Ideal Wall Research Articles

An effective layer pattern optimization model for multi-stream plate-fin heat exchanger using genetic algorithm

For multi-stream plate-fin heat exchangers (MPFHEs), a crucial factor affecting performance is the layer pattern used to distribute hot and cold streams. However, for the design of MPFHE layer pattern, so far it still remains at semi-qualitative, empirical or trial-and-error stage, lacking efficient approaches to obtain the optimum. In this paper, an effective layer pattern optimization model using genetic algorithm (GA) is developed in detail. It includes the chromosome of binary string that represents hot and cold layers alternatively arranged, the dual fitness functions that act on chromosome individuals alternatively, the constraint that requires the spacing layers between hot and cold streams to be no less than 1 and no greater than 2, the selection of optimization tool and the corresponding setting of GA parameters. When used to optimize the layer pattern of actual MPFHE, this model exhibits well. The average thermal efficiency of exchanger reaches up to 98% of that obtained under the ideal common wall temperature assumption. Furthermore, the study of this paper make it clear the association of previous classical deviation from ideal of layer pattern zigzag curve with MPFHE thermal performance, which is not a strict monotonic decreasing relation as people thought to be, but a multiple vs. multiple relation.

Analysis on Life Cycle CO<sub>2</sub> Emission of Aerated Concrete Production in China

Aerated concrete is a new type of wall material with beneficial features like light weight, heat insulation, fire prevention and low energy consumption. As a key milestone in wall materials innovation and energy-saving of building system, it has been proven to be an ideal wall material which can replace traditional clay brick through years of application and practice. This study calculated the CO2 emissions in all the stages of life cycle of aerated concrete production. Compared with clay brick, the life cycle CO2 intensity of aerated concrete block will be decreased by 67.4% with the same insulation effect. Study on the environmental loads of aerated concrete industry will provide theoretical base for the carry out of energy-saving and emission reduction, the formulation of clean production and the development of recycling economy.

Scheme for contact angle and its hysteresis in a multiphase lattice Boltzmann method

In this paper, a scheme for specifying contact angle and its hysteresis is incorporated into a multiphase lattice Boltzmann method. The scheme is validated through investigations of the dynamic behaviors of a droplet sliding along two kinds of walls: a smooth (ideal) wall and a rough or chemically inhomogeneous (nonideal) wall. For an ideal wall, the wettability of solid substrates is able to be prescribed. For a nonideal wall, arbitrary contact angle hysteresis can be obtained through adjusting advancing and receding angles. Significantly different phenomena can be recovered for the two kinds of walls. For instance, a droplet on an inclined ideal wall under gravity is impossible to stay stationary. However, the droplet on a nonideal wall may be pinned due to contact angle hysteresis. The steady interface shapes of the droplet on an inclined nonideal wall under gravity or in a shear flow quantitatively agree well with the previous numerical studies. Besides, the complex motion of a droplet creeping like an inchworm could be simulated. The scheme is found suitable for the study of contact line problems with and without contact angle hysteresis.

Ideal MHD Stability Prediction and Required Power for EAST Advanced Scenario

The Experimental Advanced Superconducting Tokamak (EAST) is the first fully superconducting tokamak with a D-shaped cross-sectional plasma presently in operation. The ideal magnetohydrodynamic (MHD) stability and required power for the EAST advanced tokamak (AT) scenario with negative central shear and double transport barrier (DTB) are investigated. With the equilibrium code TOQ and stability code GATO, the ideal MHD stability is analyzed. It is shown that a moderate ratio of edge transport barriers' (ETB) height to internal transport barriers' (ITBs) height is beneficial to ideal MHD stability. The normalized beta βN limit is about 2.20 (without wall) and 3.70 (with ideal wall). With the scaling law of energy confinement time, the required heating power for EAST AT scenario is calculated. The total heating power Pt increases as the toroidal magnetic field BT or the normalized beta βN is increased.

