Multi-zone Airflow Model Research Articles

Study of Natural Ventilation Strategies in the São Cristóvão Church in Lisbon Using a Multizone Airflow Model

Visitors have a significant impact on the indoor climate of buildings housing works of art, and the relationship between the number of visitors, the indoor air quality and the protection of exposed works of art is an important factor in the overall study of the indoor climate of heritage buildings without mechanical ventilation and/or air-conditioning systems. In view of these concerns and the lack of studies on natural ventilation in heritage buildings, this study aims to analyse the performance of natural ventilation in the São Cristóvão Church in Lisbon, Portugal. For the preparation of this study, an analysis of the natural ventilation of this church was carried out by creating a model in the CONTAM software, and the indoor air quality was analysed based on different international standards and guidelines for carbon dioxide levels and air flow rates (ACH). Estimating the current ventilation strategy, an average ACH of 0.75 h−1 was estimated during the time the church is open, and an ACH of 0.15 h−1 was estimated during the time the doors were closed. In a yearly analysis, an average ACH of 0.30 h−1 was obtained. These air exchange values guarantee EN 16798-1 category I air quality for 72% of the year and category II air quality for 18% of the year. Different natural ventilation strategies were analysed: (a) three scenarios exploring different cross ventilation scenarios; (b) a scenario assuming that the church is closed all year round; and (c) a scenario estimating an increase in the number of visitors, giving an idea of the variations in human pollutants and possible consequences. Taking into account the air infiltration and the fact that masses, an occasional situation with a high number of visitors, are always held just before the church closes, it is guaranteed that carbon dioxide levels will never exceed the limit of 350 ppm above the outdoor values imposed by EN-13779, registering a maximum of 291 ppm.

Multi-Zone Infection Risk Assessment Model of Airborne Virus Transmission on a Cruise Ship Using CONTAM

Since the onset of the pandemic, the cruise industry has faced substantial challenges, experiencing an 81% year-on-year decline in 2020. Notably, China’s cruise industry has remained shuttered for nearly two years. The conventional epidemiological investigations relying on human memory have proven inadequate in regard to swiftly and reliably identifying high-risk populations, thus leading to excessive pandemic prevention or other inappropriate measures. Furthermore, current research endeavors have not adequately addressed the critical issues of isolation location selection and the estimation of isolation scale within multi-zone indoor environments. Therefore, how to control the epidemic with minimal impact on the public has become an urgent problem. To solve the problems mentioned above, a multi-zone infection risk assessment model of airborne virus transmission was proposed to rapidly qualify the risk of infection, identify the high-risk population, and provide guidance. The model can be divided into two parts, including a multi-zone airflow model and a risk assessment model based on the Wells–Riley model. A large in-service ro-ro passenger ship already in service was used for the application of the proposed method. The results show that the number of isolated rooms shall be at least 5% of the occupancy and the isolation location can be chosen through the multi-zone infection risk assessment model. This paper provides insights into risk assessment to mitigate the spread of epidemics on a large cruise ship, and the method can be easily applied to all kinds of multi-zone indoor environments.

Probable cross-corridor transmission of SARS-CoV-2 due to cross airflows and its control

A COVID-19 outbreak occurred in May 2020 in a public housing building in Hong Kong – Luk Chuen House, located in Lek Yuen Estate. The horizontal cluster linked to the index case’ flat (flat 812) remains to be explained. Computational fluid dynamics simulations were conducted to obtain the wind-pressure coefficients of each external opening on the eighth floor of the building. The data were then used in a multi-zone airflow model to estimate the airflow rate and aerosol concentration in the flats and corridors on that floor. Apart from flat 812 and corridors, the virus-laden aerosol concentrations in flats 811, 813, 815, 817 and 819 (opposite to flat 812, across the corridor) were the highest on the eighth floor. When the doors of flats 813 and 817 were opened by 20%, the hourly-averaged aerosol concentrations in these two flats were at least four times as high as those in flats 811, 815 and 819 during the index case's home hours or the suspected exposure period of secondary cases. Thus, the flats across the corridor that were immediately downstream from flat 812 were at the highest exposure risk under a prevailing easterly wind, especially when their doors or windows that connected to the corridor were open. Given that the floorplan and dimension of Luk Chuen House are similar to those of many hotels, our findings provide a probable explanation for COVID-19 outbreaks in quarantine hotels. Positive pressure and sufficient ventilation in the corridor would help to minimise such cross-corridor infections.

