CHAPTER 5 STUDY OF THERMAL COMFORT IN A ROOM WITH INSECT PROOF SCREEN

Transcription

1 146 CHAPTER 5 STUDY OF THERMAL COMFORT IN A ROOM WITH INSECT PROOF SCREEN 5.1 INTRODUCTION In recent days, most of the buildings are equipped with insect proof screens to keep the insect not to enter inside the building space to avoid disease and to make disturbance free environment. This insect proof screen is very essential for the buildings especially at hill stations, forests and cities, which are highly disturbed by mosquitoes and flies. Mosquitoes and highly poisonous bugs cause disease like malaria, filaria, dengue, brain fewer, yellow fewer and etc. Hence, it is necessary to provide an insect proof screen behind the window openings and on any other openings of the buildings. Insect proof screens avoid night time heat loss and controls the solar radiation in day time, and as well as prevents the entry of insects (Miguel et al 1997). But insect proof screen reduces 50% of the air inflow rate and simultaneously increases the temperature of a green house (Bartsanas et al 2002). Meanwhile, the research work on residential building with insect proof screen was very rare and hence, this chapter is focused to study the effect of insect proof screen on indoor air flow characteristics and thermal comfort of a room with window openings at their adjacent walls. 5.2 STUDY OF AIR FLOW CHARACTERISTICS IN A SPACE WITH INSECT PROOF SCREEN- REVIEW Some of the research studies that include the investigation on insect proof screen are discussed as follows. Miguel et al (1999) studied the air flow

2 147 through the porous screen in a green house by means of physical model. Valera et al (2006) developed the mathematical correlation and found the pressure drop as the function of air flow velocity across the porous material. Liu Shu-zhen et al (2005) studied the effect of wind velocity and window opening height on indoor air temperature of a green house with insect proof screen. From the literature survey, it is identified that most of the research studies includes the insect proof screen in the analysis of green house with agricultural crops (Brundrett, 1993, Kosmos et al1993, Fatnassi et a , Katsoulas et al 2006). Figure 5.1 shows the effect of insect proof screen on the indoor air velocity and temperature inside a green house. Figure 5.1 Variation of air velocity and temperature for green house with and without insect proof screen (Bartzanas et al 2002)

3 148 In general, air flow through the insect proof screen can be analyzed by theoretical approach, experimental testing and numerical analysis. The theoretical method includes Bernoulli s approach (Mumoz et al 1993) and Forchheimer equation (Bailey et al 2003). But the theoretical approaches lag to predict the air flow pattern, temperature profile inside the room. In the experimental testing, wind tunnel setups with small-scale models are usually preferable, as they control the wind speed and direction. But in the experimental analysis, it is very difficult to isolate the wind force from the buoyancy force during measurement (Guohui Gan 2003). Moreover, it is essential to measure the turbulent intensity precisely which is responsible for the diffusion process of the particles, but such accurate measurements are difficult to obtain in experimental testing (Oliver Rouaud and Michel Havet 2002). Majority of residential buildings rooms are analyzed without insect proof screen. In this context, present study employs the CFD technique to investigate the air flow inside the room with windows opening at their adjacent walls along with the insect proof screen. Present study examines the effect of window opening area with insect proof screen and porosity of insect proof screen on room s thermal comfort. The window opening area is nondimensionalized with the exposed wall surface area so as to increase the generalizability of results from this analysis. 5.3 FLOW THROUGH INSECT PROOF SCREEN Flow through the porous media is attracted considerable interest in recent years because of its importance in fluidized bed combustion, enhanced oil reservoir recovery, underground spreading of chemical waste, enhanced natural gas combustion in an inert porous matrix and chemical catalytic reactors. The basic conservation equations describing the flow of a fluid through an infinitesimal volume can be written in a compact form as given in Equation (5.1) (Marcelo de Lemos 2006).

