Prediction of Evaporation Losses in Wet Cooling Towers

The accurate prediction of all aspects of cooling tower behavior is very important. Accurately predicting evaporation losses is significant because water in cooling towers is cooled primarily through the evaporation of a portion of the circulating water, which causes the concentration of dissolved solids and other impurities to increase. An empirical relation is developed on the basis of ASHRAE's rule of thumb that is simple and accurate with a wide range of applicability. The predicted values are in good agreement with experimental data as well as predictions made by an accurate mathematical model.     

Simulation of boil-off losses during transfer at a LH2 based hydrogen refueling station

Losses along the LH₂ pathway are intrinsic to the utilization of a cryogenic fluid. They occur when the fluid is transferred between 2 vessels (liquefaction plant to trailer, trailer to station storage, station storage to pump or compressor, then possibly onto fuel cell electric vehicles …) and when it is warmed up due to heat transfer with the environment. Those losses can be estimated with good accuracy using thermodynamic models based on the conservation of mass and energy, provided that the thermodynamic states are correctly described. Indeed, the fluid undergoes various changes as it moves along the entire pathway (2 phase transition, sub-cooled liquid phase, super-heated warming, non-uniform temperature distributions across the saturation film) and accurate equations of state and 2 phase behavior implementations are essential. The balances of mass and energy during the various dynamics processes then enable to quantify the boil-off losses. In this work, a MATLAB code previously developed by NASA to simulate rocket loading is used as the basis for a LH₂ transfer model. This code implements complex physical phenomena such as the competition between condensation and evaporation and the convection vs. conduction heat transfer as a function of the relative temperatures on both sides of the saturated film. The original code was modified to consider real gas equations of state, and some semi-empirical relationships, such as between the heat of vaporization and the critical temperature, were replaced by a REFPROP equivalent expression, assumed to be more accurate. Non-constant liquid temperature equations were added to simulate sub-cooled conditions. The model shows that under environmental heat transfer only the liquid phase of a LH₂ vessel would experience cooling, while the boil-off is mainly a result of evaporation from the saturation film onto the vapor phase. Under the conditions assumed for this work, it was also concluded that the actual LH₂ density was lower than the corresponding saturation density given by the working pressure of the vessel. During a bottom fill transfer, for example from a LH₂ trailer to an on-site stationary vessel, it is shown that the boil-off losses are due to the compression of the vapor phase (“pdV” force). The model indicates that the magnitude of those losses is not dependent on the regulated pressure in the receiving vessel but is rather a function of the initial pressure in the vessel, amounting to more than 12% of losses for a vessel initially at 100 psia. At last, the model is used to estimate the amount of vapor H₂ vented when depressurizing a LH₂ trailer following a LH₂ delivery.     

不同加热功率下充液率对无芯环路热管性能的影响

设计了以铝为管材、丙酮为传热工质的无芯环路热管。其蒸发段采用加热带加热,冷凝段用风冷降温。热管依靠蒸发压头使工质循环,并依靠重力作用,使冷凝液回流到蒸发段。搭建试验台并研究了不同加热功率下充液率对无芯环路热管的传热温差、传热量、热效率、热阻和当量导热系数的影响。结果表明:加热功率为150.00 W、充液率为30%时,无芯环路热管的均温性最好;传热温差和热阻均最小,分别为6.75℃、0.045 K/W。传热量132.00 W、热效率0.88、当量导热系数168 125 W/(m·K),均达到最大值。所以,该无芯环路热管在本实验研究范围内的最佳工作条件为加热功率150.00 W、充液率30%。     

Summary heat losses of trough solar water heating collectors with exposed evaporation surface

The results of analytic studies to determine the summary heat losses of trough solar water heating collectors with exposed evaporation surface are presented, and the values of the heat loss components are compared.     

生物质循环流化床飞灰烧失特性研究

选取2家典型生物质电厂除尘器处的飞灰,通过热重-傅里叶红外光谱联用（TG-FTIR）、X射线荧光光谱（XRF）、能谱（EDS）和X射线衍射（XRD）等分析手段,对飞灰的成分及其在高温下的失重特性进行研究。研究发现,生物质飞灰的高温失重机理较为复杂,在空气中加热时,碱金属氯盐的蒸发从380 ℃开始缓慢进行,600 ℃以下主要进行未燃碳的燃尽与碳酸镁的分解;600~750 ℃主要进行碳酸钙的分解;750 ℃后蒸发速度迅速上升,直至950 ℃完全蒸发;950 ℃以上发生矿物质的玻璃化反应和碱金属硫酸盐的蒸发。鉴于目前电力行业和ATSM对生物质飞灰含碳量的测试结果均不准确,该根据生物质飞灰的烧失机理,提出更加精确的氮气和空气气氛下600 ℃烧失量之差含碳量测试方法。     

