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Abstract A comprehensive analysis of enhanced condensation with phase separation concept is presented, for which mesh-membrane-tube (MMT) is suspended in a tube. A twill Dutch weaved mesh screen and two plain Dutch weaved mesh screens are used to fabricate MMT. Our study reveals that liquid leakage across MMT not only keeps better condensation on condenser wall, but also decreases pressure drop rise penalty. To modulate stratified-flow, the #1 MMT with smallest d p has the largest capillary force to pump liquid towards core region to expose more condenser surface with vapor, where d p is pore diameter. To modulate annular-flow, finest mesh wires of #1 MMT ensure the best wetting to liquid to prevent condenser wall from being impacted by satellite droplets. These mechanisms explain the best performance of #1 MMT, reaching a maximum heat transfer enhancement ratio of 1.82. The three MMTs share a single curve of nozzle discharge coefficient versus Reynolds number. The similar performance of #2 and #3 MMTs is due to identical d p / φ , where φ is mesh open porosity. Fine mesh wires and small mesh pores are suggested to enhance condensation heat transfer. MMT are recommended to be used in condenser tube upstream when vapor mass qualities larger than 0.1. Highlights • Mesh-membrane-tube (MMT) is suspended in tube to enhance condensation heat transfer. • MMT captures droplets with fine mesh wires to resist liquid accumulation on condenser wall. • MMT having smaller mesh pores enhances liquid suction from annular region to core region. • The ratio of mesh pore diameter to open porosity governs liquid flow across membrane. • An overall 82% improvement of heat transfer enhancement is observed, maximally. [ABSTRACT FROM AUTHOR]

Highlights • Condensation experiment on superhydrophobic nanograsses surface (NGS) is performed. • Fresh NGS behaves smaller drop but deteriorates heat transfer compared with polymer surface. • Collapse and breakage of nanograsses worsen heat transfer for long-term operation. • Three mechanisms to induce nanostructure failure are proposed. • There is a limit of decayed heat transfer after nanostructure failure. Abstract Dropwise condensation heat transfer (DWC) on superhydrophobic nanograsses surface (NGS) was investigated for long-term operation. For DWC of pure water-vapor on fresh NGS, two heat transfer regimes are identified: higher heat transfer coefficients with droplet jumping, and constant heat transfer coefficients with droplet rolling. The one-week operation not only deteriorates heat transfer performance, but also changes jumping or rolling mode to sliding mode. The condensation heat transfer coefficients are apparently decreased from first to third day, but they approach a limit value since the third day. In order to identify if the single-molecule-layer of polymer (SML) modified on nanograsses was destroyed, DWC on a smooth single-molecule-layer of polymer surface (SSML) was tested to display stable heat transfer with drop sliding for one-week operation, concluding no failure of the polymer layer. The collapse and breakage of nanograsses were observed to explain the decayed heat transfer versus time on NGS. Compared with SSML, the NGS has smaller droplet departure size but lower heat transfer coefficients, indicating positive and negative effects after introducing nanostructures. Three nanostructure failure mechanisms are proposed. This work suggests a new research field of the nanoscale fluid-wall interaction. [ABSTRACT FROM AUTHOR]

Onset of droplet motion is important for various applications, including dropwise condensation and water management in fuel cells. In order to determine critical conditions for onset of motion and specific motion mode, a general problem having a droplet sheared by a gas stream on inclined surface is investigated. Two criterion equations are theoretically established for onset of droplet sliding and rolling independently, including dimensionless parameters of Bond number ( Bn ), Ohnesorge number ( Oh ) and Weber number ( We ), inclination angle parameter and wettability parameters. The criterion equations predict critical gas velocity, maximum droplet radius, and “sliding angle” or “rolling angle”. Droplet sliding predictions agree well with experimental data in the literature. Criterion surfaces of sliding or rolling are constructed to verify if sliding or rolling can be initiated, influenced by both equilibrium contact angle and contact angle hysteresis. By coupling the criterion equations of sliding and rolling, we develop a mode selection criterion equation, which is only dependent on equilibrium contact angle θ e . Three regions are clarified: (1) for 126.3° <  θ e  < 147.0°, a droplet rolls if Bn  <  Bn t and slides if Bn  >  Bn t , where Bn t is the transition Bond number; (2) for θ e  < 126.3°, a droplet only slides; (3) for θ e  > 147.0°, a droplet only rolls. The theoretically determined 147.0° contact angle is newly recommended as the contact angle boundary between hydrophobicity and super-hydrophobicity. One of the applications of this work is to provide a general guideline for droplet detachment or retention in fuel cells and condensers. [ABSTRACT FROM AUTHOR]

