The effect of nanoparticle type and nanoparticle mass fraction on heat transfer enhancement in pool boiling

Determining the heat transfer performance with nanofluids is of cardinal importance in the utilization of nanofluids in thermal systems. This study presents an experimental investigation on nucleate pool boiling heat transfer of TiO2 nanoparticles/water and CuO nanoparticles/water nanofluids on a flat heater plate and aims to reveal the effect of mass fraction of nanoparticles in these nanofluids for attaining the maximum enhancement in pool boiling heat transfer. The effect of mass fraction on boiling heat transfer characteristics was studied for mass fractions varying from 0.001% to 0.2% for the heat flux range between 48.7 and 134.9 kW/m2. The experimental results showed that the heat transfer performance was improved when TiO2 nanoparticles were added to pure water, as base fluid. However, the amount of enhancement was highly dependent on mass fraction. It was realized that the lowest mass fraction (0.001%), namely the dilute TiO2 nanoparticles/water nanofluid, has the largest enhancement (around 15%). A further increase in mass fraction still augments heat transfer compared to pure water, however, the amount of enhancement decreased with mass fraction. Furthermore, the performed visualization showed that the addition of nanoparticles into the base fluid, increased the number of nucleation sites, and the bubbles had a more spherical shape along with a decrease in their size. For CuO/water nanofluids, heat transfer was enhanced at mass fractions larger than 0.001%. This enhancement could be more than 35% for the mass fraction of 0.2 wt.%. This study clearly indicates that the nanoparticle mass fraction corresponding to the best performance is highly dependent on the type of nanoparticle

Subcooled flow boiling heat transfer of γ-Al2O3/water nanofluids in horizontal microtubes and the effect of surface characteristics and nanoparticle deposition

In this study, subcooled flow boiling heat transfer characteristics of nanofluids were investigated at micro scale. For this purpose, the effect of γ-Al2O3 (gamma-alumina) nanoparticles with an average solid diameter of 20 nm was considered. In the experiments, various mass fractions were considered in horizontal smooth stainless steel microtubes with inner and outer diameters of ∼502 µm and ∼717 µm, respectively, at mass fluxes of 1200 and 3400 kg m−2 s−1. Nanoparticles were added to distilled water (base fluid) at five mass fractions (low mass fractions 0.05 wt% and 0.2 wt%; high mass fractions 0.5 wt%, 1 wt% and 1.5 wt%). According to our results, subcooled flow boiling heat transfer coefficients for nanofluids with low mass fractions were nearly the same as those of the pure water. However, heat transfer deteriorated for nanofluids with high mass fractions. Observations of dynamic light scattering measurements for low and high mass fractions before and after the experiments revealed that agglomeration of nanoparticles is an important parameter in deterioration of heat transfer at higher concentrations. Besides, Scanning Electron Microscopy images of microtube inner surfaces showed that deposition of nanoparticles and agglomerated nanoparticles on the inner surface of the microtubes also contributed to the heat transfer deterioration at high mass fractions. Generally, the deterioration in heat transfer beyond a specific mass fraction value was linked to the disturbance in the stability of suspended nanoparticles and deposition of nanoparticles upon boiling.SUNUM; FEN

Experimental study on heat transfer of multi-walled carbon nanotubes/water nanofluids in horizontal microtubes

In this study, heat transfer characteristics of multi-walled carbon nanotube based nanofluids were investigated in horizontal microtubes with outer and inner diameters of ∼1067 and ∼889 μm, respectively. Carbon nanotubes (CNTs) with outer diameter of 10–20 nm and length of 1–2 micron as non-spherical nanoparticles were used for nanofluid preparation, where water was considered as basefluid. Nanofluid was characterized using the Scanning Electron Microscopy (SEM). According to obtained results, deposited CNTs have considerable effect on the convective heat transfer inside the microtube

Experimental study on heat transfer of multi-walled carbon nanotubes/water nanofluids in horizontal microtubes

