Predicting Thermal Stresses Induced by Conjugate Heat Transfer

2009 ◽  
Author(s):  
Mohamed-Nabil Sabry
Keyword(s):  
Heat Transfer ◽  
Wall Temperature ◽  
Degrees Of Freedom ◽  
Conjugate Heat Transfer ◽  
Thermal Stresses ◽  
Uniform Heat Flux ◽  
Temperature Profiles ◽  
Heat Sources ◽  
Thermally Induced ◽  
Simulation Techniques

Thermal stresses developed in electronic systems mainly depend, not only on average temperature values, but rather on wall temperature profiles. These profiles are difficult to predict unless one uses detailed finite element or finite difference modeling and simulation techniques. This type of analysis is only suitable at final design phases were geometrical details are available or being finalized. It is not suitable at early design phases to get a rapid estimate of wall thermal gradients to orient design appropriately. Standard approaches involving correlations for the heat transfer coefficient fail to predict temperature profiles for many reasons. In fact, these correlations depend on temperature profile as an input. In most engineering applications, walls are neither infinitely conducting nor of negligible conductivity to justify the usage of either uniform temperature or uniform heat flux assumptions. Correlations addressing conjugate heat transfer would not be able to solve the problem, unless a large number of them were available covering all possible combinations of fluid and wall conditions. Besides, the case of multiple heat sources, quite common in modern systems, can never be correctly handled by such an approach. The flexible profile technology was proposed earlier to model heat transfer in either solids (conduction) or fluids (forced convection. The model depends on domain (fluid or solid) geometry and physical properties, regardless of the particular set of applied boundary conditions, including that of multiple heat sources. Combining a fluid flexible profile model with a solid one, will allow predicting wall temperature profiles, with an adjustable level of precision, depending on the number of degrees of freedom retained. It will be applied in this paper to predict thermally induced stresses in some simple test cases as a demonstrator of the potentials behind this approach.

Paper 27: Heat Transfer to Steam-Water Mixtures Flowing in Uniformly Heated Tubes in which the Critical Heat Flux has been Exceeded

1967 ◽  
Vol 182 (8) ◽  
pp. 258-267 ◽  
Author(s):  
A. W. Bennett ◽  
G. F. Hewitt ◽  
H. A. Kearsey ◽  
R. K. F. Keeys
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Wall Temperature ◽  
Heat Transfer Process ◽  
High Mass ◽  
Uniform Heat Flux ◽  
Wall Region ◽  
Temperature Profiles ◽  
Axial Temperature ◽  
Dry Out

Experiments are described on evaporative heat transfer to boiling water in upflow in a vertical electrically heated 0·497-in inside diameter tube at 1000 lbf/in2 (abs.). The main objects were to measure the surface temperature profiles in the region beyond the dry-out point in the channel where liquid ceased to flow on the channel wall, and to investigate the behaviour of the dry-out ‘interface’ between the ‘wetted wall’ and the ‘dry wall’ regions. The test section was made from ‘Nimonic’ as this can withstand the highest temperatures in the ‘dry wall’ region and also has a low temperature coefficient of electrical resistivity, thus allowing a uniform heat flux to be maintained with wide axial temperature variation. The temperature in the ‘dry wall’ region first increased rapidly with distance from the dry-out point, after which it either increased at a slower rate or, at high mass velocities, even decreased. The dry-out ‘interface’ moved reversibly down and up the channel as the heat flux was increased and decreased. Local surface temperatures showed no hysteresis with cycling of heat flux, in contrast with the pool boiling situation. A method of predicting the wall temperature profile in the ‘dry wall’ region has been developed. In this method, the heat-transfer process is considered as being in two steps: wall to superheated steam continuum, and steam continuum to water droplets. The first step was calculated from standard single-phase steam heat-transfer correlations, and the second step was calculated on the basis of simultaneous heat transfer to, and steam diffusion from, the droplets. It was important to take account of the slip between the droplets and the steam. Satisfactory agreement was obtained between measured and predicted wall temperature profiles.

Natural Convection From Protruding Heat Sources in a Channel

2003 ◽  
Author(s):  
Tunc Icoz ◽  
Qinghua Wang ◽  
Yogesh Jaluria
Keyword(s):  
Heat Transfer ◽  
Fluid Flow ◽  
Natural Convection ◽  
Rayleigh Scattering ◽  
Convection Heat Transfer ◽  
Separation Distance ◽  
Natural Convection Heat Transfer ◽  
Temperature Profiles ◽  
Heat Sources ◽  
Convection Heat

Natural convection has important implications in many applications like cooling of electronic equipment due to its low cost and easy maintenance. In the present study, two-dimensional natural convection heat transfer to air from multiple identical protruding heat sources, which simulate electronic components, located in a horizontal channel has been studied numerically. The fluid flow and temperature profiles, above the heating elements placed between an adiabatic lower plate and an isothermal upper plate, are obtained using numerical simulation. The effects of source temperatures, channel dimensions, openings, boundary conditions, and source locations on the heat transfer from and flow above the protruding sources are investigated. Different configurations of channel dimensions and separation distances of heat sources are considered and their effects on natural convection heat transfer characteristics are studied. The results show that the channel dimensions have a significant effect on fluid flow. However, their effects on heat transfer are found to be small. The separation distance is found to be an important parameter affecting the heat transfer rate. The numerical results of temperature profiles are compared with the experimental measurements performed using Filtered Rayleigh Scattering (FRS) technique in an earlier study, indicating good agreement. It is observed that adiabatic upper plate assumption leads to better temperature predictions than isothermal plate assumption.

