Condensation Heat Transfer Coefficient Research Articles

CONDENSATION HEAT TRANSFER IN SMOOTH AND THREE-DIMENSIONAL DIMPLED TUBES OF VARIOUS MATERIALS

The heat transfer and pressure drop of R410A and R32 within a smooth and an enhanced dimpled tube were measured for mass fluxes from 100 to 400 kg m<sup>-2</sup> s<sup>-1</sup>, average vapor qualities between 0.2 and 0.8, and saturation temperatures between 35°C and 45°C. The test section was 2 meters in length, and the outer and inner diameters of the tube were 9.52 and 8.32 mm, respectively. The inner surface of the enhanced tube was dimpled. Three dimpled tubes and three smooth tubes, differing by material (copper, aluminum, and stainless steel) were tested to examine the material effect. The measured condensation heat transfer coefficient (HTC) for the copper smooth tube was 1.10 to 1.16 times higher than that of the aluminum, and likewise, between 1.19 and 1.31 times higher than that of the stainless-steel tube. Similarly, the condensation HTC for the copper dimpled tube was 1.06 to 1.15 times higher than that of the aluminum dimpled tube, and 1.26 to 1.38 times higher than that of stainless-steel dimpled tube. In general, and the condensation HTC for R32 was greater than that for R410A, mainly due to the greater liquid thermal conductivity of R32. Flow patterns were observed for different vapor qualities and used to establish corresponding heat transfer mechanisms. Finally, a new correlation for dimpled tubes was proposed based a modified smooth tube correlation, which predicted the measurements within an error band of 20&#37.

PREDICTION OF NH3 CONDENSATION HEAT TRANSFER COEFFICIENT WITH ARTIFICIAL NEURAL NETWORK AND MULTIPLE REGRESSION METHODS

Soğutma sektöründe kullanılan halokarbon veya sentetik içerikli bileşiklerin çevreye olumsuz etkilerinden dolayı günümüzde kullanımları uluslararası protokollerle sınırlandırılmıştır. Bu sebeple, NH3 gibi düşük küresel ısınma ve ozon tüketme faktörlerine sahip organik bazlı soğutucu akışkanların kullanımı ön plana çıkmıştır. NH3, yüksek ısıl kapasitesi ve düşük viskozite gibi avantajlı termo-fiziksel özelliklerinden dolayı son yıllarda iklimlendirme endüstrisinde tercih edilmektedir. İklimlendirme sistemleri geliştirilirken, belli bağıntılarla sistemin ısı transfer katsayısı ve basınç düşüşü tahmin edilmelidir. Ancak NH3’ün diğer akışkanlardan farklı termo-fiziksel özelliklere sahip olması sebebiyle, literatürde NH3 için yoğuşma rejiminde ısı transfer katsayısını ve basınç düşüşünü yüksek doğruluk oranında veren bir model bulunmamaktadır. Bu çalışmada, yatay ve düz borularda yoğuşma rejiminde bulunan NH3 akışkanı ile yapılmış literatür çalışmalarından veriler alınarak çoklu regresyon ve yapay sinir ağları metotları ile ısı transferi katsayısı tahmini yapılmıştır. Sonuçlar literatürdeki bilgiler ışığında tartışılmış ve öneriler sunulmuştur.

Flow condensation heat transfer correlation development of R32-oil mixture inside a micro-fin tube

The air-conditioners employ R32 as refrigerant because it has low GWP. The heat transfer performance of R32 in the heat exchangers would be influenced by the lubricating oil cycled with R32. In order to obtain the quantitative effect of oil on the flow condensation heat transfer characteristics, flow condensation heat transfer correlation of R32-oil mixture is needed. In this study, an experimental rig for testing the flow condensation heat transfer coefficients of R32-oil mixture is established. The test conditions covering the actual working conditions of air-conditioners are designed, including the condensation temperature ranging from 40 to 50 °C, mass flux ranging from 100 to 400 kg m − 2 s − 1, vapor quality ranging from 0.1 to 0.9 and oil concentration ranging from 0 % to 5 %. The results show that the flow condensation heat transfer coefficients decrease with the increasing of oil concentration because of the high viscosity of the oil. A new condensation heat transfer correlation is developed based on the enhanced structure of the micro-fin tube and physical properties of the working fluid, and it agrees with 82 % of the experimental results within a deviation of ± 20 %.

