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Piezoelectric (PE) fan is a novel type of active cooling device which enhance heat transfer with less power consumption. In order to achieve the theoretical limit of heat transfer performance of multiple PE fans system in confined volume, the blade location effects on that were investigated in detail by conducting three-dimensional simulations. Here, the blade pitch ranges from 1.6 to 4.2 times the fan width, and the optimal values of blade pitch with respect to four typical vibration phase angles (0°, 60°, 120°, 180°) were obtained, respectively. For all vibration modes, the air volume variation curve follows a parabola trend with blade pitch. The largest air volume is obtained when fans operating in-phase mode (φ = 0°), a conclusion that differs from that in infinite space. We attribute this to the vortex action, and its formation is determined by two effects. The positive one is the attenuation of counteracting effect from normal forces with the increasing blade pitch, where the force was exerted by two blades onto their inner fluid. The negative one is weakening of the coupling effect of two jets formed by the blade tips when spacing of dual fans is gradually increased. In addition, the local hot spot phenomenon was observed in both in-phase and counter-phase mode. It was observed that the distance between local hot spots broadens with the increasing blade pitch for in-phase mode while remains substantially constant for counter-phase mode. Our work provides a design principle for heat transfer optimization of dual PE fans in confined volume and will benefit both theoretical understanding and application in thermal management of PE fan system. [ABSTRACT FROM AUTHOR]

Supercritical CO2 heat transfer in vertical tubes has been widely studied, but limited work exists for that in horizontal tubes. In this work, experiments were conducted for CO2 in a horizontal tube with inner diameter of 8 mm under wide range of pressures (5.51–20.41 MPa), covering both subcritical and supercritical conditions. The ranges of heat flux and mass flux were 97.5–404.0 kW/m2 and 510.2–1314.2 kg/m2s, respectively. The heat transfer for subcritical pressure far below critical pressure showed typical flow boiling characteristics, with a wall temperature peak along the tube corresponding to departure from nucleate boiling. For near-critical pressure, it showed characteristics similar to supercritical heat transfer, with continuous increase of wall temperature along the tube. The effects of different parameters including pressure, mass flux, and heat flux on the heat transfer behavior were analyzed and discussed. Moreover, using the pseudo-vapor quality concept of the supercritical three-regime-model introduced in a previous work, the heat transfer coefficient was found to behave very similarly between subcritical and supercritical conditions. The experimental results showed significant deviation from single-phase convection within the two-phase/two-phase-like region compared with the single-phase regions, and which validates the similarity between subcritical boiling and supercritical pseudo-boiling. [ABSTRACT FROM AUTHOR]

Droplet evaporation is a complex and fundamental topic that holds great scientific interest due to its relevance in numerous physical and biological processes. We systematically study laser-induced nanofluid droplet evaporation under varying light frequencies. Our findings indicate the existence of two spectral regimes where droplet evaporation is either enhanced or inhibited, which is in stark contrast to the constant regime observed under fixed laser power. The enhanced regime is attributed to the rapid heat transfer initiated by the formation of vapor microbubbles inside the droplet, causing an increase in the overall temperature of the droplet. Conversely, the inhibited regime is associated with reduced heat conduction inside the droplet resulting from localized cooling effects brought about by droplet evaporation. Correlations between heat transfer mechanisms and thermal responses at the droplet surface further support these observations. We also demonstrate that both convective and conductive heat transfers determine the critical light frequency to enhance droplet evaporation. Three light-driven flow patterns are additionally identified inside the droplet. These are photophobic, phototropic, and rolling flows, which are driven by the explosive bubble growth, surface tension gradients, and mass shifts in the droplet center, respectively. Understanding these properties is important for developing miniature evaporators, nanoparticle self-assembly, and various biomedicine applications requiring precise temperature and kinetic control. [ABSTRACT FROM AUTHOR]

This paper explores the supercritical heat transfer mechanism by characterizing the boiling contribution ratio qb/q, where qb is the boiling heat flux and q is the applied heat flux. Experiments are performed using nickel–chromium wire in 15 °C liquid carbon dioxide at 5.2, 7.6, 9.0, and 11.0 MPa. The evaporation heat flux qe is the amount of heat used for vapor generation, while qb is the heat transfer in the bulk liquid due to the disturbance of the flow/temperature field by vapor–liquid interface motion. A data processing procedure is developed to measure qb/q from the captured images. Similar trends appear for both supercritical pseudo-boiling and subcritical boiling. The evaporation-like regime at supercritical pressures reaches qb/q = 0.21–0.43, while the film boiling (evaporation) regime achieves qb/q = 0.08. In the supercritical-boiling-like regime, qb/q increases sharply from 0.19 to 0.65, whereas in the subcritical-nucleate-boiling regime, qb/q maintains a value of 0.30 followed by a rapid rise to 0.68 under a vigorous bubble merging and departing mechanism. At both subcritical and supercritical pressures, the heat transfer deteriorates in the evaporation regime, but is significantly enhanced by phase-change-induced flow/temperature field perturbations. The boiling curves differ in the two pressure domains. At supercritical pressures, natural convection transitions smoothly to the evaporation-like regime, then to the boiling-like regime. At subcritical pressures, a steep transition from natural convection to nucleate boiling occurs, and then, film boiling is induced through the action of surface tension. The above findings complete the inverse boiling curves in the two pressure domains. [ABSTRACT FROM AUTHOR]

Supercritical fluids have been widely used in a variety of applications, in which heat transfer under supercritical pressure is of great importance. This paper presents a critical review on supercritical heat transfer, including a summary on the past work focusing on in-tube heat transfer and the pseudo-boiling concept, the understanding of the mechanisms of supercritical heat transfer, and suggestions for future work. For supercritical fluids heated in tubes, the typical characteristics, the effects of various parameters, the buoyancy and flow acceleration effects, and the heat transfer deterioration phenomenon are all discussed, and the heat transfer correlations and numerical methodologies are summarized. For supercritical fluids cooled in tubes, the experimental observations and explanations, the proposed heat transfer correlations, and the numerical results are summarized and discussed. More importantly, this review provides a comprehensive review of the supercritical pseudo-boiling concept, including the experimentally observed pseudo-boiling phenomenon, the theoretical studies for supercritical fluids to reveal their heterogenous two-phase features, and the progress in the application of the pseudo-boiling concept on supercritical heat transfer. Finally, suggestions for future research are provided to further advance the understanding and enable accurate prediction of supercritical heat transfer. [ABSTRACT FROM AUTHOR]

