The Experts below are selected from a list of 108 Experts worldwide ranked by ideXlab platform
Antonio Campo - One of the best experts on this subject based on the ideXlab platform.
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On the Teaching of the Lumped Model for Unsteady Heat Conduction: Natural Convection Versus Forced Convection
2013Co-Authors: Antonio CampoAbstract:Within the framework of the lumped model for unsteady heat conduction, the Biot number criterion for forced convection heat exchange between a body surface and a fluid stipulates that Bi = < 0.1 where the mean Convective Coefficient is affected by the imposed fluid velocity. Conversely, when heat is exchanged by natural convection, the Biot number criterion involves a mean Convective Coefficient that depends on the temperature difference between a body surface and a fluid. Under unsteady conditions, the body surface temperature changes continually with time, and so does the mean Convective Coefficient, . As a consequence, the Biot number criterion must be modified. This gives rise to Bimax = < 0.1 where is the maximum mean Convective Coefficient happening at the initial temperature Ti and time t = 0. In this paper, the exact mean temperature distribution T (t) is deduced for a case study wherein a solid sphere is cooled by natural convection in quiescent air at ambient temperature under the premises of the lumped model. Additionally, a new modified Biot number criterion Bimax is proposed in order to facilitate quick engineering analysis and design of sphere/air ensembles related to natural convection. The physics-based equivalence interweaves the solid thermal conductivity, the fluid thermal conductivity and the extended Grashof number embracing the initial–to–fluid temperature difference.
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prediction of friction factors and axial descent of Convective Coefficients in laminar flows through internally finned tubes by way of solving two dimensional heat conduction equations
The International journal of mechanical engineering education, 2008Co-Authors: Antonio Campo, G A Ledezma, Ignacio Carvajal MariscalAbstract:This paper reports a simple numerical procedure for the prediction of the friction factors and the axial descent of the convection Coefficients in the upstream region of internally finned tubes for prevalent laminar regimes. When single-phase viscous fluids are fully developed, the velocity profiles are invariant with the axial coordinate, while the temperature distributions do vary with it. Since the 2-D momentum equation corresponds to a 2-D heat conduction equation, the 2-D velocity profiles supply the product of the friction factor and the Reynolds number, fRe. By implementing the transversal method of lines (TMOL) in the sub-domain z → 0, the 3-D energy equation is transformed into an equivalent quasi 2-D heat conduction equation. The local Convective Coefficient asymptotes hc,Z→0 changing with the axial coordinate z → 0 come from the quasi 2-D temperature distributions. The fRe and hc,Z→0 results have been synthesized for commensureate fin configurations in the sub-domain z → 0 of the heat exchange ...
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Quick identification of gases for enhancing heat transfer in turbulent pipe flows using standard correlation equations for the Convective Coefficient and the friction factor
Applied Thermal Engineering, 2005Co-Authors: Antonio CampoAbstract:Abstract This paper addresses a simple algebraic procedure for the quick identification of gases (different than air) that are capable of enhancing heat transfer in turbulent pipe flows while causing small-to-moderate accretions in pressure drops. Relying on standard correlation equations for the Convective Coefficient and the friction factor, the algebraic procedure is centered on the existing competition between the four intervening thermophysical properties: density ρ, viscosity η, specific heat capacity at constant pressure cp and thermal conductivity λ. To judge the ability of a candidate gas, a figure-of-merit articulating the Convective Coefficient h and the pressure drop Δp is chosen. Fixing the length-to-diameter ratio L/D of the pipe along with the average fluid velocity uave, it turns out that the h Δ p ratio is directly proportional to λ c p and inversely proportional to η approximately. In this way, the units of the h Δ p ratio are closely equivalent to the units of the specific heat capacity cp. A case study involves twenty five gases (pressure of 1 atm and temperature of 300 K) commonly used in turbulent forced convection pipe flows for industrial applications. It is demonstrated that the three better gases are helium, hydrogen and methane. Qualitatively, helium is superior to air by a factor of 5.22, hydrogen is superior to air by a factor of 3.58 and methane is superior by a factor of 2.24.
Joseph Virgone - One of the best experts on this subject based on the ideXlab platform.
