Numerical and Parametric Analysis for Enhancing Performances of Water Photovoltaic/Thermal System

Photovoltaic/thermal (PV/T) systems are innovative cogeneration systems that ensure the cooling of photovoltaic (PV) backside and simultaneous production of electricity and heat. However, an effective cooling of the PV back is still a challenge that affects electrical and thermal performance of the PV/T system. In the present work, a PV/T numerical model is developed to simulate the heat flux based on energy balance implemented in MATLAB software. The numerical model is validated through the comparison of the three-layer PV model with the NOCT model and tested under the operation conditions of continental temperate climate. Moreover, the effect of velocity and water film thickness as important flow parameters on heat exchange and PV/T production is numerically investigated. Results revealed that the PV model is in good agreement with the NOCT one. An efficient heat transfer is obtained while increasing the velocity and water film thickness with optimal values of 0.035 m/s and 7 mm, respectively, at an inlet temperature of 20 °C. The PV/T system ensures a maximum thermal power of 1334.5 W and electrical power of 316.56 W (258.8 W for the PV). Finally, the comparison between the PV and PV/T system under real weather conditions showed the advantage of using the PV/T.

Characteristic Analysis of Water Lubricated Plain Journal Bearing under Transient Load

Transient load has a huge impact on the life and stability of water-lubricated bearings. In this paper work, CFD software is used to analyse the dynamic characteristic of water lubricated bearings under different transient loads of 500N, 1000N, 2000N and 3000N. The water film pressure contour distribution at different transient time was given. The time-varying relationships between the different transient loads with bearing forces, the journal displacement, the maximum value of water film pressure as well as the minimum value of water film thickness are obtained. The results show that with the increase of transient load, the effective bearing area of dynamic water pressure film decreases, and the maximum pressure increases. The bearing forces, the journal displacement, the maximum value of water film pressure as well as the minimum value of water film thickness will increase fast.

Thin-water-film-enhanced TiO2-based catalyst for CO2 hydrogenation to formic acid

Carbon dioxide (CO2) hydrogenation can not only mitigate global warming, but also produce value-added chemicals. Herein, we report a novel three-phase catalytic system with an in-situ generated and dynamically updated...

Experimental Studies of the Heat Exchange Between the Water Film and the Casting Roller in the Thermal Preconditioning Chamber

When the casting roller is cooled or heated in the preconditioning chamber the water is supplied to its surface by flat-jet nozzles. The visual inspection of the model of the casting roller showed that a considerable part of it can be covered with the water film spreading from sprinkling zones. It was established that the heat conductivity in the roller body is considered to be a crucial thermal preparation factor in the conjugate heat-exchange problem for the roller of a large diameter at Bio criterion values exceeding 20. Hence, it is sufficient to provide an essential level of the heat transfer that corresponds to the heat transfer coefficient of 2000 W/(m2∙K) to provide appropriate operating conditions for the thermal preparation of the roller. The conditions are also met in sprinkling zones. Due to this fact this scientific paper studies the heat exchange conditions under the water film that spreads between the adjacent sprinkling zones. A range of changes in the flow rate of the spreading water film was determined experimentally. The conditions of heat exchange between the surface of alpha-calorimeter and the water film were analyzed depending on its flow rate and the heat meter surface temperature. A generalized correlation equation was derived. It was established that the heat exchange intensity in sprinkling zones and under the spreading water film meets technological roller treatment conditions in the preconditioning chambers. The obtained research data can be used for the rational arrangement of the collectors and flat-jet nozzles in casting roller preconditioning chambers to reduce the cold and hot water consumption and cut down operating costs.

Permitted speed decision of single-unit trucks with emergency braking maneuver on horizontal curves under rainy weather

Under adverse weather conditions, visibility and the available pavement friction are reduced. The improper selection of speed on curved road sections leads to an unreasonable distribution of longitudinal and lateral friction, which is likely to cause rear-end collisions and lateral instability accidents. This study considers the combined braking and turning maneuvers to obtain the permitted vehicle speed under rainy conditions. First, a braking distance computation model was established by simplifying the relationship curve between brake pedal force, vehicle braking deceleration, and braking time. Different from the visibility commonly used in the meteorological field, this paper defines "driver’s sight distance based on real road scenarios" as a threshold to measure the longitudinal safety of the vehicle. Furthermore, the lateral friction and rollover margin is defined to characterize the vehicle’s lateral stability. The corresponding relationship between rainfall intensity-water film thickness-road friction is established to better predict the safe speed based on the information issued by the weather station. It should be noted that since the road friction factor of the wet pavement not only determined the safe vehicle speed but also be determined by the vehicle speed, so we adopt Ferrari’s method to solve the quartic equation about permitted vehicle speed. Finally, the braking and turning maneuvers are considered comprehensively based on the principle of friction ellipse. The results of the TruckSim simulation show that for a single-unit truck, running at the computed permitted speed, both lateral and longitudinal stability meet the requirements. The proposed permitted vehicle speed model on horizontal curves can provide driving guidance for drivers on curves under rainy weather or as a decision-making basis for road managers.