A unified approach to description of the fast and slow resistive wall modes in tokamaks

The formulation of the boundary conditions is considered in the problem of the resistive wall mode (RWM) stability in tokamaks. The mode-wall interaction, usually modeled in the thin-wall approximation, is described here with account of the finite thickness of the wall and skin effect. It allows one to step beyond the standard restrictions into the area of faster RWMs than the usual “slow” RWMs near the stability threshold. The analysis is carried out with the energy balance equations incorporating the dissipation in the wall. The approach is equally applicable to the modes of any kind and allows natural matching of the exterior problem with the models for the inner region. For example, it allows one to connect the outer task to the classical energy principle for the inner area. It is shown how to calculate the RWM growth rates within this model. A general algorithm with equations applicable to arbitrary toroidal systems and its full realization in the conventional cylindrical model are described. In the latter case, it is shown that the growth rate of the “fast” RWMs essentially differs from the estimates of the standard theory of slow RWMs. The analysis proves that the RWM theory has to be complemented by the additional block of calculations for more correct formulation of the boundary conditions on the inner side of the wall than that in the theory with an ideal or thin resistive wall.

Design of Two-Band Critically Sampled Rational Rate Filter Banks With Multiple Regularity Orders and Associated Discrete Wavelet Transforms

An approach for designing two-band rational rate critically sampled nonorthogonal filter banks with perfect reconstruction and multiple regularity orders is proposed. This approach relies on an updated result on the relationship between the filters in the filter bank to achieve all the desired properties. Filter responses are optimized to approximate ideal brick wall responses, subject to linear and nonlinear constraints derived from regularity and perfect reconstruction conditions, respectively. The obtained designs are approximately shift-invariant as a result of strong frequency selectivity and multiple regularity orders. Two methods for solving the nonconvex optimization design problem are presented and the results from the proposed approach are compared against existing designs in the literature.

Dynamical Casimir effect in a cavity with a weakly non-equidistant spectrum

We study the influence of the non-equidistancy of the frequency spectrum on the Dynamical Casimir effect in a rectangular cavity with a harmonically oscillating ideal wall. The transition from the linear growth of the mean photon number of photons created from vacuum in the cavity with equidistant spectrum to the exponential growth in a weakly non-equidistant case is shown explicitly.

Kinetic versus ideal magnetohydrodynamic modelling of the resistive wall mode in a reversed field pinch plasma

Resistive wall modes (RWMs) are studied within the kinetic model proposed by Heyn et al. [Nucl. Fusion 46, S159 (2006); Phys. Plasmas 18, 022501 (2011)], which accounts for Landau damping, transit-time magnetic pumping, and Coulomb collisions in cylindrical geometry. Results for the reversed field pinch plasma are compared to the magnetohydrodynamic results obtained by Guo et al., [Phys. Plasmas 6, 3868 (1999)]. Stabilization of the external kink mode by an ideal wall as well as stabilization of the resistive wall mode by toroidal plasma rotation is obtained. In contrast to MHD modelling, which predicts a stability window for the resistive wall position, kinetic modelling predicts a one sided window only, i.e., the resistive wall must be sufficiently close to plasma to achieve rotational stabilization of the mode but there is no lower limit on the wall position. Stabilizing rotation speeds are found somewhat smaller when compared to MHD results. In addition, for the present plasma configuration, the kinetic model predicts resistive wall mode stabilization only in one direction of toroidal rotation. In the opposite direction, a destabilizing effect is observed. This is in contrast to MHD where mode stabilization is symmetric with respect to the direction of the toroidal plasma rotation.

Axially sheared and pre-stressed hollow cylinder within the context of the theory of interacting continua

Investigating the effects of altered transmural pressure is of great importance. Indeed, many data obtained from experiments on isolated dog carotid arteries and jugular veins serve to provide a foundation describing the collapse of both arteries and veins for both positive and negative transmural pressures. It has been also reported that transmural pressure induces matrix-degrading activity in porcine arteries ex vivo. An illustrative example dealing with the application of a transmural pressure and the resulting transmural filtration of fluid through a pre-strained hollow conduit subjected to combined finite deformations is investigated here. Some results that can provide additional useful insight can help in improving the method for performing prosthesis conduit material for use with living tissue, understanding problems which involves the diffusion of ideal fluid-saturated wall for various mechanical parameters such as the pre-stress/strain and applied or induced axial shear are discussed.

