Vapor Pressure Gradient Research Articles

Ultra-high freshwater production in multistage solar membrane distillation via waste heat injection to condenser

Passive solar membrane distillation (MD) is an emerging technology to alleviate water scarcity. Recently, its performance has been enhanced by multistage design, though the gains are marginal due to constrained temperature and vapor pressure gradients across the device. This makes condenser cooling enhancement a questionable choice. We argue that condenser heating could suppress the marginal effect of multistage solar MD by unlocking the moisture transport limit in all distillation stages. Here, we propose a stage temperature boosting (STB) concept that directs low-temperature heat to the condensers in the last stages, enhancing moisture transport across all stages. Through STB in the last two stages with a heat flux of 250 W m−2, a stage-averaged distillation flux of 1.13 L m−2 h−1 S−1 was demonstrated using an 8-stage MD device under one-sun illumination. This represents an 88% enhancement over the state-of-the-art 10-stage solar MD devices. More notably, our analysis indicates that 16-stage STB-MD devices driven by solar energy and waste heat can effectively compete with existing photovoltaic reverse osmosis (PV-RO) systems, potentially elevating freshwater production with low-temperature heat sources.

A practical method for the in-situ estimation of the effective thermal resistance of gas diffusion layers in PEMFC

The membrane electrode assembly of a PEM fuel cell is hotter than the gas feeding plates because heat is generated within it, and the diffusion layers are thermally resistive. The temperature gradient created, which increases as a function of the current density, is useful for evacuating the water produced in vapor form. This water transport mechanism is known as the “heat pipe effect”. The temperature gradient produces a saturation vapour pressure gradient, which in turn produces a diffusive vapor flow. Mastering water management, coupled with heat management, requires knowledge of the temperature of the MEA and therefore the effective thermal resistance of the diffusion layers. In this paper, we present an original method for estimating this thermal resistance in situ, without using heat flux sensors or directly measuring the temperature of the MEA. The method requires the ability to impose a temperature gradient between the anode and cathode plates, and to perform a hot-side water balance. Thermal resistance is deduced from the coupling between heat and water transfer. Two diffusion layers widely used by the community are tested. The GDL Sigracet 22 BB (from SGL Carbon) has a lower effective thermal resistance than the GDL Freudenberg H15C14. Variations in these thermal resistances are measured as a function of the temperature gradient imposed and of the compression ratio.

Mass transfer fundamentals in pervaporation, perstraction and sorption: A unified approach

The high affinity between an organic solute and a polymeric sorbent or membrane is the basis for separating organic contaminants from water via sorption, pervaporation and perstraction. These processes all share the same features of dissolution and diffusion, and thus their mass transfer characteristics can be correlated. However, they are often dealt with independently, and the research findings from one process are rarely applied to the other. The permeability coefficient in perstraction (for liquid permeation) based on concentration gradient as the driving force for mass transfer and the permeability coefficient for pervaporative transport expressed customarily in analog to vapor permeation can hardly be compared directly, and sometimes seemingly contradictive trends in temperature dependencies of the permeability coefficients for the two process modes may result. Looking into the mass transfer fundamentals pertaining to sorption, pervaporation and perstraction, this work attempted to provide a unified approach to the mass transfer in all these processes. While the permeability coefficient in pervaporation uses equivalent vapor pressure gradient across the membrane as the driving force, this work provided a generalized treatment by taking into account the sorption constants embedded in the permeability coefficients of the different processes so that the performance parameter for one process could be applied to another, which would be especially useful for screening and developing appropriate membrane/sorbent materials. In addition, the relationships among the selectivities of these separation processes were elucidated, which allowed for an intuitive illustration of how the process selectivity changed due to the process mode. Aniline removal from water via pervaporation, perstraction and sorption using a poly(ether-b-amide) membrane/sorbent was used as an example to illustrate and validate the approach.

