Condensation Heat Transfer Performance Research Articles

Evaluation of dropwise condensation from a vertical condenser tube impregnated with semisolid lubricant

AbstractThe quest for augmenting dropwise condensation heat transfer performance has been the driving force behind the exploration of innovative techniques and approaches to fulfill the desired objective. Mostly, earlier research on dropwise condensation was devoted to study condensation from horizontal tubes and plates but, rarely, addressed dropwise condensation from vertical tubes. An interesting topic of current research in dropwise condensation is to explore the application of Slippery Liquid Infused Porous Surfaces (SLIPSs) to improve the condensation performance. The aforesaid facts are the motivation behind the topics of interest in the present study. In this study, we explore the applicability of semisolid lubricant‐impregnated porous surfaces in enhancing dropwise condensation performance of a vertical condenser tube. The experiments conducted over a wide range of subcooling (5°C ≤ ΔT ≤ 65°C) in saturated steam environment showed significant improvement in condensation heat transfer coefficient of a vertical condenser tube impregnated with semisolid lubricants when compared to bare vertical condenser tube. The highest enhancement is found to be 280% at a subcooling of 40°C. Furthermore, these surfaces proficiently sustain dropwise condensation over a period of 72 hours without compromising heat transfer performance. The devised fabrication method is simpler, more cost‐effective, and less time‐consuming than earlier techniques used for creating SLIPSs. Additionally, an approach based on numerical optimization by a stochastic global optimization technique, namely, Genetic Algorithm, is proposed to retrieve the coefficients and the exponent in the mathematical expression of overall resistance, that is being used to compute the tube‐side dropwise convection heat transfer coefficient.

Condensation heat transfer performance of R290 in a plate heat exchanger for electric vehicle heat pumps

In the continuous improvement of electric vehicles driving range, a heat pump system with low-global warming potential (GWP) refrigerant is a promising solution. In the present work, the condensation heat transfer coefficient (HTC) and frictional pressure drop (FPD) were experimentally evaluated in an offset strip fins embedded plate heat exchanger (PHE), using an eco-friendly, low-GWP refrigerant R290 as the working fluid. A Modified Wilson Plot Method was used to predict the heat transfer of the coolant side (water/ethylene glycol; 50/50 %). The main focus of the present study revolves around the R290-to-coolant condensation experiment, wherein we analyzed the influence of mass flux (G = 20–50 kg m−2 s−1), heat flux (q = 3–12 kW m−2), and saturation temperature (Tsat = 40–55 °C) on HTC and FPD within the PHE while variable mean vapor quality (xm). According to the results, all experimental conditions were fall in the forced-convection condensation regime. The results state that HTC increases with a rise in q, G, and xm, but decreases with a surge in Tsat. Conversely, FPD increases with a rise in G and xm, and decreases with a escalation in Tsat, while q has no significant impact. Correlations were developed to predict the Nusselt number and friction factor values of R290 with mean absolute errors of 10.6 % and 11.2 %, respectively. These correlations are compared to previous research findings, signifying their efficacy in predicting the heat transfer characteristics of R290 within PHEs.

Entropy Generation of R513A Condensation Flow Inside the Horizontal Microfin Tubes

Abstract It is widely acknowledged that the pressure drop increases with enhanced heat transfer in heat exchanger tubes, and entropy generation analysis serves as an effective method to comprehensively evaluate heat transfer and pressure drops. This paper conducts experimental research on in-tube condensation heat transfer using refrigerants R513A and R134a in six test tubes, comprising both smooth and microfin tubes with outer diameters of 9.52 mm and 12.7 mm, respectively. The microfin tubes are available in two types, with 60 and 65 fins, respectively, and a helix angle of 18 deg. The experimental conditions included mass fluxes of 50–250 kg/m2 s and condensation temperatures of 35 °C, 38 °C, and 40 °C. The findings indicate that replacing R134a with R513A is feasible. The 9.52 mm tube exhibits superior overall heat transfer performance compared to the 12.7 mm tube, and the 60-fin microfin tube outperforms the 65-fin tube in terms of heat transfer efficiency. This suggests that microfin tubes with smaller diameters and an optimal number of fins are more effective in enhancing condensation heat transfer performance.

