Healthy Buildings 2017 Europe July 2-5, 2017, Lublin, Poland

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1 Healthy Buildings 2017 Europe July 2-5, 2017, Lublin, Poland Paper ID 0252 ISBN: The effect of spray cooling in long and narrow space for two jet sources with intensive turbulence at 700 K Yuliang Xie 1, Xu Zhang 1, *, Wei Ye 1 1 Tongji University, Shanghai, China * Corresponding zhangxu-hvac@tongji.edu.cn SUMMARY In this study, a spray cooling rig was designed for a reduced-scale experimental cabin that represents a long and narrow space for special industrial purposes. Two jet sources were introduced to the cabin with intensive turbulence and the highest temperature was approximately controlled at 700 K. A 4 4 TF6 nozzle matrix was installed near the cabin ceiling, along with another 4 4 AM4 nozzle matrix for cooling. We tested the spray cooling effect of the nozzles in different locations in the test chamber, and gave some experimental data. Furthermore, the effects of the distance between heat source and nozzles were analyzed and optimized arrangements for cooling were proposed for cooling down the intensive heat using the minimum amount of water and background ventilation. KEYWORDS Water spray; high temperature; water droplets; evaporative cooling; experiment 1 INTRODUCTION The characteristics that energy saving and environmental protection make the spray cooling has been widely used and studied in recent years. Water spray evaporation has been used in passive cooling (Belarbi et al. 2006) and fire control in tunnel (Grant et al. 2000, Ingason, 2008, Nmira et al. 2009). The earliest experimental data with high accuracy have been obtained (Kachhwaha et al. 1998). Kachhwaha introduced a hollow cone-shaped water spray to the wind tunnel with air flow to downstream and counter-current configuration. They measured dry bulb temperature (DBT) and humidity changes between the inlet and outlet planes. It can be seen that Spray cooling is often used to mitigating air temperature in confined spaces with internal heat sources, such as long tunnels and industrial consecutive heating/cooling processes. Local or whole-space ventilation is also commonly applied as an auxiliary solution to mix internal air and prevent extremely high temperature around heat sources. However, the combined effects of background ventilation and cooling is uncertain for that the evaporative process of the droplets can be interfered once the mass of the droplets were small enough to follow the airflow, especially when the internal heat source was in massive turbulence. This paper introduced spray cooling experiment for high temperature intensive disturbance heat source. 2 EXPERIMENTAL STUDIES The long and narrow space experimental cabin is shown in Fig. 1. It uses 3KW inverter motor driven centrifugal fan, exhaust fan set in the end of the system to ensure the stability of the inlet airflow. After the inlet section, the air enters the spray test section. After the inlet section, the

2 air enters the test section with the spray, and the heat source injects the high temperature gas from the bottom up, so the nozzle spray direction is from top to bottom to achieve a better cooling effect. The high temperature gas is produced by a diesel burner and fed into the test chamber from both sides of the pipeline. After the test section is a measuring section with a length of 1.2 m, we find that there is an approximately uniform velocity distribution in the test Figure 1. Experimental cabin and water system. section. Test section cross-sectional size is mm and 3m long. Using the thickness of 2mm steel plate as the wall of the wind tunnel, the outer layer of 50mm thick rock wool insulation to ensure that the tunnel during the experiment and the surrounding environment is almost insulated. This is quite different from the experiment of Sureshkumar et al. (2008), which validates the cooling effect of parallel and counter current spray structures under dry and hot conditions. Water system shown in Fig. 1, the water system consists of a 1.5m3 (1m 1m 1.5m) water tank, a centrifugal pump, valves and pressure gauge. The water tank interface includes top replenishment, backwater and water supply in the bottom. During the experiment, the water is pumped into the nozzle from the tank and the additional water is returned to the tank through the bypass line and the nozzle pressure is adjusted by adjusting the opening of the return valve. After the spray, the non-evaporated water is collected by the tank and then discharged to the sewer. The water system experiment is intermittent and is suspended for 30 minutes after 5 minutes of operation. Compared with the early experimental system (Kachhwaha et al. 1998), the system adds T-type thermocouples to the tank to closely monitor the tank temperature changes and suspend the test when the water temperature changes by more than 3 C. The details of the instrument and data acquisition system are shown in Fig. 2. The temperature and humidity meter is placed upstream of the nozzle to measure the inlet air DBT and WBT. The tail exhaust section is equipped with a digital pressure gauge to measure the exhaust speed. In the spray section, 27 thermocouples were placed on the three sections of the spray section to

3 measure the DBT, with 3 x 3 total of 9 thermocouples per cross section. The water temperature and pressure upstream of the nozzle are measured by thermocouple and pressure gauge, respectively. All thermocouples are fully accessible to the Fluke Calibration 2638A HYDRA Series III Data Acquisition Unit. Fluke records the data for a period of 4s, and a set of experiments to achieve steady state usually takes 30 minutes. All measurements are recorded to the computer at the same time. Figure 2. Instrumentation and data acquisition system The nozzle type used in this experiment includes the pressure mechanical atomizing nozzle TF6 and the impingement atomizing nozzle AM4. The shape and structure of the two nozzles are shown in Fig. 3 (a) and (b), respectively. The maximum diameter and flow of the two nozzles are 2.38 and 2.31 mm, 7.0 and 7.6 l / min (0.5 MPa), respectively. The particle size distribution of the two nozzles is shown in Fig. 4, and the droplet size of the AM4 nozzle is smaller than that of TF6. Figure 3. (a) TF6 nozzle shape and structure (b) AM4 nozzle shape and structure The experiment was conducted in February 2017 with a total of eight operating conditions, including two types of nozzles (TF6 and AM4) and four spray locations (rows 1, 2, 3, 4). First set the nozzle to be opened and adjust the pressure, each experiment should be running for a long time to ensure that the air flow is stable and the exhaust port temperature to meet the requirements (440 ), then open the spray for 5minutes and record all parameters.

