Modeling of supply airflow from slot line diffuser on ceiling for CFD of thermal environment in perimeter zone

Highlights

• Outlet airflow's characteristics under both thermal and isothermal conditions of the slot line diffuser are examined by a full-scale experiment.

• Turbulence statistics on the diffuser's outlet surface are calculated based on the X-type probe hotwire anemometer's measurement data.

• Outlet characteristics model for slot line diffuser was proposed based on the full-scale experiment. The model can be used to define the boundary data in CFD simulation.

• A detailed CFD simulation of the diffuser's outlet airflow was carried out and validated its accuracy.

• Detailed CFD simulation result was compared with the CFD simulations using the P.V. method and the simple outlet boundary.

Abstract

The slot line diffuser is widely used in Japan near the perimeter windows because of its blocking effect on convective heat transfer. However, research on this diffuser has mainly focused on the effects of the turbulence model on CFD simulations and longitudinal airflow distribution, while studies on its detailed outlet characteristics in three-dimensional have been limited. In order to obtain and summarize this diffuser's outlet characteristics more accurately, this paper set a slot line diffuser with an adjustable outlet area in free space. The detailed velocity and turbulence statistics distribution on the diffuser's outlet surface were obtained by a hot-wire anemometer, and the diffuser's airflow distribution in the free space was measured by an ultrasonic anemometer. Then the outlet characteristics of this diffuser were summarized using these measured data in order to make a detailed supply airflow model. At the same time, the difference of the airflow of full-size supply and that of half-size supply is characterized. Moreover, a detailed CFD simulation of the diffuser's outlet airflow was performed using the characteristics model to define the boundary conditions. The detailed CFD result with outlet characteristics model were validated by comparing with experimental data. Additionally, the prescribed velocity (P.V.) method and simple boundary simulation were also attempted to reproduce the diffuser's outlet airflow, and compared with the experiment and detailed CFD results. This study clarified this diffuser's outlet characteristics in three-dimensional, and it is considered that the outlet characteristics model can help improve the simulation accuracy of the kind of diffuser.

Introduction

With the development of construction and material technology, the thermal performance of building envelopes is enhanced, and the window to wall ratio (WWR) is increasing. On the one hand, large WWR contributes to a better living standard by providing more daylight [1] and useful heat gain [[2], [3], [4]]. On the other hand, large WWR may cause heat losses or undesirable heat gains because of the poor thermal resistances of the window glass [5]. The radiation coming from the hot/cold window glass or getting through from the sun will affect the occupant's thermal sensation directly [6]. Also, enhanced thermal performance of building's envelope (such as walls and roofs with high thermal resistance), will make the window glass with relatively large thermal conductivity act as a thermal bridge [7,8], and cause the perimeter zone with large area windows occupied a considerable part of building energy consumption [9,10].

To improve the thermal environment and decrease the energy consumption of air-conditioning in the perimeter zone, studies such as using some shading devices [[11], [12], [13]], daylight redirection system [14,15], high reflection, and insulation window material [[16], [17], [18]] were present in recent years. However, most of these studies majored in blocking out the daylight radiation or the heat transfer from outdoor air passively. To handle the heat load through the glass in the perimeter zone, some methods such as combining with air conditioning or ventilation system are needed. Airflow window (AFW) [19,20] is an efficient way to exhaust the heat inside or near the window glass by buoyance effect or the fan inside the double skin glass. However, as for the window's shading and reflection methods mentioned above, these studies mostly majored in cooling conditions. In the heating period, blocking out the daylight radiation may have the opposite effect. The efficiency of AFW may become limited as there is a risk of the cold draft [20] because of its up-out down-in airflow characteristics in the heating season.

Therefore, research concentrating on the active way, such as using the air conditioning system to improve the thermal environment in perimeter space in the heating condition is significant. Different from the cooling condition, cold radiation [21,22] and the cold draft [23,24] during the heating period will obviously affect the occupant's satisfaction with the thermal environment. Poor predicted mean vote (PMV) and predicted percentage of dissatisfied (PPD) [25,26] evaluation would be obtained unless the room air temperature is raised integrally to offset the cold air draft and radiation effect. To avoid consuming much more energy when increasing the room air temperature, heating the window glass, neutralizing the cold airflow by hot air, blocking or decreasing draft velocity was considered adequate.

Perimeter heater and slot line diffuser are widely in Japan used to improve the perimeter thermal environment. For the perimeter heater, a traditional perimeter air conditioning equipment, was usually installed right beside the windowsill. Several research [27,28] have examined and improved its heating efficacy, such as plume airflow from the heater can heat up the window glass simultaneously as offsetting the cold draft by heat and buoyancy.

