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[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] The paper presents the results of calculated research on determining the thermal technical indicators of a combined solar desalinization and drying plant. The structure of the plant is developed and proposed. A mathematical model is developed that describes the thermal processes occurring in the plant based on heat-balance equations solved using the Laplace method.
[en] Steam condensation tests were conducted on titanium corrugated tubes at the pressure of 5 kPa and 10 kPa, which are the operating pressures of a power plant condenser. Seven 22.2 mm O.D titanium corrugated tubes had 6.4 mm ≤ p ≤ 22.1 mm and 0.2 mm ≤ e ≤ 0.6 mm. The condensation heat transfer coefficient increased as the corrugation pitch or the corrugation depth decreased. The tube having p = 6.4 mm and e = 0.2 mm yielded the highest heat transfer coefficient with the enhancement from 57 % to 75 %. Comparison with Mehta and Raja Rao correlation revealed that the data are predicted within ±15 %.
[en] Highlights: • CFD analyses are performed on heat transfer of super critical water in rod bundles. • Low y+ approaches may lead to unreliable results when facing high thermal loads. • The use of wall functions is suggested as a more prudent approach. - Abstract: The present paper summarises the results of the simulation of heat transfer to supercritical water in rod-bundle geometries by a CFD code, using wall function models. Two different sets of experimental data were considered, concerning both relatively high and low mass flux conditions and inlet temperature spanning from low to near-critical values. In past analyses, the unsuitability of low-Reynolds number turbulence models was observed in predicting heat transfer in rod bundles, when both high mass and heat flux values were imposed; in fact, large overestimations of wall temperatures were reported in such conditions. This motivated to try simpler models, such as the wall function approach, in order to investigate if, though expectedly not very accurate, they could at least reproduce the experimental data at an acceptable level. As reported in the present paper, the selected models, which adopt a “high y+” wall treatment (indicating wall functions in the STAR-CCM+ code), seem reasonably able at reproducing the general observed experimental trends. The present understanding of the phenomena and the available modelling techniques unfortunately do not allow obtaining better results when the wall or fluid temperature approach the pseudocritical value. However, the comparison with results obtained by low-Reynolds models shows that, at least when facing operating conditions similar to the ones considered in the present work, a downgrading of the adopted modelling techniques may be beneficial and allows obtaining reasonable results. Analyses were also performed considering conditions far from the pseudo-critical temperature, in which low-Reynolds models had provided good performance, reporting that the wall function approach seems effective also in these cases for obtaining first guess results.
[en] This paper reports the numerical results of the mixed convection and entropy generation of Cu-water nanofluid flow in an open cavity heated from different sides with non-uniform temperature distribution. The finite volume method is used to solve the governing equations. The analysis is carried out by a range of Richardson numbers, 0.01 ≤ Ri ≤ 10, at a nanoparticle volume fraction of 0 ≤ φ ≤ 0.1, and Reynolds number Re = 200, with a cavity aspect ratio of L/H = 2. Three heating modes are considered: (A) the left wall is heated (inflow side, assisting flow); (B) the horizontal bottom wall is heated; and (C) the right wall is heated (outflow side, opposing flow). The results show that the heat transfer and the entropy generation increase with increasing Richardson number and nanoparticle volume fraction. The highest heat transfer and entropy generation are obtained with heating mode C (opposing flow). The contribution of heat transfer and fluid friction irreversibilities in the entropy generation depends on Richardson number and the heater position. The present investigation shows that the configuration with non-isothermal heater located at the bottom wall (B) has the highest performance in terms of heat transfer enhancement with minimum entropy generation. (author)
[en] A two-phase solidification process for a one-dimensional semi-infinite material is considered. It is assumed that it is ensued from a constant bulk temperature present in the vicinity of the fixed boundary, which it is modelled through a convective condition (Robin condition). The interface between the two phases is idealized as a mushy region and it is represented following the model of Solomon, Wilson, and Alexiades. An exact similarity solution is obtained when a restriction on data is verified, and it is analysed the relation between the problem considered here and the problem with a temperature condition at the fixed boundary. Moreover, it is proved that the solution to the problem with the convective boundary condition converges to the solution to a problem with a temperature condition when the heat transfer coefficient at the fixed boundary goes to infinity, and it is given an estimation of the difference between these two solutions. Results in this article complete and improve the ones obtained in Tarzia (Comput Appl Math 9:201–211, 1990).
[en] The convective heat transfer coefficient of the mold is an important parameter in the numerical simulation of the injection molding process, and it significantly affects the results. This influences many factors related to processability including the time required for filling and solidification. This study examines the previous work on this coefficient, including commercial implementations, and explains its characteristics. From real cases, the filling time, solidification time, and pressure distribution have been presented according to this coefficient, and a method for treating it is suggested.
[en] We propose a simple theoretical calculation scheme based on the phenomenological Fourier heat-flow formalism to study thermal transport behaviors in nanoscale copper rods. The axial heat transport is characterized by a new super-oscillatory feature along with small-amplitude heat spikes. It is anticipated that these atypical spikes are generated by accumulation of localized 'hotspots' that have low heat dissipation characteristics. In the case of radial transport, we witness the existence of three distinct heat regimes owing to buildup of hot electrons after experiencing ballistic scattering events. It is important to note that, even though the nanorod diameter is comparable to or smaller than the electron mean free path length, λmfp ⁓ 30 nm; multiple ballistic electronic scattering from the outer surface of the nanorods and subsequent accrual into several layers through secondary collisional events has led to concentric heat zones. The hotspots disappear when the nanorod diameter exceeds λmfp. (author)
[en] Heat transfer fluids are important component in transferring heat through heat exchangers in variety of industrial applications including solar energy. Measurement of convective heat transfer coefficients in experimental setup simulating as much actual operating conditions as possible is one reliable method. Experimenting with fully synthetic heat transfer oil meant for use in concentrated solar power plants, the paper presents experimental data for the oil run in a closed-loop indoor test setup up to high temperatures of 200 °C and at two flow rates of 900 and 1200 kg h−1. Convective heat transfer coefficients were calculated based on actual steady-state heat transfer taking place between the hot oil and cold water flowing in a counterflow shell and tube heat exchanger. It was observed that the convective heat transfer coefficient is higher at lower oil flow rate and there is more variation in the experimental values at lower flow rates of oil. On the contrary, the coefficients of convective heat transfer on the basis of empirical correlations at same two oil flow rates were calculated to be higher at higher oil flow rate with the variation uniformly patterned. With respect to calculations based on empirical correlations and experimentally observed values, a comparison of convective heat transfer coefficient “hi” for oil at the two flow rates, the empirically calculated heat transfer coefficients show an increasing trend with a definite gradient, while the experimental values show variable trend which is increasing initially with temperature, then drops slightly and then again starts to increase. In view of the fact that the empirical correlations do not take into account the nature and chemistry of the oil, it has been concluded that the experimental determination of heat transfer coefficient is reliable and feasible, though it may not necessarily correlate with the theoretically derived values.