Extended surface thermal length

The ratio of outside to inside surface for bare tubes is usually in the range of 1.1 to 1.5 depending on the tube diameter and tube wall thickness. When the thermal resistance on the outside of the tube is much greater than that on the inside, more heat can be transferred (per unit length) through tubes that have a greater ratio of outside to inside area. Such tubes are referred to as extended surface (finned) tubes and are available with external to internal surface ratios ranging from 3 to 40. 

The procedure here is similar to that adopted previously. A heat balance, as opposed to a momentum balance, is taken over an element which extends beyond the limits of both the velocity and thermal boundary layers. In this way, any fluid entering or leaving the element through the face distant from the surface is at the stream velocity u and stream temperature 0S. A heat balance is made therefore on the element shown in Figure 11.10 in which the length l is greater than the velocity boundary layer thickness S and the thermal boundary layer thickness t. 

To achieve fracture in UHTC materials, ManLabs researchers decided to notch all subsequent test specimens. Notches were 6 mm deep x 1.6 mm wide and parallel to the axis of the cylinder, extending inward from the outer surface along the entire length of the specimen. The main disadvantage of using notched specimens was the lack of adequate experimental and analytical data on the shape factor required to compare the experimental results with predicted properties as well as the inability to compare these results with those of previous evaluations. Their results showed that materials with SiC and carbon additions displayed somewhat higher steady state thermal stress resistance than the other compositions. Nonetheless, all the diboride compositions tested showed a level of thermal stress resistance considerably above any other ceramics they had tested. ... 

The infrared absorption coefficient and thermal wave decay coefficients, a(v) and flj, respectively, determine the magnitude of the photoacoustic signal. The term ot( exp —[a( +fls]x in the expression for temperature oscillation leads to a linear PA signal dependence on infrared absorption when a( thermal wave decay length, L, although it is sometimes referred to as the sampling depth, penetration depth, or thermal diflusion depth. The sample layer extending a distance L beneath the surface contributes... 

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