Micro-condenser using the phase separation concept was investigated in this paper. Lined pin fin arrays generate liquid passages and vapor passages alternatively in chip width direction. The decreased Gibbs free energy with gas–liquid interface advancing pin fin throat location is the mechanism to induce liquid flow from vapor passages to liquid passages. The decreased energy dissipation due to decreased interfacial area between the two phases accounts for pressure drop reduction. Three micro-condensers were investigated: microchannel condenser (SWM), parallel phase separation condenser (PPS with constant cross sections of fluid passages) and conical phase separation condenser (CPS with varied cross sections of fluid passages). Micro-condensers had identical project condensation surface of 25.0 mm by 3.0 mm. The etched depth was 75 μm. Water-vapor was the working fluid. Compared with SWM, phase separation condensers increased mass flow rate by 15% at similar pressure drops. PPS condenser enhances heat transfer at moderate or smaller cooling intensity, but deteriorates heat transfer at large condensed liquid flow rate, at which over liquid expansion occurs to flood all pin fin side walls. CPS condenser self-adapts variations of flow rates of the two phases to stabilize vapor–liquid interface near pin fin membrane. Pin fin side walls facing vapor passage are covered by thin liquid film to eliminate over liquid expansion. CPS condenser enhances heat transfer over entire operating parameter ranges, increasing condensation heat transfer coefficients by 74% maximally while pressure drops are decreased. CPS condenser has the best performance among the three condensers. [ABSTRACT FROM AUTHOR]

Condensation heat transfer of R245fa was investigated in a shell-tube condenser. The copper tube had an inner diameter of 14.70 mm with a 1600 mm heat transfer length. Mass fluxes and vapor mass qualities covered ranges of 198.8–504.7 kg/m 2 s and 0.291–0.976, respectively. Condensation heat transfer coefficients are increased with increases of mass fluxes and vapor mass qualities. The horizontal position yielded minimum condensation heat transfer coefficients. Deviating from the horizontal position enhances condensation heat transfer. The condenser is suggested to operate at weakly inclined flow to reach better thermal performance. Flow patterns and liquid height signals explored the condensation heat transfer mechanisms. The flow is stable for inclined down-flow but becomes unstable for inclined up-flow, which was caused by the vapor-liquid interface wave. Non-dimensional parameter analysis identified the competition between inertia force and gravity force. The vapor-liquid interface wave is the mechanism to enhance condensation heat transfer for inclined up-flow, while the decreased liquid film thickness on the tube bottom enhances heat transfer enhancement for inclined down-flow. A correlation based on the Froude number of vapor phase successfully matched the experimental data. This study is useful for the condenser design and operation such as applied in Organic Rankine Cycles. [ABSTRACT FROM AUTHOR]

We decouple a heat pipe into capillary pressure, flow resistance, condensation heat transfer, and assign specific length scale to adapt each function. We verify the synergy of various length scales to activate various functions. The strategy guides multiscale design to realize an enhancement of capillary pressure, a well management of flow resistance and an ultra-thin liquid thickness on condenser surface. The porous wick consists of a particle sub-layer and 3D mastoid process array. The tips of mastoid process directly contact the condenser wall. Four vapor chambers are formed by sintering d m = 73.8 μm without oxidation (#1), with oxidation (#2), d m = 556 nm without oxidation (#3) and with oxidation (#4), respectively. Liquid suction and heat transfer experiments were performed. Four types of evaporator temperatures versus inclination angles were observed. Small difference is found between bottom and top heating modes. The multiscale wick influences the vapor–liquid phase distribution to cause the difference between side and other heating modes. The d m = 73.8 μm particle sintering with nano-roughness successfully balance various conflicts among capillary pressure, vapor–liquid interface area, flow resistance and liquid removal from the condenser surface. Nano-roughness increases the vapor–liquid interface area to have 3–4 times of evaporation heat transfer coefficients compared with smooth particle surface. Nano-roughness increases the wettability to capture liquid from condenser, having ∼18 times of condensation heat transfer coefficients to those without nano-roughness. The d m = 556 nm particle sintering and nano-roughness are the poor match for heat pipes. This paper gives a clue to construct multiscale wicks for heat pipes and ensures better performance at varied gravity such as micro-gravity environment. [ABSTRACT FROM AUTHOR]

