Sanjay K. Roy

Forced convection heat transfer in microencapsulated phase change material slurries: flow in circular ducts

Abstract The heat transfer characteristics of microencapsulated phase change material slurry flow in circular ducts are presented in this paper. The energy equation is formulated by taking into consideration both the heat absorption (or release) due to the phase change process and the conductivity enhancement induced by the motion of the particles. The heat source or heat generation function in the energy equation is derived from solutions for freezing or melting in a sphere. The correlation for the effective conductivity of the slurry is obtained based on available analytical and experimental results. The governing parameters are found to be the particle concentration, a bulk Stefan number, the duct/particle radius ratio, the particle/fluid conductivity ratio, and a modified Peclet number. For low temperature applications, it is found that the dominant parameters are the bulk Stefan number and concentration. The numerical solutions show that heat fluxes about 2–4 times higher than single phase flow may be achieved by a slurry system.

Laminar forced convection heat transfer in microcapsulated phase change material suspensions

An experimental study using a suspension of n-eicosane microcapsules in water was conducted in order to evaluate the heat transfer characteristics of phase change material suspensions. Experiments were done for laminar, hydrodynamically fully developed flows in a circular duct with a constant wall heat flux. The temperature of the suspension entering the test section was maintained at or below the melting point of the phase change material. The primary parameters in the study were the bulk Stefan number and the volumetric concentration. In addition, a few experiments were conducted to evaluate the effect of particle diameter and degree of homogeneity of the suspension. The heat fluxes chosen for the experiments were typical of low temperature applications (below 60°C). Results show that use of phase change material suspensions can reduce the rise in wall temperature by up to 50% as compared to a single phase fluid for the same non-dimensional parameters. The most dominant parameter influencing the heat transfer was found to be the bulk Stefan number. The effect of concentration was found to be insignificant by itself, though its influence is felt indirectly through the bulk Stefan number. An increase in particle diameter by a factor of 2.5 was found to further reduce the wall temperature rise by 15%. The degree of homogeneity of the suspension had no observable effect on the wall temperatures.

Laminar forced convection heat transfer with phase change material suspensions

Results of an experimental study of laminar forced convection heat transfer in a circular duct with a phase change material emulsion (n-octadecane in water) are presented in this paper. The bulk Stefan numbers considered in this study range up to 3.0 and the concentration of phase change material range up to 30% by volume. The results show that the heat transfer characteristics for phase change material emulsions are similar to those of microencapsulated phase change material suspensions, thus confirming that the microcapsule walls do not affect the heat transfer process significantly.

An evaluation of phase change microcapsules for use in enhanced heat transfer fluids

Abstract The results of an experimental study to evaluate the properties of microencapsulated phase change materials have been presented. Two phase change materials, n-eicosane and stearic acid, have been used in the study. The microcapsules were manufactured with two different wall thickness, comprising of approximately 15% and 30% of the total microcapsule volume. Three different microcapsule sizes ranging from 50μ to 250μ have been considered. The microcapsules with thinner walls are unable to withstand repeated thermal cycling past the melting point. However, the microcapsules with thicker walls were found to be both structurally as well as thermally stable.

Turbulent heat transfer with phase change material suspensions

This paper presents an effective specific heat capacity model for turbulent heat transfer to phase change material (PCM) suspensions in a circular tube with constant wall heat flux. The model has been implemented in the form of a computer code and its numerical predictions are found to agree with previously published experimental data. Further results show that the bulk Stefan number, the non-dimensional melt temperature range and the degree of subcooling are the three parameters of importance. They also confirm that considerable reductions in wall temperatures, of the order of 50% or more, may be obtained at low to moderate Stefan numbers. For most typical cases with high wall heat fluxes, the Stefan number and the degree of subcooling determine the location of the tube where the phase change effects are predominant.

Gravity-assisted melting in a spherical enclosure : effects of natural convection

A theoretical model of graivity-assisted melting in a spherical enclosure is discussed in this paper. A sphere with a phase change material initially in the solid phase at its melting temperature is instantaneously exposed to a uniform higher temperature at the wall. The solid phase is assumed to have a higher density as compared to the liquid and drops down as it melts. The effects of natural convection on the melting process have been considered in this analysis. Suitable simplications have been made where necessary, in order to reduce computational effort and time. The non-dimensional melt time and heat transfer coefficient have been obtained as a function of the property values, operating temperatures and physical size for Md ⪡ 1, Ste ⪡ 1, 104 ⩽ Gr ⩽ 106, 10 ⩽ Pr ⩽ 100, 0.5 ⩽ Mt ⩽5.0,0 ⩽ Sb ⩽ 0.75, 0.01 ⩽ 1/Prα0 ⩽ 1.0 and 0.01 ⩽ Ste/c0p ⩽ 0.2. Natural convection is found to limit the range of applicability of previously published correlations.

Melting of a Free Solid in a Spherical Enclosure: Effects of Subcooling

The melting process within a spherical enclosure with the solid phase uniformly subcooled initially has been studied. The preliminary analysis of the problem is similar to a previous study where the degree of subcooling was zero. However, the heat transfer equation has been modified to include the effects of a temperature gradient in the solid core. As a result, a closed-form solution cannot be obtained. At every time step, the unsteady conduction equation has been solved numerically using a toroidal coordinate system, which has been suitably transformed to immobilize the moving boundary and to transform the infinite domain into a finite one. The temperature gradient at the surface is now used to solve the film equation numerically. The melt time, Nusselt number, and melt flux distributions have been obtained over a range of the parameters normally encountered in solar thermal systems.

The performance of liquid heat sinks with phase change material suspensions

Abstract Results of an experimental study to investigate the performance of liquid heat sinks with microencapsulated phase change material suspensions are presented. The volumetric concentrations of microcapsules in the suspension was varied from 0% to 28%. For the case of heating from below, the use of phase change material suspensions was found to be detrimental to the overall heat transfer because of a reduced convection due to the presence of microcapsules. In contrast, the performance of the liquid heat sink is improved when heating is from above.

A numerical study of natural convection heat transfer in a vertically eccentric spherical annulus

Abstract The results of a numerical investigation of the natural convection process between isothermal vertically eccentric spheres with hotter inner core, are being presented. The Grashof and the Prandtl numbers have been kept constant at 4×104 and 10 respectively. Eccentricities varying from −0.75 to +0.75 have been studied. From the numerical solution, it is possible to explain the average results obtained previously for the regime where the steady crescent flow pattern exists. Negative eccentricities have been found to enhance convection while positive eccentricities have the reverse effect. Results also show that heat transfer actually increases slightly for very high positive eccentricities where conduction plays an important role.

Gravity-assisted melting in a horizontal cylinder heated by external forced convection

The results of an experimental study of melting of a free solid in a cylinder heated by external forced convection have been presented. The ranges for the average wall Stefan number and Archimedes number have been varied between 0.026 to 0.053 and 8.76×106 to 3.34×108 respectively. Multiple runs have been made for most of these cases using different values of Reynolds number and free stream temperature to obtain the desired average wall Stefan number. Though the melt rate is almost identical to that for the isothermally heated capsule in about half the tests, it is strongly affected by the dynamics of the melting process in other cases, where two different melt patterns are observed.