Scaling up of nanophotonics-enabled solar direct-contact membrane desalination system under ambient condition and sun tracking: Challenges, remedies and performance report

A direct-contact membrane desalination system is fabricated and tested. A hydrophobic PVDF membrane, coated with carbon black nano-particles, is placed between feed and permeate channels. Localized heating through absorption of solar heat by the nano-particle layer produces water vapour, which gets driven through the porous membrane by the vapour pressure gradient maintained by the temperature differential between the channels. Non-requirement of bulk feed heating enhances the photothermal efficiency of nano-particle enabled systems. It is also observed to have advantages such as low scaling and easy cleaning. The major drawback in scaling up conventional membrane desalination system is gradual loss of temperature gradient with length. Here, it is observed that for significantly longer length, temperature gradient is not reduced to a low value to set a practical limit for the system size. 99.5 % salt removal is achieved while the range of flux is 0.14–0.28 kg/m2hr under solar irradiance of 331–839 W/m2 and ambient temperature condition. Two-axis tracking is observed to produce an augmentation of flux by 23.3 %. Numerous challenges, as faced during experiments, are reported in detail along with the corresponding fixes. Thus, the present study provides a technology roadmap for scaling up a nanophotonics-enabled solar membrane desalination system.

Model moisture transport in oil‐paper insulation of transformer: Theory and experiment

AbstractThe insulation performance of oil paper insulation is significantly affected by moisture such that monitoring moisture content is important. However, it is difficult to obtain the moisture content accurately due to its dynamic change under multi physical fields. Studying the moisture transportation in oil paper insulation under multi physical fields becomes a vital method in solving the problem. A multi physical model describing moisture migration was proposed, which considered the effects of temperature on moisture in different existing states in oil‐immersed paper (OIP). The temperature distribution formed a vapour pressure gradient to drive moisture migration and affected migration speed. Then, experiments and simulations of moisture migration were performed, which showed that the experiments were in good agreement with simulation. The results revealed that the temperature gradient caused uneven moisture distribution and the increased temperature reduced OIP moisture content. The established model could fully characterise moisture migration under temperature gradient, which provided a theoretical reference for predicting the risk of partial dampness and evaluating insulation performance.

Scalable In‐Plane Directional Crystallization for The Printable Hole‐Conductor‐Free Perovskite Solar Cell Based on The Carbon Electrode

AbstractPopular solution‐processed approaches for producing the active layer of perovskite solar cells (PSCs) generally have to make compromise between crystallinity and compactness by inducing a rapid crystallization process with explosive nucleation and limited growth via removing solvent quickly. Here, a practical growth‐dominated in‐plane directional crystallization technique (IPDC) with a deeply retarded crystallization process for the scalable preparation of PSCs are reported. During the low‐temperature annealing, a tiny chamber with a small height is built atop the wet perovskite precursor film to restrain the vertical diffusion and removal of solvent vapor. The chamber eliminates the vertical solvent vapor gradient and induce a horizonal in‐plane gradient of solvent vapor pressure (SVP) toward the preset exhaust port which allows the slow escape of solvent vapor to outer space. In this way, nucleation is induced preferentially near the port and the as‐formed heterogeneous nuclei then grow continuously and directionally. With IPDC, sufficient filling of perovskite with high crystallinity and obvious growth orientation is realized in non‐ordered mesoporous scaffolds. An encouraging power conversion efficiency of 19.35% and 16.53% is achieved respectively for the 0.1 and 52.3‐cm2 printable mesoscopic hole‐conductor‐free PSCs with carbon electrodes.

Micrometeorological Comparison of Canopy Temperature between Two Wheat Cultivars Grown under Irrigation in a Hot Environment in Sudan

A thorough exploration of the micrometeorological aspects influencing canopy temperature in contrasting wheat cultivars can unveil the specific mechanisms of adaptation to heat stress. However, information on wheat microclimates in hot environments for crop improvement is lacking. Here, we used a micrometeorological method to investigate wheat’s response to high temperatures. Field experiments were conducted in the Gezira Scheme, Sudan, to compare two high-yielding heat-tolerant cultivars, Imam and Bohaine, in terms of canopy temperature depression (CTD), air temperature gradient (ATG), and vapor pressure gradient (VPG) from a 2 m height to canopy level. The maximum air temperature at 2 m during the main growing season was 37 °C. Air temperature at canopy level was mostly lower in the Imam field than in the Bohaine field, and it was positively correlated with and higher than radiometric canopy surface temperature. The maximum CTD during the reproductive stage was 4.7–6.5 °C in the Bohaine field and 5.0–7.2 °C in the Imam field. ATG was also larger in the Imam field, attributed to the greater leaf area of the Imam canopy, as presumed from the NDVI difference between fields. ATG was negatively correlated with VPG in both fields, and the relationship was stronger at lower nighttime wind speeds and weaker at higher daytime wind speeds. These results indicate that the micrometeorological approach can be used to compare cultivars in high-temperature environments.