In this study, heat transfer characteristics of multi-walled carbon nanotube based nanofluids were investigated in horizontal microtubes with outer and inner diameters of 1067 and similar to 889 mu m, respectively. Carbon nanotubes (CNTs) with outer diameter of 10-20 nm and length of 1-2 micron as non spherical nanoparticles were used for nanofluid preparation, where water was considered as basefluid. Nanofluid was characterized using the Scanning Electron Microscopy (SEM). According to obtained results, deposited CNTs have considerable effect on the convective heat transfer inside the microtube

Entropy Generation Analysis of Laminar Flows of Water-Based Nanofluids in Horizontal Minitubes under Constant Heat Flux Conditions

During the last decade, second law analysis via entropy generation has become important in terms of entropy generation minimization (EGM), thermal engineering system design, irreversibility, and energy saving. In this study, heat transfer and entropy generation characteristics of flows of multi-walled carbon nanotube-based nanofluids were investigated in horizontal minitubes with outer and inner diameters of ~1067 and ~889 µm, respectively. Carbon nanotubes (CNTs) with outer diameter of 10–20 nm and length of 1–2 µm were used for nanofluid preparation, and water was considered as the base fluid. The entropy generation based on the experimental data, a significant parameter in thermal design system, was examined for CNTs/water nanofluids. The change in the entropy generation was only seen at low mass fractions (0.25 wt.% and 0.5 wt.%). Moreover, to have more insight on the entropy generation of nanofluids based on the experimental data, a further analysis was performed on Al2O3 and TiO2 nanoparticles/water nanofluids from the experimental database of the previous study of the authors. The corresponding results disclosed a remarkable increase in the entropy generation rate when Al2O3 and TiO2 nanoparticles were added to the base fluid

Increasing the stability of nanofluids with cavitating flows in micro orifices

One of the most critical challenges for nanofluids in practical applications is related to their stability and reusability since a gradual agglomeration of nanoparticles in nanofluids occurs with time and is accelerated by heating. In this study, we propose a technique to maintain the performance and stability of nanofluids with the use of cavitating flows through micro orifices to prevent agglomeration and sedimentation of nanoparticles, which will increase the durability of the nanofluids. γ-Al2O3 (gamma-alumina) nanoparticles with a mean diameter of 20 nm suspended in water were utilized. In the current approach, a flow restrictive element induces sudden pressure, which leads to cavitation bubbles downstream from the orifice. The emerging bubbles interact with the agglomerated structure of nanoparticles and decrease its size through hitting or shock waves generated by their collapse, thereby increasing the stability and reusability of nanofluids. The method does not involve any use of expensive surfactants or surface modifiers, which might alter the thermophysical properties of nanofluids, may adversely influence their performance and biocompatibility, and may limit their effectiveness.TÜBİTAK ; SUNUM ; Sabanci University Faculty of Engineering and Natural Science

Deagglomeration of nanoparticle clusters in a "cavitation on chip" device

Due to the potential of significant energy release in cavitating flows, early cavitation inception and intensification of cavitating flows are of great importance. To use this potential, we investigated the deagglomeration of nanoparticle clusters with the implementation of hydrodynamic cavitation in a microfluidic device. For this purpose, a microfluidic device with a micro-orifice geometry was designed and fabricated using standard microfabrication processes. The system was tested with distilled water in the assembled experimental setup. The flow patterns were characterized using the cavitation number and inlet pressure. Titania nanoparticles were utilized to prepare nanoparticle suspensions. The suspensions were heated to allow agglomeration of nanoparticles. The system was operated with the new working fluid (nanoparticle clusters) at different inlet pressures. After characterizing flow patterns, the flow patterns were compared with those of pure water. The deagglomeration effects of hydrodynamic cavitation on nanoparticle clusters showed the possibility to apply this method for the stabilization of nanoparticles, which paves way to the implementation of nanoparticle suspensions to thermal fluid systems for increased energy efficiency as well as to drug delivery. Our results also indicate that the presence of nanoparticles in the working fluid enhanced cavitation intensity due to the increase in the number of heterogeneous nucleation sites

