Papers by Arturo Pacheco-Vega
Volume 5: Education and Globalization, 2015
ASME 2015 13th International Conference on Nanochannels, Microchannels, and Minichannels, 2015
ASME 2012 10th International Conference on Nanochannels, Microchannels, and Minichannels, 2012
Detailed numerical simulations have been carried out to find the velocity and temperature fields ... more Detailed numerical simulations have been carried out to find the velocity and temperature fields of a rectangular channel with large aspect-ratio. The channel under analysis is aimed to cool a thermo-chromic liquid crystal material (TLC) that is able to capture laser irradiation in the terahertz range. The overall objective of the cooling system is to maintain a nearly-homogeneous temperature of the TLC layer that is not exposed to the direct laser irradiation. The fluid flow and heat transfer simulations are carried out on the basis of three-dimensional versions of the Navier-Stokes equations, along with the energy equation, for an incompressible flow, to determine values of velocity, pressure and temperature inside the channel under different operating conditions. These values are then used to find, from a specific set, the value of the channel height that allows for the most uniform temperature distribution within the expected operating conditions. Results from this analysis indicate that, for all the inlet velocities considered, there is a common value of the channel height, that represents the optimum.
Volume 11: Micro and Nano Systems, Parts A and B, 2007
Volume 5: Energy Systems Analysis, Thermodynamics and Sustainability; NanoEngineering for Energy; Engineering to Address Climate Change, Parts A and B, 2010
ABSTRACT Strategies based on the principle of heat flow and temperature control were implemented,... more ABSTRACT Strategies based on the principle of heat flow and temperature control were implemented, and experimentally tested, to increase the sensitivity of a Tian-Calvet microcalorimeter for measuring heats of adsorption. Here, both heat-flow and temperature control schemes were explored to diminish heater-induced thermal variations within the heat sink element hence obtaining less noise in the baseline signal. PID controllers were implemented within a closed-loop system to perform the control actions in an calorimetric setup. The experimental results demonstrate that the heat flow control strategy provided a better baseline stability when compared to the temperature control. A modified control strategy is then suggested to maintain a stable core temperature and signal noise level in the system.
Volume 10: Heat Transfer, Fluid Flows, and Thermal Systems, Parts A, B, and C, 2008
We consider the problem of accuracy in the algorithmic classification of thermodynamic properties... more We consider the problem of accuracy in the algorithmic classification of thermodynamic properties of fluids from a fuzzy C-means (FCM) clustering methodology. The analysis emphasizes on the way the algorithm is affected by factors such as the natural scale of the data, and the following strategies chosen for the classification: (1) data normalization, (2) transformation, (3) sample size of furnished data, and (4) the value of the fuzzy parameter. Experimental data corresponding to pressure, volume, and temperature of water are taken from the literature and used to show that the natural scaling of the data, the normalization and transformation strategies, as well as the values chosen for the fuzzy parameter are all important factors in the classification. Also, a decrease in the number of data used during the process degrades the quality of the solution. A complete consideration of the issues examined here are undoubtedly beneficial every time a FCM classification is tried on a new problem.
Volume 1: Heat Transfer in Energy Systems; Thermophysical Properties; Heat Transfer Equipment; Heat Transfer in Electronic Equipment, 2009
ABSTRACT We introduce a methodology to extract the regimes of operation from condensing heat exch... more ABSTRACT We introduce a methodology to extract the regimes of operation from condensing heat exchanger data. The methodology uses a Gaussian mixture clustering algorithm to determine the number of groups from the data, and a maximum likelihood decision rule to classify the data into these clusters. In order to assess the accuracy of clustering technique, experimental data from the literature visually classified as dry-surface, dropwise condensation, and film condensation, are used in the analysis. Though there is a discrepancy between the clustering classification and the visual one, an independent evaluation using artificial neural networks (ANNs) shows that the clustering methodology is able to both find the different regimes of operation and classify the data corresponding to each regime.
Volume 10: Heat Transfer, Fluid Flows, and Thermal Systems, Parts A, B, and C, 2008
Volume 6B: Energy, 2013
We perform numerical simulations of single-phase and twophase models of a direct methanol microfl... more We perform numerical simulations of single-phase and twophase models of a direct methanol microfluidic fuel cell (µ-DMFC). The focus of this study is on the parametric analysis of a single channel of the system, for specific sets of operating conditions, in order to map the dependence of the cell performance with respect to the geometrical parameters. Different geometries, ranging from 500 µm to 4 mm in width, and 500 µm to 4 cm in length, along with membrane thicknesses from 50 µm to 500 µm, were considered. The mathematical models are given in terms of the Navier-Stokes, the Butler-Volmer and the Maxwell-Stefan equations, along with Darcy's equation for the flow across the membrane. The difference between the single-and two-phase flow models lies upon the specific constitutive equations used. For each geometry and operating condition, the two-dimensional equations were solved by a finite element method. The conditions of operation include: flow rates and inlet weight fractions of methanol at the anode and oxygen the cathode. The results from this analysis, presented as polarization curves and power densities, indicate that fuel-cell systems with higher flow rates and inlet weight fraction of methanol achieve the best performance. However, when the concentration of methanol exceeds 2M the cell performance is negatively impacted due to crossover. Comparison of the results indicates that the two-phase model has a more restrictive domain for both the geometrical parameters and operating conditions. * Address all correspondence to this author.
