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VERTICAL PROFILE OF CONTAMINANT CONCENTRATION IN SICKROOM WITH LYING PERSON VENTILATED BY DISPLACEMENT

Tomoya Suzuki1†, Kazunobu Sagara1, Toshio Yamanaka1,

Hisashi Kotani1, and Tatsuya Yamashita2

1Department of Architectural Engineering, Osaka University, Japan 2Sanki Engineering Co, Ltd. Japan

ABSTRACT In the sickroom, high indoor air quality and thermal comfort is essential for the treatment of patients. Therefore it is proposed to use displacement ventilation for the whole room ventilation and the radiant panel for the thermal comfort of each bed. This study is intended to investigate validity of this system. This paper shows the experimental and calculated results of the displacement-ventilated room with one bed and one radiant panel. The vertical profile of contaminant concentration and temperature are measured and compared with the calculated ones. KEYWORDS Displacement ventilation, Contaminant interface, Thermal stratification INTRODUCTION The mechanism of displacement ventilation is extracted by Sandberg [1], Nielsen[2] and Skistad [3]. The theoretical model for predicting vertical profile of temperature suggested in previous study by Nielsen [4] and Mundt [5]. It is known that contaminant interface forms at the height that airflow rate of supply and airflow rate from heat source are equivalent. Contaminant concentration profile can be briefly predicted with this. In Japan, the study of displacement ventilation by floor-mounted air supply is very often conducted. Displacement ventilation is not only used at living space but also used at large space as a factory or a hall. The profile of temperature of large space can be prevented by block-model [6] (Togari et al. 1991). The equation for preventing airflow rate of heat source is suggested in previous study [7] (Mihashi and Takahashi et al.1998). The relationship between the ventilation heat loss and contaminant concentration in the lower zone was investigated previously [8] (Yamanaka and Kotani et al. 2001). It is, however, necessary to predict the effect of cold wall on the pollution of the air in the lower zone for occupants. The approach of preventing the profile of temperature and contaminant concentration is studied by macro-model [9] (Higashimoto and Yananaka et al. 2003). As observed above, a great number of studies for displacement ventilation are conducted but displacement ventilation for sickroom is hardly at all studied. Most sickrooms in Japan have four beds for each room and there is a problem of odor diffusion in the whole room from the human body or the body waste. It is also difficult to control the thermal environment individually in this type of room. Therefore it is proposed to use displacement ventilation for whole room and the radiant panel for each bed. This study is intended to inspect validity of this system. This paper shows the experimental results of the displacement-ventilated room with one bed, and it is described that theoretical predictions of the vertical distribution of contaminant concentration based on the calculation of previous study [10] (Yamanaka and Kotani et al. 2001). It is called interface layer model. This model is based on contaminant balance by convection and diffusion. The experimental results of vertical profile of contaminant concentration are compared with the calculated ones.

† Corresponding Author: Tel: 816 6879 7645, Fax: 816 6879 7646 E-mail address: suzuki_tomoya@arch.eng.osaka-u.ac.jp