Crystallization of hard spheres near wall

The classical molecular dynamics method has been used to study a thermodynamic system of hard spheres occurring near an ideal wall. It is established that, at an initial mean number density of spheres above a certain value, the system exhibits relaxation to an equilibrium state that consists of two phases. The closer the wall, hard spheres form a primitive pyramidal crystal belonging to a trigonal system; farther from the wall, hard spheres occur in an anisotropic liquid phase that is separated from the crystal by a transition layer. As the mean density increases, the process of crystallization in the configuration space takes place and the system passes to a new equilibrium state with increased thickness of the near-wall crystalline layer, at unchanged densities of both the crystal and isotropic liquid phase.

The high beta tokamak-extended pulse magnetohydrodynamic mode control research program

The high beta tokamak-extended pulse (HBT-EP) magnetohydrodynamic (MHD) mode control research program is studying ITER relevant internal modular feedback control coil configurations and their impact on kink mode rigidity, advanced digital control algorithms and the effects of plasma rotation and three-dimensional magnetic fields on MHD mode stability. A new segmented adjustable conducting wall has been installed on the HBT-EP and is made up of 20 independent, movable, wall shell segments instrumented with three distinct sets of 40 saddle coils, totaling 120 in-vessel modular feedback control coils. Each internal coil set has been designed with varying toroidal angular coil coverage of 5, 10 and 15°, spanning the toroidal angle range of an ITER port plug based internal coil to test resistive wall mode (RWM) interaction and multimode MHD plasma response to such highly localized control fields. In addition, we have implemented 336 new poloidal and radial magnetic sensors to quantify the applied three-dimensional fields of our control coils along with the observed plasma response. This paper describes the design and implementation of the new control shell incorporating these control and sensor coils on the HBT-EP, and the research program plan on the upgraded HBT-EP to understand how best to optimize the use of modular feedback coils to control instability growth near the ideal wall stabilization limit, answer critical questions about the role of plasma rotation in active control of the RWM and the ferritic resistive wall mode, and to improve the performance of MHD control systems used in fusion experiments and future burning plasma systems.

KSTAR equilibrium operating space and projected stabilization at high normalized beta

Along with an expanded evaluation of the equilibrium operating space of the Korea Superconducting Tokamak Advanced Research, KSTAR, experimental equilibria of the most recent plasma discharges were reconstructed using the EFIT code. In near-circular plasmas created in 2009, equilibria reached a stored energy of 54 kJ with a maximum plasma current of 0.34 MA. Highly shaped plasmas with near double-null configuration in 2010 achieved H-mode with clear edge localized mode (ELM) activity, and transiently reached a stored energy of up to 257 kJ, elongation of 1.96 and normalized beta of 1.3. The plasma current reached 0.7 MA. Projecting active and passive stabilization of global MHD instabilities for operation above the ideal no-wall beta limit using the designed control hardware was also considered. Kinetic modification of the ideal MHD n = 1 stability criterion was computed by the MISK code on KSTAR theoretical equilibria with a plasma current of 2 MA, internal inductance of 0.7 and normalized beta of 4.0 with simple density, temperature and rotation profiles. The steep edge pressure gradient of this equilibrium resulted in the need for significant plasma toroidal rotation to allow thermal particle kinetic resonances to stabilize the resistive wall mode (RWM). The impact of various materials and electrical connections of the passive stabilizing plates on RWM growth rates was analysed, and copper plates reduced the RWM passive growth rate by a factor of 15 compared with stainless steel plates at a normalized beta of 4.4. Computations of active RWM control using the VALEN code showed that the n = 1 mode can be stabilized at normalized beta near the ideal wall limit via control fields produced by the midplane in-vessel control coils (IVCCs) with as low as 0.83 kW control power using ideal control system assumptions. The ELM mitigation potential of the IVCC, examined by evaluating the vacuum island overlap created by resonant magnetic perturbations, was analysed using the TRIP3D code. Using a combination of all IVCCs with dominant n = 2 field and upper/lower coils in an even parity configuration, a Chirikov parameter near unity at normalized poloidal flux 0.83, an empirically determined condition for ELM mitigation in DIII-D, was generated in theoretical high-beta equilibria. Chirikov profile optimization was addressed in terms of coil parity and safety factor profile.

