Vapor absorption systems offer advantages over vapor compression systems for air-conditioning systems in some applications. They use heat as their primary energy input and hence provide opportunities to use solar energy or waste heat to drive these systems. The absorber is the most crucial component of the vapor absorption system and has the largest impact on its performance. Absorber design requires a keen understanding of the underlying heat and mass transfer processes in the absorber. The horizontal tube geometry is by far the most popular absorber geometry, due to the high absorption efficiencies achievable without incurring commensurate pressure drops. Several models have been proposed in the literature to model the heat and mass transfer during absorption over horizontal tube banks. However all of them make very simplistic assumptions about the flow profiles in the absorber. High speed flow visualization studies in the literature have shown that the flow occurs in the form of droplets and the formation and the detachment of these droplets and their impact on the tube has significant effects on the heat and mass transfer. Most absorption models in the literature neglect these flow modes and assume the solution to flow as a uniform film. The present study attempts to numerically model the heat and mass transfer in the absorber taking the realistic drop-wise and wavy film flow patterns into consideration. The impact of the fall of these droplets on the tube causes the lithium bromide solution present on the film on the tube to mix and present newer regions of the solution for vapor absorption. The impact of the droplets also causes waves that propagate axially over the liquid film on the tube. The mixing effect and the waves caused due to droplet impact play a very important role in the heat and mass transfer. Results obtained from this study will aid in better understanding of the vapor absorption process, and in the design of more efficient absorbers.

Heat actuated adsorption heat pumps offer the opportunity to improve overall energy efficiency in waste heat applications by eliminating shaft work requirements accompanying vapor compression cycles. The coefficient of performance (COP) in adsorption heat pumps is generally low. The objective of this thesis is to model the adsorption system to gain critical insight into how its performance can be improved. Because adsorption heat pumps are intermittent devices, which induce cooling by adsorbing refrigerant in a sorption bed heat/mass exchanger, transient models must be used to predict performance. In this thesis, such models are developed at the adsorbent particle level, heat/mass exchanger component level and system level. Adsorption heat pump modeling is a coupled heat and mass transfer problem. Intra-particle mass transfer resistance and sorption bed heat transfer resistance are shown to be significant, but for very fine particle sizes, inter-particle resistance may also be important. The diameter of the adsorbent particle in a packed bed is optimized to balance inter- and intra-particle resistances and improve sorption rate. In the literature, the linear driving force (LDF) approximation for intra-particle mass transfer is commonly used in place of the Fickian diffusion equation to reduce computation time; however, it is shown that the error in uptake prediction associated with the LDF depends on the working pair, half-cycle time, adsorbent particle radius, and operating temperatures at hand. Different methods for enhancing sorption bed heat/mass transfer have been proposed in the literature including the use of binders, adsorbent compacting, and complex extended surface geometries. To maintain high reliability, the simple, robust annular-finned-tube geometry with packed adsorbent is specified in this work. The effects of tube diameter, fin pitch and fin height on thermal conductance, metal/adsorbent mass ratio and COP are studied. As one might expect, many closely spaced fins, or high fin density, yields high thermal conductance; however, it is found that the increased inert metal mass associated with the high fin density diminishes COP. It is also found that thin adsorbent layers with low effective conduction resistance lead to high thermal conductance. As adsorbent layer thickness decreases, the relative importance of tube-side convective resistance rises, so mini-channel sized tubes are used. After selecting the proper tube geometry, an overall thermal conductance is calculated for use in a lumped-parameter sorption bed simulation. To evaluate the accuracy of the lumped-parameter approach, a distributed parameter sorption bed simulation is developed for comparison. Using the finite difference method, the distributed parameter model is used to track temperature and refrigerant distributions in the finned tube and adsorbent layer. The distributed-parameter tube model is shown to be in agreement with the lumped-parameter model, thus independently verifying the overall UA calculation and the lumped-parameter sorption bed model. After evaluating the accuracy of the lumped-parameter model, it is used to develop a system-level heat pump simulation. This simulation is used to investigate a non-recuperative two-bed heat pump containing activated carbon fiber-ethanol and silica gel-water working pairs. The two-bed configuration is investigated because it yields a desirable compromise between the number of components (heat exchangers, pumps, valves, etc.) and steady cooling rate. For non-recuperative two-bed adsorption heat pumps, the average COP prediction in the literature is 0.39 for experiments and 0.44 for models. It is important to improve the COP in mobile waste heat applications because without high COP, the available waste heat during startup or idle may be insufficient to deliver the desired cooling duty. In this thesis, a COP of 0.53 is predicted for the non-recuperative, silica gel-water chiller. If thermal energy recovery is incorporated into the cycle, a COP as high as 0.64 is predicted for a 90, 35 and 7.0°C source, ambient and average evaporator temperature, respectively. The improvement in COP over heat pumps appearing in the literature is attributed to the adsorbent particle size optimization and careful selection of sorption bed heat exchanger geometry.

