Modeling heat and mass transport phenomena at higher temperatures in solar distillation systems - The Chilton-Colburn analogy

In the present investigation efforts have been devoted towards developing an analysis suitable for heat and mass transfer processes modeling in solar distillation systems, when they are operating at higher temperatures. For this purpose the use of Lewis relation is not new although its validity is based on the assumptions of identical boundary layer concentration and temperature distributions, as well as low mass flux conditions, which are not usually met in solar distillation systems operating at higher temperatures associated with considerable mass transfer rates. The present analysis, taking into consideration these conditions and the temperature dependence of all pertinent thermophysical properties of the saturated binary mixture of water vapor and dry air, leads to the development of an improved predictive accuracy model. This model, having undergone successful first order validation against earlier reported measurements from the literature, appears to offer more accurate predictions of the transport processes and mass flow rate yield of solar stills when operated at elevated temperatures.     

Analysis of the heat and mass transfer processes in solar stills - The validation of a model

The outcome of the earlier systematic research work on the theoretical modeling of the complex transport phenomena occurring in solar stills was the development of the fundamental Dunkle’s model, already known almost four decades ago. Although it has been based on several simplified assumptions, this model has extensively been employed over the years as a convenient and sufficiently accurate predictive tool for solar stills working under ordinary operating conditions. However, it has occasionally been reported that it fails under unusual operating conditions, mainly corresponding to higher average temperatures, usually leading to higher distillate yields. The aim of the present investigation was to relax the initially established simplified assumptions of the fundamental Dunkle’s model and to evaluate the comparative accuracy of both, the refined and the earlier fundamental models against an extensive body of previously reported measurements from the literature, both field and laboratory. The comparative presentation of results indicates that although both models are impressively correct for ordinary low temperature operating conditions where the humid air thermophysical properties are close to those of dry air, the saturation vapor pressure at the brine and condensing plate temperatures are negligible compared to barometric pressure and the familiar Jakob’s dimensionless Nusselt-Rayleigh correlation for natural convection heat transfer appears to be valid, they both fail at higher operational temperatures. It appears that as far as Dunkle’s simplified model is concerned, this occurs not only owing to the first two counteracting effects but also to the effect of the dimensionless convective heat transfer correlation affecting also the accuracy of the refined model, which fails to predict precisely the natural convection conditions at higher Rayleigh numbers representing conditions of strong turbulence in the solar still cavity. Assuming a constant asymptotic value of the exponent n = 1/3 which persists over a broad region of high Rayleigh numbers relevant to solar still operation, an improved value of the proportionality constant C around the value of 0.05 was estimated for the accurate prediction of measurements, at least as far as the available data from the literature is concerned.     

Effect of Relative Humidity on the Prediction of Natural Convection Heat Transfer Coefficients

Results of an investigation into the sensitivity of natural convection heat transfer correlations with respect to relative humidity are presented. Given the relatively small values of natural convection heat transfer coefficients, small changes in the thermophysical properties can have a significant impact on the values predicted by theoretical/empirical correlations. In this study, the thermophysical properties are assumed to be those of a dry air and water vapor mixture. The mole fractions are determined as a function of relative humidity. Several widely used natural convection heat transfer correlations have been examined to determine the impact of varying the relative humidity on the predicted Nusselt number. The results show a general trend of an increasing Nusselt number with relative humidity. The results presented in this paper provide an engineering tool for obtaining accurate values of natural convection heat transfer coefficients for a moist air environment using only the thermophysical properties of dry air.     

Effect of thermophysical properties models on the predicting of the convective heat transfer coefficient for low concentration nanofluid

The term of nanofluid refers to a solid–liquid mixture with a continuous phase which is a nanometer sized nanoparticle dispersed in conventional base fluids. In order to study the heat transfer behavior of the nanofluids, precise values of thermal and physical properties such as specific heat, viscosity and thermal conductivity of the nanofluids are required. There are a few well-known correlations for predicting the thermal and physical properties of nanofluids which are often cited by researchers to calculate the convective heat transfer behaviors of the nanofluids. Each researcher has used different models of the thermophysical properties in their works. This article aims to summarize the various models for predicting the thermophysical properties of nanofluids which have been commonly cited by a number of researchers and use them to calculate the experimental convective heat transfer coefficient of the nanofluid flowing in a double-tube counter flow heat exchanger. The effects of these models on the predicted value of the convective heat transfer of nanofluid with low nanoparticle concentration are discussed in detail.     