Modulated heat transfer tube (MHTT) is called by suspending porous-membrane-tube (PMT) in a large tube to form an annular region and a core region. PMT was made by sintering copper powders in a circular membrane shape, which is lyophilic to capture liquid and make liquid flowing towards PMT inside. Vapor phase is prevented from entering PMT. Thus, vapor void fractions near the tube wall are increased to enhance condensation heat transfer. Experiments were performed with a copper tube inner diameter of 14.81 mm and a heat transfer length of 1200 mm. R245fa was the working fluid. Great attention was paid to explain condensation heat transfer coefficients influenced by mass fluxes, vapor mass qualities and inclination angles. It was found that MHTT had 85% improvement of condensation heat transfer coefficients compared with bare tube (BT), maximally. The heat transfer enhancement factors of MHTT were increased with mass fluxes and/or vapor mass qualities. MHTT had largest condensation heat transfer coefficients for the vertical upflow, due to the promoted liquid mass and momentum exchange across the porous membrane. MHTT had smallest condensation heat transfer coefficients for the horizontal flow. The droplet capture and liquid flow to PMT are the heat transfer enhancement mechanism for annular flow modulation. Alternatively, the capillary pumping effect held liquid within PMT to decrease liquid height in the annular region, accounting for the heat transfer enhancement mechanism for stratified flow modulation. This paper finds a new clue to use PMT in a large tube for phase change heat transfer enhancement. [ABSTRACT FROM AUTHOR]

Condensation heat transfer of R245fa in a tube with the diameter of 14.81 mm and length of 1200 mm was investigated. The R245fa mass fluxes, inlet vapor mass qualities and inclination angles covered the ranges of 191.3–705.4 kg/m 2 s, 0.191–0.947 and −90° to 90°, respectively. The non-dimensional analysis yielded the importance of inertia force and gravity force to influence condensation heat transfer. Surface tension force played unimportant role. Condensation heat transfer coefficients ( h ) showed the general increase trend versus inclination angles ( θ ) and the non-monotonic behavior in the full inclination angle range. The maximum h occurred at θ = 30° and −15°. The h values were sensitive to θ near the horizontal positions. The main flow patterns in the condenser tube were stratified-smooth flow, stratified-wavy flow and intermittent flow. For inclined upflow, the liquid layer thickness and interface wave competed with each other to influence heat transfer. An optimal inclination angle existed at which heat transfer coefficients reached maximum. For inclined downflow, condensation heat transfer was balanced by the liquid layer thickness and buoyancy force. The flow was more stable. Heat transfer coefficients were well correlated with the general increase term multiplied by the non-monotonic variation term. The parameters in the correlation depended on the Froude number and vapor mass qualities. The correlation successfully explained the effects of mass fluxes, vapor mass qualities and inclination angles on condensation heat transfer coefficients. Based on the present finding, the condenser is recommended to operate at the inclination angle of −15° or 30°, at which heat transfer coefficients are maximum and pressure drops are minimum. The exactly horizontal flow weakens the condenser performance. [ABSTRACT FROM AUTHOR]

Based on the volume-of-fluid (VOF) method in the FLUENT code, many phase-change models have been proposed and applied to simulate evaporation and condensation problems. To further improve the accuracy, in this article a new phase-change model is built using user-defined functions (UDFs). The accuracy of this new phase-change model is verified by two evaporation problems (a one-dimensional Stefan problem and a two-dimensional film boiling problem) and one condensation problem (single steam bubble condensation in subcooled water). The simulation results of this new model show good agreement with the classical analytical or numerical results, proving its accuracy and feasibility. [ABSTRACT FROM AUTHOR]

