Development of an experimental set-up to investigate heat transport in convective air flows with phase transition

The heat transfer in mixed convective air flows with phase transition is a phenomenon which occurs in nature and a plethora of technical applications. When condensing materials form droplets, condensation leads to an increased heat transfer rate and therefore is advantageous in heat exchangers \cite{Schmidt-1930}, whereas in other technical applications condensation is oftentimes undesirable. For instance, fogging on the windshield or headlights of motor vehicles leads to restrictions when driving and a reduction of optical transparency influences road safety. In addition, a considerable amount of thermal energy is required for defogging, which can have a negative effect on the range of electric cars \cite{WesthoffFAT}. The design and modification of these components with regard to optimizing dehumidification or preventing misting is often based on experience and intuition. Despite the enormous progress of numerical methods in recent years, there is still a lack of reliable and applicable models for the numerical simulation of condensation in general, and in particular of droplet condensation on surfaces. Due to the complexity of the physical processes that determine the heat transport and thus the fogging on the panes, reliable simulations are expensive and time-consuming, and therefore not suitable for industrial applications. Thus, the objective of the present study is to develop a model, using dimensionless numbers, which allows the prediction of the condensation behavior on panes based on a reduced parameter space. In such a configuration the mass transport of the vapor by phase transition, the resulting latent and the sensible heat transfer are determined by the physical processes of convection and diffusion, the boundary conditions and the material properties of the surfaces. In addition, the mass transfer of water vapor changes even with the smallest changes in the boundary conditions. A major challenge of this study is therefore to design and construct an experimental setup with the appropriate measurement technology, which meets the requirements of ensuring defined boundary conditions, reproducibility of the experiments and measurement accuracy. The setup corresponds to a generic replica of a vehicle headlamp

Calculation of the three-point contact angle for water droplets on a surface from a rearward droplet observation

Evaluation method to determine the three-phase contact angle from microscope images using a ray-tracing algorithm

Reynolds number and humidity dependency of dropwise condensation in moist convective air flows

In moist air flows with phase transition is the convective flow strongly influenced by the sensible and the latent heat transfer. Additionally, the droplet shape (contact angle), the surface properties and the spatial distribution of the droplets also have a strong influence. Hence, empirical models or numerical calculations often fail to predict the heat and mass transfer due to the large number of parameters and the mutual interplay of the different heat transport mechanisms. To obtain a reliable prediction of the mass and the heat transfer, time-consuming and cost-intensive experiments or computationally expensive numerical simulations are necessary. Neither is feasible in the development and design process for industrial applications. It would be too costly and time consuming. Therefore, the method of predicting the droplet size distribution and the corresponding heat transfer by means of a scalar model is of vital interest. To overcome this issue we have been developing a prediction model based on the scaling of system characteristic numbers. Part of this approach is to investigate the effect of large-scale flow structures on the dynamics of the droplet size distribution and the corresponding sensible heat transfer and the condensation mass transfe

Measurement of the turbulent heat fluxes in mixed convection using combined stereoscopic PIV and PIT

The results of simultaneous measurements of velocity and temperature fields in a turbulent mixed convection airflow are analyzed and discussed. To access local temperature and velocity fields in airflows, we present a combination of stereoscopic particle image velocimetry and particle image thermometry. The obtained flow fields make it possible to determine the local convective heat fluxes, thus givinginsight into the dynamics of plumes and Taylor-Görtler-like vortices. The evaluated mean local heat fluxes further reveal that the main convection roll transports a substantial amount of heat alongthe coolingplate and back to the heated bottomplate. Yet, the associated mean turbulent heat fluxes remain positive as they are dominated by the correlation of the temperature and the vertical velocity component. More specifically, a statistical analysis of the local heat fluxdistribution reveals that Taylor-Görtler-like vortices lead to more skewed distributions of the turbulent convective heat fluxes than plumes

A semi-empirical model for the prediction of heat and mass transfer of humid air in a vented cavity

A semi-empirical model to predict the mass transfer rate of water from humid air in mixed convection together with the global heat transfer in a novel experimental set-up is presented. The cuboidal sample consists of isothermally cooled and heated plates with ventilation channels driving a mixed convective flow with inlet channel Reynolds numbers between 210 and 1270, Grashof numbers up to 8.46 , and with relative humidities from 29% to 83% (at 25 C). The volumetric velocity field was measured by means of tomographic particle image velocimetry together with the fluid temperature and humidity. The measurement results are used to develop a one-dimensional model to predict the global heat and mass transfer by quantifying the dependency of the Nusselt and Sherwood number on the experimental boundary conditions. A relative deviation between the measurement results and the model prediction below 1% for the sensible heat transfer is reported, while the prediction of the vapor-mass transfer rate exhibits an average relative deviation below 6%