Convective Heat Transfer of an Air Bubble in Water With Variable Thermophysical Properties

A numerical study of convective heat transfer of an air bubble in water with variable thermophysical properties is considered. Two-dimensional simulations of multifluid flows with heat transfer include the Navier–Stokes, energy, and volume of fluid (VOF) advection equations. The solver computes the flow field and temperature by solving the systems of Navier–Stokes equations and the energy equation using the finite–volume method with the SIMPLE algorithm and tracks the position of interface between two fluids with different fluid properties by the VOF method with piecewise linear interface construction technique. Empirical correlations in terms of temperature for thermophysical properties are considered in the simulations. The convective heat transfer model is assessed with a benchmark problem of cooling of water and compared with previous literature data showing good agreement. Finally, a numerical study of the effect of the bubble diameter in the range from 2 mm to 3 mm on heat transfer is performed.     

Experimental Study of Condensation in a Shell and Tube Heat Exchanger in the Presence of a Noncondensable Gas

In this paper, an experimental study of the condensation of water vapor from a binary mixture of air and low‐grade steam has been depicted. The study is based upon diffusion heat transfer in the presence of high concentration of noncondensable gas. To simplify the study, experimental analysis is supported by empirical solutions. The experimental setup is custom designed for testing a new shell and tube type heat exchanger supplied by the manufacturer. Air–vapor mixture at 80 °C (max) and 20.2% relative humidity enters the heat exchanger at a mass flow rate of 480 kg/h and condenses 27 kg/h vapor using cooling water at an inlet temperature of 7 °C to 10 °C and mass flow rate of 3500 kg/h. By using the experimental data of constant inlet air mass fraction, mixture gas velocity, and different volumetric flow rate of the cold fluid, the local heat transfer coefficients are obtained. The main objective of this work is to establish an approximate value for surface area and overall heat transfer coefficient of a horizontal shell and tube condenser used in process space. Under designed working conditions, the condenser is found to work efficiently with 90% vapor condensation by mass.     

Combined heat and moisture convective transport in a partial enclosure with multiple free ports

Combined heat and moisture transport in an enclosure with free ports has been investigated numerically. Enclosed moist air interacts with the surrounding air through these free-vented ports. The governing conservation equations were solved numerically using a control volume-based finite difference technique. Appropriate velocity boundary conditions at each ports are imposed to achieve overall mass conservation across this system. Air, heat and moisture transport structures are visualized respectively by streamlines, heatlines and masslines. Effects of buoyancy ratio, thermal Rayleigh number on convective heat/moisture transfer rate and flow rate across each free-vented port are discussed. Particularly, Numerical results demonstrate that the convective heat and moisture transport patterns and transport rates on horizontal ports greatly depend on properties of porous medium, while the air exchange rate on vertical port is almost unaffected by the buoyancy ratios for most situations.     

Saturated solar ponds: 2. Parametric studies

The effects of following parameters on the performance of saturated solar ponds are studied: thickness of upper convective zone, nonconvective zone, and lower convective zone; starting time of the pond; water table depth below the pond; ground thermal conductivity; transmissivity of salt solution; incident radiation; ambient air temperature, humidity, and velocity; thermophysical properties of salt solution; pond bottom reflectivity; convection, evaporation, radiation, and ground heat losses; temperature and rate of heat removal; type of salt. Magnesium chloride and potassium nitrate salt ponds located at Madras (India) are considered for the parametric study. A comparison is also made with an unsaturated solar pond.     

