Filmwise Condensation Research Articles

Nanoarray-Embedded Hierarchical Surfaces for Highly Durable Dropwise Condensation.

Durable dropwise condensation of saturated vapor is of significance for heat transfer and energy saving in extensive industrial applications. While numerous superhydrophobic surfaces can promote steam condensation, maintaining discrete microdroplets on surfaces without the formation of a flooded filmwise condensation at high subcooling remains challenging. Here, we report the development of carbon nanotube array-embedded hierarchical composite surfaces that enable ultra-durable dropwise condensation under a wide range of subcooling (ΔTsub = 8 K–38 K), which outperforms existing nanowire surfaces. This performance stems from the combined strategies of the hydrophobic nanostructures that allow efficient surface renewal and the patterned hydrophilic micro frames that protect the nanostructures and also accelerate droplet nucleation. The synergistic effects of the composite design ensure sustained Cassie wetting mode and capillarity-governed droplet mobility (Bond number < 0.055) as well as the large specific volume of condensed droplets, which contributes to the enhanced condensation heat transfer. Our design provides a feasible alternative for efficiently transferring heat in a vapor environment with relatively high temperatures through the tunable multiscale morphology.

Wick-free paradigm for high-performance vapor-chamber heat spreaders

In this study, a novel vapor chamber featuring wickless wettability-patterned components is designed, fabricated, and tested. This is the first phase-change heat transfer apparatus comprised entirely of wick-free elements that transport liquid by way of wetting forces generated by precise surface-wettability patterning. Our approach takes advantage of the phase-changing properties of water within a closed loop composed of two opposing, wickless wettability-patterned plates, one acting as an evaporator, and the other designed as a condenser. The plates feature different geometric wettability patterns, each especially designed/engineered to transform a flat rectangular copper plate to a functional component of a low-profile vapor chamber. The wettability pattern of the condenser facilitates spatially controlled dropwise and filmwise condensation and offers an optimal way to move the condensate through specifically built wedge tracks utilizing capillary forces. The wettability pattern laid on the evaporator enables the collection/accumulation and transport of the returning condensate fluid to the heat-input portion where evaporation must be strongest for maximum heat removal. Using this system, we effectively cooled a 0.9 cm2 heated area by spreading the heating power (∼200 W) over a 30× larger condenser area. The best performance metrics were thermal resistance 0.18 K/W at 10 W, in-plane thermal conductivity 1.17 kW/(m·K) at 22 W, and maximum heat flux ∼ 2.2 MW/m2. The present wick-free vapor-chamber heat-spreader apparatus is the first of its type and the simplicity of its design along with its moderate dimensions (making it light and thin) are important advantages that make this system well-suited for a wide range of thermal management applications.

Droplet sweeping to enhance heat transfer during dropwise condensation

It is well known that dropwise condensation (DWC) can achieve heat transfer coefficients (HTCs) up to 5-8 times higher as compared to filmwise condensation (FWC). The interaction between the condensing fluid and the surface defines the condensation mode. Coatings that present low surface energy and high droplet mobility are a solution to promote DWC instead of FWC on metallic substrates. In the present paper, the effect of vapor velocity during DWC has been investigated over a sol-gel coated aluminum surface and a graphene oxide coated copper surface. Heat transfer coefficients and droplets departing radii have been measured at constant saturation temperature and heat flux, with average vapor velocity ranging between 3 m s−1 and 11 m s−1. A recent method developed by the present authors to account for the effect of vapor velocity on the droplet departing radius is here presented. The results of the proposed method, when coupled with the Miljkovic et al. [1] heat transfer model, are compared against experimental data.

