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[en] Compared to the pure refrigerants, the zeotropic refrigerant mixtures have the obvious temperature glide during phase change. Therefore, the Lorenz cycle can be approached with this special attribute. By analysing the heat transfer in the counter flow heat exchanger, a new evaluation method for zeotropic refrigerant mixtures based on the variance of the temperature difference between the refrigerant and heat transfer fluid (HTF) is proposed in this paper. For approaching to the Lorenz cycle and perfect glide matching, the zeotropic mixture which has smaller variance in the heat exchanger should be chosen in the refrigeration cycle. The variance of temperature difference is affected by two factors which are the temperature difference between the inlet and outlet of HTF and the linear relationship between the refrigerant temperature and enthalpy, respectively. The smallest variance of the zeotropic refrigerant can be obtained by setting the temperature difference of HTF to be the optimal temperature difference.
[en] A simplifying approach for calculating the radiant energy is achieved by introducing the concept of net transmittance, resulting in a novel variation of the net radiation method that provides an easy way for solving a variety of situations. In particular, a closed form for the net radiation between two grey plates through a radiation shield formed by a series of partially transparent partially reflecting partially absorbing plates is found. In addition, the method is generalized to cylindrical and spherical geometries
[en] Highlights: •LES analyses are performed for a simple problem with supercritical pressure water. •Cases with and without conjugate heat transfer at the heating wall are considered. •Larger wall temperature oscilattions were observed without conjugate heat transfer. •The need to perform resolved CFD analyses with conjugate heat transfer is suggested. -- Abstract: The present paper reports the results obtained from Large Eddy Simulation (LES) analyses concerning the effect of conjugate heat transfer with walls when dealing with heat transfer to supercritical pressure fluids. A specific operating condition was investigated with and without walls showing a clearly understandable effect on turbulence of the actual characteristics of the wall, something further complicating turbulence modelling, already quite difficult in this field. Unlike the experience of past studies, which considered conjugate heat transfer by LES or DNS when dealing with constant property fluids, and resulted in a limited influence on the observed phenomena, in the present work relevant effects are instead identified. In fact, in the case of supercritical pressure fluids, the strong changes in fluid properties close to the pseudo-critical threshold may provide a strong feedback on the velocity field and then on turbulence; in particular, the presence of a wall with realistic properties strongly damps the large temperature and fluid properties fluctuations obtained when imposing a constant heat flux. Consequently, unlike fluids in standard conditions, heat transfer to supercritical fluids seems to be depending on the actual fluid-and-wall coupling, thus adding a further challenging aspect in this already complicated topic. Though further analyses are underway for confirming the observed behaviour, the presented findings related to a simple example open new scenarios in the development of heat transfer correlations and CFD models to be used for supercritical fluids. In fact, the available data, both experimental and by DNS, can no more be considered independent from the imposed boundary conditions at the wall and the effect of the wall properties should be seriously taken into account.
[en] Highlights: • A stochastic optimization approach is proposed to design organic heat transfer fluids. • Risk metrics are used to design fluids that withstand strong variability in system conditions. • Non-intuitive mixture compositions are identified. - Abstract: Over 50% of the heat generated in industry is in the form of low-grade heat (with operating temperatures below 370 °C). Recovering heat from these sources with standard Rankine cycles (using water as working fluid) is inefficient and expensive. Organic working fluids have become an attractive alternative to mitigate these inefficiencies. In this work, we address the problem of designing flexible multi-component organic fluids capable of withstanding variability in heat source temperatures and efficiencies of individual cycle equipment units. The design problem is cast as a nonlinear stochastic optimization problem and we incorporate risk metrics to handle extreme variability. We show that a stochastic optimization framework allows us to systematically trade-off performance of the working fluid under a variety of scenarios (e.g., inlet source temperatures and equipment efficiencies). With this, it is possible to design working fluids that remain robust in a wide range of operational conditions. We also find that significant flexibility of the working fluid can be obtained by using optimal concentrations as opposed to using single component mixtures. We also find that state-of-the-art nonlinear optimization solvers can handle highly complex stochastic optimization problems that incorporate detailed physical representations of the system.
