Stavros Yanniotis

Boiling on the surface of a rotating disc

Abstract An experimental unit to study heat-transfer characteristics while boiling at subatmospheric pressure of a horizontal smooth spinning disc was designed and built. Evaporation experiments were carried out with the speed of rotation varying from 0 to 1000 rpm and the feed flow rate from 1 to 5 litre/min. The boiling temperature varied between 40 and 50 °C. Water and corn syrup were used as test liquids. Results are presented from the experimental measurement of the heat flux on the disc as a function of the wall superheat. The heat-transfer coefficient for water, in the low range of heat flux (10–30 kW/m2), increased from about 2 to 9 kW/m2K when the speed of rotation increased from 0 rpm to 1000 rpm. In the upper range of heat flux (60–100 kW/m2), it increased from about 5 to 16 kW/m2K for the same increase in the speed of rotation. The feed flow rate between 1 and 5 litre/min had no significant influence on the heat-transfer coefficient in the range of 200–600 rpm. With the 60 °Brix corn syrup, the heat-transfer coefficient at 10 °C temperature difference increased from about 0.8 to about 2.3 kW/m2K when the speed of rotation increased from 0 to 1000 rpm. A simple theoretical relation was derived which predicts the heat-transfer coefficient for water at inlet Reynolds numbers 500–1000 relatively well, but underpredicts the results for corn syrup at inlet Reynolds numbers 10–100.

A new method for interpolating and extrapolating water activity data

Abstract A new method is developed in which the water activity of a food substance is plotted against the water activity of a reference substance at the same ‘number of monolayers’. The plot gives a straight line for water activity values above one monolayer for all food items that have been examined. Using this plot it is possible to extrapolate sorption isotherms and obtain sorption isotherms knowing only two experimental points. Examples of interpolation and extrapolation are presented.

Effect of Particle Orientation during Thermal Processing of Canned Peach Halves: A CFD Simulation

The objective of this work was to apply Computational Fluid Dynamics (CFD) to study the effect of particle orientation on fluid flow, temperature evolution, as well as microbial destruction, during thermal processing of still cans filled with peach halves in sugar syrup. A still metal can with four peach halves in 20% sugar syrup was heated at 100 °C for 20 min and thereafter cooled at 20 °C. Infinite heat transfer coefficient between heating medium and external can wall was considered. Peach halves were orderly placed inside the can with the empty space originally occupied by the kernel facing, in all peaches, either towards the top or the bottom of the can. In a third situation, the can was placed horizontally. Simulations revealed differences on particle temperature profiles, as well as process F values and critical point location, based on their orientation. At their critical points, peach halves with the kernel space facing towards the top of the can heated considerably slower and cooled faster than the peaches having their kernel space facing towards the bottom of the can. The horizontal can case exhibited intermediate cooling but the fastest heating rates and the highest F process values among the three cases examined. The results of this study could be used in designing of thermal processes with optimal product quality.

Volatile profiles of healthy and aflatoxin contaminated pistachios

The volatile profile of four different groups of dried pistachios namely: H: healthy, NC: naturally contaminated with aflatoxin, AC: artificially contaminated with aflatoxigenic strains of the fungi Aspergillus flavus and ANT: artificially contaminated with non-toxigenic strains of the fungi A. flavus, was determined. The volatiles were isolated by the HS-SPME method and determined by GC-FID and GC-MS, whereas aflatoxin by HPLC. Seventy two volatile compounds were identified almost equally distributed among the above four studied groups. The predominant chemical compounds were monoterpenes, alcohols, ketones, aldehydes, esters and hydrocarbons. The monoterpenes, mainly determined as α-pinene and α-terpinolene were detected in all samples. Even though the general volatile profile was similar among groups, some differences were detected between healthy and contaminated groups of samples. When some key volatiles such as eight-carbon and seven-carbon alcohols and aldehydes were used along with the species-specific sesquiterpenes and the other terpenes detected, a correct classification was obtained in H, NC, AC and ANT groups, as was demonstrated by cluster and discriminant analyses. This evidence provides a potential tool for distinguishing contaminated samples on the basis of characteristic volatile patterns.

Design and Energy Analysis of Absorption-Driven Multiple Effect Evaporators

Absorption-driven multiple effect evaporators have been reported to achieve significant energy savings compared to conventional systems. This paper explores limiting economic circumstances and heat integration options which complicate the decision between absorptiondriven and steam-driven systems. A simple way of performing preliminary feasibility studies is illustrated through a low boiling point elevation example. A situation is presented where absorption-driven evaporators can never cost less than steam-driven ones.