4 149 t x j u j x j S (5.1) where is the general variable (not to confuse with the porosity to be introduced later), u j is the j th velocity component, is the density, and and S are the diffusion coefficient and source terms, respectively. The value of and its corresponding parameters ( and S take different forms according to the conserved quantity (mass, momentum, energy, chemical species, turbulent kinetic energy, etc.). The Equation (5.1) is rewritten in Cartesian coordinates form for three dimensions and in the case of mass conservation, = 1, = S = 0. t u x x v y y w z z 0 (5.2) When the equation 5.1 is written for the flow through porous media, the medium of porosity is to be accounted. Therefore, the continuity Equation (5.2) can be modified and expressed in vector form as in Equation (5.3). t u D (5.3) where is the medium porosity defined as the non dimensional ratio of pore volume to total (fluid plus solid) volume, is the density based on total volume and u D is the superficial velocity defined as the volumetric flow rate divided by unit of total cross-sectional area. The well-known Darcy law of motion is given in Equation (5.4). K u D ( p g) (5.4)

5 150 where p is the pressure based on total area. The quantity K is referred to permeability of the medium, the unit of which is Darcy, defined as the permeability of a porous medium to viscous flow (for the flow of one ml of a liquid of one centipoise viscosity under a pressure gradient of one atm/cm across one cm 2 in 1 sec). Darcy s law made a relation for the pressure loss for the flow through the porous medium. p (5.5) K where p is the pressure loss, is the fluid viscosity, and is the fluid velocity. This Equation (5.5) is suitable for the flow having Reynolds number lesser than unity. A modified equation was further developed by Forchheimer by adding an extra squared fluid velocity term with the Darcy s equation for the Reynolds et al 2006). number more than one as given in Equation (5.6) (Valera Y 2 p (5.6) K K where Y is the dimensionless inertial factor and it is the function porosity ( ). 5.4 STUDY OF INDOOR AIR FLOW CHARACTERISTICS AND THERMAL COMFORT FOR A ROOM WITH INSECT PROOF SCREEN The importance of analyzing the room with insect proof screen was discussed in the previous section 5.3. However, in this section a detailed study on indoor airflow characteristics and thermal comfort of a room with insect

6 151 proof screen is discussed with the results obtained from the CFD simulation. The test case room with insect proof screen attached to the window opening is modeled in the Gambit software as shown in Figure 5.2. The test case room is of size (5m x 5m x 4m) having the window openings at their adjacent walls along with insect proof screen. In the previous chapter 2 and 3 the room interior height is taken as 4.8 m and from that chapter the effective travel of height is identified as 3.8m, since the indoor air temperature at above the height of 3.8m increases drastically. Hence in this chapter the test room interior height is considered as 3.8 and this height will be taken in forth coming chapters. Figure 5.2 Test case room model attached with insect proof screen CFD Methodology The three dimensional model of the room with window openings along with insect proof screen is created in GAMBIT software and placed inside a atmospheric box of size 30m x 30m x 20m (W x L x H). The model was meshed with the structured tetrahedral T gird element with the grid size of 0.5. The boundary conditions and the material properties of the building

7 152 materials are same as discussed in the section Additionally, the insect proof screen is simulated as a porous medium. The dimensionless inertial factor, Y and Permeability, K for a porous medium are calculated from the following Equations (5.7) and (5.8) respectively (Miguel et al 1997) and specified as an additional boundary conditions at the window openings Y (5.7) K (5.8) 3-dimensional double precision segregated solver is used to solve the flow domain. Segregated solver solves the continuity, momentum and energy equations sequentially. Standard k- turbulence model is employed and second order upwind method is specified. All the cases are iterated up to the convergence level of VALIDATION OF CFD SIMULATION FOR THE FLOW THROUGH INSECT PROOF SCREEN For the validation of air flow simulation through the insect proof screen, a comparison is made between the simulated pressure drops across the insect proof screen with experimental result carried out by Valera et al (2006). Initially, a two dimensional wind tunnel model with insect proof screen of porosity value Is created as same in the experimental test (Valera et al 2006).The velocity and the static pressure of air flow through the insect proof screen are predicted from the CFD analysis. The velocity of air entering the wind tunnel inlet is increased from 0.1 m/s to 8 m/s. Around 12 cases are analysed under different inlet air velocity and the air velocity is measured at a

