Air Wet-bulb Temperature Research Articles

외기 온·습도 변화에 따른 종이재질 전열교환 엘리먼트 성능에 관한 연구

초록: ,. ·., · · ,...Abstract: An enthalpy exchanger in which heat and moisture transfer occur between the indoor and outdoorair operates at various outdoor conditions. In this study, the effect of the outdoor-air temperature and humidityon the performance of an enthalpy exchanger was experimentally investigated. An apparatus wasspecially-made to accurately measure the incoming and outgoing dry- and wet-bulb air temperatures as well asthe flow rates. Tests were conducted in constant-temperature and constant-humidity chambers at differentoutdoor temperatures and humidities. It is shown that the effectiveness of latent-heat exchange increases as therelative humidity increases; further, this effect exhibited minimal dependence on the absolute humidity.However, the effectiveness of sensible-heat exchange is independent of both temperature and humidity.

Boundary Conditions for Artificial Snow Production in the Austrian Alps*

Abstract To assess how meteorological conditions favorable for the production of artificial snow vary in time and space, wet-bulb temperatures are calculated using temperature and humidity data of 14 Austrian stations between October and April for 1948–2007 (station altitudes 585–3105 m MSL). Technical specifications of snow guns are used to define a wet-bulb temperature threshold value of −2°C for snowmaking and a relationship between wet-bulb temperature and snowmaking capacity. The Mann–Kendall nonparametric-trend test is used to detect monotonic long-term changes in air temperature, relative humidity, wet-bulb temperature, and number of snowmaking days. It is applied multiple times to overlapping time periods to capture significant trends on different time scales. Results show a marked, common air- and wet-bulb seasonal mean (October–April) temperature increase between +1.5° and +3.1°C from 1980 to 1990 for a majority of stations with no trends thereafter. The number of snowmaking days per season decreased by −20 to −34 for half of the stations in the period around 1979–2003. No altitudes were especially affected by changes in the analyzed variables. The estimated volume of produced artificial snow shows high interannual variability and exhibits no trends at an hourly resolution over the last two decades.

Analysis of Dehumidification Effects on Cooling Capacity of an Evaporative Cooler

In this study, effect of desiccant wheel, heat exchanger and cooling coil will be evaluated on decreasing the wet bulb temperature of entering air to cooling tower and decreasing the outlet cold water temperature. For this purpose, change effect of desiccant wheel parameters will be investigated on wet bulb temperature of outlet air from heat exchanger. After that, optimum parameters and minimum wet bulb temperature will be selected. Then, outlet cold water temperature will be achieved for various cooling coil surface temperature with definition of by pass factor and also by using optimum desiccant wheel parameters and entrance air wet bulb temperature to tower related to cooling coil surface temperature. To calculate wet bulb temperature, a mathematical model will be used that shows physical properties of air. After that a nomograph will be used to predict effect of decrease of entrance air wet bulb temperature on reducing the outlet water temperature and it will be done for several cities in Iran. At the end, an equation will be used to calculate required water to air mass flow rate for each outlet cold water temperature. With considering of known circulating water mass flow rate, required air for tower would be calculated and suitable desiccant wheel can be selected.

Experimental study of cooling effects of a passive evaporative cooling wall constructed of porous ceramics with high water soaking-up ability

As a passive cooling strategy aimed at controlling increased surface temperatures and creating cooler urban environments in summer, the authors developed a passive evaporative cooling wall (PECW) constructed of porous ceramics. These ceramics possess a capillary force to soak water, which means that their vertical surface is wet up to a level higher than 100 cm when their lower end is placed in water. The present paper describes an experiment that clarifies the cooling effects of a prototype PECW constructed of pipe-shaped ceramics. The PECW is capable of absorbing water and allows wind penetration, thus reducing its surface temperature by means of water evaporation. Passive cooling effects such as solar shading, radiation cooling, and ventilation cooling can be enhanced by incorporating PECWs into the design of outdoor or semi-outdoor spaces in parks, pedestrian areas and residential courtyards. The following findings were understood from an experimental data collected over a summer period. Wet vertical surfaces of the ceramic pipe reached a height of over 1 m at an outdoor location exposed to solar radiation. Wet surface conditions can be maintained throughout successive sunny days during summer. A slight difference in the vertical surface temperatures of the ceramic pipe was found. The air passing through the PECW was cooled, and its temperature can be reduced to a minimum value by several degrees during summer daytime. It was also found that the surface temperature of the shaded ceramic pipe can be maintained at a temperature nearly equal to the wet-bulb temperature of outdoor air.

