Georgios Dimitrakis - Academia.edu (original) (raw)

Papers by Georgios Dimitrakis

Fuel Processing Technology, Sep 1, 2014

The work carried out in this paper assessed how processing conditions and feedstock affect the qu... more The work carried out in this paper assessed how processing conditions and feedstock affect the quality of the coke produced during microwave coke making. The aim was to gather information that would support the development of an optimised microwave coke making oven. Experiments were carried out in a non-optimised 2450 MHz cylindrical cavity. The effect of treatment time (15-120 min), power input (750 W-4.5 kW) and overall power input (1,700-27,200 kWh/t) on a range of maximise the advantage of using microwaves). Understanding and quantifying the rapidly changing dielectric properties of the coal and coke materials is vital in addressing both of these challenges.

Ultramicroscopy, 2017

This paper describes traceable measurements of the dielectric permittivity and loss tangent of a ... more This paper describes traceable measurements of the dielectric permittivity and loss tangent of a multiphase material (particulate rock set in epoxy) at micron scales using a resonant Near-Field Scanning Microwave Microscope (NSMM) at 1.2GHz. Calibration and extraction of the permittivity and loss tangent is via an image charge analysis which has been modified by the use of the complex frequency to make it applicable for high loss materials. The results presented are obtained using a spherical probe tip, 0.1mm in diameter, and also a conical probe tip with a rounded end 0.01mm in diameter, which allows imaging with higher resolution (≈10µm). The microscope is calibrated using approach-curve data over a restricted range of gaps (typically between 1% and 10% of tip diameter) as this is found to give the best measurement accuracy. For both tips the uncertainty of scanned measurements of permittivity is estimated to be±10% (at coverage factor k=2) for permittivity ⪝10. Loss tangent can b...

Separation and Purification Technology, 2021

Abstract This work presents a comparative study of microwave and conventional (conductive) heatin... more Abstract This work presents a comparative study of microwave and conventional (conductive) heating for adsorbent regeneration. A binary gas mixture representative of pre-dried flue gas from coal-fired power plants (15% v/v CO2 in N2) was passed through a rotatory fixed-bed adsorption column filled with a zeolite molecular sieve (13X) and an activated carbon (Norit R2030CO2). The impact of the two regeneration methods on both the textural properties and the carbon capture performance (CO2 uptake capacity, regeneration efficiency, and rate of regeneration) were assessed and compared after consecutive adsorption/desorption cycles. Overall, Norit R2030CO2 maintained stable adsorption capacity and regeneration efficiency with both conventional and microwave heating but slightly better with the latter. Additionally, power consumption per adsorbent unit mass and per adsorbate removed were reduced with microwave regeneration by 18.69 and 17.76% respectively compared to conventional regeneration. In the case of 13X, adsorption capacity and regeneration efficiency were found to be relatively stable after a drop in the first cycle in both heating modes, whereas power requirement was found higher in microwave regeneration than in conventional regeneration. NoritR2030CO2 showed a slightly higher maximum desorption rate when regenerated with microwave heating compared to conductive heating. Contrarily, the maximum desorption rate for the molecular sieve is higher with conductive heating as opposed to microwave heating. However, the data indicated a bigger desorption rate with the microwave regeneration in both adsorbents in later stages of the heating process (i.e. from min 16th for NoritR2030CO2 and from min 18th for 13X until process completion). The breakthrough time (tb) of Norit R2030CO2 was unaffected by cyclic operation or the heating methods, whereas in 13X this value varied over the cycles. The latter observation indicates that 13X requires either longer times or higher temperatures to achieve full regeneration compared to Norit R2030CO2. It can be concluded that microwave-assisted regeneration presented slight advantages over regeneration with conductive heating in delivering more steady capture capacity and regeneration efficiency for Norit R2030CO2 under the test conditions employed here whereas 13X exhibited indifference.

