Nesreen Ghaddar

Numerical investigation of incompressible flow in grooved channels. Part 1. Stability and self-sustained oscillations

Incompressible moderate-Reynolds-number flow in periodically grooved channels is investigated by direct numerical simulation using the spectral element method. For Reynolds numbers less than a critical value R c the flow is found to approach a stable steady state, comprising an ‘outer’ channel flow, a shear layer at the groove lip, and a weak re-circulating vortex in the groove proper. The linear stability of this flow is then analysed, and it is found that the least stable modes closely resemble Tollmien–Schlichting channel waves, forced by Kelvin–Helmholtz shear-layer instability at the cavity edge. A theory for frequency prediction based on the Orr–Sommerfeld dispersion relation is presented, and verified by variation of the geometric parameters of the problem. The accuracy of the theory, and the fact that it predicts many qualitative features of low-speed groove experiments, suggests that the frequency-selection process in these flows is largely governed by the outer, more stable flow (here a channel), in contrast to most current theories based solely on shear-layer considerations. The instability of the linear mode for R > R c is shown to result in self-sustained flow oscillations (at frequencies only slightly shifted from the originating linear modes), which again resemble (finite-amplitude) Tollmien-Schlichting modes driven by an unstable groove vortex sheet. Analysis of the amplitude dependence of the oscillations on degree of criticality reveals the transition to oscillatory flow to be a regular Hopf bifurcation.

MODELING AND SIMULATION OF SOLAR ABSORPTION SYSTEM PERFORMANCE IN BEIRUT

An analytical study is performed on solar energy utilization in space cooling of a small residential application using a solar lithium bromide absorption system. A simulation program for modeling and performance evaluation of the solar-operated absorption cycle is done for all possible climatic conditions of Beirut. The results have shown that for each ton of refrigeration it is required to have a minimum collector area of 23.3 m2 with an optimal water storage tank capacity ranging from 1000 to 1500 liters for the system to operate solely on solar energy for about seven hours a day. The monthly solar fraction of total energy use in cooling is determined as a function of solar collector area and storage tank capacity.

Modeling of heat and moisture transport by periodic ventilation of thin cotton fibrous media

Abstract In walking conditions, the air spacing between the fabric layer of a porous clothing system and the human skin changes with the walking frequency. This change will cause air penetration in and out of the clothing system depending on the fabric air permeability. The air passing through the fabric can considerably reduce the heat and moisture transfer resistance of the clothing system and its suitability for a given thermal environment. In this work, the coupled convection heat and moisture exchange within the clothing system subject to sinusoidal air layer thickness variation about a fixed mean is experimentally investigated and theoretically modeled to predict the periodic fabric regain, the fabric temperature and the transient conditions of the air layer located between the fabric and the skin. Experiments were conducted in environmental chambers under controlled conditions using a sweating hot plate at 35 °C that represents the human skin and a gear motor to generate the oscillating fabric motion. The first set of experiments was done using a dry isothermal hot plate to measure the sensible heat transfer. The second set of experiments was conducted with an isothermal sweating hot plate and the total heat (sensible and latent) transport from the plate was recorded. A mathematical model was developed for the heat and mass transport through the air spacing layer and the fiber clothing system. In the fabric, a three-node adsorption model was used to describe the effect of fabric motion (ventilation) on the sensible and latent heat flows from the human skin under different environmental conditions. The fiber model was linked to the transport model of the oscillating air spacing layer that falls between the fiber and the fixed boundary (human skin). The transport equations were solved numerically. The sensible and latent heat transport quantities at the moist solid boundary were calculated. A reasonable agreement was observed between the model predictions of heat loss or gain from the hot plate and the experimentally measured results.

Experimental and Numerical Investigation of the Effect of Phase Change Materials on Clothing During Periodic Ventilation

A numerical and experimental investigation is conducted of periodic ventilation pro cesses in fabric containing microcapsules of phase change materials (PCM). When PCMS are added to textiles, they release heat as the liquid changes to a solid state and absorb heat as the solid returns to a liquid state. In this work, PCMS are incorporated in a numerical three-node fabric ventilation model to study their transient effect on body heat loss during exercise when subjected to sudden changes in environmental conditions from warm indoor air to cold outdoor air. The results indicate that the heating effect lasts approxi mately 12.5 minutes depending on PCM percentage and cold outdoor conditions. Heat released by PCMS decreases the clothed-body heat loss by an average of 40-55 W/m2 for a one-layer suit depending on the frequency of oscillation and crystallization temperature of the PCM. The experimental results reveal that under steady-state environmental condi tions, the oscillating PCM fabric has no effect on dry resistance, even though the measured sensible heat loss increases with decreasing air temperature of the chamber. When a sudden change in ambient conditions occurs, the PCM fabric delays the transient response and decreases body heat loss.

Energy conservation of residential buildings in Beirut

Energy conservation measures on buildings have a significant role to play in reducing the burden of the energy bill on the Lebanese economy. The residential sector is one of various sectors that energy measures can be applied to. Such measures include the use of insulation materials, double-glazed windows, shading, efficient air-conditioning systems, economical lighting and reduction of infiltration rates. It is demonstrated through detailed energy analysis of typical residential and office buildings that strict conservation is benificial on the micro- and macro-economic levels. A code of practice is suggested to establish acceptable standards for energy use in residential buildings and ranking is done of energy measures based on economical indices.