DEVELOPMENT OF AN EVAPORATIVE COOLING WALL CONSTRUCTED OF POROUS CERAMICS WITH HIGH WATER SOAKING-UP ABILITY

The authors have developed a passive cooling wall (PCW) constructed of a porous ceramics. The ceramics has a capillary force to soak up water and allow its vertical surface to be wet to a level higher than 1 m when its lower end is placed in water. The present paper describes experiments to investigate the developed ceramics performance and cooling effects of a prototype PCW constructed of the pipe-shaped ceramics. The following findings were understood from experimental data monitored in a summer period. Wet vertical surfaces of the ceramic pipe reached a height of more than 1 m. Surface wet conditions can be maintained throughout successive sunny days in the summer. It was found that there is a slight difference in vertical surface temperatures of the ceramic pipe on a sunny summer day. The air passing through the PCW was cooled and its temperature can be reduced by several degrees in the summer daytime. It was also found that surface temperatures of the shaded ceramic pipe can be kept at a temperature nearly equal to the wet-bulb temperature of outdoor air.

Theoretical and experimental study of a cross-flow induced-draft cooling tower

The main objective of this study is to find a proper solution for the cross-flow water cooling tower problem, also to find an empirical correlation's controlling heat and mass transfer coefficients as functions of inlet parameters to the tower. This is achieved by constructing an experimental rig and a computer program. The computer simulation solves the problem numerically. The apparatus used in this study comprises a cross-flow cooling tower. From the results obtained, the 'characteristic curve' of cross-flow cooling towers was constructed. This curve is very helpful for designers in order to find the actual value of the number of transfer units, if the values of inlet water temperature or inlet air wet bulb temperature are changed. Also an empirical correlation was conducted to obtain the required number of transfer units of the tower in hot water operation. Another correlation was found to obtain the effectiveness in the wet bulb operation.

Evaporation of intercepted rainfall from isolated evergreen oak trees: Do the crowns behave as wet bulbs?

A new approach is suggested for estimating evaporation of intercepted rainfall from single trees in sparse forests. It is shown that, theoretically, the surface temperature of a wet tree crown will depend on the available energy and windspeed. But for a fully saturated canopy under rainy conditions, surface temperature will approach the wet bulb temperature when available energy tends to zero. This was confirmed experimentally from measurements of the radiation balance, aerodynamic conductance for water vapour and surface temperature on an isolated tree crown. Net radiation over a virtual cylindrical surface, enclosing the tree crown, was monitored by a set of radiometers positioned around that surface. Aerodynamic conductance for the tree crown was derived by scaling up measurements of leaf boundary layer conductance using the heated leaf replica method. Thermocouples were used to measure the average leaf surface temperature. Results showed that a fully wet single tree crown behaves like a wet bulb, allowing evaporation of intercepted rainfall to be estimated by a simple diffusion equation for water vapour, which is not restricted by the assumptions of one-dimensional transfer models usually used at the stand scale. Using this approach, mean evaporation rate from wet, saturated tree crowns was 0.27 or 0.30 mm h −1, when surface temperature was taken equal to the air wet bulb temperature or estimated accounting for the available energy, respectively.

Numerical investigation on the heat and mass transfer in a direct evaporative cooler

A simplified mathematical model is developed to describe the heat and moisture transfer between water and air in a direct evaporative cooler. The mass of evaporated water is treated as a mass source of air flow, and the related latent heat of water evaporation is taken as a heat source in the energy equation. The momentum caused by water evaporation is taken into account in the momentum equations. The effective air viscosity and diffusion coefficient are decided experimentally. The models and methods are validated by comparing the numerical results with those of experiment for the same evaporative cooler. The influences of the inlet frontal air velocity, pad thickness, inlet air dry-bulb and wet-bulb temperatures on the cooling efficiency of the evaporative cooler are calculated and analyzed. The cooling effects of the direct evaporative cooler are predicted for use in four different regions in northwest China using the present numerical method and local weather data for air conditioning design. The predicted results show the direct evaporative cooler with high performance pad material may be well applied for air conditioning with reasonable choices for the inlet frontal velocity and pad thickness.

