A review on the two-phase pressure drop characteristics in helically coiled tubes

Due to their compact design, ease of manufacture and enhanced heat transfer and fluid mixing properties, helically coiled tubes are widely used in a variety of industries and applications. In fact, helical tubes are the most popular from the family of coiled tube heat exchangers. This review summarises and critically reviews the studies reported in the pertinent literature on the pressure drop characteristics of two-phase flow in helically coiled tubes. The main findings and correlations for the frictional two-phase pressure drops due to: steam-water flow boiling, R-134a evaporation and condensation, air-water two-phase flow and nanofluid flows are reviewed. Therefore, the purpose of this study is to provide researchers in academia and industry with a practical summary of the relevant correlations and supporting theory for the calculation of the two-phase pressure drop in helically coiled tubes. A significant scope for further research was also identified in the fields of: air-water bubbly flow and nanofluid two phase and three-phase flows in helically coiled tubes

A review on the heat and mass transfer phenomena in nanofluid coolants with special focus on automotive applications

Engineered suspensions of nanosized particles (nanofluids) are characterized by superior thermal properties. Due to the increasing need for ultrahigh performance cooling in many industries, nanofluids have been widely investigated as next-generation coolants. However, the multiscale nature of nanofluids implies nontrivial relations between their design characteristics and the resulting thermo-physical properties, which are far from being fully understood. This pronounced sensitivity is the main reason for some contradictory results among both experimental evidence and theoretical considerations presented in the literature. In this Review, the role of fundamental heat and mass transfer mechanisms governing thermo-physical properties of nanofluids is assessed, from both experimental and theoretical point of view. Starting from the characteristic nanoscale transport phenomena occurring at the particle-fluid interface, a comprehensive review of the influence of geometrical (particle shape, size and volume concentration), physical (temperature) and chemical (particle material, pH and surfactant concentration in the base fluid) parameters on the nanofluid properties was carried out. Particular focus was devoted to highlight the advantages of using nanofluids as coolants for automotive heat exchangers, and a number of design guidelines was suggested for balancing thermal conductivity and viscosity enhancement in nanofluids. This Review may contribute to a more rational design of the thermo-physical properties of particle suspensions, therefore easing the translation of nanofluid technology from small-scale research laboratories to large-scale industrial applications

Transient internal forced convection under dynamic thermal loads: in clean-tech and automotive applications

This research aims to address the thermal behavior of emerging engineering applications with dynamic thermal characteristics. Such applications include: i) clean-tech systems, e.g., powertrain and propulsion systems of Hybrid/Electric/Fuel Cell Vehicles (HE/E/FCV); ii) sustainable/renewable power generation systems (wind, solar, tidal); and iii) information technology (IT) systems (e.g., data centers, e-houses, and telecommunication facilities). In this research, transient internal forced-convection was used to model the thermal characteristics of the cooling systems in the above-mentioned applications. In addition, sinusoidal heat flux was considered, since arbitrary loads can be modeled by a superposition of sinusoidal waves using a Fourier transformation series. Additionally, benchmark driving cycles were used to investigate the thermal characteristics of a cooling system in the context of the real-world application of HE/E/FCV. Firstly, the energy equation was solved analytically for a steady tube flow under an arbitrary time-dependent thermal load. Then sinusoidal heat flux was taken into account, and closed-form relationships were obtained to predict the temperature distribution inside the fluid and the Nusselt number. Finally, the presented results were validated using a commercially available software program: ANSYS Fluent. In the next step, the energy equation was solved analytically for tube flow with an arbitrary flow rate and a given time-dependent heat flux. Sinusoidal heat flux and flow rate were then taken into account; closed-form series solutions for the temperature distribution and Nusselt number of the tube flow were presented. An independent numerical simulation was also performed to validate the models.Additionally, new testbeds were designed and built and a comprehensive experimental study was performed to analyze the thermal behavior of a tube flow under arbitrary time-dependent heat flux. It was shown that there was an excellent agreement between the experimental data and the predictions of the developed models.As a result of the above work, a new model was developed that predicts the minimum instantaneous flow rate to maintain the temperature at a given level under an arbitrary time-dependent heat flux. Compared to conventional steady-state designs, the developed model can result in up to a 50% energy savings while maintaining the temperature of the system below the targeted value

Mixed convection heat transfer and pressure drop characteristics of the copper oxide-heat transfer oil (CuO-HTO) nanofluid in vertical tube

In this paper, the mixed natural-forced convection is experimentally investigated for the heat transfer oil-copper oxide (HTO-CuO) nanofluid flow upward in a vertical tube. The flow regime is laminar and the temperature of the tube surface is constant. The effect of the nanoparticles concentration on the heat transfer rate and the pressure drop is studied as Richardson number varies between 0.1 and 0.7. It is observed that the mixed convection heat transfer rate increases with both the nanoparticles concentration and Richardson number. New correlations are proposed to predict the Nusselt number of the nanofluid flow with the reasonable accuracy. As the heat transfer enhancement methods usually accompany with increment in the pressure drop, the figure of merit is evaluated experimentally. As such the maximum figure of merit of 1.31 is achieved using the 1.5% concentration of the nanoparticles in Richardson number of 0.7. This study provides a platform to design next generation of low flow rate nanofluid-based heat exchangers and may improve the accuracy of predicting the mixed convection characteristics of nanofluid flows