Subcooled flow boiling heat transfer of γ-Al2O3/water nanofluids in horizontal microtubes and the effect of surface characteristics and nanoparticle deposition

In this study, subcooled flow boiling heat transfer characteristics of nanofluids were investigated at micro scale. For this purpose, the effect of γ-Al2O3 (gamma-alumina) nanoparticles with an average solid diameter of 20 nm was considered. In the experiments, various mass fractions were considered in horizontal smooth stainless steel microtubes with inner and outer diameters of ∼502 µm and ∼717 µm, respectively, at mass fluxes of 1200 and 3400 kg m−2 s−1. Nanoparticles were added to distilled water (base fluid) at five mass fractions (low mass fractions 0.05 wt% and 0.2 wt%; high mass fractions 0.5 wt%, 1 wt% and 1.5 wt%). According to our results, subcooled flow boiling heat transfer coefficients for nanofluids with low mass fractions were nearly the same as those of the pure water. However, heat transfer deteriorated for nanofluids with high mass fractions. Observations of dynamic light scattering measurements for low and high mass fractions before and after the experiments revealed that agglomeration of nanoparticles is an important parameter in deterioration of heat transfer at higher concentrations. Besides, Scanning Electron Microscopy images of microtube inner surfaces showed that deposition of nanoparticles and agglomerated nanoparticles on the inner surface of the microtubes also contributed to the heat transfer deterioration at high mass fractions. Generally, the deterioration in heat transfer beyond a specific mass fraction value was linked to the disturbance in the stability of suspended nanoparticles and deposition of nanoparticles upon boiling.SUNUM; FEN

Entropy Generation Analysis of Laminar Flows of Water-Based Nanofluids in Horizontal Minitubes under Constant Heat Flux Conditions

During the last decade, second law analysis via entropy generation has become important in terms of entropy generation minimization (EGM), thermal engineering system design, irreversibility, and energy saving. In this study, heat transfer and entropy generation characteristics of flows of multi-walled carbon nanotube-based nanofluids were investigated in horizontal minitubes with outer and inner diameters of ~1067 and ~889 µm, respectively. Carbon nanotubes (CNTs) with outer diameter of 10–20 nm and length of 1–2 µm were used for nanofluid preparation, and water was considered as the base fluid. The entropy generation based on the experimental data, a significant parameter in thermal design system, was examined for CNTs/water nanofluids. The change in the entropy generation was only seen at low mass fractions (0.25 wt.% and 0.5 wt.%). Moreover, to have more insight on the entropy generation of nanofluids based on the experimental data, a further analysis was performed on Al2O3 and TiO2 nanoparticles/water nanofluids from the experimental database of the previous study of the authors. The corresponding results disclosed a remarkable increase in the entropy generation rate when Al2O3 and TiO2 nanoparticles were added to the base fluid

Experimental and numerical investigation of inlet temperature effect on convective heat transfer of γ-Al2O3/water nanofluid flows in microtubes

Nanofluids are the combination of a base fluid with nanoparticles with sizes of 1-100 nm. In order to increase the heat transfer performance, nanoparticles with higher thermal conductivity compared to that of base fluid are introduced into the base fluid. Main parameters affecting single-phase and two-phase heat transfer of nanofluids are shape, material type and average diameter of nanoparticles, mass fraction and stability of nanoparticles, surface roughness and fluid inlet temperature. In this study, the effect of inlet temperature of deionized (DI) water/alumina (Al2O3) nanoparticle nanofluids was both experimentally and numerically investigated. Nanofluids with a mass fraction of 0.1% were tested inside a microtube having inner and outer diameters of 889 and 1067 micrometers, respectively, for hydrodynamically developed and thermally developing laminar flows at Reynolds numbers of 650, 1000, and 1300. According to the obtained numerical and experimental results, the inlet temperature effect was more pronounced for the thermally developing region. The performance enhancement with nanoparticles was obtained at rather higher Reynolds numbers and near the inlet of the microtube. There was a good agreement between the experimental and numerical results so that the numerical approach could be further implemented in future studies on nanofluid flows