Experimental research on condensation flow and heat transfer characteristics of immiscible binary mixed vapors on different wettability wall surfaces

In the process of biomass gasification syngas waste heat recovery, water and organic compounds in the syngas will condense at the same time, resulting in immiscible condensates adhering to the wall, which can reduce the heat exchanger heat transfer efficiency and working life. Changing the wettability of the wall surface can affect the condensate flow state on the wall surface, thus affecting the condensation heat transfer performance. Water and organic cyclohexane were used as immiscible working fluids in this paper, the condensation flow and heat transfer characteristics of the mixed vapors on hydrophilic, super-hydrophilic, and super-hydrophobic wall surfaces were experimentally investigated. The results showed that cyclohexane in immiscible condensates existed as a liquid film on hydrophilic, super-hydrophilic, and super-hydrophobic wall surfaces, while the water existed in droplets on the hydrophilic and super-hydrophobic wall surfaces, and there were two forms of channels and droplets on the super-hydrophilic wall surface. The condensation heat transfer coefficient on the super-hydrophobic wall surface was higher than that on the hydrophilic wall surface and the super-hydrophilic wall surface, with a maximum of 34% higher than hydrophilic and 45% higher than super-hydrophilic, which was related to the smaller droplet departure diameter on the super-hydrophobic surface. In addition, the super-hydrophilic wall surface had better heat transfer performance than the hydrophilic wall surface only when the immiscible condensate was a "film-channel" flow pattern. The results can provide guidance for enhancing the condensation heat transfer of the water and organic compounds binary mixed vapors.

Effect of Surface Renewal on the Drop Size Distribution in Dropwise Condensation within a Hybrid Surface

The description of liquid drop growth and drop distribution are two key models in evaluating the thermal performance of dropwise condensation (DWC) heat transfer. The drop size distribution describes the growth process of small drops by direct condensation and large drops by coalescence. The present work investigates the effect of surface renewal and coalescence intensities of DWC within a hybrid surface. Additionally, it examines the validity of the current empirical expression of the drop size distribution that is developed for DWC without considering surface renewal and coalescence intensities. The simulation work illustrates the drop growth process and surface renewal as drops depart and merge with neighboring film regions. The simulation results show that in hybrid DWC, the area fraction occupied by drops (f) lies between 0.28 to 0.296 for the ratio of maximum drop diameter to DWC region width (RD) from 0.125 to 1 and a total temperature drop (∆T) of 2, 4, and 8 ℃. Thus, the drop population is less sensitive to RD, and an average f of 0.288 is generalized. On the other hand, the surface renewal for DWC within the hybrid surface shows improvement for RD > 0.5 with the highest enhancement of 64 to 85% taking place at RD = 1, mainly due to the merging effect. In addition, results for drop size distribution profiles of DWC within the hybrid surface are characterized by a lower population of large drops and a higher population of small drops than full DWC. Additionally, the constant exponent (n) in the literature’s empirical expression is replaced by a polynomial series as a function of drop effective and maximum radii. The impact of surface renewal on coalescence intensity is presented in a relatively steeper slope on the logarithmic scale.

Modeling Dropwise Condensation on Hydrophobic Microgrooved Surface.

