Multiple Solutions And Inverse Problem Of Indoor Air Convection

Several fundamental studies, including multiple stable flows, inverse air convection, resonant heat transfer and double diffusive convection in porous media have been conducted in this dissertation, to satisfy the realistic requirements from building ventilation, airborne pollutant transport, migration of heat and moisture through envelop, control of indoor air environment, prediction of biological warfare agent release from a terrorist attack and hostile air environment.Firstly, obeying unique boundary and operating conditions, multiple stable flows, i.e., two or more stable steady flow solutions could be obtained when different initial conditions are used. In practice, the expected airflow and heat and mass transport structures could be achieved upon imposing different initial conditions, which is used to create advantageous air environment in work zone, inhibit the diffusion of pollutants, and enhance the replacement ventilation. In this dissertation, the multiple steady solutions of natural convection and building ventilation are numerically and experimentally investigated.A detailed numerical study has been conducted to investigate the steady double diffusive natural convection in a rectangular enclosure with the simultaneous presence of discrete heat and moisture sources. Numerical results are particularly presented to illustrate the effects of the buoyancy ratio, strip pitch and thermal Rayleigh number on the multiple steady flow patterns and the associated heat and mass transfer, for both destabilizing and stabilizing solutal buoyancy forces. Numerical results demonstrate that the expected fluid flow, heat and mass transport structures could be obtained upon imposing the corresponding initial conditions and parameters.Three-dimensional nonlinear aerodynamic structures of airflow in a slot-ventilated compartment with three ports are revealed using Computational fluid dynamics (CFD). Numerical results are particularly presented to illustrate the effects of the inlet airflow velocity, enclosure width, and supplying ports elevation on the multiple flow patterns and the associated ventilation flow rates. It is shown that the room airflow rate can be promoted or inhibited, depending strongly on the jet velocity, enclosure width and elevation of supplying ports.Three-dimensional nonlinear aerodynamic structures of airflow in a slot-ventilated compartment with three ports are observed using Particle image velocimetry (PIV) and water tunnel model. Particle image velocimetry (PIV), which is normally used for measuring velocities in liquids and gases, was adopted to measure velocities in flows and validate numerical simulation. Effect of inlet water velocities on the multiple fluid flows was also experimentally analyzed.Secondly, the fluid flow field is usually calculated by CFD with the known boundary and initial conditions. However, unknown characteristics of heat and pollutant sources should be determined from the knowledge of the temperature or concentration measurements taken inside the flow through analyzing and calculating the inverse problem of indoor air convection. Numerical solution of inverse convection problem would directly contribute to enhancement of indoor air environment, refrainment from terrorist attack, and examination of hostile environment. In this dissertation, inverse natural convection in isotropic and anisotrpic medium and inverse mixed convection have been numerically conducted, where the unknown profiles of heat flux along a vertical wall are determined with the known boundary conditions and the measured temperatures inside the domain.An iterative Fletcher-Reeves conjugate gradient method is firstly adopted to estimate the boundary heat fluxes in a fluid-saturated enclosure, where the fluid flow is dynamically coupled with the heat convection of Ra≤107. The sets of direct, sensitivity and adjoint equations required for the solution of the inverse problem are formulated in terms of an arbitrary domain in two dimensions. The pressure-correction method is utilized to solve the continuum direct, sensitivity and adjoint problems by enforcing global mass and energy conservations. The effects of position and number of temperature sensors, heat flux profiles and noise data on the solution of inverse convection problem are also addressed.A numerical implementation of estimating boundary heat fluxes in an enclosure saturated with anisotropic medium is subsequently proposed. Particularly, the flow field is dynamically coupled with the heat convection in the fluid and the heat conduction in the solid domain. The accuracy of the heat flux profile estimations is shown to depend strongly on the body size and relative thermal conductivity of the solid material. Effects of functional form of the unknowns, sensors number and position, and measurement errors on the accuracy of estimation are also highlighted.An inverse mixed convection problem, combing with internal buoyancy flow and external forced flow, is solved using the Fletcher-Reeves conjugate gradient method to estimate the unknown boundary heat flux in a ventilated room. The interactions between forced and free convections are studied with plots of vectors, streamlines, isotherms and heatlines. The accuracy of the heat flux profile estimations is shown to depend strongly on the external flow intensity, thermal source strength, heat flux profile, sensor position, and measurement errors.Thirdly, natural convection affords a means of thermal control, which eliminates the fan or pumps for forced convection and provides a noise- and vibration-free environment. However, natural convection is not an effective mode of heat transfer, and compared to forced convection or boiling, associated thermal resistances are large. Fortunately, resonance is the best way to improve the heat transfer performance of natural convection. Resonance is a phenomenon associated with the eigenmodes of a system, which is essentially independent of the kind of external forcing imposed. If the system is exposed to an external forcing with the correct natural frequency, resonance takes place in which the eigenmodes are excited and amplified. The increasing interest of natural convection in an enclosure with time-periodic boundary conditions is attributable to the relevance of such transient processes in many technological applications, including the electronic apparatus design problem, the fluid motion in rooms and solar collectors heated periodically by the daily solar radiation.In this dissertation, two-dimensional calculation and theoretical analysis have been firstly performed for laminar natural convection induced by two discrete heating elements flush-mounted to one vertical wall of a square enclosure. Scale analysis predicts the resonance frequency and points out that geometry and property of fluid could affect the resonance frequency markedly. Mechanical details of fluid flow and average heat transfer characteristics across heaters and centerline of the enclosure are scrutinized. Apart from these, conjugate natural convection heat transfer in an enclosure subject to periodic temperature boundary conditions is also investigated. A close inspection is concentrated on the effects of the body size and thermal conductivity ratio on the resonant frequencies for unsteady state. The results are presented in terms of amplitude of cycle averaged Nusselt number, fluctuating velocity and temperature contours and its intensity.Finally, double diffusive natural convection in porous medium is numerically and analytically studied, whose flows resulting from the combined action of both temperature and concentration has surged in view of its fundamental importance in various engineering problems. Prominent among these are the migration of moisture through air contained in fibrous insulation, contaminant transport in saturated soils, grain storage, food processing and storage, to name just a few. This dissertation reports a numerical and analytical study of natural convective heat and mass transfer through a vertical porous layer subjected to localized heating and salting from one side, which has received considerable attention for porous insulation, grain storage and food processing. Streamlines, heatlines and masslines in the system are produced to elucidate the flow structure transition from solutal-dominated opposing to thermal dominated and solutal-dominated aiding flows, respectively. At the same time, the scale analysis sorts out successfully many effects that influence the outcome of discrete numerical experiments. This study is significant for optimizing the building heat transfer, designing filters and solar collectors.