COUPLED HEAT AND MASS TRANSFER PROBLEM WITH DEPENDENT HEAT CONDUCTIVITY PROPERTIES AND ITS SEMI-ANALYTICAL SOLUTION

Present paper proposes a computation method for a coupled heat and mass transfer problem within the porous media where the heat conductivity properties of the media undergo changes caused by the mass transfer. Heat and mass transfer processes are coupled through the inclusion of evaporation and condensation phenomena which in turn require the solution to an another the vapour transfer problem. This complex coupled problem re-sults in a system of both linear and non-linear second order partial differential equations that are spatially discre-tized by Finite Element Method. Temporal integration is carried out analytically. Thus, the proposed system of equations covering linear vapor transfer problem, linear filtration problem and non-linear heat transfer problem is transformed into a system of both linear and non-linear first order ordinary differential equations being solved by semi-analytical method. Picard approach of successive iterations is used for linearization of the equations. Convergent solution is achieved which is demonstrated on a sample problem herein below. Proposed method gives good insight on the processes taking place within the structures being subjected to temperature and vapor pressure gradients, including the residual effects of moisture accumulation, and assesses its impact on heat con-ductivity of materials that the structures consist of. Present study is a part of more extensive research on the ap-plication of semi-analytical methods in heat transfer problems, therefore it is not exhaustive and complete. Shortcomings of the method and its possible work-arounds as well as the topics for further studies are discussed in Conclusions and Further Studies section.

Numerical and experimental investigation of the melt removal mechanism and burr formation during laser cutting of metals

During laser fusion cutting, burr forms when the molten metal does not sufficiently exit the interaction zone. When it forms on the lower edge of the cut flank, burr becomes a factor limiting quality. Previous research has shown that a temporally regular and spatially localized melt flow can prevent the formation of burr. However, the high dynamics of the subprocesses involved can cause intrinsic instabilities that disrupt the flow and reduce the efficiency of the melt ejection. This paper presents a study on the correlation between process parameters, melt flow properties, and burr formation. It includes an experimental observation of the melt-flow dynamics using high-speed videography. In addition, a Computational Fluid Dynamics model was set up to examine fundamental flow properties, some of which are not observable experimentally. The dependency of the burr formation on the liquid Weber and Reynolds numbers is analyzed, and it is demonstrated how the magnitude and allocation of vapor pressure gradients in the kerf decisively affect melt ejection and burr formation. Additionally, a previously unknown melt ejection regime is identified in the thick section range, which occurs at feed rates close to the maximum cutting speed under specific high-power process conditions. This regime is characterized by a significantly increased process efficiency that could open up a new high-speed process window.

The Effect of Operating Parameters on Total Cross-membrane Flux in a PVDF Flat Sheet Membrane

Membrane distillation (MD) is an emerging thermal membrane technology that involves water vapor driven by a vapor pressure gradient over a hydrophobic membrane. MD faces several challenges, one of which is the flux of water vapor. The total cross-membrane flux in membrane distillation was investigated in this paper using the co-current PVDF flat sheet for direct contact. membrane distillation applications. The goal of this research is to improve total cross-membrane flux. The effect of various operational parameters is studied, including feed inlet temperature (333.15–358.15 K), feed flow rate (1–2.5 kg/s), permeate inlet temperature (288.15–313.15 K), and feed inlet NaCl concentration (0.035 to 0.485 kg/kg). To acquire a good value of total cross-membrane flux, their interactions with the total cross-membrane flux are studied in this work. The obtained results were computed during MATLAB-Simulations under several scenarios adopting the Trial-&-Error approach. This last inputs various parameters' values and thus draws the required curves to be discussed and analyzed. The results indicated that the PVDF flat sheet membranes provide a significantly higher total cross-membrane flux at higher feed input temperatures, producing a 73.2075 kg/(m2 h) at a feed inlet temperature of 358.15 K, a permeate inlet temperature of 293.15 K, and a flow rate of 2.5 kg/s, with a feed inlet NaCl concentration of 0.035 kg/kg. Feed inlet temperature significantly affected the total flux through the membrane; however, flow rate, permeate inlet temperature, and feed inlet NaCl concentration had a less significant effect.