8 153 distance of 90mm before insect proof screen. The static pressure is measured at a distance of 45mm before and after the insect proof screen. From the predicted value of static pressure, the pressure drop due to insect proof screen is calculated and a comparison is made with the experimental pressure drop given by Valera et al (2006) as shown in Figure 5.3. Figure 5.3 Comparison of pressure drop across the insect proof screen between CFD simulation and experimental values (Valera et al 2006) In Figure 5.3, the maximum error is found as 8.77% for the inlet air velocity of 0.15 m/s. Such a comparison is agreeable and similar pattern of trend for pressure drop across the insect proof screen is noticed for other porosity values. 5.6 EFFECT OF INSECT PROOF SCREEN ON INDOOR AIR CONDITION As expected, insect proof screen attached to the room window significantly reduces the air flow and increases the inside temperature of the room. Figure 5.4 presents the velocity vector plot for the two cases (without and with insect proof screen) in a horizontal plane, parallel to the floor at the

9 154 height of 2m from the floor. For the given reference wind speed of 1m/s, air enters in to the room through the window opening, with an average velocity of m/s for the room without insect proof screen and m/s for the room with insect proof screen. The insect proof screen reduces the air inflow velocity by 75%. Insect proof screen causes recirculation of air inside the room to a lower velocity but is having better mixing of air in the internal space. Even though there is an undesirable recirculation of air inside the room which helps to mix the air in the internal space, the insect proof screen intern creates the local heating inside the room because of poor exit of indoor air. Figure 5.4 Velocity vector (m/s) plot at the plane XZ and at a height of 2m from ground (a) without insect proof screen and (b) with insect proof screen Figure 5.5 shows the temperature distribution in the horizontal plane parallel to the floor at a height of 2m from the datum. A temperature rise has been observed from the two cases especially at the regions for the smaller value of the velocity magnitude of the internal air. Major portions of the room without insect proof screen is influenced by the temperature value of K, whereas, the same room with insect proof screen the temperature is raised to 310 K. Providing the room with insect proof screen raises the temperature by 3 due to insufficient removal of stale air and low mass flow rate of incoming air. In the room without insect proof screen, the temperature at the right side of the room is low in comparison with the left side. This is

10 155 due to the fact that both the windows are located only at the right side of the room. Figure 5.5 Temperature (K) plot at the plane XZ and at a height of 2m from ground (a) without insect proof screen and (b) with insect proof screen Effect of Window Openings Area with Insect Proof Screen Area on Indoor Air Flow Characteristics In this section, area of window opening with insect proof screen is varied, to investigate its effect on the indoor conditions of air. A window opening area is generalized with the exposed wall surface area and referred as a non dimensional number, a i *. In this study, the value of a i * is varied from 0.05 to 0.45 with an increment of 0.2 and the porosity of insect proof screen, is kept constant as These cases are simulated by the CFD technique and the variations in the indoor temperature and velocity along the mid lines are shown in Figures 5.6 to 5.7 respectively. Figure 5.6 shows the temperature variations along the mid lines by changing the a i * value and keeping value as constant.

11 156 Figure 5.6 Temperature variation along the midlines by varying a i * for a constant =0.35 For all a i * values, temperature along X 1 X 2 decreases from 0.5 to 4.5 m. This is due to the fact that both the windows are positioned at the right side of the room and hence the air flow is comparatively dominant on the right side. Temperature along Y 1 Y 2 is almost constant up to a height of 1.75 m from the ground and decreased slightly between 1.75 m and 3m. This decrease in temperature is available only at the height of 1.75m to 3m due to the location of window at the same level. Temperature along Z 1 Z 2 is constant up to a distance of 4m, and beyond that, the temperature shoots. This

12 157 determines the effective travel of air in the z direction is up to 4 m distance. Increasing the value of a i *, size of the window opening increases and allows more volume of air, which in turn decreases the inside air temperature. Similarly, temperature along Y 1 Y 2 and Z 1 Z 2 also decreases by increasing a i * value. Table 5.1 Minimum temperature at various locations along the midlines and the corresponding a i * value Midline X1X2 Midline Z1Z2 Midline Y1Y2 a i * Temperature (K) a i * Temperature (K) a i * Temperature (K) Figure 5.7 shows the variations in air velocity along the mid lines for various values of a i *.

