Numerical Investigation of Air Change Effectiveness …4th International Conference On Building...

4thInternationalConferenceOnBuildingEnergy,Environment

NumericalInvestigationofAirChangeEffectivenessinanOfficeRoomwithImpingingJetVentilation

M. Cehlin 1, U. Larsson 1, HJ. Chen 21 Department of Building, Energy and Environmental Engineering,

Faculty of Technology and Environmental Engineering, University of Gävle, Sweden 2 RISE Research Institutes of Sweden, Division of Built Environment - Energy and circular economy,

Borås, Sweden

SUMMARYProviding occupant comfort and health with minimum use of energy is the ultimate purpose of heating, ventilating and air conditioning systems. This paper presents the air-change effectiveness (ACE) within a typical office room using impinging jet ventilation (IJV) in combination with chilled ceiling (CC) under different heat loads ranging from 6.5 - 51 W per square meter floor area. In this study, a validated CFD model based on the v2f turbulence model is used for the prediction of air flow pattern and ACE. The interaction effect of chilled ceiling and heat sources results in a complex flow with air circulation. The thermal plumes and air circulation in the room result in a variation of ACE within the room but also close to the occupant. For all studied cases, ACE is above 1.2 close to the occupants, indicating that IJV is more energy efficient than mixing ventilation.

INTRODUCTIONProviding occupants comfort and a healthy environment with minimum use of energy is the ultimate purpose of heating, ventilating and air conditioning (HVAC) systems. Building ventilation directly affects indoor air quality, and influences occupants’ heath and productivity. Among various types of ventilation, the most widely known and used ventilation methods are mixing ventilation (MV) and displacement ventilation (DV). In MV air is supplied with high momentum at ceiling level in order to dilute the contaminated and cool/warm room air with cleaner and cooler/warmer supply air to lower the contaminant concentration and regulate the temperature. DV on the other hand relies on low momentum cold air supplied directly into the occupied zone at floor level, causing thermal stratification within the room. Hence, DV is restricted to only being used for cooling of rooms. As an alternative ventilation strategy, impinging jet ventilation (IJV) was developed in the 1990s (Karimipanah and Awbi, 2002), and this ventilation strategy combines the advantages of both the MV and DV systems. In impinging jet ventilation, air with high momentum is discharged downwards, distributing fresh air along the floor further into the room compared to DV, and it can therefore be used for both cooling and heating of large spaces (Karimipanah and Awbi, 2002; Ye et al., 2016).

Air distribution devices are used in various HVAC systems including variable air flow volume (VAV) systems, and constant air volume (CAV) systems. The diffusers rarely operate in their design condition, since jet behavior from diffusers may vary in both VAV and CAV systems. This is a major challenge for ventilation systems, as thermal discomfort or low ventilation effectiveness may appear due to varying operation conditions.

A common measure for evaluation of ventilation effectiveness related to indoor air quality (IAQ) is air change effectiveness (ACE). ACE relates the local mean age of air (MAA) in a region within the breathing level against the nominal time constant of the ventilation system. MAA is defined as the average time for air to travel from the air supply to any location in a ventilated space. The distribution of the mean age of air therefore reflects the airflow pattern in the ventilated room. Perfect mixing of the indoor air is used as a reference case, giving uniform MAA in the room and ACE=1.0. Hence, ACE is a measure of how well the outdoor air is distributed to the occupied zone compared to a perfect mixing case. ASHRAE Standard 62.1 (2013) specifies minimum ventilation rate for different type of conditions. This required ventilation rate is increased or decreased to take into account the ventilation effectiveness. ASHRAE Standard 129-1997 (2002) specifies how to modify the minimum ventilation rate defined in Standard 62.1 to account for ACE. Therefore, innovative ventilation systems such as IJV are of interest to investigate since they are more effective in maintaining acceptable indoor air quality than mixing (Karimipanah and Moshfegh, 2007).

Local MAA can be determined by experimental measurements but also by numerical modelling. There are three strategies for numerically predicting local MAA: the steady-state method, the transient method, and the particle marker method. In the majority of numerical studies regarding ventilation effectiveness in buildings, local MAA is calculated using the steady-state method by solving an additional partial differential equation describing the transport of mean age of air (e.g. Roos, 1998; Noh et al., 2008; Bartak et al., 2001; Kwon et al., 2011; Chen et al., 2015; Vachaparambil et al., 2018).

