Complex Thermal Flow Research Articles

Lattice Boltzmann simulation of complex thermal flows via a simplified immersed boundary method

A simplified immersed boundary (IB) lattice Boltzmann method (LBM) is developed in this study for simulating incompressible flows with heat transfer. The no-slip and constant temperature boundary conditions are both enforced by correcting the distribution functions. Similar to the previous distribution function correction (DFC)-IB method (S. Tao et al., 2021 [59]), the kinetic nature of LBM has been retained in the present scheme, which directly modifies the distribution functions other than models the interface as a source of IB force in the boundary treatment. However, this study presents two improvements. First, we reduce the number of distribution functions that need to be modified, and hence the computation load of the method can be reduced significantly. Furthermore, the correction term can be determined directly and explicitly, replacing the calculation of a local position-related adjustment parameter in the previous DFC-IB method. As a result, the computed velocity and temperature at the interface are consistent with their desired counterparts, and hence the Dirichlet boundary conditions can be accurately specified. The present simplified DFC IB method is validated in the simulations with both stationary and moving boundaries, including the flow past a hot cylinder, mixed convection in a square lid-driven cavity, and sedimentation of a particle in a long channel. The results demonstrate the accuracy and efficiency of the method.

Distribution function correction-based immersed boundary lattice Boltzmann method for thermal particle flows

A novel immersed boundary lattice Boltzmann method (IB-LBM) is proposed to simulate the complex thermal particle flows. In the present scheme, the boundary condition is directly implemented by correcting the distribution function at the neighboring points around the interface, similar to the original LBM. Furthermore, an adjustment parameter is introduced for ensuring the accuracy in the boundary treatment. Those two improvements both facilitate the alleviation of computational load, for evaluation and incorporation of the boundary force and the iterative procedure involved in the previous methods. Three simulations of complex thermal flows with moving boundary, including the mixed convection in a cavity containing a rotating cylinder, the sedimentation of a cold particle in a hot fluid and the drafting-kissing-tumbling dynamics of two hot settling particles in a vertical channel are performed to validate the present IB-LBM for. The results are found to have good agreement with those available in the literature.

Simulating Turbulent Thermal Plasma Flows for Nanopowder Fabrication

This article presents descriptions of theoretical models and numerical methods for simulating turbulent thermal plasma flow with nanopowder growth. Turbulence models must express turbulent and laminar states because both states co-exist with thermal plasmas showing large density variation and transport properties. Time-dependent 3D simulations are conducted based on Large Eddy Simulation using a dynamic Smagorinsky model. Results show significant difference depending on temporal and spatial discretization schemes and velocity–pressure coupling algorithms. Simulation results demonstrate that advanced numerical methods with high-order accuracy should be used for long and robust computations capturing steep gradients of nanopowder concentration and plasma temperature and 3D dynamic motions of multiscale vortices, which are turbulent features of thermal plasma flows with low Mach numbers. A thermal plasma jet generates a double-layer structure of inner high-temperature thick vortex rings and outer low-temperature thin vortex rings near the nozzle exit. Flowing downstream, these vortices interact, deform, and break up. Consequently, plasma transits to a complex thermal flow. The widely spreading distribution of multiscale vortices agrees with experimental observations, which are not simulated using conventional methods. Nanopowder is generated from material vapour by nucleation and condensation at interfacial regions between plasma and cold gas. Those regions include numerous vortices. Therefore, the vortices convey the nanopowder, producing a complex distribution of nanopowder. Simultaneously, the nanopowder diffuses and increases in size, decreasing in number by interparticle coagulation. Cross-correlation analysis suggests that a nanopowder distribution distant from a plasma jet can be controlled through temperature fluctuation control at the upstream plasma fringe.

Design and operability of multi-stream heat exchangers for use in lng liquefaction processes

Among the diverse types of heat exchanger technology, the geometrical features of plate-fin exchangers provide the required flexibility for the incorporation of large number of streams in a single unit. The main design and operating issues related to multi-stream exchangers are linked to the complex thermal flows that take place inside the unit. Several simplifications have been proposed to reduce this complexity and derive design methodologies. One of these simplifications is the uniform channel heat load which gives rise to the uniform wall temperature assumption. This simplification assumes that at any plane cut normal to the flow of the fluids inside the exchanger, the temperature of the separating walls is the same. One way of practically achieving such condition is by appropriate surface selection. The thermal performance of secondary surfaces has been extensively studied and vast information on their thermo-hydraulic performance has been published. A design criterion that has emerged in the last decade is to engineer secondary surfaces to achieve the required thermal performance. Such approach is taken advantage of in this work so that the concept of uniform wall temperature in design can be effectively met. This paper shows the derivation of a surface design methodology. The approach is demonstrated on a case study using triangular shape fins.