The Canadian Journal of Chemical Engineering, 2021

International Journal of Multiphase Flow, 2018

Churn flow is an important intermediate flow regime occurring in between slug and annular flow pa... more Churn flow is an important intermediate flow regime occurring in between slug and annular flow patterns in two-phase flow, with profound implications in chemical and petroleum industry. The majority of studies to date in churn flow have been carried out mainly using water or liquids of low viscosities and limited information exists regarding the behaviour of high viscosity liquids which resemble realistic process conditions. In this paper, a study that investigated churn flow and its characteristics in high viscosity oils (360 and 330 Pa.s) and large diameter columns (240 and 290mm) is presented for a first time. Transition to churn flow regime starts when the structure velocity, length and frequency of the liquid bridges, which appear at the end of slug flow, increase. In churn flow, gas flows at the core of the oil column with a wavy passage, leaving the top surface open to atmosphere with a possibility of creating a very long bubble. The average length of the bubbles seen to decrease with increasing the gas flow rate. While, no considerable change is observed in void fraction, structure velocity and film thickness at this flow pattern.

Journal of Food Engineering, 2021

Abstract There is currently no recognised equation, or set of equations, that can be used to adeq... more Abstract There is currently no recognised equation, or set of equations, that can be used to adequately describe moisture dependant dielectrics. This study addresses this issue so that moisture dependant dielectric properties can be directly input into drying models, where the microwave or radio frequency source is always limited to a single frequency. This was achieved by adapting water activity equations to describe the moisture dependant behaviour of the loss factor and the dielectric constant of hygroscopic inhomogeneous materials at microwave and radio frequencies. These equations were fitted to thirty moisture dependant loss factor and dielectric constant data sets. The adapted water activity equations proved to be very effective at describing dielectric behaviour, with the best equation fits to the thirty moisture dependant dielectric data sets having an average Mean Relative Error of 2.99%. The suitability of the equations are discussed, and specific equations are recommended for fitting to different types of moisture dependant dielectric response.

Journal of Food Engineering, 2019

Composites Part A: Applied Science and Manufacturing, 2000

The main objectives of this research work are to develop conductive knitted fabric composite mate... more The main objectives of this research work are to develop conductive knitted fabric composite materials and to determine their electromagnetic shielding effectiveness (EMSE). Polypropylene is the matrix phase and glass fibers are the reinforcement phase of the composite material. Copper wires are incorporated as conductive fillers to provide the electromagnetic shielding properties of the composite material. The amount of copper in the composite material is varied by changing the yarn composition, fabric knit structure and stitch density. The EMSE of various knitted fabric composites is measured in the frequency range of 300 kHz to 3 GHz. The variations of EMSE of knitted fabric composites with fabric structure, stitch density and yarn compositions are described. Suitability of conductive knitted fabric composites for electromagnetic shielding applications is also discussed.

1. Eng., Ph.D.; Institute of Geotechnics of Slovak Academy of Sciences; Watsonova 45, 040 01 Kosi... more 1) Eng., Ph.D.; Institute of Geotechnics of Slovak Academy of Sciences; Watsonova 45, 040 01 Kosice, Slovak Republic; email: znamenackova@saske.sk, tel.: (+421)55 792 2619 2) RNDr., Ph.D.; Institute of Geotechnics of Slovak Academy of Sciences; Watsonova 45, 040 01 Kosice, Slovak Republic; email: sdolinska@saske.sk, tel.: (+421)55 792 2619 3) RNDr., Ph.D.; Institute of Geotechnics of Slovak Academy of Sciences; Watsonova 45, 040 01 Kosice, Slovak Republic; email: lovasm@saske.sk, tel.: (+421)55 792 2630 4) Eng., Ph.D.; Institute of Geotechnics of Slovak Academy of Sciences; Watsonova 45, 040 01 Kosice, Slovak Republic; email: hredzak@saske.sk, tel.: (+421)55 792 2600 5) Dpl. Eng., Ph.D.; National Centre for Industrial Microwave Processing, University of Notthingham, UK; email: georgiosdimitrakis@notthingam.ac.uk, tel.: (+44)1159514170