Ventilation systems have restricted cooling capacity, which might cause thermal discomfort for occupants in rooms with high heat loads (Alamadari et al., 1998). Therefore, when high heat load exists in a room, a cooling system such as chilled ceiling (CC) is usually considered to supplement the cooling load. Many studies have been conducted to evaluate the performance of displacement ventilation in combination with chilled ceiling. However, very few studies have been carried out to explore the performance of IJV in combination with chilled beam (e.g. Chen et al., 2013). To the author’s knowledge no study has been conducted to analyze the ventilation effectiveness for IJV in combination with CC.

The aim of this paper is to numerically examine the effects, in particular the interaction effects, of chilled ceiling and heat sources on ACE within an impinging jet ventilated room. Five different cases are set up including the variables of cooling load of chilled ceiling, heat load composition and air flow rate.

ISBN: 978-0-646-98213-7 COBEE2018-Paper218 page 641


METHODSThe room under consideration has a dimension of 4.2×3.6×2.5m, which can be furnished like a single-person or two-person office. Air is supplied through a duct and discharged at a height of 0.8 m above the floor, and evacuated from the exhaust located below the ceiling at the same side wall. The outlet of the supply device is semi-elliptical with an opening area of 0.0166 m2. For more information, see Chen et al. (2012). One rectangular box is used to represent the four lamps installed along the same side of the room. PC and table are attached to the side wall z = 3.6 m, and the ceiling lighting is placed at the position directly above the head.

In a previous publication (Chen et al., 2013), a specific numerical setup within the current design space was extensively validated using experimental data regarding air temperature and air velocities. In the present study, the computational setup is exactly the same as in Chen et al. (2013). The commercial finite volume code ANSYS Fluent 17.2 is used to numerically solve the governing equations. V2f model is used as turbulence model. The pressure and velocity coupling is handled with the SIMPLE algorithm. The convection terms are discretized with the second-order upwind scheme, while second-order central differencing is used for the viscous terms.

Figure 1. Layout of the physical model used in the case studies. In all the cases except Case 4 (see Table 1) a single-person office is considered, which only contains one group of internal heat load located near side wall z = 3.6 m.

A sketch of the computational domain and the coordinate system used is presented in Figure 1. Based on exactly the same mesh strategy as in Chen et al. (2013), the grid size used in the case studies varies between 5.3 (one occupant) and 9.1 million (two occupants), and the perspective view of mesh configuration is shown in Figure 2.

Figure 2. Mesh configuration for the case with two occupants (9.1 million cells).

At the inlet, uniform velocity is specified with turbulence intensity 10% and hydraulic diameter 0.1265 m. The surrounding walls are assumed to be adiabatic, and the floor, ceiling, window as well as internal heat loads are imposed with constant heat flux. Details can be found in Table 1.

The calculation of the local mean age of air is made by solving an additional partial differential equation. It is derived from the concentration equation with the assumption of uniform production of contaminants throughout the room. It is a passive scalar equation since the variable does not interact with the velocity field (Sandberg and Sjöberg, 1983; Davidson and Olsson, 1987). A user-defined function has been implemented in ANSYS Fluent for calculation of local MAA and the air-change effectiveness. For incompressible steady-state conditions, one additional convection-diffusion equation needs to be solved, taking the following form:

!!"#

𝑢%𝜏 = !!"#

()*)+ (

*!,!"#

+ 𝑆, (1)

Where 𝜏 is the local mean age of air, 𝜈 and 𝜈/ are the laminar and turbulent kinematic viscosities, respectively, 𝜎 laminar Schmidt number of air and 𝜎/ is the turbulent Schmidt number for the age of air.

In Eq. 1, the source term 𝑆, is equal to 1. The boundary conditions for the solution of 𝜏 are zero value at air supply and zero gradient at air exhaust and wall surfaces.

The local ACE values are calculated by

𝐴𝐶𝐸 = ,567,

(2)

𝜏89:=;<

(3)

Where V (m3) is the room air volume and q (m3/s) is the inlet air volume flow rate.



Case studies A total of five simulation cases were carried out to investigate the effects of chilled ceiling cooling power, heat load composition as well as its location, and supplied air conditions on IJV performance in an office room. Heat load in the room is contributed by both internal heat loads (i.e., from occupants and electrical equipment) and external heat loads due to solar radiation (imposed on floor and window). The incoming solar radiation is divided into two parts: one part is the solar radiation absorbed by the window as well as the transmitted heat due to temperature difference between indoors and outdoors, and the other part is the directly transmitted solar radiation to the room which is assumed to be evenly distributed over the floor in the CFD model.

