Peak Heat Flux

Figure 2.38 Effect of liquid subcooling on peak heat flux, for heaters of various sizes in isopropanol. (From Elkssabgi and Lienhard, 1988. Copyright 1988 by American Society of Mechanical Engineers, New York. Reprinted with permission.)... 

Fig. 33. Effect of pressure on the peak heat flux and critical AT. The data include water, ethanol, benzene, propane, pentane, and hexane on a horizontal chromium plate (C2).

The peak heat flux for nucleate pool boiling is indicated as point a in Fig. 9-3 and by a dashed line in Fig. 9-8. Zuber [7] has developed an analytical expression for the peak heat flux in nucleate boiling by considering the stability requirements of the interface between the vapor film and liquid. This relation... 

An interesting peak heat-flux phenomenon is observed when liquid droplets impinge on hot surfaces. Experiments with water, acetone, alcohol, and some of the Freons indicate that the maximum heat transfer is observed for temperature excesses of about 165°C, for all the fluids. The peak flux is a function of the fluid properties and the normal component of the impact velocity. A correlation of experimental data is given in Ref. 30 as... 

Calculate the peak heat flux for boiling water at atmospheric pressure on a horizontal cylinder of 1.25-cm OD. Use the L.ienhard relation. 

Estimate the peak heat flux for boiling water at normal atmospheric pressure. 

EXAMPLE 10-2 Peak Heat Flux in Nucleate Bolling... 

The predicted distribution of net heat flux at the pipe surfaces is illustrated in Figure 11.19. It is interesting to note that the process fluid temperature distribution had a noticeable effect on the heat transfer. Cells A and B had the largest heat fluxes at the bottom, with considerably less heating at the top, whereas cells C and D had relatively uniform heat flux. It is also evident that, in cells A, B, and C, the peak heat flux occurred near the center of the tube bank (near the symmetry plane in the model). 

The virtual test results showed that the cells with the greatest nonuniformity (A and B) had the largest heat fluxes at the bottom and considerably less heating at the top. This was caused by radiation from the flames, which were concentrated in the bottom of the furnace. Further, the peak heat fluxes that occured near the center of the tube banks were caused by the interaction of opposing burner jets, which met in the center of the furnace. The hot gases from the burner jets must turn... 

FIGURE 5.10 Schematic view of heat flux as a function of A7i On the left of the maximum (the peak heat flux) is the nucleate boiling regime where cylinders of vapor form as the peak is approached. On the right of the peak is the region of film boiling. 

Assume that the peak is characterized by the critical condition for Kelvin-Helmholtz instability and for this critical wavelength the fastest growing wavelength from Rayleigh-Taylor instability can be used. Show that it can be used to obtain a correlation for the peak heat flux. Also assume where needed that... 

Radiant heat flux is defined as heat intensity on a specific tube surface. Thus, heat flux represents the combustion intensity and is analogous to how hard a fired heater is run. More specifieally, keeping the firing rate within safe limits is equivalent to maintaining the peak heat flux at less than the design limit because high firebox temperatures could cause tubes, tube-sheet support, and refiractory failures. What is the peak flux and why is it so important to keep it within the limit These questions will be answered next. 

Figure 4. Illustrative example of the dynamic responses of a flat copper cooler and copper cooler with a hot face pattern, due to a slag spall occurring at time zero. A reduced peak heat flux and lower total energy loss are indicated for a copper cooler with a hot face pattern.

In designing a forced convection cooled furnace component, the most critical factor is the expected peak heat load or heat flux that it is expected to withstand under emergency conditions, as visualized for a slag spall in Figure 4. Peak heat flux is often difficult to estimate and is scenario dependent. Not all emergency scenarios can be identified a priori. If previous operational data for similar furnaces are available (for at least one entire smelting campaign). 

The steady state heat flux that a cooler must withstand is often known with some accuracy, either due to calculation (e.g. from Equations [1-5]), piloting or previous commercial operation. The peak heat flux can be considered a known-unknown [39], i.e. it is known that it will occur, but the magnitude is uncertain. Estimations can be made using heat transfer modeling for certain dynamic scenarios, such as a simulated spall or partial refractory failure. These values can be compared to the mean heat flux and a suitable safety factor applied to the nominal value for design purposes. It is recommended that peak design heat flux, should be at least 4 times higher... 

MEG, Dowtherm A, and Galden HT 200 (marked in blue) may experience boiling at the copper interface depending on the system operating pressure and local peak heat fluxes, as these are 2-phase fluids. 

There is a suggestion that peak heat fluxes in the evaporators of rotating heat pipes increase as the one-fourth power of acceleration (Costello and Adams, 1960). While the condenser performance has been less well documented, high G forces allow very thin film thicknesses and continuous irrigation of the surface, reducing the thermal resistance across it. Because of the sealed nature of heat pipes and other rotating devices, even further enhancement of condenser performance could be achieved by promoting drop-wise condensation. 

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