The use of additives in the base fluid like water or ethylene glycol is one of the techniques applied to augment the heat transfer. Newly an innovative nanometer sized particles have been dispersed in the base fluid in heat transfer fluids. The fluids containing the solid nanometer size particle dispersion are called ‘nanofluids’. Two main categories were discussed in detail as the single-phase modeling which the combination of nanoparticle and base fluid is considered as a single-phase mixture with steady properties and the two-phase modeling in which the nanoparticle properties and behaviors are considered separately from the base fluid properties and behaviors. Both single phase and two phase models have been presented in this paper. This paper intends to provide a brief review of researches on nanofluid flow and heat transfer via semi analytical and numerical methods. It was also found that Nusselt number is an increasing function of nanoparticle volume fraction, Rayleigh number and Reynolds number, while it is a decreasing function of Hartmann number.

Nanofluid convective heat transfer using semi analytical and numerical approaches: A review

Entropy generation of nanofluid and hybrid nanofluid flow in thermal systems: A review

MHD mixed convection and entropy generation of nanofluid filled lid driven cavity under the influence of inclined magnetic fields imposed to its upper and lower diagonal triangular domains

The forced convection of nanofluid flow in a long microchannel is studied numerically according to the finite volume approach and by using a developed computer code. Microchannel domain is under the influence of a magnetic field with uniform strength. The hot inlet nanofluid is cooled by the heat exchange with the cold microchannel walls. Different types of nanoparticles such as Al2O3 and Ag are examined while the base fluid is considered as water. Reynolds number are chosen as Re=10 and Re=100. Slip velocity and temperature jump boundary conditions are simulated along the microchannel walls at different values of slip coefficient for different amounts of Hartmann number. The investigation of magnetic field effect on slip velocity and temperature jump of nanofluid is presented for the first time. The results are shown as streamlines and isotherms; moreover the profiles of slip velocity and temperature jump are drawn. It is observed that more slip coefficient corresponds to less Nusselt number and more slip velocity especially at larger Hartmann number. It is recommended to use Al2O3-water nanofluid instead of Ag-water to increase the heat transfer rate from the microchannel walls at low values of Re. However at larger amounts of Re, the nanofluid composed of nanoparticles with higher thermal conductivity works better.

The effects of different nano particles of Al2O3 and Ag on the MHD nano fluid flow and heat transfer in a microchannel including slip velocity and temperature jump

In this study, natural convection in a nano-fluid filled cavity having different shaped obstacles (circular, square and diamond) installed under the influence of a uniform magnetic field and uniform heat generation was numerically investigated. The cavity was heated from below and cooled from the vertical sides while the top wall was assumed to be adiabatic. The temperatures of the side walls vary linearly. The governing equations were solved by using Galerkin weighted residual finite element formulation. The numerical investigation was performed for a range of parameters: external Rayleigh number (104 ≤ RaE ≤ 106), internal Rayleigh number (104 ≤ RaI ≤ 106), Hartmann number (0 ≤ Ha ≤ 50), and solid volume fraction of the nanofluid (0 ≤ ϕ ≤ 0.05). It is observed that the presence of the obstacles deteriorates the heat transfer process and this is more pronounced with higher values of Re E . Averaged heat transfer reduces by 21.35%, 32.85% and 34.64% for the cavity with circular, diamond and squared shaped obstacles compared to cavity without obstacles at RaI = 106. The effect of heat transfer reduction with square and diamond shaped obstacles compared to case without obstacle is less effective with increasing values of Hartmann number. Second law analysis was also performed by using different measures for the normalized total entropy generation.

Natural convection and entropy generation of nanofluid filled cavity having different shaped obstacles under the influence of magnetic field and internal heat generation

In this paper, mixed convection as well as second law of thermodynamics analysis in a three-dimensional microchannel filled with a nanofluid under a magnetic field are numerically studied. The temperature fields, variation of horizontal velocity, thermal resistance, pressure drop, Hartmann and Reynolds numbers are investigated. Moreover, heat, frictional and magnetic entropy generation are surveyed in different volume fractions. Analyzing the results of numerical simulations indicates that with increasing Hartmann number, maximum horizontal velocity along the centre line and the inlet and outlet thermal resistance decrease in the microchannel. On the other hand, by enhancing the strength of the imposing magnetic field, heat entropy generation mitigates, while frictional and magnetic ones increase. However, the increasing of two last is very small compared to heat entropy generation. The ratio of Nuavg/(pressure drop) is greater than 10. Therefore, the thermal gain of this microchannel fairly dominates the loss of pressure reduction.

MHD mixed convection and entropy generation in a 3-D microchannel using Al2O3–water nanofluid

The aerobic wastewater treatments depend on the aeration. Hence, the size of the bubbles used in the aeration system may play a crucial role in this regard. This study attempted to investigate the effects of aeration bubble size on wastewater treatment efficiency of a “sequence batch reactor” (SBR) system at a laboratory scale using a novel designed fine/nanobubble forming instrumentation system. Based on the presence of microorganisms in the stationary phase, chemical oxygen demand removal efficiency on the 15th day (80.0 and 95.0%) was majorly better than on the 10th and 15th days in fine and nanobubble aeration systems. Moreover, with increasing sludge age, the “sludge volumetric index” (SVI) increased up to 170.0 ml g−1 on the 15th day. In addition, sludge rate and F/M ratio were much higher and expressively less in the nanobubble system rather than in the fine-bubbles system in which sludge was majorly denser. Therefore, the sludge was more easily deposited and the percentage of dry sludge was higher compared with the fine-bubble system. Thus, oxygen and specific oxygen uptake rate consumption were significantly reduced. The efficiency of the phosphorus removal was estimated to be between 54.0–60.0% for nanobubble aeration, compared to the general systems such as the SBR (10–20%) under similar conditions. In addition, the efficiency of the nitrogen removal in the nanobubble aeration system with different densities of 40.0, 50.0, and 60.0 ml g−1 was found as 99.0%, relatively higher compared to fine bubble with 96.0% nitrogen efficiency. In conclusion, a nanobubble aeration system could give considerably promoted efficiencies in all terms of the tested treatment effective parameters.

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M. Haji Ahmadi

Papers

MHD mixed convection and entropy generation in a 3-D microchannel using Al2O3–water nanofluid

Efficient Wastewater Treatment via Aeration Through a Novel Nanobubble System in Sequence Batch Reactors