Prediction of evaporation losses in evaporative fluid coolers

The accurate prediction of various aspects of thermal behavior of evaporative fluid coolers is very important for both design and rating calculations. Exactly predicting evaporation losses is significant since the process fluid is cooled primarily by evaporation of a portion of the recirculating water that causes the concentration of dissolved solids and other impurities to increase. An empirical relation to predict evaporation losses is developed on the basis of the rule of thumb recommended by manufacturers, which is simple and accurate with a wide range of applicability. The predicted values are in good agreement with the numerical values obtained from the calibrated model where the maximum error was found to be approximately 4% but often less than 2%.     

The impact of fouling on performance evaluation of evaporative coolers and condensers

Fouling of evaporative cooler and condenser tubes is one of the most important factors affecting their thermal performance, which reduces effectiveness and heat transfer capability with time. In this paper, the experimental data on fouling reported in the literature are used to develop a fouling model for this class of heat exchangers. The model predicts the decrease in heat transfer rate with the growth of fouling. A detailed model of evaporative coolers and condensers, in conjunction with the fouling model, is used to study the effect of fouling on the thermal performance of these heat exchangers at different air inlet wet bulb temperatures. The results demonstrate that fouling of tubes reduces gains in performance resulting from decreasing values of air inlet wet bulb temperature. It is found that the maximum decrease in effectiveness due to fouling is about 55 and 78% for the evaporative coolers and condensers, respectively, investigated in this study. For the evaporative cooler, the value of process fluid outlet temperature T_p,out varies by 0.66% only at the clean condition for the ambient wet bulb temperatures considered. Copyright © 2005 John Wiley & Sons, Ltd.     

The effect of longitudinal heat conduction in cross flow indirect evaporative air coolers

The effect of the longitudinal heat conduction in the exchanger wall of a compact-plate cross flow indirect evaporative cooler is investigated. A NTU method is used to study the heat and mass transfer characteristics. A block iterative numerical method is used to solve the coupled conservation equations for the primary fluid, the secondary fluid and the liquid film. The model was validated using previously published data. The exchanger performance deterioration due to the conduction effect has been determined for various design and operating conditions. The results indicate that the thermal performance deterioration of the evaporative coolers may become significant for some typical operating conditions and could be as high as 10%, while it lies less than 5% for most conservative conditions.     

Heat and fluid transport in an evaporative capillary pump

The thermal and fluid mechanical properties of an evaporative capillary pump are described by means of characteristics that have been determined by tests. The capillary pump, which has been newly designed for an application in future space projects, serves a double function as the heat absorbing element in a closed loop as well as a pump circulating the working fluid. The working fluids used were CCI₃F (freon R11) and liquid anhydrous ammonia. The paper describes the associated heat and mass transport in the capillary pump, and the physical process of the capillary pumping is explained. Furthermore, a model is introduced which explains the boiling heat transfer in the porous structure of the capillary pump.     

Novel design and modelling of an evaporative cooling system for buildings

Passive evaporative cooling has great potential as an alternative to conventional air‐conditioning in arid hot climates because of its low cost and zero pollution. This paper describes a novel evaporative cooling system with an automatic wind‐tracking device to improve its operating efficiency. The design and operating principles are discussed. A mathematical model is simplified by the assumption of convective heat and mass transfer of staggered streamlets of water. A computer program has been developed to calculate the deflection and length of spray water streamlets, as well as evaporative water mass, minimum cooled water temperature and required cooling time. A typical example illustrates that approximately 20 kg water are evaporated and around 26 min are required for 980 kg of water to be cooled from 28°C to the wet bulb temperature of 19.2°C of ambient air in a typical arid hot climate (relative humidity = 0.30, dry bulb temperature = 32°C and wind velocity = 4 m s^?1). The application of adsorbents, would allow the evaporative cooling system to be applied in hot, humid climates, in addition to hot climates with low humidity. Copyright © 2006 John Wiley & Sons, Ltd.     

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.     

A novel interconnector design of SOFC

In this study, a novel interconnector design is proposed, which is named as the X-type interconnector. The solid oxide fuel cell (SOFC) models are established for the conventional interconnector and the X-type interconnector. The results indicate that the design of the X-type interconnector is beneficial to the transport of gas in SOFC, which has a higher oxygen concentration under rib than that of the conventional interconnector. Furthermore, compared with the X-type interconnector, the potential difference in cathode is larger for the conventional interconnector, which indicates that the X-type interconnector reduces the current path and improves the performance of SOFC. For any porosity and conductivity of anode and cathode, the X-type interconnector is superior to the conventional interconnector. Moreover, when the cathode conductivity is smaller, the advantage of the X-type interconnector becomes more remarkable.     

