Local heat flux

In addition, manufacturers boiler operational manuals provide hardcopy data ratings for heat transfer coefficients, local heat flux, fuel utilization, furnace heat release rates, maximum continuous rating... 

Authoritative guidelines on operating practices indicate that boilers having a maximum local heat-flux of between 44,000 and 82,780 Btu/hr/ft2/°F (79,250-149,000 Btu/hr/ft2/°K, 250-470 kW/m2/°K) should be considered highly rated. 

In typical cause-and-effect mode, where chlorides penetrate the deposit or where a localized overconcentration of hydroxyl ions occurs, the magnetite film is disrupted and particular forms of very damaging corrosion occurs. In addition, where localized heat flux exceeds design limits within a boiler and may be accompanied by departure from nucleate boiling (DNB) conditions, overheating and metal failure may also occur. 

ASME recommends that for Industrial WT, high-duty, primary-fuel-fired, turbine drive boilers, where local heat fluxes are greater than 1.5 x 105 Btu/hr/ft2 (> 473.2 kW/m2), use values for at least the next highest pressure range. 

Boilers operating up to 40 bar (580 psig), but which have a local heat flux density greater than 300 kW/m2 should be treated as those in the column 41 to 60 bar. 

We will discuss the question in terms of the interface between the two-phase liquid (liquid-gas) and the wall. For this, consider the transient local heat flux as... 

Comparisons of rod bundle data with the Columbia correlation and other existing correlations were made using COBRA-IIIC code for predictions of all correlations. The DNBR (or CHFR) reported is not the critical power ratio as used by other authors. The DNBR errors reported by Reddy and Fighetti (1983) are based on the following analysis The measured local heat flux at the experimental location of the first or higher-rank CHF indications is compared with the predicted CHF calculated using local conditions from the subchannel analysis for the... 

Cooper, M. G., and A. J. P. Lloyd, 1966, Transient Local Heat Flux in Nucleate Boiling, Proc. 3rd Ini. 

Hill, K. W F. E. Motley, F. F. Cadek, and J. E. Casterdine, 1974, Effects on Critical Heat Flux of Local Heat Flux Spikes or Local Flow Blockage in Pressurized Water Reactor Rod Bundles, ASME Paper 74-WA/HT-54, ASME, New York. (5)... 

Ignition sources of BS 5852 Parts 1 and 2 Theoretical heat of combustion approx. Flame height Flame temp. Local heat flux. Rate of burning Duration of flaming ... 

A couple points are emphasized 1) the authors have found no published effort to determine the forces on the barrel as a function of material type and solids conveying discharge pressure, and 2) unlike conventional barrels that are thick to induce a maximum thermal capacitance and strength, the barrel in this device was machined to a minimum thickness in order to enhance heat transfer and thus allow a good estimate of the inner wall temperature from the outside temperature measurements and the local heat flux. 

Local heat flux and driving force x = A(l/T) in a heat exchanger. The temperature increase of the cold stream is AT. 

The total rate of heat added to the control volume through the control surfaces can be expressed in terms of the local heat flux q as follows ... 

Consider the typical case of heat transfer between one fluid inside a tube and another fluid outside the tube, shown in cross section in Fig. 11. Heat is transferred by convection from the hot fluid (taken arbitrarily to be the fluid inside the tube) to the fouling deposit (if any) on the inside surface, through the fouling deposits and tube wall by conduction, and then by convection to the fluid outside the tube. At the point where the inside fluid temperature is T and the outside is t. the local heat flux inside the tube is... 

The local heat flux from the surface to the fluid at any x value may be computed by Fourier s heat conduction law,... 

Consider the system shown in Fig. 5-7. The temperature of the wall is T ., the temperature of the fluid outside the thermal boundary layer is T, and the thickness of the thermal boundary layer is designated as 8,. At the wall, the velocity is zero, and the heat transfer into the fluid takes place by conduction. Thus the local heat flux per unit area, q", is... 

Fig. 7. Unit cell containing a wet particle packing for calculating the effective thermal conductivity. A cross-section showing the local heat fluxes at steady state (from Kohout et al., 2004).

Whereas the circumferential variations of the local wall shear stress (i.e., the momentum flux) in itself are not of interest in the study of the BSR, the analogous variations in mass flux or surface concentration are indeed. In Ref. 15 a graph is presented of the local heat flux relative to the circumferential average, for the constant-temperature boundary condition, as a function of a and s/dp. These data are based on a semianalytical solution of the governing PDE, following the procedure described by Ref. 8 (see Section II.B.2). At a relative pitch of 1.2 the local flux at a = 0 is ca. 64% lower than the circumferential average at a relative pitch of 1.5 the flux at a = 0 is still ca. 20% lower than the circumferential average. In the case of a constant surface temperature, the local heat fluxes are directly proportional to the local Nusselt (or Sherwood) numbers. 

Despite the fact that the test section was thermally well insulated, a power balance taking into account heat transfer with the surroundings was necessary to determine the local heat flux i x (and the local fluid temperature where x is the distance from the test section inlet. The heat flux to the fluid is... 

Figure 4. Liquid interface and distribution of the local heat flux at internal wall for Re=300.

Figure 10. Interface shape, distribution of local heat flux and pressure in rivulet cross-section. Distance from liquid inlet equals to 5 cm, Re=2.5 and AT=0.5K.

In order to calculate the shape coefficient S in general, the heat flow Q has to be determined by integration of the local heat flux... 

Recall from the first part of this course, the local heat flux through the interface can be related to the local temperatures using any one of three types of local heat transfer coefficients ... 

To determine the effective conductivity, we will use the relationship between the average heat flux and the particle dipole, S, discussed in Problem 9-26. To compute the dipole, we must determine the local heat flux on the surface of the particle. 

It may be noted that the functional dependence of the local heat flux onx is the same as the shear stress. This is a consequence of the fact that the thickness of the thermal boundary layer varies with x in the same way as the momentum boundary-layer thickness. Furthermore, the form of the correlations for large and small Prandtl numbers are also quite similar. However, this latter observation may be somewhat misleading. In the case Pr 1, the heat... 

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