Recently, our research group proposed the phase separation condenser tube, in which a mesh cylinder was inserted to form the flow structure of 'gas near the tube wall and liquid in the tube core', significantly enhance the condensation heat transfer. But the bubble leakage towards the core region may worsen the heat transfer enhancement. In order to prevent the bubble leakage, the critical criterion was proposed based on the Young-Laplace equation, considering the inertia force, viscous force and pulsating flow. It was found that the critical criterion depends on the dimensionless parameter G, the We number and a coefficient C. The numerical model was developed in terms of the volume of fluid method to predict the two-phase laminar flow in the phase separation condenser tube. The results show that the bubble leakage takes place at the bubble tip, which is agreed with the experimental observations. The critical curve distinguishing the non-bubble-breaking and bubble-breaking was obtained by comparing the bubble dynamics at different G* and We. The coefficient C was determined. The critical criterion for the bubble leakage is given as $$ G^{*} \times We = \left( {0.22We^{0.99349} - \frac{{G^{*} }}{{4.7 \times 10^{3} \times We^{0.00651} }} + \frac{196.39}{{We^{0.00651} }}} \right) \times \frac{{4{ \cos }\alpha }}{\Delta \varTheta W} $$ , providing the design and operation guidance for the phase separation condenser tube. [ABSTRACT FROM AUTHOR]

Purpose – Based on the numerical research on the relationship between the flow pattern transition and the condensation heat transfer in circular microchannels, the purpose of this paper is to bring forward a concept of external separation circular microchannel to regulate and control the flow pattern for enhancing the condensation heat transfer. Design/methodology/approach – The numerical research is based on the volume of fluid method and the vapor-liquid phase change model proposed by the present authors. Findings – By numerical research on the condensation process of water in a general circular microchannel, it is discovered that, with the increase of the inlet velocity and the reduction of the temperature difference between the saturation temperature and the channel wall temperature, the bubble detachment frequency is raised and the water vapor condensation length is extended, representing an exponential growth. Therefore, for the condensation process with low temperature difference and high mass flow rate, it is in urgent need to regulate and control the flow pattern. Originality/value – To prevent the flow pattern in the general circular microchannel converted from annular flow to slug flow and then to bubble flow, this paper brings forward a concept of external separation circular microchannel, which regulates and controls the flow pattern by discharging partial liquid from the annular wall opening. After regulation and control, the flow pattern is converted from original periodic annular flow/slug flow/bubble flow to current stable annular flow. Accordingly, the heat transfer performance is enhanced and the condensation length is lowered remarkably. [ABSTRACT FROM AUTHOR]

Abstract: This paper reports condensation heat transfer in a horizontal tube with a 14.81 mm inside diameter and a 1200 mm heat transfer length, by using the phase separation concept. The hollow mesh cylinder was formed by packaging two layers of mesh screen surface. The outer layer had the pore size of 76 μm. This invention constructs a multiscale condenser tube. The modulation of stratified flow and annular flow was focused on. Liquid was held within the mesh cylinder to increase the vapor covered tube wall surface to enhance the heat transfer for stratified flows. For annular flows, liquid droplets in the vapor core were captured by the mesh screen surface to increase the vapor void fractions near the tube wall, enhancing the hat transfer. The measurements showed that the heat transfer coefficients could be more than two times of those in the bare tube. The total thermal resistance was decreased by 45.6%, maximally. The heat transfer enhancement ratios were increased with the vapor mass fluxes, Gx in, for the condensation flow along the whole tube length. A pressure analysis explained the increased heat transfer trend. The increment of heat transfer enhancement ratios was suppressed with a liquid flow part in the tube. [Copyright &y& Elsevier]

