Jianlei Niu

Performance comparisons of desiccant wheels for air dehumidification and enthalpy recovery

Desiccant wheels have two major applications: air dehumidification and enthalpy recovery. Since the operating conditions are different, heat and mass transfer behaviors in the wheels are quite different. In this paper, the performances of desiccant wheels used in air dehumidification and enthalpy recovery are compared with each other. To accomplish this task, a two-dimensional, dual-diffusion transient heat and mass transfer model which takes into account the heat conduction, the surface and gaseous diffusion in both the axial and the thickness directions is presented. Effects of the rotary speed, the number of transfer units, and the specific area on the performance of the wheel are investigated and compared in the two situations. The cycles that the desiccant and air undergo in the wheel are plotted in psychrometric charts to demonstrate the different heat and moisture transfer mechanisms during the dehumidification and enthalpy recovery processes.

Energy requirements for conditioning fresh air and the long-term savings with a membrane-based energy recovery ventilator in Hong Kong

A study of the energy requirements for conditioning ventilation air and the yearly performance of a membrane-based energy recovery ventilator (MERV) in Hong Kong is carried out. The weather data are classified into six process regions in the psychrometric chart and the percentage of annual hours in each region is determined to describe different energy requirements. The variations in the amount of required heating and cooling energy are calculated for different indoor temperature and humidity set points. It is found that the required annual energy is primarily used to remove moisture from fresh air, with only a small fraction used for sensible cooling. The energy for heating in cold weather is negligible. Energy recovery ventilators are employed to study the possible annual energy savings. Hour-by-hour calculations disclose that in hot and humid regions like Hong Kong, about 58% of the energy required for conditioning fresh air could be saved annually with an MERV, which recovers both latent and sensible energy, while only about 10% of the energy could be saved with a traditional sensible-only energy recovery ventilator (SERV). The more humid the weather, the more superior is an MERV in comparison with an SERV.

Indoor humidity behaviors associated with decoupled cooling in hot and humid climates

Abstract Radiant cooling with independent air dehumidification/ventilation is a complementary cooling and ventilation technology that has the potential to provide better thermal comfort, air quality and energy consumption than conventional all-air systems. However, in hot and humid regions, fears for the risk of condensation on ceiling panels limit its market penetration. To address this problem, in this paper, indoor humidity behaviors associated with decoupled cooling in hot and humid climates are investigated. Room mean temperature, mean humidity, maximum RH on the ceiling panel surfaces, annual condensation hours and annual primary energy consumptions are predicted with systems combining chilled ceiling with various air dehumidification and ventilation strategies, using a building energy simulation code ACCURACY. The effects of night air infiltration rates and the ratios of air flow rates of return air to those of fresh air on the indoor humidity performance and annual condensation hours are discussed. The results indicate that dehumidification and ventilation prior to cooling panels operation is required to reduce condensation risks in hot and humid climates. It is also revealed that a 1 h in advance dehumidification/ventilation in summer could completely eliminate the condensation problems.

The airborne transmission of infection between flats in high-rise residential buildings: Tracer gas simulation

n Abstractn n Airborne transmission of infectious respiratory diseases in indoor environments has drawn our attention for decades, and this issue is revitalized with the outbreak of severe acute respiratory syndrome (SARS). One of the concerns is that there may be multiple transmission routes across households in high-rise residential buildings, one of which is the natural ventilative airflow through open windows between flats, caused by buoyancy effects. Our early on-site measurement using tracer gases confirmed qualitatively and quantitatively that the re-entry of the exhaust-polluted air from the window of the lower floor into the adjacent upper floor is a fact. This study presents the modeling of this cascade effect using computational fluid dynamics (CFD) technique. It is found that the presence of the pollutants generated in the lower floor is generally lower in the immediate upper floor by two orders of magnitude, but the risk of infection calculated by the Wells–Riley equation is only around one order of magnitude lower. It is found that, with single-side open-window conditions, wind blowing perpendicularly to the building may either reinforce or suppress the upward transport, depending on the wind speed. High-speed winds can restrain the convective transfer of heat and mass between flats, functioning like an air curtain. Despite the complexities of the air flow involved, it is clear that this transmission route should be taken into account in infection control.n n

Energy saving possibilities with cooled-ceiling systems

Abstract Based on hour-by-hour simulation studies, this paper presents the annual energy consumption characteristics of water-panel type cooled-ceiling systems in comparison with conventional all-air systems. In particular, some unique energy saving measures associated with cooled-ceiling systems are numerically studied. The results indicate that a cooled-ceiling system can save much of the fan energy required in all-air systems, and that in the temperate Dutch climate, the system has a compatible energy performance with a VAV (variable air volume) system for office-building cooling purposes. The energy saving possibilities studied include using ceiling water to pre- or re-heat ventilation air and using a cooling tower for free cooling. The paper also demonstrates the possibilities of using a simulation technique to estimate non-conventional energy efficient system designs.

