Performance Of Displacement Ventilation Research Articles

Analysis of Thermal Performances in a Ventilated Room Using LBM-MRT: Effect of a Porous Separation

This article demonstrates the feasibility of porous separation on the performance of displacement ventilation in a rectangular enclosure. A jet of fresh air enters the cavity through an opening at the bottom of the left wall and exits through an opening at the top of the right wall. The porous separation is placed in the center of the cavity and its height varies between 0.2 and 0.8 with three values of thickness, 0.1, 0.2, and 0.3. The heat transfer rate was calculated for different intervals of Darcy (10−6 ≤ Da ≤ 10), Rayleigh (10 ≤ Ra ≤ 106), and Reynolds (50 ≤ Re ≤ 500) numbers. The momentum and the energy equations were solved by the lattice Boltzmann method with multiple relaxation times (LB-MRT). Schemes D2Q9 and D2Q5 were chosen for the velocity and temperature fields, respectively. For porous separation, the generalized Darcy–Brinkman–Forchheimer model was adopted. It is represented by a term added in the standard LB equations. For the dynamic domain, numerical simulations revealed complex flow structures depending on all control parameters. The results showed that the thermal field, mainly in the second compartment, is very dependent on the size and permeability of the porous separation. However, they have no influence on the transfer rate.

The comparison of design airflow rates with dynamic and steady-state displacement models in varied dynamic conditions

A temperature-based method is usually applied in displacement ventilation (DV) design when overheating is the primary indoor climate concern. Different steady-state models have been developed and implemented to calculate airflow rate in rooms with DV. However, in practical applications, the performance of DV depends on potentially dynamic parameters, such as strength, type and location of heat gains and changing heat gain schedule. In addition, thermal mass affects dynamically changing room air temperature. The selected steady-state and dynamic models were validated with the experimental results of a lecture room and an orchestra rehearsal room. Among the presented models, dynamic DV model demonstrated a capability to take into account the combination of dynamic parameters in typical applications of DV. The design airflow rate is calculated for the case studies of dynamic DV design in the modelled lecture room in both dynamic and steady-state conditions. In dynamic conditions of heavy construction in 2–4 hours occupancy periods, the actual airflow rate required could be 50% lower than the airflow rate calculated with the steady-state models. The difference between steady-state and dynamic multi-nodal model is most significant with heavyweight construction and short occupancy period (17%–28%). In cases with light construction, the dynamic DV model provides roughly the same airflow rates for four-hour occupancy period than the Mund’s model calculates. The dynamic model can significantly decrease the design airflow rate of DV, which can result in a reduction of investment costs and electrical consumption of fans.

Influence of shape and heat release of thermal passenger manikins on the performance of displacement ventilation in a train compartment

The influence of sensible heat release on the performance of displacement ventilation was studied in a generic laboratory representing the lower cabin of the German Aerospace Center (DLR)’s Next Generation Train. Tests at variable mean cabin temperatures were conducted with human subjects, thermal manikins and heating mats at different but constant heat release rates. Moreover, tests with thermal manikins and an adaptive heat release rate were performed. Flow fields, surface temperature distributions as well as local temperatures and velocities were evaluated. The study revealed that the shape of the heat sources is negligible if only global quantities, such as the global enthalpy flux or the heat removal efficiency, are evaluated. The latter was found to be independent of the amount of released heat. However, it strongly reacts to the choice of probe positions used for the mean cabin temperature. Regarding the comfort-relevant flow parameters, both the shape and the heat release rate are highly relevant to temperature stratifications and flow fields in the passenger zone. The manikins with adaptive heat release proved to be a major improvement in terms of realistic simulation of the human metabolism for investigations regarding the performance of ventilation systems. However, the determination of the mean cabin temperature receives increased relevance as it is prone to limit the closeness to reality.

