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research-article  
Roberta J C da Fonseca, Guilherme C Fraga, Rogerio Brittes da Silva and Francis Franca
J. Heat Transfer   doi: 10.1115/1.4038548
This paper presents a methodology for the application of the weighted-sum-of-gray-gases (WSGG) model to systems where the medium is bounded by nongray surfaces. The method relies on the assumption that each gray gas absorption coefficient is randomly spread across the entire wavenumber spectrum. It follows that, in the spectral integration of the radiative transfer equation (RTE), the local emission term can be computed by the joint probability of emission from the subsections of the spectrum related to each gray gas coefficient and from each wall emissivity band. One advantage of the proposed methodology is that it allows the use without any modification of WSGG correlations that are available in the literature. The study presents a few test cases considering a one-dimensional, non-uniform medium slab composed of H2O and CO2, bounded by nongray surfaces. The accuracy of the methodology is assessed by direct comparison with line-by-line (LBL) calculations.
TOPICS: Radiative heat transfer, Emissions, Gases, Absorption, Slabs, Emissivity, Carbon dioxide, Probability, Water
research-article  
Sayan Sadhu, Maddali Ramgopal and Souvik Bhattacharyya
J. Heat Transfer   doi: 10.1115/1.4038541
A high-temperature natural circulation loop using supercritical carbon dioxide as loop fluid is modeled to study the effects of operating variables and relevant design parameters on loop performance. The steady state system model duly considers the axial conduction through loop fluid as well as loop wall, and heat transfer with surroundings. The heat source is considered to be a heater with controlled heat flux and the heat sink is modelled as an end heat exchanger with water as the external cold fluid. The governing conservation equations for mass, momentum, and energy are non-dimensionalized and are solved numerically discretizing in finite volume method. The numerical results are validated against experimental results reported in the literature in terms of modified Grashof number (Grm) and Reynolds number (Re). Results show that heat loss to the ambient affects the loop performance significantly for the high-temperature loop. It is also observed that the heat input at which the circulation becomes maximum can be increased by increasing either the diameter and/or the loop height. However, better performance is obtained with larger diameter tubes instead of longer loop heights. Axial conduction is seen to have a negligible effect on the overall loop performance. Boussinesq approximation appears to be reasonable as the operating conditions of the supercritical loop are away from the critical point.
TOPICS: Steady state, Water, High temperature, Supercritical carbon dioxide, Fluids, Heat, Heat conduction, Design, Heat exchangers, Approximation, Finite volume methods, Heat losses, Heat sinks, Heat transfer, Reynolds number, Heat flux, Momentum
research-article  
R. Balasubramaniam and Enrique Rame
J. Heat Transfer   doi: 10.1115/1.4038520
We analyze the condensation of a quiescent vapor, that is in equilibrium with its liquid, induced by a stagnation point flow in the liquid. The liquid flow brings subcooled liquid from far away to the interface. The ensuing heat transfer causes the vapor to condense. A similarity formulation for the liquid and vapor flow fields and the liquid temperature field is pursued, and a perturbation solution is performed when the ratio of the product of viscosity and density of the vapor to that of the liquid is small. A two-term higher order asymptotic solution is shown to be in excellent agreement with numerical results. The reduction in the rate of condensation due to the presence of a non-condensable gas in the vapor that is insoluble in the liquid is also analyzed.
TOPICS: Flow (Dynamics), Condensation, Vapors, Viscosity, Equilibrium (Physics), Stagnation flow, Subcooling, Heat transfer, Temperature, Density
research-article  
Swastik Acharya, Sumit Agrawal and Sukanta K. Dash
J. Heat Transfer   doi: 10.1115/1.4038478
Natural convection heat transfer from a vertical hollow cylinder suspended in air has been analyzed numerically by varying the Rayleigh number (Ra) in laminar (10^4 = Ra = 10^8) regime. The simulations have been carried out by changing the ratio of length to pipe diameter (L/D) in the range of 1<=L/D<=20. Full conservation equations have been solved numerically for a vertical hollow cylinder suspended in air using algebraic multi-grid solver of FLUENT 13.0. The flow field around the vertical hollow cylinder have been observed to be pretty interesting and visually stimulating for small and large L/D. It has been found that the average Nusselt number(Nu) for vertical hollow cylinder suspended in air increases with the increase in Rayleigh number and the Nu for both the inner and the outer surface also increases with Ra . However, with the increase in L/D, average Nu for the outer surface increases almost linearly whereas the average Nu for the inner surface decreases quadratically and attains asymptotic value at lower L/D for low Ra. In this study the effect of parameters like L/D and Ra on Nu are analyzed, and a correlation for average Nusselt number has been developed for laminar regime. These correlations are accurate to the level of ±6% .