Scaling of heat and mass transfer in turbulent convection in moist air-flow

Modeling mass transfer-rates on cooled surfaces from humid air is important to determine whether fogging occurs on technical surfaces like automotive windshields, headlights, or inside airplane fuselage. Industrial grade thermal design tools for i.e. automotive headlights do not reliably predict the occurrence of condensation or the duration of defogging in the early design stage. In order to improve the prediction of the latent heat transfer and to quantify the influence of changing boundary conditions, experimental investigations in a mixed convective air-flow with phase transition have been performed

Generation, Detection and Analysis of Aerosol Spreading Using low-cost Sensors

In the context of the Corona pandemic the investigation of aerosol spreading is utmost important as the virus is transported within the aerosol exhaled by an infected person. Addressing this, an aerosol-exhaling thermal manikin, a sensor system allowing for spatially resolved measurements of the aerosol concentration and its application within an aircraft cabin is presente

Automated measurement of the number and growth of water droplets in mixed convection

Condensation of humid air at subcooled surfaces occurs in various technical applications i.e. windows, camera lenses, windshields or headlights. Misting of these surfaces is unwanted or even poses a safety risk. Providing a better understanding of the associated mass transfer during evaporation or condensation is the requirement for solving such problems and is the aim of our work. Considering the latter, the vast majority of publications investigate condensation of steam with small fractions of non-condensable gas, while the aforementioned applications are exposed to low fractions of water vapor. The mass transfer in channel flow has been reported by i.e. Zheng et al. [1] or Westhoff et al. [2] but is rarely investigated in experimental studies with more complex flow structures

Evaluation of three measurement techniques for water-vapor mass transfer in case of droplet condensation

Mass transfer in moist air flows with droplet condensation is governed by the mutual interaction of convective and latent heat transfer. To characterize the physical mechanisms of such flows, it is necessary to measure the condensation rate precisely. For this purpose, we applied three sophisticated measurement techniques to determine the mass transfer for droplet condensation on a cooled surface. The experimental set-up consists of a rectangular box with a mixed convective airflow. Droplet condensation occurs on a subcooled panel, which has a polymer surface with a droplet contact-angle of 80.07(28) degrees. Time series of the total water mass on the cooled surface, the mass transfer-rate, and total heat transfer are measured in a Reynolds number range from 300 to 900 and relative humidities between 29% and 83% (at 25°C air temperature). The considered measurement methods are: strain gauges for total water mass, self-calibrated capacitive humidity probes for mass difference between the inlet and outlet of the sample, and microscopy for the surface droplet-distribution. By means of these measurement techniques a total mass-transfer uncertainty lower than 0.01% of the maximum value is obtained with an uncertainty of the mass-transfer rate below 0.01 mg/s (1% of maximum value). At the conference we would like to present the evaluation and comparison of these measurement techniques, lead a discussion on their respective advantages and disadvantages and present results to verify the scaling of latent and convective heat transfer in mixed convective air flows with droplet condensation

Analysis of Aerosol Spreading in a German Inter City Express (ICE) Train Carriage

This paper focuses on the propagation of aerosols in a rolling stock passenger compartment. Extensive measurements were carried out in our stationary test vehicle DIRK, an ICE 2 rail car, operated in a climate chamber. It is shown that the propagation of aerosols only occurs for a distance of a few seat rows. Furthermore, the maximum percentage of particles exhaled by a passenger and inhaled by another passenger is less than 0.35%. A mouth-nose-protection (surgical mask) at the aerosol source reduces this value to a maximum of 0.25%. Moreover, the use of a mouth-nose-cover reduced the propagation lengths. Here, only the effect of the mask at the source was considered, a further reduction of inhaled aerosols will be achieved when the receivers also wear masks. It is concluded that, for this type of passenger coach, the indirect propagation of aerosols, i.e., via the HVAC system, is nearly irrelevant compared to the direct propagation from one passenger to another. However, there is a non-zero aerosol transport via the HVAC system, resulting in local inhaled particles far away from the source of around 0.015–0.026%, which is more than one order of magnitude lower than on the most highly contaminated seats