Foods, 2021

Non-uniform temperature distribution within solid food is a major problem associated with microwa... more Non-uniform temperature distribution within solid food is a major problem associated with microwave heating, which limits industrial applications. Therefore, an experimentally validated 3D model was proposed to study the effect of microwave applicator geometry on the electromagnetic field distribution and heating pattern of shrimp under different processing conditions. Simulation results were compared with physical experiments, in which a cooked peeled shrimp sample was heated using two different laboratory-scale microwave applicators (rectangular and cylindrical cavities). For the rectangular applicator, the temperature distribution within the shrimp, when examined in cross-section, was more homogeneous compared to that of the cylindrical applicator. The results showed the influence of the complex shape of the food on the temperature distribution during microwave heating, as well as of process parameters (input power and geometry cavity). Moreover, this modelling method could provi...

Microwave non-destructive testing (MNDT) methods represent an effective solution in detecting def... more Microwave non-destructive testing (MNDT) methods represent an effective solution in detecting defects within composite structures with relatively low electrical conductivity. They offer the advantage to overcome the problems of traditional NDT techniques such as coupling, danger coming from ionizing radiation, limited depth of operation, large wavelengths, time consuming post processing. Near-field microwave and millimetre non-invasive inspections have been successfully used for detecting defects such as disbond and delamination in complex structures. In dielectric materials, they can be used for dielectric properties characterization, degree of porosity evaluation, degree of ageing, anisotropy, dielectric mixture constituents determination, state of cure. When it comes to the analysis of carbon fibre reinforced polymers (CFRPs), the use of these non-destructive techniques is restricted by the composite relatively high conductivity of about 10 S/m. In this paper, the investigation o...

Chemical Engineering and Processing - Process Intensification, 2019

Microwave technology is gaining popularity as a tool for chemical process intensification and an ... more Microwave technology is gaining popularity as a tool for chemical process intensification and an alternative to conventional heating. However, in flow systems non-uniform temperature profiles are commonly encountered and hence methods to characterise and improve them are required. In this work, we studied the effects of various operational parameters - microwave power, inlet flow rate, tube orientation and pressure - on the electric field and temperature profiles of water flowing in a PTFE tube (2.4 mm internal diameter), placed in a commercial single-mode microwave applicator. A finite element model was developed to estimate the longitudinal temperature profiles and the absorbed microwave power, while in situ temperature monitoring was performed by a fibre optic probe placed at multiple locations inside the tube. The water temperature inside the tube increased by increasing the microwave power input and temperature profiles stabilised beyond 20 W, while the percentage absorbed micr...

Uvod Mikrovlnna energia si v sucasnom obdobi razi cestu do mnohých oblasti ľudskej cinnosti. Využ... more Uvod Mikrovlnna energia si v sucasnom obdobi razi cestu do mnohých oblasti ľudskej cinnosti. Využiva sa pri inovacii a intenzifikacii technologických procesov v roznych priemyselných odvetviach. Mikrovlnný ohrev je založený na interakcii mikrovĺn s polarnymi molekulami latok. Vplyvom kmitaveho pohybu molekul dochadza k „treniu“, ktoreho dosledkom je vznik tepelneho efektu. Tento poznatok bol efektivne využitý na rýchly ohrev potravin, pretože vsetky potraviny obsahuju polarne molekuly vody. Postupom casu ziskavali mikrovlnne zariadenia stale vacsi význam ako zdroje tepelnej energie, predovsetkým pri urýchleni chemických reakcii. Mikrovlnna technika sa zacina zavadzať do chemických laboratorii a v sucasnosti sa využiva hlavne na tepelný rozklad vzoriek pri chemickej analýze prvkov. Uplatneniu mikrovlnneho žiarenia je venovana znacna pozornosť svetových výskumných skupin. Možnosti využitia mikrovĺn boli odskusane v procesoch susenia roznych materialov, napr. dreva, textilii, kavy, pap...

Materials Science and Engineering: A

Journal of Cleaner Production

Reaction Chemistry & Engineering

Significant increases in the reaction rate and final conversion of a model polyesterification are... more Significant increases in the reaction rate and final conversion of a model polyesterification are reported by the use of microwave heating. These increases have been demonstrated when both organometallic...