Energy and exergy analysis of counter flow wet cooling towers

Cooling tower is an open system direct contact heat exchanger, where it cools water by both convection and evaporation. In this paper, a mathematical model based on heat and mass transfer principle is developed to find the outlet condition of water and air. The model is solved using iterative method. Energy and exergy analysis infers that inlet air wet bulb temperature is found to be the most important parameter than inlet water temperature and also variation in dead state properties does not affect the performance of wet cooling tower. .

Aplicação de métodos numéricos para estimativa de variáveis psicrométricas

O ar atmosférico é formado por uma composição de ar seco e vapor d'água, cujos valores da quantidade de vapor, em termos quantitativos, são variáveis em função dos elementos climáticos, tais como: temperatura e pressão. Portanto, para estimar os valores de umidade relativa do ar, são necessárias informações completas de séries de dados de temperatura do ar (Tar), pressão atual de vapor (e a) e temperatura de bulbo úmido (Tu). O trabalho apresenta como objetivo a elaboração de rotina computacional para encontrar valores da temperatura do termômetro de bulbo úmido (Tu) em função de outras variáveis, para uma série de dados obtidos, mediante a aplicação de métodos numéricos (Método de Newton-Raphson e Método da Secante), para fins de complementação de uma série de dados diários para Piracicaba - SP. Verificou-se que os métodos numéricos de Newton-Raphson e da Secante apresentaram bom desempenho na simulação dos resultados, porém o método da Secante apresentou-se como o melhor para encontrar os valores de Tu, mediante a resolução numérica da equação psicrométrica.

Thermodynamic study of wet cooling tower performance

An analytical model was developed to describe thermodynamically the water evaporation process inside a counter-flow wet cooling tower, where the air stream is in direct contact with the falling water, based on the implementation of the energy and mass balance between air and water stream describing thus, the rate of change of air temperature, humidity ratio, water temperature and evaporated water mass along tower height. The reliability of model predictions was ensured by comparisons made with pertinent experimental data, which were obtained from the literature. The paper elaborated the effect of atmospheric conditions, water mass flow rate and water inlet temperature on the variation of the thermodynamic properties of moist air inside the cooling tower and on its thermal performance characteristics. The analysis of the theoretical results revealed that the thermal performance of the cooling tower is sensitive to the degree of saturation of inlet air. Hence, the cooling capacity of the cooling tower increases with decreasing inlet air wet bulb temperature whereas the overall water temperature fall is curtailed with increasing water to air mass ratio. The change of inlet water temperature does not affect seriously the thermal behaviour of the cooling tower. Copyright © 2005 John Wiley & Sons, Ltd.

MEASURING AND MODELLING TRANSPIRATION VERSUS EVAPOTRANSPIRATION OF A TOMATO CROP GROWN ON SOIL IN A MEDITERRANEAN GREENHOUSE

The main sources of water vapour in a greenhouse are plant transpiration and the evaporation of water from soil (evapotranspiration). These processes are influenced by environmental factors such as solar radiation and water vapour pressure deficit, as shown by the Penman-Monteith equation. Most previous studies have been conducted in Northern countries and with soiless crops, which means that they only refer to transpiration. In Mediterranean greenhouses many crops are still grown on soil, so evaporation adds another component to the water vapour balance. The purpose of this study was: 1 – to measure the evapotranspiration (soil and crop) and transpiration (crop), of a tomato crop grown on a heavy soil using a lysimeter; 2 – to develop models as a function of the inside solar radiation and water vapour pressure deficit; 3 – to validate the models using data from different days and 4 to compare the models. Experiments were conducted in a Mediterranean unheated greenhouse covered with co-extruded PE-EVA-PE film, with a soil grown tomato crop, located in Lisbon. The orientation was east-west and ventilation was achieved by continuous apertures located on the side walls over its entire length. Trickle ferti-irrigation tubes were located between each two rows of plants. Climatic parameters, such as dry and wet bulb air temperatures, crop and leaf temperatures and solar radiation were measured and recorded using a data logger. Information on the evolution of the crop was also recorded. To compute evapotranspiration and transpiration a lysimeter was used and data of irrigation, drainage and soil moisture content were recorded. For the evaluation of the transpiration, the lysimeter was covered with a plastic film to prevent evaporation from the soil. To develop the models, data were recorded over periods of several days at different stages of plant development and another set of data from different periods was used for model validation. Comparison between measured and estimated data shows good agreement for both models. Differences between transpiration and evapotranspiration were very small, which reveals that, under the conditions experienced, evaporation from the soil can be neglected when trickle ferti-irrigation is used. INTRODUCTION Evapotranspiration of greenhouse crops is a very important tool for greenhouse climate control and irrigation management (Stanghellini, 1987; Yang et al., 1990; Jolliet and Bailey, 1992; Baille et al., 1994; Baptista et al., 2000; Rana and Katerji, 2000; Seginer, 2002). Consequently, it has been studied by several authors and several models Proc. IC on Greensys Eds.: G. van Straten et al. Acta Hort. 691, ISHS 2005 314 were published. Most of those models are based on the Penman-Monteith equation, being the transfer of the water vapour between the leaf and the air calculated as:

