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.     

Modelling of a direct evaporative air cooler using artificial neural network

This paper proposes the use of artificial neural networks (ANNs) to predict various performance parameters of a direct evaporative air cooler. For this aim, an experimental evaporative cooler was operated at steady‐state conditions, while varying the dry bulb temperature and relative humidity of the entering air along with the flow rates of air and water streams. Using some of the experimental data for training, a three‐layer feed‐forward ANN model based on back propagation algorithm was developed. This model was used for predicting various performance parameters of the cooler, namely the dry bulb temperature and relative humidity of the leaving air, mass flow rate of the water evaporated into the air stream, sensible cooling rate, and effectiveness of the cooler. Then, the performance of the ANN predictions was tested by applying a set of new experimental data. The predictions usually agreed well with the experimental values with correlation coefficients in the range of 0.969–0.993, mean relative errors in the range of 0.66–4.04%, and very low root mean square errors. This study reveals that, as an alternative to classical modelling techniques, the ANN approach can be used successfully for predicting the performance of direct evaporative air coolers. Copyright © 2007 John Wiley & Sons, Ltd.     

Recent researches in indirect evaporative cooler V: relative thermal performance of buildings coupled to direct and indirect evaporative cooler

A mathematical model has been developed to evaluate the relative thermal performance of a building coupled with an indirect or direct evaporative cooler. Using periodic analysis for taking into account thermal storage of building envelope, explicit expressions have been obtained for room air temperature and room air humidity. For comparing their performance under different climatic conditions, numerical calculations have been made taking meteorological parameters for a typical day for Delhi (composite climate), Jodhpur (hot-dry climate) and Madras (hot-humid climate). It is found that the indirect evaporative cooler is a more effective and energy efficient system than the air-conditioner; it can hence be commercially used for computer and electronic exchange applications as well as for human comfort in a variety of climatic conditions, whereas direct evaporative cooler has limited use (only in hot-dry and composite climates). © 1997 by John Wiley & Sons, Ltd.     

Heat and mass transfer from a horizontal tube of an evaporative heat dissipator

In this investigation, the mass transfer coefficient with simultaneous water and air flow has been determined experimentally. The coefficient has also been calculated theoretically from the convective heat transfer coefficient of air by applying the Lewis relation for an air-water mixture. The ratio of experimental to theoretical mass transfer coefficients has been found to lie between 0.80 to 9.35. A new term ‘evaporative effectiveness’' is defined as the ratio of energy dissipated in evaporative cooling to that in simple water cooling. Its variation is studied and found to range from 0.85 to 1.78. Correlations for design purposes are recommended in this paper.     

膜加湿器热质交换过程的理论分析

膜加湿器是保证质子交换膜燃料电池(PEMFC)正常高效运行的重要组成部分.以燃料电池的板式膜加湿器为研究对象,根据热质交换原理对膜加湿器的传热传质过程进行了理论计算,分析了空气质量流量、膜内加湿侧进口温度和膜内加湿侧进口湿度对传热传质过程的影响.在传热方面:当空气质量流量不同时,随着膜内加湿侧进口温度的变化,膜内的热流量变化趋势不一致;当膜内加湿侧进口相对湿度为95%时,随着空气质量流量的变化,膜内热流量变化不大.在传质方面:当加湿侧进口相对湿度不变时,膜中水传输速率随着空气质量流量的增大而减小;当空气质量流量不变时,膜中水传输速率随着加湿侧进口相对湿度的增大而增大.     

Design and performance analysis of a small solar evaporative cooler

In this paper, the process of design, manufacturing, and performance analysis of a simple yet innovative solar evaporative cooler is illustrated. This investigation aims to evaluate the performance of a very small cooler with minimum energy consumption (10 W). A solar evaporative cooler was made out of various simple electrical and mechanical components. The cooler was tested in a specified room with the volume of 510?×?310?×?320 cm³. In order to evaluate the performance of the cooler, the air temperatures at different locations in the room was measured and compared during five consecutive days. The results showed that the performance of the cooler is relatively reasonable during the summer. However, its performance can be improved using high-performance cooling pads and solar panels. This simple evaporative cooler benefits from the ease of manufacturing process, cost effectiveness, and high portability.     

Theoretical analysis of heat and mass transfer to fluids flowing across a flat plate

Analytical solutions for heat and mass transfer to fluids flowing across an isothermal flat plate were obtained. Laminar, inviscid and irrotational flow were assumed in the solution of the energy equation. The derived new relations were comparable with the experimental data and the solutions of other investigators.     

