The present investigation can be divided into two parts: (a) experiments made to examine the mechanism of dropwise condensation of steam with particular reference to the stability of drop promoting surfaces as affected by the material of cooled surface, the drop promoter, the surface finish, the rate of heat transmission, and the presence of non-condensable gas. and (b) a theoretical analysis of the beat transmission through individual droplets, the transient heat transfer through exposed areas, the statistical study of drop size distribution, and the estimation of steam side coefficient. An apparatus was developed to examine qualitatively,the behaviour of drop promoting surfaces on a small scale. It is considered that sufficient evidence was found to show that steam in contact with a cooled surtace condenses as a thin liquid film which later breaks into droplets. surfaces treated to give dropwise condensation deteriorate into mixed condensation in due time, and the duration tor which a treated surface maintains dropwise condensation varies between a few hours to several days, depending on many factors among which· the presence of non-condensable gas must not be overlooked. An approximation to the heat transmission through individual droplets has been worked out with assumed heat flow lines. The result, checked by the relaxation method. is correct within . + 10%. An analysis Of the transient heat transfer through exposed areas was made neglecting the increasing resistance of any accumulating liquid. The drop size distribution was analyzed tor one drop promoting surface at three different heat transmission rates. Based on this drop size distribution, the heat transmission through the drops was estimated by assuming they were held at rest on a cooled surface conducting heat under a steady state. . The estimated coefficient comes within the range or experimental results of many investigators.

Enhancement of condensation heat transfer by wrapping of fine WIres on a condenser tube and Marangoni condensation of binary mixtures have been studied. For wire-wrapped tubes enhancement is due to modification of the profile of the condensate surface which leads to axially-directed pressure gradients and local thinning of the condensate film. Approximate theories do not agree well with limited available data prior to the present work. A systematic experimental investigation has been conducted using three fluids with widely different properties. Five wire diameters and a range of winding pitch have been used. Maximum heat-transfer enhancement ratios of 3.7, 2.2 and 2.3 for R-I13, ethylene glycol and steam respectively were obtained. The effect of inundation for steam condensation on wire-wrapped tubes has also been investigated. Extensive data exist for Marangoni condensation of steam-ethanol mixtures on small plane ve.rtical surfaces. Here the practically more relevant case C?f a horizontal tube has been studied. Apparent differences between the vertical plate and horizontal tube data are shown to be due to circumferential variation of tube surface temperature. Enhancement ratios up to around 3.7 have been obtained with as little as 0.05% mass fraction of ethanol in the boiler feed. For wire-wrapped tube and Marangoni condensation, a copper condenser tube (outside diameter 12.2 mm) fitted with four embedded wall thermocouples was cooled internally by water using a wide range of flow rates. The coolant temperature rise was measured to within 0.01 K using a ten-junction thermopile while the coolant temperature rise ranges were 0.11 to 0.77 K, 0.89 to 9.28 K and 1.00 to 6.98 K for the wire-wrap tests with R-I13, ethylene glycol and steam respectively and 1.24 to 29.1 K for Marangoni condensation. The effect on the boiler performance for water-ethanol mixtures has also been investigated.

It is a well-known phenomenon that during Marangoni condensation of binary mixtures, a small concentration of more volatile constituent with smaller surface tension gives significant heat transfer enhancements. This is due to surface tension gradients causing instability in condensate film, resulting in a pseudo-dropwise mode of condensation which resembles closely to dropwise condensation of pure fluid on the hydrophobic surface, consequently, the film gets thinner with lower thermal resistance across the condensate film and thus higher heat transfer coefficient is achieved. Marangoni condensation of steam-ethanol mixtures has been widely investigated in the past. However, Marangoni condensation of self-rewetting fluids e.g. steam-butanol is yet to be investigated where the constituent in a small concentration is a less volatile component. Marangoni condensation of steam-ethanol, steam-butanol and steam-propanol mixtures has been investigated on a horizontal smooth tube at an atmospheric pressure. For all experiments, concentrations by mass in the boiler feed when cold prior to start of the experiment were 0.001%, 0.005%, 0.01%, 0.025%, 0.05%, 0.1%, 0.5% and 1.0%. The coolant temperature rise was measured accurately with a ten-junction thermopile. Tube wall temperature was measured using four thermocouples embedded in the test tube wall. Effects of pressure and vapour velocity over a wide range of vapour-to-surface temperature difference have been investigated. Care was taken to avoid error due to the presence of air in the vapour. Marangoni condensation of steam-butanol and steam propanol mixtures show significant heat transfer enhancements compared with that of steam-ethanol mixtures. Higher Heat flux and heat-transfer coefficients were observed. For the steam-ethanol mixtures, enhancement ratio (heat flux or heat-transfer coefficient divided by the corresponding value for pure steam condensation on a horizontal smooth tube for the same vapour-to-surface temperature difference and vapour velocity) of 5.5 was found at an ethanol concentration of 0.01%. For steam-butanol mixtures, the maximum enhancement ratio was found to be 11 at a concentration of 0.005% and 0.01%. For steam-propanol mixtures, the maximum enhancement ratio of 8.5 was found at the same mass concentrations as steam-butanol mixtures. Enhancement ratio was generally higher at lower ethanol concentrations, increases at first with increasing vapour-to-surface temperature difference and subsequently decreases at high vapour-to-surface temperature difference. Finally, a semi-empirical model was proposed to predict the Marangoni condensation of steam-ethanol mixtures based on the vapour phase diffusion theory of Sparrow and Marchall (1969) and pure steam dropwise theory of Rose (2002).

