Burnout point

Clays used in foundries include hydrous alumina silicates, known as bentonites. Their properties provide cohesion and plasticity in the green state and also high strength when dried. There are three clays that are commonly used in foundries western bentonite [sodium bentonite, burnout point 1290°C (2350°F)], southern bentonite [calcium bentonite, burnout point 1065°C (1950°F)], and fire clay [kaolinite, burnout point 1425°C (2600°F)]. 

Between the two roots of Eq. (5-140), one must take into consideration the higher value in order to assure the positivity of (z — L0). Substituting this value of qM into Eq. (5-138) yields a transcendental equation its solution gives the value of z corresponding to the burnout point. 

Silvestri, M., 1966, On the Burnout Equation and on Location of Burnout Points, Energia Nucleare 13(9). (5)... 

C Draw the boiling curve and identify the burnout point on the curve. Explain how burnout is caused. Why is the burnout point avoided in the design of boilers ... 

Fig, 24. Effect of subcooling on burnout point. Water was boiled on stainless-steel at one atmosphere with no forced convection (G2). 

Bernath (B2) has presented a correlation which is shown to fit the burnout points for water at various pressures and velocities, in systems of various geometries. 

Bernath also presents a correlation (for water) for the temperature of the solid at the burnout point. 

Critical heat flux (CHF), also known in the literature as burnout point, is generally related to a drastic decrease in the heat transfer coefficient and is observed not only under pool boiling but also under convective boiling conditions. The CHF condition is observed when the liquid supply to the heated surface is blocked and the surface is covered by a layer of vapor, such that the heat is transferred from the surface to the liquid by conduction and convection through a vapor layer. When heat is dissipated from a device which the imposed parameter is the heat flux, viz. microprocessors, fuel cells, spacecraft payloads and fuel elements in nuclear reactors, exceeding the CHF may result in an irreversible damage of the thermally controlled device. 

It can also be seen that in the nucleate-boiling region the heat transfer characteristics of the greased surface are somewhat different than those exhibited by the other surfaces. The burnout point for the greased surface was approximately 50 greater than for the nongreased surfaces. 

Figure 8 shows a similar graph for a 45 orientation. At this orientation the burnout point for a greased surface is about 100 greater than that obtained with the nongreased surfaces. In addition, ATis considerably increased for the greased... 

Local heat fluxes as large as 40,000 Btu/hr-ft were obtained. The fluid quality X at the burnout point varied from 0.25 to 0.93 for hydrogen and 0.80 to 0.97 for nitrogen. 

For the range of conditions covered herein, an exponential equation was developed for hydrogen which describes the tube-wall-temperature profile in the film-boiling region. This equation applies only to an electrically heated tube having the same temperature—resistance characteristic, orientation, and dimensions as the one used in the present study. Analysis showed that, as a first approximation, the prime variables affecting the shape and absolute values of the temperature profile included flow rate, maximum nucleate flux, and distance downstream of the burnout point. The resultant equation for hydrogen is ... 

Surface-temperature profiles associated with minimum film flux for hydrogen showed a humped shape beginning just downstream of the burnout point, as shown by two tjrpical tube-wall-temperature profiles in Fig. 8. The previous temperature equation does not describe the humped profile. Additive terms, believed to be primarily functions of fluid quality and flow rate for a given fluid, are required. It is not known whether or not a humped wall-temperature profile is always associated with a minimum film flux or may also occur with a maximum nucleate heat-flux condition for certain combinations of flow, fluid quality, and heat flux. 

L = tube length from entrance to burnout point (nucleate boiling length), ft L = distance along tube measured from burnout point, ft Ng = Boiling number gP[Pg.268]

Q = heat input from tube entrance to burnout point, Btu/hr q = heat flux per unit surface area, Btu/hr-ft ... 

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