Improved condenser design and condenser-fan operation for air-cooled chillers

Air-cooled chillers traditionally operate under head pressure control via staging constant-speed condenser fans. This causes a significant drop in their coefficient of performance (COP) at part load or low outdoor temperatures. This paper describes how the COP of these chillers can be improved by a new condenser design, using evaporative pre-coolers and variable-speed fans. A thermodynamic model for an air-cooled screw-chiller was developed, within which the condenser component considers empirical equations showing the effectiveness of an evaporative pre-cooler in lowering the outdoor temperature in the heat-rejection process. The condenser component also contains an algorithm to determine the number and speed of the condenser fans staged at any given set point of condensing temperature. It is found that the chiller’s COP can be maximized by adjusting the set point based on any given chiller load and wet-bulb temperature of the outdoor air. A 5.6–113.4% increase in chiller COP can be achieved from the new condenser design and condenser fan operation. This provides important insights into how to develop more energy-efficient air-cooled chillers.

The sting at the end of the tail: Model diagnostics of fine‐scale three‐dimensional structure of the cloud head

AbstractThis is the second in a series of papers on ‘the sting at the end of the tail’—a mesoscale region of potentially damaging surface winds that can occur close to the evaporating tip of a cloud head wrapped around the bent‐back front of an extratropical cyclone. In the first paper the sting phenomenon was identified from a purely observational study of the great storm of October 1987. In the present paper an observationally validated forecast run from a high‐resolution numerical weather prediction model is used to examine the evolving three‐dimensional structure of the sting phenomenon. It is shown that the damaging surface winds in the October 1987 storm were due to a well‐defined mesoscale ‘sting jet’ (SJ), identifiable as a coherent ensemble of trajectories originating in cloudy air at about 650 hPa. There is evidence of multiple mesoscale slantwise circulations, and the SJ appears to form within the descending part of these circulations. Air within the SJ descends to the 900 hPa level over a period of about 4 h, during which time it accelerates from less than 20 to above 45 m s−1 with extremes greater than 50 m s−1. Although the wet‐bulb potential temperature of air in the SJ remains constant during its descent to 900 hPa, evaporation leads to a reduction of up to 5 K or more in dry‐bulb potential temperature in some parts of the jet. The SJ is situated behind the primary cold front, and is distinct from the associated warm‐conveyor‐belt low‐level jet; it is also distinct from the cold‐conveyor‐belt low‐level jet which remains below and behind it as the SJ skirts the bent‐back front. © Crown copyright, 2005.

Refinement of the Transfer Characteristic Correlation of Wet-Cooling Tower Fills

Transfer characteristic correlations given in the literature for wet-cooling tower fills are generally only a function of the air and water mass flow rates. This is a gross simplification of a very complex heat and mass transfer (evaporative cooling) process. In addition to the effects of the air and water mass flow rates, effects of the inlet water temperature, air drybulb temperature, wetbulb temperature, and fill height on the transfer characteristic, or Merkel number, are investigated in the present study. The accuracy of two different empirical equations is also evaluated. It is found that the transfer characteristic correlations for wet-cooling tower fills are functions of the inlet water temperature and fill height but not of the air drybulb and wetbulb temperatures.

Modeling of Thermal Performance of a Cooling Tower Using an Artificial Neural Network

In the present study, the ability of an artificial neural network model to evaluate the thermal performance of a cooling tower, which used in the heating, ventilating, and air conditioning industries to reject heat to the atmosphere, is examined. The network is trained with the following experimental values: the ratio of the water mass flow rate to air mass flow rate, the inlet water temperature, and the outlet water temperature, and the inlet air wet-bulb temperature are selected as input variables, while the output is the coefficient of performance. It is concluded that a well-trained neural network provides fast, accurate, and consistent results, making it an easy-to use tool for preliminary engineering studies.

