Bijay Shrestha

Modeling of Vegetation Permittivity at Microwave Frequencies

When materials are to be treated with microwaves for different purposes such as drying, online moisture measurements, disinfestation, and remote sensing, thorough knowledge of the material dielectric properties becomes extremely important. Unlike for other materials, measurement of vegetation dielectric properties is very complicated due to the nature of the materials themselves. Hence, vegetation dielectric models that require fewer accurately and easily measurable physical quantities are of great importance. Therefore, dielectric models that only require the measurement of moisture content (MC) have been investigated for alfalfa, a plant material that has high commercial value in national and international markets. The models were developed by measuring the dielectric properties by using an open-ended coaxial probe at frequencies ranging from over 300 MHz to 18 GHz at 22 degC and at an MC ranging from 12% to 73% in wet basis

Prediction of moisture content of alfalfa using density-independent functions of microwave dielectric properties

The use of density-independent functions of the dielectric properties of chopped alfalfa, calculated from microwave reflection coefficients from 300 MHz to 18 GHz, was studied for determining moisture content in the range from 12% to 73%, wet basis, at bulk densities from 0.139 to 0.716 g cm−3 at 20 °C. Prediction of moisture content with worst-case relative errors of about 3% or less over the range from 20% to 73% confirmed promising prospects for use of such density-independent functions for reliable moisture measurement for important plant materials.

Microwave Dielectric Properties of Alfalfa Leaves From 0.3 to 18 GHz

Dielectric properties (i.e., permittivity) are essential in designing, simulating, and modeling microwave applications. The permittivity of stacked leaves of alfalfa (Medicago sativa) were measured with a network analyzer and a coaxial probe, and the effect of moisture content (MC: 12%-73% wet basis), frequency (300 MHz to 18 GHz), bound water (Cole-Cole dispersion equation), temperature (-15°C and 30°C), leaf-orientation, and pressure (0-11 kPa) were investigated. The measured permittivity increased with MC. A critical moisture level (CML) of 23% was reported, below which the permittivity decreased with increasing frequency at 22°C. Above CML and up to 5 GHz, the dielectric constants followed the Cole-Cole dispersion, and the dielectric loss factors consisted of ionic and bound water losses. Above 5 GHz, the behavior of the dielectric constant was similar to that of free water, and the polar losses became dominant. Above 0°C, the measured permittivity followed a trend similar to that of free saline water and was characterized by the Debye equation. Below 0°C, it was dominated by nonfreezing bound and unfrozen supercooled moistures. The relaxation parameters and the optimum pressure (9 kPa) for the leaf measurements were determined. The effects of variations among the samples, and their orientations had negligible effects on the measured permittivity.

Dielectric Behaviour of Whole-Grain Wheat with Temperature at 27.12 MHZ: A Novel Use of a Liquid Dielectric Test Fixture for Grains

This paper is a step towards fulfilling the gap seen in temperature dependence of permittivity (ϵ) of grains at radio frequencies. A commercial device especially built for liquids was exploited for the first time in measuring ϵ of bulk wheat samples at various temperatures. The dielectric constant (ϵ′) of the wheat increased almost linearly with temperature, and varied between 3.82 and 5.95. The loss factor (ϵ″) increased non-linearly with temperature with values between 0.07 and 0.93. Regression models predicted ϵ′ and ϵ″ of the wheat with RMSE of 0.14 and 0.034, and R2 values of 0.97 and 0.99, respectively.

Thermal Conductivity, Specific Heat, and Thermal Diffusivity of Saponaria vaccaria Seed Particles

The seeds of Saponaria vaccaria (prairie carnation) are the major source of saponins, which are used for the treatment of cancers and various infections. The thermal conductivity, specific heat, and thermal diffusivity of Saponaria vaccaria seed particles were determined at temperatures ranging from 25°C to 55°C, moisture content from 15.35% to 80% dry basis, and effective density from 516.5 to 1328.05 kg m-3. The thermal conductivity was measured by the transient technique using a line heat source for the particles with and without air, and pores filled with ethanol-water solutions of 10%, 40%, 70%, and 100% volumetric ethanol concentrations. The values ranged from 0.057 to 0.625 W m-1 °C-1. Specific heat was measured by differential scanning calorimetry and ranged from 1118.57 to 3023.78 J kg-1 °C-1. Thermal diffusivity was calculated from thermal conductivity, specific heat, and density and ranged from 0.60 × 10-7 to 1.58 × 10-7 m2 s-1. Particle and bulk densities of the particles followed a parabolic relationship with moisture content. Thermal conductivity increased with moisture content. Specific heat increased linearly with temperature and exhibited ascending-descending trends with moisture content. Thermal diffusivity followed descending-ascending trends with moisture content. Regression models for all the properties were developed based on the experimental data.

