow were not found during the entire condensation process. Observed flow patterns agreed with Tandon’s flow pattern map. Four kinds of correlations are employed to predict the condensing heat transfer coefficient, and the result reveals that Cavallini’s correlation agrees with the experimental results relatively well. The measured pressure drop has a good agreement with the Lockhart–Martinelli correlation.

Determination of flow regimes and heat transfer coefficient for condensation in horizontal tubes

This study explores condensation of FC-72 in horizontal tubes. Using high-speed video motion analysis, dominant condensation flow regimes are identified for different combination of mass velocities of FC-72 and cooling water. Additionally, detailed heat transfer measurements are used to explore both axial and circumferential variations of the condensation heat transfer coefficient. Four different regimes are identified: stratified, stratified-wavy, wavy-annular with gravity influence, and wavy-annular without gravity influence. In the latter regime, which is achieved at high FC-72 mass velocities, annular film transport is dominated by vapor shear with negligible gravity effects. Using different types of regime maps, prior relations for transitions between regimes are assessed, and new, more accurate transition relations developed. The heat transfer coefficient is shown to be highest near the inlet, where quality is near unity and the film thinnest, and decreases gradually along the condensation length because of axial thickening of the liquid film. This study also explores the predictive capabilities of prior heat transfer correlations and a control-volume-based annular flow model. The experimental data of both the local and average condensation heat transfer coefficients show fair to good agreement with predictions of prior and popular correlations. But superior predictions in both trend and magnitude are achieved with the annular flow model.

Performance of Vertical Tube Passive Condenser with the Effects of Noncondensable and Secondary Cooling Water

An experimental work was carried out on a passive containment cooling system (PCCS) test facility where the effects of change in secondary cooling pool water level and the presence of a noncondensable on the PCCS heat transfer characteristics were investigated. Two condensation flow regimes, complete condensation and flow-through condensation relevant to initial and late stages of PCCS operation, were investigated where secondary cooling pool water level decreases with time. A single tube and a tube bundle test section were used. Steady-state tests with half full and half secondary pool levels were performed using single and four-tube bundle test sections. Transient tests were carried out on the tube bundle test section where the secondary pool water decreases continuously due to boil-off. Transient tests carried out with secondary pool water level change showed that the system pressure for complete condensation mode increases with decrease in water level; however, rate of condensation is almost constant. If the PCCS is operated in through-flow mode the system pressure (primary side pressure) is constant; however, the condensate rate decreases, indicating that some of the steam does not condense. A decrease in pool water level to the top header initially decreases the inlet steam pressure, indicating slight heat transfer enhancement due to efficient cooling of PCCS tubes due to two-phase mixture. When the water level is below three-fourths of the tube height the inlet steam pressure increases with decrease in the water level, indicating decrease in PCCS heat transfer rate. The presence of a noncondensable also reduced PCCS condensation heat transfer.

Flow regime transitions during condensation in microchannels

Heat transfer and pressure drop for two-phase flow inside tubes are closely related to the corresponding flow mechanisms. The flow patterns formed in microchannels during condensation differ from those observed in conventional tubes. Using an extensive R134a condensation flow-regime database (1 < Dh < 4.91 mm, 150 < G < 750 kg m−2 s−1), new flow regime transition criteria are proposed. The data are used to understand the physical mechanisms and the governing influences for each of the identified flow regimes and develop dimensionless transition criteria. These criteria can be utilized to identify the flow regimes and transitions for various fluids, operating conditions and channel sizes, thereby generalizing their applicability. This mechanistic determination of condensation flow regimes in different operating conditions and geometries will assist in the development of models for predicting condensation heat transfer and pressure drop, enabling the development of optimized microchannel heat exchangers.

Single- and Multi-Constituent Condensation of Fluids and Mixtures with Varying Properties in Micro-Channels

The relative magnitudes of the forces governing the transfer of heat, mass, and momentum during microscale condensation are fundamentally different from those in macroscale geometries, primarily due to the increasing importance of surface tension. A systematic series of experiments on condensation flow regimes, pressure drop, and heat transfer was conducted using innovative visualization and measurement techniques for condensation of synthetic and natural refrigerants and their azeotropic and zeotropic mixtures through micro-channels with a wide range of diameters (0.1 < DH < 5 mm), shapes, and operating conditions. These experiments resulted in flow-regime-based heat transfer and pressure drop models with very good predictive capabilities for such micro-channel geometries.