No description available.

Fossil fuel is an important energy source, but is unavoidabiy running out. Since the cellulosic material is the most abundant source of organic matter, the ethanol, which is produced from cellulosic waste materials, is gaining more and more attention. These materials are cheap, renewable and their availability makes them superior compared to other raw materials. The cellulose must be hydrolyzed to glucose before it can be fermented to ethanol. The enzymatic hydrolysis of cellulose using cellulase enzymes is the most widely used method. The production cost of cellulase enzymes is the major cost in ethanol manufacture. To optimize the cost of ethanol production, enzyme stability needs to be improved through maintaining the activity of the enzymes and by optimizing the production of the cellulase. The aim of researchers, engineers and industrials is to get more biomass for the same cost. The filamentous fungus Trichoderma reesei has a long history in the production of the cellulase enzymes. This production can be influenced strongly by varying the growth media and culture conditions (pH, temperature, DO, agitation,... ). At present, it is my opinion that no modelling study has included both the hydrodynamic and kinetic aspects to investigate the effect of shear and mass transfer on the morphology of microorganisms that influence the rheology of the broth and production of cellulase. This thesis presents the development of a mathematical model for cellulase production and the growth of biomass in an airlift bioreactor. The kinetic model is coupled with the methodology of two-phase flow using mathematical models based on the bubble break-up and coalescence to predict mass transfer rate, which is one of the critical factor in the fermentation. A comparison between the results obtained by the developed model and the experimental data is given and discussed. The design proposed for the airlift geometry by Ahamed and Vermette enables us to get a high mass transfer and production rate. The results are very promising with respect to the potential of such a model for industrial use as a prediction tool, and even for design.

An investigation of binary fluid heat and mass transfer in ammonia-water absorption was conducted. Experiments were conducted on a horizontal-tube falling-film absorber consisting of four columns of six 9.5 mm (3/8 in) nominal OD, 0.292 m (11.5 in) long tubes, installed in an absorption heat pump. Measurements were recorded at both system and local levels within the absorber for a wide range of operating conditions (nominally, desorber solution outlet concentrations of 5 - 40% for three nominal absorber pressures of 150, 345 and 500 kPa, for solution flow rates of 0.019 - 0.034 kg/s.). Local measurements were supplemented by high-speed, high-resolution visualization of the flow over the tube banks. Using the measurements and observations from videos, heat and mass transfer rates, heat and vapor mass transfer coefficients for each test condition were determined at the component and local levels. For the range of experiments conducted, the overall film heat transfer coefficient varied from 923 to 2857 W/m²-K while the vapor and liquid mass transfer coefficients varied from 0.0026 to 0.25 m/s and from 5.51×10^-6 to 3.31×10^-5 m/s, respectively. Local measurements and insights from the video frames were used to obtain the contributions of falling-film and droplet modes to the total absorption rates. The local heat transfer coefficients varied from 78 to 6116 W/m²-K, while the local vapor and liquid mass transfer coefficients varied from -0.04 to 2.8 m/s and from -3.59×10^-5 (indicating local desorption in some cases) to 8.96×10^-5 m/s, respectively. The heat transfer coefficient was found to increase with solution Reynolds number, while the mass transfer coefficient was found to be primarily determined by the vapor and solution properties. Based on the observed trends, correlations were developed to predict heat and mass transfer coefficients valid for the range of experimental conditions tested. These correlations can be used to design horizontal tube falling-film absorbers for ammonia-water absorption systems.