Condensation effects on energy performance of building window systems

Low-e products have been accepted widely as an energy-efficient solution or energy retrofitting strategy for glazing systems, including single-pane windows, which may reduce the emission of radiant infrared energy and improve the insulating properties of overall windows. However, the addition of a low-e layer may increase the condensation risk, and in turn, significantly diminish the window’s thermal performance. To better understand the condensation effects of low-e windows on overall building energy use, we extended the classic Nusselt filmwise condensation model to establish and validate a simplified double-film-based analytical model, and then characterized the condensation-dependent thermal dynamics of low-e coated single-pane glazing. A series of whole building energy use comparisons between simulations ignoring and considering the condensation effect were conducted. The results indicate that if the condensation effect is taken into account, the low-e single-pane glazing may lead to adverse effects on the energy-saving purpose in certain climates relative to the original single-pane glazing. More careful and comprehensive energy analyses are needed to use low-e products for window energy retrofits.

CONDENSATION PHASE CHANGE BEHAVIORS ON A ROUGH SURFACE CHARACTERIZED BY FRACTAL CANTOR

Understanding the fundamental mechanisms of vapor condensation on rough surfaces is crucial to a wide range of industrial applications. A hybrid thermal lattice Boltzmann model of the condensation heat transfer process on downward-facing rough surfaces characterized by the Cantor fractal is developed and numerically analyzed to investigate the condensation phase change behaviors on rough hydrophobic and hydrophilic surfaces. The dynamic behaviors of vapor condensation, including the evolutions of vapor–liquid interface, heat flux, condensate mass, and temperature distribution, on the hydrophilic and hydrophobic rough surfaces are presented and compared with corresponding smooth surfaces. The results indicate that the rough surface preferred a filmwise condensation under hydrophilic conditions but a hybrid dropwise–filmwise condensation under hydrophobic conditions. On the rough hydrophobic surface, the liquid film can rapidly adsorb droplets, maintaining a high-efficiency dropwise condensation. The absorption of droplets accelerates the liquid film growth and detachment process on the rough hydrophobic surface, which reduces the time-averaged thermal resistance of the filmwise region. These two behaviors together enhance condensation heat transfer on the downward-facing rough hydrophobic surface. Besides, stable dropwise condensation could also be formed on smooth hydrophilic surfaces and has better heat transfer performance than corresponding hydrophobic surfaces under the same heat transfer condition.

Investigation of the effect of maximum droplet radius on condensation heat transfer on hydrophobic-hydrophilic hybrid surfaces with the effect of Marangoni convection

Condensation occurs in two modes, including filmwise and dropwise condensation. On a hybrid surface, the maximum radius of the condensed droplet on the surface in the dropwise region significantly affects the rate of heat transfer. In this paper, the effect of maximum droplet radius on heat transfer on hybrid surfaces is investigated. In this paper, the effect of Marangoni convection-current due to the temperature gradient- is considered as a resistance in the heat transfer process. The results indicate that an optimum value of the maximum droplet radius exists to enhance the condensation heat transfer of steam in the hybrid surface that is dependent on the surface subcooling degree, filmwise region width and the contact angle. As the surface subcooling degree increases from 1.0 K to 10 K, the optimum maximum radius of the droplet in the dropwise region increases from about 0.25 mm to 0.3 mm. Also, the optimum maximum radius of the droplet increases from about 0.25 mm to 0.35 mm when the filmwise region width is increased from 0.5 mm to 1.0 mm. Finally, the results of applying the effect of Marangoni convection indicate a 30% increase in the heat transfer coefficient in the case of applying Marangoni convection.