Conjugate Thermal Transport in Gas Flow in Long Rectangular Microchannels

2009 ◽  
Author(s):  
Zhanyu Sun ◽  
Yogesh Jaluria
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Conjugate Heat Transfer ◽  
Fused Silica ◽  
Maximum Temperature ◽  
Bulk Temperature ◽  
Uniform Heat Flux ◽  
Substrate Thickness ◽  
Variable Properties ◽  
Axial Conduction

This paper is directed at the numerical simulation of pressure-driven nitrogen slip flow in long microchannels, focusing on conjugate heat transfer under uniform heat flux wall boundary condition. This problem has not been studied in detail despite its importance in many practical circumstances such as those related to the cooling of electronic devices and localized heat input in materials processing systems. For the gas phase, the two-dimensional momentum and energy equations are solved, considering variable properties, rarefaction, which involves velocity slip, thermal creep and temperature jump, compressibility, and viscous dissipation. For the solid, the energy equation is solved with variable properties. Four different substrate materials are studied, including commercial bronze, silicon nitride, pyroceram and fused silica. The effects of substrate axial conduction, material thermal conductivity and substrate thickness are investigated in detail. It is found that substrate axial conduction leads to a flatter bulk temperature profile along the channel, lower maximum temperature, and lower Nusselt number. The effect of substrate thickness on the conjugate heat transfer is very similar to that of the substrate thermal conductivity. That is, in terms of axial thermal resistance, the increase in substrate thickness has the same impact as that caused by an increase in its thermal conductivity. By comparing the results from constant and variable properties models, it is found that the effects of variation in substrate material properties are negligible.

scholarly journals Large Eddy Simulations with Conjugate Heat Transfer (CHT) modeling of Internal Combustion Engines (ICEs)

2019 ◽  
Vol 74 ◽  
pp. 51 ◽  
Author(s):  
Angela Wu ◽  
Seunghwan Keum ◽  
Volker Sick
Keyword(s):  
Heat Transfer ◽  
Surface Temperature ◽  
Wall Temperature ◽  
Conjugate Heat Transfer ◽  
Internal Combustion Engines ◽  
Internal Combustion ◽  
Large Eddy Simulations ◽  
Temperature Fields ◽  
Large Eddy ◽  
The Impact

In this study, the effects of the thermal boundary conditions at the engine walls on the predictions of Large-Eddy Simulations (LES) of a motored Internal Combustion Engine (ICE) were examined. Two thermal boundary condition cases were simulated. One case used a fixed, uniform wall temperature, which is typically used in conventional LES modeling of ICEs. The second case utilized a Conjugate Heat Transfer (CHT) modeling approach to obtain temporally and spatially varying wall temperature. The CHT approach solves the coupled heat transfer problem between fluid and solid domains. The CHT case included the solid valves, piston, cylinder head, cylinder liner, valve seats, and spark plug geometries. The simulations were validated with measured bulk flow, near-wall flow, surface temperature, and surface heat flux. The LES quality of both simulations was also discussed. The CHT results show substantial spatial, temporal, and cyclic variability of the wall heat transfer. The surface temperature dynamics obtained from the CHT model compared well with measurements during the compression stroke, but the absolute magnitude was 5 K (or 1.4%) off and the prediction of the drop in temperature after top dead center suffered from temporal resolution limitations. Differences in the predicted flow and temperature fields between the uniform surface temperature and CHT simulations show the impact of the surface temperature on bulk behavior.

Conjugate Thermal Transport in Gas Flow in Long Rectangular Microchannel

2011 ◽  
Vol 133 (2) ◽  
Author(s):  
Zhanyu Sun ◽  
Yogesh Jaluria
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Conjugate Heat Transfer ◽  
Fused Silica ◽  
Maximum Temperature ◽  
Bulk Temperature ◽  
Uniform Heat Flux ◽  
Substrate Thickness ◽  
Variable Properties ◽  
Axial Conduction

This paper is directed at the numerical simulation of pressure-driven nitrogen slip flow in long microchannels, focusing on conjugate heat transfer under uniform heat flux wall boundary condition. This problem has not been studied in detail despite its importance in many practical circumstances such as those related to the cooling of electronic devices and localized heat input in materials processing systems. For the gas phase, the two-dimensional momentum and energy equations are solved, considering variable properties, rarefaction, which involves velocity slip, thermal creep and temperature jump, compressibility, and viscous dissipation. For the solid, the energy equation is solved with variable properties. Four different substrate materials are studied, including commercial bronze, silicon nitride, pyroceram, and fused silica. The effects of substrate axial conduction, material thermal conductivity and substrate thickness are investigated in detail. It is found that substrate axial conduction leads to a flatter bulk temperature profile along the channel, lower maximum temperature, and lower Nusselt number. The effect of substrate thickness on the conjugate heat transfer is very similar to that of the substrate thermal conductivity. That is, in terms of axial thermal resistance, the increase in substrate thickness has the same impact as that caused by an increase in its thermal conductivity. By comparing the results from constant and variable property models, it is found that the effects of variation in substrate material properties are negligible.

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