Subrata Sengupta

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.

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.

Empirical heat transfer and frost thickness correlations during frost deposition on a cylinder in cross-flow in the transient regime

This paper reports on results of an experimental investigation where the emphasis was placed on obtaining empirical correlations for the frost thickness–time history and the heat transfer coefficient–time history for a cylinder in humid air cross-flow. The facility employed for the investigation consisted of a low-velocity wind tunnel comprised of a rectangular test section, a transition section and a honeycomb placed at the tunnel entrance. An external refrigerator was used to cool an antifreeze solution having a mixture of 90% methanol and 10% ethylene glycol. Measured parameters included, among other things, the heat transfer coefficient as well as the frost thickness.

Laminar flow and heat transfer in a finned tube annulus

Abstract Laminar flow and heat transfer magnitudes in a finned tube annulus are obtained numerically. External circular fins on the inner tube are periodic. Pressure drop and heat transfer characteristics of the fins are obtained in the periodically fully developed region by varying geometric and flow parameters. Geometric parameters are annulus radius ratio (0.3 to 0.5), fin height/annular gap (0.33 to 0.67) and fin spacing/annular gap (2 to 5). Flow parameters are Reynolds number (100 to 1000) and Prandtl number (1 to 5). Comparisons are made with a plain tube annulus having the same length, heat transfer surface area, volume flow rate, and Reynolds number. The entire inter-fin space was found to be occupied by recirculating flow except at Reynolds numbers less than 500. A major contribution to heat transfer is made from the fin side at the downstream end. At Prandtl numbers less than 2, the use of fins may not be justified because the increase in pressure drop is more pronounced than the increase in heat transfer. At a Reynolds number of 1000 and A Prandtl number of 5, the heat transfer increases by a factor of 3.1, while the pressure drop increases by a factor of 2.3.

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.

LAMINAR FLOW AND HEAT TRANSFER IN BLOCKED ANNULI

Laminar flow and heal transfer in annular passages with axially nonuniform inner tubes are obtained numerically. A characteristic feature of these passages is that the flow separates in the streamwise direction. An axisymmetric coordinate system with an algebraic transformation in the radial direction kas been used. Fully elliptic vorticity-slream function and energy equations in the transformed coordinates are solved using an iterative alternate direction implicit (ADI) method. In an annulus with a smooth blockage, the flow separates immediately downstream of the blockage at Reynolds numbers greater than 100. The main features of the flow are established at a Reynolds number of 1000. The pressure drop is drastic near the maximum constriction. The heat flux is also high in the constricted region. A sharp increase in the heat transfer occurs where the fluid reattaches itself to the wall. The increase in the total pressure drop is about an order of magnitude greater than that in the average Nusselt number.

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.

Three-dimensional time-dependent simulations of hydro-thermal behaviour of Biscayne Bay

The purpose of the paper is to evaluate the performance of rigid-lid formulations in predicting velocity and temperature fields in domains where the applicability of rigid-lid formulations is not immediately obvious. A test was conducted to determine whether local depth scaled vertical exchange coefficients yield better simulations compared to constant vertical coefficient formulations. Extensive infrared imagery was used to evaluate the models ability to predict temperature yields in Biscayne Bay, Fla. Two cases, one with constant vertical eddy transport coefficient, and the other with vertical transport coefficient scaled with local depth are discussed.

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.