Condensation of refrigerant R450A inside a mini-channel of rectangular cross section with an internal longitudinal groove

The increase in jeopardizing the natural life and environmental resources together with high rates of energy consumption are still in focus of international agencies and scientific communities. For this issue, global awareness has been oriented to uncover alternatives to harmful species and to magnify effectiveness of thermal systems. For the present study, the refrigerant R450A was investigated as a potential substitute to R134a during condensation inside two mini-channels which are rectangular cross section channel (SRC) and a modified channel (MRC). The latter was characterized by an internal longitudinal groove. Wall subcooling, vapor superheat, and mass velocity were considered while the condensing temperature was kept constant. Numerical CFD simulation was realized for the condensation inside the channel using ANSYS-CFX 15.0 with additional user defined expressions to describe condensate layer thickness inside the channel, which in turn was utilized for the condensation heat transfer coefficient (HTC). Predicted data was compared with experimental one in terms of HTC and flow patterns. The predicted data of HTC showed an average deviation by about 6.6% compared to experimental data for R134a.The refrigerant R450A resulted in lower condensation HTC than R134a. The MRC resulted in an average enhancement in the HTC by about 55.7% and a reduction in the pressure drop by about 10.4% compared to the SRC. The enhancement of the MRC in HTC was remarkable at high degree of superheat, low mass velocity, and low wall subcooling. Accordingly, the novel MRC represents a successful potential design for condensation of refrigerants in mini-channels.

Numerical evaluations on enhanced steam–air condensation outside S-type tubes under natural convection conditions

Steam-air condensation is extensively utilized in various industrial fields. In pursuit of better efficiency in applications, this study promoted a novel structure of heat transfer enhancing tube named S-type tube. It combines the superior heat transfer performance of short tubes and inclined tubes, demonstrating a high potential for enhanced condensate heat transfer. To evaluate the heat transfer property and obtain the optimized S-type tube structure, three-dimensional CFD methods were used, coupled with the liquid film model in STAR-CCM + software. The effcts of wall sub-cooling, pressure, air mass fraction and downward forced velocity, bend radius, horizontal section length on the steam–air condensation heat transfer characteristics outside S-type tubes were investigated. It was identified that S-type tubes can enhance the steam–air condensation heat transfer capability, and the enhanced heat transfer ability increases with the increase of pressure and the decrease of air mass fraction, and remains basically unchanged with changes in sub-cooling. The condensation heat transfer coefficients outside S-type tubes are 0.9 ∼ 1.71 times that of vertical tube range 0.2 MPa to 0.8 MPa. The main mechanisms affecting the condensation heat transfer are summarized: the scouring effect between tube segments to enhance heat transfer and the air layer pileup effect to inhibit heat transfer. The heat transfer enhancement capacity for the S-type tube is determined by the relative magnitude between the scouring effect and the pileup effect. Both effects increase as bend radius decreases; therefore, selecting an appropriate bend radius is crucial.

A new heat transfer model based on non-equilibrium theory for annular flow condensation-part Ⅱ: Ternary zeotropic mixtures

