Half-cooling Time Research Articles

Heat transfer within a multi-package: Assessing the impact of package design on the cooling of strawberries

Perishable horticultural products may necessitate multi-packaging for preservation. The optimized primary and secondary packaging designs allow to reduce cooling time and ensure uniform product temperature, thereby enhancing fruit quality. This study examined how the secondary package design impacts the cooling of strawberries in airtight clamshells (AC) during precooling. The AC design simulates heat transfer within Modified Atmosphere Package (MAP) where there is no direct interaction between the external cooling air and the internal environment of the clamshell. Laboratory experiments were conducted to simulate one level of a pallet, using artificial material instead of real strawberries in order to have a better control of thermophysical properties and thus to better investigate heat transfer mechanisms. The thermal performance of an existing tray design was compared to three new alternative designs. The effect of air headspace, vent holes area and inlet airflow rate on the cooling efficiency was investigated.Experimental results revealed significant cooling heterogeneities among different AC positions, with the largest observed disparities being 1.8 h for half cooling time (HCT) and 3.7 h for seven-eight cooling time (SECT) in the current tray design. Incorporating vent holes into the current commercialized tray design demonstrated superior cooling performance, with 8% improvement of the overall average HCT. Analysis showed increasing the thickness of the air headspace above the AC increased 91% and 113% the overall average HCT and SECT, respectively. The research found that airflow distribution in a tray has a critical effect on the heat transfer between the AC walls and the surrounding air temperature. Thus, the packaging design is crucial in ensuring proper ventilation around the ACs.The alternative designs or operating under a lower airflow rate revealed a potential to cut down on energy use for ventilation. Specifically, when the airflow rate was reduced by one-quarter, there was a remarkable 94% decrease in energy usage. However, this benefit is counterbalanced by a 100% increase in the overall average HCT, which might adversely affect the product quality. Hence, optimizing the packaging design is essential to ensure the right balance between energy efficiency and product quality.The HCT exhibited a linear correlation with the external resistance, which is influenced by the airflow behavior, whatever the AC positions and tray designs.

Experimental simulation of temperature non-uniformity in a loaded container along air cargo supply chain: Mango case study

The cooling time of mango boxes (initial temperature 19 °C) at different positions in a Unit Load Device (ULD) located in a test room of 6 °C was monitored. Plaster mangoes were used due to the high perishability of real mangoes. The influence of box designs (closed and top-open) on the cooling time were studied. Both box designs showed shorter Half-cooling time (HCT) at the bottom than at the top. Closed boxes exhibited more uniform temperatures in the ULD. Heat transfer in closed boxes was primarily conduction, while natural convection played a significant role in top-open boxes. Then, the ULD was exposed to variable ambient temperatures to simulate real conditions in an air cargo supply chain. The results demonstrate that the critical step is the airport tarmac; after 2 h for example, the temperature at the top rose to 50 °C (with external air at 32 °C), resulting in a 10 °C increase in the mangoes in the top box.

Experimental and numerical investigation of spherical food product during forced convection cooling

Purpose of this study is to make improvements in the heat transfer behaviour during cooling of food products in forced convection environment. In the present study the effect of air velocity during pre-cooling of spherical food product i.e. orange is seen. The experiments are performed on an air blast apparatus and the results are compared with a 3D CFD model. This model is studied at (0.5, 1.0, 2.0, 2.5 and 3.0 m/s) velocities for orange. It is observed that the cooling is improved significantly up to a particular speed and increasing the speed above its critical speeds not enhance cooling significantly i.e. 2.0 m/s, hence is not advisable to increase the speed above its critical speed as it leads to loss of energy which in turn increases the overall operational cost. . The half cooling time(HCT) and the seven-eight cooling time(SECT) consequently decreases by 44.27% and 43.31% respectively for orange by a rise in air-inlet velocity from0.5 - 3.0 m/s.