Thermal performance of a eutectic mixture of lauric and stearic acids as PCM encapsulated in the annulus of two concentric pipes

Thermal performance characteristics of a eutectic mixture of lauric and stearic acids as phase change material (PCM) during the melting and solidification processes were determined experimentally in a vertical two concentric pipe-energy storage system. This study deals with three important subjects: The first one is to determine the eutectic composition ratio of the lauric acid (LA) and stearic acid (SA) binary system, and to measure its thermophysical properties by DSC. The second one is to establish the thermal characteristics of the mixture such as total melting and solidification times, the heat transfer modes in melted and solidified PCM, and the effect of Reynolds and Stefan numbers as inlet heat transfer fluid (HTF) conditions on the phase transition behaviors. The final one includes the calculations of the heat transfer coefficients between the outside wall of the HTF pipe and the PCM, and heat fractions during the melting and solidification processes of the mixture, and also the discussion of the effect of inlet HTF parameters on these characteristics. The LA–SA binary system in the mixture ratio of 75.5:24.5 wt % forms a eutectic, which melts at 37°C and has a latent heat of 182.7 J g⁻¹, and, thus, these properties make it an attractive phase change material used for passive solar space heating applications such as building and greenhouse heating with respect to the climate conditions. The experimental results indicated that the mixture encapsulated in the annulus of two concentric pipes has good thermal and heat transfer characteristics during the melting and solidification processes, and it has potential for heat storage in passive solar space heating systems.     

Coupled heat and mass transfer in a counter flow hollow fiber membrane module for air humidification

Hollow fiber membrane based air humidification offers great advantages over the traditional methods because the liquid water droplets are prevented from mixing with the process air, while water vapor can permeate through the membranes effectively. The novelty in this research is that the coupled heat and moisture transport in a hollow fiber membrane module for air humidification is investigated, both numerically and experimentally. The air stream and the water stream flow in a counter flow arrangement. It is found that the membranes play a key role in humidification performances. For sensible heat transfer, both the liquid side and the membrane side resistance can be neglected, while the total heat transfer coefficients are determined by the air side heat transfer coefficients. In contrast, in mass transfer, only the liquid side resistance can be neglected, while the total mass transfer coefficients are co-determined by membrane properties and the air side convective mass transfer coefficients.     

Thermodynamic and thermophysical effects enabling high-forced convection heat transfer coefficients in supercritical fluids

Abstract

The strong variation of thermophysical properties of working fluids operating in the vicinity of the critical point makes this thermodynamic domain attractive to several energy applications. Therefore, herein a two-dimensional numerical method is used to investigate the effect of local thermophysical property variations on the local and overall thermal performance of internal convective heat transfer in a pipe in 324 operational conditions. Focusing on carbon dioxide and water as heat transfer fluids, an association of the variation of key thermophysical properties with thermohydraulic performance metrics is proposed, namely: (a) the local and (b) mean convective heat transfer coefficient and (c) the maximal temperature obtained at the tube wall. It is shown that there is an optimal combination of parameters such as mass flow rate, operating pressure, wall heat flux, and inlet temperature that, when properly selected, allow for a minimal maximal wall temperature. As expected, optimality is strongly associated with the Widom—or pseudo critical—line that extends from the critical point. Interestingly, however, contrary to what is observed in constant-property fluids, high heat transfer coefficient or minimal maximum temperature lead to different sets of optimal operating conditions. This difference is explained by how thermophysical properties vary locally along heat exchangers, which significantly affects overall heat transfer.     

Finite-volume modelling of heat and mass transfer during convective drying of porous bodies – Non-conjugate and conjugate formulations involving the aerodynamic effects

In this study, a numerical procedure is outlined and representative results for heat and mass transfer during convective drying of porous bodies are presented. The Luikov model was implemented and applied both on individual samples of construction materials and agricultural products, as well as on a drying-chamber scale, with parallel flow of a hot air stream over rectangular slabs which represent the product to be dried. In the latter case the configuration is an experimental dryer in which the heat source is a solar air collector with evacuated tubes. A general approach was developed that allows a selection between modelling of phenomena either in the drying solid only, or considering an extended simulation domain encompassing, apart from the solid body, the flow of air as well. In the second case, the solution of the flow field is pursued along with a conjugate heat/mass transfer problem coupling the solid and fluid phenomena and in both cases phase change (evaporation) was taken into account. For the numerical simulation, the finite-volume method was used. The validation of the model was based on experimental and numerical results from the literature and results from simulations that were conducted in the pursuit of the energetic optimization of an experimental solar dryer of NCSR “Demokritos” are presented. In the latter case, the effect of the particular flow field features developing for a single and a double-plate configuration on the heat/mass transport and drying rates is demonstrated. Such a methodology could be used to analyze the transport phenomena in any type of convective dryer, including those utilizing solar energy as the heat source.     