In the past decades, attention has been paid to the flowcondensation heat transfer characteristics of zeotropic mixtures, but most of the research has been carried out on binary mixtures, with very few studies performed on multicomponent mixtures that contain ternary and more components. The condensing process of multicomponent mixtures is significantly more complex since there are interactions between the components except for mixture effects, and these mechanisms are still unclear. Therefore, existing prediction methods either being computationally complicated or fail to accurately describe the condensation characteristics of multicomponent zeotropic mixtures. Focused on this issue, a new heat transfer correlation for ternary zeotropic mixtures in annular flow condensation was proposed based on our previously developed heat transfer model for binary zeotropic mixtures. The ternary mixture is regarded as a combination of two binary mixtures by analyzing the relationship of the condensation rate difference between components and the concentration gradient, and binary interaction coefficients are introduced to estimate the relative contribution of the two independent concentration gradients in the condensing vapor of ternary zeotropic mixtures. The newly developed correlation is simple and agrees well with the condensation heat transfer coefficients of ternary zeotropic mixtures, with an average relative deviation (MRD) of -1.6 %, an average absolute relative deviation (MARD) of 13.9 %, and 90.2 % of the data points falling within ±30 % deviation band for a total of 307 annular flow data.

Experimental investigation and correlation development of flow condensation with R123/MWCNTs nanorefrigerant inside a horizontal tube

The current experimental study is focused on examining the behaviour of heat transfer of pure R123 and R123/multi-walled carbon nanotubes nanofluid condensing in a smooth horizontal pipe at a range of different working conditions, like the mass velocity of about 200–500 kg/m2s, multi-walled carbon nanotubes volume fraction of approximately 0.01, 0.05, and 0.1 %, vapour quality of approximately 0.05–0.9, and saturation temperature of about 35.3 and 40.9 °C. The heat transfer coefficient of pure R123 and nanorefrigerant is compared in this investigation. According to the results, employing multi-walled carbon nanotubes improves the heat transfer coefficient for all selected volume fractions. It is observed that the heat transfer coefficient for both pure and nanorefrigerant falls as the saturation temperature rises from 35.3 to 40.9 °C. For a mass velocity of about 400 kg/m2s, 0.1 % fraction of multi-walled carbon nanotubes results in the highest heat transfer coefficient increase of 29 % compared to the base refrigerant. The mean absolute error analysis is evaluated to forecast the accuracy of the experiments by comparing the experimentally captured condensation heat transfer data with available correlations, which shows good agreement. A new correlation for flow condensation heat transfer coefficient is suggested, with an acceptable mean absolute percentage error of about 9.6 %.

A Comprehensive Study on the Effects of Surface Roughness on Power Consumption and Heat Transfer Performance in Novel In-Tube Condensation r123yf Refrigerant Systems

This study investigates the impact of surface roughness on the condensation Heat Transfer Coefficient (HTC) of R123yf refrigerant in a uniquely designed test rig with pre-condenser and post-condenser for quality regulation. In mining industries, the mining equipment consumes more amount to power for the Air Conditioning system. To reduce the power consumption of copper metal inside the surface morphology needs to be changed. Experiments were conducted in a 750 mm long, 8.4 mm internal diameter copper tube, with regard to mass flux varying from 150 kg.m-2.s-1 to 300 kg.m-2.s-1, refrigerant quality varying from 0.3 to 0.8, and saturated temperature from 35°C to 45°C. The impact of varying surface roughness (1.5 μm, 2.5μm, 3.4 μm, and 6.7 μm) on HTC was examined. The plane tube condensation experimental results agreed with M M Shah's heat transfer coefficient correlations, with a 20% absolute mean deviation. It was observed that increasing surface roughness enhanced the HTC. The optimal surface roughness was determined to be 3.4 μm, as it resulted in a significant increase in HTC (26.25%) with a moderate rise in power consumption (15.40%). Higher surface roughness led to a drastic increase in power consumption, making it less desirable for practical applications. Through a systematic analysis of the experimental data, this study identifies the critical trade-off between surface roughness and power consumption, providing guidance for the design of energy-efficient condensation heat transfer systems using R123yf refrigerant. By pinpointing an optimal surface roughness for copper tubes used in refrigeration, this study offers a pragmatic solution for enhancing the efficiency of air conditioning systems in mining equipment, a sector notorious for high energy expenditures. The findings indicate a direct application in mining operations where equipment cooling demands contribute significantly to energy consumption. Implementing the recommended surface modifications can lead to substantial energy savings, thereby reducing operational costs and improving the sustainability of mining activities. Furthermore, the adaptation of R123yf refrigerant systems with optimized surface roughness could serve as a model for energy conservation measures across various facets of the mining industry..