正丁醇水溶液液滴的蒸发特性实验研究

自湿润流体是一种具有特殊的表面张力特性的二元流体,了解其蒸发传热特性对于揭示其强化传热机理十分重要.为了探究添加自湿润流体液滴的蒸发特性,采用液滴形状分析仪(DSA100)研究了不同温度(30、40、50、60℃)下铜底板上去离子水、正丁醇水溶液(质量分数为0.5％)液滴的蒸发特性.结果 表明:加入少量正丁醇溶液并不影...     

Evaluation of gasoline evaporation during the tank splash loading by CFD techniques

This paper presents a CFD simulation of gasoline evaporation during the splash loading of gasoline tank. The volume of fluid based (VOF) interface tracking method in conjunction with a mass transfer model was used. Evaporation rate was determined by a source term at the interface. The effect of different parameters such as loading velocity, temperature and initial vapor mass fraction on gasoline lost was determined. The results show that by increasing the loading velocity, the total evaporated mass decreases. Based on CFD simulations a correlation has been proposed to evaluate the amount of gasoline losses during the fuel tank splash loading.     

Performance enhancement of air-cooled open cathode polymer electrolyte membrane fuel cell with inserting metal foam in the cathode side

In this study, a novel way to improve performance of the air-cooled open cathode polymer electrolyte membrane fuel cell is introduced. We suggest using a metal foam in the cathode side of the planar unit fuel cell for the solution to conventional problems of the open cathode fuel cell such as excessive water evaporation from the membrane and poor transportation of air. We conduct experiment and the maximum power density of the fuel cell with metal foam increases by 25.1% compared with the conventional fuel cell without metal foam. The open cathode fuel cell with metal foam has smaller ohmic losses and concentration losses. In addition, we prove that the open cathode fuel cell with metal foam prevents excessive water evaporation and membrane drying out phenomena with numerical approach. Finally, we apply the metal foam to the air-cooled open cathode fuel cell stack as well as the planar unit cell.     

Modelling of energy flows in potato crisp frying processes

Food frying is very energy intensive and in industrial potato crisp production lines frying is responsible for more than 90% of the total energy consumption of the process. This paper considers the energy flows in crisp frying using a First Law of Thermodynamics modelling approach which was verified against data from a potato crisp production line. The results indicate that for the frying process considered, most of the energy used is associated with the evaporation of water present in the potato and on the surface of potato slices. The remainder is from evaporation of frying oil and air of the ventilation system and heat losses from the fryer wall surfaces by convection and radiation. The frying oil is heated by an industrial gas furnace and the efficiency of this process was calculated to be 84%. The efficiency of the overall frying process which was found to be of the order of 70% can be improved by employing exhaust heat recovery and optimising other operating and control parameters such as exhaust gas recirculation.     

Natural convection phenomena in inclined cells with finite side-walls—A numerical solution

The effects of Rayleigh number, aspect ratio and angle of inclination on the heat transfer through an inclined air-cell are studied via the numerical solution of the relevant two dimensional governing equations. This is done for two side wall boundary conditions, namely that of perfectly insulating and infinitely conducting side walls. In addition, the effects of a finite thickness wall with a finite value of thermal conductivity is studied, by solving the two dimensional conduction equation in the wall and matching the values of temperature and heat flux at the common boundaries between the two regions. It is shown that the perfectly in sulating and infinitely conducting boundary conditions are the two extremes of the real case and, depending on the values of the fluid parameters and the value of the wall thermal conductivity, the two fixed boundary conditions can be either accurate or inaccurate representations of the real case. A discussion of the relevance of this work to solar absorbers is included, with the major conclusion being that, depending on the aspect ratio of the cell used, cellular structures can be effective in reducing convective losses in inclined absorbers.     

Eulerian multiphase study of direct steam generation in parabolic trough with OpenFOAM

