R. Velraj

HEAT TRANSFER ENHANCEMENT IN A LATENT HEAT STORAGE SYSTEM

Abstract Commercial acceptance and the economics of solar thermal technologies are tied to the design and development of efficient, cost-effective thermal storage systems. Thermal storage units that utilize latent heat storage materials have received greater attention in the recent years because of their large heat storage capacity and their isothermal behavior during the charging and discharging processes. One major issue that needs to be addressed is that most phase-change materials (PCM) with high energy storage density have an unacceptably low thermal conductivity and hence heat transfer enhancement techniques are required for any latent heat thermal storage (LHTS) applications. In the present paper the various heat transfer enhancement methods for LHTS systems are discussed. Three different experiments to augment heat transfer were conducted and the findings are reported.

Experimental analysis and numerical modelling of inward solidification on a finned vertical tube for a latent heat storage unit

The enhancement of heat transfer in a thermal storage system consisting of a cylindrical vertical tube with internal longitudinal fin arrangement is discussed in this paper. This configuration that forms a V-shaped enclosure for the phase change material (PCM) gives maximum benefit to the fin arrangement. A theoretical model that also accounts for the circumferential heat flow through the tube wall is developed using enthalpy formulation and is employed in conjunction with the fully implicit finite difference method to solve the solidification in the convectively cooled vertical tube. A generalised enthalpy-temperature (H-T) relationship, developed by Date, is suitable for constant phase change temperature and this is modified in the present work to accommodate materials having either constant or a range of phase change temperatures. The numerical model is validated with experimental data. The instantaneous surface heat flux and the solidified fraction for different number of fins and tube radii, that are of interest for the design of latent heat thermal storage system consisting of an array of vertical cylindrical tubes containing PCM inside it, are also discussed in this article.

Heat transfer enhancement study of a LHTS unit containing dispersed high conductivity particles

A theoretical analysis is presented for the performance study of a Latent Heat Thermal Storage (LHTS) system that contains a phase change material (PCM) dispersed with high conductivity particles. The effect of fraction of dispersed particles in the PCM on energy storage time and heat flux is presented for laminar and turbulent flows, and also analytical expressions are presented for various quantities of interest to study the energy storage capabilities. The combined effect of thermal and flow properties of both the heat transfer fluid (HTF) and the PCM-mixture is also included in the study. It is observed that there exists an optimum fraction of particles to be dispersed in the PCM for maximum energy storage/extraction.

Studies on Fanning Friction (f) and Colburn (j) Factors of Offset and Wavy Fins Compact Plate Fin Heat Exchanger–A CFD Approach

The objective of this work is to address the uncertainties in the estimation of the Fanning friction (f) and Colburn (j) factors in a compact offset plate fin heat exchanger, and the generation of flow friction and heat transfer correlations in the form of f and j for compact wavy plate fin heat exchangers. A typical offset fin has been analyzed using FLUENT software (a general purpose computational fluid dynamics (CFD) simulation tool) for estimation of f and j data. These results are compared with the results of the available correlations in the literature and in-house experimental results. Uncertainties in estimation of f and j data are reviewed and the variations in the results are highlighted. In the case of wavy fin configuration, very limited correlations are available in the literature. Hence, with the objective of developing correlations, 18 different types of wavy fins geometries have been analyzed using a CFD tool to find out the f and j data in the laminar and turbulent regions. Multiple regression analysis has been carried out on the set of data to generate the correlations for f and j. These correlations have been compared with the experimental results of wavy fin reported in the literature.

APPLICATION OF DRIFT-FLUX MODEL IN LIQUID-SOLID CIRCULATING FLUIDIZED BED

Solids circulation rate and solids holdup are experimentally investigated in a liquid-solid circulating fluidized bed that covers a wide range in solid and liquid flow rates, particle size, and density. The drift-flux model, which is extensively used to describe the flow behavior of gas-liquid systems, is modified in the present work and for the first time applied to the experimental data of a liquid-solid circulating fluidized bed. The distribution parameter Co is evaluated using the drift-flux model from the present experimental data and also from the data available in the literature. It is found from the analysis that the value of the distribution parameter lies in the range of 0.78 to 0.99, which is a reverse trend to the liquid-gas system. The weighted average drift velocity (usj) is well correlated to the terminal velocity of the particle. The solid holdup predicted using this model agrees with the experimental results.