High-Performance Planar Thermal Diode With Wickless Components

Abstract The characterization “thermal diode” (TD) has been used to portray systems that spread heat very efficiently in a specific direction but obstruct it from flowing in the opposite direction. In this study, a planar vapor chamber (VC) with a wickless, wettability-patterned side and an opposing wick-lined side is fabricated and tested as a thermal diode. When the chamber operates in the forward mode, heat is naturally driven away from the heat source; in the reverse mode, the system blocks heat flow, thus acting as a thermal diode. The low-profile assembly takes advantage of the phase-changing properties of water inside a sealed chamber. The wettability-patterned plate—when on the cooled side, e.g., forward operation mode—enables spatially controlled dropwise condensation (high heat transfer rate) and filmwise condensation (high drainage rate), thus facilitating an efficient transport mechanism of the condensed medium on superhydrophilic wedge tracks by way of Laplace pressure-driven capillary forces. The same chamber acts as a thermal blocker when the wick-covered plate is on the cooled side (reverse operating mode), trapping the condensate in the wick pores and blocking heat flow to the opposite side. The system's thermal behavior is similar to the theoretical electrical diode. This work explores the effect of the condenser's wettability pattern design and the chamber's fluid charging ratio (CR). With this system, thermal diodicities exceeding 20 have been achieved, and are tunable by altering the wettability pattern. The thermal rectification concept and its proper quantification in terms of possible definitions are discussed. The present vapor chamber—thermal diode design could be well-suited for an extensive range of thermal-management applications, ranging from aerospace, spacecraft, and smart-building construction materials, to electronics protection, electronics packaging, refrigeration, thermal control during energy harvesting, thermal isolation, etc.

Dynamics of water condensation on a switchable surface originated from molecular orientations.

In heat transfer systems, how water condenses on the surface is critical to the energy efficiency of the system. With fixed surface wettability, hydrophilic surfaces enhance the nucleation rate but result in filmwise condensation due to pinning effect, which impedes the heat transfer between water vapor and surface during droplet growth. A hydrophilic surface with high drop mobility is realized with static tailored wettability surfaces, while tunable surfaces have potential in more comprehensive manipulation in condensation with different scale in time and scale. However, the mechanism has rarely been investigated and elucidated. In this paper, we investigate water condensation on a tunable surface originated from surface tension distribution control. The surface tension distribution under applied electric field is modeled and tested. We demonstrate that the surface tension manipulated by liquid crystal orientation alters the nucleation site density. Also, the periodic surface tension distribution aligns condensed water drops and decelerates the radius growth of droplets. The mechanism of active water condensation manipulation can be further applied to other tunable surfaces for various applications such as atmospheric water generator, heat transfer systems, and desalination systems.

Enhancing heat transfer performance of a two-phase closed thermosyphon using a polymer-coated hydrophobic condenser

• Durability of thin polymer coating is improved by combining adhesion promoter layers. • Droplet removal and re-nucleation enhance condenser heat transfer performance of TPCT. • Dropwise condensation can provide a 6 times higher condenser heat transfer coefficient. • 74% lower overall thermal resistance than bare and superhydrophobic TPCTs. A two-phase closed thermosyphon (TPCT) is a passive heat transfer device that transfers heat from one point to another by two-phase fluid circulation. Dropwise condensation on a hydrophobic surface, where discrete droplets grow on a condenser surface, promises higher condensation heat transfer coefficients (≈40–70 kW/m 2 ⋅K) than conventional filmwise condensation. Recent researchers have investigated the effect of dropwise condensation on the heat transfer performance of a TPCT. However, their measured condenser heat transfer coefficients (<25 kW/m 2 ⋅K) were much lower than expected, and the heat transfer enhancement mechanism of a hydrophobic TPCT has not been investigated in depth. Here, we achieved a higher condenser heat transfer coefficient of ~66 kW/m 2 ⋅K in a TPCT using a polymer-based hydrophobic coating film and investigated the heat transfer enhancement mechanism by performing experiments and analytical models. A thin polymer film with an optimized thickness (≈60 nm) was combined with adhesion promoter layers to achieve stable and high-performance dropwise condensation consisting of enhanced surface roughness and a coupling agent. The internal two-phase flow patterns and condensation behaviors were systematically evaluated for the heat transfer performance, which was compared to untreated bare and CuO nanostructured superhydrophobic surfaces. We revealed that rapid droplet removal and active re-nucleation on the polymer-coated hydrophobic surface in a TPCT led to 6 times higher condenser heat transfer coefficients and 68–74% smaller overall thermal resistance than those of the bare and superhydrophobic TPCTs. These results are helpful to understand the effect of dropwise condensation on the heat transfer performance of a TPCT and provide a direction for developing more efficient TPCTs.