Heat transfer characteristics of R-1234yf, an eco-friendly alternative refrigerant in condenser for semiconductor chiller

This study aims to optimize a semiconductor chiller condenser that uses R-1234yf refrigerant, focusing on its flow pattern and heat transfer characteristics during condensation while considering the tube diameter. The heat transfer coefficient (HTC) decreases as the pressure of R-1234yf increases. This decline is attributed to changes in its physical properties, such as the density ratio between the liquid and gas phases and the thermal conductivity of the liquid phase, according to the change in saturation pressure. The HTC of R-1234yf increases with the mass flow rate (MFR) because the Reynolds number in the tube increases as MFR increases, enhancing heat transfer. Furthermore, the local condensation HTC (CHTC) of R-134a exceeds that of R-1234yf across the entire vapor quality range, primarily due to the higher liquid-phase density of R-1234yf. The correlations by Dobson–Chato [21] and Bashar et al. [10] were found to be suitable for predicting CHTC values for R-1234yf. The predicted values closely align with the experimental values, with an error range of less than ± 10 %, thus indicating that the correlations by Dobson–Chato and Bashar et al. can be directly applied to the design of condensers that use R-1234yf refrigerant.

Investigating impacts of condensation on thermal performance in greenhouse glazing and operational energy use for sustainable agriculture

Condensation frequently manifests in environmentally controlled agricultural structures (such as greenhouses) due to the utilisation of specific covering materials and the often meticulous management of high relative humidity to facilitate optimal plant growth. The formation of condensation can significantly influence the thermal and optical characteristics of the covering materials, which also affects the covering insulation and the growth of plants. This study was conducted to investigate the impacts of surface condensation on the thermal performance of various greenhouse covering types. These options feature low-emissivity (Low-E) coatings, infrared reflective (IR) additives, UV protective films, and anti-condensation additives. The physical properties, specifically emissivity and wettability, were examined to understand their roles in modifying the thermal transfer dynamics of greenhouse coverings. A scale physical model was made to test the thermal performance of the coverings with and without condensation. The results indicated the effect of condensation on the overall heat transfer coefficient (U-factor). Especially when the surface exhibited characteristics of low emissivity, it resulted in a notable near-tripling of the U-factor. These widely accepted and advocated strategies of Low-E and IR reflective products may inadvertently lead to enhanced operational energy consumption when considering the influence of condensation. Notably, this research has also devised a regression function that establishes a correlation between the contact angle and the condensation heat transfer coefficient. This tool facilitates swift analysis and estimation of the effects of condensation on the U-factor. In the end, the potential and future work on condensation effects is also discussed.

Experimental investigation on condensation heat transfer coefficient and frictional pressure drop of low GWP refrigerant R1234yf on a wavy fin surface

Refrigerant R1234yf is an ideal substitute to replace R134a due to its lower value of global warming potential (GWP). The thermophysical properties of R1234yf are similar with R134a and thus it is considered as a potential replacement for R134a in thermal fluid systems. Further, it was noticed that there is a lack of analysis on the condensation of R1234yf in a brazed plate fin compact heat exchanger (CHE) employing wavy fins. In the present work, condensation heat transfer coefficient and frictional pressure drop of refrigerant R1234yf are investigated in a brazed plate fin compact heat exchanger with wavy fin surfaces. The test condenser with wavy fin surface was selected to examine the condensation heat transfer coefficient and frictional pressure drop using refrigerants R134a and R1234yf for varying mass flux and saturation temperatures. The present study also discusses the dependency of specific kinetic energy of refrigerant on frictional pressure drop. Further, using the experimental data of wavy fin surfaces, suitable correlations for the condensation heat transfer coefficient and frictional pressure drop of R1234yf were proposed. The present paper also reports the comparison made between refrigerants R134a and R1234yf. Comparison between R1234yf and R134a shows that condensation heat transfer coefficient and frictional pressure drop of R1234yf is 9–16 % and 10–17 %, respectively, lower than of R134a in the mass flux range 15–44 kg/m2s.