Assessing the mechanisms thought to govern ice and snow friction and their interplay with substrate brittle behavior

Sliding friction on ice and snow is characteristically low at temperatures common on Earth’s surface. This slipperiness underlies efficient sleds, winter sports, and the need for specialized tires. Friction can also play micro-mechanical role affecting ice compressive and crushing strengths. Researchers have proposed several mechanisms thought to govern ice and snow friction, but directly validating the underlying mechanics has been difficult. This may be changing, as instruments capable of micro-scale measurements and imaging are now being brought to bear on friction studies. Nevertheless, given the broad regimes of practical interest (interaction length, temperature, speed, pressure, slider properties, etc.), it may be unrealistic to expect that a single mechanism accounts for why ice and snow are slippery. Because bulk ice, and the ice grains that constitute snow, are solids near their melting point at terrestrial temperatures, most research has focused on whether a lubricating water film forms at the interface with a slider. However, ice is extremely brittle, and dry-contact abrasion and wear at the front of sliders could prevent or delay a transition to lubricated contact. Also, water is a poor lubricant, and lubricating films thick enough to separate surface asperities may not form for many systems of interest. This article aims to assess our knowledge of the mechanics underlying ice and snow friction.

Study on the Influence of Silica Fume (SF) on the Rheology, Fluidity, Stability, Time-Varying Characteristics, and Mechanism of Cement Paste

In this study, the rheology, fluidity, stability, and time-varying properties of cement paste with different substitute contents of silica fume (SF) were investigated. The result showed that the effects of SF on macro-fluidity and micro-rheological properties were different under different water–cement ratios. The addition of SF increased the yield stress and plastic viscosity in the range of 2.61–18.44% and 6.66–24.66%, respectively, and reduced the flow expansion in the range of 4.15–18.91%. The effect of SF on cement paste gradually lost its regularity as the w/c ratio increased. The SF can effectively improve the stability of cement paste, and the reduction range of bleeding rate was 0.25–4.3% under different water–cement ratios. The mathematical models of rheological parameters, flow expansion, and time followed the following equations: τ(t) = τ0 + k0t, η(t) = η0eat, and L(t) = L0 − k1t, L(t) = L0 − k1t − a1t2. The SF slowly increased the rheological parameters in the initial time period and reduced the degree of fluidity attenuation, but the effect was significantly enhanced after entering the accelerated hydration period. The mechanism of the above results was that SF mainly affected the fluidity and rheology of the paste through the effect of water film thickness. The small density of SF particles resulted in a low sedimentation rate in the initial suspended paste, which effectively alleviated the internal particle agglomeration effect and enhanced stability. The SF had a dilution effect and nucleation effect during hydration acceleration, and the increase of hydration products effectively increased the plastic viscosity.

Adsorption of Gas-Phase Cyclohexanone on Atmospheric Water Films

The fate of atmospheric volatile organic compounds (VOCs) strongly depends on the partitioning processes on the surface of aerosols, which are coated with a thin water film. However, the behavior of VOCs in the aqueous film of aerosols is difficult to measure. In this work, the interfacial partition constant of cyclohexanone was determined using a novel flow-tube reactor. A thin, aqueous film placed in the reactor was exposed to cyclohexanone gas. The subsequent partitioning was measured using chromatography techniques. The quality control tests were first conducted to ensure the accuracy of the adsorption experiments. The cyclohexanone concentration was then plotted as a function of film thickness to obtain the partitioning constants. As the thickness of the water film decreased, the aqueous concentration of cyclohexanone increased, indicating that surface adsorption played a dominant role in the uptake of cyclohexanone. According to the temperature dependence of the interfacial partition constant, the solvation enthalpy and entropy of cyclohexanone were obtained. The results of this study would help to elucidate the effect of atmospheric water film on the gas–aerosol partitioning of VOCs, and thus can help to better understand the fate of VOCs in the atmosphere.