Heat and mass transfer in direct contact hygroscopic condensation

A theoretical analysis of direct contact hygroscopic-condensation of cold vapor on hot films is presented. The condensation of the relatively low temperature, low pressure, vapors on a hot film of an hygroscopic brine solution may occur due to the reduced vapor pressure of a sufficiently concentrated solution. The driving force for condensation is the difference between the partial pressure of water in the brine and the partial pressure of the condensing water vapor. The condensation is also governed by simultaneous mass transfer mechanisms, due to a non-isothermal absorption, with a possible opposing thermal driving force in the condensing vapor phase. The overall performance is determined by the accumulating effects of the various resistances to heat and mass transfer. The present study is aimed to elucidate the controlling mechanisms associated with this absorption-condensation process, and suggest overall transfer rates at the laminar and turbulent flow regimes.     

Rheological properties of paraffin suspensions of surface-modified alumina powder for low-pressure injection moulding

Due to the hygroscopic nature of fine alumina powders, the presence of water may have a detrimental effect on the rheological properties of a suspension in melted paraffin for low-pressure injection moulding (LPIM). For this reason, a modification of the powder surface from hydrophilic to hydrophobic is essential for the production of high-quality moulded ceramics. In our paper, the efficiency of the application and the chemisorption of protective monomolecular layers of a long-chain carboxylic acid or its salts will be presented. The effect of the powder treatment on the shear viscosity and the viscoelastic properties of the suspensions for LPIM as well as on their stability in terms of the detrimental effect of water will be discussed.This paper was presented at the first Annual European Rheology Conference (AERC) held in Guimarães, Portugal, September 11-13, 2003.     

Influence of high pressure processing on the linear viscoelastic properties of egg yolk dispersions

The effect of high-pressure treatments on the gelation of egg yolk dispersions was studied by using differential scanning calorimetry (DSC) and small amplitude oscillatory shear (SAOS). The influence of pressure of processing and pH were analysed. The DSC results suggest a progressive decrease in thermal denaturation enthalpy as pressure level increases related to protein denaturation. SAOS was used to evaluate the effect of different pressure levels on the linear viscoelastic behaviour of egg yolk dispersions. An increase in the pressure level produces a dramatic change in the linear viscoelastic behaviour undergoing a sol–gel transition. High hydrostatic pressure (HHP) processing was also analysed as a function of pH and solids content. The results obtained confirm that the impact of high pressure on aggregation and network formation can be modulated by pH. HHP processing of egg yolk systems is highly affected by protein concentration particularly when it is compared to heat processing.     

Small and large strain rheology of wheat gluten

 The rheological properties of wheat gluten were studied under both small and large deformation and compared with those of the parent flours. The limiting strain of linear viscoelastic behaviour of gluten doughs, 3 × 10⁻², was an order of magnitude larger than that of the flour doughs, 10⁻³. The role of starch in the lower limiting strain of flour doughs was indicated by the exponential decrease in the limiting strain of gluten-starch mixtures with greater quantities of starch. Large strain measurements showed gluten doughs possessed greater shear and elongational viscosities than flour doughs and these differences were greatest at lower shear and elongation rates (0.01 and 0.1 s⁻¹). The larger viscosities of flour and gluten doughs at the low strain rates help to stabilise and prevent the collapse of gas bubbles during bread fermentation and baking. Increasing starch levels in gluten-starch mixtures, at either constant or optimal water levels, lowered the elongational viscosity. Dynamic measurements were, however, more sensitive to the level of water added to the gluten-starch mixtures. The storage modulus decreased with increasing starch levels when constant water levels were used to prepare the mixtures, but when optimal water levels were used the storage modulus increased. Gluten and starch are major contributors to the large and small strain rheological properties of flour doughs; however, gluten-starch mixtures were unable to duplicate exactly the rheological properties of flour doughs, indicating that other flour components such as pentosans, lipids and water soluble proteins also influence dough rheology. Received: 20 March 2001 Accepted: 11 July 2001     

Insight into the interaction between water and surfactant aerosol particles based on molecular dynamics simulations

The ubiquitous surfactant significantly influences the hygroscopic growth of atmospheric aerosol particles. However, knowledge on the morphology of surfactant particles after the adsorption of water is insufficient. In this study, the interaction between water and particles composed of surface active malonic acid (MA) or adipic acid (AA) are simulated based on the molecular dynamics method. The key point is the combined effect of temperature and water content on the structural properties of particles and the surface propensity of surfactants at the equilibrium state. Results show that demixed structure 1 with the adsorption of water clusters on acid grain, mixed structure and demixed structure 2 with acids coating on water droplet can be observed. With temperature increasing from 160 K to 330 K the surface propensity of MA and AA increases first and then weakens. Near the standard atmospheric temperature (280–330 K), the surface propensity of MA and AA increases with increasing water content and alkyl group, and its sensitivity to temperature and water content varies regularly. Moreover, all surfactants at the particle surface orient their hydrophobic groups toward the gas. These findings improve our insight into the surfactant partitioning and further assist in more accurate prediction of the particle hygroscopic growth.     