To improve the critical heat flux (CHF) in boiling heat transfer, an idea of triggering three-dimensional mixed boiling was proposed, and the corresponding surface with an extended thin film (ETFS) was designed by adding a film structure on the plain surface (PS). With ethanol as working fluid, experiments were conducted to explore mixed boiling heat transfer and CHF enhancement mechanisms on ETFS. The results demonstrate that ETFS induces spatially uneven temperatures on the boiling surface, which triggered mixing boiling in the spatial dimension—the coexistence of nucleate and film boiling—leading to a significant CHF enhancement. Nucleate boiling in the falling liquid along the extended surface is the key to stabilizing the surface temperature during the formation of dry spots on the base surface. Compared with PS and the traditional pin–fin surface, ETFS increased CHF by approximately 110% and 45%, respectively. Analysis indicated that mutual interference between nucleate boiling and film boiling effectively inhibits heat transfer deterioration caused by the deactivation of nucleating points. Through decoupling analysis of the mutual interference process in mixed boiling, it is shown that at heights of 2.0, 4.0, 6.0, and 8.0 mm, the contributions of extended film to total heat transfer are 12%, 64%, 37%, and 24%, respectively, indicating an optimal extended film height of 4.0 mm for maximum boiling heat transfer performance. This novel boiling surface with extended film structure not only improves CHF, but also reduces design costs, offering valuable guidance for radiator design purposes. [ABSTRACT FROM AUTHOR]

The jet always sweeps to the leftmost and rightmost points in the sweeping jet and film composite cooling (SJF) process, resulting in a different coolant flow in each film hole. The film can not easily cover the outer surface evenly under the scouring of the mainstream. This work presents a case study to analyze the effects of two mainstream variables on the film areodynamic and cooling performance of the SJF. Three different mainstream velocities (Vm= 10 m/s, 50 m/s, 90 m/s) and three different mainstream turbulence intensities (Tu = 1%, 10%, 20%) are discussed. Results indicate that the increase of mainstream velocity yields to better film attachment. When the mainstream velocity increases from 10 m/s to 50 m/s, the overall cooling effectiveness and total pressure loss coefficient are reduced by 17.68% and 98.60%, respectively. When the mainstream velocity increases from 50 m/s to 90 m/s, the overall cooling effectiveness and total pressure loss coefficient are almost unchanged. The effect of turbulence intensity on the overall cooling effectiveness and total pressure loss coefficient are relatively small. The increase of mainstream turbulence intensity enhances the disturbance of the mainstream to the coolant from the middle film holes, and the distribution of adiabatic film cooling effectiveness is more uneven when the mainstream turbulence intensity is raised to 10% and 20%. In the research scope of present work, the flow structure, total pressure loss coefficient and overall cooling effectiveness are the most expected under the conditions of lower turbulence intensity and higher mainstream velocity (Tu = 1%, Vm= 90 m/s). [ABSTRACT FROM AUTHOR]

Flooding may take place for in-tube condensation, causing unstable flow and deteriorated heat transfer. Here, the phase separation principle is proposed to eliminate flooding. Comparative experiments of condensation were performed in both bare tube (BT) and modulated heat transfer tube (MHTT) with a mesh membrane tube (MMT) insert. The working fluid is water-steam under a sub-atmospheric pressure. It was observed that slug flow exists at small mass fluxes and vapor mass qualities in BT. Due to the periodic formation of liquid column over the tube cross section, flooding indeed takes place, causing unstable flow, deteriorated heat transfer, and large pressure drop. The MHTT completely eliminated flooding, converting the unstable flow into a stable flow. Heat transfer coefficients are 7.47 times of those in BT, maximally, accompanying reduced pressure drops. High-speed visualization and theoretical analysis indicated that smaller pore size provides larger capability to prevent the vapor phase penetrating the mesh screen, resulting in larger driving force for liquid suction toward the MMT inside. The MMT provided a tunnel for liquid transportation in the upward direction, which is the mechanism to eliminate flooding. The modulation of the annular flow pattern was also performed and analyzed by using the phase separation principle, and the results indicated the effectiveness of MMT in the annular flow regime. This work is benefit for applications, such as air-cooled condenser, whose performance is important to influence the whole system performance for power generation. [ABSTRACT FROM AUTHOR]

Using supercritical CO2 (S-CO2) instead of water steam as heat transfer fluid for coal-fired power plants is another way to further improve the power generation efficiency. In this paper, a multi-field coupling model that integrates fluid flow, heat transfer and thermal stress is proposed to solve the complex problem. The effects of the key operating parameters on the performances of the cooling wall are numerically investigated. More details of the characteristics of fluid dynamics, thermal stress, pressure drop, coupled heat transfer and the whole flow and temperature fields in the S-CO2 cooling wall are revealed and discussed. A comparison study is made between the cooling wall tube and waterwall tube. Both temperature and thermal deformations of cooling wall tube are much higher than that of water. Numerical simulation results indicate that inclination angle has little effects on the temperature distribution, while increase of pressure drop is obvious as the inclination angle increases. Increasing tube diameter can effectively reduce both the pressure drop and the temperature of the cooling wall tube. As a result, a new insight is introduced for the design of S-CO2 power cycle. [ABSTRACT FROM AUTHOR]

Plasmonics offer unprecedented control over light and stimulate fundamental research and engineering applications in solar energy. The surface plasmon resonance is responsible for both enhanced light scattering and absorption. The plasmon excitations in nanostructures can be tuned to control the hot-carrier emission. The damping dissipation of the kinetic energy of surface plasmons releases heat at nanoscale, which can be used to create high-performance nano-heaters and/or radiators. Spectrally and/or thermally engineered plasmonic nanomaterials attract considerable attention for solar energy application due to their distinct thermoplasmonic properties. In light of these advances, this paper provides a critical review of current research in thermoplasmonics with focus on its physic mechanisms, structure tuning strategies and solar energy applications. Basic mechanism of thermoplasmonics is described from the photothermal conversion and heat transfer physics to thermal-induced processes. Structure tuning strategies including self-tunable plasmons, plasmon coupling strategies and active plasmons with tunable gap distances are then fully discussed in terms of their principles and structures. Based on the flourishing development of novel thermoplasmonic structures, potential applications ranging from solar collector, solar radiator, thermo-photovoltaic, solar desalination and sterilization, solar degradation and catalysis, to solar fuels, and solar fertilizers are additionally highlighted. The advantages of using plasmonics over the conventional technologies are identified, and the areas where important basics involved when the thermoplasmonics bringing into application are stressed throughout the text. Finally, we provide our views on future challenges in solar thermoplasmonics, together with a few suggestions for further developments of this technology. This work would bring new insights and inspire innovative works on designing thermoplasmonics for solar energy application. [ABSTRACT FROM AUTHOR]