Fuel Processing Technology, Sep 1, 2014

The work carried out in this paper assessed how processing conditions and feedstock affect the qu... more The work carried out in this paper assessed how processing conditions and feedstock affect the quality of the coke produced during microwave coke making. The aim was to gather information that would support the development of an optimised microwave coke making oven. Experiments were carried out in a non-optimised 2450 MHz cylindrical cavity. The effect of treatment time (15-120 min), power input (750 W-4.5 kW) and overall power input (1,700-27,200 kWh/t) on a range of maximise the advantage of using microwaves). Understanding and quantifying the rapidly changing dielectric properties of the coal and coke materials is vital in addressing both of these challenges.

Ultramicroscopy, 2017

This paper describes traceable measurements of the dielectric permittivity and loss tangent of a ... more This paper describes traceable measurements of the dielectric permittivity and loss tangent of a multiphase material (particulate rock set in epoxy) at micron scales using a resonant Near-Field Scanning Microwave Microscope (NSMM) at 1.2GHz. Calibration and extraction of the permittivity and loss tangent is via an image charge analysis which has been modified by the use of the complex frequency to make it applicable for high loss materials. The results presented are obtained using a spherical probe tip, 0.1mm in diameter, and also a conical probe tip with a rounded end 0.01mm in diameter, which allows imaging with higher resolution (≈10µm). The microscope is calibrated using approach-curve data over a restricted range of gaps (typically between 1% and 10% of tip diameter) as this is found to give the best measurement accuracy. For both tips the uncertainty of scanned measurements of permittivity is estimated to be±10% (at coverage factor k=2) for permittivity ⪝10. Loss tangent can b...

Separation and Purification Technology, 2021

Abstract This work presents a comparative study of microwave and conventional (conductive) heatin... more Abstract This work presents a comparative study of microwave and conventional (conductive) heating for adsorbent regeneration. A binary gas mixture representative of pre-dried flue gas from coal-fired power plants (15% v/v CO2 in N2) was passed through a rotatory fixed-bed adsorption column filled with a zeolite molecular sieve (13X) and an activated carbon (Norit R2030CO2). The impact of the two regeneration methods on both the textural properties and the carbon capture performance (CO2 uptake capacity, regeneration efficiency, and rate of regeneration) were assessed and compared after consecutive adsorption/desorption cycles. Overall, Norit R2030CO2 maintained stable adsorption capacity and regeneration efficiency with both conventional and microwave heating but slightly better with the latter. Additionally, power consumption per adsorbent unit mass and per adsorbate removed were reduced with microwave regeneration by 18.69 and 17.76% respectively compared to conventional regeneration. In the case of 13X, adsorption capacity and regeneration efficiency were found to be relatively stable after a drop in the first cycle in both heating modes, whereas power requirement was found higher in microwave regeneration than in conventional regeneration. NoritR2030CO2 showed a slightly higher maximum desorption rate when regenerated with microwave heating compared to conductive heating. Contrarily, the maximum desorption rate for the molecular sieve is higher with conductive heating as opposed to microwave heating. However, the data indicated a bigger desorption rate with the microwave regeneration in both adsorbents in later stages of the heating process (i.e. from min 16th for NoritR2030CO2 and from min 18th for 13X until process completion). The breakthrough time (tb) of Norit R2030CO2 was unaffected by cyclic operation or the heating methods, whereas in 13X this value varied over the cycles. The latter observation indicates that 13X requires either longer times or higher temperatures to achieve full regeneration compared to Norit R2030CO2. It can be concluded that microwave-assisted regeneration presented slight advantages over regeneration with conductive heating in delivering more steady capture capacity and regeneration efficiency for Norit R2030CO2 under the test conditions employed here whereas 13X exhibited indifference.

