Freezing times of regularly shaped food items

The freezing of food is one of the most significant applications of refrigeration. In order for freezing operations to be cost-effective, it is necessary to optimally design the refrigeration equipment. This requires estimation of the freezing times of foods. Numerous semianalytical/empirical methods for predicting food freezing times have been proposed. The designer of food refrigeration facilities is thus faced with the challenge of selecting an appropriate estimation method from the plethora of available methods. Therefore, a review of selected semi-analytical/empirical food freezing time prediction methods applicable to regularly shaped food items is given in this paper. The performance of these various methods is evaluated by comparing their results to experimental freezing time data obtained from the literature.     

Sensitivity and uncertainty analysis of heat-exchanger designs to physical properties estimation

We have used Monte Carlo methods to study the sensitivity and uncertainty of heat exchanger designs to physical properties estimation. The determination of appropriate confidence intervals for the overall heat-transfer coefficient and total required heat-exchange area plays a very important role in heat-exchanger thermal designs. The physical property models used exhibit systematic and random errors, which can change depending on how the models are used. Of particular interest in this work were the errors in properties estimation at high temperatures. Experimental physical properties of hot-gas mixtures at high temperatures (i.e., combustion gases) are very limited, and heat-exchanger simulation and design relies heavily on predictive models for physical properties. In this work, case studies of heat-exchanger performance and design under the influence of random and systematic errors on property estimation are presented. The results show that the performance and design can be very sensitive to errors in physical properties. The analysis method proposed can be used to identify when and which, properties play a significant role in the error propagation for this type of equipment. Further, the methodology proposed can be used to study the uncertainty in heat-exchanger performance and cost introduced by these types of errors.     

Effect of uncertainties in physical properties on forced convection heat transfer with nanofluids

Nanofluids are considered to offer important advantages over conventional heat transfer fluids. However, at this early stage of their development, their thermophysical properties are not known precisely. As a result, the assessment of their true potential is difficult. This fact is illustrated by analyzing their thermohydraulic performance for both laminar and turbulent fully developed forced convection in a tube with uniform wall heat flux. Two different models from the literature are used to express these properties in terms of particle loading and they lead to very different qualitative and quantitative results in two types of problems: replacement of a simple fluid by a nanofluid in a given installation and design of an elementary heat transfer installation for a simple fluid or a nanofluid.     

Thermophysical Properties of a New Environment Friendly Alternative—Trifluoroiodomethane

Trifluoroiodomethane (CF₃I) is considered as a promising refrigerant alternative, especially as a component in mixtures, to replace CFC-12. But reliable thermophysical property data for CF₃I are still limited. The investigations on thermophysical properties of CF₃I developed by us are summarized in this paper. Experimental data of critical parameters, and the correlations of saturated liquid and vapor density, enthalpy of vaporization, vapor pressure, PVT properties, second virial coefficient, ideal-gas heat capacity, surface tension, viscosity and thermal conductivity are given in the present paper.     

Influence of thermophysical properties on two-phase flow convective boiling of refrigerant mixtures

In this paper, an analytical study on the influence of thermophysical properties on heat transfer characteristics of two-phase flow boiling of some refrigerant mixtures in air/refrigerant horizontal enhanced surface tubing is presented.Correlations were proposed to predict the thermophysical properties of refrigerant mixtures such as thermal conductivity and viscosity as well as their impact on the heat transfer characteristics such as average heat transfer coefficients, and pressure drops of R-507, R-404A, R-410A, and R-407C in two-phase flow boiling inside enhanced surface tubing. In addition, it was found that the refrigerant mixture's pressure drop is a weak function of the mixture's composition.It was also evident that the proposed improved correlations for predicting the thermophysical properties were applicable to the entire heat and mass flux, investigated in the present study. The deviation between the experimental and predicted value using new and improved correlations for the heat transfer coefficient and pressure drop were <±20 %, for the majority of data.     

Natural convection due to horizontal temperature and concentration gradients—1. Variable thermophysical property effects

Typically for single component fluids, the variation of thermophysical properties is negligible except in the presence of large temperature differences, and, therefore, has no appreciable effect on the heat transfer. In contradistinction, thermophysical properties can vary significantly due to concentration differences which affect the heat and mass transfer. This work examines the effects of thermophysical property variation on the heat and mass transfer in a cavity due to natural convection driven by combined thermal and solutal buoyancy forces. Results indicate that thermophysical property variations can appreciably influence heat and mass transfer and velocity distribution.     

Comparative review of turbulent heat transfer of nanofluids

Three essential aspects of the turbulent-flow, convective heat transfer of nanofluids relevant to their applications are comparatively reviewed in detail based on both theoretical analyses and experimental data. These aspects are: (a) selection – the comparison criteria of the thermophysical property-related heat transfer performance of nanofluids and their base fluids, (b) design – the predictions of the heat transfer coefficients of nanofluids based on homogeneous fluid models by using nanofluid effective thermophysical properties, and (c) effectiveness – the enhancements of the heat transfer coefficients of nanofluids over their base fluids. This review, including research from the inception of nanofluids to date, quantifies the accuracy of bases for future nanofluid evaluation.     

