A.R. Balakrishnan

Nucleation site density in pool boiling of saturated pure liquids: Effect of surface microroughness and surface and liquid physical properties

An experimental investigation on the nucleation density during nucleate pool boiling of saturated pure liquids at low to moderate heat fluxes is described. The surface-liquid interaction during the boiling phenomena and its effect on the nucleation site density and thereby the heat flux is examined. Stainless steel and aluminum with different surface finishes obtained by polishing the surfaces with different grades of emery paper was used in the study. A parameter, Ra, called the arithmetic average roughness or the centerline average was used to characterize the surface microroughness. The parameter Ra is defined as the average values of the peaks and valleys on the surface. This was measured experimentally by using a profilometer. The liquids used in the study were distilled water, carbon tetrachloride, n-hexane, and acetone. The nucleation site density at different heat fluxes for various surface-liquid combinations was measured by using high-speed photography. The data showed that the nucleation site density depends on the surface microroughness, the surface tension of the liquid, the thermophysical properties of the heating surface and the liquid, and the wall superheat. A correlation in terms of the wall superheat, the Prandtl number, a surface-liquid interaction parameter (the ratio of the thermal conductivity, density, and specific heat of the solid to the liquid), and a dimensionless surface roughness parameter has been proposed. The correlation proposed matches data obtained in the present study. The correlation also matches data in the literature obtained on copper and nickel surfaces of various surface finishes, further validating the correlation and the mechanism suggested in this study.

Parametric studies on thermally stratified chilled water storage systems

Abstract An analysis of the stratification decay in thermally stratified vertical cylindrical cool storage systems is presented using a one dimensional conjugate heat conduction model. The degree of thermal stratification depends upon the length to diameter ratio, wall thickness to length ratio, the thermo-physical properties of the material of the storage tank, the type and thickness of the insulation and the design of the admission system for both cold and warm water. A parametric study of the stratified chilled water storage tanks in charging, discharging and stagnation modes of operation is made. The thermoclines degrade due to the heat transfer from the ambient, thermal diffusion in the storage tank, axial wall conduction and mixing due to admission of the fluid in the storage tank during charging and discharging. The degree of thermal stratification in storage tanks is expressed in terms of either heat capacity (thermal capacitance ratio) or modified Biot Number. A mixing parameter accounts for the effect of mixing on thermal stratification in both charge and discharge cycles.

Experiments on stratified chilled-water tanks

Abstract This paper presents experimental studies on thermal stratification in chilled-water storage systems. A fibreglass storage tank in static and dynamic modes of operation is used. The parameters varied are aspect ratio, flow rates, initial temperature difference and thickness of insulation. Emphasis is given to the effects of mixing at the inlet, leading to the definition of the mixing coefficient. The thermocline decay increases with the value of mixing coefficient, which is expressed as a function of Reynolds number and Richardson number.

Nucleate pool boiling heat transfer of pure liquids at low to moderate heat fluxes

Abstract A model based on an additive mechanism of heat transfer is proposed for pool boiling of single component systems. The contributing modes of heat transfer are: (i) the heat transferred as latent heat to the evaporating microlayer, (ii) the heat transferred by transient conduction during re-formation of the thermal boundary layer and (iii) the heat transferred by turbulent natural convection from the heating surface not influenced by the bubbles. The heat flux due to the evaporating microlayer is estimated from the instantaneous microlayer thickness during the bubble growth period. An estimate of the nucleation site density is obtained from a literature correlation that includes the boiling surface characteristics. Experimental data from the literature and the present study show very good agreement with the model, validating the postulated mechanism.

EFFECTS OF THERMAL BOUNDARY CONDITIONS ON ENTROPY GENERATION DURING NATURAL CONVECTION

A comprehensive numerical study on entropy generation during natural convection is studied in a square cavity subjected to a wide variety of thermal boundary conditions. Entropy generation terms involving thermal and velocity gradients are evaluated accurately based on the elemental basis set via the Galerkin finite element method. The thermal and fluid irreversibilities during the conduction and convection dominant regimes are analyzed in detail for various fluids (Pr = 0.026,988.24) within Ra = 103–105. Further, the effect of Ra on the total entropy generation and average Bejan number is discussed. It is observed that thermal boundary conditions significantly affect the thermal mixing, temperature uniformity, and the entropy generation in the cavity. A case where the bottom wall is hot isothermal with linearly cooled side walls and adiabatic top wall is found to result in high thermal mixing and a higher degree of temperature uniformity with minimum total entropy generation.