Experimental Study on Gaseous Moisture Absorption and Swelling of Red-Bed Mudstone in Central Sichuan, China under Different Relative Humidity Environments

Aiming to resolve the engineering hazards caused by the long-term micro-swelling of red-bed mudstone, gaseous moisture absorption-swelling tests under four ambient humidities were conducted (60%, 70%, 85% and 99%) by developing a set of experimental devices. Based on the moisture absorption characteristics of mudstone, the effects of a vapor pressure gradient on the equilibrium time, moisture absorption rate and swelling rate were discussed. Combined with its mineral composition and pore structure, the correlation mechanism between gaseous moisture absorption and swelling was explained in depth. The experimental results suggested that the whole process was long-term and slow. The swelling rate was lower than the moisture absorption rate by two orders of magnitude. As a result, the duration of a stable swelling strain was two orders of magnitude higher than that of a stable moisture absorption. The characteristic curve of moisture absorption went through three stages: the rapid stage, dominated by crystal layer adsorption; the intermittent expansion stage, dominated by adsorbed water film; and the decelerated stage, dominated by capillary condensation. In the intermittent expansion stage, as the relative humidity declined, the duration of an intermittency increased, and the number of intermittencies decreased. The evolution of the swelling rate obviously lagged the proportion of moisture absorption. This hysteresis effect reached its maximum when the moisture absorption transitioned from the intermittent expansion stage to the decelerated stage. Moreover, the equilibrium time, moisture absorption ratio and swelling strain all had a non-linear proportional relationship with the relative humidity. In this case, the critical value was 85%. Finally, a unified mathematical expression for the gaseous moisture absorption curve was acquired and the relationship between the swelling strain and the increment of moisture content was fitted; this will provide the necessary research basis for a subsequent simulation of its volume change under changing ambient humidity.

Effect of Maize and Chickpea Intercropping under the Different Row Ratios on Air Temperature and Vapoure Pressure Profile in Crop Canopy

An experiment was laid out to study the effect of maize and chickpea intercropping under the different row ratios on air temperature and vapoure pressure profile in crop canopy at Agronomy farm, Bansilal Amritlal College of Agriculture, Anand Agricultural University, Anand (Gujarat), India during the rabi season of the year 2021-2022. The soil of the experimental field was loamy sand. The experiment was laid out in a randomized block design with six treatments replicated four times. The treatments details are T1: Sole maize, T2: Sole chickpea, T3: Maize paired row, T4: Maize + chickpea (1: 1), T5: Maize + chickpea (1 : 2), T6: Maize + chickpea (2 : 2). During the early crop growth inversion profile of air temperature was observed because crops canopy was short. Air temperature was lowest at ground level, with increase in height air temperature increase. At upper canopy air temperature is lower than lower canopy because active transpiration by the dense foliage at upper canopy lower down the air temperature. During the early crop growth lapse profile of vapour pressure was observed. Highest value of vapour pressure was observed at the ground, but with an increase in height vapour pressure decreased. In the case of within crop canopy, strong gradient of vapour pressure was observed in upper crop canopy and weak gradient at lower crop canopy.

