Forced Convective and Nucleate Flow Boiling Heat Transfer to Alumnia Nanofluids

A large number of experiments have been performed to quantify the forced convective and nucleate flow boiling heat transfer coefficient of Al2O3 water based nanofluid. The employed test loop provides conditions to investigate the influence of operating parameters such as heat flux, flow mass flux and volumetric concentration of nanofluids (0.5, 1 and 1.5%). Results demonstrate that two heat transfer regions are observed namely forced convective and nucleate boiling region. Investigating on the operating parameters illustrated that with increasing the heat flux and flow rate of nanofluid, heat transfer coefficient of nanofluid dramatically increases. In contrast, with increasing the volumetric concentration of nanofluid, controversial condition is observed such that increases the heat transfer coefficient in forced convective region is reported while reduction of heat transfer coefficient is seen for nucleate boiling zone. Obtained results were then compared to Chen and also Gungor-Winterton well-known correlations. Results of this comparison show that experimental data are in a good agreement with those of obtained by correlations

Boiling Heat Transfer of Alumina Nano-Fluids: Role of Nanoparticle Deposition on the Boiling Heat Transfer Coefficient

This paper focuses on the thermal performance of alumina nano-fluids during the quenching process of a surface at the boiling condition, which can be a good answer to the controversial results available in the nano-fluid related literature. For this purpose, an experimental study is conducted to investigate the potential application of alumina/water nano-fluid for cooling a stainless steel rod under the flow boiling heat transfer mechanism. Nano-fluids are prepared by dispersing the 5, 50 and 80nm alumina nanoparticles into the deionized water. The experimental facility provides conditions to quantify the heat transfer coefficient in forced convection and nucleate boiling heat transfer domains at different operating conditions. In terms of operating time, the experiments are divided into two domains namely short time study and extended time study. For the short time study (0-60 minutes of study with neglecting the role of time on the deposition of nanoparticles) enhancement of heat transfer coefficient is reported for all nano-fluids, however for nano-fluid with smaller nanoparticle size, higher thermal performance is registered. In extended time study (60-1000 minutes) heat transfer coefficient is found to be considerably deteriorated for all nano-fluids. This work demonstrates that the reason for deterioration of heat transfer coefficient is referred to the surface roughness, nanoparticle size, static contact angle and thermal fouling resistance parameters. These four parameters are simultaneously determinative factors, which strongly control the thermal behaviour of nano-fluids over the extended time and are the exact reasons for the controversies raised in the literature

Boiling Thermal Performance of TiO2 Aqueous NanoFluids as a Coolant on a Disc Copper Block

This work focuses on potential application of nano-fluids in cooling of high heat flux surfaces. For this purpose, experimental studies have been performed to quantify the heat transfer coefficient of Titana (TiO2) aqueous nano-fluids under different operating conditions. Boiling mechanism is established on a disc copper made heater at different heat flux, mass concentration of nano-fluids and sub-cooling temperatures. Results demonstrated that heat transfer coefficient of Titana nano-fluids are relatively higher than that of the base fluid. Heat and mass concentration of nano-particles can intensify the pool boiling heat transfer coefficient, while sub-cooling temperature can only have impacts on bubble formation. Also, visual study demonstrates that fouling formation of nano-particles can intensify the bubble transport due to the intensification of nucleation sites in the boiling surface.

Experimental studies o

Experimental investigations on the influences of different contaminants to deionized water have been conducted under the sub-cooled flow boiling heat transfer inside the vertical annulus. Many experiments have been performed to investigate the influence of different operating parameters on the flow boiling heat transfer coefficient in the upward flow of contaminated water under the atmospheric pressure. The experimental apparatus provides the particular conditions to investigate the influence of heat flux (up to 132 kW/m2), flow rate (1.5–3.5 l/min), sub-cooling level (Max. 30 °C), and concentration of contaminants (1–5% by volume). According to the results, with increasing the heat flux and flow rate, the flow boiling heat transfer coefficient and rate of bubble formation significantly increase. Results also demonstrated that adding contaminants to the deionized water causes the flow boiling heat transfer coefficient to be deteriorated. Likewise, sub-cooling level may only influence on the onset of nucleate boiling and heat flux corresponding to beginning of nucleate boiling phenomenon which is called inception heat flux

Energy harvesting materials: overview

The rapidly growing concerns about climate change and the environmental issues of traditional energy resources have led to search for sustainable energy technologies. Energy harvesting includes a range of technologies to capture freely available energy from the environment and convert it to more useful forms including electrical, thermal, and mechanical energies. The efficiency and performance of energy harvesting techniques are directly related to the characteristics of the employed materials. This article discusses the major attributes and potentials of different materials employed in energy harvesting technologies. An introduction to the basic concept of energy harvester technologies is provided with a brief discussion on the mechanism of each material to harvest energy. The article further includes sections on piezoelectric, thermoelectric, magnetic, graphene, ferroelectret, thermoelastic and photovoltaic materials as used in energy harvesting technologies