The internal heat transfer performance and flow structures of a sweeping jet and film composite cooling on a flat plate were numerically studied. Sweeping jet and film composite cooling consists of a fluidic oscillator and 20 cylindrical film holes; the direct jet is formed by removing the feedback from the fluidic oscillator, which is different from the traditional cylindrical nozzle. Four different mass flow rates of coolant were considered, and the inclination angle of the film hole was 30°. The Conjugate Heat Transfer method (CHT) and Unsteady Reynolds Averaged Navier Stokes equation (URANS) were employed. The results indicated that the flow resistance coefficients of the sweeping jet were larger than those of the direct jet, and the Nusselt number monotonously increased with the increase in the mass flow rate. Compared to the direct jet, the sweeping jet had a more spatially uniform heat removal rate, and the area-averaged Nusselt number was slightly lower. Therefore, the sweeping jet and film composite cooling caused the distribution of the flat plate heat transfer to be more uniform. It is worth noting that the novel direct jet nozzle in the present work had considerable area-averaged impingement cooling effectiveness. [ABSTRACT FROM AUTHOR]

Leidenfrost droplet possesses ultra-low flow resistance, but it is challenging to obtain large thrust force for fast transportation and regulate the direction of droplet motion. Here, for the first time, we demonstrate a novel mechanism for the control of droplet dynamics by explosive boiling. Our system consists of two surfaces that have different functions: a smooth surface running in the Leidenfrost state for droplet levitation and a skirt ring edge surface (SRES) as an explosive boiling trigger. For droplet-wall collision with SRES, micro/nanoscale roughness not only enhances energy harvesting from the skirt ring to the droplet due to increased radiation heat transfer but also provides nucleation sites to trigger explosive boiling. The symmetry breaking of explosive boiling creates a thrust force that is sufficient to propel the droplet. The suppression of the thrust force relative to the inertia force regulates the droplet trajectory as it passes through a target location. We show orbit lines passing through a focusing spot that is ∼1% of the Leidenfrost surface area around its center with a maximum traveling speed of ∼85 cm/s, which is ∼2 times of that reported in the literature. The scale law analysis explains the droplet size effect on the self-propelling droplet dynamics. Our work is attractive for applications under the conditions of the required traveling speed and direction of the droplet. [ABSTRACT FROM AUTHOR]

The heat transfer and pressure drop (Δ P) of supercritical fluid are crucial for the safety of advanced power cycles. Here, experiments were performed for sCO 2 in a horizontal tube with inner diameter (d in) of 10 mm, covering in the ranges of (G): 496.7–1346.2 kg/m2, heat flux (q w): 97.4～400.3 kW/m2 and pressure (P): 7.53–23.51 MPa. Inspired by the similarity of the outer wall temperature (T ow) distributions between the sCO 2 flow and those of stratified-wavy-flow in subcritical pressures, the multiphase flow theory is introduced in subcritical pressures to deal with the sCO 2 heat transfer. In the two-phase-like (TPL) regime, heat transfer coefficients (HTC) and friction factors (f) of sCO 2 are found to significantly deviate from the correlations based on the single-phase flow theory. Further, the vapor-like Froude number Fr VL is proposed to develop new correlations for both the heat transfer and the flow resistance covering the whole data range. The present paper establishes a connection between the flow resistance and heat transfer in supercritical pressures, which is important for the design and operation of sCO 2 power cycles. • Supercritical heat transfer and flow characteristics of CO 2 is investigated in a horizontal tube. • Thick vapor-like film can reduce the frequency of negative temperature differences occurring. • Flow resistance and heat transfer in the two-phase-like regime both strongly deviates from the single-phase convection. • Fr VL was proposed to be the similarity criterion number to connect flow resistance and heat transfer for horizontal flow. • The new correlations with Fr VL number for heat transfer and friction factor are established. [ABSTRACT FROM AUTHOR]

We investigate droplet evaporation, which is a natural phenomenon but the mechanism is not well understood. We are surprised to find that sunlight irradiation does not always enhance droplet evaporation, which is against the common sense that "Sun accelerates water evaporation." This is true at least for short-time evaporation. A whole droplet lifetime consists of two regimes of evaporation: a light induced deterioration regime and an acceleration regime. The deterioration regime is explained by the decreased temperature difference from the droplet bottom to apex, weakening Marangoni flow to hinder conduction heat transfer from the substrate to the droplet. The enhanced regime is explained by the reduced light energy reflection via the droplet surface. The substrate conduction heat transfer and radiation heat transfer of light are coupled to dominate evaporation. The two mechanisms create opposite contributions, resulting in a constant evaporation rate for sunlight irradiation on a droplet. However, natural light decreases the evaporation rate vs time. Hence, evaporation rates with and without sunlight irradiation cross at a specific time. Our work enhances the fundamental understanding of droplet evaporation and provides a useful guideline for efficient solar energy utilization. [ABSTRACT FROM AUTHOR]