Direct steam generation (DSG) in parabolic trough solar collectors is a feasible option for economic improvement in solar thermal power generation. Three-dimensional Eulerian two-fluid simulations are performed under OpenFOAM to study the turbulent flow in the evaporation section of the parabolic trough receiver and investigate the phase change, and pressure drop of water as a heat transfer fluid. First, the model's validity has been tested by comparing the numerical results of a laboratory scale boiler with the available correlations and semi-correlations of boiling flows from the literature. Simulations agreed well with Rouhani–Axelsson correlation for horizontal tubes, with a mean relative error of less than 7.1% for all studied cases. However, despite a mean relative error of less than 13.19% compared to the experimental data in the literature, the reported pressure drop factor remains valid; overprediction remains tolerable for most engineering applications. Second, the scaling effect on the mathematical model, from laboratory to commercial-scale configuration, was tested with experimental data of the DISS test loop in Platforma Solar de Almeria, Spain. The Monte Carlo Ray Tracing method under the Tonatiuh package allowed for obtaining the nonuniform heat flux distribution. Due to the large size of the evaporation section in the DISS loop (eight collectors), each collector is considered independently in the simulations. Thus, simulations follow each other, taking the numerical results of each collector output as input data in the next collector and so on until the last. The numerical results showed an excellent agreement for the void fraction with 3.53% against the Rouhani–Axelsson correlation. Frictional pressure losses are within a 17.06% error of the Friedel correlation, in the range of previous work in the literature, and the heat loss is less than 4.69% error versus experimental correlation.     

Performance enhancement of high temperature latent heat thermal storage systems using heat pipes with and without fins for concentrating solar thermal power plants

A key drawback of using latent heat thermal storage systems for concentrating solar thermal power plants is the low thermal conductivity of the phase change material during the melting and solidification processes. This paper investigates an approach for reducing the thermal resistance by utilising axially finned heat pipes. A numerical model simulating the phase change material melting and solidification processes has been developed. This paper also includes the models of the evaporation and condensation of the heat pipe working fluid. The results show that by adding four axial fins and including the evaporation and condensation, the overall thermal performance of the storage system is enhanced significantly compared to having bare heat pipes. After 3 h a total of 106% increase in energy storage is obtained during the charging process. The results also show that the combined effect of incorporating the evaporation/condensation process and adding the fins leads to a threefold increase in the heat storage during the first 3 h. During the discharge process, there was a 79% increase in energy discharged and also the combined effect of incorporating the evaporation/condensation as well as adding the fins results in an almost four fold increase in the heat extracted within the first 3 h. A parametric analysis has also been carried out to analyse the effect of the finned heat pipe parameters after incorporating evaporation and condensation of the heat pipe working fluid.     

Effects of nanoparticles on nanofluid droplet evaporation

Laponite, Fe₂O₃ and Ag nanoparticles were added to deionized water to study their effect of evaporation rates. The results show that these nanofluid droplets evaporate at different rates (as indicated by the evaporation rate constant K in the well known D²-law) from the base fluid. Different particles lead to different values of K. As the particle concentration increases due to evaporation, K values of various Ag and Fe₂O₃ nanofluids go through a transition from one value to another, further demonstrating the effect of increasing nanoparticle concentration. The implication for the heat of vaporization (h_fg) is discussed.     

Performance enhancement of a split air conditioner using porous material inside the evaporator pipes

In this experimental study, a porous material is used inside the pipes of the evaporator as the main heat exchanging device in the air conditioning cycle. The used porous material consists of stainless steel balls of different diameters. As a case study, refrigerant R454B, which is a drop-in replacement to refrigerant R410A, is used as a working fluid in the air conditioner thermodynamic cycle. Four different porosities were used during the experimental tests; 100% (empty tube), 46%, 40%, and 33%. This study investigated the influence of variation of porosity as well as outside air temperature and refrigerant evaporation temperature on the cycle coefficient of performance, evaporation capacity, pressure drop, and power consumption during the compression process. Measured evaporation temperatures and indoor temperatures during tests were in the range of 1.5–12°C and 18–25°C, respectively. The use of porous material in the evaporation heat exchanger resulted in a considerable increase in the cycle evaporation capacity and coefficient of performance. Varying porosity from 100% to 33% resulted in an average percent increase of cycle evaporation capacity and coefficient of performance by 48.8% and 84.3%, respectively. Also, decreasing porosity from 100% to 33% resulted in an average percent increase in power consumption during the compression process by about 27%. An average percent increase of power consumption of compressor by about 25.9% is also reported, when evaporation temperature increased from 1.5°C to 12°C. Increasing outside air temperature from 27.1°C to 39.5°C resulted in decreasing evaporation capacity and coefficient of performance by 35.2% and 34.5%, respectively, and in increasing compressor power consumption by about 14.3%. A considerable pressure drop was recorded during the evaporation process when using porous material. The volumetric evaporation capacity, as well as compressor discharge temperature, are increased by increasing evaporating temperature and by decreasing evaporator porosity. The increase in air ambient temperature resulted in a considerable increase in refrigerant mass flow rate.     