Controls and rates of evaporation from a water supply reservoir in the Colorado Front Range

Evaporation can be a major component of a reservoir’s water balance in arid, populated regions such as the western United States, yet accurate measurements are scarce. In an effort to fill this observation gap, direct measurements of evaporation from Standley Lake, a water supply reservoir in the Colorado Front Range, were collected via the eddy covariance method from March 1, 2020 through February 28, 2021. Evaporation followed a seasonal pattern, with the spring, summer, and early fall experiencing daily cycles that peaked just after midday, with measurable evaporation occurring each day for approximately 4–6 h. In the late fall and winter, the daily cycles decayed and episodic enhancements in evaporation were common, most often lasting between 18 and 36 h, some longer. Evaporation peaked in June, aligning with peak net radiation, and summertime evaporation was predominantly driven by diurnal variations in wind speed and the vapor pressure gradient. Interestingly, fall and winter saw multiple periods of negative (downward) sensible heat transfer which helped drive wintertime evaporation. The study period evaporation totaled 1377 mm, with 35 % of that occurring in summer, 27 % in fall, 21 % in spring, and 17 % in winter. This study shows that evaporation is a significant part of the energy balance of Standley Lake, and with continued observations, enables a better understanding of the impacts a changing climate and increased water demands have on water resources.

Diffusionlike Drying of a Nanoporous Solid as Revealed by Magnetic Resonance Imaging

Drying plays a central role in various fabrication processes and applications of functional nanoporous materials, most prominently in relation to energy storage and conversion. During such processes, liquid coexists with air inside the sample, leading to transport as a result of concentration gradients of vapor and/or liquid. Experimentally, it is extremely challenging to unravel this transport phenomenology inside the hidden geometry of porous media. Here, we observe the drying of a model nanoporous material (monolithic mesoporous silica glass, Vycor) with magnetic resonance imaging. We show that, for various boundary conditions (air-flux intensities), no dry region develops, but the sample desaturates in depth. This desaturation is almost homogeneous throughout the sample for weak air flux, while saturation gradients can be observed for sufficiently strong air flux. We demonstrate that the transport of water is mainly ensured by liquid flow towards the free surface, resulting from a gradient of vapor pressure, associated with local saturation (via the desorption curve), leading to a gradient of liquid pressure (via the Kelvin law). Assuming otherwise standard hydrodynamic characteristics of the nanoconfined liquid, this results in a diffusionlike model, which appears to represent experimental data very well in terms of the spatial distribution of water over time inside the sample for various boundary conditions (air-flux intensities). Finally, we propose a predictive model of the detailed drying characteristics of a nanomaterial from knowledge of its pore size, permeability, and desorption curve. This provides an insight into the rational design of drying-based processes employing functional nanoporous materials and allows for a mechanistic understanding of drying phenomenologies in natural nanoporous media.

Geopolymer concrete for net-zero buildings: Correlating paste chemistry with monolith hygrothermal performance

Geopolymers are among the materials that are being studied for use in Net-Zero Buildings. The main reasons for their popularity are their low embodied energy and capacity to passively control indoor microclimate especially when they are cellular. The performance of cellular geopolymers as passive thermal and moisture regulators depends on their manufacturing parameters. To achieve an optimal design of geopolymers for the desired application understanding the influences of these parameters is crucial. Among these salient parameters are the initial water content and alkalinity of the reaction mixture. The focus of this paper is to investigate the extent of these factors' influence on the thermal, hydric, phase composition, and microstructure of aerated geopolymers. In this study, two groups of samples were prepared by varying initial water content and alkaline molarity of the reaction mix. The results showed that the vapor permeability and moisture effusivity of the samples prepared by adjusting the initial water content showed similar hydric properties. The vapor permeability of the samples measured at 7–75%RH and 7–37%RH vapor pressure gradients are ≈3 kg/m.s. Pa and ≈2.3 kg/m.s Pa respectively. Their moisture buffer capacities are very similar as well (ranging from 4.93 to 5.14 g/ (m2%RH)). The slim differences in hydric properties are attributed to minor differences in porosity, degree of reaction, and porogen decomposition. However, the thermal conductivity and density of the samples increased as the water content is raised. This is because of differences in the amount of water trapped in the interstitial sites of the inorganic polymer. On the other hand, the hygrothermal and physical properties of the three samples prepared at different molarities showed a non-monotonic trend (for instance; Density (g/cm3): 0.50, 0.40, 0.44; Moisture buffer capacity (g/ (m2%RH)): 4.65, 5.65, and 4.79). This is because the fluctuations in alkaline molarity resulted in significant differences in the degree of geopolymerization, carbonation, and porosity of the samples. Despite the differences, all the geopolymers samples showed a potential to efficiently modulate indoor environment compared to conventional building materials.