TOPICS: Heat transfer, Natural convection, Numerical analysis, Cylinders, Rayleigh number, Engineering simulation, Simulation, Algebra, Pipes, Flow (Dynamics)
research-article  
Kuljeet Singh and Ranjan Das
J. Heat Transfer   doi: 10.1115/1.4038479
The present work establishes an improved experimentally-validated analysis to predict performance and exergy-related parameters of a mechanical draft cooling tower involving wooden splash fills. Unlike earlier studies, which account for the effect of at most three tower inlet parameters for the exergy analysis, the present study simultaneously considers all five inlet parameters affecting the tower exergy performance. To simultaneously predict outlet air and water conditions, an optimization algorithm involving discrete functions of dry and wet bulb temperatures is used in conjunction with the mathematical model derived from mass and energy conservations within the control volume involving Bosnjakovic correlation. From practical point of view, five inlet parameters such as; dry-bulb temperature, relative humidity, water temperature, water and air flow rates are selected for the exergy analysis. Thereafter, the influence of all inlet parameters on the tower performance is analyzed on various important exergy-related factors. The quantitative analysis reveals that the inlet air humidity, water inlet temperature and the inlet water mass flow rate significantly influence the air and water exergy changes. The present study also reveals that among the five inlet parameters, the water temperature, air humidity and air mass flow rate are primarily responsible for the exergy destruction. Furthermore, it is observed that the second law efficiency is mainly governed by the inlet air flow rate. The present study is proposed to be useful for selecting the tower inlet parameters to improve exergy performance of mechanical cooling towers.
TOPICS: Exergy, Cooling towers, Water, Temperature, Water temperature, Air flow, Flow (Dynamics), Exergy analysis, Optimization algorithms
research-article  
Hien Nhan Hoang, Chul-Hwa Song, In-Cheol Chu and Dong-Jin Euh
J. Heat Transfer   doi: 10.1115/1.4038481
Most of the existing empirical correlations for wall heat transfer during flow boiling show a limited predictability stemming mainly from so-called the suppression and enhancement factors, which are introduced to describe the boiling heat transfer hypothetically by a combination of nucleate pool boiling and single phase forced convection. There is no physical basis strongly supporting the determination of these factors. This study, to avoid such limitations, presents a distinctive approach to the modeling of wall boiling heat transfer utilizing the physical concept of wall heat flux partitioning. A new correlation of local boiling heat transfer coefficient is composed of primary heat transfer mechanisms of transient conduction and forced convection. Heat transfer areas of these mechanisms replace the suppression and enhancement factors in the new correlation and are determined empirically by dimensionless analysis. Based on an experimental database of 3,187 points collected for saturated boiling of various working fluids flowing inside channels of different configurations, the new correlation is obtained and compared with existing correlations widely used. The evaluation highlights much better predictability of the present correlation. While the other correlations show relatively large scattering with over 30% deviation from the experimental data, the newly proposed correlation shows an excellent agreement with a deviation of less than 10%. The good predictability would be from the well-structured physical basis and make the new correlation promising in practical boiling heat transfer analysis.
TOPICS: Boiling, Flow (Dynamics), Heat transfer, Forced convection, Modeling, Databases, Nucleate pool boiling, Transient heat transfer, Heat flux, Heat transfer coefficients, Scattering (Physics), Fluids, Radiation scattering, Electromagnetic scattering
Technical Brief  
Saikat Mukherjee, Saikat Datta and Arup Kumar Das
J. Heat Transfer   doi: 10.1115/1.4038480
A Phase change heat transfer from liquid to gas is studied in nanoscopic framework using molecular dynamics. Water on structured Si substrate is observed from molecular viewpoint after employing heat flux at a constant rate. Initially, we observe that water settles down on the substrate occupying the free space within the notch to obtain its static shape maintaining intra molecular configuration based on attractive and repulsive forces in neighboring hydroxyl bonds. Upon applying heat flux we observe that molecular vibration increases which repels neighbors to make the packing loose. Molecular dilution initiated at the notch and then proceeds to the rest domain. Progressive loosening of the molecules leads to the formation of vapor bubbles which increase in size with time. The rate of growth of this bubble is studied as a function of surface geometry parameters such as notch height, notch width, notch type and notch spacing. Present simulations enrich the knowledge of surface characteristics on boiling heat transfer from fundamental principle in the molecular domain.
TOPICS: Heat transfer, Boiling, Molecular dynamics, Bubbles, Water, Heat flux, Engineering simulation, Vibration, Geometry, Molecular configurations, Shapes, Vapors, Vacuum, Packing (Shipments), Simulation, Packings (Cushioning)
research-article  
Rajkumar Sarma and Pranab Mondal
J. Heat Transfer   doi: 10.1115/1.4038451
We focus on the entropy generation minimization for the flow of a viscoelastic fluid through a parallel plate microchannel, under the combined influences of applied pressure gradient, interfacial slip and conjugate heat transfer. We use the simplified Phan-Thien-Tanner model (s-PTT) to represent the rheological behaviour of the viscoelastic fluid. Using the thermal boundary conditions of third kind, the transport equations are solved analytically to obtain the velocity and temperature distributions in the flow field, which are further used to calculate the entropy generation rate in the analysis. In this study, the influential role of following dimensionless parameters on entropy generation rate is examined: the viscoelastic parameter, slip coefficient, channel wall thickness, thermal conductivity of the wall, Biot number and Peclet number. We show that, there exists particular value of the above mentioned parameters, leading to a minimum entropy generation of the system. We believe that results of this analysis could be of helpful in optimum design of microfluidic system/devices, typically used in thermal management such as in electronic device, micro reactor, micro heat exchanger etc.
TOPICS: Entropy, Viscoelastic fluids, Flow (Dynamics), Heat transfer, Wall thickness, Microchannels, Rheology, Thermal conductivity, Microfluidics, Design, Heat exchangers, Boundary-value problems, Pressure gradient, Temperature distribution, Thermal management

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