Half-cooling time of cabbage stored in a refrigerated room

Cabbage heads stored in containers in a high-capacity cooling room were evaluated by the half-cooling times. The temperature of the heads and the surrounding air in their immediate vicinity was measured by a puncture thermometer. The shortest half-cooling times were measured in the immediate vicinity of the evaporator; in contrast, very long half-cooling times were measured in a container on the opposite side of the cooling room. The measured values were longer by one order of magnitude. When cabbage heads are inside the container furthest from the evaporator, without being covered by a layer of stacked containers, then the heat transfer passes directly into the ambient air, thus, the half-cooling times in this container were not the longest. The head cabbage cooling, evaluated by the half-cooling time (in hours) and the end of cooling time (in hours), in the chambers to a storage temperature (0 °C) are directly proportional. The heads in a container 14 m from the evaporator, in the bottom position, but not covered by other containers had a half-cooling time of 194.7 hours, with a total time of 973.5 h, because the air circulating around the container, permanently cooled down the stored heads. The total calculated cooling time will be extended 5.0 times. For the heads in the same position, but 7 m from the evaporator, the heat dissipation at the bottom position was so slow that the half-cooling time was 225 hours, and the total time was 1 125 hours (46.88 days), which was the longest cooling time for the cabbage heads.

Commercial forced-air precooling of strawberries: A temperature distribution and correlation study

Forced-air precooling of strawberries is rarely ideal due to the non-uniform cooling rate inside precooling facilities and the simple heuristics used to select precooling duration. The aim of this work was to investigate the product temperature distribution during commercial forced-air precooling of strawberries, quantify the relationship between the ambient and product temperature and assess precooling durations. Our field studies showed that the half-cooling time (HCT) on the inside of precooling tunnels is approximately twice the HCT on the outside, facing the incoming air, but is relatively constant along the length of the tunnel and vertically within pallets. The HCT on the outside of the tunnel is strongly correlated to the HCT on the inside, enabling temperature predictions from a limited number of sensors. Temperature variations of up to 7 °C were observed at the end of precooling, suggesting that precooling should be extended in some instances to improve uniformity.

An Evaluation of the Performance of Forced Air Cooling on Cooling Parameters in Transient Heat Transfer at Different Layers of Pomegranate

The quality of horticultural products can be promoted using high techniques. One of these methods is precooling applied before storage and leads to increased shelf and storage life of the fruit. For this reason, the effect of forced air cooling was conducted to investigate the cooling rate at the center (aril), spongy tissue (peel) and leathery skin (rind) of pomegranate (Punica granatum L.). Airflow velocity as an effective factor in cooling products at three levels of 0.5, 1, and 1.3 m s-1 and temperature of 7.2 °C was considered. Cooling parameters including lag factor and cooling coefficient were calculated from experimental data. Then, half-cooling time and seven-eighths cooling time were obtained at different layers of pomegranate. Cooling heterogeneity was analyzed at different air velocity and at different layers of pomegranate. The results showed that increase in air velocity from 0.5 to 1.3 m s-1, reduced the half-cooling time and seven-eighths cooling time. After 5000 seconds, the change of air velocity had a slight influence on decreasing temperature of different layers of pomegranate. Cooling heterogeneity at the air velocity of 0.5 m s-1 was low and then increased at the air velocity of 1 m s-1. Finally, at the air velocity of 1.3 m s-1, it was declined. The overall results illustrate that the applied methodology in this research, which explains unsteady heat transfer in the cooling process, can be performed in pomegranate or similarly shaped fruits.

Forced-air cooling of polylined horticultural produce: Optimal cooling conditions and package design

A 3-D computational fluid dynamics (CFD) model of palletised polylined kiwifruit packages undergoing forced-air cooling was used to determine an optimal operating point. Two operating conditions (pressure drop and flowrate across the pallet) were tested and the results evaluated based on cooling rate, uniformity, energy requirement and pallet throughput per week. An optimal operating point of 100Pa (0.25Lkg−1s−1) ensured relatively rapid cooling of the produce, without incurring excessive operational costs due to the energy requirements.Based on the results an alternative package was designed with the aim of redistributing the incoming refrigerated airflow to channel cool air through the pallet layers before directing it towards the slowest cooling packages, located at the back of the pallet. Evaluating the new design at the optimal conditions for the current package showed that at constant flowrate both pressure drop and energy requirement to achieve half-cooling time (HCT) were reduced by 24%, while improving cooling uniformity and pallet throughput per week. Alternatively, keeping the pressure drop constant required a similar energy input, while further increasing the cooling uniformity and pallet throughput.