Response of global lightning activity to air temperature variation

It is an issue of great attention but yet not very clear whether lightning activities increase or decrease on a warmer world. Reeve et al. presented that lightning activities in global land and the Northern Hemisphere land have positive response to the increase of wet bulb temperature at l000hPa. Is this positive response restricted only to wet bulb temperature or in land? What is the response of global lightning activities (in both land and ocean) to the global surface air temperature variation like? This paper, based on the 5-year or 8-year OTD/LIS satellite-based lightning detecting data and the NCEP reanalysis data, makes a reanalysis of the response of the global and regional lightning activities to temperature variations. The results show that on the interannual time scale the global total flash rate has positive response to the variation in global surface air temperature, with the sensitivity of 17±7% K−1. Also, the seasonal mean flash rate of continents all over the world and that of continents in the Northern Hemisphere have sensitive positive response to increase of global surface air temperature and wet bulb temperature, with the sensitivity of about 13±5% K−1, a bit lower than estimation of 40% K−1 in Reeve et al. However, the Southern Hemisphere and other areas like the tropics show no significant correlation.

Experimental Evaluation of One, Two, and Three Stage Evaporative Cooling Systems

The thermal performance of four different arrangements of evaporative air coolers have been evaluated experimentally during the hot summer of Kuwait. The systems include one-stage direct evaporative cooling (DEC), one-stage indirect evaporative cooler (IEC) linked to an external cooling tower, two-stage indirect/direct evaporative coolers (IEC/DEC), and three-stage system of evaporative cooling and mechanical vapor compression (IEC/DEC-MVC). Two variables are used to evaluate the system thermal performance, the thermal effectiveness and the energy efficiency ratio (EER). The data show that the IEC/DEC has the highest EER, followed by the DEC, IEC/DEC-MVC, and IEC. The DEC has the lowest effectiveness, succeeded by the DEC/IEC, IEC, and IEC/DEC-MVC. Coupling MVC with IEC/DEC extends the cooling range and can cool the ambient air dry bulb temperature over a range of 40°C to 15°C at any value of ambient air wet bulb temperature. Two experimental correlations have been developed for each one of the tested systems. The first one relates the effectiveness to water to air mass flow ratio (L/G) or Reynolds number (Re), while the second correlation relates the EER with effectiveness and L/G and/or Re. These relationships are very important in designing and optimizing the studied evaporative cooling units.

Thermal performance of a closed wet cooling tower for chilled ceilings: measurement and CFD simulation

A closed wet cooling tower, adapted for use with chilled ceilings in buildings, was tested experimentally. The thermal efficiency of the cooling tower was measured for different air flow rates, water flow rates, spray flow rates and wet bulb air temperatures. CFD was also used to predict the thermal performance of the cooling tower. Good agreement was obtained between CFD prediction and experimental measurement. Copyright © 2000 John Wiley & Sons, Ltd.

The modelling and potential applications of a simple solar regenerated grain cooling device

Abstract Cooling stored grains is an effective method of slowing the growth of populations of pest insects and maintaining desirable properties of the grains, such as seed viability. In temperate and sub-tropical climates, ventilation or aeration with ambient air at night usually results in reducing the wet-bulb temperature of the intergranular air, and this has a beneficial effect on storage. However, the relative humidity of ambient air at night is usually high, and this limits its cooling capacity. This paper describes a device that reduces the humidity of ambient air almost isothermally, and which therefore enables grain to be cooled to lower temperatures than those attainable using ambient air. The grain cooling device is simple to build using readily available materials, and requires only sheet metal working skills for its fabrication. Furthermore, the energy consumption of the device is very low and it uses solar energy to regenerate beds of desiccant. A mathematical model of the device is presented which enables the heat and mass transfer processes that occur in the device to be studied in detail. The model is validated against previously published experimental results. It is indicated that when operating in a subtropical climate the cooling device is capable of reducing the wet-bulb temperature of ambient air from about 17.3°C to about 12.3°C. Under these conditions the cooling device provides typically 50 times more cooling energy than electrical energy consumed. The model demonstrates that the degree of grain cooling that is possible decreases with increasing air flow rate, but the quantity of grain that can be cooled increases. The model is used to explore the effects of aerating grain with air conditioned by the solar cooler on insect population growth, the rate of pesticide decay and seed viability.