Transient heat transfer

FIGURE 15.72 Effect of contact angle on transition boiling heat transfer. Transient tests with saturated water on 100-mm copper surface (from Shoji et al. [183], with permission from ASME). 

To understand the heat and moisture flow characteristics of textile fabrics, many mathematical models have been propounded. Matty computational tools like Computational Fluid Dynamics (CFD), artificial neural networks, fuzzy logic and many more are also being used to understand the complex relationships between the clothing parameters and the perception of comfort. This chapter deals with the studies on heat and mass transfer properties of textile assemblies. The phenomena covered here are diy steady state heat transfer, transient heat transfer, moisture vapor and liquid moisture transfer and coupled heat and moisture transfer properties of fibers, fiber bundles, fibrous materials and other textile stmctures. The processes involved in each and the woik done on modeling and simulation of the transfer processes till date, from the point of view of clothing comfort have been discussed. 

Fypass Flow Effects. There are several bypass flows, particularly on the sheUside of a heat exchanger, and these include a bypass flow between the tube bundle and the shell, bypass flow between the baffle plate and the shell, and bypass flow between the shell and the bundle outer shroud. Some high temperature nuclear heat exchangers have shrouds inside the shell to protect the shell from thermal transient effects. The effect of bypass flow is the degradation of the exchanger thermal performance. Therefore additional heat-transfer surface area must be provided to compensate for this performance degradation. 

Operation of a reactor in steady state or under transient conditions is governed by the mode of heat transfer, which varies with the coolant type and behavior within fuel assembHes (30). QuaHtative understanding of the different regimes using water cooling can be gained by examining heat flux, q, as a function of the difference in temperature between a heated surface and the saturation temperature of water (Eig. 1). 

Maintenance of isothermal conditions requires special care. Temperature differences should be minimised and heat-transfer coefficients and surface areas maximized. Electric heaters, steam jackets, or molten salt baths are often used for such purposes. Separate heating or cooling circuits and controls are used with inlet and oudet lines to minimize end effects. Pressure or thermal transients can result in longer Hved transients in the individual catalyst pellets, because concentration and temperature gradients within catalyst pores adjust slowly. 

Slime masses or any biofilm may substantially reduce heat transfer and increase flow resistance. The thermal conductivity of a biofilm and water are identical (Table 6.1). For a 0.004-in. (lOO-pm)-thick biofilm, the thermal conductivity is only about one-fourth as great as for calcium carbonate and only about half that of analcite. In critical cooling applications such as continuous caster molds and blast furnace tuyeres, decreased thermal conductivity may lead to large transient thermal stresses. Such stresses can produce corrosion-fatigue cracking. Increased scaling and disastrous process failures may also occur if heat transfer is materially reduced. 

Grolmes, M. A., A Simple Approach to Transient Two-Phase Level Swell, Multi-Phase Flow and Heat Transfer III. Part A Fundamentals. Proceedings of the Third Multi-Phase Flow and Heat Transfer Symposium—Workshop, Miami Beach, FL, April 18-20, 1983. 

Steady state models of the automobile catalytic converter have been reported in the literature 138), but only a dynamic model can do justice to the demands of an urban car. The central importance of the transient thermal behavior of the reactor was pointed out by Vardi and Biller, who made a model of the pellet bed without chemical reactions as a onedimensional continuum 139). The gas and the solid are assumed to have different temperatures, with heat transfer between the phases. The equations of heat balance are ... 

Chaudhari AM, Woudenberg TM, Albin M, Goodson KE (1998) Transient liquid crystal thermometry of microfabricated PCR vessel arrays. J Microelectromech Sys 7 345-355 Cheng P, Wu WY (2006) Mesoscale and microscale phase heat transfer. In Greene G, Cho Y, Hartnett J, Bar-Cohen A (eds) Advances in heat transfer, vol 39. Elsevier, Amsterdam Choi SB, Barron RF, Warrington RQ (1991) Fluid flow and heat transfer in micro- tubes. ASME DSC 40 89-93... 

Part 1. Presentation of the model. Int J Heat Mass Transfer 47 3375-3385 Tiselj I, Hetsroni G, Mavko B, Mosyak A, Pogrebnyak E, Segal Z (2004) Effect of axial conduction on the heat transfer in micro-channels Int J Heat Mass Transfer 47 2551-2565 Triplett KA, Ghiaasiaan SM, Abdel-Khalik SI, Sadowski DL (1999) Gas-liquid two-phase flow in microchannels. Part I. Two-phase flow patterns. Int J Multiphase Flow 25 377-394 Tsai J-H, Lin L (2002) Transient thermal bubble formation on polysihcon micro-resisters. J Heat Transfer 124 375-382... 

The problems of micro-hydrodynamics were considered in different contexts (1) drag in micro-channels with a hydraulic diameter from 10 m to 10 m at laminar, transient and turbulent single-phase flows, (2) heat transfer in liquid and gas flows in small channels, and (3) two-phase flow in adiabatic and heated microchannels. The smdies performed in these directions encompass a vast class of problems related to flow of incompressible and compressible fluids in regular and irregular micro-channels under adiabatic conditions, heat transfer, as well as phase change. 

At present there is no small-scale test for predicting whether or how fast a fire will spread on a wall made of flammable or semiflammable (fire-retardant) material. The principal elements of the problem include pyrolysis of solids char-layer buildup buoyant, convective, tmbulent-boundary-layer heat transfer soot formation in the flame radiative emission from the sooty flame and the transient natme of the process (char buildup, fuel burnout, preheating of areas not yet ignited). Efforts are needed to develop computer models for these effects and to develop appropriate small-scale tests. 

Cavitations generate several effects. On one hand, both stable and transient cavitations generate turbulence and liquid circulation - acoustic streaming - in the proximity of the microbubble. This phenomenon enhances mass and heat transfer and improves (micro)mixing as well. In membrane systems, increase of fiux through the membrane and reduction of fouling has been observed [56]. 

To provide designers with formulas for predicting real or potential boiling heat transfer behavior, in both steady and transient states. 

Celata, G. P., M. Cumo, F. D Annibale, G. E. Farello, and T. Setaro, 1985, Critical Heat Flux in Flow Transients, in Advances in Heat Transfer, Vol. IV, ENEA Publicatione, Rome. (5)... 

Fauske, H. K., 1971, Transient Liquid-Metal Boiling and Two-Phase Flow, Proc. Int. Seminar Heat Transfer in Liquid Metals, Trogir, Yugoslavia, September. (3)... 

Hall, W. B., and W. G. Harrison, 1966, Transient Boiling of Water at Atmospheric Pressure, Proc. Third Int. Heat Transfer Conf, vol. 3, p. 186, AIChE, New York. (5)... 

Schraub, F. A., 1969, Spray Cooling Heat Transfer Effectiveness during Simulated Loss-of-Coolant Transients, ASME Paper 69WA/NE-8, ASME, New York. (4)... 

Tachibana, F., M. Akiyama, and H. Kawamura, 1968, Heat Transfer and Critical Heat Flux in Transient Boiling, 1. An Experimental Study in Saturated Pool Boiling, J. Nuclear Sci. Technol. of Japan 5(3) 117. (5)... 

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