Sture Holmberg

A Modified Low-Reynolds-Number k-ω Model for Recirculating Flows

A modified form of Wilcoxs low-Reynolds-number k-ω model (Wilcox) is proposed for predicting recirculating flows. The turbulent diffusion for the specific dissipation rate, ω, is modeled with two parts: a second-order diffusion term and a first-order cross-diffusion term. The model constants are re-established. The damping functions are redevised, which reproduce correct near-wall asymptotic behaviors, and retain the mechanism describing transition as in the original model. The new model is applied to channel flow, backward-facing step flow with a large expansion ratio (H/h = 6), and recirculating flow in a ventilation enclosure. The predictions are considerably improved

On the assessment of ventilation performance with the aid of numerical simulations

Abstract The assessment of ventilation performance is discussed. New local indices are developed with the aid of numerical simulations to quantify air diffusion and contaminant dispersion. The local purging effectiveness, Asp, is an index for evaluating the contribution of each inlet in a multi-inlet system. The local specific contaminant-accumulating index, α, can be used to indicate the tolerance of a ventilation flow to contaminants. Asp and α can be derived from transport equations. A method based on age-variation analysis is used to define Asp and the Expected Contaminant Dispersion Index (ECDI). The latter is an index for forecasting contaminant dispersion emitted at a specific location with unknown source strength. These new scales and methods can be used to assess ventilation performance.

Demand-controlled ventilation in new residential buildings: consequences on indoor air quality and energy savings

The consequences on indoor air quality (IAQ) and potential of energy savings when using a variable air volume (VAV) ventilation system were studied in a newly built Swedish building. Computer simulations with IDA Indoor Climate and Energy 4 (ICE) and analytical models were used to study the IAQ and energy savings when switching the ventilation flow from 0.375 l·s−1·m−2 to 0.100 l·s−1·m−2 during unoccupancy. To investigate whether decreasing the ventilation rate to 0.1 l·s−1·m−2 during unoccupancy, based on Swedish building regulations, BBR, is acceptable and how long the reduction can last for an acceptable IAQ, four strategies with different VAV durations were proposed. This study revealed that decreasing the flow rate to 0.1 l·s−1·m−2 for more than 4 h in an unoccupied newly built building creates unacceptable IAQ in terms of volatile organic compounds concentration. Hence, if the duration of unoccupancy in the building is more than 4 h, it is recommended to increase the ventilation rate from 0.100 l·s−1·m−2 to 0.375 l·s−1·m−2 before the home is occupied. The study showed that when the investigated building was vacant for 10 h during weekdays, increasing the ventilation rate 2 h before occupants arrive home (low ventilation rate for 8 h) creates acceptable IAQ conditions. In this system, the heating requirements for ventilation air and electricity consumption for the ventilation fan were decreased by 20% and 30%, respectively.

Surgical clothing systems in laminar airflow operating room: a numerical assessment

This study compared two different laminar airflow distribution strategies - horizontal and vertical - and investigated the effectiveness of both ventilation systems in terms of reducing the sedimentation and distribution of bacteria-carrying particles. Three different staff clothing systems, which resulted in source strengths of 1.5, 4 and 5 CFU/s per person, were considered. The exploration was conducted numerically using a computational fluid dynamics technique. Active and passive air sampling methods were simulated in addition to recovery tests, and the results were compared. Model validation was performed through comparisons with measurement data from the published literature. The recovery test yielded a value of 8.1 min for the horizontal ventilation scenario and 11.9 min for the vertical ventilation system. Fewer particles were captured by the slit sampler and in sedimentation areas with the horizontal ventilation system. The simulated results revealed that under identical conditions in the examined operating room, the horizontal laminar ventilation system performed better than the vertical option. The internal constellation of lamps, the surgical team and objects could have a serious effect on the movement of infectious particles and therefore on postoperative surgical site infections.