Abstract: Condensation heat transfer has been studied in the past century due to its wide applications in energy and power systems. The key scientific issue is the thick liquid thickness near the tube wall along the condenser tube length. The fabricated microstructures on the inner wall are the conventional technique to improve the performance. Here a passive phase separation concept was proposed to create distinct phase distribution. An empty cylinder made of a single layer of mesh pore surface was suspended in a tube, dividing the tube into an annular region and an inner region. The mesh pore surface prevents gas phase entering the inner region but sucks liquid towards the inner region. Thus largest possibility for gas directly contacted with the inner wall surface is ensured. An air/water two-phase flow experiment was performed and the stratified flow pattern modulation was investigated. When the liquid level in the horizontal tube is relatively higher, the liquid can be thoroughly within the mesh cylinder to form the “gas-floating-liquid” mode. The whole inner tube wall surface is covered by the gas phase. If the liquid content is relatively smaller, partial liquid can be sucked into the mesh cylinder. The contact area between the inner tube wall and gas is increased. The stratified flow pattern modulation is expected to significantly enhance the condensation heat transfer under low mass fluxes which is being verified by our continuous experiment. [Copyright &y& Elsevier]

The passive phase separation concept was proposed to modulate flow patterns for heat transfer enhancement. By the flow pattern modulation, the gas tends to be near the wall and the liquid tends to be in the tube core. Experiment has been performed to verify the fresh idea and the flow pattern modulation mechanism was analyzed qualitatively. This paper focuses on the numerical simulation of the bubble dynamics for a single bubble in the vertical phase separation condenser tube to quantitatively explore the flow pattern mechanism, based on a multiscale grid system and the volume-of-fluid (VOF) method. It is found that: (1) the modulated liquid film thickness can be decreased by 70% compared to that in the bare tube region; (2) the modulated bubble traveling velocity can be doubled, causing the increased liquid velocity and velocity gradient in the annular region to weaken the fluid boundary layer; (3) the significantly increased bubble traveling velocity in the annular region promotes the mass and momentum exchange between the annular region and the core region, and yields the self-sustained pulsating flow in the core region. The above three factors are benefit for the performance improvement of the heat transfer facilities. [ABSTRACT FROM AUTHOR]

Abstract: A passive phase separation concept was proposed to modulate flow pattern in a condenser tube. An empty mesh cylinder is suspended in the condenser tube. The miniature mesh pores prevent gas bubble entering the mesh cylinder but capture liquid into the mesh cylinder, ensuring largest possibility for cold tube wall directly contacted with gas to form the perfect thin liquid film condensation heat transfer. We performed the air–water two-phase flow experiment. It was found that for a relatively higher liquid height in the horizontal tube, all liquid can be captured by the mesh cylinder to form the “gas-floating-liquid” mode. If the liquid height is small in the horizontal tube, partial liquid can be sucked by the mesh cylinder, the contact area between tube wall and gas is increased. When plug flow reaches the mesh cylinder surface, elongated saddle bubbles are formed in the annular region to envelop the mesh cylinder surface. When bubbly flow in the horizontal tube approaches the mesh cylinder area, miniature bubbles can merge to form large bubbles in the annular region. For the later two cases, all the gas flow rate is flowing in the annular region and the inside mesh cylinder is the liquid. [Copyright &y& Elsevier]

Abstract: Chaotic behavior of closed loop pulsating heat pipes (PHPs) was studied. The PHPs were fabricated by capillary tubes with outer and inner diameters of 2.0 and 1.20mm. FC-72 and deionized water were used as the working fluids. Experiments cover the following data ranges: number of turns of 4, 6, and 9, inclination angles from 5° (near horizontal) to 90° (vertical), charge ratios from 50% to 80%, heating powers from 7.5 to 60.0W. The nonlinear analysis is based on the recorded time series of temperatures on the evaporation, adiabatic, and condensation sections. The present study confirms that PHPs are deterministic chaotic systems. Autocorrelation functions (ACF) are decreased versus time, indicating prediction ability of the system is finite. Three typical attractor patterns are identified. Hurst exponents are very high, i.e., from 0.85 to 0.95, indicating very strong persistent properties of PHPs. Curves of correlation integral versus radius of hypersphere indicate two linear sections for water PHPs, corresponding to both high frequency, low amplitude, and low frequency, large amplitude oscillations. At small inclination angles near horizontal, correlation dimensions are not uniform at different turns of PHPs. The non-uniformity of correlation dimensions is significantly improved with increases in inclination angles. Effect of inclination angles on the chaotic parameters is complex for FC-72 PHPs, but it is certain that correlation dimensions and Kolmogorov entropies are increased with increases in inclination angles. The optimal charge ratios are about 60–70%, at which correlation dimensions and Kolmogorov entropies are high. The higher the heating power, the larger the correlation dimensions and Kolmogorov entropies are. For most runs, large correlation dimensions and Kolmogorov entropies correspond to small thermal resistances, i.e., better thermal performance, except for FC-72 PHPs at small inclination angles of θ <15°. [Copyright &y& Elsevier]