<p>Evaporative cooling is a highly energy efficient alternative to conventional vapor compression cooling system. The sensible cooling effect of evaporative cooling systems is well documented in the literature. Direct evaporative cooling however increases the relative humidity of the air as it cools it. This has made it unsuitable for data centers and other applications where humidity control is important. Desiccant-based dehumidifiers (liquid, solid or composites) absorb moisture from the cooled air to control humidity and is regenerated using waste heat from the data center. This work is an experimental and theoretical investigation of the use of desiccant assisted evaporative cooling for data center cooling according to ASHRAE thermal guidelines, TC 9.9. The thickness (depth) of the cooling pad was varied to study its effect on sensible heat loss and latent heat gain. The velocity of air through the pad was measured to determine its effect on sensible cooling. The flow rate of water over the pad was also varied to find the optimal flow for rate for dry bulb depression. The configuration was such that the rotary desiccant wheel (impregnated with silica gel) comes after the direct evaporative cooler. The rotary desiccant wheel was split in a 1:1 ratio for cooling and reactivation at lower temperatures. The dehumidification effectiveness of a fixed bed desiccant dehumidifier was compared with that of a rotary desiccant wheel and a thermoelectric dehumidifier. A novel condensate recovery system using the Peltier effect was proposed to recover moisture from the return air stream, (by cooling the return air stream below its dew point temperature) thereby optimizing the water consumption of evaporative cooling technology and providing suitable air quality for data center cooling. The moisture recovery unit was found to reduce the mass of water lost through evaporation by an average of fifty percent irrespective of the pad depth.</p> <p> </p>

Abstract : The harsh conditions of electrolytic bath in aluminium electrolysis cell have been an obstacle against the understanding of mass transfer that is at the origin of the aluminium production process. This knowledge is of great importance due to the impact that it could have on the functional parameters of the cell like current efficiency. Numerical modelling is a way to overcome the difficulties and to shed light over the hidden aspects of the electrochemical process. The electrolyte typically used in an aluminum electrolysis cell is composed of different ions moving in the electromagnetic field generated by the high intensity current needed for this industrial application. The behaviour of these ions is under the influence of concentration gradients (diffusion) and depends also on other phenomena in the cell like bath flow (convection) and electric field (migration). In this study, the coupling between these fields is treated for 1D and 2D models of the cell. The relative importance of migration and diffusion are compared for two different categories of electroactive and electroinactive ions in a transient model. For both categories of ions, migration is the dominant form of mass transfer in the very first stages of electrochemical process. However, diffusion becomes the dominant mechanism of mass transfer for electroactive ions in developed boundary layers. In 2D model, there is a concentration gradient between interelectrode and near sidewalls region. Consequently, there is a diffusion of ions in and out of the interelectrode space to diminish the depletion or overconcentration of certain electroactive ions like Al[subscript 2]OF[subscript 6][superscript -2] and AlF[subscript 4][superscript -] at the electrodes. Furthermore, the impact of convection and bath equilibrium in addition to a more suitable mass transfer model has been studied on a parallel plate electrodes reactor. Finally, an open source library is developed and built on OpenFoam (an open source C++ CFD platform) that is capable of solving mass transfer equations for different models. The description and findings of this thesis will shed light on the mass transfer mechanisms in both bulk region and boundary layers, and can be used for further studies in this field. / Résumé : L’étude des mécanismes de transfert de masse des ions dans le bain électrolytique dans une cellule d’électrolyse d’aluminium se heurte aux conditions sévères qui y sont rencontrées : haute température, milieu corrosif, etc. Cependant, il est important de connaitre ces mécanismes de transfert en raison de leurs grands impacts sur les paramètres indicatifs du procédé d’électrolyse, par exemple l’efficacité du courant. Le calcul numérique est une façon de surmonter ces difficultés et d’éclairer les aspects moins connus du procédé de production d’aluminium. L’électrolyte utilisé pour l’électrolyse est composé par différents ions qui se déplacent dans un champ électromagnétique. Ce dernier est généré par le courant électrique intense qui passe par la couche d’aluminium et le bain. Le comportement dynamique des ions est sujet à leur gradient de concentration (la diffusion), à l’écoulement du bain (la convection) et au champ électrique (la migration). Dans le cadre de cette étude, le mouvement des ions est analysé et l’importance relative de la diffusion et de la migration est comparée en régime transitoire pour deux classes d’espèces électroactives et non-électroactives. Pour ces deux types d’espèces, on observe que la migration est le mécanisme dominant de transfert de masse dès les premières phases de l’électrolyse. Cependant, la diffusion devient graduellement le mécanisme le plus important aux électrodes pour des espèces électroactives comme Al[indice inférieur 2]OF[indice inférieur 6][indice supérieur -2] et AlF[indice inférieur 4][indice supérieur -]. Le champ électrique et le champ de concentration ont été simulés à partir d’un modèle 2-D. Les résultats montrent qu’il y a un gradient de concentration entre l’espace inter-électrodes et la région proche de la couche de gelée. Par conséquent, il y a diffusion des espèces entre ces deux régions qui vient diminuer le gradient de concentration et ainsi éviter l’épuisement des ions Al[indice inférieur 2]OF[indice inférieur 6][indice supérieur -2] ou la surconcentration des ions AlF[indice inférieur 4][indice supérieur -]. En outre, un code libre a été développé et implémenté sur OpenFOAM (une plateforme libre de librairies C++). Ce code est capable de résoudre simultanément les équations du champ électrique, du transfert de masse et de Navier-Stokes. Les principaux apports de cette thèse, tel que les modèles et résultats obtenus, peuvent éclairer les mécanismes de transfert de masse dans le bain et aux électrodes et ainsi améliorer leur compréhension.