Experimental studies of a hygroscopic condenser-evaporator heat exchanger based on concentration differences of vertically falling films

A concentration-driven power cycle motivated by differences in vapor partial pressures (boiling point rise) and latent heats of brine and water is studied. The condensation of relatively low-pressure, low-temperature vapor occurs on the free interface of a relatively hot falling film of a hygroscopic salt solution due to the reduced vapor pressure of the brine. The heat released is transferred to the evaporating/cooling water film on the other side of a vertical plate separating the brine and water films. The process is maintained because the latent heat of condensation on the brine film is higher than the latent heat of evaporation of pure water. The condensation driving force is the difference between the partial pressure of condensing water vapor and that of water in the brine solution. The simultaneous mass and heat transfer mechanisms associated with this nonisothermal absorption can occur even against an opposing thermal driving force in the condensing vapor phase. Complementing earlier studies by the same authors, a vertical film-type condenser-evaporator heat exchanger is considered. The experimental study deals with the effects of the various parameters involved in this rather unique process and the mechanisms that control them. The experimental results prove the potential of operating this new heat transfer modality and provide the background for the theoretical determination of the optimal performance of this direct-contact power cycle.     

Effects of variable fluid properties on natural convection of MHD fluid from a heated vertical wavy surface

In the present paper, the influence of temperature-dependent fluid properties, density, viscosity and thermal conductivity on MHD natural convection flow from a heated vertical wavy surface is studied. It is assumed that, the fluid density and the thermal conductivity vary as exponential and linear functions of temperature, respectively. However, the fluid viscosity is assumed to vary as a reciprocal of a linear function of temperature. The model analysis used here is more relevant to liquid flow. Using the appropriate variables, the wavy surface are transformed into a flat one. The transformed boundary layer equations are solved numerically, using implicit-Chebyshev pseudospectral method, for several sets of values of the physical parameters, namely, the temperature dependent fluid properties parameters, the magnetic parameter, the amplitude-wavelength ratio parameter, and the Prandtl number. The numerical values obtained for the velocity, temperature, shearing stress, and the Nusselt number are presented through graphs and tables for several sets of values of the parameters. The effects of the physical parameters on the flow and heat transfer characteristics are discussed. The results were compared with numerical solutions of previous works. The present results are found to be in good agreement.     

Rheology and microstructure of gluten and soya-based o/w emulsions

Highly concentrated oil-in-water (o/w) emulsion stabilised by means of gluten and soya protein isolate (SPI) at low pH have been characterized by means of linear dynamic viscoelasticity and droplet size distribution analysis (DSD). The microstructure of these emulsions has been characterized at a colloidal level by using confocal laser scanning microscopy (CLSM) and light microscopy (LM). These emulsions always exhibited a behaviour characteristic of highly flocculated emulsions with a mechanical spectrum showing a well-developed plateau region. DSD results generally showed log normal bimodal profiles. Microstructure images revealed occurrence of a close packing of droplets with a broad distribution of sizes participating in the formation of a three dimensional flocculated network. The Mason model of elasticity of compressed emulsions has been used to correlate viscoelastic and microstructural parameters giving adequate fitting but underestimating the elastic properties obtained for the highest concentration of gluten. These deviations may be explained in terms of an enhancement of the elastic network formed in the aqueous phase in which the glutenin fraction must play an important role. Paper presented at the Annual European Rheology Conference (AERC) 2005, April 21-23, Grenoble, France.     

Slip effects on heat and mass transfer in MHD micropolar fluid flow over an inclined plate with thermal radiation and chemical reaction

The influence of partial slip, thermal radiation, chemical reaction and temperature‐dependent fluid properties on heat and mass transfer in hydro‐magnetic micropolar fluid flow over an inclined permeable plate with constant heat flux and non‐uniform heat source/sink is studied. The transverse magnetic field is assumed as a function of the distance from the origin. Also it is assumed that the fluid viscosity and the thermal conductivity vary as an inverse function and linear function of temperature, respectively. With the use of the similarity transformation, the governing system of non‐linear partial differential equations are transformed into non‐linear ordinary differential equations and are solved numerically using symbolic software MATHEMATICA 7.0 (Wolfram Research, Champaign, IL). The numerical values obtained for the velocity, microrotation, temperature, species concentration, skin friction coefficient and the Nusselt number are presented through graphs and tables for several sets of values of the parameters. The effects of various physical parameters on the flow and heat transfer characteristics are discussed.Copyright © 2011 John Wiley & Sons, Ltd.     

骨料堆积注浆再生混凝土的抗压强度及其影响因素

为提高再生粗骨料利用率及降低再生混凝土质量控制难度,本文采用骨料堆积注浆法制备早强型再生混凝土。利用Griffith断裂力学理论及"实际水料比与名义水料比"的差异,分析了骨料性能对混凝土强度的影响,并通过扫描电子显微镜(SEM)分析了再生混凝土界面过渡区(interfacial transition zone,ITZ)的特点。结果表明,骨料强度及吸水率均对再生混凝土强度具有影响,且骨料吸水作用对混凝土强度提高具有促进作用,再生混凝土的界面结合强度较高。