Based on the heat loads from occupants, electrical equipment and solar radiation, the supply airflow rate and temperature are set at 0.023 m3/s and 17 °C (except for case 1 where the flow rate is 0.010 m3/s) to insure good thermal comfort in the room. To make the comparisons under the same resultant thermal comfort conditions, the parameter of operative temperature is chosen and calculated as the mean temperature of the average seated air temperature and radiant temperature. The average seated air temperature is calculated based on heights of 0.1, 0.6, and 1.1 m, considering the fact that no single point can represent the thermal stratified environment, and the radiant temperature is taken at a height of 0.6 m. Under the condition with higher heat load in the room, lower room air temperature is controlled to compensate for the higher radiation temperature needed to achieve the similar perceived thermal comfort condition for the occupant. Based on the method described above, the obtained operative temperatures for all the studied cases are kept between 24.1 ºC – 24.5 ºC, the value for each case is listed in Table 1.

Table 1. Major parameters used in the case studies. Case Heat

load (W/m2)

Heat load distribution IHL EHLF EHLW (W) (W) (W)

Ts(oC)

Qv (l/s)

CC cooling load (W)

R (-)

Top(oC)

1 6.5 95a 0 0 17 10 0 0 24.1 2 17 257b 0 0 17 23 0 0 24.4 3 34 257 76 181 17 23 288 0.56 24.4 4 51 257 130 384 17 23 567 0.72 24.5 5 51 514c 76 181 17 23 567 0.72 24.2

IHL: internal heat load EHLF: external heat load imposed on floor EHLW: external heat load imposed on window R: Portion of chilled ceiling cooling power to total cooling power a one occupant (95 W) b one occupant (95 W) + PC (120 W) + fluorescent lamps (42 W) c two occupants (190 W) + two PC (240 W) + fluorescent lamps (84 W)

ACE is evaluated at 1.1m above floor and at a cross section in the middle of the room. Also, MAA at the surface of the occupants are investigated.

RESULTSCase 1 is the case with no heating from the floor or from the window, only from the person, and with an inlet flow to the room of 10 l/s. Figures 3 and 4 show that the middle part of the room behind the person has a local ACE lower than 1 and it shows that the large region in the middle of the room is poorly ventilated, especially above 1.1 m. From the floor up to around 0.5 m the local mean air age is high, due to high momentum of the supply air. Also, air around the workstation between the occupant and the side wall x=3.6m is maintained at good air quality.

Figure 3. Contour plots of ACE at level 1.1 m above floor.

Case1

Case3

Case5

Case2

Case4prel



The figures show that the buoyancy force from the heat source is not so strong compared to the impulse force from the inlet device. The table has a major effect on the flow pattern in front of the person creating a wake, which reduces the local ACE around the workplace.

Case 2 is a case with heating from the workplace, compared to case 1 where heat from the computer and the lamp is added. In addition, the inlet flow is increased to 23 l/s. The flow pattern in the room now turns more against the workstation because of the higher buoyancy effect generated from the heat sources, this behavior lowers the local mean age of air for the zone around the person. The right corner on the opposite wall from the inlet device shows a lower mean age of air than the left corner on the same wall, confirming that the flow turns against the workstation. The mixing region the middle of the room is now decreasing.

Case 3 has a higher energy load than case 2. Now heat from the surroundings is also added, simulated by heat from window and floor. Case 3 has a cooling ceiling (R = 56%) to relieve the cooling of heat sources in the room from the ventilation system. The total heat generated from sources is 34 W/m2. The inlet flow is 23 l/s. Highest value of the local mean age air is still in the middle of the room but ACE is now above 1 which is better than case 1 and 2. When the power of the heat sources has increased, the buoyancy effect related to the impulse from the inlet device increases, which generates more stratification of the air in the room. ACE values around the workstation and the occupant are lower than for case 2.

Case 4 has a higher total heat source at 51 W/m2 (with 50% located on the window) creating a strong upward flow from the window to the upper parts. Now the heat from the window increased the buoyancy effect so much that it has changed the flow pattern in the room. An important feature of the air flow pattern in this case is the large air circulation which can be seen in Figure 5. This air circulation is enhanced and enlarged as the heat flux on the window and the cooling power of the CC increase (Chen et al., 2013). The zone with mixed air in the middle of the room has now increased again. The zone up to around 0.5 meter from the floor is well ventilated and shows a low mean age of air. The zone close to the person is still well ventilated but the local mean air of age has increased compared to the other cases.