Using the pseudo-boiling concept, supercritical heat transfer is analogized to the boiling at subcritical pressures. The proposed supercritical-boiling-number SBO represents the competition between the bubble expansion induced momentum force and the inertia force. Sudden changes between normal heat transfer and heat transfer deterioration are found when crossing critical SBO , which is experimentally determined for CO 2 , H 2 O, R134a and R22. The linear law of supercritical heat transfer is discovered. Our study supports the heterogeneous structure of supercritical fluids. • Supercritical-boiling-number SBO dominates supercritical heat transfer mechanism. • Sudden changes in heat transfer behaviors are found when crossing critical SBO. • Critical SBO values are experimentally determined for CO 2 , H 2 O, R134a and R22. • Our study supports the heterogeneous structure of supercritical fluids. Supercritical fluids such as CO 2 , water and organic fluids are frequently applied in power systems. The accurate prediction of heat transfer deterioration (HTD) is important to keep the safe operation of advanced power systems such as solar driven supercritical carbon dioxide Brayton cycle. As described in textbooks, it is impossible to identify liquid from gas beyond the critical point, thus supercritical fluid is assumed to have homogeneous structure with a single-phase. The single-phase assumption cannot explain and predict supercritical heat transfer (SHT). Instead, we investigate SHT by the pseudo-boiling concept. Heat transfer is analogized between supercritical pressure and subcritical pressure to create a new non-dimensional supercritical-boiling-number SBO , representing the bubble expansion induced momentum force against the inertia force when it is coupled with the ratio of liquid density with respect to vapor density. Our study reveals sudden changes from normal heat transfer (NHT) to heat transfer deterioration (HTD) with obvious temperature peak when crossing a critical SBO , which is 5.126 × 10−4, 2.018 × 10−4, 1.653 × 10−4 and 1.358 × 10−4 for CO 2 , H 2 O, R134a and R22, determined by a large quantity of database. Our work paves a new way to understand the SHT mechanism and supports the heterogeneous structure of liquid-like fluid and vapor-like fluid for supercritical fluids. [ABSTRACT FROM AUTHOR]

• Pseudo-boiling of supercritical fluid is investigated using Pt wire and NiCr wire. • Bubble-like interface is tracked by the grey extracting method based on image files. • Evaporation-like mode occurs for Pt wire, both evaporation-like and boiling-like modes occur for NiCr wire. • Intermittent boiling-like heat transfer is caused by the piston motion of the wire. • Continuous boiling-like heat transfer is caused by the temporal-spatial temperature variations of the wire. In this study, we identify whether evaporation-like (EL) mode and boiling-like (BL) mode occur under supercritical pressures by investigating the heat transfer of platinum (Pt) and nickel-chromium (NiCr) wires, with a length of 22.0 mm and diameter of 70 μm, in carbon dioxide liquid at 8 and 10 MPa. Using a direct photographic technique based on the light refractive principle, we analyse the brighter/darker pattern in a varied-density field. The vapor–liquid interface is quantified using the grey extraction method. Our results show that beyond the limit of natural convection (NC), only the EL mode is identified for the Pt wire. However, for the NiCr wire, the EL mode, the transition from EL to BL (TEB) mode, and the BL mode are observed. The opposing gravity and drag forces cause downward bending of the Pt wire and upward bending of the NiCr wire. In the TEB region, the varying importance of interfacial expansion velocity over the buoyancy velocity accounts for the piston motion of the NiCr wire. During falling of the wire, the lighter fluid penetrating the heavier fluid causes interfacial instability to trigger the intermittent BL heat transfer. Beyond the TEB region, continuous BL heat transfer is observed. We introduce the small-perturbation theory to achieve a new non-dimensional parameter, H 3. For the Pt wire, a smaller (either negative or positive) H 3 explains why only the EL mode appears. For the NiCr wire, the negative H 3 in the TEB region proves that the intermittent BL heat transfer is caused by the piston motion of the wire, but the positive and ∼30 times larger H 3 indicates that the continuous BL heat transfer is caused by the temporal-spatial temperature variations. Furthermore, we observe that the BL mode occurs in a vapor-like layer instead of wall cavities, which is different from subcritical boiling. The BL mode displays random and disordered features, like those of subcritical boiling. This work elaborately explores common and uncommon features between supercritical pseudo-boiling and subcritical boiling, and uncovers the secret of two modes of pseudo-boiling. [Display omitted] [ABSTRACT FROM AUTHOR]

The objective of this paper is to provide the optimal choice of single-reheating or double-reheating when considering residual flue gas heat in S-CO2 coal fired power system. The cascade utilization of flue gas energy includes three temperature levels, with high and low temperature ranges of flue gas heat extracted by S-CO2 cycle and air preheater, respectively. Two methods are proposed to absorb residual flue gas heat Qre in middle temperature range. Both methods shall decrease CO2 temperature entering the boiler T4 and increase secondary air temperature Tsec air, whose maximum value is deduced based on energy conservation in air preheater. The system is analyzed incorporating thermodynamics, boiler pressure drop and energy distribution. It is shown that at a given main vapor temperature T5, the main vapor pressure P5 can be adjusted to a value so that Qre is completely eliminated, which is called the main vapor pressure adjustment method. For this method, single-reheating is only available for higher main vapor temperatures. The power generation efficiency for single-reheating is obviously higher than double-reheating. If residual flue gas heat does exist, a flue gas heater FGC is integrated with S-CO2 cycle, which is called the FGC method. Both single-reheating and double-reheating share similar power generation efficiency, but single-reheating creates less residual flue gas heat. We conclude that single-reheating is preferable, and the pressure adjustment method achieves obviously higher power generation efficiency than the FGC method. [ABSTRACT FROM AUTHOR]

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]

Abstract The challenge of the nanofluid solar energy harvesting lies in the strong coupling between nanoparticles generated heat and flow/temperature fields in various length and time scales. In this paper, the solar-driven evaporation of nanofluid droplet is investigated. The droplet evaporation rates behave a sharply increased regime and a constant regime when crossing a critical nanofluid concentration of c i,cr = 10 ppm. We decouple the total solar energy absorption into two parts in terms of the optical bands, and the whole droplet into two spatial domains. Our study reveals a small length scale starting from contact line to behave higher temperature and larger temperature gradient there, newly recognized as a boundary layer effect. Correspondingly, a contact line region CLR is defined, while the remain is the bulk volume region BVR. The observed coffee-ring behaves a self-assembly of nanoparticles in CLR. We found that even though CLR is two magnitudes smaller compared to the whole droplet, it dominates almost all of the electromagnetic energy absorption, while BVR contributes almost all of the infrared energy absorption. Our finding successfully explains the two regimes of droplet evaporations. With increase of initial nanofluid concentrations, the droplet evaporation rates are raised due to more heat generation induced by enhanced nanoparticles deposition in CLR. The increased evaporation rates stop at a critical concentration of 10 ppm, corresponding to sufficient nanoparticles deposition. Beyond the critical condition, extra nanoparticles are hidden beneath the effective nanoparticles layers to cause unchanged evaporation rate. The present work is useful to develop novel solar steam generation device. The evaporation induced coffee-ring can be regarded as a novel "fabrication" method of nanostructure for plasmonic heating. The pure water evaporation of liquid film or droplet on a prepared coffee-ring array offers great convenience for solar energy utilization. Graphical abstract fx1 Highlights • Solar-driven evaporation of nanofluid droplet is investigated. • A critical nanofluid concentration distinguishes two regimes of droplet evaporations. • The boundary layer effect is identified near droplet contact line during evaporation. • Coffee-ring nanoparticles dominate the light energy absorption in visible region. • The two regimes of drop dynamics correspond to insufficient and extra coffee-ring nanoparticles. [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]