Controlling the gelation time of sodium silicate gelants for fluid management in hydrocarbon reservoirs

An efficient gel-based treatment of fractures and high-permeability layers requires the precise placement of the gelant within the formation, which is highly dependent on the gelation time. This study proposes an optimal method that reduces the uncertainties associated with the exact placement of sodium silicate gelants in a formation. This method is based on the control of the gelation time, so that these potent and eco-friendly blocking agents can be used more effectively in water shut-off, profile modification, and carbon storage projects. We proposed a new pre-flush operation and gelation protocol for sodium silicate gelants; gelation time experiments were conducted using a rheometer and through characterization of gel strength codes. To improve the pre-flush operation, the masking potential of citric acid is examined in this study. We show that citric acid is capable of complexing formation water ions, thus small amounts of citric acid can delay the gelation time by up to 8 h. In previous studies, silica nanoparticles were used as nucleation promoters in sodium silicate gelants, which required large amounts of surfactant for their stabilization. As an alternative, we propose a new gelation mechanism that is based on the adsorption tendency and nucleation ability of silica nanoparticles. We show that, based on their adsorption tendency, silica nanoparticles initially form an H+ layer around themselves and do not participate in the gelation reaction. At the same time, they can reduce the reaction rate of silicates, which postpones the gelation time by up to 14% (at 0.15 wt% of silica nanoparticles in defined concentrations of sodium silicate and acid). However, we observed that adding silica nanoparticles at a concentration less than 0.15 wt% or greater than 0.3 wt% accelerates gelation. But when the H+ layer breaks down, the nanoparticles intensify the gelation process through nucleation. The mechanistic insights are derived from an experimental study in which the concentrations of silica nanoparticles, sodium silicate, and gelling agent are varied. Also, experiments in a wide range of temperatures and concentrations of silica nanoparticles, sodium silicate, and citric acid showed that shielding silica nanoparticles with H+ ions released from the gelling agent keeps the nanoparticles stable in the gelant without the use of other additives. Accordingly, this mechanism, in addition to having the ability to decrease the gelation rate, increases the nucleation capacity, and reduces the complexity and cost of the system. The results of this work provide guidelines for preparing sodium silicate gelants and optimizing their gelation process.</p

Energy saving in thermal energy systems using dimpled surface technology – A review on mechanisms and applications

Recently, dimpled surfaces have found great attention due to their abilities in energy management and heat transfer enhancement, low weight, small values of pressure drop penalty, simple fabrication, and small maintenance costs. Many experimental and numerical studies are accomplished to investigate the potentials of dimpled surface technology in energy management of various thermal energy systems. This paper presents a comprehensive review about the developments and current status of this technology employed in different thermal energy systems including heat exchangers, mini/micro energy systems, two-phase flow energy systems, jet impingement cooling systems, and solar thermal energy systems. Moreover, comprehensive discussions concerning the flow structures and heat transfer behaviours in dimpled surfaces and different geometries and arrangements of dimples are provided. Finally, based on current status of studies in this field, some suggestions are suggested for future researches. The results showed that the main reasons for the convective heat transfer improvement achieved by dimples are flow reattachment, flow impingement, and upwash flow at the downstream region of the dimples, while the heat transfer can be reduced due to flow separation and recirculation in the upstream region of the dimples

Applications of nanofluids in thermal energy transport

In recent years, many heat transfer investigations have been conducted to explore the potential applications of so-called “nanofluids.” These are a new class of operating fluids that include one or more types of nanoscale particles dispersed in an ordinary liquid. A large number of experiments and simulations have shown that these fluids have great potential for improving the efficiencies of various thermal systems. Due to the growing practical and scientific significance of nanofluids, this chapter focuses on nanofluid technology and provides a state-of-the-art view of this topic and its engineering applications. First, different mechanisms of heat transfer improvement by nanofluids are briefly discussed. Then, descriptions of the thermophysical properties and stability of some modern nanofluids, including magnetic nanofluids, graphene nanofluids, hybrid nanofluids, and carbon nanotube-based nanofluids, are provided. In the third part, the applications of nanofluids in advanced thermal systems such as thermosyphons, electronics cooling modules, heat pipes, microchannel heat sinks, heat exchangers, and refrigeration systems are reviewed. Finally, the last section of the chapter puts forward a summary and prospective of future research directions