Computational modeling of airflow and heat transfer in a vented box during cooling: Optimal package design

Optimization of fresh fruit packaging designs is required to reduce energy loss by minimizing the precooling time and to enhance fruit quality by providing more uniform cooling without inducing chilling injuries. In this work, a computational fluid dynamics (CFD) model is developed to study the airflow patterns and heat transfer inside an existing container and a newly developed container. The CFD model employs an unsteady-state approach based on a two-equation eddy-viscosity turbulence model (SST-κ–ω model). The cooling performance of the existing container and the new container are evaluated experimentally and numerically with the CFD model. The CFD results reveal a complex and uneven distribution of the airflow inside the existing vented package. Such airflow leads to a non-uniform temperature distribution over the produce, with a maximum temperature difference of ∼8 °C between two layers of stacked produce. For the new boxes, the half-cooling time and coefficient of temperature variation are about twofold less than those for the existing boxes, and the maximum temperature difference is ∼2.5 °C between two layers of stacked produce. Thus, the new package design clearly shows significant improvements in cooling performance. The numerical model is verified by comparing the simulation results to those of experiments, and the predicted results are consistent with the measured results. The maximum temperature deviation is less than 1.5 °C, and the maximum root-mean-square error and average relative error for produce temperature are 1.452 °C and 13.6%, respectively. This research provides a reliable theoretical and experimental basis for improving airflow and produce-temperature uniformity and for minimizing energy consumption during the forced-convection cooling of produce.

Theoretical Half-Cooling Time of Mass-Average Temperature of Fruits and Vegetables

Theoretical Half-Cooling Time of Mass-Average Temperature of Fruits and Vegetables

Modelling turbulent flow and heat transfer using macro-porous media approach used to predict cooling kinetics of stack of food products

A semi-empirical model was established in order to predict two-dimensional turbulent flow and temperature evolution of each product through stacked spherical food during cooling or heating by forced convection. The model compares the products to a macro-porous medium. It uses the Darcy–Forchheimer momentum equation to predict the average superficial velocity. Turbulence intensity inside the porous medium, is obtained from an original transport equation for turbulent kinetic energy in which generation and dissipation are explicitly related to superficial velocity. The energy equation for fluid and solid phases is considered taking into account one-dimensional heat conduction inside the spherical products and heat transfer between air and product surface depending on local velocity and turbulence intensity. Few parameters of the model have to be experimentally determined for arranged spherical products. The same approach can be used for other shapes and arrangements. The experimental thermal heterogeneity, characterised by half-cooling time dispersion, was in agreement with predicted values in the case of a two-dimensional pattern with two baffles.

Indirect Measurement Method for Laminar to Turbulent Airflow through Horticultural Produce Simulators

Sixty-four plastic balls were instrumented for use as horticultural produce simulators and strategically distributed in an orthogonal matrix along with 448 other plastic spheres to simulate precooling of column-stacked produce. This research tool was based on the correlation between the cooling rate of produce simulators and the air approach velocity. Since a range of airflow rates was already recognized as creating particular difficulties for indirect air velocity measurement, particular care was taken with the air velocity measurement of the theoretical range of each airflow pattern, namely, laminar, transient, and turbulent. Correlations were determined by measuring the half-cooling time of each instrumented ball positioned at a fixed location inside a ball matrix that was inserted into an open-ended ventilation tunnel and subjected to various airflows. The results obtained allowed the laminar flow pattern to be distinguished from the transient and turbulent patterns using a specific correlation relating the half-cooling time to the air velocity for this type of airflow pattern. However, a single equation was used to correlate half-cooling time to air velocity for transient and turbulent airflow patterns. The new calibration decreased the experimental error and standard deviation obtained with the indirect measurement method system previously developed.