[en] Research interest in convective heat transfer using suspensions of nano-sized solid particles has been growing rapidly over the past decade, seeking to develop novel methods for enhancing the thermal performance of heat transfer fluids. Due to their superior transport properties and significant enhancement in heat transfer characteristics, nanofluids are believed to be a promising heat transfer fluid for the future. The stability of nanofluids is also a key aspect of their sustainability and efficiency. This review summarizes the recent research findings on stability, thermophysical properties and convective heat transfer of nano-sized particles suspended in base fluids. Furthermore, various mechanisms of thermal conductivity enhancement and challenges faced in nanofluid development are also discussed. (topical review)
[en] Nanofluid has a potential to become a promising coolant in many diverse industrial processes. However, that opportunity faces several challenges that need to be solved through a long road of nanofluid research programs. Three kinds of the challenges that will be studied in this paper are: 1) determination of nanofluid thermophysical properties, 2) heat transfer characteristics of nanofluid, and 3) the stability factor of nanofluid. This paper also assesses the issue that must be addressed when nanofluid is utilized in nuclear technology applications. The radiation safety aspect of nanofluid utilization in nuclear reactor technology must be taken into account. The comprehensive and multidisciplinary research and assessment are crucial to be carried out in order to ensure the practical applications of nanofluid as new and potential heat transfer fluid. (paper)
[en] A large number of simple test cases have been set up to test so far as possible all the facilities available in TAU. Wherever possible the results are compared with analytical solutions. Comparisons are also made with HEATRAN and ANSYS. (author)
[en] Highlights: • Prepared samples showed excellent stability after 14 days of preparation. • Thermal conductivity has been enhanced and the maximum enhancement was 40%. • A new correlation to predict the thermal conductivity has been proposed. • The nanofluid showed great potential to be used as a coolant fluid. • The convective heat transfer coefficient enhanced by increasing solid concentration. - Abstract: The present research aims to suggest a three-step guideline towards selecting a proper Nanofluid regarding the heat transfer effectiveness. To do so, employing two-step technique, the nanofluid’s samples were prepared in various nanoparticles’ concentrations (0.125, 0.25, 0.5, 0.75, and 1%) of MWCNT-ZnO hybrid nanoparticle in a thermal oil. The samples’ stability has been examined employing the Zeta potential analysis. The samples’ thermal conductivity has been experimentally measured at various temperatures (15, 25, 35, 45, and 55 °C) and solid concentrations. After that, a three-step guideline to select a proper nanofluid as a heat transfer fluid has been proposed. Then, for both the internal laminar and turbulent regimes, variations of pumping power due to adding hybrid nanoparticle has been theoretically studied. Furthermore, the possible effects of adding nanoparticles on the convective heat transfer coefficient in a microchannel heat sink have been investigated. The results declared that the convective heat transfer coefficient had been enhanced by 42%. It is concluded that the produced nanofluid, as coolant fluid, would bring a certain benefit in heat transfer applications. These pre-assessment process would ease the decision-making process in selecting a new coolant which possesses superior heat transfer properties in comparison to the conventional coolants (i.e., water and oil).
[en] Highlights: • A tube-in-tank latent thermal energy storage (LTES) unit using composite PCM is built. • Thermal performances of the LTES unit are experimentally and numerically studied. • Thermal performances of the LTES unit under different operation conditions are comparatively studied. • A 3D numerical model is established to study the heat transfer mechanisms of the LTES unit. - Abstract: Paraffin is a commonly used phase change material (PCM) which has been frequently applied for thermal energy storage. A tube-in-tank latent thermal energy storage (LTES) unit using paraffin as PCM is built in the present study, which can be used in many applications. In order to enhance the thermal performance of the LTES unit, the composite PCM is fabricated by embedding copper foam into pure paraffin. The performances of the LTES unit with the composite PCM during the heat charging and discharging processes are investigated experimentally, and a series of experiments are carried out under different inlet temperatures and inlet flow velocities of the heat transfer fluid (HTF). The temperature evolutions of the LTES unit are obtained during the experiments, and the time-durations, mean powers and energy efficiencies are estimated to evaluate the performance of the LTES unit. Meanwhile, a three-dimensional (3D) mathematical model based on enthalpy-porosity and melting/solidification models is established to investigate the heat transfer mechanisms of the LTES unit and the detailed heat transfer characteristics of the LTES unit are obtained. It can be concluded that the LTES unit with the composite PCM shows good heat transfer performance, and larger inlet flow velocity of the HTF and larger temperature difference between the HTF and PCM can enhance the heat transfer and benefit the thermal energy utilization. Furthermore, a LTES system with larger thermal energy storage capacity can be easily assembled by several such LTES units, which can meet versatile demands in applications.