Dropwise condensation heat transfer on water-repellent surfaces is inherently linked to the mode of droplet departure from the surface. When a microgrooved hydrophobic surface is exposed to condensation, multiple spontaneous droplet removal pathways for surface renewal are manifested. We present numerical modeling of dropwise condensation on a microgrooved hydrophobic surface. Our model is an extension of the well-established one-dimensional modeling approach involving estimation of overall condensation heat transfer through the integration of individual droplet contributions. The model presented here accounts for all the surface renewal mechanisms observed on the microgrooved hydrophobic surface: growth and coalescence of condensate droplets within the microgroove and on the ridges, imbibition of the microgrooves with condensate, bulge formation, spontaneous dewetting of the microgrooves, and shedding of large drops through gravity. The modeling results show that the microgrooves trigger condensate shedding from the surface much earlier compared to a planar hydrophobic surface. As a result, the microgrooved hydrophobic surface maintains a much lower area coverage and attains a significantly higher condensation heat flux compared to a planar surface. The model also enables isolation of the relative contributions of the four mechanisms, wherein it is observed that the spontaneous dewetting transition of microgrooves dominates the other mechanisms in terms of the overall surface renewal rate. This is in contrast to the planar hydrophobic surface where droplet shedding under gravity is the main surface renewal mechanism. Finally, we also evaluate the effect of microgroove geometry on the condensation heat transfer performance. The model predicts that hydrophobic microgrooves with depth of ∼200 μm and narrow widths below ∼100 μm can yield enhanced thermal performance.

On the effect of static and dynamic contact angles on humid air condensation heat transfer

Surface modification is a widely utilized technique for enhancing condensation heat transfer. Two main properties of surfaces in manipulation of condensation heat transfer are the contact angle and contact angle hysteresis. This study focuses on the influence of contact angle (CA) and contact angle hysteresis (CAH) on humid air condensation. For this, hydrophilic and hydrophobic surfaces with varying CA and CAH values were fabricated and tested in a humidity-controlled climate chamber at different relative humidity levels. Three hydrophilic surfaces samples with a contact angle of approximately 70° and CAH values of 10°, 20°, and 42° were tested. Two hydrophobic surfaces with a contact angle of approximately 110° and CAH values of 21° and 39°were also prepared as well as a hydrophobic surface with a contact angle of 96° and a CAH of 43°. The role of CA and CAH in different stages of condensation cycle was investigated. Our findings show that while CA plays the main role in droplet nucleation, CAH has a significant impact on droplet coalescence and departure. Increasing CAH while keeping CA constant has a negative effect on condensation heat transfer in all wettability levels. However, the relationship between changes in CA while keeping CAH constant does not have the same trend in the condensation heat transfer performance for every case. Changing CAH for lower CAH values led to a greater impact on enhancing condensation heat transfer than higher values. Moreover, increasing CAH on hydrophobic surfaces had a more significant effect than on hydrophilic surfaces. Additionally, decreasing CAH had a more pronounced effect on improving condensation heat transfer than increasing CA. The findings emphasize on the importance of considering both the contact angle (CA) and contact angle hysteresis (CAH) in the surface design to have the optimum condensation heat transfer performance.

Condensation Heat Transfer Experiments of R410A and R32 in Horizontal Smooth and Enhanced Tubes

Abstract A horizontal smooth copper tube and two enhanced tubes (a dimpled tube and a diamond-shaped tube) with a length of 2 m and an outer diameter of 9.52 mm were studied in a two-phase condensation experiment using R410A and R32 as heat transfer working fluids. The condensation heat transfer performance of different heat transfer tubes at various mass fluxes ranging from 150 to 400 kg/(m2s) and different vapor qualities ranging from 0.8 to 0.2 was studied experimentally. For R410A, the heat transfer coefficients (HTCs) of the dimpled tube and the diamond-shaped tube were found to be 1.39–1.52 times and 1.30–1.35 times higher than those of the smooth tube, respectively. For R32, the HTCs were found to be 1.46–1.59 times higher for the dimpled tube and 1.26–1.35 times higher for the diamond-shaped tube than for the smooth tube. The diamond-shaped tube was found to be effective in directing the flow of the liquid phase and in thinning the liquid film thickness. The dimpled tube was found to promote droplet entrainment and disturbance. Both of those effects contributed to improving the HTCs in the two enhanced tubes investigated. The HTCs of R410A were found to be lower than those of R32 because of the smaller specific heat capacity, smaller latent heat of condensation, and relatively poorer thermal conductivity of R410A. Taking the combined effect of heat transfer and pressure drop into account, a performance enhancement factor (PEF) was computed and was found to range from 0.9 to 1.51. Based on the experimental data collected, a new empirical correlation equation has been proposed with a maximum error band of 10%. Flow pattern maps for the tested tubes have also been generated and are presented in the paper. Both of the two enhanced tubes were found to promote the development of annular flow. The HTCs were found to be highest when the flow pattern was of the annular flow type.