A three-dimensional theoretical model for predicting transient thermal behavior of thermoelectric coolers

A simulation model is developed and used to predict transient thermal behavior of the thermoelectric coolers. The present model amends the previous models, in which the P–N pair is simply treated as a single bulk material so that the temperature difference between the semiconductor elements was not possible to evaluate. Based on the present simulation model, the thermoelectric cooler is divided into four major regions, namely, cold end (region 1), hot end (region 2), and the P-type and N-type thermoelectric elements (regions 3 and 4). Solutions for the three-dimensional temperature fields in the P-type and the N-type semiconductor elements and transient temperature variations in the cold and the hot ends have been carried out. The magnitude of the coefficient of performance (COP) of the thermoelectric cooler are calculated in wide ranges of physical and geometrical parameters. To verify the numerical predictions, experiments have been conducted to measure the temperature variations of both the cold and the hot ends. Close agreement between the numerical and the experimental data of the temperature variations has been observed.     

A novel induction machine design suitable for inverter-drivenvariable speed systems

Induction machines designed for inverter-driven variable speed systems are different from those fed directly from a utility power line. In this paper, a novel design approach for inverter driven induction machines is presented and implemented. This is followed by an investigation on sizing equations and rotor slot shape specifically for this purpose. The proposed approach permits the integration of the design of machines with inverters, comprehensive performance analysis, and system optimization, resulting in 20-30% higher power density for the induction machine than those designed for direct utility power supplies by conventional methods. Simulation analysis and experimental results are presented to substantiate the conclusions     

A new ideal evaporative freezing cycle

A new ideal evaporative freezing cycle for freezing of water is proposed and analyzed by using the conservation of energy and the conservation of mass principles. The proposed cycle utilizes low temperature heat sources such as solar energy, geothermal energy, and waste heat, and consists of a freezing chamber, an air-to-air heat exchanger, a desiccant chamber, an air-to-water heat exchanger, and a fan through which air circulates at atmospheric pressure. The operating principles of the cycle is based on the fact that as dry air picks up moisture from water, the water vapor absorbs heat primarily from the remaining body of the water, and thus the water is cooled and frozen. It is shown that the proposed system can produce 28.4 g ice/kg dry air circulated at most and have a thermal coefficient of performance up to 0.47. The proposed evaporative freezing cycle offers a viable alternative to the conventional refrigeration methods and provides refrigeration by using the inexpensive source of thermal energy source. Also, various aspects of the cycle proposed is discussed.     

A novel heat dissipation fin design applicable for natural convection augmentation

In this study, a comparative study of heat sink having various fin assembly under natural convection is investigated. The fin pattern includes a rectangular, a trapezoidal and an inverted trapezoidal configuration. Tests were performed in a well controlled environmental chamber having a heat load ranging from 3 to 20 W. From the test results, the heat transfer coefficient of the conventional rectangular fins is higher than that of the trapezoidal fins while the heat transfer coefficient of the inverted trapezoidal fins is higher than the trapezoidal one by approximately 25%, and it exceeds that of convectional rectangular fin by about 10%. The heat transfer improvements of the inverted trapezoidal fin are mainly associated with a larger temperature difference and inducing more air flow into the heat sink.     

A novel design of a heat exchanger for a metal-hydrogen reactor

A metal-hydride reactor equipped by a spiral heat exchanger is experimentally studied. The inserted exchanger provides significant insights into the problem of minimizing the total storage time by manipulating the operating parameters. Performance studies are carried out by varying the supply pressure, volume of the tank, absorption temperature, and overall heat transfer coefficient.     

A novel design in composities of various materials for solar selective coatings

This study describes the development of multilayer metal-dielectric graded index solar selective coatings in which the metallic volume fraction increases with depth, from top (air–film interface) to bottom (film–substrate interface). The work is based on computer simulation followed by validation through fabrication of the coatings and optical measurements. The influence of the choice of the number of layers present in a graded index composite selective absorber and results obtained for a new destructive interference bilayer (four-layer system) coating, designed using the computer model, were studied. The design and optimization of the composite coating was undertaken using a computer tool developed within this program of research employing Bruggeman and Maxwell–Garnett effective medium formalisms. The design tool enabled all key design parameters, with the exception of particle size and orientation, to be varied systematically to permit the sensitivity of the optical properties of the selective absorber coating to be studied.The model was validated with a supporting program of experimental research in which many different selective absorbers were prepared by co-sputtering of metal and dielectric materials.Although the best compositional gradation can be achieved by increasing the number of layers, the variation in optical performance beyond a certain number of layers is minimal. The destructive interference produced between adjacent layers contributes to the absorptance. The effect of the number of layers (single, four and 10) has been calculated for various materials such as nickel, vanadium, tungsten, cobalt and chromium based coatings. Solar absorptance of 0.98 and 0.96 was achieved by simulation and experimental findings with less than 0.07 thermal emittance at 300 K for 200 nm thick, 4-PGSAC (four-layer system) of V : Al₂O₃ composites. Other designs showed lower optical performance for all the material combinations regardless of their individual optical properties. Use of such thin film coating on the absorbers of solar thermal appliances can reduce thermal losses significantly, which could be of importance to the relevant industry.     