Abstract The objective of this paper is to develop a criterion to predict the onset of heat transfer deterioration (HTD) for supercritical CO 2 heat transfer. A new mechanism is proposed by assuming supercritical pseudo-boiling. Before bulk fluid reaches pseudo-critical temperature (T pc), the tube cross-section contains a vapor layer and a liquid-like fluid. The saturation temperature interface is defined at T = T pc , inside and outside which are a vapor layer (T > T pc) and a subcooled liquid (T < T pc). The subcritical boiling number is extended to supercritical "boiling" number, defined as SBO = q w /(Gi pc), where q w , G and i pc are heat flux, mass flux, and CO 2 enthalpy at T pc , respectively. SBO , by coupling the density ratio between "liquid" and "vapor", represents the competition between vapor expansion induced momentum force and inertia force. The experiment of supercritical CO 2 heat transfer is performed in a 10.0 mm inner diameter tube, covering ranges of P = 7.5–21.1 MPa, G = 488–1600 kg/m2s and q w = 74–413 kW/m2. Surprisingly, the onset of HTD is found to occur at a critical SBO which is 5.126 × 10 − 4 and the critical heat flux is expressed as q C H F = 5.126 × 10 − 4 G i pc . It is shown that our new criterion is also suitable for other experiments reported in literature, providing a general guidance to design and operate S CO 2 heaters to avoid HTD. Graphical abstract Image 1 Highlights • Similarity principle deals with heat transfer deterioration at supercritical pressure. • The analysis results in a new supercritical "boiling" number SBO. • SBO comprehensively considers the effects of a set of variables. • CO 2 heat transfer experiment is performed over wide parameter ranges. • The critical SBO of 5.126 × 10−4 distinguishes two regimes of heat transfer. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Molecular dynamics simulation was performed in nano-Janus-interface system. • Temperature gradients can be steepened by lowering hot wall wettabilities. • Heat transfer is switched by varying wall wettabilities and applied forces to liquid. • The finding helps to develop temperature sensitive nano-energy-devices. Abstract Janus-interface-system is referred as liquid confined between hydrophilic and hydrophobic surfaces, nanoscale energy transfer in Janus-interface-system is not understood. Here, we investigate Poiseuille flow by non-equilibrium molecular dynamics (NEMD) using argon as the working fluid. The two solid walls hold different temperatures and wettabilities. It is found that for pure heat conduction, temperature jumps are negative on hot wall but positive on cold wall to create positive heat flow from hot wall to cold wall (called positive heat transfer). However, the convective energy transfer in Janus-interface-system always behaves positive temperature jumps on the two walls due to viscous heating. We show that, lowering hot wall wettabilities creates more significant velocity and temperature slippages on hot wall than those on cold wall to steepen liquid temperature gradients in nanochannel. We further show that heat flow sign can be switched between positive and negative, by (1) keeping super-hydrophilic hot wall but changing cold wall wettabilities, (2) keeping super-hydrophilic-hot-wall/hydrophobic-cold-wall but varying external forces applied to liquid, and (3) keeping super-hydrophilic-hot-wall/hydrophobic-cold-wall but varying hot wall temperatures. All the three cases yield non-symmetry velocity profile and more sensitive changes of temperature jumps on cold wall than those on hot wall for heat transfer switch. The transition between positive and negative heat transfer occurs at the zero temperature gradient in the channel. The findings not only enhance the understating of nanoscale energy transfer in Janus-interface-system, but also provide novel working principle for nano-devices behaving temperature sensitive nature. [ABSTRACT FROM AUTHOR]

For coal fired power plant, the supercritical carbon dioxide Brayton cycle (S-CO 2 ) is difficult to absorb flue gas heat in a wide temperature range of 120–1500 °C. Here, novel methods are developed to cascade utilize flue gas heat, in which energies in high, moderate and low temperature levels are extracted by top cycle, bottom cycle or flue gas cooler (FGC), and air preheater, respectively. The cascade utilization shall satisfy the criterion that CO 2 temperature entering boiler for top cycle equals to CO 2 temperature leaving boiler for bottom cycle . The separate-top-bottom-cycle (STB) is proposed, in which no any component is shared by top and bottom cycles. Six possible bottom cycles are studied. The thermodynamics analysis is coupled with heat transfer and pressure drop analysis for whole power plant. It is found that the main vapor pressure of bottom cycle can be the “best” parameter to be adjusted over a wide range of 15–35 MPa to couple and optimize top and bottom cycles. Then, the parameter coordination principle is proposed to share specific components for top and bottom cycles. Thus, the separate cycles are converted into a connected cycle to simplify the whole system layout. The connected cycle has a power generation efficiency of 51.82% at main vapor parameters of 700 °C/35 MPa, significantly higher than available supercritical water-steam Rankine cycle power plant. The findings in this paper give a clue to further raise the power generation efficiency for large scale S-CO 2 coal fired power plant. [ABSTRACT FROM AUTHOR]

When supercritical carbon dioxide (S CO 2 ) Brayton cycle is used for coal fired power plant, the significantly increased flow rate causes extremely large boiler pressure drops, and residual flue gas energy extraction becomes difficult. This paper contains two consecutive parts to resolve these issues. The first part deals with general analysis. Results show that, introducing intercooling and/or reheating into cycle apparently elevate thermal efficiencies, but cycle performance is obviously deteriorated by large pressure drops. Partial flow strategy was proposed in part 2 to yield boiler module design. Both flow rate and length for each module are cut to be half, reducing pressure drop to 1/8 of that with total flow mode. Surprisingly, we show that CO 2 boiler pressure drop can be equivalent to or even smaller than that for supercritical water-steam boiler. Three flue gas energy extraction schemes are proposed. The case A scheme not only keeps lower exit flue gas temperature (∼120 °C), but also maintains acceptable secondary air temperature. Finally, a 1000 MWe S CO 2 power plant design is given. With main vapor parameters 620 °C/30 MPa, thermal efficiency and power efficiency are 51.22% and 48.37% respectively, showing advantages over supercritical water-steam Rankine cycle. Future works are recommended on S CO 2 power plant design. [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]