Experimental Study on Condensation Heat Transfer Performance of Hydrophilic/Hydrophobic Microstructured

In order to study the condensation and heat transfer characteristics of similar microstructure surfaces, two similar microstructure surfaces, cylindrical and circular, were fabricated by femtosecond laser technology on a 0.5 mm silicon wafer. The cylindrical surface is superhydrophobic when the contact angle is more than 150°, and the circular surface is hydrophilic when the contact angle is less than 90°. The difference in condensation heat transfer characteristics between superhydrophobic and hydrophilic microstructures was analyzed, and a visual condensation experimental platform was built. Experimental research showed that: At the same flow rate, the heat transfer coefficient of the superhydrophobic surface and the hydrophilic surface decreases significantly with the increase of the surface subcooling degree, but the heat transfer coefficient of the cylindrical surface is still much larger than that of the circular surface. In addition, the heat transfer performance of the hydrophobic microstructure surface is better than that of the hydrophilic surface at medium and high-speed cooling water flow rates. Although the surface microstructures are similar in shape, the heat transfer performance of cylindrical microstructures is much better than that of circular microstructures under the same conditions, and the heat flux of cylindrical microstructures is 2.2 times that of circular microstructures.

Wicking assisted condenser platform with patterned wettability for space application

Vapor condensation is extensively used in applications that demand the exchange of a substantial amount of heat energy or the vapor-liquid phase conversion. In conventional condensers, the condensate removal from a subcooled surface is caused by gravity force. This restricts the use of such condensers in space applications or horizontal orientations. The current study demonstrates proof-of-concept of a novel plate-type condenser platform for passively removing condensate from a horizontally oriented surface to the surrounding wicking reservoir without gravity. The condensing surface is engineered with patterned wettabilities, which enables the continuous migration of condensate from the inner region of the condenser surface to the side edges via surface energy gradient. The surrounding wicking reservoir facilitates the continuous absorption of condensate from the side edges. The condensation dynamics on different substrates with patterned wettabilities are investigated, and their condensation heat transfer performance is compared. The continuous migration of condensate drops from a superhydrophobic to a superhydrophilic area can rejuvenate the nucleation sites in the superhydrophobic area, resulting in increased heat transport. The proposed condenser design with engineered wettability can be used for temperature and humidity management applications in space.

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.

Hydrocarbon condensation flow and heat transfer in wave helical pipes: Effect of structure parameters

In this study, two types of wave-structured pipes for heat exchangers that are easy to process are proposed, and the condensation pressure drop and heat transfer performances for hydrocarbon refrigerant upward flow in wave helical pipes are investigated. A numerical method based on the inhomogeneous two-fluid model is introduced and validated by experimental data in literature. The effect of wave parameters (wave height and wave length) on heat transfer, pressure drop and flow patterns are studied. At lower vapor quality conditions, it is found that as wave height increases, frictional pressure drop and heat transfer coefficient exhibit a trend of initially increasing, followed by decreasing, and then increasing again. However, under higher vapor quality conditions, frictional pressure drop and heat transfer coefficient continue to increase. Besides, both the frictional pressure drop and heat transfer coefficient increase with the decrease of wave length. The averaged enhancement on frictional pressure drop for vertical wave pipes and horizontal wave pipes are respectively 1.279–5.148 and 1.238–5.808 times, while the average augmentation in heat transfer coefficient is 1.243–3.006 and 1.316–3.579 times, respectively. The comprehensive thermal performance of wave pipes was assessed using the performance evaluation criterion. The range of averaged performance evaluation criteria (under different vapor qualities) in vertical wave pipes and horizontal wave pipes are respectively 1.150–1.737 and 1.230–2.026. The horizontal wave pipe when the wave height is 5 mm and wave length is 5.24 mm shows the best overall thermal performance, with an averaged performance evaluation criterion of 2.026. The two types of wave-structured pipes proposed in this paper can increase the pipe length of the spiral-wound heat exchanger at the same volume, while also improving the performance of the spiral-wound heat exchanger. The outcomes of the study can offer some assistance in creating simple but effective spiral wound heat exchangers.