METHOD OF EXPERIMENT Measurement of the vertical profile of temperature The displacement ventilated room is set up in an air conditioned room. It has 2.68m heights and 3.0m widths and 3.0m length, as shown in Figure 1. Walls are thermally insulated with a 50mm thick insulated material. Room air temperature, wall surface temperature and contaminant concentration are measured. Air supply is located at the bottom of the room. The temperature of fresh supply air is controlled by FCU. The inlet is a grille half-cylinder diffuser. It is installed on the floor along the rear wall. The exhaust is a 500mm×500mm square opening located in the center of the ceiling. Heat source is a mannequin twisted around with heating-cable. It is located on the bed in the center of displacement ventilated room. Its heat generation is controlled at 40W as sensible heat load of slumbering human. To simulate heat sources other than human, additional heat sources (two black lumps) are used. In the condition of using only one additional heat source, it is placed at “A1” (see Figure 2) and its heat emission rate is 60W. In the condition of using two additional heat sources, they are placed at “A1” and “A2” and each heat emission rate is 30W. Table 1 shows the experimental cases. Flow rate varies in a range of 50-300m3/h. The height of additional heat sources is changed at 0.5m, 1.0m or 1.5m in the condition of supply air flow rate of 250m3/h and heat emission rate of 30W+30W (indicated as ※ in Table 1). Carbon dioxide, CO2, is used as tracer gas. The CO2 supply tube is located on the mannequin. CO2 flow rate is controlled at 0.5L/min or 1.0L/min. To obtain a constant CO2-flow rate, a mass flow controller is used. Measurement points for air temperature, wall surface temperature and CO2 concentration are summarized in Figure 2. Air temperatures (P1, P2, P3, P6, P7, P8 and P9) are measured at 30 points vertically (i.e. 210 points for all). Air temperatures above a mannequin (P4 and P5) are measured at 18 points vertically (i.e. 36 points for all). Wall surface temperatures (W1-W12) are measured at 6 points vertically (i.e. 72 points for all). CO2 concentrations (P1, P3, P5, P7 and P9) are measured at 12 points vertically (i.e. 60 points for all).

Figure 1. Experimental bed room with displacement ventilation Figure 2. Measurement points

Table 1. Experimental cases

EXPERIMENTAL RESULTS Vertical temperature distribution The measured results of vertical profile of temperature of air and wall surface are shown in Figure 3. The measured temperature is the averaged value of the same height. Their abscissas mean temperature minus supply air temperature. If walls are thermally insulated perfectly, exhaust air temperature is theoretically higher than supply air temperature by about 1.1℃ in the case of 100m3/h without additional heat source, and theoretically, increasing of supply flow rate makes the temperature difference between exhaust air and supply air smaller. But Figure 3 (1) indicates that no definite difference caused by supply air flow rate exits and differences between exhaust air and supply air temperature are only around 0.4℃ in all cases. Although it is not clear from the result of this experiment, one of the cause is considered to be heat loss through walls. Figure 3 (2) shows the effect of heat load from additional heat source on vertical temperature profiles. Its result can be explained well that exact thermal stratification is formed in the case with additional heat source. But an influence of the number of additional heat sources can’t be seen. Figure 3 (3) shows the effect of the heights of additional heat sources on vertical temperature profiles. The influence of the heights of additional heat source on the vertical temperature profile is not seen. Wall surface temperatures are comparatively lower than air temperatures. It is expressed that in the cases with additional heat sources, the wall surface temperatures are raised because of radiation from additional heat sources.

Figure 3. Vertical profile of temperature (Top: air temperature, Below: wall surface temperature)

Vertical profile of CO2 concentration The experimental results of vertical profile of the measured CO2 concentration are shown in Figure 4. The measured concentration is the averaged value of the same height. The concentration minus supply air concentration is normalized by the concentration difference between supply and exhaust. The experimental results indicate that the contaminant interface is formed at upper level of the room, but it becomes obscure with increasing supply air flow rate (Figure 4 (1)). There are some cases that normalized concentration in upper part of the room doesn’t reach 1.0. It is assumed that concentration

is influenced by the exhaust opening located at the center of the ceiling. That is because tracer gas rising with plume from the mannequin is exhausted by the exhaust opening located at the center of the ceiling and can’t spread in the upper part of the room. In some cases the higher concentration in the lower part of the room are seen. This is due to downdraught along the cold walls. Figure 4 (2) shows that additional heat source makes the contaminant interface clearer, but the height of interface moves lower because of the plume of additional heat source. Any effect of the height of additional heat source on the vertical profile of CO2 concentration is not seen (Figure 4 (3)). Further experiments will be carried out in the future.