Development of suitability maps for ground-coupled heat pump systems using groundwater and heat transport models

The thermophysical properties of subsurface materials (soils, sediments and rocks) and groundwater flow strongly affect the heat exchange rates of ground heat exchangers (GHEs). These rates can be maximized and the installation costs of the ground-coupled heat pump (GCHP) systems reduced by developing suitability maps based on local geological and hydrological information. Such maps were generated for the Chikushi Plain (western Japan) using field-survey data and a numerical modeling study. First, a field-wide groundwater model was developed for the area and the results matched against measured groundwater levels and vertical temperature profiles. Single GHE models were then constructed to simulate the heat exchange performance at different locations in the plain. Finally, suitability maps for GCHP systems were prepared using the results from the single GHE models. Variations in the heat exchange rates of over 40% revealed by the map were ascribed to differences in the GHE locations, confirming how important it is to use appropriate thermophysical data when designing GCHP systems.     

The influence of binary mixture thermophysical properties in the analysis of heat and mass transfer processes in solar distillation systems

Although a substantial amount of research work has already been devoted to various aspects of modeling the convective and mass transport processes in solar distillation systems, it appears that the role of thermophysical and transport properties of the working medium and their effect on the thermal behavior and performance analysis of such systems has been left almost completely unnoticed. The working medium in these systems, which is a binary mixture of water vapor and dry air in equilibrium, appears to exhibit a completely different set of properties than dry air, especially at saturation conditions and at the higher region of the solar still operational temperature range. An analysis is presented aiming to signify the effect of binary mixture thermophysical properties on the transport processes and the associated quantities and evaluate the thermophysical properties of the working medium in these systems, based on contemporary data for dry air and water vapor. The derived results, in the form of convenient algebraic correlations, are employed to investigate the effect of using the appropriate thermophysical properties on the calculation of the convective heat and mass transfer, as well as the distillate mass flow rates. According to the results from the present investigation, although the use of improper dry air data leads to a significant overestimation of the convective heat transfer coefficient, the errors associated with the use of improper dry air properties is a moderate overestimation of distillate output which is estimated to be up to 10% for maximum average still temperatures of 100 °C.     

Theoretical simulation of vacuum cooling of spherical foods

A model has been developed for predicting the temporal temperature and mass of spherical solid foods during vacuum cooling. This paper discusses the effects of product thermophysical properties, convection heat transfer coefficient, latent heat of evaporation as well as vacuum environmental parameters that govern the heat and mass transfer of product. The temporal trends of total system pressure, product temperature such as surface temperature, centre temperature, mass-average temperature, the mass of product were predicted. The model accounts for the change of temperature of solid product systematically during vacuum cooling by means of simulation. This paper also compares the results of the computer algorithm with experimental results taken from published literature.     

利用Delphi和Access实现对溴化锂水溶液热物性数据的可视化查询

通过对Access2003数据库基本结构的分析,介绍了利用Delphi访问建立在Ac-cess2003上的物性数据库的方法,并实现了对溴化锂水溶液热物性数据的可视化查询。     

The influence of a vertical ground heat exchanger length on the electricity consumption of the heat pumps

The use of heat pumps combined with vertical ground heat exchangers for heating and cooling of buildings, has significantly gained popularity in recent years. The design method for these systems, as it is proposed by ASHRAE, is taking into account the maximum thermal and cooling loads of the building, the thermophysical properties of the soil at the area of installation and a minimum Coefficient of Performance (COP) of the heat pumps. This approach usually results in larger than needed length of the ground heat exchanger, thus increasing the installation cost.A new analytical simulation tool, capable to determine the required ground heat exchanger length has been developed at the Process Equipment Design Laboratory (PEDL) of the AUTh. It models the function of the system as a whole over long time periods, e.g. 20 years, using as input parameters the thermal and cooling loads of the building, the thermophysical properties of the borehole and the characteristic curves of the heat pumps. The results include the electricity consumption of the heat pumps and the heat absorbed from or rejected to the ground.The aim of this paper is to describe the developed simulation algorithm and present the results of such a simulation in a case study. It is proved that the total required length of the ground heat exchanger is less than that calculated using the common numerical method.     

Thermal Engineering Tools in Performance-Based Design for Fire Safety

Fouling of heat exchangers' heat transfer areas is one of the serious problems of certain process plants. A waste incinerating plant is a typical example of such a process plant. The main process stream in a waste incinerating plant (i.e., produced stream of flue gas) and its thermophysical characteristics and properties significantly influence operating, maintenance, and investment costs of installed equipment and its service life. Moreover, properties of flue gas can vary over a wide range. This contribution deals with the issue of the fouling mechanism at the heat transfer area of tubular heat transfer equipment installed in plants for thermal processing of wastes and presents a mathematical model developed for fouling tendency prediction and for prevention in design and operating of tubular heat transfer equipment designed for applications in waste incinerating plants. Obtained results were compared with experimental data published in worldwide available literature and a very good agreement was found.     

