Pressure drop forced convection

The radiant section of an industrial boiler may typically contain only 10 per cent of the total heating surface, yet, because of the large temperature difference, it can absorb 30-50 per cent of the total heat exchange. The mean temperature difference available for heat transfer in the convective section is much smaller. To achieve a thermally efficient yet commercially viable design it is necessary to make full use of forced convection within the constraint of acceptable pressure drop. 

The conclusion to be drawn from the above examples and many others is that softness in a boiling system, preceding the boiling channel inlet, may cause flow oscillations of low frequency. It is probably the pressure perturbations arising from the explosive nature of nucleate boiling that initiates the oscillation, and the reduced burn-out flux which follows probably corresponds to the trough of the flow oscillation, as a reduction in flow rate always drops the burn-out flux in forced-convection boiling. 

The data presented in the previous chapters, as well as the data from investigations of single-phase forced convection heat transfer in micro-channels (e.g., Bailey et al. 1995 Guo and Li 2002, 2003 Celata et al. 2004) show that there exist a number of principal problems related to micro-channel flows. Among them there are (1) the dependence of pressure drop on Reynolds number, (2) value of the Poiseuille number and its consistency with prediction of conventional theory, and (3) the value of the critical Reynolds number and its dependence on roughness, fluid properties, etc. 

Yen T-H, Kasagi N, Suzuki Y (2003) Forced convective boiling heat transfer in micro-tubes at low mass and heat fluxes. Int J Multiphase Flow 29 1771-1792 Yu W, France DM, Wambsganss MW, Hull JR (2002) Two-phase pressure drop, boiling heat transfer, and critical heat flux to water in a small-diameter horizontal tube. Int J Multiphase Flow 28 927-941... 

The heat transfer coefficient at the inside wall and pressure drop through the coil can be estimated using the correlations for flow through pipes see Section 12.8 and Volume 1, Chapters 3 and 9. Correlations for forced convection in coiled pipes are also given in the Engineering Sciences Data Unit Design Guide, ESDU 78031 (2001). 

Fauske, H. K., and M. A. Grolmes, 1970, Pressure Drop for Forced Convection Flashing Sodium, in Liquid Meta Heat Transfer and Fluid Mechanics, J. C. Chen and A. A. Bishop, Eds., pp. 135-143, ASME, New York. (3)... 

Green, S. J., G. W. Mauer, and A. Weiss, 1962, Burnout and Pressure Drop Studies for Forced Convection Flow of Water Parallel to Rod Bundles, ASME Paper 62-HT-43, Natl. Heat Transfer Conf., Houston, TX. (3)... 

Tippets, F. E., J. A. Bond, and J. R. Peterson, 1965, Heat Transfer and Pressure Drop Measurements for High Temperature Boiling Potassium in Forced Convection, Proc. Conf. on Applied Heat Transfer Instrumentation to Liquid Metal Experiments, ANL-7100, p. 53-95, Argonne National Lab., Argonne, IL. (3)... 

Whalley, P. B., P. Hutchinson, and G. F. Hewitt, 1973, The Calculation of Critical Heat Flux in Forced Convective Boiling, Rep. AERE-R-7520, European Two Phase Flow Group Meeting, Brussels. (5) Whalley, P. B., P. Hutchinson, and G. F. Hewitt, 1974, The Calculation of Critical Heat Flux in Forced Convection Boiling, Heat Transfer 1974, vol. IV, pp. 290-294, Int. Heat Transfer Conf., Tokyo. (5) Wichner, R. P, and H. W. Hoffman, 1965, Pressure Drop with Forced Convection Boiling of Potassium, Proc. Conf. on Applications of Heat Transfer Instrumentation to Liquid Metals Experiments, ANL-7100, p. 535, Argonne National Lab., Argonne, IL. (3)... 

Flame impingement. This is often caused by dirty burner tips, lack of combustion air, poorly designed burners, high burner tip pressure, improper adjustment of the burner, or improper draft. I have seen a heater in Cartagena, Colombia, with the flames being forced outward against the upper radiant wall tubes. The problem was an extreme positive pressure in the firebox, due to excessive pressure drop of the flue gas in the fouled convective section. 

For a subcooled liquid in forced convection, boiling inside a tube, pressure drop considerations are important. As the burnout condition is approached, the volume of vapor formed becomes great and causes a back pressure. This back pressure can decrease the liquid flow rate so that the tube condition becomes serious. Burnout may occur because of this choking. Kreith and Foust (Kl) discuss this problem and present a graph for predicting choking for boiling water. 

Let the medium between the flat surfaces of two bodies (now a fluid because of practical reasons) flow with a mean velocity V (Fig. 1.13). This flow results from either an imposed pressure drop or an induced buoyancy, respectively called forced and natural convection. Lettheinlettemperatureofthefluidbe72. (Note that the fluid temperature need not be Tz- Selection of Tz for this temperature eliminates temperature gradient near plate 2 and simplifies the following development.) The convection heat transfer from plate 1 is defined as the conduction in the fluid next to plate 1 (in view of the fact... 

S. M. MacBain and A. E. Bergles, Heat Transfer and Pressure Drop Characteristics of Forced Convection Evaporation in Deep Spirally Fluted Tubing, in Convective Flow Boiling, J. C. Chen ed., pp. 143-148, Taylor Francis, Washington, DC, 1996. 

Bell-Delaware Method. Pressure drop and heat transfer calculations (the step 6 of the above thermal design procedure) constitute the key part of design. Tubeside calculations are straightforward and should be executed using available correlations for internal forced convection. The shellside calculations, however, must take into consideration the effect of various leakage streams (A and E streams in Fig. 17.30) and bypass streams (C and F streams in Fig. 17.30) in addition to the main crossflow stream B through the tube bundle. Several methods have been in use over the years, but the most accurate method in the open literature is the above mentioned Bell-Delaware method. This approach is based primarily on limited experimental data. The set of correlations discussed next constitutes the core of the Bell-Delaware method. 

T. J. Rabas and J. Taborek, Survey of Turbulent Forced-Convection Heat Transfer and Pressure Drop Characteristics of Low-Finned Tube Banks in Cross Flow, Heat Transfer Eng., Vol. 8, No. 2, pp. 49-62,1987. 

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