A.R. Balakrishnan

Bio: A.R. Balakrishnan is an academic researcher from Indian Institute of Technology Madras. The author has contributed to research in topics: Heat transfer & Nucleate boiling. The author has an hindex of 21, co-authored 44 publications receiving 1592 citations. Previous affiliations of A.R. Balakrishnan include University of Waterloo.

Effects of thermal boundary conditions on natural convection flows within a square cavity

Abstract: A numerical study to investigate the steady laminar natural convection flow in a square cavity with uniformly and non-uniformly heated bottom wall, and adiabatic top wall maintaining constant temperature of cold vertical walls has been performed. A penalty finite element method with bi-quadratic rectangular elements has been used to solve the governing mass, momentum and energy equations. The numerical procedure adopted in the present study yields consistent performance over a wide range of parameters (Rayleigh number Ra, 103 ⩽ Ra ⩽ 105 and Prandtl number Pr, 0.7 ⩽ Pr ⩽ 10) with respect to continuous and discontinuous Dirichlet boundary conditions. Non-uniform heating of the bottom wall produces greater heat transfer rates at the center of the bottom wall than the uniform heating case for all Rayleigh numbers; however, average Nusselt numbers show overall lower heat transfer rates for the non-uniform heating case. Critical Rayleigh numbers for conduction dominant heat transfer cases have been obtained and for convection dominated regimes, power law correlations between average Nusselt number and Rayleigh numbers are presented.

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

Abstract: 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.

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.

Flow and pressure drop fluctuations in a vertical tube subject to low frequency oscillations

Abstract: Heat transfer and other equipment mounted on off-shore platforms may be subjected to low frequency oscillations. The effect of these oscillations, typically in the frequency range of 0.1–1 Hz, on the flow rate and pressure drop in a vertical tube has been studied experimentally in the present work. A 1.75 m-long vertical tube of inner diameter 0.016 m was mounted on a plate and the whole plate was subjected to oscillations in the vertical plane using a mechanical simulator capable of providing low frequency oscillations in the range of 8–30 cycles/min at an amplitude of 0.125 m. The effect of the oscillations on the flow rate and the pressure drop has been measured systematically in the Reynolds number range 500–6500. The induced flow rate fluctuations were found to be dependent on the Reynolds number with stronger fluctuations at lower Reynolds numbers. The effective friction factor, based on the mean pressure drop and the mean flow rate, was also found to be higher than expected. Correlations have been developed to quantify this Reynolds number dependence.

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.

Introduction to the Finite Element Method

Abstract: This chapter introduces the finite element method (FEM) as a tool for solution of classical electromagnetic problems. Although we discuss the main points in the application of the finite element method to electromagnetic design, including formulation and implementation, those who seek deeper understanding of the finite element method should consult some of the works listed in the bibliography section.

Thermal energy storage technologies and systems for concentrating solar power plants

Abstract: This paper presents a review of thermal energy storage system design methodologies and the factors to be considered at different hierarchical levels for concentrating solar power (CSP) plants. Thermal energy storage forms a key component of a power plant for improvement of its dispatchability. Though there have been many reviews of storage media, there are not many that focus on storage system design along with its integration into the power plant. This paper discusses the thermal energy storage system designs presented in the literature along with thermal and exergy efficiency analyses of various thermal energy storage systems integrated into the power plant. Economic aspects of these systems and the relevant publications in literature are also summarized in this effort.

Pool boiling characteristics of nano-fluids

Abstract: Common fluids with particles of the order of nanometers in size are termed as ‘nano-fluids’ which have created considerable interest in recent times for their improved heat transfer capabilities. With very small volume fraction of such particles the thermal conductivity and convective heat transfer capability of these suspensions are significantly enhanced without the problems encountered in common slurries such as clogging, erosion, sedimentation and increase in pressure drop. This naturally brings out the question whether such fluids can be used for two phase applications or in other words phase change in such suspensions will be assistant or detrimental to the process of heat transfer. The present paper investigates into this question through experimental study of pool boiling in water–Al 2 O 3 nano-fluids. The results indicate that the nano-particles have pronounced and significant influence on the boiling process deteriorating the boiling characteristics of the fluid. It has been observed that with increasing particle concentration, the degradation in boiling performance takes place which increases the heating surface temperature. This indicates that the role of transient conduction in pool boiling is overshadowed by some other effect. Since the particles under consideration are one to two orders of magnitude smaller than the surface roughness it was concluded that the change of surface characteristics during boiling due to trapped particles on the surface is the cause for the shift of the boiling characteristics in the negative direction. The results serve as a guidance for the design of cooling systems with nano-fluids where an overheating may occur if saturation temperature is attained. It also indicates the possibility of such engineered fluids to be used in material processing or heat treatment applications where a higher pre-assigned surface temperature is required to be maintained without changing the fluid temperature.

Heat transfer characteristics of thermal energy storage system using PCM capsules: A review

Abstract: Thermal energy storage has recently attracted increasing interest related to thermal applications such as space and water heating, waste heat utilization, cooling and air-conditioning. Energy storage is essential whenever there is a mismatch between the supply and consumption of energy. Use of phase change material (PCM) capsules assembled as a packed bed is one of the important methods that has been proposed to achieve the objective of high storage density with higher efficiency. A proper designing of the thermal energy storage systems using PCMs requires quantitative information about heat transfer and phase change processes in PCM. This paper reviews the development of available latent heat thermal energy storage technologies. The different aspects of storage such as material, encapsulation, heat transfer, applications and new PCM technology innovation have been carried out.

An overview of thermal energy storage systems

Abstract: Due to humanity's huge scale of thermal energy consumption, any improvements in thermal energy management practices can significantly benefit the society. One key function in thermal energy management is thermal energy storage (TES). Following aspects of TES are presented in this review: (1) wide scope of thermal energy storage field is discussed. Role of TES in the contexts of different thermal energy sources and how TES unnecessitates fossil fuel burning are explained. Solar power generation, building thermal comfort and other niche applications of TES are presented. (2) Insight into classes of TES storage materials with details like their physical properties, cost, operational performance and suitability to application requirements is provided. (3) Insight into types of TES systems is presented. TES systems are classified using different types of criteria. Most common TES systems like seasonal TES systems, CSP plant TES systems, TES systems of domestic solar thermal applications, heat and cold storages of building HVAC systems etc are described. Active TES systems like thermocline, packed bed, fluidized bed, moving bed etc are analyzed. Passive TES systems implemented in buildings, textiles, automobiles etc are presented. TES systems operating in cold, low, medium and high temperature ranges are listed. Design parameters, operational issues and cost model of TES systems are discussed.