Experimental study and general correlation for condensation heat transfer coefficient inside microfin tubes

This study investigated the condensation heat transfer characteristics of the pure refrigerants R1234yf and R32 in a microfin tube with a small diameter of 3.5 mm outside diameter and 3.18 mm inner mean diameter. Experiments were carried out at mass velocities between 50 and 200 kg m-2 s-1, saturation temperatures of 20 and 30 °C, and vapor quality between 0 and 1. The effects of vapor quality, mass velocity, saturation temperature, and refrigerant properties on condensation heat transfer characteristics were investigated. A new correlation is also developed to predict condensation heat transfer coefficients that can be applied to horizontal microfin tubes with inner diameters between 2.5 and 8.9 mm. The new correlation is validated with the database containing 1240 experimental data points from 10 independent research groups. The comparison results show that the three existing correlations overestimate the heat transfer coefficient data with mean deviation greater than 30.0 %. Meanwhile, the new correlation predicted the present experimental data and other researchers' data with mean deviation of 19.8 %.

Condensation enhancement experiment investigation of pure steam and steam-air mixture over the chrome-plated tube with different plating thickness

The condensation enhancement research of pure steam and steam with non-condensable gas is of great significance for reactor safety. In this study, to evaluate the influence of chrome-plated tubes on condensation enhancement, pure steam and steam with air condensation heat transfer experiments are conducted, and the plating thicknesses of 1 μm (tube #1) and 10 μm (tube #2) are considered. The results show that dropwise condensation (DWC) occurred in the part of tube #1 surface, while tube #2 surface is filmwise condensation (FWC). For the pure steam condition, the steam condensation heat transfer coefficient (HTC) of tube #2 is higher than that of tube #1, and both are greater than the theoretical analysis of Nusselt. Thus, the condensation enhancement effect of the chrome-plated tubes is remarkable. For the steam with air condition, a new condensation HTC expression is proposed, which is related to gas pressure, wall subcooling, and air mass fraction. Compared with the widely used experimental correlations and the experimental data of polishing tubes, it is found that the condensation enhancement performance is not significant. This is because the dominant thermal resistance affecting heat transfer is the air layer at the gas-liquid interface. Thus, enhancing the disturbance of the gas layer is the key to the condensation enhancement of steam with non-condensable gas.

Flow condensation inside a multiport mini channel and a rectangular mini channel with pin fin array

In this paper, an experimental study of flow condensation was carried out of R134a in a multiport mini channel and a mini channel with pin fin array. The former comprised 20 parallel rectangular channels with an equivalent diameter of 0.64 mm. The latter is a narrow rectangular mini channel containing 10 rows of diamond pin fins with a staggered configuration, and height and longitudinal/transverse pitch of 0.5 mm and 2 mm respectively. To acquire the local value of heat flux and heat transfer coefficient, air jet impingement cooling devices were adopted to condense the vapor of refrigerants. The effects of vapor quality, mass flux, heat flux, and saturation pressure on flow condensation heat transfer coefficient were investigated with the operating conditions: vapor quality from 1 to 0, mass flux from 160 to 450 kg/(m2s), heat flux from 10.0 to 39.8 kW/m2, and saturation pressure from 600 to 1500 kPa. The experimental results indicated that the pin fin array significantly improves the condensation heat transfer coefficient up to nearly 186 % compared to the multiport mini channel in the high vapor quality region. For both channels, the local heat transfer coefficient increases with an increase in vapor quality, mass flux, and heat flux whereas decreases with increase in saturation pressure. The influence of heat flux and mass flux on heat transfer coefficient was more pronounced in the high vapor quality region than in the low vapor quality region. A sensitivity analysis was performed at different vapor quality levels. The most influential parameters in the smooth channel and the pin fin array are saturation pressure and mass flux, respectively. The relative contribution of heat flux is more obvious in the high vapor quality region. Some of the existing correlations have good prediction performance for the smooth channel experimental data in this paper, and the corresponding MAD is within 20 %. However, their deviations are much greater for the applications of pin fin array.

