[en] A numerical study of a turbulent buoyant helium jet developing in a two-vented cavity is conducted based on a well-resolved numerical simulation. A sufficiently large exterior region is modelled in the computational domain to approach the natural inlet/outlet conditions of the vents, leading to a good agreement between the numerical results and available experimental particle image velocimetry (PIV) measurements. For a release of helium with a flow rate of 5 Nl.min^-1 in an air-filled cavity about 15 cm high, the flow structure and the helium dispersion are analysed illustrating the strong confinement effect enhanced by a cross-flow from the lower vent. By tracking the jet axis deviation and comparing the classic plume description in terms of global fluxes estimated from DNS results to the Morton et al.' theory [1], we quantify the effect of the confinement and the cross-flow on the flow structure. Finally, we highlight a blocking zone at mid-cavity height originated from confinement, where helium accumulates. (authors)

[en] Near-field radiative heat transfer (NFRHT) between rough surfaces, due to its widespread presence in engineering practice of near-field energy utilization, requires indepth studies, especially from the perspective of physical mechanism. In this paper, an effective multilayer model is built to approach the NFRHT between random rough surfaces of silicon carbide (SiC). Using the effective medium theory (EMT), the effective dielectric function of each layer is obtained, which forms a gradient distribution of dielectric function along the depth of the medium. The influence of the effective dielectric function on surface phonon polaritons (SPhPs) is analyzed, showing that the effective layers with small filling fraction of SiC feature lower SPhP resonance frequencies than SiC bulk. The coupling of SPhPs from the gradient distribution of dielectric function produces new surface modes that dominates the NFRHT. Investigation on the effect of root mean square height (RMS height, σ) reveals that the peaks of local density of states (LDOS) and spectral heat flux are red-shifted as σ increases, while the spectral heat flux below the peak frequency gets larger. This can be attributed to the coupling of SPhPs inside the rough layer. We also found the total net heat flux between rough surfaces separated by an average distance exceeds that between smooth plates and increases with increasing σ, which offer a new way to enhance NFRHT. Finally, this work provides a reference for the simulation and understanding of the NFRHT between rough surfaces.

[en] Results of the study on SO2 reduction in a utility boiler furnace by means of furnace sorbent injection are presented in this paper with analysis of major influential parameters. The Ca-based sorbent injection process in pulverized lignite fired boiler furnace with tangentially arranged burners is simulated. In simulations sorbent particles are distributed among the burner tiers, where they are injected together with coal, and also through sorbent injection ports located above the burners. The sorbent reactions model was adapted to be efficiently implemented in the code for CFD simulations of complex processes considering both the calculation time and the results accuracy. The sorbent particles reaction model was simplified with several assumptions to allow for faster calculations and significantly reduce simulation time without loss in calculation precision during the particle tracking in boiler furnace. Two phase gas-particle flow is modeled, with coal and sorbent particles reactions and interactions with gaseous phase. Test-cases based on fuels with different composition and combustion organization were simulated in details, and results showed that significant increase in reduction of SO2 at furnace exit could be achieved by proper sorbent injection. The sorbent injection locations were analyzed with special care to enable maximum SO2 capture in the case-study furnace under investigated conditions. Most of the test-cases with low SO2 capture had one or more of the following problems: intensive particle sintering, low local temperatures (leading to low calcination rates), or bad particles distribution. Significant SO2 retention was possible when the process was organized in such a way that particles were exposed to optimal temperature range, and injected in the furnace zones with high SO2 concentration simultaneously. It was shown that better results can be achieved by injection of sorbent through multiple burner tiers, with SO2 emission reduction efficiency around 60% at the furnace exit in several well optimized test-cases. © 2018 Elsevier Ltd

[en] We present an experimental and theoretical investigation of single-phase heat transfer under exponential power inputs. We conduct forced flow experiments with water, covering a broad range of mass fluxes (from 0 to 19, 300 kg/m²/s), bulk temperatures (from 25 to 100 degrees C), pressures (from 0.1 to 1.2 MPa), exponential power escalation periods (from 2.5 to 200 ms), and considering two different heater and channel geometries. We use a high-speed infrared thermometry technique to measure the space- and time-dependent heat transfer coefficient between the heated surface and the coolant, building a database covering 73 different experimental conditions. We consider turbulence as a diffusive process and develop an analytic model that can predict 80 percent of our database within a ± 10 percent error, and the entire database within a ± 20 percent error. We discuss the presence of three heat transfer regimes, i.e., transient conduction, transient turbulent diffusion, and quasi-steady turbulent heat transfer, and derive analytically the two associated transition criteria. These transitions depend on the power escalation period, fluid properties, and are connected to the profile of the turbulent diffusion properties across the boundary layer. (authors)

[en] Forced convective boiling is of great interest for several applications in the power and process industry, particularly in nuclear plants. Under certain nominal, incidental or accidental conditions, a boiling crisis may occur resulting in the meltdown of the heating surface. It is then essential to predict as accurately as possible the thermal-hydraulic conditions leading to the occurrence of this boiling crisis. Such an objective cannot reasonably be achieved without a good description of the associated two-phase flow. The objective of the present study is twofold: (1) to identify the necessary key parameters for correctly de-scribing boiling flows, and (2) to present in a didactic way an original stationary and local model involving these parameters. This new model is primarily based on four mixture balance equations, a sub-model for the local vapor generation rate, and a turbulence sub-model inspired by the pioneering work. The results obtained with this original boiling flow model are then compared to an extensive experimental data set obtained on a R12/R134a experimental facility. The comparison clearly demonstrates that this new model contains the fewer necessary submodels to describe the structure of a boiling two-phase flow under pressurized water reactor conditions. Subcooled boiling is acceptably described by the model. However, for higher values of void fraction, the model always predicts a nonexistent void fraction peak near the heating wall and overpredicts the wall and liquid temperatures. This behavior may be explained by: (i) the inadequacy of the radial turbulence modeling, (ii) the use of Prandtl's analogy whose validity under boiling conditions is questionable, and (iii) too simplistic a model for the vapor generation rate. (authors)

