Non-adiabatic Boundary Condition Research Articles

Water boiling time in a spent fuel pool with adiabatic and non-adiabatic boundary conditions

The Spent Fuel Pool (SFP) of Angra II, from Brazil, has received standard spent fuel (SF) assemblies of Uranium dioxide (UO2) discharged from Pressurized Water Reactors (PWR) of the Nuclear Power Plants (NPP) of Angra since the beginning of its operation. However, in case of using Mixed Oxide (MOX) or Thorium-based fuels, it would require further thermal studies of wet storage. It includes the determination of the water boiling time (Tb) of the SFP in case of breakdown of its external cooling system (ECS). This work presents studies of Tb of a simulated SFP storing mixed SF discharged from PWRs. The types of mixed SF studied include MOX plus UO2, oxide of thorium/uranium (U-Th)O2 plus UO2, and oxide of Thorium/transuranic (TRU-Th)O2 plus UO2. The simulations were implemented in CFX Ansys considering the top of the SFP as either adiabatic or non-adiabatic wall. Tb is considerably higher when the non-adiabatic boundary condition is used.

Exact solution of a non-stationary cavity with one intermode interaction

A non-stationary one-dimensional cavity can be described by the time-dependent and multi-mode effective Hamiltonian of the so-called dynamical Casimir effect. Due to the non-adiabatic boundary conditions imposed in one of the cavity mirrors, this effect predicts the generation of real photons out of vacuum fluctuations of the electromagnetic field. Such photon generation strongly depends on the number of modes in the cavity and their intermode couplings. Here, by using an algebraic approach, we show that for any set of functions parameterizing the effective Hamiltonian, the corresponding time-dependent Schrödinger equation admits an exact solution when the cavity has one intermode interaction. With the exact time evolution operator, written as a product of eleven exponentials, we obtain the average photon number in each mode, a few relevant observables, and some statistical properties for the evolved vacuum state.

Investigation of a High Karlovitz, High Pressure Premixed Jet Flame with Heat Losses by LES

ABSTRACT Large-eddy simulations (LES) are presented for a lean preheated high pressure jet flame experiment for which detailed in situ data is available, using a finite rate chemistry (FRC) approach in a gas-turbine model combustor at high Karlovitz number. The impact of the different combustion models on the flame stabilization in the simulation is investigated and the predicted carbon monoxide (CO) and nitric oxide (NO x ) emissions are analyzed. For the FRC approach, the DRM19 reaction mechanism and a new inhouse skeletal mechanism are applied. The more detailed DRM19 mechanism is extended to include OH* species, the new skeletal mechanism includes CO and NO x reaction paths. An industry relevant tabulated chemistry approach is assessed on the ability to predict this lifted flame, where the flamelet tables are calculated from the detailed GRI-3.0 reaction mechanism. A dynamic thickened flame approach is applied to resolve the flame on the numerical grid including a model for the turbulence chemistry interaction. Adiabatic and non-adiabatic simulations are compared, where the impact of heat losses due to chamber cooling and thermal radiation are considered. Velocities, temperatures, fuel mass fractions and CO and NO x mass fractions at different axial locations are in good agreement to the experiments when heat losses are considered. The significant flame lift was correctly predicted by the FRC approach with DRM19 chemistry when non-adiabatic boundary conditions were applied. This provides evidence that the flame is stabilized by flame propagation assisted by auto ignition and that ignition-delay times of mixtures composed of fresh and burnt gases need to be captured by the applied models.

Numerical analysis of natural convection of alumina–water nanofluid in H-shaped enclosure with a V-shaped baffle

Numerical analysis of the free convection heat transfer in an H-shaped cavity loaded with alumina–water nanofluid is performed in this work. The entire bottom wall of the H-shaped enclosure and the top rib are at the hot temperature, the two sidewalls are at the lower temperature, and the others are insulated. This study aims to elucidate the influences of different variables on the heat transfer of nanofluid, namely the Rayleigh number (Ra = 104–106), nanoparticle volume fraction percentage (ϕ = 0–2%), aspect ratio (AR = 0.2–0.4), installation of baffle, location of baffle in the cavity, and the types of baffle boundary condition. To model the buoyancy force, the Boussinesq approximation is employed. The characteristics of flow and thermal transmission are presented in streamlines and isotherms. The results show that the heat transmission is improved by Rayleigh number enhancement and nanoparticle volume fraction due to the enhanced temperature gradient and the nanofluid thermal characteristics. In addition, the baffle installation at the bottom rib under a non-adiabatic boundary condition improve the heat transfer because the bottom wall has a larger mean temperature than the top wall.

