External convection

Hereby I is the integral of the temperature difference (PCM temperature, T, minus ambient temperature, Too) vs. time. It considers constant specific heat for the substance and tube, and constant external convection coefficient. 

These complementary observational constraints indicate that another process participates to the transport of AM in solar-type stars, while MC and turbulence are successful in more massive stars. The two most likely candidates are the large-scale magnetic field which could be present in the radiative zone and the internal gravity waves (hereafter 1GW) which are generated by the external convective zone. As we just explained, the observations suggest that the efficiency of this process is finked to the growth of the convective enveiope. This is a characteristics of 1GW. 

Another experiment of interest is that by El Wakil et al. (49), in which the center and peripheral temperatures were measured for a vaporizing droplet subjected to mild forced convection. The measurements show that there are essentially no differences between these two temperatures, not even during the initial transient heating period. Visual observations also revealed the existence of fairly rapid internal circulations. These imply that the assumption of a uniform droplet temperature may be quite realistic for droplet vaporization with some external convective motion. 

A reasonably complete analysis for this problem has not been performed. However, many important contributions towards analyzing a particular aspect of the problem do exist and are discussed briefly according to the intensity of the external convection, which can be represented by the droplet Reynolds number Re. In all the discussions it is assumed that the droplet surface tension is suflBciently great to maintain its spherical shape. 

Note that the critical radius of insulation depends on the thermal conductivity of the insulation k and the external convection heat transfer coefficient h. The rate of heat transfer from the cylinder increases with the addition of insulation for r2 < r teaches a maximum when rj = r , and starts to decrease for 2 > Thus, insulating the pipe may actuaUy increase the rale of heat transfer from the pipe instead of decreasing it when T2 < r -... 

C WTiat is external forced convection How does it differ from internal forced convection fian a heat transfer system involve both internal and external convection at the same time Give an example. 

C.P. Naveira, M. Lachi, R. M. Cotta, and J. Padet, Hybrid Formulation and Solution for Transient Conjugated Conduction-External Convection, Int. J. Heat Mass Transfer, 52, 112-123 (2009). 

Raithby and Hollands, in the chapter on natural convection, have developed correlation equations for external convection from isothermal bodies. In the correlation equations, the conduction Nusselt number is based on the shape factors developed in these sections. 

Heat Transfer on Convection Duct Walls. For this boundary condition, denoted as , the wall temperature is considered to be constant in the axial direction, and the duct has convection with the environment. An external heat transfer coefficient is incorporated to represent this case. The dimensionless Biot number, defined as Bi = heDhlkw, reflects the effect of the wall thermal resistance, induced by external convection. 

Heat Transfer on Walls With External Convection. Figure 5.3 presents the results obtained by Hsu [30] for the thermal entrance problem with the convective duct wall boundary condition without consideration of viscous dissipation, fluid axial conduction, flow work, or internal heat sources. As limiting cases of the boundary condition, the curves corresponding to Bi = 0 and Bi = °° are identical to Nu H and Nu T, respectively. Significant viscous dissipation effects have been found by Lin et al. [31] for larger Bi values. 

Generally, conduction and radiation can be modelled exphdtly and external convection provides few problems for general purpose computer codes but experimental evidence may be required to support modelling assumptions and basic data used to represent internal convection and radiation. Radiation reflection will be important in gas filled packages, and insufficient knowledge of thermal emissivities may restrict the final accuracy. A sensitivity study with different emissivities can be used to show that the assumptions are adequate or to provide conservative (i.e. maximum) limits on calculated temperatures. 

Modeling supercritical extraction of solid material is based on the mass balances that are relevant for the internal diffusive transport of the extract within the solid matter and the external convective transport of the extract from the solid surface to the solvent fluid. 

Answer by Author Heat transfer calculations indicate that a marked reduction in the thermal gradient can be achieved by use of insulation. It would assist in conduction of heat from top to bottom, and it would also eliminate external convective effects. 

Assuming that the external convective velocity in physical space is independent of the property coordinate (e.g., particle size), the convective term takes a simpler form ... 

Charge-transfer resistance represented in Eq. 5-24, is a non-linear element controlled by the Butler-Volmer relationship [1, pp. 45-48], with strong dependence on the electrochemical potential. External convection will enhance mass transport and minimize diffusion effects, keeping the assump-... 

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