Energy-saving potential of deeply retrofitting building enclosures of traditional courtyard houses – A case study in the Chinese Hot-Summer-Cold-Winter zone

It is increasingly believed that energy retrofits are a protection tool to preserve cultural heritage for future generations. However, few studies are available regarding the deep retrofits of traditional courtyard houses in the Chinese Hot-Summer-Cold-Winter zone, although the winter conditions in these buildings are very uncomfortable based on existing studies. Besides, few studies analyzed the actual energy performance of still-occupied traditional buildings in this region. Therefore, this study investigated the energy-saving potential of deeply retrofitting the building enclosures of a traditional courtyard house in Tongren in southern China. The investigation was based on calibrated hygrothermal simulation with WUFI®Plus applying the multi-zone airflow modeling. For the calibration, short-term monitoring (two weeks) was conducted in winter and measured the air temperature, relative humidity, and heating energy consumption. The calibration showed satisfactory results by assessing the Normalized-Mean-Bias-Error, the Coefficient of Variation of Root-Mean-Square Error, and the Coefficient of determination. In contrast, the application of Goodness of Fit shows poorer performance and requires further discussion for short monitoring periods. According to this study, improving the U-values of building envelopes to meet at least the current Chinese energy-efficiency standard can reduce more than 56% energy demand for heating and cooling, with an estimated payback time of 2.5 years for the material cost. With growing expectations of building occupants on indoor thermal comfort, the improvement of the energy performance of traditional buildings will become more critical for their long-term preservation and should be implemented in China's energy efficiency policies and initiatives in the future.

Potential of cross-ventilation channels in an ideal typical apartment building predicted by CFD and multi-zone airflow model

Natural ventilation is a passive environmental technology which can improve indoor air quality and cool down the building without consuming energy. However, due to the increasing densities of cities, land prices are constantly increasing. To save building area, apartment houses often use one elevator hall for multiple households (i.e. three-, four-, six-, or eight-households). But when an apartment is on the leeward side, the wind pressure ventilation is low. Therefore, this study proposes establishing a horizontal airflow channel—cross-ventilation channel—in the elevator hall to improve air exchange rate. The paper aim to predict its effect. Computational fluid dynamics (CFD) simulation verified by wind tunnel test, and the multi-zone airflow model are applied to predict the air flow rates in seven cities in China, which represented the five climate zones of China. It is found that the horizontal airflow channel can obviously improve the ventilation capacity of the leeward apartment, especially for south-side apartment. The area ratio of channel-to-ground increases by 1%, and the ventilation rate increases by average 5% in this study. It could be used as an alternative technology for improving natural ventilation in apartment buildings.

Empirical validation of a multizone building model coupled with an air flow network under complex realistic situations

Building energy simulation (BES) assesses the energetic and thermal behaviour of buildings and can be used for the evaluation of energy conservation measures (ECMs). However, building modelling under realistic conditions can be challenging for simulation programs. The aim of this work is to test whether a multizone building model developed with a simulation tool can handle the complex situations of an in-use building. Monitoring data from the Twin Houses empirical validation experiment of the International Energy Agency, Energy in Buildings and Communities (IEA EBC) Annex 71 is used for that purpose. The multizone building model is developed with the energy simulation software TRNSYS coupled with an air flow network in TRNFLOW. The innovative aspect of this work lies in the validation of a combined multizone building model under stochastic occupancy gains, taking into account room-wise heating profiles, ventilation, and interior air flow currents due to window and door operation. The results of a coheating test showed that the errors in the simulated heating power for the whole house are a coefficient of variation of the root mean square error (CV(RMSE)) of 12.72% and a normalised mean bias error (NMBE) of −8.35%. Nevertheless, more difficulties were found in separately predicting the heating power of each individual room. However, the combined multizone air flow model was able to predict indoor temperatures in all rooms with root mean square error (RMSE) and mean absolute error (MAE) values of<1 °C.