Heat transfer by new families of straight and pin fins: exact solutions

Fins are surface extensions used to increase the rate of heat transfer from a heated surface to the surrounding cooler fluid or from a heated fluid to the surface. In this article, we describe new families of straight and pin fins for which the temperature distribution along the fin as well as fin effectiveness and efficiency can be computed in closed form. The profile of the new straight fin is a circular arc centered on the fin’s longitudinal axis, while the profile of the new pin fin is a more complex concave parametric curve. Although the pattern of temperature distribution in these fins is remarkably similar to that in the classic straight and pin fins of constant thickness, the geometry of the new fins is strikingly different; in particular, the maximum length of the new fins is finite. We also discovered that both classic fins of constant thickness and novel fins described in this work have the following equivalence property: if the ratio of the heat transfer coefficient on the fin’s tip to that on the lateral surface is not too high, then the temperature distribution for a fin with a non-adiabatic boundary condition at the tip is identical to that for a longer fin of the same kind with adiabatic tip. Our analysis of heat transfer by fins is only based on standard homogeneity, steady-state, and one-dimensionality assumptions. In particular, we completely dispense with the “length-of-arc” assumption that underlies most of the previous works.

Large-Eddy Simulation of the lean-premixed PRECCINSTA burner with wall heat loss

Swirl burners are widely used in aeronautical engines and industrial gas turbines. These burners enhance the flame stabilization by bringing back hot products to the reactive zone and their topology ensures a good compactness. However, the outer recirculation zones created in these burners induce wall heat loss that affects the flame structure. This paper proposes a numerical strategy based on Large-Eddy Simulation (LES) and non-adiabatic boundary conditions with a skeletal chemistry approach coupled to the Dynamic Thickened Flame model (TFLES). Simulations were performed on the PRECCINSTA burner with meshes up to 877 millions elements. Results demonstrated the impact of the addition of wall heat loss on the lift-off of the external flame front, leading to a flame topology change from a M-shape to a V-shape. This lift-off height increases with the grid resolution showing the strong influence of the mesh resolution on the flame topology. Comparisons of the non-adiabatic LES results on the finest mesh with the experimental data show an unprecedented agreement. The flame quenching process is analyzed and exhibits the role of strain rate which controls the level of penetration of cooled products within the inner reaction zone.

Non-adiabatic flow characteristics of micro impeller

Non-adiabatic working condition is one of the major causes of performance deterioration in micro gas turbine engines. Complex micro scale geometry, low Reynolds number operating condition and high surface to volume ratio all lead to severe heat transfer. This paper first established a simple heat transfer model to determine appropriate non-adiabatic boundary condition for computational fluid dynamics (CFD) simulations. Isothermal wall temperature is identified as a heat transfer boundary based on model analysis in combination with material selection for pre-design of the engine and verified by the experiment carried out on directed structure applied in the model. A series of numerical simulations with adiabatic and non-adiabatic boundary conditions is then carried out to study the flow characteristics of high speed, low Reynolds number micro impeller. The physical nature for significant performance degradation related to flow behavior changes due to heat transfer effect is revealed by detailed analysis of typical flow features extracted from the comparative investigation. The result established the basis for heat transfer modeling of micro impeller purposing implications for design modification in order to attain high efficiency and better performance.

Large-eddy simulation of stratification effects on dispersion in urban environments

This paper focuses on investigating thermal buoyancy effects on dispersion of approach flows and local heat transfer from/to buildings by taking the DAPPLE site as the test case but for varying wind direction and magnitude. Firstly, based on the BT tower data, only weakly unstable conditions (i.e. Richardson number |Ri|<1) of approach flows were considered, with adiabatic boundary conditions at the ground and building surfaces. It was found that the predicted concentration in the far field was significantly reduced. Secondly, numerical experiments were performed, i.e. using various Richardson numbers. It was found that the modelled mean concentration for bulk Richardson number −0.9 was in reasonable agreement with field data at all stations. Thirdly, to understand the effects of weakly local unstable conditions, non-adiabatic boundary conditions at ground and building surfaces were implemented as well as weakly unstable conditions in the approach flows. No significant difference was found comparing with those using adiabatic boundary conditions. These simulations suggest that stratification effects on dispersion in weakly unstable conditions (e.g. in London) are not negligible.

Molten-salt thermal energy storage in thermoclines under different environmental boundary conditions

Operation during the charge and discharge cycles of molten-salt thermoclines used for solar thermal energy storage depends strongly on the environmental boundary conditions to which the tanks are exposed. A comprehensive model which accounts for thermal transport in the molten-salt heat transfer fluid and the filler material in the tank is developed for exploring the effects of boundary conditions on thermocline performance. Heat loss from the tank under non-adiabatic boundary conditions is found to distort the temperature and salt flow distributions relative to the uniform conditions found in adiabatic thermoclines; as a result, the outflow temperature drops more rapidly in the former case. Such effects of non-adiabatic boundaries become insignificant at large salt-flow Reynolds numbers. As the Reynolds number increases beyond 250, the discharge efficiency of non-adiabatic thermoclines approaches that of the adiabatic counterparts. In the case of significant heat loss at the walls, the discharge efficiency of thermoclines increases with increasing Reynolds number, a trend that is opposite to that in adiabatic thermoclines.