Daniel R. Rousse

Numerical predictions of two-dimensional conduction, convection, and radiation heat transfer. II. Validation

Abstract The research presented in this paper involves the detailed formulation of a Control-Volume Finite Element Method (CVFEM) for the solution of combined-mode heat transfer in participating media. The proposed numerical method accounts for emitting, absorbing, and scattering media in regularly- and irregularly-shaped geometries. In the proposed CVFEM, the calculation domain is divided into three-node triangular finite elements: the geometrical flexibility associated with finite element methods is preserved. Each element is further subdivided in such a way that upon assembly of all elements, complete control volumes are formed around each node in the calculation domain. To account for the directional nature of radiation heat transfer, a spherical envelope, surrounding each node in the calculation domain, is discretized in adjacent non-overlapping solid angles (or control angles). Element-based interpolation functions for the dependent variables, and the subdomain-type method of weighted residuals are used to derive algebraic approximations to the governing equations. Interpolation schemes that guarantee positive contributions to the coefficients in the algebraic discretization equations are used to approximate the convective and radiative fluxes across control-volume surfaces.

Heat recovery in greenhouses: a practical solution

Abstract In recent years, passive infiltration of air into greenhouses has been reduced drastically. However, very low air exchange rates can lead to abnormally high levels of humidity which can damage harvests. Hence, farmers have to ventilate. In an effort aimed at reducing heating costs related to ventilation, a heat exchanger was designed to be used as a dehumidifier. A CDN

A numerical study of the melting of phase change material heated from a vertical wall of a rectangular enclosure

2000 air-to-air multi-pipe counterflow heat exchanger unit was installed in a greenhouse used for the experimental cultivation of hydroponic tomatoes and cucumbers during the winter of 1996. The first series of tests, carried out between March and May 1996 in a 576 m 3 enclosure, demonstrated that average efficiencies of η =84% and 78% were obtainable with air volumetric exchanges rates of 0.5 and 0.9 change/h, respectively. Latent heat was found to play a major role in the overall heat transfer, contributing about 40% of the total energy exchanged in some situations. The unit made of plastic is durable and rot and rust resistant. Its efficiency, mainly because of its very low level of compactness, was found to be very good in the presence of frost and ice. A commercial implementation is now considered as well as experiments in broiler houses.

Optical tomography reconstruction algorithm with the finite element method: An optimal approach with regularization tools

This article presents the first research effort of our group to formulate, implement and validate a numerical method in order to optimise the design of solar passive walls involving phase change materials (PCMs). The fusion of ice, gallium and the commercially available PCM 27 (hydrated salt), engineered by Cristopia and later embedded within an experiment unit, was studied. Comparisons against other prediction methods and experimental data for the fusion of gallium were carried out with good agreement of the solutions. The proposed enthalpy-based method is found to be excellent to predict the fusion, but still fails to reproduce adequately the exact solidification pattern measured for the PCM 27. Further research is going on to improve the model.

A novel technique to monitor carburizing processes

Optical tomography is mathematically treated as a non-linear inverse problem where the optical properties of the probed medium are recovered through the minimization of the errors between the experimental measurements and their predictions with a numerical model at the locations of the detectors. According to the ill-posed behavior of the inverse problem, some regularization tools must be performed and the Tikhonov penalization type is the most commonly used in optical tomography applications. This paper introduces an optimized approach for optical tomography reconstruction with the finite element method. An integral form of the cost function is used to take into account the surfaces of the detectors and make the reconstruction compatible with all finite element formulations, continuous and discontinuous. Through a gradient-based algorithm where the adjoint method is used to compute the gradient of the cost function, an alternative inner product is employed for preconditioning the reconstruction algorithm. Moreover, appropriate re-parameterization of the optical properties is performed. These regularization strategies are compared with the classical Tikhonov penalization one. It is shown that both the re-parameterization and the use of the Sobolev cost function gradient are efficient for solving such an ill-posed inverse problem.

