Kamal Abdel Radi Ismail

Numerical and experimental study on the solidification of PCM around a vertical axially finned isothermal cylinder

Abstract This paper presents the results of a numerical and experimental investigation realized on finned tubes with the objective of using them in thermal storage systems. The model is based upon the pure conduction mechanism of heat transfer, the enthalpy formulation approach and the control volume method. The finite difference approximation and the alternating direction scheme are used to discretize the basic equations and the associated boundary and initial conditions. The model was validated by comparison with available results and additional experimental measurements realized by the authors. The number of fins, fin length, fin thickness, the degree of super heat and the aspect ratio of the annular spacing are found to influence the time for complete solidification, solidified mass fraction and the total stored energy. The results confirm the importance of the fins in delaying the undesirable effects of natural convection during the phase change processes. Also, this study indicates the strong influence of the annular space size, the radial length of the fin and the number of fins on the solidified mass fraction and the time for complete phase change. Based upon experimental observations and the tendencies of the numerical results, a metallic tube fitted with four–five fins of constant thickness equal to the tube wall thickness and of radial length around twice the tube diameter should be a compromise solution between efficiency, increase in the heat flow rate and the loss of available storage capacity.

Numerical simulation of a variable speed refrigeration system

Abstract This work presents two numerical models to simulate the transient and steady state behavior of a vapor compression refrigeration system. The condenser and the evaporator were divided into a number of control volumes. Time dependent partial differential equations system was obtained from the mass, energy and momentum balances for each control volume. As the expansion valve and the compressor both have very small thermal inertia, the steady state models were applied for these components. Transient and steady state models numerical predictions were compared and good agreement was found. Further simulations were performed with the objective of verifying the possibility of controlling the refrigeration system and the superheating of the refrigerant in the evaporator outlet by varying the compressor speed and the throttling valve sectional area. The results indicate that the proposed models can be used to formulate an algorithm for controlling a refrigeration system.

A parametric study on possible fixed bed models for pcm and sensible heat storage

Abstract This paper presents the results of a comparative numerical investigation on packed bed thermal models suitable for sensible and latent heat thermal storage systems. Four basic groups of models were investigated, that is, the continuous solid phase models, Schumanns model, the single phase models and the thermal diffusion models or models with thermal gradient inside the particles. For each of these models a computer code was written and optimized. The models were first evaluated in relation to the computational time consumed to solve a specific test problem. The models were then compared in relation to the influence of particle size, void fraction, particle material, flow rate variations, working fluid inlet temperature variations and finally wall thermal losses. The results obtained were compared, analysed and discussed

Numerical and experimental study of spherical capsules packed bed latent heat storage system

A numerical model to simulate a storage system composed of spherical capsules filled with PCM placed inside a cylindrical tank fitted with a working fluid circulation system to charge and discharge the storage tank. The simplified transient one-dimensional model is based on dividing the tank into a number of axial layers whose thickness is always equal or larger than a capsule diameter. It is also assumed that the temperature of the working fluid is uniform and equal to the average temperature of the layer. The solidification process inside the spherical capsule is treated by using a conductive one-dimensional phase change model with convective boundary condition on the external surface. The convection present in the liquid phase of the PCM is treated by using an effective heat conduction coefficient in the liquid region of the PCM. The solution of the differential equations is realized by the finite difference approximation and a moving grid inside the spherical capsules. The geometrical and operational parameters of the system are investigated both numerically and experimentally and their influence on the charging and discharging times was investigated.

Thermally effective windows with moving phase change material curtains

This paper presents the results of a numerical and experimental study on thermally efficient windows. Experimental investigation using spectrophotometry was realized on simple and composite glass samples filled with air or phase change material. The transmittance and reflectance tests indicated large reduction in the infrared and ultraviolet radiations while maintaining good visibility. The number of glass sheets, their thickness and the gap between them were also investigated. The numerical model is based upon one-dimensional formulation of the composite window. The program was optimized and the predicted results are presented and discussed.

A parametric study on ice formation inside a spherical capsule

This paper reports the results of a numerical study on the heat transfer during the process of solidification of water inside a spherical capsule. The governing equations of the problem and associated boundary conditions were formulated and solved using a finite difference approach and a moving grid scheme. The model was optimized and the numerical predictions were validated by comparison with experimental results realized by the authors. The model was also used to investigate the effects of the size and material of the shell, initial temperature of the phase change material and the external temperature of the spherical capsule on the solidified mass fraction and the time for the complete solidification.

PCM thermal insulation in buildings

Departamento de Engenharia Termica e Fluidos Faculdade de Engenharia Mecânica - UNICAMP, Caixa Postal 6122, CEP 13083-970, Campinas (SP)

Solidification of pcm inside a spherical capsule

This paper presents a numerical study of the solidification of pcm enclosed in a spherical shell. The mathematical model is based on pure conduction in the pcm subject to boundary conditions of constant temperature or convection heat transfer on the external surface of the spherical shell. The model is validated by comparison with available similar models and the agreement is found to be satisfactory. The model was then used to predict the effect of the size of the spherical shell, shell thickness, shell material, initial pcm temperature and the external wall temperature on the solidified mass fraction and the time for complete solidification.

Thermal performance of a pcm storage unit

This paper presents a two-dimensional model for the phase change, conduction based heat transfer problem around a tube immersed in the pcm. The energy equation is written in the enthalpy form, and the heat and flow problems are coupled by an energy balance on the fluid element flowing inside the tube. The numerical solution is based upon the average control volume technique and the ADI finite difference representation. The results obtained show the effects of the variation of the ratio of the radius of the inner to the outer tube, Biot number, Stefan number and the working fluid inlet temperature on the solidified mass fraction, NTU and effectiveness.

Parametric study on composite and PCM glass systems

This paper presents the results of a numerical and experimental study on thermally efficient windows. An experimental investigation using spectrophotometry was realized on simple and composite glass windows filled with air or PCM. The transmittance and reflectance tests indicate large reductions in the infrared and ultraviolet radiations while maintaining good visibility. The number of glass sheets, their thickness and the gap between them were also investigated. The numerical model is based upon a one dimensional formulation of the composite window. The program was optimized, and the predicted results were compared with experimental measurements.