Abraham J. Salazar

Analysis of Fin-Tube Joints in a Compact Heat Exchanger

This article deals with an analysis of fin-tube joints as functions of topological alterations of the joint fillet size. Based on numerical predictions of a joint topology formed by the surface tension driven reactive flow of molten metal, and subsequently verified by empirical evidence gathered through both laboratory and industrial testing, the topology alterations were identified for thermal integrity studies. Subsequently, thermal characteristics of corresponding fin-tube joints were determined in terms of two models of the thermal contact resistance. Model predictions of the fin efficiency with an altered topology of the joint zone were compared with the simulation results from a computational fluid dynamics study, and the results fit well. Numerical predictions of joint topology were devised using an in-house-developed finite-element code, and verified by the Surface Evolver code. Such prediction provided quantitative joint topology information that was needed in assessments of the joint thermal performance. Experimental data were obtained using a computer-controlled transparent hot zone with an ultra-high-purity nitrogen background atmosphere under tightly controlled conditions, and also by an analysis of the state-of-the-art manufacturing process data obtained from an industrial setting. It is demonstrated that a value of fin efficiency, assumed as recommended by traditional sizing design procedures, may drastically differ from actual values.

Theoretical analysis of the Exponential Transversal Method of Lines for the diffusion equation

Recently a new approximate technique to solve the diffusion equation was proposed by Campo and Salazar. This new method is inspired on the Method of Lines (MOL) with some insight coming from the method of separation of variables. The proposed method, the Exponential Transversal Method of Lines (ETMOL), utilizes an exponential variation to improve accuracy in the evaluation of the time derivative. Campo and Salazar have implemented this method in a wide range of heat/mass transfer applications and have obtained surprisingly good numerical results. In this paper, the authors study the theoretical properties of ETMOL in depth. In particular, consistency, stability and convergence are established in the framework of the heat/mass diffusion equation. In most practical applications the method presents a very reduced truncation error in time and its different versions are proven to be unconditionally stable in the Fourier sense. Convergence of the solutions is then established. The theory is corroborated by several analytical/numerical experiments.

Similarity between unsteady-state conduction in a planar slab for short times and steady-state conduction in a uniform, straight fin

The principal goal of this study is two-fold. First, to elucidate an analogy between unsteady-state conduction in a planar slab for short times and steady-state conduction in a straight fin of uniform cross section. Second, to present approximate analytical solutions of the transient heat conduction equation for short times in a plane having a uniform initial temperature and subjected to a uniform surface temperature (Dirichlet boundary condition). Use was made of a hybrid computational method, theTransversal Method Of Lines (TMOL) to bypass the classical solution techniques for partial differential equations and exploit the physical analogy with the steady-state, heat conduction in a straight fin. The resulting quasi-steady, approximate analytical solution is very easy to employ and is suitable for obtaining quality short-time temperature distributions in the slab.ZusammenfassungHauptziel der Untersuchung ist zum einen, eine Analogie zwischen kurzzeitiger instationärer Wärmeleitung in einer ebenen Platte und stationärer Leitung in einer geraden Rechteckrippe gleichbleibenden Querschnitts aufzuzeigen, zum anderen sollen analytische Näherungslösungen der instationären Wärmeleitungsgleichung für kurze Zeiten hergeleitet werden, wobei eine Platte mit gleichförmiger Anfangstemperatur unter Aufprägung einer gleichförmigen Oberflächentemperatur (Dirichletsche Randbedingung) zugrunde gelegt sei. Mit Hilfe einer hybriden Berechnungsmethode (Transversal Method of Lines: TMOL) läßt sich die klassischer Lösungstechnik für partielle Differentialgleichungen umgehen, und die physikalische Analogie zur stationären Wärmeleitung in einer geraden Rippe ausnutzen. Die resultierende, quasistationäre, analytische NäherungslÖsung ist leicht auszuwerten und eignet sich zur Ermittlung des Temperaturfeldes in der Platte für kurze Zeiten.

