Influence of relayed fans and low level exhausts on performance of attachment ventilation under heating mode

https://doi.org/10.1016/j.jobe.2021.102155Get rights and content

Highlights

  • Impact of relayed fans on heating performance of attachment ventilation.

  • Impact of low level exhausts on heating performance of attachment ventilation.

  • Head and foot temperature difference less than 3 °C.

  • Relayed fans increase temperature efficiency under high level exhausts.

Abstract

Compared with displacement ventilation, attachment ventilation characterized by high supply air momentum has the potential to generate warm air lake on the floor under winter heating mode. However, the airflow can not keep attaching to the floor and moves upwards by thermal buoyancy. The influence of relayed fans and low level exhausts on performance of attachment ventilation under heating mode was explored. One/two relayed fans and five dimensionless exhaust heights were investigated. Performance evaluating index include vertical temperature gradient, temperature efficiency, predicted mean vote (PMV), predicted percentage of dissatisfied (PPD) and Draught risk (DR). Simulation results showed that relayed fans and low level exhausts effectively increase the attachment length of warm air lake in winter. Operation of relayed fans reduces vertical temperature difference, maintaining the head and foot temperature difference less than 3 °C. Low level exhausts effectively increase occupant zone temperature, reduce airflow short circuit and improve temperature efficiency. The operation of relayed fans slightly increase the temperature efficiency under high level exhausts. Two relayed fans decrease the temperature efficiency under low level exhausts. Better performance can be achieved by using only one relayed fan.

Introduction

Heating, ventilation and air conditioning (HVAC) system is used to eliminate indoor thermal loads to create a good indoor thermal environment, and distribute sufficient fresh air to dilute indoor pollutants [1]. Space heating is necessary to ensure indoor thermal environments in cold season. Radiative heating systems and convective heating systems are the two methods to warm up spaces through providing hot surfaces and supplying hot air respectively [2]. Convective heating systems have more potential for application because of better thermal responses.

Ventilation and air distribution methods include mixing ventilation (MV), displacement ventilation (DV), and so forth. MV is the traditional air distribution method for removing indoor contaminants and creating uniform indoor thermal environments. The whole spaces including human occupied zone and ambient zone are conditioned by the supply air. The unnecessary air conditioning for spaces outside occupied zone may cause energy waste especially for tall spaces. Previous studies reported airflow short circuit in the upper zone when MV is applied for space heating [[3], [4], [5]]. DV, as one of stratified ventilation manners, was widely used because of good air quality and high energy efficiency. DV is characterized as low air supply speed and not recommended for space heating in winter [6].

A new ventilation and air distribution method, named attachment ventilation, was pointed out [[7], [8], [9], [10]]. The air lake which generated by attachment ventilation is similar to that generated by DV. Air supply momentum and buoyancy force are two main factors affecting indoor airflow patterns. High speed air lake of attachment ventilation can be utilized for space heating. Warm air lake heats lower part of the occupied zone. Lower temperature at head level and higher temperature at ankle level can increase the thermal comfort [11,12]. Stratified ventilation systems, such as impinging jet ventilation (IJV), were also investigated for space heating in recent years [[13], [14], [15]]. Limited extension area of warm air lake may be the disadvantage of stratified ventilation for application in heating mode. Same problem may also occurs for attachment ventilation application in winter. For spaces with long depth, the deflection of warm air lake seems to be inevitable.

The importance of exhaust location is reported [[16], [17], [18], [19]]. Ye conducted the optimization research on exhaust height by TOPSIS method for IJV [18]. The results indicated that optimal range of exhaust height is 1.2–1.5 m for best performance of IJV. Cheng conducted experimental and numerical studies on stratified ventilation systems to investigate the effects of exhaust height [19]. Results showed that ceiling level exhausts and return grilles at upper boundary of occupied zone perform best and is recommended in practice. Some researchers applied low level exhausts to enhance ventilation performance in heating mode [5,20,21]. Amai experimentally compared the temperature efficiency and ventilation efficiency with different exhaust locations in heating mode [20]. The results indicated that locating exhausts on the floor level increases temperature efficiency by 70% and ventilation efficiency by 30% respectively, compared with locating exhausts on ceiling level.

Relayed fans were used to remove contaminants in tunnels by extending jet flow distance. Wang firstly applied relayed fans into industrial premises combined with nozzle supply systems to improve thermal environments [22,23]. The measurements indicated that the air diffusion performance can be enhanced and dissatisfaction percentage can be reduced when secondary airflow relay system is properly configured. Sun mounted vertical fans on workstation/chair to optimize DV airflow distribution in vertical direction [24,25]. Wang utilized adjustable fan network to improve airflow pattern to maintain required indoor thermal conditions coping with different distribution of indoor heat sources [26].

The two methods, low level exhausts and relayed fans, may have positive effects on expending attachment distance of warm air lake in winter. In this study, the indoor thermal environment of attachment ventilation in heating mode is studied. The different operation conditions of relayed fans, different supply air temperature and exhausts heights are taken into consideration to assess the thermal environment.

Section snippets

Numerical aspects

Computational fluid dynamics (CFD) simulation method has been widely used in the past 40 years to predict airflow pattern, temperature distribution and contaminant distribution. Appropriate turbulence model should be selected and boundary layer grid should be reasonably generated. For attachment ventilation, it is important to accurately predict the departure point of attachment ventilation jet on vertical wall surface, the re-attachment point on horizontal floor and thermal plumes generated by

Results

Performance of attachment ventilation, combined with relayed fans and different exhaust heights, is evaluated in terms of airflow distribution, temperature stratification and thermal comfort indexes.

Discussion

In this study, whether relayed fans and low level exhausts can improve the performance of attachment ventilation in winter conditions has been explored. The main task of optimizing the performance of attachment ventilation under winter conditions is to enhance supply air momentum, reduce buoyancy effect and extend warm air lake. Relayed fans effectively extend the attachment length and slightly increase the draught risk at ankle height. Application of attachment ventilation and relayed fans in

Conclusions

The influence of exhaust locations, operation conditions of relayed fans and supply air temperatures on attachment ventilation performance under winter heating mode is explored. The following conclusions are drawn.

  • 1)

    Relayed fans and low level exhausts effectively increase attachment length. Operation of relayed fans reduces vertical temperature difference, maintaining the head and foot temperature difference less than 3 °C along jet direction;

  • 2)

    Low level exhausts effectively increase occupied zone

Author statement

This manuscript has not been published previously nor is it currently under consideration for publication in any other journals. All authors know and approve of its submission as a research article for publication in Journal of Building Engineering.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The study was supported by the National Key R&D Program of China (No. 2019YFC0605105).

References (30)

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