A general correlation for evaporative heat transfer in micro/mini-channels

Experimental results of the saturated-flow boiling heat transfer in micro/mini-channels for both multi- and single-channel configurations were obtained from the literature. The collected database contains more than 3700 data points, covering a wide range of working fluids, operational conditions, and different micro-channel dimensions. The whole database was analyzed by using various existing correlations to verify their respective accuracies. However, none of the existing correlations could predict the data sets precisely. Using the boiling number, Bond number and Reynolds number, a general correlation for evaporative heat transfer in micro/mini-channels was established. In addition, the Bond number in predicting heat-transfer coefficients can be used as a criterion to classify a flow path as a micro-channel or as a conventional macro-channel.     

A model of an evaporative cycle for heat and power production

The paper discusses the modeling of an evaporative gas turbine cycle that is to be erected in the Laboratory of Heat and Power Engineering at the Lund Institute of Technology. The equations describing the following components are presented: the compressor, the combustion chamber, the expander, the recuperator and the humidifier. The use of the models are discussed.     

Low pressure evaporative cooling of micron-sized droplets of solutions and its novel applications

For free molecular regime the mathematical model of low pressure evaporative cooling of binary droplets in gas flow is developed. The model includes five ordinary differential equations and takes into account effects such as the release of the latent heat of condensation of both components and the release of the latent heat of dissolution. Simulations were made for weak aqueous solutions of ammonia. It was discovered that compositions of gas flow and the aqueous solution affect the rate of evaporative cooling of droplets. The ratio of mass flow of solution and gas flow is also an important parameter. The cooling rate of such binary droplets can reach the value of about 2 × 10⁵ K/s.As first applications we consider the air cooler based on evaporative cooling of droplets. For pressure of 20–80 Torr in aerosol reactor, it is shown that in the cooler with length of about 1 m temperature of air flow may drop to about 10–15 °C.The second application is the formation of nanoparticle in evaporating multicomponent droplet with two volatile components. Simulation was made for aqueous solution of ammonia which is widely used by experimentalists and engineers now. Effects of the number of precursors in droplet and supersaturation in droplet on the final size of nanoparticles were investigated.     

A multi-stage down-draft evaporative cool tower for semi-enclosed spaces: Aerodynamic performance

A multi-stage down-draft evaporative cool tower (DECT) was developed as an improvement to a previously developed single-stage design. The new tower incorporates a secondary air inlet, added to reduce the water consumption required to produce a desired cooling output in a tower of given maximum cross-section and primary inlet geometry. The secondary air, which may be drawn from the interior space being chilled, is cooled by evaporation in the lower section of the tower. This paper reports on the results of experiments conducted to establish the aerodynamic performance of the design prior to installation of a water spraying system. Design of the water spraying system and experiments on cooling performance are discussed in a companion paper.     

A novel design of a household clothes tumbler dryer

This paper explores the feasibility of a novel design of household clothes tumbler dryer, where the electrical elements of a standard tumbler clothes Dryer A (which heat the ambient air entering the drum) were replaced by a water-to-air finned tube heat exchanger (Dryer B). To achieve the objective of the study, experimental performances of the two dryers (e.g., Dryers A and B) were evaluated for the same operating conditions. During the experiments, Dryer B was found to have shorter drying times, lower moisture extraction rates for the same power input and hence significantly more efficient than Dryer A. This type of dryer may be adopted by the industry in the future for its relatively low cost, higher energy efficiency and environmental benefits.     

A novel two-step method to design inter-plant hydrogen network

Inter-plant hydrogen network with reuse/recycle optimization is important for saving hydrogen resource. Thus, it's necessary to optimize the inter-plant hydrogen network. In this paper, a novel two-step method that combines the pinch insight with mathematical programming is developed to optimize the inter-plant hydrogen network with purification reuse/recycle. A new transhipment model for targeting inter-plant hydrogen network is built to target the hydrogen utility consumption for each refinery. After the hydrogen consumption is determined, individual plant hydrogen networks are designed separately. The mathematical model is linear to guarantee global optimal solution. Furthermore, the model can also optimize the number of inter-plant connections of the hydrogen network. Case study indicates the effectiveness of model.