Effective diffusivity and evaporative cooling in convective drying of food material

This article presents mathematical models to simulate coupled heat and mass transfer during convective drying of food materials using three different effective diffusivities: shrinkage dependent, temperature dependent and average of those two. Engineering simulation software COMSOL Multiphysics was utilized to simulate the model in 2D and 3D. The simulation results were compared with experimental data. It is found that the temperature dependent effective diffusivity model predicts the moisture content more accurately at the initial stage of the drying, whereas, the shrinkage dependent effective diffusivity model is better for the final stage of the drying. The model with shrinkage dependent effective diffusivity shows evaporative cooling phenomena at the initial stage of drying. This phenomenon was investigated and explained. Three dimensional temperature and moisture profiles show that even when the surface is dry, inside of the sample may still contain large amount of moisture. Therefore, drying process should be carefully dealt with otherwise microbial spoilage may start from the centre of the ‘dried’ food. A parametric investigation has been conducted after the validation of the model.

Evaluating the potential use of direct evaporative cooling in Australia

Greenhouse gas emissions and associated global climate change are a significant and growing concern for the world community. In order to improve building energy efficiency, the use of evaporative cooling systems is attracting growing attention. Using a climate assessment tool, the potential use of direct evaporative coolers over different Australian climates is evaluated. It is found that overall, the potential use of direct evaporative cooling is very significant in Australian climates. Among all the eight capital cities across Australia, except for Darwin, the need of hybrid cooling for other cities is found to be insignificant,and is less than 5% if appropriate air circulation is provided on hot/warm days. It is also found that the potential use of evaporative cooling can be significantly influenced by a change in the applications or design parameters. In Brisbane, it is estimated that with an increase of sensible cooling load from 30 W/m2 to 40 W/m2 in the conditioned space, the requirement in hours of hybrid cooling can increase significantly, from 4.06% to 14.89%.

Performance analysis of solar desiccant-evaporative cooling for a commercial building under different Australian climates

This paper evaluates and compares the system performance of a solar desiccant-evaporative cooling (SDEC) system with a referenced conventional variable air volume (VAV) system for a typical office building in all 8 Australian capital cities. A simulation model of the building is developed using the whole building simulation software EnergyPlus. The performance indicators for the comparison are system coefficient of performance (COP), annual primary energy consumption, annual energy savings, and annual CO2 emissions reduction. The simulation results show that Darwin has the most apparent advantages for SDEC system applications with an annual energy savings of 557 GJ and CO2 emission reduction of 121 tonnes. The maximum system COP is 7. For other climate zones such as Canberra, Hobart and Melbourne, the SDEC system is not as energy efficient as the conventional VAV system.

Light-induced evaporative cooling in a magneto-optical trap

This paper presents an experimental demonstration of light-induced evaporative cooling in a magneto-optical trap. An additional laser is used to interact with atoms at the edge of the atomic cloud in the trap. These atoms get an additional force and evaporated away from the trap by both the magnetic field and laser fields. There remaining atoms have lower kinetic energy and thus are cooled. It reports the measurements on the temperature and atomic number after the evaporative cooling with different parameters including the distance between the laser and the centre of the atomic cloud, the detuning, the intensity. The results show that the light-induced evaporative cooling is a way to generate an ultra-cold atom source.

Pulsed evaporative cooling of ion cloud in an electron beam ion trap

A technique for producing cold ensembles of trapped highly charged ions is described. The ions, trapped in an electron beam ion trap, can undergo a drastic contraction during the pulsed mode of evaporative cooling, if a truncated Boltzmann distribution is assumed. The underlying theory and the experimental results are presented.

Numerical study of a M-cycle cross-flow heat exchanger for indirect evaporative cooling

In this paper, numerical analyses of the thermal performance of an indirect evaporative air cooler incorporating a M-cycle cross-flow heat exchanger has been carried out. The numerical model was established from solving the coupled governing equations for heat and mass transfer between the product and working air, using the finite-element method. The model was developed using the EES (Engineering Equation Solver) environment and validated by published experimental data. Correlation between the cooling (wet-bulb) effectiveness, system COP and a number of air flow/exchanger parameters was developed. It is found that lower channel air velocity, lower inlet air relative humidity, and higher working-to-product air ratio yielded higher cooling effectiveness. The recommended average air velocities in dry and wet channels should not be greater than 1.77 m/s and 0.7 m/s, respectively. The optimum flow ratio of working-to-product air for this cooler is 50%. The channel geometric sizes, i.e. channel length and height, also impose significant impact to system performance. Longer channel length and smaller channel height contribute to increase of the system cooling effectiveness but lead to reduced system COP. The recommend channel height is 4 mm and the dimensionless channel length, i.e., ratio of the channel length to height, should be in the range 100 to 300. Numerical study results indicated that this new type of M-cycle heat and mass exchanger can achieve 16.7% higher cooling effectiveness compared with the conventional cross-flow heat and mass exchanger for the indirect evaporative cooler. The model of this kind is new and not yet reported in literatures. The results of the study help with design and performance analyses of such a new type of indirect evaporative air cooler, and in further, help increasing market rating of the technology within building air conditioning sector, which is currently dominated by the conventional compression refrigeration technology.