In this investigation a parametric study of a Pulsco vent silencer is conducted. The overall obje... more In this investigation a parametric study of a Pulsco vent silencer is conducted. The overall objective is to seek the optimum system performance. The vent silencer consists of a perforated diffuser, the silencer body and a tube module, the latter being comprised by a set of perforated tubes surrounded by acoustic packing. The mathematical model for the vent silencer is built upon a modified Helmholtz equation along with the Delany-Bazley model for the tube module, coupled with the linearized-potential and compressible-flow equations for the diffuser and plenum sections. The parameters chosen in the parametric study include: porosity and perforation size of the tubes, type of acoustic packing, bulk density of the packing material, and the flow Mach number. For each set of parameters, the model equations are solved with the finite element method. The numerical results indicate that a perforation size (i.e., tube-hole diameter) of 1/8" with porosity of 0.4, packing density of 48 kg/m 3 for TRS-10 and 100 kg/m 3 for Advantex packing materials, and low Mach number flows provided the optimum transmission loss. The numerical results are in agreement with the experimental data. Inlet Outlet Acoustic Packing Perforated ow tube Perforated Di user Plenum z θ r FIGURE 2. Schematic of the computational domain.
Heat Transfer, Volume 1, 2006
2011 International Conference on Infrared, Millimeter, and Terahertz Waves, 2011
We describe our effort in the development of a low cost, wide-band detector/camera for generation... more We describe our effort in the development of a low cost, wide-band detector/camera for generation of spatially resolved images of radiation beams in a multispectral range of wavelengths, from infrared to terahertz. The detector (T-camera) utilizes a thermochromic liquid crystal (TLC) film as the sensitive element in a temperature controlled chamber in conjunction with a CCD detector array. The T-camera
Physics of Fluids, 2014
Studies of stratified spin-up experiments in enclosed cylinders have reported the presence of sma... more Studies of stratified spin-up experiments in enclosed cylinders have reported the presence of small pockets of well-mixed fluids but quantitative measurements of the mixedness of the fluid has been lacking. Previous numerical simulations have not addressed these measurements. Here we present numerical simulations that explain how the combined effect of spin-up and thermal boundary conditions enhances or hinders mixing of a fluid in a cylinder. The energy of the system is characterized by splitting the potential energy into diabatic and adiabatic components, and measurements of efficiency of mixing are based on both, the ratio of dissipation of available potential energy to forcing and variance of temperature. The numerical simulations of the Navier-Stokes equations for the problem with different sets of thermal boundary conditions at the horizontal walls helped shed some light on the physical mechanisms of mixing, for which a clear explanation was absent. C 2014 AIP Publishing LLC.
Thermochimica Acta, 2005
Mathematical models of a Tian-Calvet microcalorimeter were solved numerically by the finite-eleme... more Mathematical models of a Tian-Calvet microcalorimeter were solved numerically by the finite-element method in an effort to understand the relative importance of the three basic heat transfer mechanisms operative during gas dosing experiments typically used to determine heats of adsorption on catalysts and adsorbents. The analysis pays particular attention to the quantitative release of heat through various elements of the cell and sensor cups to assess time delays and the degree of thermal shunting that may result in inaccuracies in calorimetric measurements. Conductive transfer predominates in situations where there is high gas headspace pressure. The convection currents that arise when dosing with considerable gas pressure in the cell headspace region are not sufficiently strong to shunt significant amounts of sample heat away from being sensed by the surrounding thermopiles. Therefore, the heat capture fraction (heat sensed/heat produced) does not vary significantly with gas headspace pressure. During gas dosing under very low gas headspace pressure, radiation losses from the top of the sample bed may significantly affect the heat capture fraction, leading to underestimations of adsorption heats, unless the heat radiated from the top of the catalyst bed is effectively reflected back to the sample region or absorbed by an inert packing layer also in thermal contact with the thermopile wall.