Simulation and validation of indoor temperatures and relative humidity in multi-zone buildings under occupancy conditions using multi-objective calibration

Buildings are expected to be energy efficient but also to provide a comfortable environment to their occupants as well as to be durable. Indoor relative humidity influences the energy consumption, structure and the occupants’ comfort and health. Therefore, it is necessary to control and predict indoor environmental conditions. This work aims to simulate indoor temperatures and relative humidity in a multi-zone building model under realistic conditions, such as occupancy and moisture gains, operation of windows and doors and mechanical ventilation. The combined multi-zone air flow model is developed with TRNSYS and TRNFLOW and it is applied to a phase of the Twin Houses extended experiment of the international energy agency, energy in buildings and communities (IEA EBC) Annex 71. The main novelty of this work is to determine the masses of surface and deep moisture buffer storage by performing a multi-objective calibration of indoor temperatures and relative humidity. Results prove that building energy simulations need to model the moisture buffering of internal materials to accurately predict indoor humidity. The error in simulating relative humidity is reduced by 69% in the house with moisture gains after calibration of the masses of buffering materials. Moreover, the calibrated multi-zone building model shows a great agreement between simulated and measured data with an average root mean square error (RMSE) among thermal zones of 0.51 °C and 0.48 °C in indoor temperatures and 3.58% and 2.21% in relative humidity for the two houses in the validation period.

TDC-based horizontal leakage area estimation in multiunit residential buildings: Three correction factors

Infiltration and interzonal airflow driven by wind and stack interactions in a multi-unit residential building (MURB) are crucial factors affecting indoor air and environmental qualities. To analyze the weather-driven airflow in MURBs, it is essential to define accurately the horizontal leakage areas for serial airflow paths on typical and basement floors in the multizone airflow modeling. Considering the challenges in measuring numerous leakage areas, a horizontal leakage estimation method based on the thermal draft coefficient (TDC) is proposed, in which the leakage areas are calculated by the stack-driven pressure differences relatively easy to measure. Moreover, three correction coefficients are theoretically suggested to minimize the TDC uncertainties from (1) different household envelope areas, (2) horizontal temperature distributions on each floor, and (3) multiple shafts in real MURBs. Two validation studies were conducted in a real MURB. First, the TDC-based models were validated by CONTAMW, and the effectiveness of the three correction factors was shown in the simulation-based case. The envelope area ratio correction factor was most effective for leakage areas in dwelling units. The shaft ratio correction factor was necessary for elevator doors on typical floors and all leakage areas on basement floors. The air density correction factor could improve all leakage areas under the horizontal temperature distributions on each floor. Then, based on field measurements, the proposed method was applied to leakage estimation in the building. Compared with the existing method, the field-measurement-based validation case demonstrated a significant improvement in the leakage estimation under measurement uncertainties.

Identifying decaying contaminant source location in building HVAC system using the adjoint probability method

Building heating, ventilation and air-conditioning (HVAC) system can be potential contaminant emission source. Released contaminants from the mechanical system are transported through the HVAC system and thus impact indoor air quality (IAQ). Effective control and improvement measures require accurate identification and prompt removal of contaminant sources from the HVAC system so as to eliminate the unfavourable influence on the IAQ. This paper studies the application of the adjoint probability method for identifying a dynamic (decaying) contaminant source in building HVAC system. A limited number of contaminant sensors are used to detect contaminant concentration variations at certain locations of the HVAC ductwork. Using the sensor inputs, the research is able to trace back and find the source location. A multi-zone airflow model, CONTAM, is employed to obtain a steady state airflow field for the studied building with detailed duct network, upon which the adjoint probability based inverse tracking method is applied. The study reveals that the adjoint probability method can effectively identify the decaying contaminant source location in building HVAC system with few properly located contaminant concentration sensors.