Dropwise condensation of acetone and ethanol for a high-performance lubricant-impregnated thermosyphon

A gravity-assisted heat pipe called a two-phase closed thermosyphon (TPCT) is an efficient thermal device that transfers a large amount of heat at a relatively small temperature difference using the phase-change process. Promoting dropwise condensation on a condenser wall of a TPCT promises an increase in the effective thermal conductance of that device. Acetone and ethanol have been widely used for heat pipe applications as working fluids. Still, they pose a limitation in promoting dropwise condensation using typical hydrophobic coatings due to their lower surface energies. Recent works have shown that lubricant-impregnated surfaces (LIS) can promote dropwise condensation of low-surface-tension fluids, but no studies proved the effectiveness of the surfaces in TPCT applications. This study experimentally achieves dropwise condensation of acetone and ethanol within a TPCT using the LIS surface. We demonstrate heat transfer enhancement of a TPCT by examining temperature uniformity, condenser and evaporator heat transfer coefficients (HTCs), and overall and partial thermal resistances. We demonstrated that the LIS condenser increased the condenser HTC by 220% and decreased the overall thermal resistance of the TPCT by 44% compared to filmwise condensation. Our work also revealed a strong dependence of working fluid viscosity on dropwise condensation stability and heat transfer enhancement by comparing the two working fluids, acetone, and ethanol. This work presents the opportunity for improving thermal performance in heat pipes and applications that rely on the condensation of low-surface-tension fluids, such as HVAC, refrigeration, and organic Rankine cycle.

Modeling analysis of moist air condensation on a plate under forced convection conditions

A modeling analysis of the filmwise condensation for water steam outside a vertical plate under forced convective conditions was carried out in this study. The latent heat and sensible heat transfer under different moist air temperature, relative humidity, velocity, and wall temperature were theoretically analyzed. The model was developed based on the conservation equations of mass, momentum, energy, diffusion, and was solved numerically by finite difference methods. Meanwhile, the presence of condensate was neglected due to the low condensation load. Firstly, the results were compared with classical correlations and showed good agreement. Then the variation of latent and sensible heat transfer changing with operation conditions, such as moist air temperature, water steam mole fraction, wall temperature, and moist air velocity, were obtained and analyzed in detail. The results indicated that the latent heat transfer coefficient increased with the increase of relative humidity, moist air temperature, velocity, and cooling wall temperature. At the same time, the sensible heat transfer coefficient remains almost constant except under different moist air velocities, and latent heat transfer coefficient accounts for more than 70.3% in all the tested conditions of this study. The results obtained in this study can give a reference for the design of heat exchangers used in moist air condensation conditions.

Gradient mixed wettability surfaces for enhancing heat transfer in dropwise flow condensation

Steam condensation plays a pivotal role in a broad range of industrial applications. Dropwise condensation, which occurs on hydrophobic and superhydrophobic surfaces, has been proven to have a larger heat transfer coefficient than filmwise condensation because of enhanced droplet detachment. However, hydrophilic surfaces possess a better droplet nucleation rate compared to hydrophobic and superhydrophobic surfaces. These two characteristics (high droplet nucleation rate and better droplet removal) require counteracting properties. Many studies increased the nucleation rate by creating nanostructured surfaces. Wettability contrast mechanism is another approach, which could achieve both high nucleation rate and droplet removal, and have attracted much attention in enhancing a better heat transfer performance. In this study, gradient mixed wettability surfaces were developed on copper surfaces to improve heat transfer during steam flow condensation inside a minichannel. Circular hydrophobic islands were fabricated on a nanostructured superhydrophobic background surface. Various configurations of different island and pitch sizes are considered. Real-time images captured by a high-speed camera are used to examine the droplet dynamics and the influence of wettability on droplet size distribution. Thus, a parametric study is conducted to present the optimum conditions at different steam mass fluxes. According to the heat transfer analysis results, the heat transfer coefficient could be enhanced by 37% compared to a plain hydrophobic surface. A higher ratio of hydrophobic area provided better conditions for droplet nucleation and growth, while larger superhydrophobic regions in the downstream section was advantageous for droplet removal and sweeping. As a result, the role of mixed wettability in achieving the best performance was revealed for dropwise flow condensation.