Graphene coating reduces the heat transfer performance of water vapor condensation on copper surfaces: A molecular simulation study

Water vapor condensation is a key process for many applications. Existing studies in water vapor heterogeneous condensation have used different methods to modify the hydrophobicity of the water vapor condensation surface. One way to modify the surface properties is to use graphene coating. On a macroscale, surface modification using graphene coating can improve water vapor condensation heat transfer of various substrates. However, on the molecular scale, its effect is poorly understood. This work investigated the effect of graphene coating on water vapor condensation using molecular dynamics simulations (MDS). We examined the water vapor condensation on bare copper surfaces considering the effects of initial temperature difference, water model, and surface size, parameters that have not been investigated by previous studies employing MDS for the same water-surface configuration. One water model was then used to simulate the condensation on a copper surface with and without graphene coating. We then investigated the effect of graphene defect, the energy and vibration of graphene atoms, and the interaction between graphene and copper layers. The surface size notably influenced the condensation rate and heat transfer performance. The condensation rate and heat transfer performance were significantly reduced when the copper surface was coated by graphene. The number of water molecules condensed was 1253 molecules/ns on the bare copper surface, compared to 587 molecules/ns on the graphene-coated copper surface. Moreover, the water molecules condensed on the graphene-coated copper surface tended to return to the bulk vapor phase. Other important results are also provided. This study gives an insight into the water vapor condensation on graphene-coated copper surfaces, useful to pursuit the design and optimization of graphene-coated copper surfaces for applications that need efficient water vapor condensation, such as for industrial applications, like thermal, chemical, and nuclear.

Numerical study of condensation heat transfer performance and liquid film distribution characteristics in small-scale helium liquefiers

At present, the liquefaction rate of small-scale helium liquefiers using 4 K cryocoolers is generally low. Helium condensation, the most critical heat transfer process in the system, is seldomly studied, and the design of condenser lacks accurate theoretical guidance. In this study, a CFD model for the film condensation of helium on the surfaces of condensers in small-scale helium liquefiers was established and verified. Compared with cryogenic fluids such as nitrogen, in the condensation of helium, the temperature difference and thermal resistance between the gas-liquid interface and saturated vapor cannot be ignored under small condensation temperature differences. On different surfaces of a cylindrical condenser, the thickness of liquid film on the vertical surface is lower than those on the horizontal surfaces, and its heat transfer coefficient is 1–3 times than those of horizontal surfaces. When the cooling capacity increases from 1 W to 2 W at 4.21 K, with the increase of the condensation temperature difference and thermal resistance brought by liquid film, the average heat transfer coefficient of condenser decreases from 530 W/(m2 K) to 446 W/(m2 K), the condensation efficiency decreases by 11.7 %. This study aims to provide theoretical guidance for the optimal design of the helium condensers in small-scale helium liquefiers.