Entropy Generation During Natural Convection in a Porous Cavity: Effect of Thermal Boundary Conditions

Entropy generation plays a significant role in the overall efficiency of a given system, and a judicious choice of optimal boundary conditions can be made based on a knowledge of entropy generation. Five different boundary conditions are considered and their effect of the permeability of the porous medium, heat transfer regime (conduction and convection) on entropy generation due to heat transfer, and fluid friction irreversibilities are investigated in detail for molten metals (Pr = 0.026) and aqueous solutions (Pr = 10), with Darcy numbers (Da) between 10−5–10−3 and at a representative high Rayleigh number, Ra = 5 × 105. It is observed that the entropy generation rates are reduced in sinusoidal heating (case 2) when compared to that for uniform heating (case 1), with a penalty on thermal mixing. Finally, the analysis of total entropy generation due to variation in Da and thermal mixing and temperature uniformity indicates that, there exists an intermediate Da for optimal values of entropy generation, thermal mixing, and temperature uniformity.

On the mechanism of subcooled flow boiling of binary mixtures

Abstract Correlations have been developed for forced convection subcooled boiling of binary mixtures using data on acetone-water, isopropanol-water and n- butanol-water systems. Separate correlations for the partial boiling region and for the fully developed boiling region incorporating the effects of degree of subcooling, liquid velocity and binary composition have been proposed.

Transient analysis of energy storage in a thermally stratified water tank

A one-dimensional transient heat conduction model to describe the decay of the thermocline in a stratified water tank is presented. The problem is formulated as an initial boundary value problem and the resulting governing equations in the fluid and in the storage wall are solved numerically to obtain the temperature profiles in the wall and the fluid. The formulation considers the axial conduction of heat, both in the fluid and in the solid wall. The mixing parameters introduced in the boundary conditions at the top and bottom of the tank in the fluid region account for mixing due to inlet and outlet streams with the stored fluid. The model is applicable to the storage of both hot and chilled water. The model is validated with experimental data from the literature. The parameters that influence the operation of a stratified thermal energy storage for both heat and cool storage are examined.

Experimental Investigation of Subcooled Flow Boiling in a Minichannel

The present work presents experimental results of subcooled flow boiling heat transfer phenomena in minichannels. The experiments were conducted with high surface tension liquid (water), which clearly indicates that inertial effects are important for both momentum and thermal transport in minichannels. The test section is a horizontal AISI 316 stainless-steel minitube with internal diameter of 1.13 mm with uniform heated length of 250 mm and the tube is heated by AC electric current. The variation in wall temperature and pressure drop gives an indication of the different boiling regions along the axis of the horizontal tube. Based on sudden oscillations of the pressure drop and wall temperature, onset of nucleate boiling (ONB) is identified in the tube. During ONB, pressure drop and the fluctuations of fluid outlet temperature are observed near the exit of the test section. Predictive models for ONB were evaluated and Kandlikars ONB model was found to agree well with the present data. Comparisons of experimental heat transfer coefficient in the subcooled boiling regime with the existing correlations are also presented.

Fluid Flow and Boiling Heat Transfer in Mini Channels

Two phase flows in mini channels occur in many industrial applications such as electronic cooling, compact heat exchangers, compact refrigeration systems and in micro propulsion devices. Due to its significance, research on two phase flow in mini channels has become attractive. However, in recent times a controversy exists whether flow in minichannel is different from macro flow because there are still substantial disagreements among various experimental results. In the present study an experimental investigation is carried out for fluid flow and boiling heat transfer characteristics of mini channels with tube diameters ranging from 1–3mm. The tubes were made of SS with water as the working fluid. The variation in friction factor and Nusselt number with decrease in tube diameter for single phase flow was systematically studied. The point of Onset of Nucelate Boiling (ONB) was identified based on wall temperature profile. The effect of heat flux and mass flux on two phase pressure drop with three different tube diameters during sub cooled boiling were investigated. The results reveal that there is an unmistakable effect of tube diameter on fluid friction and onset of boiling during sub cooled boiling in tubes of mini channel dimensions.Copyright