Computational of the wind velocity effect on infiltration rates in an individual building using multi-zone airflow model

ABSTRACTThis paper presents and discusses numerical simulation of the wind velocity's effect on infiltration rates in an individual building using multi-zone modelling. The governing equations are expressed in terms of unknown pressures at points where flow paths connect. Their implementation was achieved using an advanced numerical model, which was developed via the MATLAB environment. Such a numerical approach has been validated through available results. A good agreement has been observed between available data and our study's results. Moreover, the model implemented allows to determine the reference pressure in each zone to compute the air infiltration rate by each connection. To perform this, a large range of input data is needed. These include leakage characteristics of airflow elements, wind pressure coefficients, and ventilation system characteristics. To sum up, it is found that the wind velocity increase produces a strongly variation on computed infiltration rates.

Investigating the association of healthcare-seeking behavior with the freshness of indoor spaces in low-income tenement housing in Mumbai

In this study, the strength of association of the subjective measure of the freshness of the indoor spaces and the health care-seeking behavior of the occupants was evaluated in low-income tenement houses of Mumbai. Mixed-mode research methods were employed, where the household survey data and feedback from local authorities regarding the health status of the occupants in the tenement housing were used to formulate the qualitative policy variable for assessing the built environment-health nexus. Multi-zone airflow modeling and simulations were performed to estimate the local mean age (LMA) of air, which subjectively indicated the ‘freshness’ of the space. The results show LMA was within the range of 5 min and 48 min, with the majority of units having an LMA range of 5 and 15 min. A parametric analysis of the site-based wind-flow characteristics stressed the need of early design interventions based on its orientation and obstructions, such that the natural wind flow characteristics of the site are optimally utilized for effective cross-ventilation in the living spaces. The health care-seeking behavior of the occupants for respiratory diseases was found to be infrequent in the units that had relatively higher fresh air exchanges. This study has larger implications in terms of creating a way-forward for the local authorities in sustainable renewal and rejuvenation of the low-income tenement housing in Mumbai.

Coupling fast fluid dynamics and multizone airflow models in Modelica Buildings library to simulate the dynamics of HVAC systems

Multizone models are widely used in building airflow and energy performance simulations due to their fast computing speed. However, multizone models assume that the air in a room is well mixed, consequently limiting their application. In specific rooms where this assumption fails, the use of computational fluid dynamics (CFD) models may be an alternative option. Previous research has mainly focused on coupling CFD models and multizone models to study airflow in large spaces. While significant, most of these analyses did not consider the coupled simulation of the building airflow with the building's Heating, Ventilation, and Air-Conditioning (HVAC) systems. This paper tries to fill the gap by integrating the models for HVAC systems with coupled multizone and CFD simulations for airflows, using the Modelica simulation platform. To improve the computational efficiency, we incorporated a simplified CFD model named fast fluid dynamics (FFD). We first introduce the data synchronization strategy and implementation in Modelica. Then, we verify the implementation using two case studies involving an isothermal and a non-isothermal flow by comparing model simulations to experiment data. Afterward, we study another three cases that are deemed more realistic. This is done by attaching a variable air volume (VAV) terminal box and a VAV system to previous flows to assess the capability of the models in studying the dynamic control of HVAC systems. Finally, we discuss further research needs on the coupled simulation using the models.

Air-tightness test and air infiltration estimation of an ultra-low energy building

The air-tightness performance of residential buildings in China is not fully investigated. This study intends to investigate the air-tightness performance of an ultra-low energy building in a cold area of China. The air-tightness of the building envelope was measured using blower door method and the whole year air infiltration of the building was simulated by a multi-zone network airflow model based on the air-tightness test results. A thermal infrared imager and a smoke pencil were used to find out the typical air leakage places in the building envelope. The leakage areas of the typical air leakage places are found by measuring the whole building and then sealing the corresponding component and determining the reduction in the building effective leakage area. Test results show that the air change rates of the building at the pressure difference of 50 Pa (ACH50) under the depressurization and pressurization modes are 0.5 and 0.49 h−1, respectively. Simulations show that the air change rate of building under the weather conditions of Dalian changes from 0 to 0.12 h−1 with an average of 0.05 h−1. The typical air leakage places of the building were the reserved holes in the building envelope, the junctions of the windows with the walls, the deformation parts of the outer windows and doors, and the joints and vents that imprecisely sealed. The exhaust vents were found to be the most significant source of leakage among the tested typical air leakage places.