Stretchable Anti‐Fogging Tapes for Diverse Transparent Materials

AbstractSurface wetting prevents surface fogging on transparent materials by facilitating filmwise condensation with specific chemistry, but suffers from material and geometry selectivity. Extreme environments associated with high humidity and mechanical loading further limit their anti‐fogging persistence. Here, a stretchable anti‐fogging tape (SAT) that can be applied to diverse transparent materials with varied curvatures for persistent fogging prevention is reported. The SAT consists of three synergistically combined transparent layers: i) a stretchable and tough layer with large elastic recovery, ii) an endurant anti‐fogging layer insensitive to ambient humidity, and iii) a robustly and reversibly adhesive layer. The SAT maintains high total transmittance (&gt;90%) and low diffuse transmittance (&lt;5%) in high‐humidity environments, under various modes of mechanical deformations, and over a prolonged lifetime (193 days tested so far). Two applications are demonstrated, including the SAT‐adhered eyeglasses and goggles for clear fog‐free vision, and the SAT‐adhered condensation cover for efficient solar‐powered freshwater production.

Deposition of nano-scale polymer film on micro-fins to enhance the film-wise condensation of very low surface tension substances

Dropwise condensation is extremely difficult to accomplish through the nanostructured surfaces for very low surface tension substances, such as the Freon refrigerants. To further improve the laminar film condensation heat transfer of the substances with very low surface tension, rely solely on the super hydrophobic layer might not work. In this work, a three dimensional micro-finned tube was fabricated and a layer of polymer coating was deposited outside the fins. Condensation of two substances, R1234ze(E) and R134a, were investigated at saturation temperature 30 and 40℃. Under the coupling effect of coating and fins, the condensing heat transfer coefficient can be up to 28 times larger than the smooth tube and the value can reach 43 kW/m2K at heat flux 20 kW/m2. In the lower heat flux, it can be 150% higher than the same finned tube without polymer coating. As the condensing heat transfer enhancement with only fin structure modifications would soon reach its limit, it should have a good application prospect for the micro-fins with surface treatment.

Effects and limitations of superhydrophobic surfaces on the heat transfer performance of a two-phase closed thermosyphon

A two-phase closed thermosyphon (TPCT), also called a gravity-assisted heat pipe, is an efficient heat transfer device that exploits boiling and condensation phase-change phenomena to transport large amounts of heat. Recently, jumping droplet condensation on a well-designed superhydrophobic (SHPo) surface has shown superior condensation heat transfer coefficient (HTC), exceeding ~380% and ~30% over conventional filmwise and dropwise condensation, respectively. However, SHPo surfaces within TPCTs have shown much lower HTCs than expected, and the exact cause of this has not been investigated yet. Here, we experimentally explored the effects and limitations of a SHPo surface in a TPCT by visualizing the internal flow patterns and condensation behavior according to heat flux. We constructed a TPCT device capable of visualizing the inside and fabricated a SHPo surface on a condenser wall of the TPCT. We revealed two important condensation characteristics that limit the condenser HTC of the SHPo surface and the TPCT's heat transfer performance. At high heat fluxes (≥ 90 kW/m2), the repetitive liquid collision led to an early flooding transition of the surface and a stepwise decrease in the condenser HTC. At low heat fluxes (< 90 kW/m2), counter-current flow generated by the evaporation and boiling caused the drag force enough to entrain growing and jumping droplets, which interrupted the removal of condensate from the surface and limited the condenser HTC. We quantified their effects on the condenser and TPCT's heat transfer performance using experiments and analytical models.