The Canadian Journal of Chemical Engineering, 2021

International Journal of Multiphase Flow, 2018

Churn flow is an important intermediate flow regime occurring in between slug and annular flow pa... more Churn flow is an important intermediate flow regime occurring in between slug and annular flow patterns in two-phase flow, with profound implications in chemical and petroleum industry. The majority of studies to date in churn flow have been carried out mainly using water or liquids of low viscosities and limited information exists regarding the behaviour of high viscosity liquids which resemble realistic process conditions. In this paper, a study that investigated churn flow and its characteristics in high viscosity oils (360 and 330 Pa.s) and large diameter columns (240 and 290mm) is presented for a first time. Transition to churn flow regime starts when the structure velocity, length and frequency of the liquid bridges, which appear at the end of slug flow, increase. In churn flow, gas flows at the core of the oil column with a wavy passage, leaving the top surface open to atmosphere with a possibility of creating a very long bubble. The average length of the bubbles seen to decrease with increasing the gas flow rate. While, no considerable change is observed in void fraction, structure velocity and film thickness at this flow pattern.

Journal of Food Engineering, 2021

Abstract There is currently no recognised equation, or set of equations, that can be used to adeq... more Abstract There is currently no recognised equation, or set of equations, that can be used to adequately describe moisture dependant dielectrics. This study addresses this issue so that moisture dependant dielectric properties can be directly input into drying models, where the microwave or radio frequency source is always limited to a single frequency. This was achieved by adapting water activity equations to describe the moisture dependant behaviour of the loss factor and the dielectric constant of hygroscopic inhomogeneous materials at microwave and radio frequencies. These equations were fitted to thirty moisture dependant loss factor and dielectric constant data sets. The adapted water activity equations proved to be very effective at describing dielectric behaviour, with the best equation fits to the thirty moisture dependant dielectric data sets having an average Mean Relative Error of 2.99%. The suitability of the equations are discussed, and specific equations are recommended for fitting to different types of moisture dependant dielectric response.

Journal of Food Engineering, 2019

Composites Part A: Applied Science and Manufacturing, 2000

The main objectives of this research work are to develop conductive knitted fabric composite mate... more The main objectives of this research work are to develop conductive knitted fabric composite materials and to determine their electromagnetic shielding effectiveness (EMSE). Polypropylene is the matrix phase and glass fibers are the reinforcement phase of the composite material. Copper wires are incorporated as conductive fillers to provide the electromagnetic shielding properties of the composite material. The amount of copper in the composite material is varied by changing the yarn composition, fabric knit structure and stitch density. The EMSE of various knitted fabric composites is measured in the frequency range of 300 kHz to 3 GHz. The variations of EMSE of knitted fabric composites with fabric structure, stitch density and yarn compositions are described. Suitability of conductive knitted fabric composites for electromagnetic shielding applications is also discussed.

1. Eng., Ph.D.; Institute of Geotechnics of Slovak Academy of Sciences; Watsonova 45, 040 01 Kosi... more 1) Eng., Ph.D.; Institute of Geotechnics of Slovak Academy of Sciences; Watsonova 45, 040 01 Kosice, Slovak Republic; email: znamenackova@saske.sk, tel.: (+421)55 792 2619 2) RNDr., Ph.D.; Institute of Geotechnics of Slovak Academy of Sciences; Watsonova 45, 040 01 Kosice, Slovak Republic; email: sdolinska@saske.sk, tel.: (+421)55 792 2619 3) RNDr., Ph.D.; Institute of Geotechnics of Slovak Academy of Sciences; Watsonova 45, 040 01 Kosice, Slovak Republic; email: lovasm@saske.sk, tel.: (+421)55 792 2630 4) Eng., Ph.D.; Institute of Geotechnics of Slovak Academy of Sciences; Watsonova 45, 040 01 Kosice, Slovak Republic; email: hredzak@saske.sk, tel.: (+421)55 792 2600 5) Dpl. Eng., Ph.D.; National Centre for Industrial Microwave Processing, University of Notthingham, UK; email: georgiosdimitrakis@notthingam.ac.uk, tel.: (+44)1159514170