The effect of gas diffusion layer on electrochemical effective reaction area of catalyst layer and water discharge capability

A gas diffusion layer (GDL) is one of the main components of a proton exchange membrane fuel cell (PEMFC), which helps discharge the water generated by the electrochemical reaction and promotes the diffusion of reactant gases. The GDL typically consists of a microporous layer (MPL) and a substrate, and the MPL is mainly composed of carbon black and PTFE (polytetrafluoroethylene). In this study, to optimize the effective reaction area of the catalyst layer by increasing the specific surface area of the MPL and improve the water discharge capability of GDL by increasing the porosity of the MPL, the GDLs are manufactured by varying the manufacturing methods of the MPL. The MPL design parameters considered in this study are the particle size of carbon black, the addition of smaller carbon black particles to the MPL, and the number of cracks in the MPL. The performance tendencies are quantitatively identified through polarization curves, electrochemical impedance spectroscopy, scanning electron microscopy, Brunauer-Emmett-Teller, electrochemical surface area, liquid water, water vapor, gas permeabilities, and capillary pressure gradient. The results showed that the MPL has a dominant effect on the electrochemical surface area (ECSA) of the catalyst layer and the mass transport resistance of the PEMFC.

Design and control of MOFs-based indoor humidity pump integrated into the building’s ventilated façade in hot and humid climates

Indoor humidity management is vital for both building and occupant health. Conventional methods that decouple the latent and sensible load use desiccant dehumidifiers integrated with vapor compression systems to supply cool dry air to the space and dilute the indoor generated moisture. Such methods remain energy-intensive, especially for high latent load spaces located in hot and humid climate. Direct removal of the indoor generated moisture via moisture migration through naturally ventilated building façade, proved to be a more energy-efficient alternative. However, this strategy suffers from different drawbacks in humid conditions, which impose an intermittent operation on the moisture control system. In this work, a continuous humidity pump system is proposed that integrates a metal organic frameworks-based desiccant dehumidifier to supply dry air to the ventilated building façade provided with breathable insulation. This creates a water vapor pressure gradient between the indoor and dry air to actively drive the indoor moisture to the outdoor environment, irrespective of its humidity conditions. Mathematical models are developed for the system’s subcomponents, which are then experimentally calibrated. Design and control strategies are presented for the proposed system and applied for a case study of a high latent load space located in the hot and humid climate of Beirut. Over the entire cooling season, the proposed system resulted in 66 % reduction in the investment cost and 86 % in the operating cost compared to conventionally employed hybrid desiccant – vapor compression cooling systems.

Evaporation from a large lowland reservoir – observed dynamics and drivers during a warm summer

Abstract. We study the controls on open water evaporation of a large lowland reservoir in the Netherlands. To this end, we analyse the dynamics of open water evaporation at two locations, Stavoren and Trintelhaven, at the border of Lake IJssel (1100 km2); eddy covariance systems were installed at these locations during the summer seasons of 2019 and 2020. These measurements were used to develop data-driven models for both locations. Such a statistical model is a clean and simple approach that can provide a direct indication of (and insight into) the most relevant input parameters involved in explaining the variance in open water evaporation, without making a priori assumptions regarding the process itself. We found that a combination of wind speed and the vertical vapour pressure gradient can explain most of the variability in observed hourly open water evaporation. This is in agreement with Dalton’s model, which is a well-established model often used in oceanographic studies for calculating open water evaporation. Validation of the data-driven models demonstrates that a simple model using only two variables yields satisfactory results at Stavoren, with R2 values of 0.84 and 0.78 for hourly and daily data respectively. However, the validation results for Trintelhaven fall short, with R2 values of 0.67 and 0.65 for hourly and daily data respectively. Validation of the simple models that only use routinely measured meteorological variables shows adequate performance at hourly (R2=0.78 at Stavoren and R2=0.51 at Trintelhaven) and daily (R2=0.82 at Stavoren and R2=0.87 at Trintelhaven) timescales. These results for the summer periods show that open water evaporation is not directly coupled to global radiation at the hourly or daily timescale. Rather a combination of wind speed and vertical gradient of vapour pressure is the main driver at these timescales. We would like to stress the importance of including the correct drivers of open water evaporation in the parametrization in hydrological models in order to adequately represent the role of evaporation in the surface–atmosphere coupling of inland waterbodies.