Experimental Study on the Condensation Heat Transfer on a Wettability-Interval Grooved Surface

To provide further insight into humid air condensation on hybrid surfaces, an experiment was conducted to visually investigate the condensation process on wettability-interval grooved surfaces, which had hydrophobic ridges and hydrophilic grooves. The droplet dynamic behavior and heat transfer performance of condensation on a wettability-interval grooved surface were explored and compared with four other functional surfaces, including the plain hydrophilic surface, plain hydrophobic surface, hydrophilic grooved surface, and hydrophobic grooved surface. The presence of hydrophobic ridges perpendicular to the groove direction and hydrophilic grooves allowed for the exclusion and easy spreading of droplets, respectively. Compared with the other four functional surfaces, the coupling phenomena during condensation, i.e., the spontaneous suction and directional drainage via hydrophilic grooves, were only found on the wettability-interval grooved surface. These could not only remove condensate quickly but also suppress the formation of the flooded liquid film, which was beneficial to the enhancement of heat transfer performance. It was proven by the experimental results that at subcooling 12 K, the condensation heat flux on the wettability-interval grooved surface reached 1280 W/m2, which was 1.25 times that of the plain hydrophobic surface (1030 W/m2), and 15% higher than that of the hydrophobic grooved surface (1110 W/m2). This indicated that the wettability-interval microgrooves could effectively enhance humid air condensation heat transfer performance.

A Multiscale Strategy for Constructing Finned Microporous Superhydrophobic Surface for Highly‐Efficient Condensation Heat Transfer Enhancement Enabled by Lifespan Regulation of Droplets

AbstractSpontaneous droplet jumping on micro‐/nano‐structured superhydrophobic surfaces has been exploited as an efficient means for enhancing steam condensation heat transfer. However, the good performance of such surfaces quickly decays with raising the degree of subcooling, due to the mismatch between the characteristic length scales and droplet sizes when they grow up. Herein, a novel strategy for multiscale droplet regulation is proposed by combining sub‐millimeter fin structure with a hierarchical microporous superhydrophobic surface. A superior condensation heat transfer performance is attained on such hierarchical superhydrophobic finned tube (F‐SHB), in comparison to the baseline case of superhydrophobic non‐finned (SHB) tube under well‐controlled test conditions. Although the droplet jumping is not as vigorous as that on the SHB tube, the finned geometry of the F‐SHB tube leads to a condensation heat transfer enhancement even under high degrees of subcooling up to 36 K, because of the accelerated departure of large droplets by imposing Laplace force gradient in the presence of V‐shaped sub‐millimeter fins. This multiscale enhancement strategy is shown to enable a cascading regulation over the entire lifespan of condensate droplets. The fabrication of F‐SHB tubes is facile and easy to be scaled up, showing great potential in practical steam condensation applications.

Laser Textured Superhydrophobic SiC Ceramic Surface and the Performance of Condensation Heat Transfer

Chemical modification is usually utilized for preparing superhydrophobic SiC surfaces, which has the problems of long processing time, high environmental contamination risk, and high cost. To enhance the condensation heat transfer efficiency of SiC, the superhydrophobic SiC surface was fabricated through laser texturing and heat treatment. In this study, the SiC surface was processed by laser texturing with a nanosecond laser, followed by heat treatment. Surface microstructures and compositions were investigated with SEM and XPS, and the heat transfer coefficient of the superhydrophobic SiC surface was tested. The results indicated that the laser-textured SiC surface had a super hydrophilic contact angle of 0°; after heat treatment, SiC ceramic became superhydrophobic (surface contact angle reaches 164°) because organic contamination on the original SiC surface could be cleaned by using laser texturing, which caused a chemical reaction and the formation of SiO2 on the surface. Moreover, the distribution of relatively low-energy SiOX was formed after heat treatment; then, SiC ceramic became superhydrophobic. Due to the formation of nanoscale sheet-like protrusion structures by heat treatment, the SiC superhydrophobic surface exhibited typical dropwise condensation, and the condensation heat transfer coefficient reached 331.8 W/(m2·K), which was 2.3 times higher than that of the original surface.