Numerical simulation of the disintegration of forced liquid jets using volume-of-fluid method

The present numerical study investigates the effect of finite sinusoidal velocity modulations imposed on an otherwise unperturbed cylindrical liquid jet issuing into stagnant gas using Volume-of-Fluid (VOF) methodology. Variation of the simulation parameters, comprising of the mean liquid jet velocity, modulation amplitude and frequency grouped together using a set of non-dimensional parameters, leads to the formation of a wide gamut of reproducible liquid structures such as surface waves, upstream/downstream directed bells and chains of droplets similar to those observed in experiments. The computations efficiently capture the diverse flow structures generated by the evolving modulated liquid jet inclusive of several nonlinear dynamics such as growth of surface waves, ligament interaction with shear vortices and its subsequent thinning process. The simulations identify the deterministic behaviour of modulated liquid jets to predict liquid disintegration modes under given set of non-dimensional parameters.

Accurate analytical/numerical solution of the heat conduction equation

Purpose – The purpose of this paper is to address a novel method for solving parabolic partial differential equations (PDEs) in general, wherein the heat conduction equation constitutes an important particular case. The new method, appropriately named the Improved Transversal Method of Lines (ITMOL), is inspired in the Transversal Method of Lines (TMOL), with strong insight from the method of separation of variables. Design/methodology/approach – The essence of ITMOL revolves around an exponential variation of the dependent variable in the parabolic PDE for the evaluation of the time derivative. As will be demonstrated later, this key step is responsible for improving the accuracy of ITMOL over its predecessor TMOL. Throughout the paper, the theoretical properties of ITMOL, such as consistency, stability, convergence and accuracy are analyzed in depth. In addition, ITMOL has proven to be unconditionally stable in the Fourier sense. Findings – In a case study, the 1-D heat conduction equation for a large p...

Infrared Visualization of Automotive Paint Spray Transfer Process

An Infrared thermography based visualization technique for automotive paint spray is presented. Two common automotive paint applicators were studied using this technique and the results presented. The paint applicators studied were high-speed rotary bell atomizer and low pressure air atomizer. The technique uses a uniformly heated blackbody emitter as a background. The emitted infrared energy from the background passing through the spray is attenuated by the droplets in the spray. The attenuated intensity is captured by an infrared camera, to form a two-Dimensional image of the spray flow field. From the acquired intensity image, the entire spray flow field structure is visualized and the result discussed.Copyright

Investigation of the primary breakup of round turbulent liquid jets using LES/VOF technique

*The disintegration of a round turbulent liquid jet issuing from a coaxial type atomizer into a high pressure chamber is numerically studied. The liquid-gas interface is tracked using Compressive Interface Capturing Scheme for Arbitrary Meshes (CICSAM). Large Eddy Simulation techniques are used to resolve the large scale motions in the flow while modeling the small scale statistics, required to predict the breakup mechanism. A one equation eddy viscosity model has been used to resolve the sub-grid scale stresses, which solves a transport equation for sub-grid scale kinetic energy. The advantage of using the one equation model is its inherent ability to predict backscattering, which is essential in predicting the energy exchange between the resolved and modeled scales in the currently considered multiphase regime. Numerical computations are performed using this combined LES/VOF method to investigate the primary breakup mechanism often encountered in round turbulent jets. Due to fine mesh requirements, simulations are limited to a downstream distance of 10 nozzle diameters. Our numerical study concerns the effect of relative velocity between liquid and gas phases on the turbulent jet disintegration characteristics. Injecting liquid with no co-flowing gases results in development of short wavelength perturbations on the liquid surface by the action of aerodynamic forces leading to stripping of liquid surface to form discs, ligaments and finally droplets. At the onset disc formation, the disc protrusion length to thickness (neck width near the liquid surface) ratio of the discs measured 1.9, while the ratio of disc width to jet diameter is observed to vary between 0.2-0.4. With co-flowing gas of equal velocity magnitude, suppression of liquid-gas interface instability occurs. The interface stretching and the ligament alignment configurations are modified due to decreased radial spread. Primary breakup mechanism involving disc base breakup and disc tip (ligament type) breakup have been discussed. The ligament characteristics arising from different flow configurations are clarified. I. Introduction The mechanism of primary breakup of turbulent liquid jets is of fundamental importance in various industrial processes such as spray coating, combustors, metal powder formation etc. The atomization process of liquid jets is thought to consist of two consecutive steps: primary and secondary breakup. During the primary breakup, the liquid jet exhibits large scale coherent structures that interact with the gas-phase and break into both large and small scale drops. Proceeding downstream, the drops formed due to primary breakup split further into much smaller drops. This mechanism is called secondary breakup process. The process of atomization occurs in the turbulent flow environment which results in the presence of wide range of length and time scales of motion. The foresaid variation in length and time scales differentiates the treatment of primary and secondary flow structure during atomization. The characteristics of primary breakup has significant influence on the properties of dispersed phase affecting the mixing rates with the surrounding gas, mechanisms of secondary breakup and droplet collisions, among others. Over the years, many researchers have identified that the disintegration characteristics of round turbulent liquid jets, is influenced by several parameters such as nozzle design (influence of internal turbulence, cavitation etc), injection conditions, liquid and ambient gas properties etc. The mechanism of primary breakup was earlier observed by De Juhasz et. al 1 and Spenser 2 . Further studies taken up by Grant and Middleman 3 , McCarthy and Malloy 4 identified liquid turbulence properties behind the jet instability and subsequent breakup. Later, Hoyt and Taylor 5 concluded that the drop formation due to turbulent primary breakup was associated with formation of discs and ligaments along the liquid surface and that aerodynamic effects were generally of secondary importance for turbulent primary

