Natural convection in square enclosures differentially heated at sides using alumina-water nanofluids with temperature-dependent physical properties

Laminar natural convection of Al2O3 + H2O nanofluids inside square cavities differentially heated at sides is studied numerically. A computational code based on the SIMPLE-C algorithm is used for the solution of the system of the mass, momentum and energy transfer governing equations. Assuming that the nanofluid behaves like a single-phase fluid, these equations are the same as those valid for a pure fluid, provided that the thermophysical properties appearing in them are the nanofluid effective properties. The thermal conductivity and dynamic viscosity of the nanofluid are calculated by means of a couple of empirical equations based on a wide variety of experimental data reported in the literature. The other effective properties are evaluated by the conventional mixing theory. Simulations are performed for different values of the nanoparticle volume fraction in the range 0-0.06, the diameter of the suspended nanoparticles in the range 25-100 nm, the temperature of the cooled sidewall in the range 293-313 K, the temperature of the heated sidewall in the range 298-343 K, and the Rayleigh number of the base fluid in the range 103-107. All computations are executed in the hypothesis of temperature-dependent effective properties. The main result obtained is the existence of an optimal particle loading for maximum heat transfer, that is found to increase as the size of the suspended nanoparticles is decreased, and the nanofluid average temperature is increased

Correlating equations for free convection heat transfer from horizontal isothermal cylinders set in a vertical array

Steady laminar free convection from flat vertical arrays of equally-spaced, horizontal isothermal cylinders set in free air, is studied numerically. A specifically developed computer-code based on the SIMPLE-C algorithm is used for the solution of the mass, momentum and energy transfer governing equations. Simulations are performed for arrays of 2–6 circular cylinders, for center-to-center separation distances from 2 up to more than 50 cylinder-diameters, and for values of the Rayleigh number based on the cylinder-diameter in the range between 500 and 500.000. It is found that the heat transfer rate at the bottom cylinder remains the same as a single cylinder. In contrast, the downstream cylinders may exhibit either enhanced or reduced Nusselt numbers depending on their location in the array and on the geometry of the array. Heat transfer dimensionless correlating equations are proposed both for any individual cylinder in the array and for the whole tube-array. New correlation-equations for the calculation of the heat transfer rate from a single cylinder to the surrounding air are also proposed and compared to those available in the open literature

Interactive free convection from a pair of vertical tube-arrays at moderate Rayleigh numbers

Steady laminar free convection from a pair of vertical arrays of equally-spaced, horizontal isothermal cylinders set in free air, is studied numerically. A specifically developed computer-code based on the SIMPLE-C algorithm is used for the solution of the mass, momentum and energy transfer governing equations. Simulations are performed for pairs of tube-arrays consisting of 1-4 circular cylinders, for center-to-center horizontal and vertical spacings from 1.4 to 24 cylinder-diameters, and from 2 to 12 cylinder-diameters, respectively, and for values of the Rayleigh number based on the cylinder-diameter in the range between 10(2) and 10(4). It is found that any cylinder may exhibit either enhanced or reduced Nusselt numbers with respect to the case of single tube-array, depending on its location in the array, on the geometry of the array, as well as on the Rayleigh number. Heat transfer dimensionless correlating equations are also proposed. (c) 2006 Published by Elsevier Ltd

Natural convection of water near 4°C in a bottom-cooled enclosure

A study of natural convection in water-filled square enclosures whose bottom wall is cooled at 0°C, whereas the top wall is partially or entirely heated at a temperature ranging between 10°C and 30°C is performed numerically through a computational code based on the SIMPLE-C algorithm, assuming temperature-dependent physical properties, for cavity widths in the range 1 cm-10 cm, with the main aim to point out the basic heat and momentum transfer features

Buoyancy-induced convection of water-based nanofluids in differentially-heated horizontal Semi-Annuli

