The contact angle of nanofluids as thermophysical property

Droplet volume and temperature affect contact angle significantly. Phase change heat transfer processes of nanofluids – suspensions containing nanometre-sized particles – can only be modelled properly by understanding these effects. The approach proposed here considers the limiting contact angle of a droplet asymptotically approaching zero-volume as a thermophysical property to characterise nanofluids positioned on a certain substrate under a certain atmosphere. Graphene oxide, alumina, and gold nanoparticles are suspended in deionised water. Within the framework of a round robin test carried out by nine independent European institutes the contact angle of these suspensions on a stainless steel solid substrate is measured with high accuracy. No dependence of nanofluids contact angle of sessile droplets on the measurement device is found. However, the measurements reveal clear differences of the contact angle of nanofluids compared to the pure base fluid. Physically founded correlations of the contact angle in dependency of droplet temperature and volume are obtained from the data. Extrapolating these functions to zero droplet volume delivers the searched limiting contact angle depending only on the temperature. It is for the first time, that this specific parameter, is understood as a characteristic material property of nanofluid droplets placed on a certain substrate under a certain atmosphere. Together with the surface tension it provides the foundation of proper modelling phase change heat transfer processes of nanofluids

Effect of Nanofluid Thermophysical Properties on the Performance Prediction of Single-Phase Natural Circulation Loops

Specifying nanofluids' thermophysical properties correctly is crucial for correct interpretation of a system's thermo-hydraulic performance and faster market-uptake of nanofluids. Although, experimental and theoretical studies have been conducted on nanofluids' thermophysical properties; their order-of-magnitude change is still a matter of debate. This numerical study aims to reveal the sensitivity of single phase natural circulation loops (SPNCL), which are the passive systems widely used in solar thermal and nuclear applications, to thermophysical property inputs by evaluating the effects of measured and predicted nanofluid thermophysical properties on the SPNCL characteristics and performance for the first time. Performance and characteristics of an SPNCL working with water-based-Al2O3 nanofluid (1-3 vol.%) for heating applications is evaluated for different pipe diameters (3-6 mm). The thermal conductivity effect on SPNCL characteristics is found to be limited. However, viscosity affects the SPNCL characteristics significantly for the investigated cases. In this study, Gr(m) ranges are 1.93 x 10(7)-9.45 x 10(8) for measured thermophysical properties and 1.93 x 10(7)-9.45 x 10(8) for predicted thermophysical properties. Thermo-hydraulic performance is evaluated by dimensionless heat transfer coefficients which is predicted within an error band of +/- 7% for both the predicted and measured thermophysical properties of the data. A Nu correlation is introduced for the investigated SPNCL model, which is useful for implementing the SPNCL into a thermal system

Effect of Nanofluid Thermophysical Properties on the Performance Prediction of Single-Phase Natural Circulation Loops

Specifying nanofluids’ thermophysical properties correctly is crucial for correct interpretation of a system’s thermo-hydraulic performance and faster market-uptake of nanofluids. Although, experimental and theoretical studies have been conducted on nanofluids’ thermophysical properties; their order-of-magnitude change is still a matter of debate. This numerical study aims to reveal the sensitivity of single phase natural circulation loops (SPNCL), which are the passive systems widely used in solar thermal and nuclear applications, to thermophysical property inputs by evaluating the effects of measured and predicted nanofluid thermophysical properties on the SPNCL characteristics and performance for the first time. Performance and characteristics of an SPNCL working with water-based-Al2O3 nanofluid (1–3 vol.%) for heating applications is evaluated for different pipe diameters (3–6 mm). The thermal conductivity effect on SPNCL characteristics is found to be limited. However, viscosity affects the SPNCL characteristics significantly for the investigated cases. In this study, Grm ranges are 1.93 × 107–9.45 × 108 for measured thermophysical properties and 1.93 × 107–9.45 × 108 for predicted thermophysical properties. Thermo-hydraulic performance is evaluated by dimensionless heat transfer coefficients which is predicted within an error band of ±7% for both the predicted and measured thermophysical properties of the data. A Nu correlation is introduced for the investigated SPNCL model, which is useful for implementing the SPNCL into a thermal system

Numerical study on nanofluid based single phase natural circulation mini loops: A steady 3D approach