Abstract: Multi-channel effect is important to understand transport phenomenon in phase change systems with parallel channels. In this paper, visualization studies were performed to study the multi-channel effect in a silicon triple-channel condenser with an aspect ratio of 0.04. Saturated water vapor was pumped into the microcondenser, which was horizontally positioned. The condenser was cooled by the air natural convention heat transfer in the air environment. Flow patterns are either the annular flow at high inlet vapor pressures, or a quasi-stable elongated bubble at the microchannel upstream followed by a detaching or detached miniature bubble at smaller inlet vapor pressures. The downstream miniature bubble was detached from the elongated bubble tip induced by the maximum Weber number there. It is observed that either a single vapor thread or dual vapor threads are at the front of the elongated bubble. A miniature bubble is fully formed by breaking up the vapor thread or threads. The transient vapor thread formation and breakup process is exactly symmetry against the centerline of the center channel. In side channels, the Marangoni effect induced by the small temperature variation over the channel width direction causes the vapor thread formation and breakup process deviating from the side channel centerline and approaching the center channel. The Marangoni effect further forces the detached bubble to rotate and approach the center channel, because the center channel always has higher temperatures, indicating the multi-channel effect. [Copyright &y& Elsevier]

Abstract: Condensation of steam in a single microchannel, silicon test section was investigated visually at low flow rates. The microchannel was rectangular in cross-section with a depth of 30μm, a width of 800μm and a length of 5.0mm, covered with a Pyrex glass to allow for visualization of the bubble formation process. By varying the cooling rate during condensation of the saturated water vapor, it was possible to control the shape, size and frequency of the bubbles formed. At low cooling rates using only natural air convection from the ambient environment, the flow pattern in the microchannel consisted of a nearly stable elongated bubble attached upstream (near the inlet) that pinched off into a train of elliptical bubbles downstream of the elongated bubble. It was observed that these elliptical bubbles were emitted periodically from the tip of the elongated bubble at a high frequency, with smaller size than the channel width. The shape of the emitted bubbles underwent modifications shortly after their generation until finally becoming a stable vertical ellipse, maintaining its shape and size as it flowed downstream at a constant speed. These periodically emitted elliptical bubbles thus formed an ordered bubble sequence (train). At higher cooling rates using chilled water in a copper heat sink attached to the test section, the bubble formation frequency increased significantly while the bubble size decreased, all the while forming a perfect bubble train flowing downstream of the microchannel. The emitted bubbles in this case immediately formed into a circular shape without any further modification after their separation from the elongated bubble upstream. The present study suggests that a method for controlling the size and generation frequency of microbubbles could be so developed, which may be of interest for microfluidic applications. The breakup of the elongated bubble is caused by the large Weber number at the tip of the elongated bubble induced by the maximum vapor velocity at the centerline of the microchannel inside the elongated bubble and the smaller surface tension force of water at the tip of the elongated bubble. [Copyright &y& Elsevier]

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. • A comprehensive review is performed for in-situ phase separation principle. • The phase separation principle is developed based on minimum Gibbs energy. • Phase separation is commented for evaporator, condenser and heat pipe. • Phase separation is effective to improve the phase change heat transfer performance. • Perspectives are proposed including versatile design, materials and applications. [ABSTRACT FROM AUTHOR]

