Session: K8-03: FUNDAMENTALS OF MULTI-PHYSICS TRANSPORT AND MACHINE LEARNING

Paper Number: 121457

121457 - Experimental Investigation Into Combined Fluctuating Airflow and Mist Evaporative Cooling 

Abstract: 

Outdoor environments are characterized by spatial and temporal gradients in temperature, humidity, wind speed, and radiation. Under such conditions, the relation between the thermal environment and thermal comfort (TC) can be explained using the thermal alliesthesia framework [1]. The body constantly makes thermoregulatory adaptations to maintain optimal core temperature; environmental stimuli that assist in maintaining optimal conditions are perceived pleasurable, while those that add to the thermoregulatory load are perceived unpleasant. Sensations tend to be most intense during the initial moments of a change in environmental conditions, resulting in an overshooting TC response. This “temporal alliesthesia” effect arises from the heightened discharge rate of thermoreceptors during transient conditions, which is 5 to 10 times stronger compared to steady-state conditions [2]. A promising area of research is the possibility of artificially stimulating this overshooting TC in the outdoors in a cyclic manner. Under hot outdoor conditions, this can be accomplished by exposing the body to fluctuating cool airflow, where air cooling is accomplished sustainably by mist evaporation. When this airflow contacts the body, the skin experiences recurrent, abrupt temperature drops due to low air temperature and amplified convective/evaporative skin heat loss at each flow cycle. By precisely adjusting the temperature of the airflow through controlled misting and by manipulating the fluctuation frequency of the flow, it is possible to match the physiological causes of pleasant sensations.

Applying such novel practice has the potential to reduce both the water consumption and the electricity demand required to establish comfortable outdoor thermal conditions by leveraging the understanding of human thermal perception dynamics. To accomplish this goal, two primary paths of scientific investigation are necessary. The first involves exploring the relation between human thermal perception and cyclic variations in skin temperature, and the second involves studying the fundamentals of fluctuating flows with mist cooling. Heat and mass transfer processes for constant airflow with misting have been extensively studied in literature using wind tunnel experiments [3] and computational fluid dynamics [4]. Nevertheless, previous studies have not explored the combination of fluctuating airflows and mist evaporation, which may necessitate precise synchronization to achieve a flow with desired thermal and dynamic characteristics.

This study aims to develop the basis of studying fluctuating airflows combined with mist evaporative cooling by conducting wind tunnel experiments. These experiments entail subjecting a fluctuating airflow to a spray of water droplets generated from a commercial hollow-cone nozzle configured in parallel with the airflow. The primary focus is on comprehending the heat and mass transfer processes between water droplets and the airflow. The exit cross-sectional plane of the wind tunnel is divided into uniform segments for conducting transient measurements of dry bulb temperature and relative humidity. Additionally, air velocity in the wind tunnel is measured continuously at appropriate sampling intervals. The complex two-phase flow is subject to multiple influencing factors, including the characteristics of the pattern of the airflow, its temperature and humidity, the size distribution of droplets and their initial velocity and temperature. Experiments are thus conducted under different air temperature, air humidity, airflow fluctuation pattern, and misting settings. The goal is to attain a comprehensive understanding on the effects of these parameters on the characteristics of airflow so that this understanding is used in later developments for optimizing human TC. Additionally, these experiments serve the purpose of validating numerical models for analogous flows in future studies.

[1] Parkinson, Thomas, and Richard De Dear. "Thermal pleasure in built environments: physiology of alliesthesia." Building Research & Information 43.3 (2015): 288-301. 

[2] De Dear, Richard. "Revisiting an old hypothesis of human thermal perception: alliesthesia." Building Research & Information 39.2 (2011): 108-117.

[3] Sureshkumar, R., S. R. Kale, and P. L. Dhar. "Heat and mass transfer processes between a water spray and ambient air–I. Experimental data." Applied Thermal Engineering 28.5-6 (2008): 349-360.

[4] Montazeri, H., B. Blocken, and J. Hensen. "CFD analysis of the impact of physical parameters on evaporative cooling by a mist spray system." Applied Thermal Engineering 75 (2015): 608-622.

Presenting Author: Jaafar Younes American University of Beirut

Presenting Author Biography: Jaafar Younes holds a Bachelor of Engineering in Mechanical Engineering, complemented by a Master of Science in Energy Studies. Currently, he is pursuing his Ph.D. in Mechanical Engineering, specializing in thermal and fluid track. His research centers on urban-scale challenges and mitigation measures, bioheat modeling, thermal comfort, and thermal sensation, with a particular emphasis on addressing the unique needs of the elderly population.

Authors:

Jaafar Younes American University of Beirut
Nesreen Ghaddar American University of Beirut
Kamel Ghali American University of Beirut