Uncertainty in estimating and mitigating industrial related GHG emissions

Abstract Global climate change has been one of the challenging environmental concerns facing policy makers in the past decade. The characterization of the wide range of greenhouse gas emissions sources and sinks as well as their behavior in the atmosphere remains an on-going activity in many countries. Lebanon, being a signatory to the Framework Convention on Climate Change, is required to submit and regularly update a national inventory of greenhouse gas emissions sources and removals. Accordingly, an inventory of greenhouse gases from various sectors was conducted following the guidelines set by the United Nations Intergovernmental Panel on Climate Change (IPCC). The inventory indicated that the industrial sector contributes about 29 percent to the total greenhouse gas emissions divided between industrial processes and energy requirements at 12 and 17 percent, respectively. This paper describes major mitigation scenarios to reduce emissions from this sector based on associated technical, economic, environmental, and social characteristics. Economic ranking of these scenarios was conducted and uncertainty in emission factors used in the estimation process was emphasized. For this purpose, theoretical and experimental emission factors were used as alternatives to default factors recommended by the IPCC and the significance of resulting deviations in emission estimation is presented.

NATURAL CONVECTION OVER A ROTATING CYLINDRICAL HEAT SOURCE IN A RECTANGULAR ENCLOSURE

A numerical study of laminar two-dimensional natural convection heat transfer from a uniformly heated horizontal cylinder rotating about its center, and placed in an isothermal rectangular enclosure, is performed using a spectral element method. The physical aspects of the flow and its thermal behavior are studied for a wide range of pure natural convection to mixed convection at low and high rotational speeds of the cylinder. The computer program has been validated against experimental correlations available on pure natural convection of heated bodies in enclosures. The rotation of the cylinder has been found to enhance the heat transfer. At low ratios of Rayleigh number to the square of the rotational Reynolds number, Ra / Reω 2, the maximum temperature on the cylinder surface is decreased by as much as 25–35% from similar cases with fixed cylinders. At moderate values of Ra/ Reω 2, the thermal plume rising above the cylinder is shifted in the rotation direction and the angular shift decreases as Ra / R...

Simplified Thermal Model of Spaces Cooled with Combined Positive Displacement Ventilation and Chilled Ceiling System

An improved plume-multilayer model was developed and validated to represent thermal transport in enclosures conditioned by radiant cooling and displacement ventilation systems. A novel approach was developed to estimate wall plumes for non-isothermal surfaces using the similarity solution derived for power law representation of temperature difference between the room air and the wall. The nonuniform wall plume flow rates predicted by the model agreed well with flow rates in an enclosure produced by computational fluid dynamics (CFD) simulations. Wall plumes associated with a nonuniformly heated wall with varying temperature difference between the wall and the air were found to be substantially higher than the plume predicted by isothermal wall correlation. The wall plume model is integrated with a multilayer space thermal model to predict the stratification height in the space, the vertical distribution of wall and air temperatures as a function of the chilled ceiling temperature and space air supply conditions. The plume-multilayer model results were compared with results of three-dimensional CFD simulations using commercially available software. Three test cases were considered for the simulations at cooling loads of 40 W/m2 (12.7 Btu/h·ft2), 67 W/m2 (21.2 Btu/h·ft2), and 100 W/m2 (31.7 Btu/h·ft2) and supply airflow rates per unit area of 22.5 (1.2 ft3/min·ft2), 30 (1.6 ft3/min·ft2), and 37.5 m3/h·m2 (2.1 ft3/min·ft2), respectively. The vertical wall and average air temperatures for each layer agreed well with the results of the plume-multilayer model, showing maximum errors in values of average air temperature of 0.13°C (0.23°F), 0.37°C (0.66°F), and 0.3°C (0.54°F) for the low, medium, and high load cases, respectively. The simplified model accurately predicted the stratification height at a maximum error of ±0.05 m (0.16 ft) in the three test cases. The stratification height is overestimated by 35% if wall plumes are neglected in the plume-multilayer model.

Empirical Evaluation of Convective Heat and Moisture Transport Coefficients in Porous Cotton Medium

The air penetration within a porous clothing system on a moving human being is an important physical process that considerably affects the heat and moisture resistance of the textile material. This effect of the coupled convection heat and mass exchange within the clothing system is experimentally investigated and theoretically modeled to determine the heat and mass transfer coefficients between the air penetrating the void space and the solid fiber as a function of the velocity of penetrating air. Experiments were conducted inside environmentally controlled chambers to measure the transient moisture uptake of untreated cotton fabric samples as well as the outer fabric temperature using an infrared pyrometer. The moisture uptake was conducted at three different volumetric flow rates of 0.0067, 0.018 and 0.045 m 3 /sec/m 2 of fabric area to represent airflow penetrations that could result from slow, medium, and vigorous walking, respectively. The theoretical analysis is based on a two-node adsorption model of the fibrous medium. A set of four coupled differential equations were derived describing time-dependent convective heat and mass transfer between the penetrating air and the solid fiber in terms of relevant unknown transport coefficients

Stratified storage tank influence on performance of solar water heating system tested in Beirut

This paper presents an experimental and numerical study to evaluate the performance of an active solar water heating system that uses a stratified storage tank in both natural and forced circulation operation modes. A new inlet diffuser design was used in the tank to enhance stratification, and the performance of the solar water heating system was compared to heating systems with other tank inlet designs. A simulation program is used for modeling the thermal behavior of the stratified-tank solar water heating system. Comparison of the resulting calculations with experimental data show good agreement. System efficiencies as high as 60% are observed in the Beirut climate. A substantial increase of up to 20% in the energy delivered is observed when stratification is employed in the storage tank, as compared with the fully mixed tank model. The simulation model is then applied for actual Beirut weather to predict the monthly useful energy gain from using stratified-tank solar water heating systems against the fully mixed tank model.