Three existing condensation heat transfer models are validated using 544 experimental data points for pure refrigerants and refrigerant mixtures. The Cavallini et al. (1999) model predicts well with the pure-refrigerant data sets. However, the Cavallini et al. (1999) model fails to predict the refrigerant-mixture data sets. The Yu and Koyama (1998) model, which is applicable for the pure refrigerants only, fails to predict most of the R22 data sets. The Kedzierski and Goncalves (1999) model, which is applicable for both pure refrigerants and refrigerant mixtures, yields relatively high mean absolute deviations for most of the pure-refrigerant data sets. The Kedzierski and Goncalves (1999) model does not account for the mass transfer thermal resistance in refrigerant mixtures. A new pure-refrigerant model and a new refrigerant-mixture semi-empirical model have been developed. Both the new models successfully predict the experimental data for pure refrigerant and for refrigerant mixtures.

A study of the condensation of hydrocarbons and zeotropic hydrocarbon mixtures in smooth horizontal tubes was conducted. Measurements of condensation heat transfer coefficients and frictional pressure drop were taken over a range of mass fluxes (G = 150 – 450 kg m-2 s-1), a range of reduced pressures (Pr = 0.25 - 0.95), for two tube diameters (D = 7.75 and 14.45 mm), several working fluid-to-coolant temperature differences (ΔTLM = 3 – 14°C) and temperature glides (ΔTGlide) between 7 - 14°C. The wide range of conditions investigated in this study provides considerable insight on the transport phenomena influencing condensation in pure fluids and their mixtures. The trends in heat transfer coefficient and frictional pressure gradient are discussed and compared with the predictions of correlations from the literature. The results of the experiments, combined with previous flow visualization studies on hydrocarbons, were used to develop physically consistent heat transfer and frictional pressure gradient models that are applicable to pure fluids and zeotropic mixtures. A framework was developed for zeotropic mixture condensation that recommends a specific modeling approach based on the observed trends in the heat transfer coefficient and the points of deviations from pure fluid trends. The documentation of the condensation heat transfer and pressure drop behavior of environmentally friendly refrigerants, and the development of accurate correlations, will facilitate their widespread introduction as a working fluid for refrigeration cycles. Furthermore, the accurate pure fluid models, which serve as a baseline case for zeotropic mixture modeling, yield more effective predictions of zeotropic mixture condensation, which will lead to increased efficiencies of chemical processing plants.

No description available.

Dropwise condensation (DWC) has the potential to enhance heat transfer compared to filmwise condensation (FWC). The heat transfer rates achieved by DWC depend on the drop size distribution, which is influenced by nucleation processes of newly formed drops. In DWC modeling, the nucleation site density Ns is used as an input parameter to obtain the drop size distribution of small drops. However, due to the small scale of the condensate nuclei, direct observation is difficult, and experimental data on the nucleation site density are scarce. In the literature, values in the range of 109 m−2 to 1015 m−2 can be found for Ns. In this paper, we report DWC experiments on SiO2 and 1H,1H,2H,2H-perfluorodecyltriethoxysilane (PFDTES) thin hydrophobic coatings that show significantly different nucleation site densities. Nucleation site densities are estimated from high-speed imaging of small drops during initial condensation and from model calibration using established DWC theory. We have found the values for Ns to be in the range from 1.1×1010 m−2 to 5.1×1011 m−2 for the SiO2 coating and 1011 m−2 to 1013 m−2 for the PFDTES coating. Our results show that there can be large differences in the nucleation site density under similar conditions depending on the surface properties. This underlines the importance of investigating nucleation site density specifically for each surface and under consideration of the specific process conditions used for DWC.