Experimental study on condensation heat transfer characteristics in C-shaped tube under rolling conditions

C-shaped tube heat exchanger has the advantages of good performance in heat transfer, space-saving, and reliability, and thus it has great potential to be used as passive residual heat removal heat exchanger in offshore nuclear power plants. In the present study, the condensation heat transfer characteristics in a C-shaped tube, which is the basic unit of C-shaped tube heat exchanger, were investigated experimentally, and the experiment was carried out under steady state and rolling conditions respectively. The effect of heat flux and rolling conditions on the condensation heat transfer characteristics were studied. The heat flux varies from 73.3 kW/m2 to 446.7 kW/m2; the rolling amplitude varies from 5° to 15°; and the rolling period ranges from 5 s to 20 s. The experimental results show that the condensation in the C-shaped tube is gravity-dominated. Based on the experiment data, a new correlation used to calculate the heat transfer coefficient of a C-shaped tube under steady state has been obtained. The rolling motion has significant effect on the condensation heat transfer of the C-shaped tube. With the increase of rolling angle from 0° to 15°, the condensation heat transfer performance of the C-shaped tube gradually increases and the increase rate of the condensation heat transfer coefficient is up to 18%. On the other hand, with the increase of rolling period from 5 s to 20 s, the condensation heat transfer performance of the C-shaped tube decreases. Further investigation revealed that, as the heat flux increases, the effect of rolling motion on the condensation heat transfer decreases.

Experimental study on the boiling/condensation heat transfer performance of a finned tube with a hydrophilic/hydrophobic surface

Currently, a increasing number of exchange tubes are requied for both boiling heat transfer and condensation heat transfer. Thus, it is necessary to enhance, both the boiling heat transfer performance and condensation heat transfer performance of the tube is necessary. However, few studies have been conducted in this field. A stepped lattice finned tube is suggested to enhance both boiling heat transfer and condensation heat transfer. There is room for the heat transfer enhancment of efficiency considering that wettability can also influence the heat transfer. Hence, in this study, the heat transfer performances of tubes with hybrid Hydrophilic-hydrophobic surfaces and functional fins are experimentally investigated. The condensation heat transfer coeffient and boiling heat transfer coefficient of the tube with an inner hydrophobic cavity wall and outer hydrophilic fin are nearly 3 times and 1.8 times those of the smooth tube, respectively. In boiling heat transfer, the inner hydrophobic surface can lower the bubble nucleation barrier and the outer hydrophilic surface can prevent the heat transfer surface from being covered by a vapour film. In condensation heat transfer, the outer hydrophilic surface can lower the droplet nucleation barrier and the inner hydrophobic surface contributes to dropwise condensation.

Effect of meniscus curvature on phase-change performance during capillary-enhanced filmwise condensation in porous media

Sustainably enhancing condensation heat transfer performance is a major challenge in thermal management and energy systems, since typical condensation enhancement methods (i.e., dropwise condensation with low surface energy coatings) have limited lifetime/durability, restricted compatibility with working fluids, and sustainability concerns due to the coating composition (e.g., fluorinated compounds). The robust and scalable capillary-enhanced filmwise condensation mode presented in this work demonstrates high heat transfer coefficients for water and low surface tension liquids condensing in a porous wick. Thin porous wicks offer the highest enhancements in heat transfer, however such thin porous wicks have thickness-dependent permeability, and the effective liquid thickness of the wick depends on the shape of the liquid-vapor interface. In this study, we leverage a spatially-discretized porous media model to characterize the effect of the wick thickness on condensation heat transfer performance. The model uses a spatially-varying permeability that depends on the local liquid-vapor interface shape/curvature and the resulting effective wick thickness. We apply this model to investigate the correlation between the heat transfer enhancement and various geometric factors, which enables the design of optimal porous structures for relevant phase-change application. We also predict favorable enhancement in condensation performance with a few common hydrocarbon and fluorocarbon fluid refrigerants. This study provides fundamental insight into the effects of the shape of the liquid-vapor interface on the phase-change performance in the capillary-enhanced filmwise condensation mode.