An Experimental Study on Latent Heat Recovery of Exhaust Wet Flue Gas

The experiment was conducted to investigate the heat transfer performance of wet flue gas in a vertical tube. The factors influencing the convective condensation of wet flue gas were experimentally investigated. The measured results indicate that the convective heat transfer of bulk flow and condensation heat transfer of vapor have significant contribution to the total heat transfer and the dominant transport mechanism is dependent upon the vapor fraction in mixture.     

The influences of thermophysical properties of porous media on superadiabatic combustion with reciprocating flow

The influences of thermophysical properties of porous media on superadiabatic combustion with reciprocating flow is numerically studied in order to improve the understanding of the complex heat transfer and optimum design of the combustor. The heat transfer performance of a porous media combustor strongly depends on the thermophysical properties of the porous material. In order to explore how the material properties influence reciprocating superadiabatic combustion of premixed gases in porous media (short for RSCP), a two‐dimensional mathematical model of a simplified RSCP combustor is developed based on the hypothesis of local thermal non‐equilibrium between the solid and the gas phases by solving separate energy equations for these two phases. The porous media is assumed to emit, absorb, and isotropically scatter radiation. The finite‐volume method is used for computing radiation heat transfer processes. The flow and temperature fields are calculated by solving the mass, moment, gas and solid energy, and species conservation equations with a finite difference/control volume approach. Since the mass fraction conservation equations are stiff, an operator splitting method is used to solve them. The results show that the volumetric convective heat transfer coefficient and extinction coefficient of the porous media obviously affect the temperature distributions of the combustion chamber and burning speed of the gases, but thermal conductivity does not have an obvious effect. It indicates that convective heat transfer and heat radiation are the dominating ways of heat transfer, while heat conduction is a little less important. The specific heat of the porous media also has a remarkable impact on temperature distribution of gases and heat release rate. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(5): 336–350, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20120     

Thermodynamic analysis of liquid desiccants

Liquid desiccants are widely used in many solar applications. In order to analyze the performance of the system using desiccant technology, the thermophysical properties of desiccants are essential. In particular, the vapor pressure of the liquid desiccant is one of the important properties in air dehumidification. In this paper, an attempt is made to predict this property based on a classical thermodynamics approach and it is found that the predicted values for lithium chloride agree very well with the experimental results. The desired sorption properties can also be obtained by mixing the desiccants, which is another method of developing a new cost-effective liquid desiccant. In this paper, simple mixing rules are used to predict the vapor pressure, density, and viscosity of the desiccant mixture, namely calcium chloride and lithium chloride. It is found that the interaction parameter need not be included in calculating the density and vapor pressure of the above mixture but must be included in predicting the viscosity.     

An overview of the effect of lubricant on the heat transfer performance on conventional refrigerants and natural refrigerant R-744

This review provides an overview of the lubricant on the heat transfer performance, including nucleate boiling, convective boiling, shell side condensation, forced convective condensation, and gas cooling, for conventional refrigerants and natural refrigerant R-744. Various parameters affecting the heat transfer coefficient subject to lubricant, such as oil concentration, heat flux, mass flux, vapor quality, geometric configuration, saturation temperature, thermodynamic and transport properties are discussed in this overview. It appears that the effect of individual parameter on theProd. Type: FTP heat transfer coefficient may be different from studies to studies. This is associated with the complex nature of lubricant and some compound effect accompanying with the heat transport process. In this review, the authors try to summarize the general trend of the lubricant on the heat transfer coefficient, and to elaborate discrepancies of some inconsistent studies. The lubricant can, increase or impair the heat transfer performance depending on the oil concentration, surface tension, surface geometry, and the like. For the condensation, it is more well accepted that the presence of lubricant normally will impair the heat transfer performance due to deposited oil film. However, the deterioration is comparatively smaller than that in nucleate/convective boiling. For the effect of lubricant on R-744 with convective evaporation, the general behavior is in line with the convectional refrigerant. For gas cooling, the lubricant cast significant effect on heat transfer coefficient especially for a higher mass flux or at a smaller diameter tube.     