The temperature and moisture content of biomass feedstocks both play a critical role in minimizing storage and transportation costs, achieving effective bioconversion, and developing relevant postharvest quality models. Hence, this study characterizes the heat and mass transfer occurring within baled switchgrass through the development of a mathematical model describing the relevant thermal and physical properties of this specific substrate. This mathematical model accounts for the effect of internal heat generation and temperature-induced free convection within the material in order to improve prediction accuracy. Inclusion of these terms is considered novel in terms of similar biomass models. Two disparate length scales, characterizing both the overall bale structure (global domain) and the individual stems (local domain), are considered with different physical processes occurring on each scale. Material and fluid properties were based on the results of hydraulic conductivity experiments, moisture measurements and thermal analyses that were performed using the constant head method, TDR-based sensors and dual thermal probes, respectively. The unique contributions made by each of these components are also discussed in terms of their particular application within various storage and bioconversion operations. Model validation was performed with rectangular bales of switchgrass (102 x 46 x 36 cm3) stored in an environmental chamber with and without partial insulation to control directional heat transfer. Bale temperatures generally exhibited the same trend as ambient air; although initial periods of microbial growth and heat generation were observed. Moisture content uniformly declined during storage, thereby contributing to minimal heat generation in the latter phases of storage. The mathematical model agreed closely with experimental data for low moisture content levels in terms of describing the temperature and moisture distribution within the material. The inclusion of internal heat generation was found to be necessary for improving the prediction accuracy of the model; particularly in the initial stage of storage. However, the effects of natural convection exhibited minimal contribution to the heat transfer as conduction was observed as the predominate mechanism occurring throughout storage. The results of this study and the newly developed model are expected to enable the maintenance of baled biomass quality during storage and/or high-solids bioconversion.