A two-phase model based on the double-diffusive approach is used to perform a numerical study on natural convection of water-based nanofluids in differentially- heated horizontal semi-annuli, assuming that Brownian diffusion and thermophoresis are the only slip mechanisms by which the solid phase can develop a significant relative velocity with respect to the liquid phase. The system of the governing equations of continuity, momentum, and energy for the nanofluid, and continuity for the nanoparticles, is solved by the way of a computational code which incorporates three empirical correlations for the evaluation of the effective thermal conductivity, the effective dynamic viscosity, and the thermophoretic diffusion coefficient, all based on a wide number of literature experimental data. The pressure-velocity coupling is handled through the SIMPLE-C algorithm. Numerical simulations are executed for three different nanofluids, using the diameter and the average volume fraction of the suspended nanoparticles, the cavity size, the average temperature, and the temperature difference imposed across the cavity, as independent variables. It is found that the impact of the nanoparticle dispersion into the base liquid increases remarkably with increasing the average temperature, whereas, by contrast, the other controlling parameters have moderate effects. Moreover, at temperatures of the order of room temperature or just higher, the heat transfer performance of the nanofluid is significantly affected by the choice of the solid phase material

Natural convection from a pair of differentially-heated horizontal cylinders aligned side by side in a nanofluid-filled square enclosure

A two-phase model based on the double-diffusive approach is used to perform a numerical study on natural convection from a pair of differentially-heated horizontal cylinders set side by side in a nanofluid-filled adiabatic square enclosure. The study is conducted under the assumption that Brownian diffusion and thermophoresis are the only slip mechanisms by which the solid phase can develop a significant relative velocity with respect to the liquid phase. The system of the governing equations of continuity, momentum and energy for the nanofluid, and continuity for the nanoparticles, is solved by the way of a computational code which incorporates three empirical correlations for the evaluation of the effective thermal conductivity, the effective dynamic viscosity, and the thermophoretic diffusion coefficient, all based on a wide number of literature experimental data. The pressure-velocity coupling is handled through the SIMPLE-C algorithm. Simulations are executed for three different nanofluids, using the diameter and the average volume fraction of the suspended nanoparticles, as well as the cavity width, the inter-cylinder spacing, the average temperature of the nanofluid, and the temperature difference imposed between the cylinders, as controlling parameters, whose effects are thoroughly analyzed and discussed. It is found that the impact of the nanoparticle dispersion into the base liquid increases remarkably with increasing the average temperature, whereas it increases just moderately as the nanoparticle size decreases, as well as the imposed temperature difference and the cavity width increase. Conversely, the distance between the cylinders seems to have marginal effects. Moreover, an optimal particle loading for maximum heat transfer is detected for most configurations investigated

Buoyancy-induced convection of water-based nanofluids in differentially-heated horizontal Semi-Annuli

A two-phase model based on the double-diffusive approach is used to perform a numerical study on natural convection of water-based nanofluids in differentially- heated horizontal semi-annuli, assuming that Brownian diffusion and thermophoresis are the only slip mechanisms by which the solid phase can develop a significant relative velocity with respect to the liquid phase. The system of the governing equations of continuity, momentum, and energy for the nanofluid, and continuity for the nanoparticles, is solved by the way of a computational code which incorporates three empirical correlations for the evaluation of the effective thermal conductivity, the effective dynamic viscosity, and the thermophoretic diffusion coefficient, all based on a wide number of literature experimental data. The pressure-velocity coupling is handled through the SIMPLE-C algorithm. Numerical simulations are executed for three different nanofluids, using the diameter and the average volume fraction of the suspended nanoparticles, the cavity size, the average temperature, and the temperature difference imposed across the cavity, as independent variables. It is found that the impact of the nanoparticle dispersion into the base liquid increases remarkably with increasing the average temperature, whereas, by contrast, the other controlling parameters have moderate effects. Moreover, at temperatures of the order of room temperature or just higher, the heat transfer performance of the nanofluid is significantly affected by the choice of the solid phase material

Double-Diffusive Effects on the Onset of Rayleigh-Benard Convection of Water-Based Nanofluids

