N. N. Barthakur

Potato slab dehydration by air ions from corona discharge

Space charge (air ions) produced by single corona electrodes was used to enhance drying rates from fresh slabs of potato. The drying path was traced by a beta-ray gauge which provided both sensitivity and reproducibility to the measurements of drying time. The rate of evaporation was increased 2.2 to 3.0 times when subjected to fluxes of 3.02×1012 positive ions alone or in combination with 7.31×1012 negative air ions/cm2 per s compared to that from an air-drying control slab. Electric wind caused by an ionic drag force seems to be the principal driving force for the observed enhancement in drying rates.

EVAPORATION RATE ENHANCEMENT OF WATER WITH AIR IONS FROM A CORONA DISCHARGE

Space charges (air ions) produced by a single point-to-plane corona electrode system were used to study the enhancement in the evaporation rates of water at three ion current levels. The maximum evaporation rates of 0.019 and 0.017 g·min−1 were observed at a 1 cm electrode gap for negative and positive air ions, respectively. The cumulative evaporation rates were linear with time and an ion-enhanced rate was about 4 times greater than the control. The current density distribution measurements agreed fairly well with those predicted from the Warburg law. The principal driving force for the observed evaporation enhancement was an ion drag phenomenon which created vortex motions in water when air ions were subjected to an externally applied electric field. Theoretical considerations from derived relationships in fluid mechanics demonstrate that the mass transfer coefficient is higher for positive than negative ions of the same current strength because of the mobility difference between the charges in the medium.

A comparative study of three leaf wetness sensors

The laboratory and field performance of two electrical resistance (ER) sensors of leaf surface wetness were compared with that of a beta-ray gauge (BRG). The BRG provided the most accurate measurements of wetness duration, which were in agreement with visual observations. A Campbell and a cotton cloth ER sensor consistently underestimated the duration of leaf surface wetness compared to the value obtained with the BRG in a dew chamber. However, the response of the Campbell sensor improved considerably with increase in the severity of dewfall. A superior performance of one of the two ER sensors could not be decisively established on the basis of the field experiments of 1989 and 1990 on soybean and tobacco crops, respectively. For studies where accurate measurements of surface wetness are critical, it is suggested that a beta-ray gauge should be used.

Impact of air ions of both polarity on evaporation of certain organic and inorganic liquids

Air ions of both polarity, produced by corona electrodes, were used to evaporate to dryness liquid samples of ethyl alcohol (EA), water (W), and carbon tetrachloride (CTC). Drying times were determined with a beta-ray gauge. Ion exposed samples of EA, W, and CTC dried, respectively, 2.3, 3.2, and 5.4 times faster than the corresponding control samples when exposed simultaneously to 0.94×1012 positive and 1.83×1012 negative air ions cm−2s−1 under the same laboratory conditions. Drying by corona discharge could be explained by three different mechanisms. Electric wind caused by the ionic drag is proposed as the principal driving force for the observed enhancement of evaporation. The decrease in free energy of a dielectric in the presence of an electric field compared to its absence may have increased the escaping tendency of the molecules of the treated liquids. The turbulence in the liquids created by the rotational effect on the dielectric molecules by the electric field may also be a factor in further enhancing the mass transfer rates from the samples.

Enhancement of evaporation rates from thin layers of liquids exposed to air ions

A beta-ray gauge system was used to study evaporation rates and drying times of liquids exposed to relatively high fluxes of air ions of either polarity produced by a corona discharge. Evaporation rates from thin layers of water, ethyl alcohol, and carbon tetrachloride increased considerably when exposed to air ions, compared to untreated liquids under constant environmental conditions. Drying times of liquid layers exposed to air ions were shortened by a factor of at least two over liquids that had not been exposed to ions. The drying time of a liquid layer was found to increase with the decrease of ion fluxes at the liquid surface. Threshold values of 1.9×1011 positive ions and 2.7×1011 negative ions cm−2 s−1 were established below which no increase in the evaporation rates were observed for ethyl alcohol and carbon tetrachloride. The evaporation rate of water at these same ion fluxes was significantly higher than that of the control. The present experiments indicate that a stream of air ions could play a major role in the observed enhancement of evaporation rates.

Comparisons of beta-ray and electronic leaf wetness sensors in a dew chamber

The performances of a beta-ray gauge system (sensor A) and an electronic sensor (sensor B) as leaf surface wetness detectors were compared in a dew chamber. Sensor A recorded longer surface wetness durations than sensor B under identical conditions of dew formation. The agreement between the sensors was close in recording the time of dew wetting when a white gauze cloth was used as an artificial leaf on sensor B. Leaf surface wetness duration due to the formation of dew can be measured more realistically by sensor A than sensor B. This is attributed to the utilization of a real leaf for sensor A and a physical model for sensor B as condensing surfaces.

Effects of Small Air Ions on Net Blotch Disease of Barley

Small positive air ions significantly reduced the number of lesions and percentage of leaf area covered by lesions of net blotch disease of barley. Appearance of disease symptoms was delayed by three days when plants were exposed to an atmosphere of positive ions. No such effect was observed for negative air ions. Height and dry weight were significantly stimulated by unipolar ions.

Growth and certain chemical constituents of tobacco plants exposed to air ions

Controlled experiments were performed in Faraday cages on the effects of positive and negative air ions on flue-cured tobacco plants. Continuous exposures for 15 days to air ions showed no significant differences in any plant growth characteristic between the treated and control plants. Standard errors in the measurement of the growth parameters for ion exposed plants were, however, consistently higher than those of control plants. Spatial variation in concentration gradients of air ions produced by corona discharge might have contributed to masking of the relatively small effects of air ions on biological organisms observed in previous experiments in this laboratory. No significant difference was observed between the experimental and control plants in nicotine, total alkaloid, and reducing sugar contents. Total nitrogen content was slightly higher for treated than control plants.

Leaf temperatures under controlled environmental conditions

A wind tunnel and an external microwave energy source formed the basis of an experimental technique to investigate many complex functional relationships between climatic parameters and leaf temperatures. The results of preliminary experimental plant-environment interactions are compared with published theoretical results obtained by solving the energy balance equation for a single leaf.

Air ion effect on respiration and photosynthesis of barley andAntirrhinum majus

Stem height increases of 13% and 15% for barley and snapdragon respectively were observed for plants maintained in an atmosphere of high concentrations of negative air ions compared to control plants. Dry weights of the exposed plant species to negative ions increased by approximately 18% compared to untreated plants. The effects of positive air ions were insignificant. The rates of photosynthesis and respiration measurements on continuously exposed plants to negative air ions conformed to these observations.