Airtightness impact on energy needs and airflow pattern: a numerical evaluation for mechanically ventilated dwellings in France

ABSTRACTThis paper aims to assess the impact of the envelope airtightness on both heating needs and airflow patterns for the case of a single-detached dwelling as function of the ventilation system. An original numerical approach based on the energy simulation tool TRNSYS coupled to the multi-zone airflow and contaminant transport model COMIS was used to compare the performance of four mechanical ventilation strategies. The impacts of the envelope leakage location (facades and/or ceiling), the zone configuration (one- or multi-zone model), the climatic and wind exposure conditions (exposed or surrounded by obstructions) were also investigated. The results show a significant increase of the heating needs when the air permeability at 4 Pa is increased depending on the climatic conditions. Furthermore, the desired airflow pattern, and hence the dispersion of pollutants from main rooms to service rooms can be significantly disturbed especially in the case of humidity-sensitive ventilation system.

Uncertainty Distribution of Air Infiltration Rates of Thai Detached Homes for Energy Model

Air infiltration through building envelope is considered as a path of outdoor air ventilation in modern Thai detached homes where there is no outdoor air intake in an air condition unit. At present, there are limited studies of air infiltration rate through building envelope for Thai homes and such few studies are insufficient for establishing a database of air infiltration rate for evaluating home energy performance. This present study used a multi-zone airflow model to calculate air infiltration rates in 134 homes from 137 housing development projects located in suburban areas near Bangkok. The calculated air exchange rate (air infiltration) ranged from 0.33-0.58 h-1 with an average value of 0.43 h-1 (s.d. = 0.04). Compared to the ventilation standards, the calculated air exchange rates were approximately 1.7 times greater than the required ventilation rates defined by the ASHRAE Standard 62.2 (2007) and 93% of those achieved the required ventilation rate prescribed in the Thai Building Control Act B.E. 2522 (2015). Consequently, it has a potential to reduce air infiltration rate for modern Thai detached home to save home energy consumption. The default air infiltration rate typically used in energy models provided a maximum input error ranged from 7-65% when compared to the calculated air infiltration rates.

IAQ and energy impacts of ventilation strategies and building envelope airtightness in a big box retail building

Heating, ventilating, and air conditioning (HVAC) systems in buildings are designed to provide thermally comfortable conditions and to maintain acceptable indoor air quality (IAQ). At the same time, the operating costs of HVAC systems are often a large percentage of the total energy consumption of buildings, which constitutes 40% of the primary energy consumed in the U.S. As efforts are pursued to reduce building energy use, which may involve reductions in outdoor air ventilation rates, it is important to consider the impacts of these measures on IAQ. To better understand the IAQ and energy trade-offs of various ventilation approaches, simulations were performed using the multizone airflow model, CONTAM, for a big box retail store in two climates. Annual energy simulations were also performed in EnergyPlus. Simulations included ventilating at the minimum outdoor airflow rate prescribed by the ASHRAE Standard 62.1–2013 Ventilation Rate Procedure during occupied hours and at a lower rate for 24 h a day. Additional cases included increasing the base outdoor air intake rates and changing the building envelope airtightness. Annual airflow and contaminant (carbon dioxide, formaldehyde, ozone, particulate matter, and a generic volatile organic compound) simulations were performed using emission rates from previous studies of retail buildings. This study demonstrated that ventilating at a lower rate for 24 h a day saved energy compared with ventilating at the higher rate prescribed in ASHRAE 62.1 and that the simulated indoor contaminant concentrations did not exceed common benchmarks or health guidelines.

Improving infiltration modeling in commercial building energy models

As building envelope performance and heating, ventilation, and air conditioning (HVAC) equipment efficiencies are increasingly improved to reduce building energy use, a greater percentage of the total energy loss of a building can occur through envelope leakage. Although the energy impacts of unintended infiltration on a building's energy use can be significant, current energy simulation software and design methods are generally not able to accurately account for envelope infiltration and the impacts of improved airtightness. New strategies to incorporate airflow calculations into building energy calculations are proposed, which are based on relationships between infiltration rates calculated using multizone airflow models, building characteristics, including envelope airtightness, weather conditions, and HVAC system operation. The new strategies are more accurate than current approaches in energy simulation software and easier to apply than multizone airflow modeling.