Permittivity of Mixtures of Saponaria vaccaria and Ethanol–Water Solution for RF Heating Assisted Extraction of Saponins

Saponins are used for the treatments of cancers and various infections. The use of radio frequency heating in extracting them from the mixture of particulate Saponaria vaccaria and ethanol-water solution has potentials in pharmaceutical and neutraceutical industries. This technique requires a thorough understanding of the dielectric properties of the mixtures. The dielectric properties of the mixtures were measured over the frequency ranging from 5 to 30 MHz using a computer-controlled precision LCR meter and a liquid test fixture. The effects of temperature, particle moisture content (MC), frequency, and ethanol concentration (EC) on the dielectric properties of the mixture and penetration depths were investigated at 13.56 and 27.12 MHz. The dielectric constants of the mixtures linearly increased with the particle MC and were more sensitive to it at higher EC of the solution. The dielectric loss factors increased with the particle moisture in somewhat quadratic fashion at 100% EC and linearly at 70%, 40%, and 10% ECs. The dielectric constant and the loss factor linearly increased with the temperature. The increments were more prominent at higher MCs and lower EC. The dielectric spectra of the mixtures monotonically decreased with increasing frequency. The dielectric constants and the dielectric loss factors dropped up to 33% and approximately 50%, respectively, when the frequency was increased from 5 to 10 MHz and then slowly decreased afterward. The penetration depths of the electromagnetic energy within the mixtures decreased with increasing frequency and ranged from 0.06 to 8.83 m at 27.12 MHz. Empirical, semiempirical, and theoretical models for the mixture dielectric properties were tested.

Studies on dhal recovery from pre-treated pigeon pea (Cajanus cajan L.) cultivars

Dhal recovery from three popular varieties of North Karnataka was studied using CFTRI mini dhal mill with five different treatments at three different levels. It was observed that Gulyal variety treated with mustard oil recorded maximum hulling efficiency (79.4%) and finished product (68.8%) when compared to a Maruti and Asha variety. However, acetic acid treatment recorded higher hulling efficiency (76.5%) for Maruti followed by Asha (56.9%). The plant growth promoting rhizobacteria (PGPR) treatment yielded minimum hulling efficiency and finished product recovery for all the varieties.

Radio Frequency Dielectric Properties of Bulk Canola Seeds under Different Temperatures, Moisture Contents, and Frequencies for Feasibility of Radio Frequency Disinfestation

The parallel capacitances and resistances of the bulk canola seeds were measured to determine the dielectric properties of canola seeds using a radio frequency dielectric fixture at different levels of temperature (30–80°C) and moisture content (5–11% w.b.), over the frequency range of 5 to 30 MHz. The dielectric constant (ε′) increased from 3.82 to 7.85 with increasing temperature and decreasing frequency regardless of the seed moisture contents. The dielectric loss factors (ε”) of the bulk seeds increased with increasing temperature and moisture content and decreased with frequency, ranging between 0.11 and 13.0. The penetration depth of the electromagnetic power in the bulk seeds varied from 1.30 to 48.0 m depending upon temperature, frequency, and moisture content. The distinct correlation of sensitivity of dielectric properties with moisture content was not observed except for that of penetration depth. The dielectric properties of the bulk canola seeds were higher than those of other oil seeds at various moisture contents; it might be affected by relatively higher linolenic acid content among other fatty acids. The large difference of the dielectric loss factors of insect pests cf. those of canola seeds showed potential for radio frequency disinfestation based on selective heating of insect pests in the canola seeds. The dielectric properties determined can be used for simulating temperature distribution within the bulk canola seeds during the radio frequency process.

Thermal death kinetics of adult red flour beetle Tribolium castaneum (Herbst) in canola seeds during radio frequency heating

ABSTRACT Thermal mortalities of adult red flour beetles Tribolium castaneum (Herbst) infesting canola seeds at various moisture contents and volumes were determined after radio frequency (RF) heating (i.e. temperature between 30 and 80°C). The mortality of 92% was achieved at the end temperature of 343 K for small-volume (1.96 × 10−4 m3, 0.250 kg) seeds, and the mortality of 99% at 333 K for large-volume (1.77 × 10−3 m3, 2.26 kg) seeds. Regardless of sample volume, the thermal mortalities of the test insects increased significantly after the seed temperature reached 333 K (60°C). The kinetic parameters of the thermal death of the adult T. castaneum were estimated using inverse simulation. The ordinary differential equation-based kinetic model with the Arrhenius temperature-dependent reaction rate constant was solved using the fourth-order Runge–Kutta method. The kinetics followed first-order reaction with the activation energy of 100 kJ/mol. Good agreements were observed between the mortalities predicted using the kinetic model and the experiments (R2 = 0.972–0.987) except for the small-volume seeds at 11% MC (11 g/100 g raw materials) (R2 = 0.741). The predicted lethal times (s) to achieve 95 and 99% mortalities using the kinetic model agreed well with those determined from the experiments.