Thesis (MEng)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: In this study, one and two-dimensional models are developed for the evaluation of the thermal performance of a delugeable flat tube bundle to be incorporated in the second stage of an induced draft hybrid (dry/wet) dephlegmator (HDWD) of a direct air-cooled steam condenser (ACSC). Both models are presented by a set of differential equations. The one-dimensional model is analysed analytically by using three methods of analysis which are: Poppe, Merkel, and heat and mass transfer analogy. The two-dimensional model is analysed numerically by means of heat and mass transfer analogy method of analysis whereby, the governing differential equations are discretised into algebraic equations using linear upwind differencing scheme. The two-dimensional model’s accuracy is verified through a comparison of the two dimensional solutions to one dimensional solutions. Satisfactory correlation between the one and two-dimensional results is reached. However, there is a slight discrepancy in the solutions, which is mainly due to the assumptions made in one-dimensional model. The effect of tube height, tube pitch, tube width, deluge water mass flow rate, frontal air velocity, steam, and air operating conditions on the heat transfer rate and air-side pressure drop for both wet and dry operating modes are investigated. The long tube height, large tube width, small tube pitch, and high frontal air velocity are found to increase the tube bundle’s performance. However, this performance is associated with a high airside pressure drop. The performance of the deluged flat tube bundle is found to be less sensitive to the changes in the deluge water mass flow rate and air operating conditions. Furthermore, the best configuration of a delugeable flat tube bundle is identified through a comparison to round tube bundle presented by Anderson (2014). The performance of the round tube bundle is found to be around 2 times, and 1.5 times of that of flat tube bundle, when both bundles operate as an evaporative and dry air-cooled condenser respectively. / AFRIKAANSE OPSOMMING: In hierdie studie is een en twee-dimensionele modelle ontwikkel vir die evaluering van die termiese prestasie van 'n benatbare plat buis bundel in die tweede stadium van 'n geïnduseerde ontwerp hibriede (droë / nat ) deflegmator van 'n direkte lugverkoelde stoom kondensator. Beide modelle is aangebied deur 'n stel van differensiaalvergelykings. Die een-dimensionele model is analities ontleed deur die gebruik van drie metodes van analise wat: Poppe, Merkel, en die hitte en massa-oordrag analogie. Die twee-dimensionele model is numeries ontleed deur middel van hitte en massa-oordrag analogie metode van analise waardeur , die regerende differensiaalvergelykings gediskretiseer in algebraïese vergelykings met behulp van lineêre windop differensievorming skema. Die tweedimensionele model se akkuraatheid is geverifieer deur 'n vergelyking van die twee dimensionele oplossings te een dimensionele oplossings. Bevredigende korrelasie tussen die een en twee-dimensionele resultate bereik word. Maar daar is 'n effense verskil in die oplossings, wat is hoofsaaklik te wyte aan die aannames wat gemaak in een-dimensional model. Die effek van buis hoogte, buis toonhoogte, buis breedte, vloed water massa-vloeitempo, frontale lug snelheid, stoom, en in die lug werktoestande op die hitte oordrag snelheid en lug - kant drukval vir beide nat en droë maatskappy modi word ondersoek. Die lang buis hoogte, groot buis breedte, klein buisie toonhoogte, en 'n hoë frontale lug snelheid gevind die buis bundel se prestasie te verhoog. Tog is hierdie prestasie wat verband hou met 'n hoë lug - kant drukval. Die prestasie van die oorstroom plat buis bundel gevind word minder sensitief vir die veranderinge in die vloed water massa-vloeitempo en lug werktoestande. Verder is die beste opset van 'n benatbare plat buis bundel geïdentifiseer deur 'n vergelyking met ronde buis bundel aangebied deur Anderson (2014). Die prestasie van die ronde buis bundel gevind word om 2 keer, en 1.5 keer van daardie plat buis bundel , wanneer beide bundels funksioneer as 'n damp en droë lugverkoelde kondensor onderskeidelik.

Thesis (MScEng (Process Engineering))--Stellenbosch University, 2008. / The general objective for this thesis is to assess the ability of cellular automata to model relatively complex processes or phenomena, in particular thermodynamic scenarios. The mass transfer in packing materials of distillation columns was selected as an example due to the sufficient level of complexity in the distillation process, and its importance in a wide range of applications. A literature survey on cellular automata that summarizes the information currently available in formal publications and the internet is included to provide a general overview on the basic theoretical principles and the application of cellular automata models in the process engineering industry. The literature study was also used to identify potential requirements for the new research project. The study objective includes the construction of a cellular automata model that is able to represent transition of solutes from the fluid on the micro-surfaces of packing materials to the by-passing vapour stream, as well as the steady-state equilibrium between evaporation and condensation. Iterated model parameters sufficient for the realistic modelling of mass transfer as a result of thermodynamic driving forces, are required to meet this objective. The model behaviour was compared and the parameters subsequently adjusted according to the behaviour that is theoretically expected from the system being simulated. Qualitative (although sometimes in a quantitative format) rather than quantitative observations and comparisons were made seeing that the model has not yet been calibrated. The model that has been developed to date is not able to simulate the individual effects of chemical and thermodynamic properties although a realistic simulation of the cumulative effect exerted by these factors, or change thereof, on a system has been achieved. The accuracy of the results that have been obtained by using iterated parameters cannot be guaranteed for scenarios that deviate too much from the systems that have already been modelled successfully. The trade-off between the ability of the model to incorporate the effect of polarization, its ability to represent separation, in particular the condensation of hydrophilic substances, for strong hydrophilic packing materials and its ability to incorporate a large number of species limits the range of scenarios that can be successfully modelled. The model is able to represent the effect of a declining driving force (difference between the component vapour pressure of the gas phase and that of the liquid phase) that is typical of a system which is allowed to reach equilibrium after an initial disturbance. The model is also able to represent an additional driving force for separation caused by the effect of intermolecular forces. The model also displays the potential ability to represent the effect of different surface structures of the packing material on the extent of separation achieved at steady state as well as the rate at which such steady state conditions have been achieved. The model must be correctly scaled to minimize inaccurate results. Although several adjustments are needed to eliminate some limitations, the model has proven itself worthy of further development due to its capability to represent the basic characteristics of mass transfer in packing materials.