Equilibrium and global MHD stability study of KSTAR high beta plasmas under passive and active mode control

The Korea Superconducting Tokamak Advanced Research, KSTAR, is designed to operate a steady-state, high beta plasma while retaining global magnetohydrodynamic (MHD) stability to establish the scientific and technological basis of an economically attractive fusion reactor. An equilibrium model is established for stability analysis of KSTAR. Reconstructions were performed for the experimental start-up scenario and experimental first plasma operation using the EFIT code. The VALEN code was used to determine the vacuum vessel current distribution. Theoretical high beta equilibria spanning the expected operational range are computed for various profiles including generic L-mode and DIII-D experimental H-mode pressure profiles. Ideal MHD stability calculations of toroidal mode number of unity using the DCON code shows a factor of 2 improvement in the wall-stabilized plasma beta limit at moderate to low plasma internal inductance. The planned stabilization system in KSTAR comprises passive stabilizing plates and actively cooled in-vessel control coils (IVCCs) designed for non-axisymmetric field error correction and stabilization of slow timescale MHD modes including resistive wall modes (RWMs). VALEN analysis using standard proportional gain shows that active stabilization near the ideal wall limit can be reached with feedback using the midplane segment of the IVCC. The RMS power required for control using both white noise and noise taken from NSTX active stabilization experiments is computed for beta near the ideal wall limit. Advanced state-space control algorithms yield a factor of 2 power reduction assuming white noise while remaining robust with respect to variations in plasma beta.

Rotational Stabilization of Resistive Wall Mode on JT-60U

We have carried out experiments to clarify the stabilizing effect of a plasma rotation on the resistive wall mode (RWM) that could limit the achievable-βN in high-βN plasmas above the no-wall ideal βN-limit. On JT-60U plasma rotations are controlled using neutral beams with varying combinations of net torque input while keeping βN constant. The RWM is destabilized as the plasma rotation is being reduced. Detailed measurements of the mode structure revealed that the RWM has a global structure that rotates with the order of the resistive wall time. In these experiments, it is found that the critical toroidal rotation speed for the RWM stabilization is less than 1% of the Alfvén speed. Moreover, the critical rotation does not strongly depend on βN. The results suggest that high-βN operation up to the ideal wall βN-limit could be possible by suppressing the RWM with a slow plasma rotation in fusion reactors.

Numerical calculations demonstrating complete stabilization of the ideal magnetohydrodynamic resistive wall mode by longitudinal flow

The cylindrical ideal magnetohydrodynamic (MHD) stability problem, including flow and a resistive wall, is cast in the standard mathematical form, ωA⋅x=B⋅x, without discretizing the vacuum regions surrounding the plasma. This is accomplished by means of a finite element expansion for the plasma perturbations, by coupling the plasma surface perturbations to the resistive wall using a Green’s function approach, and by expanding the unknown vector, x, to include the perturbed current in the resistive wall as an additional degree of freedom. The ideal MHD resistive wall mode (RWM) can be stabilized when the plasma has a uniform equilibrium flow such that the RWM frequency resonates with the plasma’s Doppler-shifted sound continuum modes. The resonance induces a singularity in the parallel component of the plasma perturbations, which must be adequately resolved. Complete stabilization within the ideal MHD model (i.e., without parallel damping being added) is achieved as the grid spacing in the region of the resonance is extrapolated to 0 step size.