In the framework of modern thermodynamics, “thermodynamics flow” and “thermodynamics force” are introduced to develop a real thermal efficiency expression for heat engine, which receives heat from a heat source and dissipates heat to environment to yield a work output, for the first time. Enclosed area of T-Q curves of a counter-current heat exchanger is the dissipation for heat to power conversion, representing loss of thermal energy quality. The relationship between heat load and dissipation for heat to power conversion is quantified. Such connection is written for both heating and cooling processes. Linking the thermal couplings between heating and cooling processes yields the thermal efficiency expressed as η real = C η Carnot , where C = 1 η Carnot - R 1 - η Carnot , η Carnot is the Carnot efficiency, R = R h / R c is the ratio of resistance in heating process R h divided by that in cooling process R c . The thermal efficiency theory tells us that no matter how complex a heat engine is, the engine should reach a lower resistance ratio of heating process with respect to cooling process to raise its thermal efficiency. The guidelines for design and operation of general heat engines are provided. A link between heat transfer and thermodynamics is presented in this work. As an application example, the effect of critical temperatures of organic fluids on the performance of Organic Rankine Cycles is successfully explained by the newly developed theory. [ABSTRACT FROM AUTHOR]

Supercritical fluids (SFs) are widely used in power generation systems using nuclear, solar, geothermal, and fossil energies. The heat transfer deterioration (HTD) phenomenon during heat transfer of SFs is of great importance to the design and operation of the components in supercritical power cycles. The criteria for determining the onset of HTD has been reported in the literature, but the quantitative model to obtain the position of the wall temperature peak during HTD occurrence, and the correlation to accurately predict the heat transfer characteristics at the wall temperature peak, are both missing. In this work, to determine the position and to predict the characteristics of HTD at the wall temperature peak, we developed two correlations based on experimental results for supercritical CO 2 (sCO 2) in vertical upward tubes with wide range of experimental parameters. The first correlation to predict the location of the peak is developed using experimental data for sCO 2 , which can accurately determine the dimensionless bulk fluid temperature corresponding to the wall temperature peak. Moreover, a new heat transfer correlation is developed for the Nusselt number at the wall temperature peak, which has a simple form of N u H T D = 0.00796 R e b 0.7215 P r b − 0.1568 K − 0.115 , where Re b , Pr b , and K are Reynolds number, Prandtl number, and supercritical K number, respectively. This correlation shows smaller prediction error compared with five existing correlations in the literature for not only sCO 2 , but also various other working fluids. It also outperforms the recently proposed K number correlation in predicting characteristics at the wall temperature peak during HTD conditions. The two correlations developed in this work provides the quantitative models for the HTD phenomenon, and can benefit the engineering application of supercritical power cycles. • SCO 2 heat transfer deterioration (HTD) was experimentally studied coving wide parameter ranges. • The HTD phenomenon was quantitatively analyzed based on experimental databases. • New methods were proposed to predict the location and characteristics of wall temperature overshoot. • The new heat transfer correlation for HTD has significantly improved prediction accuracy. [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]

The proposed gradient–porous-wall microchannels consist of bare channels and pin–fin array regions, fabricated by MEMS (microelectricalmechanicalsystem) technique. Boiling experiments were performed with acetone as the working fluid. Ultra-stable wall temperatures are achieved with oscillation amplitudes in the range of 0.02–0.18 °C. Bubble nucleation is found to happen in the porous wall. The generated vapor flows towards bare channels due to surface tension driving flow. The vapor ejection direction is periodically switched between neighboring channels, called the “bubble emission switch”. The bubble confinement ratio is newly defined. Bubbles become fat and slim in bare channels to generate high frequency “eye-blinking oscillation”. Bubble confinement ratios display sine function, and out-of-phase characteristic between neighboring channels. We confirm the “eye-blinking” oscillation as a density wave oscillation, propagating in the channel width direction. Because the porous-wall width is much smaller than the channel length, the “eye-blinking” frequencies are 10–100 times higher than that of the axially propagated density wave oscillation. The “integration parameter model” establishes the connection between “eye-blinking” oscillation and wall temperatures. The convective heat transfer intensity in bare channels is assumed to follow the bubble confinement ratio variation. The wall temperature oscillation amplitude is inversely proportional to the “eye-blinking” frequency. The phase angle between bubble confinement ratios and wall temperatures are 3 π /2, being the negative feedback mechanism to inhibit wall temperature oscillations. The porous-wall microchannels open a new way to eliminate flow instabilities for heat exchangers and thermal energy systems. [ABSTRACT FROM AUTHOR]

Supercritical heat transfer is important to design and operate advanced power generation system. Non-uniform heating is frequently encountered such as for sCO 2 boiler and solar receiver applications. Here, 3D numerical simulations are performed in turbulent regime, coupling solid wall and sCO 2 in tubes. Non-uniform heating is simulated by applying heat via a thin metal layer at the outer tube wall, covering half circumference of tube wall. Calculation results matched experimental data well. Heat transfer is compared between uniform heating and non-uniform heating. Numerical results are re-processed by the pseudo-phase transition theory, including a liquid-like region in tube core and a gas-like layer near the tube wall. The two layers of structure are interfaced by a terminating temperature T + which is determined by thermodynamics. Hence, supercritical heat transfer is evaluated based on the relative importance of thermal resistances induced by the liquid-like region and the gas-like film region. It is found that wall temperatures for uniform heating are higher than those for non-uniform heating, which is explained by larger thermal resistance of the gas-like film for uniform heating than that for non-uniform heating. This difference ensures better heat transfer for non-uniform heating than uniform heating. We conclude that in turbulent regime, the gas-like film dominates the magnitudes of wall temperature and the occurrence of heat transfer deterioration for both uniform and non-uniform heating. Our work is important for heat transfer facilities in supercritical domain, especially under non-uniform heating condition. • Heat transfer for sCO 2 in vertical tubes are comprehensively analyzed. • The heat transfer phenomenon for sCO 2 is analogous to the subcritical film heat transfer. • The thermal resistance mechanism is proposed to evaluate sCO 2 heat transfer. • Gas-like film is identified as the key factor affecting wall temperature, and even the HTD occurrence. [ABSTRACT FROM AUTHOR]