Different modeling strategies of infiltration rates for an office building to improve accuracy of building energy simulations

Air infiltration rates directly impact building energy consumption to a larger or smaller degree depending on the tightness of building enclosure and heating ventilation and air conditioning system operation. The relative importance of infiltration airflows has been increasing in total building energy consumption due to the improvements in building insulation and window products. The objective of this study is to compare the accuracy of building energy simulations associated with different air infiltration rates calculation approaches. This study used different sources of infiltration rate: time-dependent simulated data, AIVC database, and default settings in building energy simulations. A coupled framework associated with time-dependent infiltration rates is used by integrating computational fluid dynamics and multi-zone airflow modeling results into energy simulations. This framework is demonstrated with a case study for an office building in Michigan. The case study also uses the infiltration rates obtained from the database and default settings in energy simulation program. The comparison between simulation results and utility data shows that time-dependent infiltration rates could increase the accuracy of energy simulations with 3–11% reduction in the coefficient of variation of the root mean square error (CVRMSE), and 2–11% reduction in the normalized mean bias error (NMBE).

Consideration of Envelope Airtightness in Modelling Commercial Building Energy Consumption

As strategies for improving building envelope and HVAC equipment efficiencies are increasingly required to reduce building energy use, a greater percentage of energy loss will occur through building envelope leakage. Although the energy impacts of unintended infiltration on a building’s energy use can be significant, current energy simulation software and design methods are generally not able to accurately account for envelope infiltration and the impacts of improved airtightness. The airflow analyses capabilities of several energy simulation software tools are summarized, including whether the program calculates airflow rates for the user or accepts them as inputs. The bases of these airflow rate estimation approaches are evaluated for their physical soundness and accuracy. A new strategy to more accurately incorporate airflow calculations into energy software is also proposed, which is based on relationships between building infiltration rates calculated using detailed multizone airflow models and building characteristics, weather conditions, and building envelope airtightness.

Building leakage analysis and infiltration modelling for an Italian multi-family building

The presented paper aims at detailing the results of an investigation that was recently conducted in Italy to evaluate the contribution infiltration makes to meeting ventilation needs in a recently renovated apartment building and the corresponding energy costs. It is years that increasing importance has been placed on the energy efficiency in residential buildings as about 70% of the existing Italian residential building stock was built before 1976 (i.e. before any measure related to the energy efficiency in buildings). As existing dwellings have been traditionally considered ‘leaky’, the actions for improving their energy efficiency have often determined tighter buildings, raising concerns about whether the amount of infiltration is sufficient to provide occupants with acceptable indoor air quality (IAQ). The current state of knowledge on infiltration in multi-family buildings in terms of measuring procedures, corresponding air change rates and airflow patterns was reviewed. The air tightness of a three-storey, six-unit, multi-family building which can be considered representative of the existing recently renovated Italian building stock was characterized by means of a series of fan pressurization tests. The performed blower door tests are documented and results of the data analysis are reported and discussed. A simulation model was developed; simulations were performed to analyse in detail the winter magnitude of air infiltration. Winter is usually detrimental to IAQ, as severe outdoor weather prompts occupants to keep closed any opening that could allow cold drafts into their homes. Modelling results were validated on the basis of a 3-week monitoring campaign. The developed model enabled to estimate the variation with time of infiltration rates and therefore the influence of infiltrating air on the resulting heat loss and IAQ. Numerical predictions were derived using the EnergyPlus simulation tool which allowed to combine whole building thermal simulation and detailed multi-zone airflow modelling. Results show that, during the considered heating season (October–April), the average air change rate due to infiltration was approximately 0.1 h−1. It was concluded that infiltration cannot be relied upon to provide adequate ventilation air and, if not assisted by other means of ventilation, IAQ deterioration is likely to occur.