Kinetic analysis of the resistive wall modes in the ITER advanced tokamak scenario

It is found that n = 1 resistive wall modes in the ITER advanced scenario can be fully stabilized by modestly low rotation with a rotation frequency (normalized to the Alfvén frequency at the magnetic axis) of about Ω = 0.0075. The existence of this stabilization scheme is proved with the AEGIS-K (Adaptive EiGenfunction Independent Solution-Kinetic) code, which provides a fully kinetic (non-hybrid) and self-consistent (non-perturbative) description of the system. Wave-particle resonances, shear Alfvén continuum damping, trapped particle effect and the parallel electric effects are all taken into account. The rotation frequency for full stabilization is much larger than the diamagnetic drift frequency; therefore, finite Larmor radius effects are negligible. We also find that the rotation stabilization window opens first near the ideal wall limit.

Evaluation of optical and thermal properties of coatings for energy efficient windows

Over the past couple of decades a number of coated glazing products have appeared on the market, converting the window from an energy drain to a possible resource in the building's energy supply system. In this paper the light transmittance, total solar energy transmittance, and thermal transmittance of coated glazing currently available on the market and future dynamic electrochromic coatings are reviewed and their function in heating or cooling dominated climates discussed. Electrochromic coatings can switch between a transparent state and an absorbing or reflecting state when a small electrical potential is applied, and hence adapt their optical properties to the present condition. Energy balance calculations for the window component stress the importance of selecting the window depending on climate. In a heating dominated climate a low-e coating with a high g-value can result in a net energy gain on a south facing façade and thus be better than an ideal wall.

Lifshitz theory of atom-wall interaction with applications to quantum reflection

The Casimir-Polder interaction of an atom with a metallic wall is investigated in the framework of the Lifshitz theory. It is demonstrated that in some temperature (separation) region the Casimir-Polder entropy takes negative values and goes to zero when the temperature vanishes. This result is obtained both for an ideal metal wall and for real metal walls. Simple analytical representations for the Casimir-Polder free energy and force are also obtained. These results are used to make a comparison between the phenomenological potential used in the theoretical description of quantum reflection and exact atom-wall interaction energy, as given by the Lifshitz theory. Computations are performed for the atom of metastable ${\mathrm{He}}^{*}$ interacting with metal (Au) and dielectric (Si) walls. It is shown that the relative differences between the exact and phenomenological interaction energies are smaller in the case of a metallic wall. This is explained by the effect of negative entropy which occurs only for a metal wall. More accurate atom-wall interaction energies computed here can be used for the interpretation of measurement data in the experiments on quantum reflection.

Torsional displacement of piezoelectric fiber actuators with helical electrodes

A piezoceramic fiber wound with helical electrodes on its outer surface can be used as a torsional actuator. This paper explains the torsional actuator's working principles and points out that although such a mechanism can produce a large torsional strain, it will have a small axial strain. Detailed analyses are performed on the influences of the tubular fiber's wall thickness and the helical angle of the electrodes on the torsional strain. With the aid of the finite element analysis method, the electric field distribution in the actuator is illustrated. Theoretical analyses point out that to obtain the largest torsional strain, a 45° electrode helical angle is needed for an actuator with an ideal thin wall thickness, but the angle becomes smaller than 45° if the wall thickness cannot be ignored. Results of some deliberate experiments prove these analyses.

Ideal magnetohydrodynamics stability spectrum with a resistive wall

The eigenvalue equations describing a cylindrical ideal magnetohydrodynamics plasma interacting with a thin resistive wall are presented in the standard mathematical form, A⋅x=λB⋅x, without discretizing the vacuum regions surrounding the plasma. This is accomplished by using a finite-element basis for the plasma perturbations, and by coupling the plasma surface perturbations to the perturbed electrical current in the wall using a Green’s-function approach. The perturbed wall current introduces a single additional degree of freedom into the system, which, together with an auxiliary variable, u=ωξ, allows the system to take the standard linear form. The standard form allows the use of linear eigenvalue solvers, without additional iterations, to compute the complete spectrum of plasma modes in the presence of a surrounding resistive wall at arbitrary separation. Standard results are recovered in the limits of (i) an infinitely resistive wall (no wall), and (ii) a zero resistance wall (ideal wall).