• A microscale phase separation evaporator concept is proposed. • A new hybrid mechanism for boiling heat transfer is found. • Mass-flux-independent pressure drops are obtained. • Constant heat transfer coefficients with Bo numbers covering one order of magnitude. Microchannel evaporator has intrinsic drawback of two-phase flow instabilities with large pressure drop that is induced from bubble blockage in microchannels. Here, we propose a new concept of phase-separation evaporator, in which flow instabilities can be completely suppressed by using gradient pin-fin-porous wall microchannels. The phase-separation concept stems from the self-driven motion of nucleated bubbles from pin-fin region to bare channel region. Vapor bubbles release from porous wall at a high frequency of ~1000 Hz that ensures bare channels not totally wrapped by vapor. Thus, ultra-stable flow and heat transfer can be achieved in a wide range of operating parameters: T in = 81.9~92.6 °C, P in = 110.57~219.77 kPa, G = 112~264 kg/m2s, q = 20.79~292.82 kW/m2 and x out = 0.005~0.318. Both convective evaporation in bare channels and bubble nucleation in porous walls contribute to enhanced performance. Heat transfer coefficients are not changed as the Bo numbers ranging from 6.91 × 10–5 to 7.44 × 10–4. This is because when heat flux and/or mass flux is changed, one mechanism is strengthened while the other mechanism is weakened to achieve constant overall heat transfer coefficient. This heat transfer mechanism is different from that in conventional microchannels in which nucleation mechanism dominates at small Bo while convective mechanism dominates at large Bo. With mass fluxes in the range of 112 ~264 kg/m2s, pressure drops are not changed versus mass fluxes at a given heat flux, which are caused by the thermal driving effect in porous walls. Stable heat transfer coefficients and pressure drops ensure flexible selection of running parameters. The evaporator can operate at smaller mass flow rates, as long as dry-out does not occur, it can also operate at larger flow rates without accompanying additional pressure drops. [ABSTRACT FROM AUTHOR]

Highlights • A novel cost-effective method is proposed to fabricate biphilic surface. • Biphilic surface has the best thermal performance among three surfaces. • Drop nucleation occurs easily on hydrophilic dot for DWC on biphilic surface. • Drop on hydrophilic dot becomes the center to attract and merge neighboring drops. • Drops depart faster on biphilic surface than on hydrophobic surface. Abstract For dropwise condensation (DWC) in confined channels, droplets slide on surface and may spoil the nano-pillars, thus the nano-pillars structure is not suitable. Here, we provide a robust biphilic surface with polymer film as hydrophobic wettability. Mesh screen is sintered on copper surface to form welding junction array. Dipping the sintered package in a Teflon solution followed by baking technique forms a hydrophobic coating film. Hydrophilic dots are exposed by separating mesh screen from copper substrate. Condensation experiment was performed in a mini-channel with dimensions of 30.0 × 3.13 × 0.97 mm3 and pure water–vapor as the working fluid. Mass flux covered a range of 10.5–63.8 kg/m2 s. It is shown that condensation heat transfer coefficients on biphilic surface were 5.54 times of those on polished copper surface, and 1.89 times of those on hydrophobic polymer layer surface, maximally. Pressure drops are almost identical in condensers with biphilic surface and hydrophobic surface. For DWC on biphilic surface, growing droplet on hydrophilic dot attracts and merges neighboring droplets to generate directional-droplets-coalescence , explained by the Gibbs energy analysis. Higher droplet departure frequency was observed on biphilic surface than hydrophobic surface. Compared with hydrophobic surface, droplet on biphilic surface has different contact angle, covers several hydrophilic dots and reduces droplet thermal resistance. The above effects explain the heat transfer enhancement mechanism when biphilic surface is present. [ABSTRACT FROM AUTHOR]

The R245fa condensation heat transfer is investigated in a phase separation shell-tube condenser with mesh tubes inserted. The condenser has three copper tubes which have an inner diameter of 14.70 mm and a heat transfer length of 1600 mm. Four mesh tubes with mesh pore width of 15 μm are arranged in each copper tube. The comparative experiments are conducted for condensers with or without mesh tubes inserted. The flow pattern visualization is investigated to explore heat transfer enhancement mechanisms. The results showed that, with vapor mass qualities and mass fluxes increased, condensation heat transfer coefficients and friction pressure drops are increased, but heat transfer enhancement ratios and performance evaluation parameters are decreased. At low mass fluxes, the heat transfer coefficients behave a slow decrease trend with increase of inclination angles. The pressure difference between annular region and core region drives liquid flow towards core region, reducing liquid film thickness on tube bottom and increasing vapor void fractions near tube wall. With increase of inclination angles, the pressure difference is reduced but liquid film thickness on tube bottom is increased. For horizontal flow, there is an optimal match between pressure difference of annular region and core region and liquid-mesh tube contact area. Under such circumstance, the heat transfer enhancement ratios reach maximum. [ABSTRACT FROM AUTHOR]