Comparative study of the performance of the M-cycle counter-flow and cross-flow heat exchangers for indirect evaporative cooling - paving the path towards sustainable cooling of buildings

This paper provides a comparative study of the performance of cross-flow and counter-flow M-cycle heat exchangers for dew point cooling. It is recognised that evaporative cooling systems offer a low energy alternative to conventional air conditioning units. Recently emerged dew point cooling, as the renovated evaporative cooling configuration, is claimed to have much higher cooling output over the conventional evaporative modes owing to use of the M-cycle heat exchangers. Cross-flow and counter-flow heat exchangers, as the available structures for M-cycle dew point cooling processing, were theoretically and experimentally investigated to identify the difference in cooling effectiveness of both under the parallel structural/operational conditions, optimise the geometrical sizes of the exchangers and suggest their favourite operational conditions. Through development of a dedicated computer model and case-by-case experimental testing and validation, a parametric study of the cooling performance of the counter-flow and cross-flow heat exchangers was carried out. The results showed the counter-flow exchanger offered greater (around 20% higher) cooling capacity, as well as greater (15%–23% higher) dew-point and wet-bulb effectiveness when equal in physical size and under the same operating conditions. The cross-flow system, however, had a greater (10% higher) Energy Efficiency (COP). As the increased cooling effectiveness will lead to reduced air volume flow rate, smaller system size and lower cost, whilst the size and cost are the inherent barriers for use of dew point cooling as the alternation of the conventional cooling systems, the counter-flow system is considered to offer practical advantages over the cross-flow system that would aid the uptake of this low energy cooling alternative. In line with increased global demand for energy in cooling of building, largely by economic booming of emerging developing nations and recognised global warming, the research results will be of significant importance in terms of promoting deployment of the low energy dew point cooling system, helping reduction of energy use in cooling of buildings and cut of the associated carbon emission.

A probabilistic analysis of the future potential of evaporative cooling systems in a temperate climate

The probabilistic projections of climate change for the United Kingdom (UK Climate Impacts Programme) show a trend towards hotter and drier summers. This suggests an expected increase in cooling demand for buildings – a conflicting requirement to reducing building energy needs and related CO2 emissions. Though passive design is used to reduce thermal loads of a building, a supplementary cooling system is often necessary. For such mixed-mode strategies, indirect evaporative cooling is investigated as a low energy option in the context of a warmer and drier UK climate. Analysis of the climate projections shows an increase in wet-bulb depression; providing a good indication of the cooling potential of an evaporative cooler. Modelling a mixed-mode building at two different locations, showed such a building was capable of maintaining adequate thermal comfort in future probable climates. Comparing the control climate to the scenario climate, an increase in the median of evaporative cooling load is evident. The shift is greater for London than for Glasgow with a respective 71.6% and 3.3% increase in the median annual cooling load. The study shows evaporative cooling should continue to function as an effective low-energy cooling technique in future, warming climates.

Simulation of the future performance of low-energy evaporative cooling systems using UKCP09 climate projections

Recent activity in the development of future weather data for building performance simulation follows recognition of the limitations of traditional methods, which have been based on a stationary (observed) climate. In the UK, such developments have followed on from the availability of regional climate models as delivered in UKCIP02 and recently the probabilistic projections released under UKCP09. One major area of concern is the future performance and adaptability of buildings which employ exclusively passive or low-energy cooling systems. One such method which can be employed in an integral or retrofit situation is direct or indirect evaporative cooling. The effectiveness of evaporative cooling is most strongly influenced by the wet-bulb depression of the ambient air, hence is generally regarded as most suited to hot, dry climates. However, this technology has been shown to be effective in the UK, primarily in mixed-mode buildings or as a retrofit to industrial/commercial applications. Climate projections for the UK generally indicate an increase in the summer wet-bulb depression, suggesting an enhanced potential for the application of evaporative cooling. The paper illustrates this potential by an analysis of the probabilistic scenarios released under UKCP09, together with a detailed building/plant simulation of case study building located in the South-East of England. The results indicate a high probability that evaporative cooling will still be a viable low-energy technique in the 2050s.

