Evaluation of Air Mixing and Thermal Comfort From High Sidewall Supply Air Jets

Uniform mixing of conditioned air with room air is an essential factor for providing comfort in homes. The higher the supply flow rates the easier to reach good mixing in the space. In high performance homes, however, the flow rates required to meet the small remaining thermal loads are not large enough to maintain uniform mixing in the space. The objective of this study is to resolve this issue and maintain uniform temperatures within future homes. We used computational fluid dynamics modeling to evaluate the performance of high sidewall air supply for residential applications in heating and cooling modes. Parameters of the study are the supply velocity, supply temperature, diffuser dimensions, and room dimensions. Laboratory experiments supported the study of thermal mixing in heating mode; we used the results to develop a correlation to predict high sidewall diffuser performance. For cooling mode, numerical analysis is presented. The results provide information to guide the selection of high sidewall supply diffusers to maintain proper room mixing for heating and cooling of high performance homes. It is proven that these systems can achieve good mixing and provide acceptable comfort levels. Recommendations are given on the operating conditions to guarantee occupant comfort

Three-Dimensional Numerical Evaluation of Thermal Performance of Uninsulated Wall Assemblies: Preprint

This study describes a detailed three-dimensional computational fluid dynamics modeling to evaluate the thermal performance of uninsulated wall assemblies accounting for conduction through framing, convection, and radiation. The model allows for material properties variations with temperature. Parameters that were varied in the study include ambient outdoor temperature and cavity surface emissivity. Understanding the thermal performance of uninsulated wall cavities is essential for accurate prediction of energy use in residential buildings. The results can serve as input for building energy simulation tools for modeling the temperature dependent energy performance of homes with uninsulated walls

Casimir force in critical ternary polymer solutions

Consider a mixture of two incompatible polymers A and B in a common good solvent, confined between two parallel plates separated by a finite distance L. We assume that these plates strongly attract one of the two polymers close to the consolute point (critical adsorption). The plates then experience an effective force resulting from strong fluctuations of the composition. To simplify, we suppose that either plates have the same preference to attract one component (symmetric plates) or they have an opposed preference (asymmetric plates). The force is attractive for symmetric plates and repulsive for asymmetric ones. We first exactly compute the force using the blob model, and find that the attractive and repulsive forces decay similarly to L⁻⁴. To go beyond the blob model that is a mean-field theory, and in order to get a correct induced force, we apply the Renormalization-Group to a φ⁴ -field theory ( φ is the composition fluctuation), with two suitable boundary conditions at the surfaces. The main result is that the expected force is the sum of two contributions. The first one is the mean-field contribution decaying as L⁻⁴, and the second one is the force deviation originating from strong fluctuations of the composition that decreases rather as L⁻³. This implies the existence of some cross-over distance L* ∼ aNφ¹/² ( a is the monomer size, N is the polymerization degree of chains and φ is the monomer volumic fraction), which separates two distance-regimes. For small distances (L L*) the fluctuation force is more important.Розглядається суміш двох несумісних полімерів A і B , що добре розчиняються в спільному розчиннику, вміщена між двома паралельними пластинами, розділеними скінченною відстанню L. Ми вважаємо, що поблизу точки розчинення вони сильно притягають один з двох полімерів (критична адсорбція). При цьому пластини знаходяться під впливом ефективної сили, породженої сильними флуктуаціями суміші. Для спрощення ми припускаємо, що або обидві пластини притягають той самий компонент (симетричні пластини) або вони віддають перевагу різним компонентам (асиметричні пластини). Симетричним пластинам відповідає сила притягання, асиметричним – відштовхування. Спершу ми точно розрахували цю силу, використовуючи краплинну модель, і встановили, що сили притягання і відштовхування загасають подібним чином як L⁻⁴. Щоб вийти поза межі краплинної моделі, яка відповідає наближенню середнього поля, і з метою отримати правильний вигляд індукованої сили, ми застосували ренорм-груповий підхід до теорії поля φ⁴ ( φ – флуктуація суміші) з двома відповідними граничними умовами на поверхнях. У результаті встановлено, що шукана сила є сумою двох вкладів. Перший з них – це вклад середнього поля, що загасає якL⁻⁴, а другий – відхилення, викликане сильними флуктуаціями суміші, що зменшується радше як L⁻³. Це означає, що існує певна відстань кроссоверу L* ∼ aNφ¹/² ( a – розмір мономера, N – ступінь полімеризації ланцюжків і φ – об’ємна частка мономера), що розділяє характерні відстані двох згаданих режимів. На малих відстанях (L L*) більш важливим стає флуктуаційний вклад

Predicting flow reversals in chaotic natural convection using data assimilation

A simplified model of natural convection, similar to the Lorenz (1963) system, is compared to computational fluid dynamics simulations in order to test data assimilation methods and better understand the dynamics of convection. The thermosyphon is represented by a long time flow simulation, which serves as a reference "truth". Forecasts are then made using the Lorenz-like model and synchronized to noisy and limited observations of the truth using data assimilation. The resulting analysis is observed to infer dynamics absent from the model when using short assimilation windows. Furthermore, chaotic flow reversal occurrence and residency times in each rotational state are forecast using analysis data. Flow reversals have been successfully forecast in the related Lorenz system, as part of a perfect model experiment, but never in the presence of significant model error or unobserved variables. Finally, we provide new details concerning the fluid dynamical processes present in the thermosyphon during these flow reversals

Prediction of Air Mixing From High Sidewall Diffusers in Cooling Mode: Preprint

Computational fluid dynamics modeling was used to evaluate the performance of high sidewall air supply in cooling mode. The research focused on the design, placement, and operation of air supply diffusers located high on a sidewall and return grilles located near the floor on the same sidewall. Parameters of the study are the supply velocity, supply temperature, diffuser dimensions and room dimensions. Thermal loads characteristic of high performance homes were applied at the walls and room temperature was controlled via a thermostat. The results are intended to provide information to guide the selection of high sidewall supply diffusers to provide proper room mixing for cooling of high performance homes

Thermal Performance of Uninsulated and Partially Filled Wall Cavities: Preprint

Low-rise, wood-framed homes are the most common type of residential structures in the United States. Wood wall construction supports roofs efficiently and provides a stable frame for attaching interior and exterior wall coverings. Wall cavities are prevalent and increase thermal resistance, particularly when they are filled with insulating material. This paper describes detailed computational fluid dynamics modeling to evaluate the thermal performance of uninsulated or partially filled wall cavities and accounts for conduction through framing, convection, and radiation. Parameters are ambient outdoor temperature, cavity surface emissivity, cavity aspect ratio, and insulation height. Understanding the thermal performance of uninsulated or partially insulated wall cavities is essential for conserving energy in residential buildings. The results can serve as input for building energy simulation tools such as DOE2 and EnergyPlus for modeling the temperature dependent energy performance of new and older homes with uninsulated or partially insulated walls