Foods, 2021

Non-uniform temperature distribution within solid food is a major problem associated with microwa... more Non-uniform temperature distribution within solid food is a major problem associated with microwave heating, which limits industrial applications. Therefore, an experimentally validated 3D model was proposed to study the effect of microwave applicator geometry on the electromagnetic field distribution and heating pattern of shrimp under different processing conditions. Simulation results were compared with physical experiments, in which a cooked peeled shrimp sample was heated using two different laboratory-scale microwave applicators (rectangular and cylindrical cavities). For the rectangular applicator, the temperature distribution within the shrimp, when examined in cross-section, was more homogeneous compared to that of the cylindrical applicator. The results showed the influence of the complex shape of the food on the temperature distribution during microwave heating, as well as of process parameters (input power and geometry cavity). Moreover, this modelling method could provi...

Microwave non-destructive testing (MNDT) methods represent an effective solution in detecting def... more Microwave non-destructive testing (MNDT) methods represent an effective solution in detecting defects within composite structures with relatively low electrical conductivity. They offer the advantage to overcome the problems of traditional NDT techniques such as coupling, danger coming from ionizing radiation, limited depth of operation, large wavelengths, time consuming post processing. Near-field microwave and millimetre non-invasive inspections have been successfully used for detecting defects such as disbond and delamination in complex structures. In dielectric materials, they can be used for dielectric properties characterization, degree of porosity evaluation, degree of ageing, anisotropy, dielectric mixture constituents determination, state of cure. When it comes to the analysis of carbon fibre reinforced polymers (CFRPs), the use of these non-destructive techniques is restricted by the composite relatively high conductivity of about 10 S/m. In this paper, the investigation o...

Chemical Engineering and Processing - Process Intensification, 2019

Microwave technology is gaining popularity as a tool for chemical process intensification and an ... more Microwave technology is gaining popularity as a tool for chemical process intensification and an alternative to conventional heating. However, in flow systems non-uniform temperature profiles are commonly encountered and hence methods to characterise and improve them are required. In this work, we studied the effects of various operational parameters - microwave power, inlet flow rate, tube orientation and pressure - on the electric field and temperature profiles of water flowing in a PTFE tube (2.4 mm internal diameter), placed in a commercial single-mode microwave applicator. A finite element model was developed to estimate the longitudinal temperature profiles and the absorbed microwave power, while in situ temperature monitoring was performed by a fibre optic probe placed at multiple locations inside the tube. The water temperature inside the tube increased by increasing the microwave power input and temperature profiles stabilised beyond 20 W, while the percentage absorbed micr...

Uvod Mikrovlnna energia si v sucasnom obdobi razi cestu do mnohých oblasti ľudskej cinnosti. Využ... more Uvod Mikrovlnna energia si v sucasnom obdobi razi cestu do mnohých oblasti ľudskej cinnosti. Využiva sa pri inovacii a intenzifikacii technologických procesov v roznych priemyselných odvetviach. Mikrovlnný ohrev je založený na interakcii mikrovĺn s polarnymi molekulami latok. Vplyvom kmitaveho pohybu molekul dochadza k „treniu“, ktoreho dosledkom je vznik tepelneho efektu. Tento poznatok bol efektivne využitý na rýchly ohrev potravin, pretože vsetky potraviny obsahuju polarne molekuly vody. Postupom casu ziskavali mikrovlnne zariadenia stale vacsi význam ako zdroje tepelnej energie, predovsetkým pri urýchleni chemických reakcii. Mikrovlnna technika sa zacina zavadzať do chemických laboratorii a v sucasnosti sa využiva hlavne na tepelný rozklad vzoriek pri chemickej analýze prvkov. Uplatneniu mikrovlnneho žiarenia je venovana znacna pozornosť svetových výskumných skupin. Možnosti využitia mikrovĺn boli odskusane v procesoch susenia roznych materialov, napr. dreva, textilii, kavy, pap...

Materials Science and Engineering: A

Journal of Cleaner Production

Reaction Chemistry & Engineering

Significant increases in the reaction rate and final conversion of a model polyesterification are... more Significant increases in the reaction rate and final conversion of a model polyesterification are reported by the use of microwave heating. These increases have been demonstrated when both organometallic...