Evaporative cooling and cutaneous surface temperature of Holstein cows in tropical conditions

Foram estudados os efeitos da temperatura cutânea (Ts) sobre a taxa de termólise por evaporação cutânea (Es) de vacas Holandesas cronicamente expostas ao sol, considerando a pigmentação do pelame. Dezesseis vacas puras de origem foram medidas quanto à evaporação e à temperatura cutâneas às 13 h, após 6 horas de exposição ao sol, no mesmo local (flanco, pescoço e glúteo) e considerando separadamente as malhas negras e as brancas. A evaporação cutânea foi medida por meio de cápsula ventilada. Nas áreas negras a taxa de sudação (138,9 ± 8,5 g.m-2.h-1), a taxa de termólise por evaporação cutânea (93,3 ± 5,7 W.m-2) e a temperatura da superfície cutânea (33,1 ± 0,2°C) foram maiores que nas áreas brancas (109,5 ± 9,7 g.m-2.h-1, 73,6 ± 6,5 W.m-2 e 32,6 ± 0,2°C, respectivamente). Há uma relação exponencial entre evaporação e temperatura cutâneas, que pode ser representada pela equação Es = 31,5+3,67 exp{(Ts-27,9)/2,19115}, com coeficiente de determinação r²=0,68. A taxa de termólise por evaporação cutânea permanece quase constante (cerca de 48 W.m-2) até que a temperatura cutânea atinge aproximadamente 31°C.

Assessing maintenance of evaporative cooling systems in Legionellosis outbreaks

Evaporative cooling systems continue to be associated with outbreaks of Legionnaires’ disease despite widely available maintenance guidelines intended to reduce these outbreaks. Yet, the guidelines vary widely regarding the recommendations that are made to maintain evaporative cooling systems and it is unclear whether guidelines were in place or, if they were, whether they were being followed when the outbreaks of Legionnaires’ disease occurred. Thus, this study was designed to conduct two systematic reviews of (1) evaporative cooling system maintenance guidelines; and (2) published Legionnaires’ disease outbreaks. For each maintenance guideline identified in the systematic review, recommended maintenance practices were abstracted and similarities and/or differences in the reported recommendations were assessed. Following the systematic review of outbreak investigations that meet the inclusion criteria established for the study, information about the state of the evaporative cooling system during the outbreak investigation was abstracted to summarize, when reported, which maintenance practices were implemented. As expected, the recommended maintenance procedures varied greatly across the guidelines and were not always specific. Overall, the outbreak investigations tended to report similar maintenance issues that were unclear in the maintenance guidelines. Generally, these maintenance issues were biocide use, microbiological testing, frequency of general inspections, and protocols and frequency of total system cleanings. The role in which non-standardized and generalized maintenance guidelines plays in the continued association between Legionnaires’ disease and evaporative cooling systems is still not fully understood. However, this study suggests that more specific and standardized maintenance guidelines, that have been scientifically established to be effective in controlling Legionella bacteria, are needed and then these guidelines must be properly implemented in order to help reduce further Legionnaires’ disease outbreaks associated with evaporative cooling systems.^

Caracterización de la cerámica para su empleo como material de enfriamiento evaporativo en la envolvente de los edificios = Ceramic characterization towards its application as evaporative cooling material in building envelopes

