scholarly journals Effect of relative humidity on heat transfer across the surface of an evaporating water droplet in air flow

2009 ◽  
Vol 624 ◽  
pp. 57-67 ◽  
Author(s):  
RYOICHI KUROSE ◽  
AKITOSHI FUJITA ◽  
SATORU KOMORI
Keyword(s):  
Heat Transfer ◽  
Relative Humidity ◽  
Heat Loss ◽  
Water Droplet ◽  
Rear Surface ◽  
Ambient Air ◽  
Droplet Surface ◽  
Droplet Temperature ◽  
Humidity Condition ◽  
Evaporation Heat

A three-dimensional direct numerical simulation (DNS) is applied to flows inside and outside an evaporating spherical water droplet in air, and the effect of relative humidity on the heat transfer between the droplet and ambient air is investigated. The initial air temperature is set to be 15 K higher than the initial droplet temperature. The results show that the local evaporation heat loss indicates the maximum on the front of the droplet and decreases on going from the front to the rear, and the reduction on the rear becomes marked for high-droplet Reynolds numbers. This is because the evaporation rate is suppressed on the rear surface by the presence of flow separations behind the droplet. The droplet temperature decreases in the low-humidity condition, whereas it increases in the high-humidity condition. This difference is caused by the heat balance of evaporation heat loss and convective heat gain from ambient air at the droplet surface.

scholarly journals Kinetic and thermal simulation of water droplets in icing wind tunnels

2021 ◽  
Author(s):  
A. Fallast ◽  
A. R. Rapf ◽  
A. Tramposch ◽  
W. Hassler
Keyword(s):  
State Space ◽  
Test Object ◽  
Droplet Diameter ◽  
Ambient Air ◽  
Droplet Surface ◽  
Wind Tunnels ◽  
Droplet Temperature ◽  
Water Droplets ◽  
Test Conditions ◽  
The Impact

AbstractWithin the certification process of aircraft, tests under specific icing conditions are required. For such safety relevant tests—which are performed under defined and repeatable test conditions—specially equipped Icing Wind Tunnels (IWT) are required. In such IWTs, supercooled water droplets are created with the aid of a spray system injecting pre-tempered water droplets of specific diameters into the free stream air flow. Especially tests with a droplet size up to 2mm (Supercooled Large Droplets - SLDs) are of great importance. SLDs are difficult to generate under laboratory conditions in IWT since usually the available droplet flight time from the injection location to the impact position on the test object is insufficient to reliably cool down a droplet at least to freezing temperature. To investigate the limitations associated with the application of SLD, the current work provides a method to allow detailed insight into the behavior of droplets on the path from the injection spray nozzle to the test section. In this work a state space model of a single droplet is derived that combines the kinetic aspects, thermal properties as well as the governing differential equations for motion, convective heat transfer at the droplet surface and heat conduction inside the droplet. Beside the states for the droplet’s position and velocity in space, the state space vector comprises various fluid and thermodynamic parameters. The droplet-internal temperature distribution is modelled by a discrete one-dimensional spherical shell model that also incorporates the aggregate phase (freezing mass fraction) at each shell node. This approach allows, therefore, the simulation of potential droplet phase change processes (freezing/melting) as well. With the model at hand, the influence of various boundary conditions (initial droplet temperature, flow field, ambient air temperature, etc.) can be determined and evaluated. As a result, concrete measures to achieve a desired operating condition (e.g. droplet temperature at the test object) for various model assumptions can be derived. In addition, the simulation model facilitates the prediction of the droplet diameter threshold for ensuring a supercooled state upon the impact on the test object. The governing theoretical influences are described, and various simulation results for representative test conditions that occur at the Rail-Tec-Arsenal (RTA) in Vienna are presented.

scholarly journals Modelling investigation of water droplet evaporative freezing in artificial snowmaking

2019 ◽  
Vol 128 ◽  
pp. 06013
Author(s):  
Georges El Achkar ◽  
Aiqiang Chen ◽  
Rachid Bennacer ◽  
Bin Liu
Keyword(s):  
Heat Transfer ◽  
Numerical Model ◽  
Relative Humidity ◽  
Air Temperature ◽  
Water Droplet ◽  
Theoretical Models ◽  
Comsol Multiphysics ◽  
Initial Diameter ◽  
Air Space ◽  
Mass And Heat Transfer

In this paper, a modelling investigation of water droplet evaporative freezing was conducted in order to better understand the snowmaking process and hence to optimise the design of the artificial snowmaking device. To this end, mass and heat transfer theoretical models of a single water droplet cooling in an air space were established and implemented in a numerical model developed using the software COMSOL Multiphysics. The effects of the air temperature, relative humidity and velocity and the water droplet initial diameter and temperature on this process were identified and analysed, and their appropriate ranges for the snowmaking were determined.