Measurement of thickness and heat transfer coefficient of water film out of tube in an evaporative air cooler

Thickness of falling water film out of tubes is one of critical factors of heat transfer of evaporative air cooler. A new method of resistance measurement was developed to measure thickness of the film. When the resistance probe on the tip of micrometer touches the surface and bottom of the film, two corresponding sudden reductions of resistance occurs, and the difference of two graduations on the micrometer displays the thickness of the film. The film thickness of eleven angles was measured in five kinds of water flows and results varies from 0.8933mm to 1.7233 mm. Mean thickness and mean heat transfer coefficient of the film out of the tube was calculated.     

Characterization of a semi-indirect evaporative cooler

This paper presents the experimental study of a semi-indirect evaporative cooler (SIEC), which acts as an energy recovery device in air conditioning systems. Experimental measurements were achieved for the characterization of the thermal performance of the device, employing the design of experiments (DOE) methodology and an analysis of variance (ANOVA).     

Heat and mass transfer from a single horizontal tube of an evaporative tubular heat exchanger

In this investigation, water side heat transfer coefficients without air flow from a single horizontal tubes are determined. Mass transfer coefficients are determined with water and air flow from the same tube. The total energy dissipated by inside hot fluid when only water is falling is compared with that when both the air and water flow past the tube. The water side heat transfer coefficient and mass transfer coefficient are given by empirical relations h_w = 6.0(Re_p)^0.18(Re_w)^0.87 and K = 3.5(Re_p)^0.18(Re_a)^0.28 (Re_w)^0.54, respectively. The ratio of energies dissipated with water and air flow and with only water flow increases with Re_w and Re_a and its maximum value is 1.72 in the range of variables used.     

窗式直接蒸发冷却器在干燥地区居住建筑的应用研究

为了对室内空气降温的同时增加大量新风的补充,采用直流式全新风模式的蒸发冷却空调给室内降温。对使用窗式直接蒸发冷却器降温的新疆乌鲁木齐市天山区某住宅房间送风温湿度进行测试,数据表明,进风口温度在29℃,出风口温度为22℃,湿球效率为72%,室内温度稳定在25℃,相对湿度为59%,温湿度满足舒适性要求。     

Effect of interfacial phenomena on evaporative heat transfer in micro heat pipes

A three-dimensional steady-state model for predicting heat transfer in a micro heat pipe array is presented. Three coupled models, solving the microregion equations, the two-dimensional wall heat conduction problem and the longitudinal capillary two-phase flow have been developed. The results, presented for an aluminium/ammonia triangular micro heat pipe array, show that the major part of the total heat input in the evaporator section goes through the microregion. In addition, both the apparent contact angle and the heat transfer rate in the microregion increase with an increasing wall superheat. It is also shown that the inner wall heat flux and temperatures as well as the contact angle decrease all along the evaporator section.     

LHP圆盘式蒸发器三维传热与流动分析

通过补充相变蒸发界面传热传质热力学关系式,构建了回路热管(loop heat pipe,LHP)圆盘式蒸发器的流动与传热多区域耦合分析三维数学物理模型,并基于FLUENT软件对某种甲醇-不锈钢平板型蒸发器内的流动与传热情况进行了数值求解。数值分析结果表明,蒸发器内的传热与流动受几何结构影响明显,表现出较强的方向差异;不同热负荷条件下,补偿腔内流体的流动与传热特性呈现出较大差别,受到回流液速度和温度、毛细芯界面蒸发质量流量、毛细芯反向导热和侧壁漏热等多种因素共同影响。计算方法和研究结果,可以为平板型蒸发器内流动与传热特性的定量分析提供依据。     

Computational analysis of heat and mass transfer in a micro-structured PEMFC cathode

This paper presents a two-dimensional computational model of micro-structured cathodic electrodes featuring nano-porous gas diffusion media that increase the surface to volume ratio of the deposited catalyst layer. A key objective of the modeling effort is to elucidate the Knudsen diffusion within the nano-porous electrode. In addition, the model is non-isothermal and resolves the natural convection in the ambient air directly above the cathode. A major component of the analysis is the identification of mass transport limitations introduced by the micro-structured electrode and the nano-porous gas diffusion layer. Furthermore, the potential for passive regulation of temperature and oxygen supply is investigated. This analysis of the natural convection is unique because it examines buoyancy effects above a horizontal plate with variation in thermal conductivity, segregated regions of porous media, and discrete heating locations. The results of this study indicate that this electrode configuration is feasible and does not suffer from insurmountable mass transfer limitations.     