Case 5 differs from the other cases by having two workstations, and with this, more heat sources and another configuration of them in the room. However, the total amount of heat generated in the room is the same as in case 4 but now the major part of the heat sources are located at the workstations. Figures 3 and 4 show that the zone with the oldest air is not in the middle of the room as in the other cases, but is now located above the computers. The local mean age air in the middle of the room is still higher compared to the lower parts of the room, but ACE is above 1. The figures also show the airflow in the room is divided into two main zones. The extra heat sources divide the flow and therefore the impulse from the inlet will have a decreasing effect of the flow pattern in the room.

Figure 4. Contour plots of ACE at cross section x = 2.1m.

Case2

Case3

Case5

Case4

Case1



Figure 5. Streamlines for case 4 (surface of release is the window).

For all five cases the average mean age of air of the surface of the occupants is well below the nominal time constant (which is equal to MAA at the exhaust), see Table 2. The nominal time constant for case 1 is 3760 s and for the other four cases 1635 s. Case 2 provides the best air quality around the occupant. The results also indicate that case 5 provides the same level of air quality (if neglecting contaminants from occupants) as case 3, even though two workstations are located in the room resulting in higher heat load and higher chilled ceiling cooling power (with R = 72%) than case 3.

Table 2. Area weighted average of MAA over the occupants’ surface.

Case 1 Case 2 Case 3 Case 4 Case 5

Person1 3057 s 1141 s 1258 s 1348 s 1285 s

Person2 N/A N/A N/A N/A 1287 s

DISCUSSIONFor the two cases with no chilled ceiling cooling load it is a clear tendency that the amount of heat load located in the workstation region strongly influences the flow pattern in the room, and hence the air-change effectiveness. The higher buoyancy effect generated from the heat sources, together with higher supply flow rate, creates better air-change effectiveness in the whole room. The poorly ventilated regions in case 1 with ACE below 1 do not exist in case 2.

Under the situations with higher heat load in the room, a CC system is used together with IJV to remove surplus heat and maintain a comfortable operative temperature in the room. The effect of heat load distribution in the room, as well as the interaction effect of chilled ceiling and heat load can be seen by comparing case 2 – 5. As can be seen in Figure 4, the

overall mixing is increased with higher external heat loads and CC load, and hence the stratification levels are decreased as well as ACE. This increment of mixing is significantly related to the flow pattern, which is influenced by the interaction effect of thermal plumes and chilled ceiling. The interaction of CC load and high heat load creates a complex air flow pattern with strong air circulation in the room. Comparing case 4 and 5 indicates that the location of the heat load has a significant influence on ACE. The room air circulation increases when the external heat load, especially window heat load, is larger than the internal heat load versus the opposite. Also, by comparing case 3 with 5 (same external heat load) the room average ACE as well as ACE close to the occupants is more or less unchanged by adding one more workstation. Hence, the window heat load has a major role for the level air circulation in the room.

It is worth mentioning that all studied cases result in good local air quality (ACE above 1.2) at the breathing level where the occupant is located. The studied cases have also been reported to have acceptable thermal comfort, including local thermal comfort at foot level and vertical temperature gradient, in the region of the workstation (Chen et al., 2013). Therefore, IJV in combination with CC has a good potential to work well in an office room.

The influence of increasing the heat loads even more in the room on indoor air quality in terms of air freshness and ACE for IJV in combination with CC will be investigated in the future. Also, the function of IJV in the room for winter conditions (heating mode) will be investigated.

CONCLUSIONSThe results from the present study show that the flow pattern and ACE within an office room using IJV system is influenced by the interaction effect of chilled ceiling and heat sources. An important feature of the air flow pattern is the large air circulation in the room which is enhanced and enlarged as the heat flux on the window and the cooling power of the CC. The thermal plumes and air circulation in the room result in a large variation in ACE within the room but also at the breathing level where the occupant is located. The supply air is “drawn” to the heat sources (occupants, computer, window) resulting in low local mean age of air and high local ACE in the vicinity of the heat sources. Distribution of the heat sources has a major influence on the mean age of air and hence ACE close to the occupants. All studied cases result in good local air quality (ACE above 1.2) at the breathing level where the occupant is located. This indicates that IJV systems as well as IJV/CC systems are more energy-efficient than mixing ventilation due to the fact that they require lower airflow rate, and hence fan power, for the same level of air quality in the breathing zone. However, more studies have to be performed to reveal possibilities and limitations of impinging jet ventilation in combination with chilled ceiling for different heat load composition, position and power.

ACKNOWLEDGEMENTSThe authors gratefully acknowledge the support received by personnel at the laboratory of ventilation at University of Gävle.



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