Superhydrophobic (SHS) and lubricant-infused surfaces (LIS) exhibit exceptional non-wetting characteristics that make them attractive for energy production applications including steam condensation and fouling mitigation. The dissertation work focuses on application of non-wetting surfaces to energy production using a systematic approach examining each component of surface fabrication in three functional areas. First, SHS and LIS are fabricated using robust, scalable methods and tested for durability in heated, wet conditions and under high-energy water jet impingement. Clear performance differences are shown based on surface texturing, functionalizing agent, and infused lubricant. Second, SHS and LIS are applied to tube exteriors and evaluated for their ability to produce sustained dropwise condensation in a typical power plant condenser environment. The surfaces are shown to produce heat transfer coefficients up to 7-10 times that of film-wise condensation, with condenser effectiveness of 0.92 or better compared to effectiveness of about 0.6 in conventional condensers. Third, LIS on the interior of tubes are assessed in accelerated mineral fouling conditions. LIS are shown to mitigate calcium sulfate and calcium carbonate fouling under laminar conditions. The results of the study bear profound benefits to reducing the levelized cost of condensers and water uptake in thermoelectric power plants, that currently consume about 50% of the total water use in the U.S. / Doctor of Philosophy / Creating durable, hybrid surfaces for improved steam condensation and fouling mitigation would provide substantial impact to power generation worldwide. Bioinspired, non-wetting surfaces, such as superhydrophobic (SHS) and lubricant-infused surfaces (LIS) exhibit exceptional non-wetting characteristics that make them attractive for energy applications. Each of these non-wetting technologies, however, faces durability and scalability challenges that make them unfeasible for widespread, practical adoption. As a result, decades of materials science research have stagnated in the research laboratories with limited demonstrations of dropwise condensation and fouling mitigation in static situations. The dissertation work focuses on application of SHS and LIS to energy production using a systematic approach examining each component of surface fabrication in three functional areas. First, SHS and LIS are fabricated using robust, scalable methods and tested for durability using ASTM standard static and dynamic evaluation methods. Clear performance differences are shown based on surface texturing, functionalizing agent, and infused lubricant. Second, dropwise steam condensation on the surfaces are shown to exhibit heat transfer performance an order of magnitude greater than film-wise condensation in a typical power plant condenser environment. The surfaces are shown to produce heat transfer coefficients up to 7-10 times that of film-wise condensation, with condenser effectiveness of 0.92 or better compared to effectiveness of about 0.6 in conventional condensers. This work presents for the first time, a non-dimensional correlation for a priori prediction of LIS heat transfer performance given known qualities of the LIS. Third, challenges of fouling mitigation in power plants have been studied for over a decade. This work demonstrates for the first time that LIS applied to the interior of tubes mitigate calcium sulfate and calcium carbonate fouling in both static and laminar flow conditions.

Eco-friendly non-CFC refrigerants were introduced in the Air Conditioning and Refrigeration industry during the last few years to reduce damage to the stratospheric ozone layer. The HFC refrigerant R-134a, which has zero Ozone Depletion Potential (ODP), is being used extensively as a replacement for R-12 and also in some centrifugal chillers as a replacement for R-11. However, the disadvantage of R-134a is its comparatively high global warming potential (GWP). Owing to energy crisis and also to reduce the indirect warming impact resulting from electrical energy usage, the new refrigeration systems should be operated at the lowest possible condensing temperatures. In view of this, several active and passive techniques for augmentation of condensation heat transfer and reduction of condensation temperature are gaining increasing attention. Passive augmentation methods are more popular than active ones. To this end, micro-finned tubes of various geometrical shapes are being explored for compact heat exchangers in the refrigeration industry as the best choice. Towards understanding the enhancement in condensation heat transfer coefficients in micro-finned tubes, a test facility has been fabricated to measure the condensing coefficients for R-134a refrigerant. Condensation experiments have been conducted on single plain and finned tubes of outer diameter 19 mm with a refrigerant saturation temperature of 400C and tube wall temperatures 350C, 320C, 300C and 280C respectively. Water is used as the cooling medium inside the tubes with the flow rate varying from 180 lph to 600 lph. The condensing coefficient typically ranged from 0.9 – 1.4 kW/(m2 K) for plain tubes and from 4.2 to 5.8 kW/(m2 K) for the finned tubes. The results of the plain v tube are found to compare favourably with the Nusselt’s theory, leading to a validation of the experimental procedure. Upon comparing the results of finned and plain tubes, it is found that provision of fins result in an enhancement factor of 3.6 to 4.6 in the condensation heat transfer coefficients. This level of enhancement is larger than that resulting from the enhanced surface area of the finned tube surface, suggesting that, apart from the extended area, the surface tension forces play an important role in the augmentation process by driving the condensate from the fin crests to the valleys in between the fins. The measured augmentation factors have also been cross-checked using the Wilson plot method. Detailed error analysis has been performed to quantify the uncertainty in the condensation heat transfer coefficient. The performance of a bank of tubes has been determined based on the measurements carried out on practical condensers of two large chillers with refrigerating capacities of 500 TR and 550 TR. On comparing the finned tube bank results and the single finned tube results, it is found that the average condensation heat transfer coefficient in a bank of tubes having N rows varies as N ¯1/6. The deterioration is in agreement with the relation proposed by Kern.

The master's thesis deals with actual possibilities of acquisition water from air humidity in order to obtain fresh water, focusing on energy intensity of vapor-compression refrigeration for it is production. There is a basic determine study for Czech climatic zone in selected localities. For graded cooling capacity dependencies describing the acquisition of water from the air, where is the energy intensity ranges on average between 0,3 ÷ 0,5 kWh/l. For the selected family house are set the individual variants of non-potable rainwater management.