13 158 Figure 5.7 Velocity variation along the midlines by varying ai * for a constant =0.35 The velocity of indoor air increases gradually along X 1 X 2 form X=0.5m to 1.5m, maintains constant value between 1.5m to 3.5m and from 3.5m to 4.5m the velocity increases slightly. This trend depends particularly on the basis of window location. Air enters the room through the wind ward side window and turns immediately by 90 and vents through the leeward side which is located at the adjacent wall. Hence, the velocity of air gradually increases from the right side wall focusing toward the centre of the room and

14 159 maintains a constant value at the centre and consequently increases towards the left side wall. Velocity of air along the midline Y 1 Y 2 increases from the height of 1.5 m to 3m from the ground and above 3m the velocity of air falls down drastically. Similarly, velocity along Z 1 Z 2 falls down drastically up to a distance of 1.75m and beyond Z=1.75m, the velocity increases gradually up to distance of Z=4m. Beyond Z= 4m, the air velocity gets reduced and this shows the effective travel of air is up to distance of 4m in the z direction. Table 5.2 Maximum velocity at various locations along the midlines and the corresponding a i * value Midline X1X2 Midline Z1Z2 Midline Y1Y2 a i * Velocity (m/s) a i * Velocity (m/s) a i * Velocity (m/s) Effect of Insect Proof Screen Porosity on Indoor Air Flow Characteristics In the second study, the insect proof screen porosity value is changed from 0.1 to 0.9 with an increment of 0.2 on both the window openings. Figures 5.8 and 5.9 show the temperature and velocity along the midlines by changing the porosity value of the insect proof screen.

15 160 Figure 5.8 Temperature variation along the midlines by varying constant a i *=0.15 for a Increasing the porosity value of Insect proof screen causes the air to travel with high velocity which in turn lowers temperature of indoor air. The temperature along the midline X1X2, is maximum near X=0.5m and reduces drastically towards X= 4.5m. This trend is common to all the values and for = 0.1, high magnitude of temperature is noticed along the X1X2 midline. This temperature magnitude along the X1X2 midline, gets reduces gradually

16 161 by increasing the value. For the midline Y1Y2, the temperature rises slightly from Y=0.5m to 2.5m and reduces slightly up to height of Y=3m and above 3m a short rise in temperature is noticed. This trend of temperature variation is especially for = 0.1 and for rest of the values, the temperature reduces slightly from Y=0.5m to Y= 3m and above Y=3m a short rise in temperature is noticed. The temperature along Z1Z2 midline is almost constant between Z = 1m to 4m for all the temperature is noticed for values and in this midline also, the maximum =0.1 and reduces significantly by increasing the insect screen porosity,. Table 5.3 Minimum temperature at various locations along the midlines and the corresponding value Midline X1X2 Midline Z1Z2 Midline Y1Y2 Temperature (K) Temperature (K) Temperature (K) The velocity variations along the midlines are shown in Figure 5.9. In the midline X1X2, the velocity rises slightly up to a distance of X=1m and beyond X=1m, the velocity decreases up to distance of X=2.25m and thereafter a significant rise in velocity is identified up to X=4.5m. This trend

17 162 is common to =0.1to 0.5 and for rest of the values the velocity along the midline X1X2 is almost constant. Figure 5.9 Velocity variation along the midlines by varying for a constant a i *=0.15 For the midline Y1Y2, the velocity reduces gradually from Y=0.5m to 2m and drastic rise in velocity is noticed up to height of Y=3m and above 3m a significant fall in velocity up to Y=4m is identified. For the midline Y1Y2 also, the maximum velocity is obtained for =0.9 and this velocity gets