Enhanced condensation heat transfer using porous silica inverse opal coatings on copper tubes

Phase-change condensation is commonplace in nature and industry. Since the 1930s, it is well understood that vapor condenses in filmwise mode on clean metallic surfaces whereas it condenses by forming discrete droplets on surfaces coated with a promoter material. In both filmwise and dropwise modes, the condensate is removed when gravity overcomes pinning forces. In this work, we show rapid condensate transport through cracks that formed due to material shrinkage when a copper tube is coated with silica inverse opal structures. Importantly, the high hydraulic conductivity of the cracks promote axial condensate transport that is beneficial for condensation heat transfer. In our experiments, the cracks improved the heat transfer coefficient from ≈ 12 kW/m2 K for laminar filmwise condensation on smooth clean copper tubes to ≈ 80 kW/m2 K for inverse opal coated copper tubes; nearly a sevenfold increase from filmwise condensation and identical enhancement with state-of-the-art dropwise condensation. Furthermore, our results show that impregnating the porous structure with oil further improves the heat transfer coefficient by an additional 30% to ≈ 103 kW/m2 K. Importantly, compared to the fast-degrading dropwise condensation, the inverse opal coated copper tubes maintained high heat transfer rates when the experiments were repeated > 20 times; each experiment lasting 3–4 h. In addition to the new coating approach, the insights gained from this work present a strategy to minimize oil depletion during condensation from lubricated surfaces.

In-situ phase separation to improve phase change heat transfer performance

Phase change is widely applied in thermal-power conversion, cooling and thermal management, influencing efficiency, safety and cost of energy systems. The complicated mass, momentum and energy exchanges between two-phases causes large pressure drops, critical heat fluxes and flow instabilities. Here, a review on phase separation principle for phase change heat transfer is provided. The phase separation principle is presented in a theoretical form. Design and operation of phase separation for evaporator, condenser and heat pipe are commented. Because phase separation assigns pathways for liquid and vapor, it improves pool boiling performance and eliminates flow instability for micro-evaporator. For micro-condenser, the lined pin fins array creates liquid and vapor passages. The expanded liquid passage adapts increased flow rate to increased flow passage area for liquid. Pin fins are effective for filmwise condensation to increase heat transfer coefficients at low cost of pumping power. Phase separation for macro-condenser was fulfilled by suspending mesh screen membrane tube in condenser tube to confine vapor in annular region, yielding enhanced thin film condensation. Phase separation was commented on smart heat pipe. Superhydrophilic evaporator and superhydrophobic condenser generate nucleate mechanism, achieving increased heat transfer coefficients when heat loads increase. Perspective of phase separation was discussed.

CONCEPTUALIZATION, THERMAL ANALYSIS, AND MANUFACTURING OF NANO-TEXTURED MICRO-STRUCTURED SURFACES FOR ENHANCED CONDENSATION HEAT TRANSFER