Systematic study of person-to-person contaminant transport in mechanically ventilated spaces (RP-1458)

It is essential to investigate person-to-person contaminant transport in mechanically ventilated spaces to improve air distribution design and reduce the infection risk from airborne infectious diseases. This article provides a systematic study of the effects of ventilation mode, ventilation rate, and person-to-person distance on person-to-person contaminant transport. This study first collected available cases of person-to-person contaminant transport from the literature to create a database. Then this investigation identified the limitations of the existing data and added a number of cases to complete the database. The additional cases were generated by using a Reynolds-averaged Navier-Stokes (RANS)-Eulerian model that was validated by experimental data from an occupied office with under-floor air-distribution (UFAD) systems. The database shows that the overall performance of displacement ventilation and the UFAD systems was better than that of mixing ventilation. A higher ventilation rate was beneficial in reducing person-to-person contaminant transport to some extent. Person-to-person contaminant exposure increased rapidly with a decrease in person-to-person distance when the distance was smaller than 1.1 m. Generally speaking, person-to-person distance is an important parameter when compared with ventilation mode and ventilation rate.

The Optimized Operation of a Solar Hybrid Desiccant/Displacement Ventilation Combined with a Personalized Evaporative Cooler

The study investigates by modeling and experimentation the performance of displacement ventilation (DV) aided with personalized evaporative cooler (PEC) system that operates in humid climate and uses a solid desiccant (SD) dehumidification system regenerated by parabolic solar concentrator thermal source. Predictive component models of the conditioned space, the SD, the solar concentrator system, and the PEC were developed and used for the prediction of associated operational energy consumption while utilizing an optimized control strategy. The control strategy seeks optimal values of supply air flow rate and temperature and the desiccant regeneration temperature for both cases with and without the aid of the PEC while meeting space load, indoor air quality, and thermal comfort requirements. Energy consumption was calculated for the optimized strategy using genetic algorithm optimizer and the integrated DV/SD-PEC models. The results agreed well with experimental data obtained from tests on a DV climatic chamber. In addition, votes of comfort recorded by participants in the experiment using PEC were very similar to predicted comfort. The optimized hybrid system performance was applied to a typical office space of area of 64 m2. The operation of the hybrid system with PECs resulted in a higher supply air temperature. The increment in supply air temperature is 1.1°C when PEC is used compared to case without PEC. This increase in temperature showed that an energy saving of 13.5% is achieved for the PEC aided hybrid system.

Performance of “ductless” personalized ventilation in conjunction with displacement ventilation: Impact of disturbances due to walking person(s)

The performance of the novel “ductless” personalized ventilation in conjunction with displacement ventilation (DV) was compared with the performance of DV alone under realistic conditions involving disturbances due to walking of one or two persons. An office room with two workstations was arranged in a full-scale test room. Two thermal manikins were used as sedentary occupants at the workstations. Two pollution sources, namely exhaled air by one of the manikins and passive pollution on the table in front of the same manikin were simulated. The performance of the ventilation systems was evaluated with regard to the quality of inhaled air and thermal comfort of the seated “occupants”. The walking person(s) caused mixing of the clean and cool air near the floor with the polluted and warmer air at higher levels and disturbed the displacement principle which resulted in a decrease of the inhaled air quality. The performance of the “ductless” PV under the tested conditions was better as opposed to DV alone. Thus in practice the “ductless” PV will be superior to DV alone as regards perceived quality of inhaled air. The location of a walking person was found to be important. Person(s) walking close to the displacement diffuser will cause greater disturbance.

Effect of Air Supply Temperature on the Performance of Displacement Ventilation (Part II) - Indoor Air Quality

The effect of air supply temperature on the performance of a displacement ventilation (DV) system has been investigated. This paper is the second part of a 2 part series and the results are reported with regard to Indoor Air Quality (IAQ). The study is based on a typical office building in Hong Kong, under local thermal and flow boundary conditions. The criteria for IAQ were measurements of carbon dioxide levels, mean age of air and 3 volatile organic compounds (VOCs): toluene, benzene and formaldehyde. The change in supply temperature from 19°C to 22°C was found to have no significant effect on the level and distribution of contaminants in the room. The DV system was found to operate satisfactorily under the range of supply temperatures studied.