Sick Buildings or Not: Indoor Air Quality and Health Problems in Schools

Poor indoor air in schools has become a wide-spread problem with serious effects on occupant health. Resultant costs can be considerable at both local and national government levels. These include absenteeism and rehabilitation as well as building alterations and even demolition and rebuilding. This project aims to show factors contributing to health problems in Swedish schools. It includes a literature survey and particle measurements taken during various activities. Due to the fact that today there is no standard for indoor air quality (IAQ) in schools, in this project we used the outdoor air surrounding the building as an indicator. Results showed that indoor school environments had high airborne pollution levels, to a degree that probably causes health problems for many people. Regarding IAQ, this project shows the importance of taking into consideration choices in activities and furnishing of the building.

General flow and thermal boundary conditions in indoor air flow simulation

The concepts of general flow and thermal boundary conditions are introduced to treat the interaction between indoor and outdoor thermal environments in computational fluid dynamics (CFD) simulations. The results from a multi-zone model are used to supply the general flow boundary conditions. The energy balance equation at wall-air interfaces is used to supply the thermal boundary conditions. An example calculation in a 13-room building shows that infiltration influences indoor air flow patterns considerably. The air flow in a room ventilated by displacement is measured and simulated. Two surface coatings are considered, i.e. black walls and aluminum walls. Implementation of these two boundary conditions is essential in predicting air flow patterns, air quality, and thermal comfort in a real building.

Three distinct surgical clothing systems in a turbulent mixing operating room equipped with mobile ultraclean laminar airflow screen: A numerical evaluation

Two types of mobile screens producing ultraclean local laminar airflow were investigated as an addition to turbulent mixing operating room ventilation. The exploration was carried out numerically using computational fluid dynamics. Surface and volumetric particle counts were simulated on the operating and instrument tables with and without the additional mobile airflow screen. Three different source strengths (the mean bacteria-carrying particle value emitted from one person per second) due to staff clothing variety were considered. Model validation was performed through result comparisons with experimental data from the literature. Results confirmed that the mobile screen units reduced the airborne bacteria to an acceptable level for infection-prone surgeries. No significant particle concentration differences existed in the periphery of the operating room. Lower source strength resulting from a clothing system with high protective capacity reduced particle concentration.

A Study of Sources of Airborne Pollutants and Poor Hygiene in Schools

Poor indoor air quality is a large problem in Swedish schools, since the health of occupants may be affected. Resources are consumed without identification of utility indicators and there is risk of problems, even after remedial measures have been taken. This can mean both unnecessary suffering for many people and considerable resources being wasted. The building itself is often in focus and other building-related problems may be neglected. The hypothesis of the present work is that other factors than the building itself have decisive influence on indoor air quality. An assessment of these nonbuilding-related reasons for bad indoor air quality has been made in the present study using particle measurements. Results show that it is possible to decrease emissions in indoor air by over 90% through identifying and eliminating activity-related sources of airborne contaminants.

Characteristics of Particles and their Behaviour in Ventilation Air

Abstract The behaviour of particles in air flow is important for identifying those in various locations in ventilated space. The main reason for this study is to propose a new modelling concept to determine a realistic distribution of particles of different sizes in a space. The goal for this investigation is to divide particles into groups according to their behaviour in air and to improve the existing settling model. The growth of particle aerodynamic diameter in higher relative humidity is also presented. Here, growth of diameter and coagulation is used to determine the control volume concentration. The finite volume method is used to describe the particle concentration in the computational domain. A background to particle properties and theory for calculations are given for this purpose. Results from the literature survey reveal that modelling needs a more systematic approach to cover all sizes of particles with clear classification by behaviour.

Simulation of room flows with small ventilation openings by a local grid-refinement technique

Small ventilation openings in large rooms constitute a numerical resolution difficulty in indoor air flow simulation. A local grid-refinement technique is introduced to add local grid points in the small-opening regions. The method requires a relatively small increase in computer storage. A multi-grid algorithm, which turns out to be an efficient and accurate solves is used to solve the discrete system of equations on both global and local grids. A method for implementing the wall function in the solution procedure is described. Solutions of a three-dimensional isothermal mixing ventilation problem at Reynolds number of 5000 are compared with the experimental results in the literature. The method is then applied in a three-dimensional scaled model room ventilated by a plaque diffuser. The calculations show good agreement with the smoke visualisations of the flow pattern close to the diffuser. The method can be used to predict air flows with more realistic ventilation openings and more applications in large enclosures are expected in the future.