[en] Reactive gaseous films between two evaporating liquids are studied both analytically and numerically, under simplifying assumptions. For high enough heats of reaction, a semi-analytical solution is exhibited. This solution is used to validate an Arbitrary Lagrangian-Eulerian method. Computations are carried out for low heats of reaction, showing how the system behavior is strongly influenced by the value of the heat of reaction compared to the heats of vaporization of reactants. (authors)

[en] An alternative methodology is proposed here to overcome the excessive cost of large eddy simulations (LES) of full-length heated rod bundle calculations, while improving the inaccurate results typically obtained with Reynolds-averaged Navier-Stokes equations (RANS). While the cost of the full-length LES is generally too high, LES of a small section of a single rod is usually affordable. The idea presented here consists of using the information granted by the small LES calculation to determine the appropriate turbulence viscosity or turbulent thermal diffusivity that can be used to solve only for the temperature field in a pseudo-RANS approach. The study has been performed with single-rod simulations with a P/D of 1.12 and 1.24, considering rod lengths that are representative of reactor applications, for the cases of uniform heat flux and a more realistic cosine-like axial heat distribution. Here, the spectral element code Nek5000 has been used for all LES, RANS, and pseudo-RANS simulations. The recently proposed Nek5000 steady-state solver has been used for solving the temperature field in the pseudo-RANS approach and has proved significantly faster than transient schemes. Prediction of thermal quantities is compared with classical linear and nonlinear RANS models. LES for the full-length rods has also been performed and is used as a reference. Results of the proposed method show significant improvements with respect to those obtained with RANS.

[en] Various fluids such as water, gases (helium), molten salts (FLiNaK, FLiBe) and liquid metal (sodium) are used as a coolant of advanced small modular reactors (SMRs). The printed circuit heat exchanger (PCHE) has been adopted as the intermediate and/or secondary heat exchanger of SMR systems because this heat exchanger is compact and effective. The size and cost of PCHE can be changed by the coolant type of each SMR. In this study, the crossflow PCHE analysis code for advanced small modular reactor has been developed for the thermal design and cost estimation of the heat exchanger. The analytical solution of single pass, both unmixed fluids crossflow heat exchanger model was employed to calculate a two dimensional temperature profile of a crossflow PCHE. The analytical solution of crossflow heat exchanger was simply implemented by using built in function of the MATLAB program. The effect of fluid property uncertainty on the calculation results was evaluated. In addition, the effect of heat transfer correlations on the calculated temperature profile was analyzed by taking into account possible combinations of primary and secondary coolants in the SMR systems. Size and cost of heat exchanger were evaluated for the given temperature requirement of each SMR

[en] This paper presents an investigation of transient pool boiling heat transfer phenomena in water at atmospheric pressure under exponentially escalating heat fluxes on plate-type heaters. Exponential power escalations with periods ranging from 5 to 100 ms, and subcooling of 0.25 and 75 K were explored. What makes this study unique is the use of synchronized state-of-the-art diagnostics such as infrared (IR) thermometry and high-speed video HSV, which enabled accurate measurements and provided new and unique insight into the transient boiling heat transfer phenomena. The onset of nucleate boiling (ONB) conditions were identified. The experimental data suggest that ONB temperature and heat flux increase monotonically with decreasing period and increasing subcooling, in accordance with the predictions of a model based on transient conduction and a nucleation site activation criterion. Various boiling regimes were observed during the transition from ONB to fully developed nucleate boiling (FDNB). Onset of the boiling driven (OBD) heat transfer regime and overshoot (OV) conditions were identified, depending on the period of the power escalation and the subcooling. Forced convection effects have also been investigated and are discussed in the companion paper (Part II). (authors)

[en] This paper presents an investigation of forced convection effects on transient boiling heat transfer of water on plate-type heaters, at atmospheric pressure, under exponentially escalating heat fluxes. It complements the work performed under pool boiling conditions presented in the companion paper (Part I). Infrared (IR) thermometry and high-speed video (HSV) were used to gain insight into the physical phenomena and generate data that can be used for development and validation of accurate models of transient flow boiling heat transfer. Exponential power escalations with periods in the range from 5 to 500 ms, and subcooling of 10, 25 and 75 K were explored. The Reynolds number was varied from 25,000 to 60,000, depending on the subcooling. Single-phase heat transfer, onset of the boiling driven (OBD) heat transfer regime and overshoot (OV) conditions were identified. The experimental data suggest that during the single-phase heat transfer regime, forced convection is the dominant heat transfer mechanism for long periods, whereas transient conduction is more important for short periods. A criterion based on the normalized time scale for turbulent heat transfer is shown to capture all single-phase heat transfer data on a single curve. At a given period, OBD heat flux and wall superheat, as well as OV wall superheat increase with increasing subcooling and increasing Reynolds number. For a given Reynolds number and a given subcooling, they decrease with increasing periods at short periods, when the dominant heat transfer mechanism is transient conduction, whereas they barely change for long periods, as the dominant single-phase heat transfer mechanism is forced convection. Once boiling is fully developed, the heat transfer coefficient is proportional to the wall superheat to the fourth power and increases with increasing subcooling. (authors)