An exergy analysis of a solar-driven ejector refrigeration system

Exergy analysis is used as a tool to analyse the performance of an ejector refrigeration cycle driven by solar energy. The analysis is based on the following conditions: a solar radiation of 700 W/m², an evaporator temperature of 10 °C, a cooling capacity of 5 kW, butane as the refrigerant in the refrigeration cycle and ambient temperature of 30 °C as the reference temperature. Irreversibilities occur among components and depend on the operating temperatures. The most significant losses in the system are in the solar collector and the ejector. The latter decreases inversely proportional to the evaporation temperature and dominates the total losses within the system. The optimum generating temperature for a specific evaporation temperature is obtained when the total losses in the system are minimized. For the above operating conditions, the optimum generating temperature is about 80 °C.     

Heat and mass transport impact on MHD second-grade fluid: A comparative analysis of fractional operators

The effect of the magnetic flux plays a major role in convective flow. The process of heat transfer is accompanied by a mass transfer process; for instance, condensation, evaporation, and chemical process. Due to the applications of the heat and mass transfer combined effects in different fields, the main aim of this paper is to do a comprehensive analysis of heat and mass transfer of magnetohydrodynamic (MHD) unsteady second-grade fluid in the presence of ramped conditions. The new governing equations of MHD second-grade fluid have been fractionalized by means of singular and nonsingular differentiable operators. To have an accurate physical significance of imposed conditions on the geometry of second-grade fluid, the constant concentration with ramped temperature and ramped velocity is considered. The fractional solutions of temperature, concentration, and velocity have been investigated by means of integral transform and inversion algorithm. The influence of physical parameters and flow is analyzed graphically via computational software (MATHCAD-15). The velocity profile decreases by increasing the Prandtl number. The existence of a Prandtl number may reflect the control of the thickness and enlargement of the thermal effect.     

Thermal analysis of five unglazed solar collector systems for the heating of outdoor swimming pools

We have analysed measurements from five outdoor swimming pools located in Switzerland and heated by unglazed solar collectors. The main contributions to the daily energy balance of the swimming pools are evaluated. They include the active and passive solar gains, as well as the heat losses related to radiation, evaporation, convection, and water renewal (in order of importance). Coherent results are obtained using multilinear regressions in order to determine the best fitting values of the empirical parameters involved in the thermal equations.     

Determination of required core temperature for thermal comfort with steady-state energy balance method

In this study, apart from the other studies related to thermal comfort, it is combined that the fundamental equations given in the steady-state energy balance and the empirical relations expressing effects of the thermoregulatory control mechanisms of the body. In the first section of this simulation, body core temperature is calculated by using the equations expressing thermoregulatory control mechanism, the required skin temperature and sweat rate values. Variation of the calculated body core temperature is investigated with the activity level based on required skin temperature and sweat rate values. In the second section of the simulation, heat losses from the body (convection, radiation, evaporation, and respiration) and ratio of the each heat loss mechanism to total heat loss are calculated and discussed in detail.     

Power,power density and efficiency optimization of an endoreversible Braysson cycle

The performance optimization of an endoreversible Braysson cycle with heat resistance losses in the hot- and cold-side heat exchangers is performed by using finite-time thermodynamics. The relations between the power output and the working fluid temperature ratio, between the power density and the working fluid temperature ratio, as well as between the efficiency and the working fluid temperature ratio of the cycle coupled to constant-temperature heat reservoirs are derived. Moreover, the optimum heat conductance distributions corresponding to the optimum dimensionless power output, the optimum dimensionless power density and the optimum thermal efficiency of the cycle, and the optimum working fluid temperature ratios corresponding to the optimum dimensionless power output and the optimum dimensionless power density are provided. The effects of various design parameters on those optimum values are studied by detailed numerical examples.     

Flow and thermal field in sessile droplet evaporation at various environmental conditions

Sessile droplets' evaporation is a complex process that involves fluid flow coupled with heat and mass transfer. In this study, mathematical modelling of sessile droplet evaporation on hydrophobic substrates is developed and simulations are carried out on COMSOL. The model results are validated with the data available in the literature. Postvalidation, the simulation of droplet evaporation is carried out on the various substrate hydrophobicities and various environmental conditions. For these conditions, contours are plotted for temperature, velocity, and mass concentration for the droplet and moist air domain. The result shows that Marangoni convection plays a very important role in droplet evaporation. A high rate of evaporation is observed at the droplet interface at low relative humidity and a large degree of subheating. The effect of air velocity on the evaporation rate is studied, however, its effect is very marginal as compared to relative humidity and degree of subheating. The heat flux at the three-phase contact line is large for a smaller Prandtl number fluid. Overall, the evaporation rate increases with increasing the Prandtl number because it has a large value of Marangoni convection.