Convective heat transfer and second law analysis of non-Newtonian fluid flows with variable thermophysical properties in circular channels

This study presents an analytical solution, for fully developed non-Newtonian fluid flows in circular channels under isoflux thermal boundary conditions based on perturbation techniques. Since the physical properties are generally a function of temperature and may not be assumed constant under certain circumstances, the change in viscosity and thermal conductivity with temperature was taken into account. Viscous dissipation term was also included in the performed analysis. In this study, first closed form expressions for velocity, temperature distributions, and Nusselt numbers corresponding to constant thermophysical properties were given in terms of governing parameters. Then, numerical calculation was performed to obtain the values of Nusselt number and global entropy generation for variable thermophysical properties. The results revealed that neglecting the property variation significantly affects heat transfer characteristics and entropy generation, in which the deviation from the constant physical property assumption may reach up to about 32.6%.     

A comprehensive review of fundamentals,preparation and applications of nanorefrigerants

Nanofluids attract researchers in many ways for their enhanced heat transfer properties. Nanorefrigerant is one kind of nanofluids. It has better heat transfer performance than traditional refrigerants. Thermal conductivity, viscosity and density are the basic thermophysical properties that must be analyzed before performance analysis. This paper presents a comprehensive review of nucleate pool boiling, flow boiling, condensation and two-phase flow of refrigerant-based nanofluids. The effects of nanolubricants on boiling and two-phase flow phenomena are presented as well. Furthermore, studies of applications and preparation of refrigerant-based nanofluids are presented. For the limited studies done so far, there are some controversies from one study to another. Based on results available in the literatures, it has been found that nanorefrigerants have a much higher and strongly temperature-dependent thermal conductivity at very low particle concentrations than conventional refrigerant. This can be considered as one of the key parameters for enhanced performance for refrigeration and air conditioning systems. Because of its superior thermal performances, latest up to date literatures on this property has been summarized and presented in this paper as well.     

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.     

Concentration of liquid foods using desiccants: An experimental study

Liquid foods are often partially dehydrated to reduce their water content so as to enhance their storage stability. Evaporation, which consumes large amounts of energy, is the most widely used operation for concentrating liquid foods. The paper describes a novel system to concentrate liquid foods by using liquid desiccants. In the proposed system, water vapour, evaporated from the liquid food at reduced pressure, passes to the liquid desiccant, where it is absorbed while liberating its latent heat. The liquid desiccant temperature thereby increases. The warm liquid desiccant can then be used to provide a portion of the thermal energy for the evaporation of the water in the liquid food. A vertical, double-falling-film evaporator which uses calcium chloride solution was built and was used to concentrate liquid foods. The experimental set-up was tested to study the influence of the liquid-desiccant flow rate, the liquid-food flow rate, and the liquid-desiccant concentration and temperature. Some of the experimental results are discussed in this paper.     

Thermophysical properties of some paraffins applicable to thermal energy storage

An experimental study has been conducted with the aim to investigate and evaluate thermophysical properties of technical grade paraffins appropriate for solar energy storage applications.The results obtained involve the kinetics of phase transition and latent heat of phase change, the thermal cycling stability at heating and cooling and specific heat for the temperature range of application.A method for the automatic computer controlled thermal cycling has been developed. The dynamic mode of precise uniform rising and lowering of the temperature at various rates has been accomplished. Nine hundred thermal cycles have been carried out on two different mixtures of technical grade paraffins, varied by their composition.The change of the DSC kinetics measured by Mettler TA 3000 and the relative thermodynamic values of the phase transitions were not registered after cycling. The calculated enthalpies of three paraffin mixtures were found to depend on their oil content and the distribution of atoms defined by chemical and gas chromatographic analyses.     

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.     

Property models and theoretical analysis of novel solid oxide fuel cell with triplet nano-composite electrode

Triplet nano-composite electrodes are actively examined experimentally, but there is a shortage of theoretical study. Theoretical models are helpful for understanding the experiments and provide guidance for design optimization of the novel electrode. Here new models for computing the electrode electronic and ionic conductivities, TPB length and hydraulic radius are presented. The novel properties determined by the models are used in a multi-physics numerical model that couples the intricate interdependency among electric conductions, electrochemical reaction and gas transport in SOFC. The theoretical I–V relations and hydraulic radius are in good agreement with the experiments, validatingtheproposed property models. The property models are then used to examine the influence of microstructure and material composition. The results show that: (i) Larger core-particle size and smaller nano-particle size are helpful for improving electrode properties; (ii) The required nano-particle loading is determined by the desired electronic conductivity instead of the desired TPB length.