• The proposed dropwise-filmwise condensation model includes comprehensive drop detachment modes. • Only considering the double-side-suction mode underpredicts the heat transfer coefficients. • Optimal condition exists due to competition between heat transfer and area ratio of hydrophobic region. • Optimal width of hydrophobic stripe is mainly dominated by geometrical parameters of δ p and W FWC. • Biphilic surface does not always enhance heat transfer if geometrical parameters are not properly selected. Condensation heat transfer on biphilic surface is investigated. The surface periodically populates hydrophobic stripes each having a coating layer thickness δ p and a width W DWC , and hydrophilic stripes each having a width W FWC. The proposed model includes dropwise condensation on hydrophobic stripe, filmwise condensation on hydrophilic stripe, and droplet detachment radius r max criterion for heat-mass coupling between the two wettabilities regions. The r max is the minimum of detachment radii determined by droplet removal modes of double-sides-suction DSS, one-side-suction OSS and sliding, where DSS is a special case of OSS for droplet located at hydrophobic stripe centerline. Simulation results matched the measured heat transfer data well. Optimal width of hydrophobic stripe W DWC o is found to be dominated by δ p and W FWC , but other parameters weakly influence W DWC o. Interfaced by a δ p − W FWC transition curve, a heat transfer regime map is presented to contain Regime I for possible heat transfer enhancement and Regime II for heat transfer deterioration. Regime I enhances heat transfer if W DWC approaches W DWC o , but may deteriorate heat transfer if W DWC deviates W DWC o too much. The maximum heat transfer enhancement ratio is 1.67 compared with purely hydrophobic surface. Regime II always deteriorates heat transfer. Our work provides a general guideline to design biphilic surface for performance improvement. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Highlights • Superhydrophilic-hydrophobic network hybrid surface was investigated using stainless steel material. • Hybrid surface can control condensate droplet diameter and have excellent condensation heat transfer performance. • Superhydrophilic grooves can limit the growth of large condensate droplets. • There is optimum grid spacing between grooves to enhance condensation heat transfer. Abstract Superhydrophilic-hydrophobic (SHPi-HPo) network hybrid surface was designed to investigate the condensation heat transfer using stainless steel as substrate material. The SHPi-HPo surface was comprised of superhydrophilic network grooves and hydrophobic regions. Hydrophobic (HPo) surface was prepared with fluorocarbon coating using polytetrafluoroethylene (PTFE) as the matrix resin and micro-nano silicon dioxide (SiO 2) as additive to control surface roughness. Three kinds of SHPi-HPo surfaces were tested, having a grid spacing of 1.5, 2.5 and 3.5 mm and named as SHPi-HPo-1, SHPi-HPo-2 and SHPi-HPo-3, respectively. To study the effects of the wall subcooling, steam mass flux, cooling water temperature, cooling water mass flow rate and grid spacing, a series of experiments were conducted and a high speed camera was used to visualize the condensation process. The results show that SHPi-HPo surface can well control condensate droplet diameters and its condensation heat transfer performance is better than that of smooth hydrophilic (HPi) and HPo surfaces. This is attributed to SHPi-HPo surface sucking away droplets in time and limiting the growth of large condensate droplets through the superhydrophilic grooves. At wall subcooling ΔT w = 6.3 K, the heat transfer coefficient of SHPi-HPo-2 surface is 2.7 and 3.4 times that of HPi and HPo surfaces, respectively. For SHPi-HPo surface, there is optimum grid spacing between superhydrophilic grooves to enhance condensation heat transfer. Among three SHPi-HPo surfaces, the heat transfer coefficient of SHPi-HPo-2 surface has the best condensation heat transfer performance, about 0–10% higher than that of SHPi-HPo-1 surface, and at ΔT w = 9 K, the heat transfer coefficient is 1.7 times that of SHPi-HPo-3 surface. [ABSTRACT FROM AUTHOR]