Condensation Performance of Symmetrical and Parallel Capsule Plate Heat Exchangers

Condensation characteristics of symmetrical and parallel capsule plate heat exchangers were experimentally investigated in this article. Plate heat exchangers with parallel capsule corrugation, symmetrical capsule corrugation, and chevron-type corrugation are manufactured as the test sections, each containing 41 plates. Pure steam and water are the hot and cold fluids, respectively. The experiments covered a range of equivalent Reynolds number from 9,000 to 25,000, mass flux of condensing fluid from to 18 to 49 kg/(m2·s), liquid Prandtl number from 1.37 to 1.71, average vapor quality from 0.5 to 0.7, and condensation pressure from 110 to 240 kPa. The correlations of condensation heat transfer coefficient and friction factor of the symmetrical and parallel capsule plate heat exchangers are developed. Compared with chevron-type plate heat exchangers, capsule plate heat exchangers show low friction pressure drop, and 33% higher energy efficiency index for condensation flow and heat transfer. Specifically, the energy efficiency index of the symmetrical capsule plate heat exchanger is 8.6% higher than that of the parallel capsule plate heat exchanger, indicating the superiority of counter-rotating longitudinal vortices over co-rotating longitudinal vortices in condensation, similar to what is observed in single-phase flow and heat transfer.

Analysis of influence of inlet vapor quality on heat transfer and flow pattern in mini-channels during flow condensation process

Micro/mini-channel heat sinks are extensively utilized in heat dissipation systems that involve high heat flux components. These heat sinks are preferred due to their compact size and excellent heat transfer efficiency. The selection of a refrigerant condensation system for the heat dissipation system under vapor conditions is crucial. In order to investigate the heat transfer characteristics of flow condensation in a mini-channel, a visual experiment system was used with a hydraulic diameter of 2 mm and a volume flowrate of 30 mL/min, 60 mL/min, 90 mL/min, 120 mL/min respectively. The study focused on the effect of inlet vapor quality on the flow pattern and heat transfer characteristics of flow condensation in the mini-channel. The results indicate that the flow pattern in flow condensation process can be classified into annular flow, transition flow, slug flow and bubble flow in sequence along the flow direction. As the inlet vapor quality (xin) increases, the dominance of annular flow gradually increases, making it difficult to capture the bubble flow with high volume flowrate and high inlet vapor quality. This phenomenon is observed under experimental conditions when: 1) xin is equal to or greater than 0.75 at a volume flowrate of 60 mL/min, 2) xin is equal to or greater than 0.55 at a volume flowrate of 90 mL/min, and 3) xin is equal to or greater than 0.25 at a volume flowrate of 120 mL/min. The increase in inlet vapor quality is beneficial to the improvement of condensation heat transfer coefficient. In this experiment, it was observed that the local heat transfer coefficient was decreased by up to 83.7 % when comparing the lowest vapor quality to the highest at the same volume flowrate, emphasizing the significance of inlet vapor quality in the flow condensation during the heat transfer process.

Visualization study of co-existing boiling and condensation heat transfer in a confined flat thermosyphon

Flat thermosyphon (FTS) has gained wide attention in solving heat dissipation problems of data centers. The miniaturization of densely packed electronics has led to a demand for compact heat sinks. In this paper, a confined FTS consisting of an evaporator, condenser and customized quartz-glass chamber was developed. Bubble behaviors were investigated by the visualization approach under vacuum conditions. Effects of the space height, nominal heating power, and liquid filling ratio on the thermal characteristics were involved. Experimental results found that the bubble behaviors with 10 mm height are different from others. For 10 mm height, bubble contacts the condensation surface and then bounces back to the evaporator. The quantitative analysis shows that the boiling heat transfer coefficient with 10 mm height is 65.5 % of that with 25 mm height, and a condensation heat transfer coefficient with 10 mm height is 58.9 % of that with 20 mm height. Increasing heating power deteriorates the condensation heat transfer via forming a bubble film on the condensation surface. A small filling ratio (30 %) with 10 mm height is beneficial for the phase change heat transfer. It is hoped that these results can offer guidance for designing effective cooling devices.