Phase change and heat transfer characteristics of a eutectic mixture of palmitic and stearic acids as PCM in a latent heat storage system

The phase change and heat transfer characteristics of a eutectic mixture of palmitic and stearic acids as phase change material (PCM) during the melting and solidification processes were determined experimentally in a vertical two concentric pipes energy storage system. This study deals with three important subjects. First is determination of the eutectic composition ratio of the palmitic acid (PA) and stearic acid (SA) binary system and measurement of its thermophysical properties by differential scanning calorimetry (DSC). Second is establishment of the phase transition characteristics of the mixture, such as the total melting and solidification temperatures and times, the heat transfer modes in the melted and solidified PCM and the effect of Reynolds and Stefan numbers as initial heat transfer fluid (HTF) conditions on the phase transition behaviors. Third is calculation of the heat transfer coefficients between the outside wall of the HTF pipe and the PCM, the heat recovery rates and heat fractions during the phase change processes of the mixture and also discussion of the effect of the inlet HTF parameters on these characteristics. The DSC results showed that the PA–SA binary system in the mixture ratio of 64.2:35.8 wt% forms a eutectic, which melts at 52.3 °C and has a latent heat of 181.7 J g⁻¹, and thus, these properties make it a suitable PCM for passive solar space heating and domestic water heating applications with respect to climate conditions. The experimental results also indicated that the eutectic mixture of PA–SA encapsulated in the annulus of concentric double pipes has good phase change and heat transfer characteristics during the melting and solidification processes, and it is an attractive candidate as a potential PCM for heat storage in latent heat thermal energy storage systems.     

Transient analysis of direct contact evaporation and condensation within packed beds

Transient one-dimensional conservation equations have been developed to analyze the heat and mass transfer within direct-contact evaporators and condensers. The conservation equations are solved numerically to predict water, air/vapor mixture and packed bed temperatures and humidity ratio within the evaporator and the condenser. The heat and mass transport models account for the transient variations within the packed-bed due to time varying inlet air and water temperatures and humidity. Direct contact evaporator and condenser experimental facilities have been configured. The measured water and air/vapor mixture temperature differences agree well with those predicted using the transient model. The measured humidity difference is slightly greater than that predicted, and the maximum deviation is on the order of 20%.     

Numerical analysis of greenhouse-type solar stills with high inclination

A numerical analysis has been carried out to evaluated the technical feasibility of single effect solar stills with a cover slope of up to 60°. The aim of the research is to establish the overall distillate productivity and the relative importance of the heat and mass transfer mechanisms when water diffusion is minimal and convection dominates. Experimental measurements of the distillate obtained from an already working unit are related to the numerical results by means of an “equivalent” Prandtl number that simplifies the analysis, allowing for the moist air in the inside of the cavity to be represented as a perfect gas.     

Thermodynamic analysis of two-component,two-phase flow in solar collectors with application to a direct-expansion solar-assisted heat pump

Two-phase flow of pure chlorofluorocarbon (CFC) refrigerants in solar collector tubes has been examined in previous studies in connection with applications in direct-expansion, solar-assisted heat pumps (DX-SAHP). The present work extends the thermodynamic analysis of solar collectors to the multicomponent and multiphase domain to cover newly proposed refrigerant mixtures which are potential candidates for replacing CFCs in future DX-SAHP systems. A computational methodology is developed to determine the size of a solar collector of a DX-SAHP that uses a binary refrigerant mixture whose thermodynamic and transport properties are predicted from a computer code. The energy equation for the elemental collector tube control volume, incorporating the local thermodynamic and heat transfer characteristics, is integrated to determine the tube length for a given set of inlet and exit thermodynamic states of the refrigerant mixture. Effects of various parameters such as the collector mass-flow rate and operating pressure, tube diameter and absorbed solar radiation on the collector tube length, heat transfer coefficient, and the local refrigerant temperature in the tube are also considered.