Dropwise condensation has much higher heat transfer efficiency than the filmwise condensation, hence it will be widely applied in power generation, waste heat recovery, refrigeration etc. The spontaneous coalescence-induced droplet jumping on superhydrophobic surfaces was an effective way to maintain highly efficient dropwise condensation. In our study, the coalescence-induced droplet jumping process was numerically studied through transient three-dimensional diffuse interface method. The effect of contact angle and its hysteresis, droplet size, viscosity and gravity on the jumping process was studied in details, energy conversions were also provided among the droplet surface energy, viscous dissipation, kinetic energy and gravitational energy. Our study showed that contact angle had negative effect on the jumping velocity, with the decreasing contact angle, both peak jumping velocities and radius range for droplet jumping would be reduced. For the capillary-inertial process the normalized droplet jumping velocity was at the order of 0.2. Although more than half of the released surface energy could be converted into the kinetic energy, only less than 10% of the released surface energy could be converted into the translational kinetic energy for droplet jumping. The contact angle hysteresis had significant effect on the droplet jumping, the larger advancing contact angle could help improve the droplet jumping velocity, while the lower receding contact angle could reduce the droplet jumping velocity. Our results might provide useful guideline for design of efficient dropwise condensation heat transfer through coalescence-induced droplet jumping on superhydrophobic surfaces. [ABSTRACT FROM AUTHOR]

The integration temperature difference Δ T i considers the heat transfer routes, linking the heat transfer process with the thermodynamic behavior of heat exchangers. The first and second non-dimensional integration temperature differences are defined as Δ T i, h ∗ = Δ T i / T h, i and Δ T i, s ∗ = Δ T i / ( T h, i - T 0 ) respectively, where T h,i is the heat source temperature and T 0 is the environment temperature. This paper is the first to experimentally verify the significance of the non-dimensional integration temperature differences on organic Rankine cycle (ORC) systems. The first non-dimensional temperature difference is shown to have linear relationship with the revised entropy generation numbers ( N s ). With increases of the second non-dimensional integration temperature difference, the expander powers, system thermal and exergy efficiencies had parabola distributions. They simultaneously reached maximum at Δ T i, s ∗ = 0.282 , under which the vapor cavitation in the expander disappears and the exergy losses of heat exchangers are acceptable to elevate the expander efficiency. Beyond the optimal point, the ORC performance is worsened either due to the vapor cavitation in the expander, or due to the poor thermal matches in the evaporator and condenser. The second non-dimensional integration temperature difference comprehensively reflects the effects of heat source temperatures, heating powers and organic fluid flow rates and pressures, etc. It balances exergy destructions of various components to optimize the system. Thus, it can be an important parameter index to maximize the power or electricity output for a specific heat source. The usefulness of the integration temperature difference and the future work are discussed in the end of this paper. [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]

An ORC (organic Rankine cycle) was developed with R123 as the working fluid. The heat capacity is in ∼100 kW. The match between pump and expander is investigated. Lower pump frequencies ( f < 10 Hz) adapt the whole range of expander torques, yielding stable flow. Higher pump frequencies ( f > 10 Hz) adapt low expander torques only, and cause unstable flow and pump cavitation for larger expander torques. Ultra-low expander torques generate sufficiently high vapor superheatings to decrease expander efficiencies. Ultra-high expander torques achieve saturation vapor at the expander inlet, causing liquid droplets induced shock wave to worsen expander performance. An optimal range of expander torques exists to have better expander performance. A liquid subcooling of 20 °C is necessary to avoid pump cavitation. Expander powers and efficiencies show parabola shapes versus expander torques, or vapor superheatings at the expander inlet. The optimal vapor superheating is 13 °C. The cavitation mechanisms and measures to avoid cavitation are analyzed. This paper notes the overestimation of ORC performance by equilibrium thermodynamic analysis. Assumptions should be dependent on experiments. Future studies are suggested on organic fluid flow, heat transfer and energy conversion in various components. [ABSTRACT FROM AUTHOR]

This paper creates new method that uses microflows through mesh pores to modulate flow and temperature fields. Modulated heat transfer tube (MHTT) was constructed by suspending consecutive conical-mesh inserts in a tube. Because there are too many 3D mesh pores (10–100 μm size) for a conical mesh insert, numerical simulations in laminar flow regime were performed by an 3D to 2D conversion of mesh pores applying equal equivalent diameter criterion and total flow area criterion of mesh pores. The multiscale grid generation linked micron scale of mesh pores and macroscale of the tube. Covering the present data ranges, MHTT had Nusselt numbers which are 1.4–4.1 times of that in a bare tube, PEC (performance evaluation criterion) was up to 2.2, demonstrating excellent heat transfer enhancement at low flow rate pumping cost. The perfect MHTT performance comes from the distinct flow field: an attached hydraulic boundary layer having large velocity and its gradient near the tube wall, a weak circulating flow region upstream of the mesh insert and a weakly positive flow region downstream of the mesh insert. A thin thermal boundary layer on the tube wall was reached to enhance heat transfer. The periodic unit length S and diameter of the conical mesh insert δ were optimized. The varied slopes of Nu versus Re were found and explained while increasing Re . There are best matches of PPI (pores per inch) and Re (Reynolds number). Low PPI mesh insert was suggested at low Re , high PPI mesh insert deformed streamlines to deviate from the perfect flow and temperature fields. Meanwhile, high PPI mesh insert had better performance at high Re . Low PPI mesh insert had good performance, which can be further improved by raising PPI . Because metallic mesh screen is commercialized and cheap, MHTT has wide potential engineering applications. [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]

Boiling/evaporation heat transfer in a microchannel with pin fin structure was performed with water as the working fluid. Simultaneous measurements of various parameters were performed. The chip wall temperatures were measured by a high spatial-time resolution IR image system, having a sensitivity of 0.02 °C. The flow pattern variations synchronously changed wall temperatures due to ultra-small Bi number. The wavelet decomposition method successfully identified the noise signal and decoupled various temperature oscillations with different amplitudes and frequencies. Three types of temperature oscillations were identified according to heat flux q and mass flux G . The first type of oscillation occurred at q / G < 0.62 kJ/kg. The approximation coefficient of wavelet decomposition decided the dominant cycle period which was ∼3 times of the fluid residence time in the microchannel, behaving the density wave oscillation characteristic. The detail coefficients of wavelet decomposition decided the dominant cycle period, which matched the flow pattern transition determined value well, representing the flow pattern transition induced oscillation. For the second type of oscillation, the wavelet decomposition decoupled the three oscillation mechanisms. The pressure drop oscillation caused the temperature oscillation amplitudes of 5–10 °C and cycle periods of 10–15 s. The density wave oscillation and flow pattern transition induced oscillation are embedded with both the pressure rise and decrease stages of the pressure drop oscillation. The third type of oscillation happened at q / G > 1.13 kJ/kg, having the density wave oscillation coupled with the varied liquid film evaporation induced oscillation. The liquid island, retention bubble induced nucleation sites and cone-shape two-phase developing region are unique features of microchannel boiling with pin fin structure. This study illustrated that pressure drop oscillation and density wave oscillation, usually happened in large size channels, also take place in microchannels. The flow pattern transition and varied liquid film evaporation induced oscillations are specific to microchannel boiling/evaporation flow. [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]