Figure 4. Experimental results of vertical profile of CO2 concentration

Figure 5. Diagram of interface layer model

MODEL VALIDATION Interface layer model In order to predict contaminant concentrations in the displacement ventilated room, interface layer model [10] is used. This model is based on contaminant balance by convection and diffusion. The outline of this model is described in Figure 5. In this model, “interface layer” including the contaminant interface is assumed. This layer has a vertical width of W, and there is diffusive transfer of the contaminant across the interface. The diffusion consists of molecular diffusion and turbulent diffusion. At the upper edge and lower edge of the interface, no diffusion is assumed and convective flow transfer the contaminant. The position of the interface in the interface layer is expressed by ratioη. Basic equations of this model will be described below. The left side of the equation means inlet flow into the layer of contaminant and the right side of the equation means outlet flow. The value of diffusion coefficient (D+Dt) is used as 0.4496×10-3[m2/s] by previous study[8].

Contaminant balance of layer ② (see Figure 5) is express as:

( ) ( )(( )(1 ) ) ( ) (1)2e dr yu e d d dr yu t f

CC Q C C C Q D D Ay

η ∂= − − + + +

∂　,

Where 2:Floor area[m ]fA

:Contaminant concentration [-]C

:Contaminant concentration in the exhaust air[-]eC

:Contaminant concentration below interface layer[-]dC 2: Diffusion coefficient by molecular[m /h]D

2: Diffusion coefficient by turbulence[m /h]tD 3

( ) : Flow rate flowing from the area above interface layer[m /h]dr yQ

:Height above floor[m]y

: Height of the upper end of interface layer[m]uy

:Relative height coefficient of contaminant interface[-]η

:Thickness of the interface layer[m]W

The contaminant balance of the layer ③ can be expressed by:

( ) ( )1( ) (( )( ) )

2d ur yd t f e d d ur ydCC Q D D A C C C Qy

η∂ −+ + = − +

∂　,(2)

Where 3( ) : Flow rate flowing from the area below interface layer[m /h]ur yQ

:Height of the lower end of interface layer[m]dy The contaminant balance of the layer ④ can be expressed by:

( ) ( ) ( ) ( ) ( ) ( )1 1 1 1 1 1

1( ) (( )(1 ) )( ) (( )( ) )( )2 2

m m m m m m

d s d yd k e d yu k e d d d yi k d yu k e d d d yd k d yi kk k k k k k

C Q Q C Q C C C Q Q C C C Q Qη η= = = = = =

−+ = + + − + − + + + −∑ ∑ ∑ ∑ ∑ ∑ 　,(3)

Where 3:Supply airflow rate [m /h]sQ 3

( ) : Downward airflow rate along cold walls at height of [m /h]d y kQ y

: Height of the contaminant interface[m]iy The contaminant balance of the whole room can be expressed by:

u es

NMC CQ

= = ,(4)

Where :Contaminant concentration above interface layer[-]uC 3:Contaminant emission rate per heat source[m /h]M

: Number of the human[-]N Heights and concentrations of interface layer can be calculated from four equations described above.

Plumes modeling The human is considered as a line heat source with assumption that the plume is generated from overall the human. The airflow rate of plume from a line source is given below in previous study [2]:

130.014l cQ P zs= 　,(5)

It is quoted from the paper [11] that the length of the line heat source is 0.5m and located at 0.8m below the human. The airflow rate of the plume from a panel can be expressed by:

82.43 24.72hQ y= + ,(6)

Where :Convective heat emission rate [W]cP 3:Flow rate of the plume above the heat source [m /h]lQ

3( ) : Flow rate above a human at height of [m /h]h yQ y

: Length of the line source[m] s

: height above line source ( 0.5) [m]z z y= −

From the previous paper [8], the flow rate along a wall is calculated as: 1255

( ) ( ) 12.10u y d yQ Q T y l y∆ = ∆ = ∆ ∆ ,(7)

Where : Width of the wall [m]l

1 2

3( ) 1 2: Increment of flow rate along a warm wall between and [m /h]u y yQ y y∆

1 2

3( ) 1 2: Increment of flow rate along a cold wall between and [m /h]d y yQ y y∆

:Temperature difference between the wall surface and the room [K]T∆ Airflow rate from additional heat source is referred from theoretical formula [3] for point heat source.