Maxwell nanofluid heat and mass transfer analysis over a stretching sheet

The Buongiorno model Maxwell nanofluid flow, heat and mass transfer characteristics over a stretching sheet with a magnetic field, thermal radiation, and chemical reaction is numerically investigated in this analysis. This model incorporates the effects of Brownian motion and thermophoresis. The governing partial differential equations are transformed into a coupled nonlinear ordinary differential equation by using the similarity transformation technique. The resultant nonlinear differential equations are solved by using the Finite element method. The sketches of velocity, temperature and concentration with diverse values of magnetic field parameter (0.1 ≤ M ≤ 1.5), Deborah number (0.0 ≤ β ≤ 0.19), radiation parameter (0.1 ≤ R ≤ 0.7), Prandtl number (0.5 ≤ Pr ≤ 0.8), Brownian motion parameter (0.1 ≤ Nb ≤ 0.7), thermophoretic parameter (0.2 ≤ Nt ≤ 0.8), Chemical reaction parameter (1.0 ≤ Cr ≤ 2.5) and Lewis number (1.5 ≤ Le ≤ 3.0) have investigated and are depicted through plots. Moreover, the values of the Skin-friction coefficient, Nusselt number, and Sherwood numbers are also computed and are shown in tables. The sequels of this analysis reviewed that the values of Skin-friction coefficient and Sherwood number intensified with hiked values of Deborah number (β), whereas, the values of Nusselt number decelerate as values of (β) improves.     

Theoretical analysis of heat transfer in laminar pulsating flow

Pulsation effect on heat transfer in laminar incompressible flow, which led to contradictory results in previous studies, is theoretically investigated in this work starting from basic principles in an attempt to eliminate existing confusion at various levels. First, the analytical solution of the fully developed thermal and hydraulic profiles under constant wall heat flux is obtained. It eliminates the confusion resulting from a previously published erroneous solution. The physical implications of the solution are discussed. Also, a new time average heat transfer coefficient for pulsating flow is carefully defined such as to produce results that are both useful from the engineering point of view, and compliant with the energy balance. This rationally derived average is compared with intuitive averages used in the literature. New results are numerically obtained for the thermally developing region with a fully developed velocity profile. Different types of thermal boundary conditions are considered, including the effect of wall thermal inertia. The effects of Reynold and Prandtl numbers, as well as pulsation amplitude and frequency on heat transfer are investigated. The mechanism by which pulsation affects the developing region, by creating damped oscillations along the tube length of the time average Nusselt number, is explained.     

Thermal and hydraulic performance of a modified two-stage evaporative cooler

The thermal and hydraulic performance of a modified two-stage evaporative cooler is evaluated. Variables considered are the mode of operation, packing thickness, mass flow rate of the water flowing to the precooler, and the mass flux of water flowing over the packing media. The effectiveness of the system increased with the increase of the mass flow rate of water flowing to the precooler, decreasing the mass flux of water flowing to the packing, and with the increase of the packing thickness. The effectiveness of the system with structured packing was higher than that with sheathy leaf base or natural fiber packing.The air-side pressure drop per unit length in the direction of air flow was nearly constant when the structured packing was used. For the sheathy leaf and natural fiber packings, the air pressure drop increased at a uniform rate as the mass flux of water flowing over the packing increased. The air pressure drop was lowest for the setup with the structured packing.     

Recent research on an indirect evaporative cooler. Part VI: evolution of design pattern for indirect evaporative cooler

A rule of thumb for indirect evaporative cooler has been derived in terms of the size of floor area to be cooled and design parameters of tube type IEC, viz. process stream air flow rate and number of tubes; the three Indian climate zones, namely hot–dry, composite and warm–humid, represented by three cities of Jodhpur, Delhi and Madras, respectively, have been considered. Copyright © 1999 John Wiley & Sons, Ltd.     

Numerical analysis of heat and mass transfer in the capillary structure of a loop heat pipe

The heat and mass transfer in the capillary porous structure of a loop heat pipe (LHP) is numerically studied and the LHP boiling limit is investigated. The mass, momentum and energy equations are solved numerically using the finite element method for an evaporator cross section. When a separate vapor region is formed inside the capillary structure, the shape of the free boundary is calculated by satisfying the mass and energy balance conditions at the interface. The superheat limits in the capillary structure are estimated by using the cluster nucleation theory. An explanation is provided for the robustness of LHPs to the boiling limit.