The role of convection in electrohydrodynamic drying
Introduction
Electrohydrodynamic (EHD) drying requires a strong, non-uniform electric field, in order to initiate corona discharge and ionic wind (Martynenko and Kudra, 2016). Such ionic wind consists of flow of ions and of air molecules. Ion flow is created by unipolar ions (negative or positive), which are driven by the electric field to the grounded collecting electrode with an average velocity of typically 30–300 m/s (Robinson, 1961). Elastic collisions of the accelerated ions with neutral air molecules induce airflow with typical airspeed 1.0–10 m/s (Defraeye and Martynenko, 2018). Experimental measurements of electrically-induced airflow confirmed that the spatial distribution of air velocity near the material surface is related to the ion flow field (Rickard et al., 2006, Kawamoto et al., 2006). So far, it is commonly accepted that ionic wind accelerates convective drying due to a disruption of the vapor boundary layer (Moreau, 2007).
The exact role of the electrical effects (electrostatic field and ion flow) on the drying process is however still unclear. Drying in non-uniform electrostatic fields provides higher drying rates, while uniform electrostatic fields do not affect mass transfer (Zheng et al., 2011). Drying rates can also be enhanced due to the presence of air ions or cold plasma (Misra et al., 2018). This conclusion is supported by early findings that air ionization, even without an electrostatic field, could facilitate evaporation (Krueger et al., 1958, Hart and Bachman, 1968). As such, it is also likely that ion flow or an electrostatic field could directly enhance moisture transfer by polarization of water molecules and changing the thermodynamic state of the water at the surface of the material (Misra et al., 2018).
To date, these different mechanisms of EHD-induced moisture removal are not yet well understood (Martynenko and Kudra, 2016). One key reason is that researchers were not able to separate electrical effects on EHD drying from purely convective moisture transfer. To do so, one would need to design an appropriate experimental setup that is able to separate the convective effect on the drying process from possible contributions of the electrostatic field and ion flow. To have a fair comparison of convective drying with and without the presence of an electrostatic field and ion flow, the airflow field around the product should be equivalent in both cases, which is challenging to achieve.
The fundamental question we target in this study is to what extent the three factors - airflow, ion flow or electrostatic field - are contributing to EHD moisture transfer. For this purpose, a targeted experimental setup is designed, which sheds more light on the different mechanisms of EHD drying.
Section snippets
Material
The material used to measure the drying rate was an ensemble of wetted paper tissue (Cascades Tissue Group, Canada) with 2 mm thickness, a 100 × 60 mm2 surface area and a smooth flat surface. This tissue was fully saturated with tap water to a weight of 35–40 g, with an initial moisture content of 6.0 g/g on a dry basis. The water holding capacity of the sample was sufficient to maintain a uniform saturated vapor pressure over the surface during a 1-h drying experiment. This implies that: (i)
Results
The drying rate was calculated as the change of mass with time for six combinations: two airflow generation methods (BLW, EHD) and three sample configurations as shown in Fig. 3. In all cases, the mass of the sample reduced linearly with time, indicating a constant drying rate. As such, the drying process kinetics was determined by the environmental conditions (air temperature, humidity, and airspeed) and was not restricted by the moisture transport inside the sample. The experimental data,
Comparison of EHD flow to convective airflow
Experiments with electrically-induced airflow (EHD) show significantly higher drying rates than mechanically-driven (BLW) airflow for all configurations. The highest enhancement ratio of EHD compared to BLW airflow was 2.7 in the “above mesh” configuration, decreasing to 2.2 at 5 mm below the mesh. The effect of electrical forces (electrostatic field and ion flow), as quantified as a ratio of EHD drying rate above and below mesh, was about 30%. At the same time, the EHD-induced drying rate just
Impact of mesh on the drying rate
Experiments with mechanically-induced airflow (BLW) showed that the effect of the mesh on the convective airflow could not be entirely neglected. A small, but significant difference between drying rate above (2.29 g/h in configuration (a)) and below (1.86 g/h in configuration (b)) reflected the aerodynamic impact of the mesh collector as it alters the velocity distribution and the turbulence level of the airflow. A reduction in turbulence level downstream of a mesh is a common effect of a mesh
Convective and electrical effects in EHD drying
From Fig. 4 and Table 1 it follows that the highest EHD drying rate was achieved above the mesh, when material was fully exposed to the electrostatic field and ion flow. A significant difference (30%) between drying rate inside (above mesh) vs. outside of the electrostatic field and ion flow (below mesh) suggests that electrostatic field and ion flow may play a role in the enhancement of the EHD drying rate. The resulting higher drying rate is likely related to charge transfer at the
Conclusions
- 1.
This paper presents a new methodological approach for studying the contribution of electrical and convective effects in EHD drying by using a metal mesh collecting electrode and three experimental settings with a wet sample above the collector, below the collector and 5 mm below the collector.
- 2.
EHD drying provides significantly higher drying rates than standard convective drying at a comparable airspeed range.
- 3.
Excluding the electrical effects (electrostatic field and ion flow) revealed the major
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