El presente trabajo de investigación determina las características de la cerámica que más eficientemente se comporta a evaporación y a enfriamiento. Con el objeto de ser empleado como material integrado en la envolvente de los edificios para reducir su carga de refrigeración. La cerámica es un buen material para ser empleado para la refrigeración por evaporación. Es un sólido poroso inerte que, tras ser sometido a cocción a temperaturas por encima de los 900ºC, resulta uno de los materiales que mejor se comportan como contenedor de agua en su red capilar para, posteriormente, ir liberándola por evaporación al mismo tiempo que se enfría su superficie. La metodología general de investigación, se divide en tres etapas: Búsqueda y análisis del estado de la técnica y de la investigación. Estudio teórico de la eficacia del enfriamiento evaporativo como estrategia de enfriamiento pasivo en la arquitectura. Etapa experimental, desarrollada en tres fases: una primera de definición de los parámetros determinantes del Enfriamiento Evaporativo en piezas cerámicas, una segunda de selección cerámica y diseño de ensayos experimentales y una tercera de caracterización de la cerámica bajo criterios de evaporación y de enfriamiento. El recorrido por el estado de la cuestión ha identificado las aplicaciones tecnológicas y las investigaciones científicas que emplean el Enfriamiento Evaporativo con piezas cerámicas como técnica de enfriamiento. Como resultado se ha obtenido una tabla de clasificación de sistemas de enfriamiento evaporativo y se ha constatado que el conjunto de las aplicaciones están centradas en el diseño de piezas o sistemas pero que, sin embargo, no existe una definición de las características de la cerámica para su empleo como material de enfriamiento por evaporación. El estudio teórico de la eficacia del empleo del enfriamiento evaporativo como estrategia de enfriamiento pasivo en la arquitectura se ha realizado mediante cálculos de porcentaje de ampliación de horas en confort con empleo de técnicas de enfriamiento evaporativo directo e indirecto (EED y EEI). Como resultado se obtienen unos mapas para el ámbito español de potencial de aplicación del EED y EEI. Los resultados permiten afirmar que mediante EE se puede llegar a confort en prácticamente la totalidad de las horas de los días más cálidos del año en muchas localidades. La metodología experimental se ha desarrollado en tres fases. En la fase inicial, se han definido los parámetros determinantes del enfriamiento evaporativo en un medio cerámico mediante ensayos experimentales de capacidad de evaporación y de caracterización. Se realizaron un total de 12 ensayos. Se determinó que el material cerámico tiene una gran influencia en la capacidad de evaporación y enfriamiento en las piezas cerámicas, apoyando la hipótesis inicial y la necesidad de caracterizar el material. La primera fase empírica se centró en la selección cerámica y el diseño de los ensayos experimentales de comportamiento hídrico. Se seleccionaron muestras de 5 tipos de cerámica. Se realizaron 4 tipos de ensayos de caracterización y 6 tipos de ensayos experimentales de comportamiento hídrico (total 123 muestras ensayadas). Los resultados obtenidos son de dos tipos, por un lado, se determinó cuál es el tipo de cerámica que más eficientemente se comporta a EE y, por otro, se rediseñaron los ensayos de la última fase experimental. Para la segunda fase experimental se seleccionaron cerámicas de fabricación manual abarcando el mayor número de localidades del ámbito español. Se realizaron ensayos de caracterización de 7 tipos y ensayos de comportamiento hídrico de 5 tipos (total 197 muestras ensayadas). Los resultados de caracterización han permitido aportar unos rangos de las características de la cerámica que más eficientemente se comporta en los ensayos de comportamiento hídrico. Al final de la investigación se ha caracterizado el material cerámico aportando características acerca de su porosidad, capacidad de absorción, color, rugosidad y mineralogía. Así como datos de referencia de su comportamiento hídrico. Además se ha desarrollado una metodología de ensayo específica que permite evaluar la capacidad de enfriamiento eficiente de una pieza cerámica. ABSTRACT The purpose of this research is to determine the characteristics of ceramic materials having the most efficient performance in terms of evaporation and cooling, so that they can be integrated in building envelopes to reduce cooling loads. Ceramics are suitable materials for cooling through passive evaporation. After being fired at temperatures over 900 °C (1,652 °F), the capillary network of this inert porous medium turns to be excellent to retain water, which is progressively liberated by evaporation while the material surface gets colder. Research methodology has involved the following steps: Search and analysis on the state of the art in technology and research. Theoretical study on the efficiency of evaporation as passive cooling strategies in buildings. Experimental stage developed in three phases, namely: definition of parameters determining evaporative cooling in ceramic elements; ceramic selection and design of experimental tests; characterization of ceramic materials under evaporation and cooling criteria. Search and analysis on the state of the art in this field have been useful to identify technology applications and scientific research where ceramics are employed for evaporative cooling. The resulting table shows that applications are wholly focused on the design of pieces and systems. Nonetheless, there is lack of definition of material characteristics in this scope. The theoretical study on efficiency of the passive strategy applied to buildings has been realized by calculation of the percentage increase in comfort hours through direct/indirect evaporative cooling techniques (DEC/IEC). The mapping of their potential application in Spain clearly shows that comfort conditions can be reached in almost all the hours of the hottest days in many towns. In the initial phase of the experimental stage, parameters determining evaporative cooling in ceramic media have been defined. For this purpose, characterization tests and evaporation and cooling rates experiments have been carried out; the number of samples tested amounted to 12. It has been concluded that material characteristics have great influence on these rates, which supports the initial hypothesis and the need for their characterization. The first empirical phase has focused on ceramic selection and design of water behaviour experimental methods. The samples covered five different kinds of ceramic materials. Four different characterization tests and six different water behaviour experiments were carried out; the number of samples tested amounted to 123. The experimental testing procedures served to determine the most efficient types of ceramic materials in terms of evaporative cooling efficiency and, at the same time, made it necessary to change the original designed experimental test for the last phase. In the second phase, a number of varied hand-made ceramic tiles have been selected. Seven different characterization tests and five different water behaviour tests were carried out; the number of samples amounted to 197. The results of characterization served to establish a range of features in ceramic materials according to their efficiency in water behaviour experiments. Finally, ceramic materials have been characterized according to porosity, water absorption, colour, surface roughness and mineralogy. Also, reference data regarding water behaviour have been included. Moreover, an innovative and specific experimental test to evaluate cooling efficiency of ceramic tiles has been developed.