Transient CFD simulation of the respiration process and inter-person exposure assessment

n Abstractn n It is known that the person-to-person spreading of certain infectious diseases is related with the transmission of human exhaled air in the indoor environments, and this is suspected to be the case with the severe acute respiratory syndrome (SARS) outbreak. This paper presents the numerical analysis of the human respiration process and the transport of exhaled air by breathing, sneezing, and coughing and their potential impact on the adjacent person in a modeled room with displacement ventilation. In order to account for the influence of the thermal plume around the human body, a three-dimensional computational thermal manikin (CTM) with an accurate description of body geometry was applied. Some of the results were compared with those from former simulations and experiments. It was found that personal exposure to the exhaled air from the normal respiration process of other persons is very low in a modeled room with displacement ventilation. Personal exposure to pollution caused by sneezing or coughing is highly directional. When two occupants face each other the cross-infection may happen due to the long transport distance of the exhalation.n n

Laminar fluid flow and mass transfer in a standard field and laboratory emission cell

Abstract The field and laboratory emission cell (FLEC) is becoming a standard method of characterizing pollutant emissions from building materials. Based on this method, the material and the inner surface of the FLEC cap form a cone-shaped cavity. The airflow is distributed radially inward over the test surface through a slit in a circular-shaped channel at the perimeter of the chamber. After mass transfer, the air is exhausted through an outlet in the center. Usually, emission rate profiles are obtained using such cells. However, the local convective mass transfer coefficients are now needed. In this study, laminar fluid flow and mass transfer in a standard FLEC are investigated. The velocity field and moisture profiles are obtained by solving Navier–Stokes equations numerically. The whole geometry, including the air inlet and outlet, channel, air slit, and emission space, are included in the numerical modeling domain. The mean convective mass transfer coefficients are calculated and compared with the experimental data. In the test, distilled water is used in the FLEC lower chamber to substitute the emission surface. Mass transfer data are obtained by calculating humidity differences between the inlet and outlet of a gas stream flowing through the FLEC. The study concentrates on assessing the variations of velocity and humidity profiles, as well as convective mass transfer coefficients, in the cell.

Personalized Ventilation for Commercial Aircraft Cabins

Complaints about cabin air quality and persistent reports of spreading infections on commercial flights indicate that continued investigations of cabin air systems and effective measures to improve cabin air quality are required. This study used a computational fluid dynamics technique to investigate the dispersion characteristics of sneezed/ coughed particles by both the Eulerian and Lagrangian methods. These particles can be transported to a location more than three rows in front of the sneezing person, and less than 20% of the particles were exhausted, whereas the remainders are deposited owing to the high surface-to-volume ratio. Personalized ventilation, through the distribution of fresh air directly in the breathing zone, was able to shield up to 60% of air pollutants in a passengers inhalation.

Mass transfer of volatile organic compounds from painting material in a standard field and laboratory emission cell

The field and laboratory emission cell (FLEC) is becoming a standard method of characterizing pollutant emissions from building materials. It is significant to use the emission profiles from FLEC to scale the emissions of building materials in real buildings. The dynamics of mass transfer in such an FLEC are the key to perform this task. In this study, the mass transfer mechanisms of the total volatile organic compounds from a wet painting in an FLEC are experimentally and numerically investigated. A three-dimensional mass transfer model, which takes into account the convective mass transfer between the material and the air, the diffusion in the paint film and in the substrate, is developed. The emissions from a water-based emulsion paint are quantified to assess the model. The concentration fields in the film and substrate are calculated to demonstrate the processes of internal volatile organic compounds diffusion.

Indoor climate in rooms with cooled ceiling systems

By using thermal dynamics simulation and CFD (computational fluid dynamics) techniques, thermal comfort and indoor contaminant distributions are analysed for a room with: (1) a conventional displacement ventilation system, (2) a cooled ceiling with displacement ventilation system, and (3) a cooled ceiling with ceiling air supply system. The numerical simulation results indicate that a cooled ceiling reduces the vertical temperature gradients while still maintaining the ventilation effectiveness much higher than one at a load of nearly 50 W/m2 floor area. By comparison, maximum cooling capacities of the three systems are discussed, specifically in respect to their thermal comfort performances.