4 Subsequently, the pressure is reset and the process is repeated for other locations and different types of nozzles. Figure 4. Typical droplet diameter distribution of AM4 and TF6. Water pressure 0.5 MPa. 3 RESULTS The complete experimental data of the nozzle back pressure of 0.5MPa is limited by the length of the article cannot be placed in the article. In the following table, the experimental data of the fourth row of TF6 and AM4 nozzles are placed before and after. The first two columns of Table 1 give the inlet air DBT and WBT. The next two columns give the temperature of the water, that is, the temperature of nozzle inlet and outlet after cooling. In the next two columns, the average DBT and WBT of the air at the end of the test section are given. Table 1. The fourth row of experimental data (a) Inlet air Water Outlet air DBT WBT Tin Tout DBT WBT (a) Row4 TF6 Before After (b) Row4 AM4 Before After In 9 columns of Table 2 give the three cross-section temperature distributions of the test section and the distances to the inlet are 0.67 m, 1.17 m, 1.67 m respectively. Each section has nine DBT temperature measuring points, and there are 27 points in total. The first row of data is 3 sections (section 1, section 2, section 3), below each section, first by row (left, middle, right), and then for each row by position (top, middle and bottom), Each condition contains two types of data before and after. The following discussion of significant features. Fig. 5 (a) shows the change of DBT and Fig. 5 (b) shows the difference about moisture content (d) and the relative humidity (φ) in the test section before and after. There are some obvious trends in both graphs. First, because the average particle size of AM4 smaller than TF6, Figure 5 (a) can be seen AM4 cooling effect is better than TF6 in the first three rows. However, The fourth row of nozzles below the high temperature high-speed exhaust, AM4 droplets in the

5 small particle size, so the impact of high temperature and high temperature air flow than TF6, cooling effect poor than TF6. At the same time as the spray near the high temperature airflow, AM4 s DBT changes gradually decreased and TF6 s Table 2. The fourth row of experimental data (b) Section 1 (0.67m) Section 2 (1.17m) Section 3 (1.67m) Left Center Right Left Center Right Left Center Right (a) Row4 TF6 Before After (b) Row4 AM4 Before After Top Middle Bottom Top Middle Bottom Top Middle Bottom Top Middle Bottom gradually increased. It can be seen from Fig. 5 (b) that the relative humidity of AM4 is higher than that of TF6, which shows the advantage of small particle size droplets in evaporation efficiency, but with the increase of the disturbance intensity TF6 relative humidity also showed an upward trend, the first three rows of moisture content MA4 high and the fourth row of TF6 high, that is, under the strong disturbance of large diameter droplets cooling performance better. (a) DBT change (b) Specific humidity and relative humidity change Figure 5 Experimental data of different types and locations. 4 DISCUSSION The biggest source of error in this experiment may be that the burner may be shut down during the course of the work because of the long burning time. In addition, since some of the thermocouples are placed under the nozzle, the temperature is likely to be affected by the droplet during the spray, which is more pronounced for the thermocouple placed at the bottom.

6 5 CONCLUSIONS According to the current work, some experimental data of spray cooling under high temperature and intensive disturbance heat source are given. For a certain amount of spray pressure and disturbance intensity, droplets of small particle size can produce more cooling, but as the spray approaches the strong heat source, the droplet cooling capacity of the small particle size is gradually reduced, and the large particle size droplet cooling capacity gradually increased. The average temperature of the test section can be reduced by 8 C at 0.5 MPa spray pressure and the first row of AM4 nozzle. 6 REFERENCES Belarbi, R., Ghiaus, C., & Allard, F Modeling of water spray evaporation: Application to passive cooling of buildings. Solar Energy, 80(12), Grant, G., Brenton, J., & Drysdale, D Fire suppression by water sprays. Progress in Energy and Combustion Science, 26(2), Ingason, H. (2008). Model scale tunnel tests with water spray. Fire Safety Journal, 43(7), Nmira, F., Consalvi, J. L., Kaiss, A., Fernandez-Pello, A. C., & Porterie, B. (2009). A numerical study of water mist mitigation of tunnel fires. Fire Safety Journal, 44(2), Kachhwaha, S. S., Dhar, P. L., & Kale, S. R Experimental studies and numerical simulation of evaporative cooling of air with a water spray Ⅰ. horizontal parallel flow. International Journal of Heat and Mass Transfer,41(2), Kachhwaha, S. S., Dhar, P. L., & Kale, S. R Experimental studies and numerical simulation of evaporative cooling of air with a water spray II. Horizontal counter flow. International Journal of Heat and Mass Transfer, 41(2), Sureshkumar, R., Kale, S. R., & Dhar, P. L Heat and mass transfer processes between a water spray and ambient air i. experimental data. Applied Thermal Engineering,28(5 6),