1 53 3( )p c

p

gQ k P ZC Tρ

= ,(8)

Where 2:Gravitational acceleration[m/h ]g

:Mean temperature of the room[K]T 3:Airflow rate from additional heat source[m /h]pQ

3:Volumetric specific heat[Wh/m K]pCρ

Calculated results Figure 6 shows flow rates of plume from the human, walls, additional heat sources and total airflow rate. In the condition that air temperatures are higher than wall surface temperatures, downdraught is causes from wall surface. Figure 7 shows the comparisons of the calculated results of vertical profile of contaminant concentration with the experimental ones. The measured concentration is the averaged value of the same height. The calculated results data taking account of the airflow along warm or cold walls is “Calculated results (1)”. And without considering the airflow along walls is “Calculated results (2)”. The calculated results don’t coincide with the experimental ones in the case that supply air volume flow is 150m3/h or 200m3/h because experimental ones in upper part of the room don’t reach to 1.This is because the location of the exhaust opening is at the center of the ceiling (Figure 7(3), (4)). In the case that supply air volume flow is other than 50m3/h or 100m3/h, “Calculated results (1)” are lower than “Calculated results (2)” (Figure 7(3)-(12)). This is due to the effect of upward conductive airflow along the warm wall surfaces on location of interface layer. The downdraught from cold walls affects the concentration of the lower part of the room. “Calculated results (1)” is therefore higher in the lower part of the room in the case of supply air volume flow of 50m3/h and 100m3/h (Figure 7(1), (2)). “Calculated results (1)” don’t coincide with the experimental ones so much without additional heat source (Figure 7(1)-(6)). But the “Calculated results (1)” with additional heat source relatively coincide with experimental ones. This is because the thermal stratification becomes strong due to the thermal plume from the additional heat source. It can be said such a vertical profile of temperature is need for the displacement ventilation. It is considered that in practical usage thermal stratification can be made in displacement ventilated sickroom with heat sources other than human (TV, table lump or refrigerator etc…). It is, therefore, interface layer model is right for predicting the contaminant concentration in the sickroom with lying person ventilated by displacement.

Figure 6. Airflow rates from the heat sources in displacement ventilated room

Figure 7. Calculated and measured results of vertical profiles of contaminant concentration

DISCUSSION In pervious study [8], the value of diffusion coefficient (D+Dt) in displacement ventilated room is examined. The value of diffusion coefficient is identified from the measurement result of contaminant concentration of previous experiments. The mean value of diffusion coefficient is 0.4496×10-3[m2/s]. As a result, the value of diffusion coefficient is substitute as 0.4496×10-3[m2/s] in the interface layer model. There are some cases that normalized concentration in upper part of the room doesn’t reach 1.0. It is assumed that concentration is influenced by the exhaust opening located at the center of the ceiling. That is because tracer gas rising with plume from the mannequin is exhausted by the exhaust opening located at the center of the ceiling and can’t spread in the upper part of the room. In further work, the experiment will be carried out with location of the exhaust opening at corner of the ceiling.

Calculated results don’t coincide with the experimental ones so much without additional heat source. But the Calculated results with additional heat source relatively coincide with experimental ones. This is because the thermal stratification becomes strong due to the thermal plume from the additional heat source. It can be said such a vertical profile of temperature is need for the displacement ventilation. It is considered that in practical usage thermal stratification can be made in displacement ventilated sickroom with heat sources other than human (TV, table lump or refrigerator etc…). It is, therefore, interface layer model is right for predicting the contaminant concentration in the sickroom with lying person ventilated by displacement. In this paper, the temperatures and contaminant concentrations in displacement ventilated room are measured and compared with calculated results by the interface layer model. As a result it is indicated that the contaminant concentration can be predicted with using interface layer model. In further work, the temperature profile in the room will be tried to predict. REFERENCES 1. David Etheridge and Mats Sandberg: BUILDING VENTILATION-Theory and

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