Anaerobic digestion and gasification coupling for wastewater sludge treatment and recovery

This paper presents the principle and testing of a novel technique developed for carburizing processes monitoring. The technique relies upon an experimental device made of a thin iron foil with a carburizing atmosphere on one side and a decarburizing atmosphere on the other. The principle of carburizing control is based on the fact that when steady-state of carbon diffusion is reached across the thin iron foil, the measured mass flux of carbon on the decarburizing side is related to the inflow of carbon into the parts during the carburizing treatment. Hence, as a probe could be inserted directly into a given furnace, it would provide an in situ control facility. The proposed device could then be used for controlling atmospheric or low-pressure (vacuum) carburizing treatments. The results presented here are limited to atmospheric conditions. Nevertheless, they gave the incentive to the researchers to pursue the development of the device to allow for measurements in a low-pressure furnace and to refine the experimental bench to quantify thoroughly the phenomena involved within the foil.

Lyapunov Function and Sliding Mode Control Approach for the Solar-PV Grid Interface System

Sewage sludge management is an energy intensive process. Anaerobic digestion contributes to energy efficiency improvement but is limited by the biological process. A review has been conducted prior to experimentation in order to evaluate the mass and energy balances on anaerobic digestion followed by gasification of digested sludge. The purpose was to improve energy recovery and reuse. Calculations were based on design parameters and tests that are conducted with the anaerobic digester of a local wastewater treatment plant and a small commercial gasification system. Results showed a very significant potential of energy recovery. More than 90% of the energy content from sludge was extracted. Also, approximately the same amount of energy would be transferred in both directions between the digester (biogas) and the gasifier (thermal energy). This extraction resulted in the same use of biogas as the reference scenario but final product was a totally dry biochar, which represented a fraction of the initial mass. Phosphorus was concentrated and significantly preserved. This analysis suggests that anaerobic digestion followed by dehydration, drying and gasification could be a promising and viable option for energy and nutrient recovery from municipal sludge in replacement of conventional paths.

A Consistent Interpolation Function for the Solution of Radiative Transfer on Triangular Meshes. I—Comprehensive Formulation

This paper deals with control of a solar-photovoltaic (PV) power-generating system interfaced with the grid. A sliding mode control approach is used for achieving maximum power tracking control of a solar-PV array. The Lyapunov function-based control approach is designed and modeled for the dc-ac inverter to serve the functions of an active power injection to the grid, balanced grid currents at unity power factor and load currents harmonics compensation. The proposed approaches eliminate the need of adjustment of system parameters under changing loads and generation scenario. The effectiveness of the proposed control strategies is established using its stability analyses. The performance of the solar-PV power-generating system with the proposed control algorithms is demonstrated using simulation and experimental studies under various operating conditions.

Short-pulsed laser transport in absorbing and scattering media: time-based versus frequency-based approaches

This article presents a first-order skewed upwinding procedure for application to discretization numerical methods in the context of radiative transfer involving gray participating media. This scheme: (1) yields fast convergence of the algorithm; (2) inherently precludes the possibility of computing negative coefficients to the discretized algebraic equations; (3) reduces false scattering (diffusion); (4) is relatively insensitive to grid orientation; and (5) produces solutions completely free from undesirable oscillations. Theses attributes render the scheme attractive, especially in the context of combined modes of heat transfer and fluid flow problems for which computational time is a major concern. The suggested scheme has been validated by application to several basic test problems discussed in a companion article.

An overview of phase change materials and their implication on power demand

Optical tomography (OT) is a promising non-intrusive characterization technique of absorbing and scattering media that uses transmitted and/or reflected signals of samples irradiated with visible or near-infrared light. The quality of OT techniques is directly related to the accuracy of their forward models due to the use of inversion algorithms. In this paper, forward models for transient OT approaches are investigated. The system under study involves a one-dimensional absorbing and scattering medium illuminated by a short laser pulse; this problem is solved using a discrete ordinates–finite volume (DO–FV) method in both time and frequency domain. Previous works have shown that time-domain approaches coupled with first order spatial interpolation schemes cannot represent the physics of the problem adequately as transmitted fluxes emerge before the minimal physical time required to leave the medium. In this work, the Van Leer and Superbee flux limiters, combined with the second order Lax–Wendroff scheme, are used in an attempt to prevent this. Results show that despite significant improvement, flux limiters fail to completely eliminate the physically unrealistic behaviour. On the other hand, results for transmittance obtained from the frequency-based method are accurate, without physically unrealistic behaviours at early time periods. The frequency-dependent approach is however computationally expensive, since it requires approximately five times more computational time than its temporal counterpart when used as a forward model for transient OT. On the other hand, the great advantages of the frequency-based approach is that limited windows of temporal signals can be calculated efficiently (in transient OT), and it can also be used as a forward model for steady-state, frequency-based and transient OT techniques.