18 163 reduced by decreasing the value. For the midline Z1Z2, the maximum velocity is obtained near Z =0.5 m and this velocity reduces drastically towards Z= 3m and beyond Z= 3m a slight rise in velocity up to Z=4m and sudden short fall after Z=4m are noticed. This trend is common to all values and however the maximum velocity magnitude is obtained for =0.9 and reduces gradually by decreasing insect proof screen porosity, value. Table 5.4 Maximum velocity at various locations along the midlines and the corresponding value Midline X1X2 Midline Z1Z2 Midline Y1Y2 Velocity (m/s) Velocity (m/s) Velocity (m/s) Thermal Comfort Index- PMV Contour for Room with Insect Proof Screen The thermal comfort index- PMV value is predicted for the room with insect proof screen for the above analyzed cases. The values of metabolic rate =70 W/m 2 =, work completed =30W and thermal resistance of clothing = 0.11 (m 2 k)/w and relative humidity =60%. Figure 5.10 shows the PMV contour at the mid plane of the room for different a i * values with a

19 164 constant porosity value of 0.3. For the a i * value of 0.05, the zone nearer to window opening is experienced by the PMV value of 2.56 and Figure 5.10 PMV contour at the mid plane of the room for various a i * values at constant =0.35 The PMV at the center of the room is 2.74 and is increasing towards the non window wall side up to The PMV contour patterns are almost identical for all the a i * values from 0.15 to 0.45, but the magnitude of PMV gets reduced gradually. The PMV value is highest at the left wall and gradually decreasing towards right wall. Also all the room corners are having a high value of PMV. For the tested room model, the right side portion of the room is considered as a comfort zone than left side. By increasing the a i * value, the size of the window open increases and hence the PMV value gets reduced. For the a i * value of 0.15, the right side portions are having the PMV value of 2.71, 2.8 and 2.9 whereas the left side portions are affected by 2.53,

20 and 2.35, the central region has In a i * of 0.45, the right side of the room is having PMV value of 2.41 and 2.47, while the left side is having 2.31 and 2.38 with the central region of PMV Increasing the window opening allows more volume of fresh air in to the room and vents out properly without internal recirculation. Smaller opening creates recirculation and lead to local heating. Figure 5.11 shows the contour plots of PMV for different porosity values, for a constant a i * value of Figure 5.11 PMV contour at the mid plane of the room for various values at constant a i *=0.15 For the porosity value of 0.1, the PMV value is changed drastically from 2.32 to The uniform comfort is not obtained and all the room zones are influenced by hot conditioned air. Insect proof screen of low porosity value allows less volume of air to enter in to the room and hence causes insufficient transfer of heat from the room interior. Further increase in

21 166 value, reduces the PMV value and offers better comfort to the occupants. However, most of the insect proof screens are available with the porosity value in the range of 0.3 to 0.5. For the porosity value of 0.5, the right of the room is having the PMV value as 2.51 and 2.4 and the left side is having 2.59 and Similarly, for the porosity values of 0.7 and 0.9 the PMV contour patterns are identical but their PMV values are lower. For the porosity value of 0.9, centre region of the room is having the PMV value as Insect proof screens restricts the entry of harmful insects, on the other hand reduces the comfort feel of the occupants. Hence, the proper selection of insect proof screen is required on the basis of existing insects size. 5.7 REGRESSION MODEL FOR INDOOR TEMPERATURE OF A ROOM WITH INSECT PROOF SCREEN Finally, in this study a regression analysis is conducted on the predicted indoor temperature of a room with insect proof screen. The indoor temperature is predicted as the average temperature for the XZ plane at every 0.2m from the ground surface up to the roof surface. The indoor temperature is generalized with the atmospheric temperature as T* avg and the regression models for various window opening area with insect proof screen, a i * and porosity of insect proof screen, cases are given in the following section. In the first part of the regression analysis, the indoor temperature for various a i * cases are predicted from the CFD simulation, generalized with the atmospheric temperature as (T* avg ) and plotted in Figure In the Figure 5.12, the T* avg is high near Y=3.4m and reduces significantly at Y=3.2 m and below Y = 3.2 m almost constant value of T* avg is noticed up to the floor. This trend is common to all the a i * values and the magnitude of T* avg is maximum for a i * = 0.05 and reduces gradually while increasing the value of a i * value.