Effects of the properties and the structural configurations of fatty acid methyl esters on the properties of biodiesel fuel: a review

Biodiesel is a renewable fuel that is suitable for both stationary and automotive engines. It consists of fatty acid methyl esters and fatty acid ethyl esters when vegetable oils are transesterified with methanol and ethanol respectively. The structural configuration and properties of individual fatty acid methyl esters can significantly influence the biodiesel properties. The purpose of the present work is to review the effects of the properties and the structural configurations of fatty acid methyl esters on various biodiesel properties. The structural configuration includes the molecular structure, the chain length, the saturated or unsaturated nature, the degree of unsaturation and the position of double bonds. Important properties of fatty acid methyl esters are reviewed, and a large number of data consisting of their reported values are given. The viscosity, the cetane number, the heat of combustion, the density, the bulk modulus and compressibility, the iodine value, the oxidative stability, the low-temperature properties, the boiling point, the lubricity, the saponification value, the surface tension, the specific heat, the latent heat of vaporization and the flash point are reviewed in the present study. The reported mathematical equations for estimating the properties of fatty acid compounds and biodiesel are presented. A few studies that recommend enrichment of a particular fatty acid (or acids) which is possible through genetic modification to improve the overall biodiesel properties are also cited. The contribution of a particular fatty acid (or acids) favours some properties while it has an undesirable effect on other properties. Hence the specification of one particular fatty acid profile to improve all the properties of biodiesel is not possible. Considering the low-temperature properties and the oxidative stability, a combination of both saturated fatty acids and unsaturated fatty acids is necessary. Therefore, it is important to decide whether a biodiesel should contain a larger amount of saturated fatty acid esters or unsaturated fatty acid esters in order to obtain better fuel properties. The inherent genetic modification of the fatty acid profile could be the best possibility for addressing several fuel property issues simultaneously.

Free Cooling Potential and Technology Options for Thermal Energy Management of a Commercial Building in Bangalore City, India

ABSTRACTA significant portion of the energy consumption in buildings is the use of energy for heating, ventilation and air conditioning (HVAC) systems. Free cooling is one of the novel methods of energy saving in building air conditioning systems. In the present study, the free cooling potential for Bangalore City in India was determined by using hourly weather data for a year and also estimating the cooling load requirement for every month, considering 25 ± 3°C as the comfort level. The different technology options suitable for different months are reported in the present work. It is summarized from the study that Bangalore City does not require any mechanical cooling throughout the year. The results of the present study will be very useful for building architects and engineers to achieve the objective of zero energy buildings.

Investigation on phase change material-based flat plate heat exchanger modules for free cooling applications in energy-efficient buildings

ABSTRACT A phase change material (PCM)-based flat plate heat exchanger has been developed for free cooling of buildings. It has several PCM modules with an air passage between one module and another. The performance of this heat exchanger is analysed using FLUENT software on the symmetric portions of two PCM modules and an air passage. The apparent heat capacity method is used to model the latent heat value of the PCM, and the profile obtained from the digital scanning calorimeter analysis is suitably approximated. The charging and discharging characteristics of the PCMs at various velocities and inlet temperatures of the air are also analysed. The results of charging analysis are compared with the experimental results. The time-wise variation of the temperature of the PCM and air, instantaneous and cumulative heat transfer during the charging and discharging processes for various parameters of interest are reported. The results are very useful for the architects and building engineers in designing energy-efficient buildings. Abbreviations: CFD: computational fluid dynamics; DSC: digital scanning calorimeter; HTF: heat transfer fluid; LHTES: latent heat thermal energy storage; PCM: phase change material; RTD: resistance temperature detector

Analytical solutions for planar and cylindrical axisymmetric melting with heat capacity effects of flowing stream and PCM

This paper deals with the analysis of axisymmetric melting of PCM surrounding a tube and a channel carrying a hot heat transfer fluid. The analysis considers the effect of heat capacities of both the wall and the melt-layer. From this, closed form analytical solutions are obtained. Portions of the equations are presented in graphical form to study the individual and combined influence of the heat capacity parameters during the process. The present results are useful in the design and analysis of thermal energy storage systems.

Experimental Investigation of a Cascaded Latent Heat Storage System for Diesel Engine Waste Heat Recovery

Thermal storage plays a vital role in improving the overall efficiency of a waste heat recovery system. A phase change storage system exhibits high energy storage density and isothermal behavior during the charging and discharging processes. In the present work, the cascading arrangement of the phase change storage system is studied to analyze the improvement in the efficiency of the storage system during the charging process, and to compared it with a single storage tank system. The performance parameters, such as the amount of heat recovered, the heat lost, charging rate, charging efficiency, and the percentage of energy saved, are investigated in detail.