A numerical study on the Rayleigh-Benard convection in a shallow cavity filled with different metal-oxide water-based nanofluids is presented through a two-phase model, which accounts for the effects of the Brownian diffusion and thermophoresis, at variable properties with temperature. Numerical simulations are executed for different values of the average volume fraction of the nanoparticles, different aspect ratios of the enclosure, as well as for temperature difference between the bottom and the top walls. It is found that the dispersion of the nanoparticle into the base fluid increases the stability of the nanofluid layer, determining the conditions for the onset of convection also with reference to the confinement of the nanofluid

Thermal inertia of hollow wall blocks: actual behavior and myths

In the context of growing requirements to save energy in buildings and high objectives for Net Zero Energy Buildings (NZEBs) in Europe, strong emphasis is placed on the thermal performance of building envelopes, and in particular on thermal inertia to save cooling energy. High thermal inertia of outer walls leads to a mitigation of the daily heat wave, reducing cooling peak load and energy demand. Moreover, building envelopes with high heat capacity act as heat storages, increasing the effectiveness of natural ventilation for thermal comfort through a night-day energy shifting. Even though there are some papers available in the open literature on dynamic heat transfer through hollow bricks, yet common calculation methods are applicable to homogeneous layers only. That is the case of ISO 13786 regulation "Thermal performance of building components - Dynamic thermal characteristics - Calculation methods", for example. On the other hand, hollow blocks are very commonly used in building envelopes. Thus, available methods are not suitable for prediction of dynamic thermal performances. On the other hand, the widely common assumption that high mass means high thermal inertia leads to the use of higher mass blocks or bricks. Yet, numerical and experimental studies on thermal inertia of hollow envelope-components have not confirmed this general assumption, even though no systematic analysis has been found in the open literature. In this framework, numerical simulations of the thermal performance of hollow bricks have been done with a specifically-developed finite-difference computational code. Three common basic shapes with different void fraction and thermal properties have been analyzed with a triangular pulse solicitation, in order to highlight the relevance of front mass and other parameters on the thermal inertia, measured through heat wave delay. Results show that wall front mass is often misleading as thickness, number of cavities and clay thermal diffusivity are more important

Treatments and overall survival in patients with Krukenberg tumor

BACKGROUND: Krukenberg tumor (KT) is a rare secondary ovarian tumor, primarily localized at the gastrointestinal tract in most cases. KT is related to severe prognosis due to its aggressiveness, diagnostic difficulties and poor treatment efficacy. Several treatments have been used, such as cytoreductive surgery (CRS), adjuvant chemotherapy (CT) and/or hyperthermic intraperitoneal chemotherapy (HIPEC). To date, it is still unclear which treatment or combination of treatments is related to better survival. OBJECTIVE: To assess the most effective therapeutic protocol in terms of overall survival (OS). METHODS: A systematic review of the literature was performed by searching MEDLINE, Scopus, EMBASE, ClinicalTrial.gov, OVID, Web of Sciences, Cochrane Library, and Google Scholar for all studies assessing the association of treatments with OS in KTs. The effectiveness of each treatment protocol was evaluated by comparing the OS between patients treated with different treatment protocols. RESULTS: Twenty retrospective studies, with a total sample size of 1533 KTs, were included in the systematic review. Therapeutic protocols used were CRS in 18 studies, CT in 13 studies, HIPEC in 7 studies, neoadjuvant CT in 2 studies, and some combinations of these in 6 studies. Seven studies showed that CRS significantly improved OS compared to other treatments or association of treatments without it. 11 studies showed that CRS without residual (R0 CRS) had a significantly better OS than CRS with residual (R + CRS). Five studies showed that CT significantly improved OS, but other five showed it did not. Two studies showed that HIPEC in association with CRS improved OS, while another study showed that efficacy of HIPEC was comparable to CT. Two studies evaluated neoadjuvant CT, but results were conflicting. CONCLUSION: CRS and in particular R0 CRS are the treatments showing the clearest results in improving OS in KT patients. Results about CT are conflicting. HIPEC appears effective both alone and in combination with CRS, and also related to fewer adverse effect than CT. The usefulness of neoadjuvant CT is still unclear. The association of R0 CRS with HIPEC seems to be the most effective and safe therapeutic protocol for KT patients