Renewable energy or low grade energy utilizations need high performance of phase change heat exchangers. Suspending micromembrane cylinder in tube (called modulated heat transfer tube) increases the void fractions or vapor qualities near the tube wall to significantly enhance heat transfer. The R123 condensation heat transfer with horizontal position was investigated. The results for the bare tube and modulated heat transfer tube were tested case by case. It is found that the modulated heat transfer tube significantly enhance the condensation heat transfer, with the heat transfer enhancement factors (EF) covering the range of 1.118-2.124. The comprehensive performance evaluation criteria (PEC) had the range of 0.71-1.66, with most of runs behaving PEC larger than 1.0. For the two-phase outlet conditions, the EF values are increased with increases in the vapor mass fluxes, Gx, where G is the mass flux and x is the inlet vapor mass quality. The EF values are inversely related to Gx for the subcooled liquid outlet cases. The enhanced heat transfer mechanisms are analyzed with the observed images together with the discussion of pressure and velocity distributions. [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]

The available studies indicate the significantly enhanced pool boiling heat transfer by the modulated porous wick sintered on the heater wall. This paper sinters the modulated porous wick as the primary wick on the evaporator wall for loop heat pipes (LHPs). Three types of evaporators were fabricated to integrate with other LHP components: MWE (microchannel/wick evaporator), MME (modulated monoporous wick evaporator) and MBE (modulated biporous wick evaporator). Experiments were performed with water as the working fluid at various tilt angles. The major findings are (1) MBE LHP significantly shortens start-up time and stabilizes all temperatures during the steady operation; (2) MBE LHP decreased the evaporator wall temperatures by 20∼50°C at moderate or high heat loads compared with MWE LHP at similar conditions; (3) MBE LHP achieved the evaporator wall temperature of 63°C at the heat load of 200W for the anti-gravity operation, under which the heat flux attained 40W/cm2, which is 1.7∼6.7 times of those reported in references; (4) MBE behaves the nucleate boiling heat transfer at small head loads, and film evaporation heat transfer at moderate or large heat loads; (5) A properly designed MBE LHP achieved better performance when the evaporator is above the condenser; (6) Effect of liquid charge ratios was studied with the best liquid charge ratio of 51.3%. The best geometric parameters for porous stacks and vapor channels, as well as the best particle size were obtained. The MBE multiscale behavior majorly accounts for the performance improvement: small pores (∼μm scale) creating great capillary force for liquid suction, large pores (∼10μm scale) between clusters increasing surface area for liquid film evaporation, and vapor channels (∼mm scale) for vapor venting. Besides, small contact thermal resistance and reduced heat flow path in the evaporator also improve the LHP performance. [ABSTRACT FROM AUTHOR]

Abstract: The flow field modulation concept was proposed. A mesh cylinder was suspended in a tube, dividing the tube cross section into an annular region and a core region. The invention is called the modulated heat transfer tube (MHTT). The numerical simulation was performed in laminar flow regime at constant heat flux boundary condition with water as the working fluid. An equal flow area criterion was proposed for the conversion of 3D to 2D mesh pores. The non-uniform grids link micron scale of mesh pores with meter scale of tube. The results show double-peak velocity distribution over tube cross section. The near wall region has larger velocity and velocity gradient, accounting for the heat transfer enhancement mechanism. For any specific Reynolds number, there is a critical length beyond which heat transfer is deteriorated. Therefore, a set of short mesh cylinders was suspended in the tube. The configuration is called the improved modulated heat transfer tube (IMHTT). It is shown that the IMHTT ensures significant heat transfer enhancement over the whole tube length. This study provides a new enhancement mechanism and tube configuration for heat transfer, having a wide engineering application potential. [Copyright &y& Elsevier]

Highlights: [•] A phase separation concept was proposed to modulate flow patterns for vertical upflows. [•] The tube was divided into an annular region and a core region. [•] The gas bubbles flow in the annular region and liquid majorly flows in the core region. [•] Thin liquid film and pulsating flow have the potential to enhance the heat transfer. [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 multiphase heat transfer could be enhanced by creating thin liquid film on the wall. The phase separation concept is called due to the separated flow paths of liquid and gas over the tube cross section to yield thin liquid film. Our proposed heat transfer tube consists of an annular region close to the wall and a core region, interfaced by a suspending mesh cylinder in the tube. The heat transfer tube is a multiscale system with micron scale of mesh pores, miniature scale of annular region and macroscale of tube diameter and length. Great effort has been made to link from micron scale to macroscale. The Volume of Fluid (VOF) method simulates air/water two-phase flow for vertical upflow. The three-dimensional system was successfully converted to a two-dimensional one by using three equivalent criteria for mesh pores. The non-uniform base grid generation and dynamic grid adaption method capture the bubble interface. The numerical results successfully reproduce our experimental results. The numerical findings identify the following mechanisms for the enhanced heat transfer: (a) counter-current flow exists with upward flow in the annular region and downward flow in the core region; (b) void fractions are exact zero in the core region and higher in the annular region; (c) the liquid film thicknesses are decreased to 1/6–1/3 of those in the bare tube section; (d) the gas–liquid mixture travels much faster in the annular region than in the bare tube; (e) three-levels of liquid circulation exists: meter-scale bulk liquid circulation, moderate-scale liquid circulation around a single-elongated-ring-slug-bubble, and microliquid circulation following the ring-slug-bubble tails. These liquid circulations promote the fluid mixing over the whole tube length and within the radial direction. The modulated parameters of void fractions, velocities and liquid film thicknesses in the annular region and three-levels of liquid circulation are greatly beneficial for the multiphase heat transfer enhancement. [ABSTRACT FROM AUTHOR]