Hitozukuri and Monozukuri in Relation to Research and Development in Surface Coating

This chapter addresses the rich set of meanings evoked by the terms monozukuri and hitozukuri, which refer to a centuries-old philosophy deeply rooted in Japanese culture. The former refers to the art of making things with excellence, skill, spirit, zeal, pride, and more. The latter refers to the need to educate and train a person to become expert in monozukuri. Together, monozukuri and hitozukuri can provide the basis for a balanced approach to using technology and enhancing human capacities, one where integration and synthesis play a more important role than specialization and analytical skills. This chapter is intended to address how monozukuri and hitozukuri can be applied to benefit twenty-first century advanced manufacturing, using automobile painting technology research and development as an example.

Computational Modeling of Relevant Automotive Rotary Spray Painting Process

The intent of this chapter is to provide a non-exhaustive but useful guide to the capabilities of computational modeling to help understand the multiple rotary phenomena coexisting in the automotive rotary spray painting process. This manufacturing process has been extensively used to coat automobile bodies in assembly plants. For the last three decades, auto makers as well as spray painting equipment manufacturers have dedicated substantial efforts to understand rotary atomization and to develop more efficient and versatile atomizers. Despite these efforts, it is accepted that the current level of understanding is not sufficient to accurately assess the operation of commercially available rotary bell spray painting systems. The reasons for this lack of understanding are two fold: the inherent complex nature of the rotary bell painting process and the traditional experience-based development of these types of systems. In addition, there is a limitation in what experimentation and theoretical studies alone can provide to enhance both the desired understanding and the efficiency of the paint application process. Based on this realization, efforts have been oriented toward using computational modeling as a new tool to develop this understanding. However, the use of computational modeling has proven to be not an easy task due to the high speed of translation and rotation of the atomizer in real scenarios, the different scales of the phenomena going from micrometer at the rotary cup paint film level to meter at the painted part and spray booth level, the complicated rheology of the paint material (especially for dispersed-type paints), etc.

The Use of Scale Model to Study Film Flow in a Rotary Atomizer Cup

The aim of this chapter is to simulate processes of liquid droplet formation and atomization in a typical automotive paint spray system with scale modeling technique. In a spray painting process, paint is sprayed by a bell sprayer cup rotating at high speed to create fine atomized paint particles. The bell sprayers typically rotate at a speed of 30,000 rpm and have liquid (paint) flowing from the center with a flow rate of 300 cc/min. Due to the centrifugal action of the rotation, the liquid flows to the exterior of the bell, where they pass through grooves leading to the formation of ligaments and henceforth droplets due to shear forces acting against the surface tension forces. We designed different types of scale models to simulate the process.