Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userHeat Transfer in Underground Rail Tunnels
99
0
0.0
(
0
)
Added by
Jean-Luc Thiffeault
Publication date
2007
fields
Physics
and research's language is
English
Ask ChatGPT about the research
No Arabic abstract
The transfer of heat between the air and surrounding soil in underground tunnels ins investigated, as part of the analysis of environmental conditions in underground rail systems. Using standard turbulent modelling assumptions, flow profiles are obtained in both open tunnels and in the annulus between a tunnel wall and a moving train, from which the heat transfer coefficient between the air and tunnel wall is computed. The radial conduction of heat through the surrounding soil resulting from changes in the temperature of air in the tunnel are determined. An impulse change and an oscillating tunnel air temperature are considered separately. The correlations between fluctuations in heat transfer coefficient and air temperature are found to increase the mean soil temperature. Finally, a model for the coupled evolution of the air and surrounding soil temperature along a tunnel of finite length is given.
rate research
Read More
We present results of interface-resolved simulations of heat transfer in suspensions of finite-size neutrally-buoyant spherical particles for solid volume fractions up to 35% and bulk Reynolds numbers from 500 to 5600. An Immersed Boundary-Volume of Fluid method is used to solve the energy equation in the fluid and solid phase. We relate the heat transfer to the regimes of particle motion previously identified, i.e. a viscous regime at low volume fractions and low Reynolds number, particle-laden turbulence at high Reynolds and moderate volume fraction and particulate regime at high volume fractions. We show that in the viscous dominated regime, the heat transfer is mainly due to thermal diffusion with enhancement due to the particle-induced fluctuations. In the turbulent-like regime, we observe the largest enhancement of the global heat transfer, dominated by the turbulent heat flux. In the particulate shear-thickening regime, however, the heat transfer enhancement decreases as mixing is quenched by the particle migration towards the channel core. As a result, a compact loosely-packed core region forms and the contribution of thermal diffusion to the total heat transfer becomes significant once again. The global heat transfer becomes, in these flows at volume fractions larger than 25%, lower than in single-phase turbulence.
We report heat transfer and temperature profile measurements in laboratory experiments of rapidly rotating convection in water under intense thermal forcing (Rayleigh number $Ra$ as high as $sim 10^{13}$) and unprecedentedly strong rotational influence (Ekman numbers $E$ as low as $10^{-8}$). Measurements of the mid-height vertical temperature gradient connect quantitatively to predictions from numerical models of asymptotically rapidly rotating convection, separating various flow phenomenologies. Past the limit of validity of the asymptotically-reduced models, we find novel behaviors in a regime we refer to as rotationally-influenced turbulence, where rotation is important but not as dominant as in the known geostrophic turbulence regime. The temperature gradients collapse to a Rayleigh-number scaling as $Ra^{-0.2}$ in this new regime. It is bounded from above by a critical convective Rossby number $Ro^*=0.06$ independent of domain aspect ratio $Gamma$, clearly distinguishing it from well-studied rotation-affected convection.
We develop a two-fluid model (TFM) for simulation of thermal transport coupled to particle migration in flows of non-Brownian suspensions. Specifically, we propose a closure relation for the inter-phase heat transfer coefficient of the TFM as a function of the particle volume fraction, particle diameter, magnitude of the particle phases shear-rate tensor, and the thermal diffusivity of the particles. The effect of shear-induced migration in the particulate phase is captured through the use of state-of-the-art rheological closures. We validate the proposed interphase heat transfer coupling by calibrating it against previous experiments in a Couette cell. We find that, when the shear rate is controlled by the rotation of the inner cylinder, the shear and thermal gradients aid each other to increase the particle migration when temperature difference between the inner and outer walls, $Delta T = T_mathrm{in} - T_mathrm{out} < 0$. Meanwhile, for $Delta T > 0$, the shear and thermal gradients oppose each other, resulting in diminished particle migration, and a more uniform distribution of the particulate phase across the gap. Within the TFM framework, we identify the origin and functional form of a thermo-rheological migration force that rationalizes our observations. We also investigate the interplay of shear and thermal gradients in the presence of recirculating regions in an eccentric Couette cell (with offset axis and rotating inner cylinder). Simulations reveal that the system Nusselt number increases with the eccentricity $E$ for $Delta T > 0$, but a maximum occurs for $Delta T < 0$ at $E = 0.4$. This observation is explained by showing that, for $E>0.4$ and $Delta T < 0$, significant flow recirculation enhances particle inhomogeneity, which in turn reduces heat transfer in the system (compared to $Delta T > 0$).
This article condenses current endeavors and improvements in the expansion of applications of the DualSPHysics code to analyze heat transfer in a nuclear reactor core. This includes the essential conservation equations and certain physical considerations, particularly the thermal conductivity variable model, considering changes in the reference density to maintain the accuracy in the solution. Conventionally, to study these sorts of systems, Eulerian methods have been developed, nevertheless, this kind of method based on well-defined mesh shows major restrictions. The DualSPHysics code, based on Smoothed Particle Hydrodynamics (SPH) technique, has shown to be a real and robust alternative since it involves a free mesh approach, and the numerical method is very well parallelized in both computational and graphical process units (CPU and GPU). The results for the improvements developed in the present work show an exceptionally good approximation with other simulation approaches and also with experimental observation in the three cases studied (1) heat transfer analysis in a bidimensional system with thermal conductivity coefficient k variable, (2) natural convection heat transfer in a horizontal cylindrical ring similar to the space between the fuel rod and the cladding and (3) heat transfer in an experimental nuclear fuel rod square arrangement like in a Pressurized Water Reactor (PWR) nuclear core. Enhancements to this code (DualSPHysics) to use it in nuclear applications are fundamental in the exploitation of this technique in crucial areas of study.
A practical application of universal wall scalings is near-wall turbulence modeling. In this paper, we exploit temperatures semi-local scaling [Patel, Boersma, and Pecnik, {Scalar statistics in variable property turbulent channel flows}, Phys. Rev. Fluids, 2017, 2(8), 084604] and derive an eddy conductivity closure for wall-modeled large-eddy simulation of high-speed flows. We show that while the semi-local scaling does not collapse high-speed direct numerical simulation (DNS) data, the resulting eddy conductivity and the wall model work fairly well. The paper attempts to answer the following outstanding question: why the semi-local scaling fails but the resulting eddy conductivity works well. We conduct DNSs of Couette flows at Mach numbers from $M=1.4$ to 6. We add a source term in the energy equation to get a cold, a close-to-adiabatic wall, and a hot wall. Detailed analysis of the flows energy budgets shows that aerodynamic heating is the answer to our question: aerodynamic heating is not accounted for in Patel et al.s semi-local scaling but is modeled in the equilibrium wall model. We incorporate aerodynamic heating in semi-local scaling and show that the new scaling successfully collapses the high-speed DNS data. We also show that incorporating aerodynamic heating or not, the semi-local scaling gives rise to the exact same eddy conductivity, thereby answering the outstanding question.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