The Effect of Heat Exchanger Tube Structure on the Condensation Heat Transfer of R1234ze(E)/R152a Inside Smooth Tubes

Abstract In this paper, the flow condensation heat transfer characteristics of the environmentally friendly nearly-azeotropic refrigerant R1234ze(E)/R152a (mass ratio of 40:60) in smooth tubes with varying structures were numerically investigated. Under the operating conditions of mass flux of 400 kg/m2/s, heat flux of 12 kW/m2, and saturation temperature of 308.15 K, this study investigated the influence of circular tube inner diameter, elliptical tube aspect ratio, and installation orientation on condensation heat transfer, while the influence on pressure drop has not been taken into account in the present study. The results indicate that the condensation heat transfer coefficient in the tube increases as the inner diameter of the circular tube decreases. The condensation heat transfer coefficient increases by 1.086 times when the circular tube inner diameter is reduced from 10.7 mm to 5 mm. Under identical operating conditions, the condensation heat transfer coefficient of the mixed refrigerant in elliptical tubes increases with an increase in the aspect ratio. The average condensation heat transfer coefficient increases by 18.21% as the aspect ratio of the elliptical tube increases from 1 to 2. Compared to a vertical elliptical tube, a horizontal elliptical tube is more favorable for condensation heat transfer within the tube.

Combination effect of sectioned microcolumn array and wettability gradient on condensation: CFD numerical approach

Droplets nucleation renewable rate and liquid film thickness are the critical factors influencing condensation heat transfer performances. In this paper, four microcolumn array structures were designed based on the premise by increasing droplets removal rate and avoiding the flooding problem. The sectioned cylindrical microcolumn array and the surface wettability gradients were coupled to enhance condensation. The liquid-vapor interface evolution of the droplets, heat flux, and condensation heat transfer coefficient were numerically investigated and compared. Additionally, the subcooling effect was studied for the sectioned cylindrical microcolumn arrays with two-stage wettability gradients. The sectioned top demonstrated fixed droplets sliding direction and the area-average heat flux on the sectioned cylindrical microcolumn array could reach a maximum of 1228.44 kW·m−2, which was 54.6% greater than the cylindrical microcolumn array. The effectiveness of the proposed structures in removing droplets and rapidly draining water was validated. The sectioned cylindrical microcolumn array with two-stage wettability gradients demonstrated the best performances, with the area-average heat flux improved by 70.6%, 10.3%, and 6.9% compared to the other three structures. As the surface subcooling increased, the condensation enhancement of the sectioned cylindrical microcolumn array with two-stage wettability gradients became more obvious.

A robust and simple correlation for internal flow condensation

Accurate prediction of the heat transfer coefficient for internal flow condensation is essential for designing and optimizing condensers. Due to advances in manufacturing techniques and new environmental and safety regulations in recent years, applications arose with extensive use of microchannel condensers and low GWP refrigerants. Consequently, a new range of condensation flow conditions is emerging, which requires the evaluation of the accuracy of available correlations and the proposition of more accurate ones. In this work, the most extensive and updated experimental database for heat transfer coefficient (HTC) on internal flow condensation was gathered from the literature, which includes 69 working fluids, 12,017 data points, channel diameters ranging from 0.067 to 20.8 mm, and horizontal and vertical flow directions for smooth tubes. Correlations proposed from 1958 to 2023 were evaluated and compared to this database to verify their prediction error. A new simple HTC correlation was developed, showing that further improvement of available correlations is still possible. The new correlation predicted the database with a mean absolute error of 23.3 %, a 5.3 % relative improvement compared to the second-best-ranked and more complex correlation. This analysis showed that a few well-known dimensionless groups can accurately model the heat transfer process during condensation in a broad range of operational conditions.