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Convection correlations analysis for air/PCM exchanger
2014Co-Authors: Anne Duchez, Joseph Virgone, Damien DavidAbstract:The air/PCM heat exchanger described in this paper consists in horizontal PCM plates separated by an air flow. This is a storage system dedicated to insure a better summer thermal comfort. It stores freshness during the night and releases it in the house during the day. The purpose of presented work is to contribute in the improvement of the modeling of air / PCM Convective exchanges. In this kind of exchanger, we are confronted to different flow patterns, which must be taken into account in the choice of the Convective Coefficient. So a detailed study of flow patterns encountered in the exchanger is first performed and appropriate correlations from the literature are selected. These one are tested on a numerical model of the heat exchanger. The results are compared to experimental data. Here, a rigorous choice of the correlations, according to the flow patterns, does not seem to improve significantly the numerical results comparing to the use of a single correlation representative of the most covering flow structure. Other model errors must be tackled before we can appreciate the improvement due to the correlation choice. Keywords: air/PCM exchanges, exchanger, convection, simulation, experimental results
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a correlation of the Convective heat transfer Coefficient between an air flow and a phase change material plate
Applied Thermal Engineering, 2013Co-Authors: A. De Gracia, Damien David, Albert Castell, Luisa F. Cabeza, Joseph VirgoneAbstract:This paper provides a new correlation to determine the heat transfer Coefficient between an air flow and a plate made of phase change material (PCM). This correlation was built for the simulation of heat storage units containing PCM plates subjected to an inlet temperature step. The presented correlation has the following form: NuPCM x;t ¼ NuPSM$f PCM. The first term NuPSM is for a plate made of traditional material. The term fPCM is a perturbation due to the phase change in the plate. Each term depends on 5 non-dimensional parameters. One of them represents the advance in the total heating or cooling process, in order to take into account the transient evolution of the Convective Coefficient. The correlations are built using the Least Squares Method, from series of CDF simulation data. The shape of the perturbation fPCM reveals a complex evolution of the temperature repartition in the PCM plate. Finally, a nodal model of the plate has been developed in order to test the provided new correlation and other correlations available in the literature. The results obtained with the present correlation show better agreements with the CFD results, which make this correlation suitable for the simulation of PCM heat storage systems.
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A correlation of the Convective heat transfer Coefficient between an air flow and a phase change material plate
Applied Thermal Engineering, 2013Co-Authors: A. De Gracia, Damien David, Albert Castell, Luisa F. Cabeza, Joseph VirgoneAbstract:This paper provides a new correlation to determine the heat transfer Coefficient between an air flow and a plate made of phase change material (PCM ). This correlation was built for the simulation of heat storage units containing PCM plates subjected to an inlet temperature step. The presented correlation has the following form: View the MathML sourceNux,tPCM=NuPSM*fPCM. The first term NuPSM is for a plate made of traditional material. The term fPCM is a perturbation due to the phase change in the plate. Each term depends on 5 non-dimensional parameters. One of them represents the advance in the total heating or cooling process, in order to take into account the transient evolution of the Convective Coefficient. The correlations are built using the Least Squares Method, from series of CDF simulation data. The shape of the perturbation fPCM reveals a complex evolution of the temperature repartition in the PCM plate. Finally, a nodal model of the plate has been developed in order to test the provided new correlation and other correlations available in the literature. The results obtained with the present correlation show better agreements with the CFD results, which make this correlation suitable for the simulation of PCM heat storage systems.
Raymond Viskanta - One of the best experts on this subject based on the ideXlab platform.
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Effect of jet-jet spacing on Convective heat transfer to confined, impinging arrays of axisymmetric air jets
International Journal of Heat and Mass Transfer, 1994Co-Authors: Aaron M. Huber, Raymond ViskantaAbstract:Abstract The effects of jet-jet spacing ( X n D ), low nozzle-plate spacings ( H D = 0.25, 1.0 and 6.0 ) and spent air exits located between the jet orifices were studied on the magnitude and uniformity of the Convective heat transfer Coefficients for confined 3 × 3 square arrays of isothermal axisymmetric air jets impinging normally to a heated surface. Local and average Nusselt numbers are presented for Reynolds number range of 3500–20 400. The local Nusselt numbers illustrate the (non)uniformity of the heat transfer and aid in understanding the variations in the average Nusselt number. The jet-jet spacing affects the Convective Coefficient by varying the influence of the adjacent jet interference and fraction of the impingement surface covered by the wall jet. The addition of spent air exits increased the Convective Coefficient and influenced the location of the optimum separation distance. In addition, significant enhancement of the uniformity and the Convective Coefficients was observed at H D = 0.25 and 1.0 when compared to H D = 6.0 .
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Impingement heat transfer with a single rosette nozzle
Experimental Thermal and Fluid Science, 1994Co-Authors: Aaron M. Huber, Raymond ViskantaAbstract:Abstract This investigation examined the magnitude and uniformity of the Convective heat transfer Coefficients for a single rosette nozzle impinging air normally to a heated surface. The local heat transfer Coefficient was measured using a heated 0.025-mm-thick stainless steel foil impingement surface coated with liquid crystals. The temperature distribution along the surface was determined by measuring the reflected light from the liquid crystal with the use of bandpass filters and an electronic digitizer. Measurements were obtained for low nozzle-to-plate spacings ( H = 3.81, 0.635 , and 0.159 cm) with and without a confinement surface. Local and average Convective Coefficients distributions illustrate the uniformity or nonuniformity of the heat transfer and assist in understanding the variations in the magnitude of the average Convective Coefficient. The Convective Coefficients for the rosette nozzle were also compared to values obtained with axisymmetric jets. Results show that the local Convective Coefficients are highly nonuniform for the rosette nozzle. From the comparison of the rosette nozzle with the axisymmetric jet, it would appear that the enhancement of the average heat transfer by this type of specialty nozzle design compared to an axisymmetric jet occurs only for small flow rates, small operating separation distances, and specific averaging areas. Small separation distances ( H ≤ 0.635 cm ) are required to minimize the Convective Coefficient degradation due to entrainment and velocity decay for the rosette nozzle.