Estudo comparativo do resfriamento de laranja valência com ar forçado e com água

Este trabalho apresenta um estudo para comparar o resfriamento rápido de laranja Valência (Citrus sinensis O.), com ar forçado e com água gelada. O sistema de resfriamento rápido com ar forçado operou com um fluxo de ar de 1.933m³/h (3 l/s por kg de produto resfriado), a uma temperatura de 1ºC e UR=88,4 ± 2,0%, com uma velocidade do ar em torno de 1m/s. Nos experimentos com água gelada foi utilizado um sistema de imersão, com uma capacidade de 0,23m³ de água a uma temperatura de aproximadamente 1ºC. Foi determinando o tempo meio e de sete oitavos do resfriamento, partindo das leituras de temperatura, perfazendo-se duas repetições em cada experimento. Os resultados mostraram que, o resfriamento do leito de frutas com ar forçado não é homogêneo, obtendo-se um tempo de sete oitavos do resfriamento que variaram de 107min a 170min, dependendo da posição do fruto no interior da embalagem. Já no sistema com água gelada, o resfriamento acontece uniformemente em todo em todo o leito de frutos, sendo o tempo médio de resfriamento de 57min.

Growth and Development of the Main Greek Oriental Tobacco Cultivars

Abstract A microwave generator and a closed-circuit wind tunnel were used to study thermoregulatory responses of tobacco leaf. Heating and cooling curves at various wind velocities showed the maximum reduction of steady-state temperature occurred at 10 cm / s. Wind gusts of high intermittency were very effective in transferring heat from leaf tissue; gust interval and dynamic equilibrium leaf temperature were found to be linearly related. Thermal-time constant and half-cooling time of a tobacco leaf were determined as a function of wind velocity under the conditions of the Newtonian law of cooling. It was determined that transpirational cooling of tobacco leaf in total darkness could occur if leaf temperature was raised above 40°C. This study confirmed that microwaves can be effectively used to study heat exchanges of flue-cured tobacco leaves in vivo under both continuous and fluctuating wind conditions.

Phase transformational kinetics and hardenability of low-carbon, boron-treated steels

The phase transformations and hardenability of 0.1 pct C boron-treated and boron-free steels containing Mn, Cr, Ni, or Cr plus Ni, and up to 1 pct Mo were studied. Continuous cooling transformation diagrams, hardenability characteristics, and diagrams of the ferrite start half-cooling time vs alloying were established. An unalloyed 0.1 pct C steel transforms diffusionally in the ferritic-pearlitic range when cooled from an austenitizing temperature, with a negligible contribution of the intermediate (bainitic) transformation occurring at very high rates of cooling. Molybdenum extends the range of the bainitic transformation and markedly delays the decomposition of austenite in the ferritic-pearlitic range. Boron treatment of the unalloyed (molybdenum-free) 0.1 pct C steel permits bainite formation over a wider range of fast cooling programs. At lower rates of cooling, the steel transforms diffusionally into ferrite and pearlite . Alloying additions of Mn, Cr, or Ni result in a slightly higher proportion of the bainitic transformation, which may occur over a wider range of cooling programs. When both nickel and chromium are present, a modest synergistic effect on the delay of the ferritic-pearlitic transformation may be noted. Introduction of molybdenum into all of the boron-treated 0.1 pct C steels strongly delays the decomposition of austenite into ferrite-pearlite structures and vastly expands the range of cooling programs that result in the formation of bainitic structures. In this important action, molybdenum is assisted to a smaller degree by alloying additions of manganese and chromium, and to a greater degree by nickel and chromium plus nickel. In all the steels studied, the alloying elements lower the temperatures of the bainitic transformation, thereby explaining, at least partly, the somewhat higher hardness for any specified cooling program. The observed beneficial effects of boron, molybdenum, and other alloying elements on the phase transformational behavior on continuous cooling are reflected in terms of higher hardenability.