In the present study, nano-micro-structured surfaces have been systematically designed and manufactured in order to generate controlled dropwise condensation mode for enhanced heat transfer during phase-change from vapor to liquid. The conceptualization of micro-structures was conducted by using thermal modelling of an individ-ual droplet pinned at a single micro-hexagonal cavity. By varying droplet radius, resulting contact angles and geo-metric dimensions of micro-structure, threshold values have been determined for the later manufacturing process. According to the calculations for instance at contact angles of 150°, a subcooling of 1 K and a maximum droplet radius of 100 µm, the edge length and the depth of the micro-structures should be kept below 50 µm and 25 µm, respectively. Ensuring these parameters, a roughly 100 % larger heat transfer coefficient would result compared to that predicted by the classical Nusselt theory for filmwise condensation. Taking into account the mathematical analysis, laser ablation technique was adopted on 7075 aluminum samples to emboss hexagonal structures with respect to the predicted dimensions. After this step, the samples were electrochemically etched in order to achieve contact angles of more than 150° to ensure superhydrophobic solid-liquid interaction at the surface. Measurements with a high-precision microscope show that most of the structure dimensions and geometric shape were precisely manufactured. The Tensiometer results disclosed that the surface topography at all samples exhibit contact angles larger than 150° for a sessile droplet with a radius of 100 µm pinned on an individual micro-hexagon.

Heat Transfer Analysis for Dropwise-Filmwise Hybrid Surface of Steam on Vertical Plate

A model is proposed to predict and evaluate the heat transfer characteristics of dropwise-filmwise hybrid surface. This is approached by modifying the original drop-size distribution, which is defined for fully dropwise condensation (DWC) and making it applicable for dropwise-filmwise condensation (DFC). The modification is achieved by simulation work to determine two parameters: the area fraction occupied by large drops f and the ratio of the maximum radius of newly formed drops to that of larger drops γ. Simulation results show a good agreement with the literature for fully DWC and provide a correlation for each parameter with respect to DWC region width in the DFC hybrid surface. The present model evaluates the heat transfer performance of DFC by utilizing these correlations. A comparison is made between the proposed model with experimental work from the literature and results show a good agreement. While changing filmwise condensation (FWC) region width significantly affects the overall heat transfer performance, utilizing smaller width to that of DWC region has the advantage over fully DWC. Furthermore, surface renewal within the hydrophobic region is controlled by adjusting the DWC region width. When the ratio of drop maximum diameter to DWC region width is unity, surface renewal is achieved by drops merging to adjacent FWC regions only. When the ratio is less than unity, surface renewal is achieved by sweeping of departing drops within the hydrophobic region and by merging. For each case, an optimum DWC region width which corresponds to the maximum DFC heat flux is defined.

Filmwise Condensation From Humid Air on a Vertical Superhydrophilic Surface: Explicit Roles of the Humidity Ratio Difference and the Degree of Subcooling

Abstract The process involving heat and mass transfer during filmwise condensation (FWC) in the presence of noncondensable gases (NCG) has great significance in a large variety of engineering applications. Traditionally, the condensation heat transfer is expressed in the literature as a function of the degree of subcooling—reckoned as the difference between the ambient dry bulb temperature and the condenser wall temperature. However, in the presence of NCG, there exists a finite gradient of vapor mass fraction near the condenser plate, which directly influences the vapor mass flux to the condenser surface, thus limiting the condensation rate. The effects of both these influencing thermodynamic parameters, i.e., the degree of subcooling and the difference of humidity ratio (between the freestream environment and on the condenser plate), have been characterized in this work both experimentally and through a mechanistic model. The vapor mass flux during condensation on a subcooled vertical superhydrophilic surface under free convection regime is experimentally measured in a controlled environment (temperature and humidity) chamber. The mechanistic model, based on the similarity of energy and species transports, is formulated for the thermogravitational boundary layer over the condenser plate and tuned against the experimental results. Further, the model is used to obtain comprehensive data of the condensate mass flux and condensation heat transfer coefficient (CHTC) as functions of the salient thermal operating conditions over a wide parametric range. Results indicate that humidity ratio difference has a more pronounced influence on the condensation mass transfer rather than the degree of subcooling. Regime maps of condensate flux and CHTC show how these can be explicitly identified in terms of the degree of subcooling and humidity ratio difference, regardless of the prevailing thermal and humidity conditions at the freestream and the condenser plate. The mechanistic model thus lends to the development of empirical correlations of condensate mass flux and CHTC as explicit functions of these two parameters for easy use in practical FWC configurations.