Numerical Heat Transfer, Part A: Applications, 2009
We consider the problem of accuracy in the algorithmic classification of thermodynamic properties... more We consider the problem of accuracy in the algorithmic classification of thermodynamic properties of fluids from a fuzzy C-means (FCM) clustering methodology. The analysis emphasizes on the way the algorithm is affected by factors such as the natural scale of the data, and the following strategies chosen for the classification: (1) data normalization, (2) transformation, (3) sample size of furnished data, and (4) the value of the fuzzy parameter. Experimental data corresponding to pressure, volume, and temperature of water are taken from the literature and used to show that the natural scaling of the data, the normalization and transformation strategies, as well as the values chosen for the fuzzy parameter are all important factors in the classification. Also, a decrease in the number of data used during the process degrades the quality of the solution. A complete consideration of the issues examined here are undoubtedly beneficial every time a FCM classification is tried on a new problem.
Measurement Science and Technology, 2010
Strategies based on the principle of heat flow and temperature control were implemented, and expe... more Strategies based on the principle of heat flow and temperature control were implemented, and experimentally tested, to increase the sensitivity of a Tian-Calvet microcalorimeter for measuring heats of adsorption. Here, both heat-flow and temperature control schemes were explored to diminish heater-induced thermal variations within the heat sink element, hence obtaining less noise in the baseline signal. PID controllers were implemented within a closed-loop system to perform the control actions in a calorimetric setup. The experimental results demonstrate that the heat flow control strategy provided a better baseline stability when compared to the temperature control. The effects on the results stemming from the type of power supply used were also investigated.
Journal of Thermal Analysis and Calorimetry, 2013
We address the issue of hardware placement in the development of robust temperature control strat... more We address the issue of hardware placement in the development of robust temperature control strategies that can be used to maintain a stable baseline during microcalorimetric experiments. The two different control loops, each defined by the location of sensor within the calorimeter that is used to achieve control, were first developed and then tested in a fully instrumented experimental system. Both control strategies were structured on proportional-integral-derivative controllers, after which calorimetric experiments were carried out to test the efficiency and robustness of the corresponding methodology. Results indicate that sensor placement plays a fundamental role in the controlled baseline stability and that is better to place the sensing device closer to the heater than to the central core. As part of this study, comparisons were also done against a previously reported control scheme based on heat-flow measurements. Results indicate that controlling only one variable, either temperature or heat flow is sufficient to compensate for heater-induced noise, but not for external fluctuations for which a combined strategy may be necessary.
Journal of Physics: Conference Series, 2012
In the present study we carry out three-dimensional fluid flow and heat transfer simulations on t... more In the present study we carry out three-dimensional fluid flow and heat transfer simulations on the external side of a compact heat exchanger to analyze the interaction between the fluid and its geometry. The overall objective is to use the resulting information for the design of more compact devices. The type of heat exchanger considered here is the common plain-fin and tube, with air flowing over the tubes and water as the inner-tube fluid. Two heat exchanger configurations, in which the tube arrangement is either in-line or staggered, conform the basic geometries. The size of the heat exchanger -regardless of the type of arrangementwhich serves as the baseline for the parametric analysis, is defined by fixing its length; i.e., the number of rows in the flow direction. For the two heat exchanger configurations examined here, the dimensional form of the governing equations, along with the corresponding boundary conditions, are solved under specific flow and temperature values using a finite element method to compute the velocity, pressure and temperature fields. From these, the heat transfer rate and pressure drop are then calculated. The computations are performed for a range in the values of the Reynolds number within the laminar regime. For all cases considered, results from this investigation indicate that the geometrical arrangement plays a major role in the amount of heat being exchanged and that, for a given device, the length needed to exchange 99% of the corresponding amount of energy that may be transferred by the baseline model, is confined to less than 30% of the size of the original device.
Journal of Physics: Conference Series, 2012
In the present study we use Karhunen-Loève (KL) expansions to model the dynamic behavior of a sin... more In the present study we use Karhunen-Loève (KL) expansions to model the dynamic behavior of a single-phase natural convection loop. The loop is filled with an incompressible fluid that exchanges heat through the walls of its toroidal shape. Influx and efflux of energy take place at different parts of the loop. The focus here is a sinusoidal variation of the heat flux exchanged with the environment for three different scenarios; i.e., stable, limit cycles and chaos. For the analysis, one-dimensional models, in which the tilt angle and the amplitude of the heat flux are used as parameters, were first developed under suitable assumptions and then solved numerically to generate the data from which the KL-based models could be constructed. The method of snapshots, along with a Galerkin projection, was then used to find the basis functions and corresponding constants of each expansion, thus producing the optimal representation of the system. Results from this study indicate that the dimension of the KL-based dynamical system depends on the linear stability of the steady states; the number of basis functions necessary to describe the system increases with increased complexity of the system operation. When compared to typical dynamical systems based on Fourier expansions the KL-based models are, in general, more compact and equally accurate in the dynamic description of the natural convection loop.
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Papers by Arturo Pacheco-Vega