Experimental study on the effects of sodium and potassium proportions on the heat transfer performance of liquid metal high-temperature oscillating heat pipes

Liquid metal high-temperature oscillating heat pipes (LMHOHPs) are highly efficient heat transfer devices which can work in high temperature environments above 500 °C. In this paper, LMHOHPs with different proportions of sodium-potassium alloy (hereinafter referred to as NaK) as working fluids were developed, and their start-up and heat transfer performance was investigated experimentally. 310S stainless steel with an inner diameter of 6mm was employed as the pipe material, and the filling ratio was kept as 50%. Under the conditions of input powers 1500∼3500 W and inclination angles 0°∼90°, LMHOHPs with five proportions of NaK (namely, 0, 10, 46, 78 and 100 wt.% of potassium) as working fluids were tested. The results showed that: (1) The wettability between NaK and 310S stainless steel was poor, but it was improved with the increase of temperature and potassium mass fraction. At 350 °C, The contact angles of NaK (K100%) and NaK (K78%) were 72.3° and 81.1°, respectively, which meant that the liquid metal changed from non-wetting state to wetting state. (2) Except that the LMHOHP with NaK (K0%) could not start up at the inclination angle of 0°, the five LMHOHPs could start up successfully and transfer heat through oscillating motion at all inclination angles and input powers. (3) When the heat flux exceeded 4.56 × 106W/m2, the flow patterns of LMHOHPs with NaK (K10%), NaK (K46%), NaK (K78%) and NaK (K100%) changed from oscillating motion to stable unidirectional circulating flow, which significantly improved their heat transfer performance. (4) Due to the influences of saturated vapor pressure gradient, viscosity and wettability, the heat transfer performance of LMHOHPs increased with the growth of potassium mass fraction. The minimum thermal resistance of the LMHOHP with NaK (K100%) was 0.071 °C/W at the input power of 3528 W and the inclination angle of 90°. This study could provide guidance for the working fluids selection of oscillating heat pipes in high temperature environments.

Seasonal changes in gas exchange and yield of 21 genotypes of Coffea arabica

Background: Coffee breeding programs in Ecuador have information on production and disease tolerance in many genotypes; however, they lack physiological information, especially on photosynthetic characteristics and their response to drought.
 Questions: Whether high genetic variability among coffee genotypes will explains the photosynthetic and production differences expected? Will the physiological response to the dry season (DS) be different between genotypes?
 Studied species: Coffea arabica L.
 Study site and dates: Pichincha canton, Manabí province, Ecuador during March-April 2017 (rainy season, RS) and June-July 2017 (DS).
 Methods: Leaf relative water content (RWC) and gas exchange of 21 coffee genotypes were measured during DS and RS. Coffee production during a period of three years was evaluated.
 Results: Significant differences were found in RWC, photosynthetic rate (A), stomatal conductance (gs) and water use efficiency (WUE) among genotypes, between seasons, an interaction effect of genotype × season. Drought caused a significant reduction in A and gs of 30 and 44 % respectively, while WUE was not affected. A positive linear relationship was found between A and gs, and a negative relationship between A and the leaf-air vapor pressure gradient (D W ) and between gs and D W . Differences in coffee production were found among genotypes.
 Conclusions: The high genetic variability of C. arabica genotypes may explain the significant differences in RWC and gas exchange and interaction genotypes x season, suggesting a differential response of each genotype to drought. Eleven of the 21 coffee genotypes were sensitive to drought, but showed different responses, suggesting possible genotypic differences in tolerance.
 
 
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