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[en] The contribution under consideration reports on a gamma denitometric measurement of gas concentrations in a vertical heated tube bundle which is flowed around by a fluid. Two measurement positions, two flow rates of the circulating fluid, two subcooling values and eleven heat fluxes were selected for the measurement. The authors of this contribution describe the test facility, measurement methodology, results and their interpretation. The measurement uncertainty is described in detail.
[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] In the present work, we check the applicability of the effective medium model (EMM) to the problems of radiative heat transfer (RHT) through so-called wire metamaterials (WMMs)—composites comprising parallel arrays of metal nanowires. It is explained why this problem is so important for the development of prospective thermophotovoltaic (TPV) systems. Previous studies of the applicability of EMM for WMMs were targeted by the imaging applications of WMMs. The analogous study referring to the transfer of radiative heat is a separate problem that deserves extended investigations. We show that WMMs with practically realizable design parameters transmit the radiative heat as effectively homogeneous media. Existing EMM is an adequate tool for qualitative prediction of the magnitude of transferred radiative heat and of its effective frequency band.
[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] Nanofluid is an innovative heat transfer fluid with superior potential for enhancing the heat transfer performance of conventional fluids. Though many attempts have been made to investigate the abnormal high thermal conductivity of nanofluids, the existing models cannot precisely predict the same. An attempt has been made to develop a model for predicting the thermal conductivity of different types of nanofluids. The model presented here is derived based on the fact that thermal conductivity of nanofluids depends on thermal conductivity of particle and fluid as well as micro-convective heat transfer due to Brownian motion of nanoparticles. Novelty of the article lies in giving a unique equation which predicts thermal conductivity of nanofluids for different concentrations and particle sizes which also correctly predicts the trends observed in experimental data over a wide range of particle sizes, temperatures, and particle concentrations.
[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] Cooling system is highly influenced by the process of convection heat transfer from the heat source to the cooling fluid. The cooling fluid usually used conventional fluid such as water. Cooling system performance can be improved by using fluids other than water such as nano fluid that is made from a mixture of water and nano-sized particles. Researchers at BATAN Bandung have made nano fluid ZrO_2 from local materials, as well as experimental equipment for studying the thermohydraulic characteristics of nano fluid as the cooling fluid. In this study, thermohydraulic characteristics of nano fluid ZrO_2 are observed through experimentation. Nano fluid ZrO_2 is made from a mixture of water with ZrO_2 nano-sized particles of 10-7-10-9 nm whose concentration is 1 g/liter. This nano fluid is used as coolant in the cooling process of natural convection. The natural convection process depends on the temperature difference between heat source and the cooling fluid, which occur in the thermal boundary layer. Therefore it is necessary to study the thermal boundary layer thickness of nano fluid ZrO_2, which is also able to determine the local velocity. Experimentations are done with several variation of the heater power and then the temperature are measured at several horizontal points to see the distribution of the temperatures. The temperature distribution measurement results can be used to determine the boundary layer thickness and flow rate. It is obtained that thermal boundary layer thickness and velocity of nano fluid ZrO_2 is not much different from the conventional fluid water. (author)