The slot line diffuser is generally installed in the system ceiling combined with the illumination system or the ceiling space near the perimeter window. Diffuser's long and slim outlet area ensures it can possess a relatively high outlet velocity at a finite supply air volume. Jetting out at a considerable rate from the diffuser's long outlet surface makes the outlet airflow of the slot line diffuser have a spread distribution range, and can work as an air curtain to block the heat or pollutant's movement. Research on the slot line diffuser is mainly concentrated on reproducing the diffuser's outlet airflow by facilitation CFD simulation [29,30], such as the prescribed velocity method (P.V. method) and momentum method [[31], [32], [33], [34]], proposing the calculation model of diffuser's wall (ceiling) jet and the effect of the buoyancy [35,36], or the airflow characteristics effect by the wall [37] and the outlet condition [35,38]. However, in these studies, turbulence model, coandă effect, buoyancy's effect was emphasized while the outlet airflow of the slot line diffuser usually be defined as a simple opening with uniform outlet velocity or the velocity distribution calculated by the jet's equation. Furthermore, these examinations on the slot line airflow mostly majored on the diffuser's longitudinal direction (short side section) and considered its outlet airflow as the two-dimensional. It is undeniable that some diffuser with the rectification unit as the punching panel, or the particular air supply method as the ductless ceiling chamber has the uniform outlet velocity distribution, but the velocity distribution still exists when the diffuser has no rectification design or effect by the connect location from the air supply duct. The velocity distribution on the diffuser's outlet surface may complication its airflow's diffusion characteristics, especially in its longitudinal direction, making the outlet airflow different from the typical jet, thereby affecting the airflow or temperature distribution in indoor space. It is essential to clarify the detailed outlet characteristics of the slot line diffuser in three dimensions, which can be used as the reference data in the HVAC's design usage and the comparing data to ensure the simulation's accuracy.

This paper focuses on clarifying the slot line diffuser's detailed outlet airflow characteristics in three-dimensional by the full-scaled experiment and proposing a modeling method to reproduce the diffuser's outlet airflow in high precision. The slot line diffuser used as the research target is the single slot with an outlet surface in a high aspect ratio. It is widely used in Japan because of its merit such as the space-saving and can easily combine with the system ceiling. The diffuser's slim geometric and the air supply duct are usually perpendicular to the diffuser's outlet direction (Chapter 2.1, Fig. 1), complicating the distribution of its outlet airflow, based on the user's feedback and the result of some simple smoking tests. Meanwhile, the slot line diffuser used in this study has two reflection panels inside its chamber. It is a new technology attempt to improve the diffuser's heating performance by adjusting the outlet area during the heating period. However, the detailed outlet airflow characteristics data of this kind of diffuser is limited. Though TAMBARA et al. [29], measure the similar kind of diffuser's airflow distribution in the free field by the PIV method, and reproduced its outlet airflow in CFD by the P.V. method, their examination were only carried on in the diffuser's latitudinal direction, and the results of the P.V. method still has some inaccuracy.

Therefore, as the first step of this study, the slot line diffuser with a variable outlet surface (1/1 outlet or 1/2 outlet mode) is introduced in Chapter 2.1. A full-scale experiment is carried out in Chapter 2.2 to obtain the slot line diffuser's outlet characteristics, such as the airflow distribution in the free field measured by the ultrasonic anemometer and the component velocity distribution on the outlet surface measured by the X-probe hot wire anemometer, under each outlet modes. An outlet characteristics model of the slot line diffuser was presented in Chapter 2.3. A line graph summarized the outlet velocity distribution in u,v,w components, and the turbulence kinetic energy k, turbulence length l and dissipation rate ε of the diffuser's outlet airflow were calculated, based on the detailed measurement data by the X-probe hot-wire anemometer.

In Chapter 2.4, a detailed CFD simulation that divided the diffuser's outlet surface into several hundred airflow boundaries was performed to reproduce the diffuser's outlet airflow in the free field under isothermal conditions. The diffuser's outlet characteristics model presented in Chapter 2.3 was used to calculate the component velocity data in each airflow boundary individually, kinetic energy k and dissipation rate ε were used to represent the airflow's turbulence statistics on the boundary surface. The boundary velocity distribution data used for detailed CFD simulation are shown in Chapter 3.1. Compared with the P.V. method or Momentum method [29,32,33], using large numbers of boundaries based on the measurement data in CFD to represent the diffuser's outlet characteristics may be complicated. However, a detailed CFD model is expected to have further accuracy and can correctly simulate the airflow near the diffuser (the space between the P.V or momentum boundary to the diffuser's inlet boundary), which the P.V. method or momentum method cannot. So for comparison and reference, the simulation used the P.V. method, and the simulation used only simple outlet boundary were attempted in Chapter 2.4.

The detailed CFD simulation results were validated by comparing with the experimental data both in contour and line graphs in Chapter 3.2. Furthermore, simulations using the P.V. method and some simple outlet conditions were carried out and compared their results with the experiment data and detailed CFD simulation in Chapter 3.2 for reference.