22 167 Figure 5.12 Variation of T* avg at plane XZ for various a i * values From the CFD predicted T* avg value, the regression models for the a i * cases are developed as a function of position of XZ plane from ground surface, Y and are given in Equations (5.9) to (5.13). T * avg = y y y y *= (5.9) T * avg *=0.15 = y y y y (5.10) T * avg *=0.25 = y y y y (5.11)

23 168 T * avg *=0.35 = y y y y (5.12) T * avg = y y y y *= (5.13) From the above developed regression models, the T* avg, value can be predicted and intern the T* avg, is used to calculate the indoor temperature, T in. This T in is used in the modified form PMV Equation (3.3) to predict the PMV value and it's comparison with the Fanger's PMV equation are given in Table 5.5. Table 5.5 Discrepancy of modified form of PMV equation with Fanger's equation for a i * cases a i * Y (m) Average temperature at plane XZ Regression models CFD Fanger's Equation PMV value Modified PMV equation Error % In the second part, the regression analysis on the generalized indoor temperature T* avg for a room with insect proof screen under various porosity values are conducted and the predicted regression models are given in Equations (5.14) to (5.18). For the development of regression models, the variation of T* avg as a function of insect proof screen porosity and position of

24 169 XZ plane from the ground surface are predicted from CFD simulation and plotted Figure 5.13 In this figure, the maximum value of T* avg is noticed near Y =3.4m and immediately a significant fall in T* avg is identified at Y= 3.1m. Below Y=3.1m, the T* avg is almost constant with small ups and downs up to the ground surface. This trend is common to all values and the trend of T* avg variation is higher for = 0.1 and decreases gradually by increasing the porosity of insect proof screen,. Figure 5.13 Variation of T* avg at plane XZ for various values The regression models to predict the T* avg at plane XZ for various porosity, values are given below:

25 170 T * avg = 0.1 = y y y y (5.14) T * avg = 0.3 = y y y y (5.15) T * avg = 0.3 = y y y y (5.16) T * avg = 0.7 = y y y y (5.17) T * avg = 0.9 = y y y y (5.18) From the above developed regression models the T* avg and indoor temperature, T i are predicted and compared with CFD simulated results. For these cases also the closeness of modified form of PMV Equation (3.3) with Fanger's equation is checked and given in Table 5.6. Table 5.6 Discrepancy of modified form of PMV equation with Fanger s equation for cases Average temperature PMV value at plane XZ Error Y (m) Regression Fanger's Modified PMV % CFD models Equation equation

26 171 From the Tables 5.5 and 5.6, the maximum error for modified form of PMV equation with Fanger's equation is noticed as -9.90% and hence it is considered that the modified form of PMV equation is having a reasonable fit. 5.8 CONCLUSION The influence of an insect proof screen on airflow and thermal comfort is numerically investigated for an office room having the window openings at their adjacent walls. The CFD technique has been applied to study the airflow and compared with the experimental result. The simulated result indicates that the insect proof screen has a considerable effect on indoor condition of air. Insect proof screen significantly reduces the air velocity and in turn increases the air temperature. With respect to the outdoor temperature (306 K), maximum temperature amplification is identified as 2.1 for the room without insect proof screen and up to 4.9 for the room with insect proof screen. In this analysis, the effect of window opening with insect proof screen and the porosity of screen on thermal comfort are studied by using Fanger s equation. For all the analyzed cases, center portion and portions nearer to window openings sides are identified as comfort zone whereas room corners, non window wall sides are comparatively at less comfort. These results concern particularly to the windows at their adjacent walls. Finally, the regression analysis is conducted for the room with insect proof screen under various window opening area and porosity of insect proof screen. These regression equations are use full to predict the indoor temperature of a room with insect proof screen without conducting the CFD technique or experimental testing. Also the closeness of developed modified equation for PMV with Fanger's equation is made and founds that the modified equation of PMV is having good fit with Fanger's equation.