The purpose of this numerical study is to investigate the effect of using nanofluid on the thermal performance of a single phase natural circulation mini loop (SPNCmL) in inclined conditions, for the first time, by using a three dimensional steady numerical model. The steady numerical study was carried for different heater powers (10, 30, 50 W), volumetric concentrations of Al2O3-DIW nanofluid (1, 2, 3%) and inclination angles (0, 30, 60, 75 degrees). A recent experimental study has been adopted from the existing literature to compare the results of the numerical method. The results indicate that, steady numerical model can predict the reaction of the SPNCmL to particle concentration, heater power and inclination angle sufficiently. The deflection (not more than 10%) from the experimental results increases as the heater power, particle concentration and inclination angle increase. Simulation results are used to visualize temperature distributions at the critical cross-sections of the loop for different working fluids, inclination angles and heater powers. Visualized temperature distributions highlight the necessity of improvement for the investigated SPNCmL. Therefore, steady three dimensional numerical studies can be used to properly define the local defects and to increase the efficiency

Numerical analysis of germicidal uv-c lamp air disinfection system

In this study, four germicidal UV-C lamps that are in a 600x600 square duct are numerically investigated. Also an UV reactor design is investigated for high resistive microorganisms. Air flow in the duct has 3000 m3/h volumetric flow rate. Simulations are conducted with commercial computational fluid dynamics solver ANSYS-FLUENT. In square duct, Lamps has 75 W UV power and their electrical efficiency depends on UV radiation generation, lamp surface temperature, contact air humidity, lamp working hours, lamp surface pollution. The radiation intensity around the lamps in the channel is evaluated by the using discrete ordinates method depending on the location. After that, the air flow on the lamps are modelled and particle motion simulation is carried out with the DPM model. The amount of UV dose received by these particles is calculated at the duct outlet, and the inactivation ratio for the general coronavirus family is examined. As a result, D90 inactivation performance is achieved in the system. The radiation distributions obtained depending on the UV power and the dose map in the duct outlet section depending on UV power are parametrically examined and presented.</p

Carbon-based Nanofluid Applications in Solar Thermal Energy

Renewable energy sources such as solar, wind and geothermal are proposed as an alternative to fossil fuels whose excessive use causes global warming. The most popular one of the renewable energy sources is considered as solar energy due to the fact that required energy is provided by the sun entire year around the world. Solar energy systems convert the solar radiation to the useful heat or electricity. In order to achieve better performance in solar thermal systems many studies have been conducted. Some of these studies suggest that heat transfer fluid could be changed with the nanofluids which can be defined as new generation heat transfer fluid. Nanofluids are suspensions of nano-sized particles such as metals, metal-oxides, and Carbon-allotropes (C), in the conventional base-fluids (water, ethylene glycol and oil). Using nanofluid enhances the efficiency and thermal performance of solar systems due to their better thermophysical and optical properties. Recently, C-based nanofluids are getting attention due to their enhanced thermal conductivity and absorptivity at even low concentrations. The results show that C-based nanofluids have a potential to use in solar energy systems: solar collectors, solar stills, photovoltaic/thermal systems

Investigation of flow and heat transfer for a split air conditioner indoor unit

Split air conditioners (SACs) include complicated components, such as the fin and tube heat exchanger (FTHE) and cross-flow fan (CFF), and their complex interaction needs to be investigated in detail. In this study, a three-dimensional representative thin-section model was introduced for modelling SAC indoor units, and heat transfer and fluid flow analysis was made to determine the characteristics of the device. In addition, the numerical method was examined by comparing the results with the heat transfer capacity experiments and Stereo Particle Image Velocimetry (SPIV) measurements at the outlet section of the device were conducted to compare the velocity distribution. The results showed that the difference between the numerical and experimental studies were within acceptable limits; therefore, the thin-section model for the numerical study is a good assumption for determining the heat transfer and flow characteristics of the SACs. (c) 2012 Elsevier Ltd. All rights reserved

Experimental visualization of the flow characteristics of the outflow of a split air conditioner indoor unit by meshed infrared thermography and stereo particle image velocimetry

Three-dimensional (3D) flow structures due to the interaction between the device edges, fan casing and cross flow fan (CFF) in the rectangular jet at the outflow of a split air conditioner (SAC) indoor unit were investigated by the stereo particle image velocimetry (SPIV) method. In addition, a novel application of infrared thermography called meshed infrared thermography (MIT), used to determine and visualize the temperature profile in an air flow field, was introduced for the first time and used to investigate the temperature distribution at the outflow section of the SAC indoor unit. The results of measurements that were made at different positions on the device were used to prepare three-dimensional reconstructions of the rectangular jet flow and temperature distribution at the outlet section. (C) 2012 Elsevier Inc. All rights reserved