Thermal lattice Boltzmann method for complex microflows.

A methodology to simulate thermal fields in complex microflow geometries is proposed. For the flow fields, the regularized multiple-relaxation-time lattice Boltzmann method (LBM) is applied coupled with the diffusive-bounce-back boundary condition for wall boundaries. For the thermal fields, the regularized lattice Bhatnagar-Gross-Krook model is applied. For the thermal wall boundary condition, a newly developed boundary condition, which is a mixture of the diffuse scattering and constant temperature conditions, is applied. The proposed set of schemes is validated by reference data in the Fourier flows and square cylinder flows confined in a microchannel. The obtained results confirm that it is essential to apply the regularization to the thermal LBM for avoiding kinked temperature profiles in complex thermal flows. The proposed wall boundary condition is successful to obtain thermal jumps at the walls with good accuracy.

Numerical Simulation of Thermal Plasma Gasification Process

Numerical analysis of plasma gasification process was carried out base on the combination of magnetohydrodynamics (MHD) and computational fluid dynamics (CFD). A two stage gasification system which consists of a heater and a plasma rector was used to enhance syngas production in the present work. Nitrogen thermal plasma jet generated by a low power plasma torch was analyzed by a self-developed MHD code, and complex thermal flow field in the plasma reactor was simulated with a commercial CFD code. The accuracy of numerical simulation was confirmed from the comparison between numerical results and experimentally measured data of arc voltage and reactor temperature. From the numerical analysis, a high temperature for the thermal cracking of methane was expected in the upper region of the plasma reactor.

Molecular tagging techniques and their applications to the study of complex thermal flow phenomena

This review article reports the recent progress in the development of a new group of molecule-based flow diagnostic techniques, which include molecular tagging velocimetry (MTV) and molecular tagging thermometry (MTT), for both qualitative flow visualization of thermally induced flow structures and quantitative whole-field measurements of flow velocity and temperature distributions. The MTV and MTT techniques can also be easily combined to result in a so-called molecular tagging velocimetry and thermometry (MTV&T) technique, which is capble of achieving simultaneous measurements of flow velocity and temperature distribution in fluid flows. Instead of using tiny particles, the molecular tagging techniques (MTV, MTT, and MTV&T) use phosphorescent molecules, which can be turned into long-lasting glowing marks upon excitation by photons of appropriate wavelength, as the tracers for the flow velocity and temperature measurements. The unique attraction and implementation of the molecular tagging techniques are demonstrated by three application examples, which include: (1) to quantify the unsteady heat transfer process from a heated cylinder to the surrounding fluid flow in order to examine the thermal effects on the wake instabilities behind the heated cylinder operating in mixed and forced heat convection regimes, (2) to reveal the time evolution of unsteady heat transfer and phase changing process inside micro-sized, icing water droplets in order to elucidate the underlying physics pertinent to aircraft icing phenomena, and (3) to achieve simultaneous droplet size, velocity and temperature measurements of “in-flight” droplets to characterize the dynamic and thermodynamic behaviors of flying droplets in spray flows.

THERMAL PLASMA DECOMPOSITION OF FLUORINATED GREENHOUSE GASES

Fluorinated compounds mainly used in the semiconductor industry are potent greenhouse gases. Recently, thermal plasma gas scrubbers have been gradually replacing conventional burn-wet type gas scrubbers which are based on the combustion of fossil fuels because high conversion efficiency and control of byproduct generation are achievable in chemically reactive high temperature thermal plasma. Chemical equilibrium composition at high temperature and numerical analysis on a complex thermal flow in the thermal plasma decomposition system are used to predict the process of thermal decomposition of fluorinated gas. In order to increase economic feasibility of the thermal plasma decomposition process, increase of thermal efficiency of the plasma torch and enhancement of gas mixing between the thermal plasma jet and waste gas are discussed. In addition, noble thermal plasma systems to be applied in the thermal plasma gas treatment are introduced in the present paper.

Some Advances in Applications of Lattice Boltzmann Method for Complex Thermal Flows

Over the past two decades, there have been enormous advances in lat- tice Boltzmann (LB) numerical simulation and modelling. The lattice Boltzmann method has become a practical and promising tool for many fluid problems. A majority of recent studies have relied on numerical computations of isothermal flows. However, much less efforts have been devoted to complex thermal flows, such as flows in porous media subjected to external magnetic force, flows with temperature-dependent properties. In this paper, an overview is made based on some accomplishments in these numerical endeavours. Along with the paper's sections, the state-of-the-art trend and the LBM advances in modelling and in com- putational aspects for specific classes of problems of major interest will be fully touched on. Concluding remarks are given and the axis of our future studies will be traced. AMS subject classifications: 76M28, 70K50, 76S05, 82D10