No abstract

The local characteristics of heat transfer from horizontal tubes immersed in fluidized beds were investigated experimentally. Steady-state heat transfer measurements were obtained in air-fluidized beds of glass beads, both for a single tube and a ten-row bare tube bundle. The test results indicated that local heat transfer coefficients are strongly influenced by angular position and gas flow rate, as well as by particle size and system pressure. The heat transfer coefficients, averaged around the circumference of the tube, exhibited a general tendency to increase with decreasing particle size and increasing system pressure. The heat transfer coefficients for a tube in an inner-row position within the bundle were found to be slightly higher than those for a tube in the bottom-row. Comparison of the average heat transfer coefficient data obtained in this study with some of the existing correlations for heat transfer from horizontal tubes showed that the correlations are unsatisfactory.

The local contact characteristics between the bed emulsion and the tube surface for horizontal tubes immersed in fluidized beds were investigated experimentally. The measurements indicated that the contact dynamics change significantly with circumferential position, gas flow rate, particle size and system pressure. From the experimental data, quantitative information on the local fluidization behavior were generated for use in heat transfer models.

Pressure drop and void fraction measurements in two-phase (air–water) flow through porous beds of randomly packed spheres have been used to determine the interfacial gas–liquid drag and the gas–solid drag for the case of zero net liquid flux through the bed. The results, presented for beds of 3.18-, 6.35-, and 12.7-mm spheres, show that the interfacial gas–liquid drag term is of the same order as the gas-solid drag term when the particle size is greater than 6 mm.

Several boiling regimes are characterized by intermittent contacts of vapor and liquid at the superheated wall surface. A microthermocouple probe was developed capable of detecting transient surface temperatures with a response time better than 1 ms. The transient temperature data were utilized to determine the time-varying heat flux under liquid contacts. The instantaneous surface heat flux was found to vary by orders of magnitude during the milliseconds of liquid residence at the hot surface. The average heat flux during liquid contact was found to range from 105 to 107 W/m2 for water at atmospheric pressure, as wall superheat was varied from 50 to 450°C.