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This numerical investigation aims to study the turbulent characteristics and thermal enhancement parameter of CuO–water nanofluids through heat exchangers enhanced with double V-cut twisted tapes. The twist ratio of the twisted tapes is 5.25, and the cut ratio (b/c) is varied from 0 (conventional twisted tape) to 1.8. The Reynolds number is in the range of 5000–15,000, and nanoparticles volume fraction is in the range of 0 < ϕ < 1.5 % . The flow is fully turbulent, and (RNG) k − ϵ turbulent model is used for the numerical analysis. The results reveal that strong turbulent kinetic energy and additional vortex flow through the cuts of the modified twisted tapes are the main reason for better fluid mixing and heat transfer enhancement. The heat transfer enhances about 14.5% for the case of ϕ = 1.5 % . Furthermore, using double V-cut twisted tapes improves the Nusselt number of the nanofluid flow inside heat exchangers about 138% compared to conventional twisted tape without cuts. The maximum value of the thermal performance η = 1.99 is achieved by using nanofluid with ϕ = 1.5 % and b/c = 1.8 at Re = 5000. [ABSTRACT FROM AUTHOR]

A large portion of world's final energy consumption is employed in thermal form. The proper techniques can be used to improve the efficiencies of thermal energy systems and decrease the energy consumption. Phase change material (PCM) storage modules can be coupled with different thermal systems to improve their thermal efficiency. This paper reviews the real potentials of coupling PCM storage modules to heat pipes and heat pumps in terms of energy saving and thermal energy management. The different systems suggested by researchers are described, and the main findings of the studies are provided. The advantages and disadvantages of these combined systems are discussed, and some suggestions for future studies are provided. The results show that heat pipes show great heat transfer ability and have good efficiency in improving heat transfer within PCMs. Moreover, heat pipes can be incorporated into PCMs to decrease the thermal resistance. The purpose of PCM coupled with heat pipe is to incorporate the large thermal conductivity of heat pipe with the large latent heat capacity of PCM. PCMs have ability to balance the unconformity between supply and demand in heat pumps and decrease the size of storage tank used in a heat pump module. [ABSTRACT FROM AUTHOR]

Entropy generation analysis for the Cu–water nanofluid flow through a heat exchanger tube equipped with perforated conical rings is numerically investigated. Frictional and thermal entropy generation rates are defined as functions of velocity and temperature gradients. Governing equations are solved by using finite volume method, and Reynolds number is in the range of 5000–15,000. The effects of geometrical and physical parameters such as Reynolds number, number of holes and nanoparticles volume fraction on the thermal and viscous entropy generation rates and Bejan number are investigated. The results indicate that the thermal irreversibility is dominant in most part of the tube. But it decreases with increasing the nanoparticle volume fraction. Frictional entropy generation reduces with increasing the number of holes from 4 to 10. This is because of stronger velocity gradient near the perforated holes. Bejan number decreases with augment of Reynolds number. [ABSTRACT FROM AUTHOR]

In this paper, a novel study on the heat transfer characteristics in a shell and tube heat exchanger with wavy cosine corrugated wall in structure of tube bundle is investigated experimentally. The optimization procedure is performed by means of response surface methodology in different wavy starting lengths (0 mm ≤ b ≤ 120 mm), hot water flow rates (7 L min−1 ≤ Qh ≤ 11 L min−1), cold water flow rates (11 L min−1 ≤ Qc ≤ 19 L min−1) and wavy wavelengths (0 mm ≤ λ ≤ 80 mm) to obtain maximum effectiveness and the overall heat transfer coefficient. An exergetic sustainability analysis has been done to show how exergy efficiency affects sustainability. The results show that the effectiveness and the overall heat transfer coefficient increase with the cold water flow rates. Additionally, enhancement of the wavy starting lengths decreases the heat exchanger effectiveness and the overall heat transfer coefficient. From sustainability point of view, the corrugated tube is more sustainable than smooth tube. [ABSTRACT FROM AUTHOR]

Two-phase liquid flow in micro-/mini-channels has several thermal advantages in comparison with single-phase liquid flow, such as heat transfer rate, thermosiphon effect and isothermal heat sinks. Multiphase flows in microfluidic devices are usually used as an effective way to improve the cooling process, since phase change process can provide high heat transfer coefficient in comparison with single-phase flows. On the other hand, high surface-to-volume ratio of microheat exchangers can intensify the overall heat rejection from the system. In the current research, three models, namely (a) the formation of interior and exterior phase, (b) hydrodynamics of droplets during condensation process in a diffusive oblique fin (mini-channel) with hydrophilic walls and (c) liquid evacuation from the channel, are investigated by means of a novel perspective of pseudo-potential lattice Boltzmann model. The effects of channel slope angle on velocity profiles and outlet mass flow rate of the condensed liquid are analyzed. The results show that the slope angle plays an important role at the rate of droplet nucleation and on evacuation time. It is observed that the variation of liquid profile at the channel outlet can be neglected at angles more than 45°. Finally, it is revealed that the required time for droplets coalescence and discharging liquid from the mini-channel can be controlled by optimizing the channel angle. [ABSTRACT FROM AUTHOR]

In this paper, the magnetic nanofluids and magnetic field are used to provide a coating around the wall of a channel. The magnetic field is induced by the direct current wire. Iron oxide is used as magnetic nanoparticles. A finite volume method is used to solve the Navier–Stokes equations, and the Eulerian–Lagrangian approach is employed to track the magnetic nanoparticles. The effects of magnetic strength, the position of current wire, and the diameter of magnetic nanoparticles on the trajectory of magnetic nanoparticles and coating efficiency are investigated by providing contours and diagrams. The results show that the length of coating decreases by about 55% with the increase in the particle diameter in the range of 500 nm to 1 μm. Further, the coating efficiency, defined as the ratio of the number of trapped particles on the wall to the number of injected particles at the inlet of the channel, improves by increasing the magnetic strength and decreasing the vertical position of the current wire. [ABSTRACT FROM AUTHOR]

The present work investigates the natural convection of nanofluids in Rayleigh-Benard problem. New models, particularly some enhancement factors, are presented in this research to calculate the heat transfer characteristics of nanofluids around critical Rayleigh numbers, including the criteria in which the creation of Rayleigh-Benard and transition to turbulent flow occurs. The present models are compared with available models in the literature to reveal discrepancies of theoretical models in prediction of nanofluids heat transfer characteristics. This research is conducted for SiO2, Cu,CuO and Al2O3 as nanoparticle and water as the base fluid and discussed the SiO2-water nanofluid in more detail. Results indicate that the Nusselt number and convective heat transfer ratio are reduced for SiO2-water nanofluid for almost all volume fractions and as a result using SiO2 as nanoparticle into water (the base fluid) is not desired for a natural convection in Rayleigh-Benard problem. Moreover, the onset of natural convection is delayed due to the reduction in enhancement factor. [ABSTRACT FROM AUTHOR]