A. De Gracia - One of the best experts on this subject based on the ideXlab platform.
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a correlation of the Convective heat transfer Coefficient between an air flow and a phase change material plate
Applied Thermal Engineering, 2013Co-Authors: A. De Gracia, Damien David, Albert Castell, Luisa F. Cabeza, Joseph VirgoneAbstract:This paper provides a new correlation to determine the heat transfer Coefficient between an air flow and a plate made of phase change material (PCM). This correlation was built for the simulation of heat storage units containing PCM plates subjected to an inlet temperature step. The presented correlation has the following form: NuPCM x;t ¼ NuPSM$f PCM. The first term NuPSM is for a plate made of traditional material. The term fPCM is a perturbation due to the phase change in the plate. Each term depends on 5 non-dimensional parameters. One of them represents the advance in the total heating or cooling process, in order to take into account the transient evolution of the Convective Coefficient. The correlations are built using the Least Squares Method, from series of CDF simulation data. The shape of the perturbation fPCM reveals a complex evolution of the temperature repartition in the PCM plate. Finally, a nodal model of the plate has been developed in order to test the provided new correlation and other correlations available in the literature. The results obtained with the present correlation show better agreements with the CFD results, which make this correlation suitable for the simulation of PCM heat storage systems.
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A correlation of the Convective heat transfer Coefficient between an air flow and a phase change material plate
Applied Thermal Engineering, 2013Co-Authors: A. De Gracia, Damien David, Albert Castell, Luisa F. Cabeza, Joseph VirgoneAbstract:This paper provides a new correlation to determine the heat transfer Coefficient between an air flow and a plate made of phase change material (PCM ). This correlation was built for the simulation of heat storage units containing PCM plates subjected to an inlet temperature step. The presented correlation has the following form: View the MathML sourceNux,tPCM=NuPSM*fPCM. The first term NuPSM is for a plate made of traditional material. The term fPCM is a perturbation due to the phase change in the plate. Each term depends on 5 non-dimensional parameters. One of them represents the advance in the total heating or cooling process, in order to take into account the transient evolution of the Convective Coefficient. The correlations are built using the Least Squares Method, from series of CDF simulation data. The shape of the perturbation fPCM reveals a complex evolution of the temperature repartition in the PCM plate. Finally, a nodal model of the plate has been developed in order to test the provided new correlation and other correlations available in the literature. The results obtained with the present correlation show better agreements with the CFD results, which make this correlation suitable for the simulation of PCM heat storage systems.
Zhijing Feng - One of the best experts on this subject based on the ideXlab platform.
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Experimental determination of Convective heat transfer Coefficient in WEDM
International Journal of Machine Tools and Manufacture, 2007Co-Authors: Gang Cheng, Fuzhu Han, Zhijing FengAbstract:An experimental determination method of the Convective heat transfer Coefficient in wire electro-discharge machining (WEDM) is introduced. A special device is developed to measure the average temperature increment of the wire after a period of short circuit discharges, and the thermal load imposed on the wire is also tracked and recorded in advance. Then, based on the thermal model of the wire, the Convective Coefficient can be calculated accurately. Some tuning experiments are carried out inside and outside a previously cut profile to examine the influence of the kerf on the Convective Coefficient. As soon as the wire cuts into the workpiece, the Convective Coefficient will decrease more than 30%. With this method, the effect of the coolant flushing pressure on the Convective Coefficient is also estimated. If the pressure is raised from 0.1 to 0.8 Mpa, the Convective Coefficient will increase more than 20%, and thus ameliorate the cooling condition of the WEDM process.
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Experimental determination of Convective heat transfer Coefficient in WEDM
International Technology and Innovation Conference 2006 (ITIC 2006), 2006Co-Authors: Gang Cheng, Fuzhu Han, Zhijing FengAbstract:An experimental determination method of the Convective heat transfer Coefficient in WEDM is introduced. A special device is developed to measure the average temperature increment of the wire after a period of short circuit discharges, and the thermal load imposed on the wire is also tracked and recorded in advance. Then, based on the thermal model of the wire, the Convective Coefficient can be calculated accurately. With this method, the effect of the coolant flushing pressure on the Convective Coefficient is estimated. Experiments are also carried out inside and outside a previously cut profile to examine the influence of the kerf on the Convective Coefficient.