Effects of Ambient Air Relative Humidity and Surface Temperature on Water Droplet Spreading Dynamics

2018 ◽  
Author(s):  
M. Jadidi ◽  
M. A. Farzad ◽  
J. Y. Trepanier ◽  
A. Dolatabadi
Keyword(s):  
Contact Angle ◽  
Relative Humidity ◽  
Surface Temperature ◽  
Air Temperature ◽  
Water Droplet ◽  
Superhydrophobic Surface ◽  
High Speed ◽  
Ambient Air ◽  
Dew Point ◽  
Droplet Spreading

Water droplet impact on horizontal glass, aluminum, and superhydrophobic surfaces is experimentally investigated using high speed imaging. Experiments are performed at three different relative humidities (i.e. 10, 20 and 30%) and three surface temperatures (i.e. 20, 2 and −2°C) to ascertain their effects on droplet spreading and recoil behaviors. In this study, the droplet Weber number, Reynolds number, and the ambient air temperature are fixed at 16.2, 1687, and 23°C, respectively. The high-speed images of impact, spreading and recoil of the droplets as well as the temporal variations of droplet spreads are prepared. Results show that the ratio of surface temperature to dew point temperature (which depends on the air temperature and relative humidity) has a significant influence on droplet spreading, recoil, and contact angle. When this ratio is less than one, condensation and frost formation become important. Decreasing the mentioned ratio (it can be done by decreasing the surface temperature or increasing the relative humidity) causes the droplet spreading factor for hydrophilic surfaces to increase significantly. For superhydrophobic surface, decreasing this ratio (within the mentioned range) does not influence the maximum spreading. However, the recoiling phase is slowed down and the droplet detachment time form the superhydrophobic surface is increased noticeably. In addition, the equilibrium contact angle decreases as the mentioned ratio decreases.

scholarly journals A Comprehensive Mathematical Model for Simulation of Latent and Sensible Heat Losses from Wet-Skin Surface and Fur Layer of a Cow

2010 ◽  
Vol 7 (1) ◽  
pp. 83-96
Author(s):  
Z.A. Khan ◽  
I.A. Badruddin ◽  
M. Mokhtar ◽  
W.M.W. Muahamad
Keyword(s):  
Mathematical Model ◽  
Relative Humidity ◽  
Heat Loss ◽  
Ambient Air ◽  
Skin Surface ◽  
Heat Losses ◽  
Air Velocity ◽  
Hot Environment ◽  
Total Body ◽  
Body Heat

This paper presents a comprehensive mathematical model that calculates heat and mass transfer from the skin surface and fur layer of a cow. The model predicts evaporative, respiratory, convective, radiant and urine heat losses for different levels of skin and fur wetness, air velocity, ambient temperature, relative humidity, breathing rate and volume of urine discharged per unit time. Effect of various environmental conditions on skin temperature and consequently heat loss from cow’s body is discussed. It is found that the heat loss through respiration is significant and can not be ignored. Evaporative cooling and respiratory heat losses are, decreased due to increased relative humidity because of increased saturation of ambient air with water particles. It is also found that very small proportion of total body heat is lost through urine in stressful hot environment.

scholarly journals Experimental study of evaporation of droplets of water ethanol solution at high relative air humidity

2021 ◽  
Vol 2057 (1) ◽  
pp. 012059
Author(s):  
A N Sterlyagov ◽  
M I Nizovtsev ◽  
V N Letushko
Keyword(s):  
Relative Humidity ◽  
Ethyl Alcohol ◽  
High Speed ◽  
Moisture Absorption ◽  
Water Solution ◽  
Ambient Air ◽  
Droplet Surface ◽  
Ethanol Solution ◽  
High Relative Humidity ◽  
Air Humidity

Abstract A change in the temperature of suspended droplets of a water solution of ethanol, evaporating at high relative humidity, is experimentally investigated using high-speed microphotography and infrared thermography. The data obtained for ethyl alcohol droplets show a significant dependence of the surface temperature of evaporating droplets on relative humidity of the ambient air. It should be noted that at high relative humidity, a significantly smaller decrease in the temperature of droplet surface than at low humidity was observed during evaporation. This relates to the processes of moisture absorption and condensation on the surface of an evaporating droplet of ethyl alcohol at high air humidity, accompanied by the release of heat.

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