Determination of the Reference Temperature for a Convective Heat Transfer Coefficient in a Heated Tube Bank

The paper presents a theoretical analysis of heat transfer in a heated tube bank, based on the Nusselt number computation as one of the basic dimensionless criteria. To compute the Nusselt number based on the heat transfer coefficient, the reference temperature must be determined. Despite the value significance, the quantity has several different formulations, which leads to discrepancies in results. This paper investigates the heat transfer of the inline and staggered tube banks, made up of 20 rows, at a constant tube diameter and longitudinal and transverse pitch. Both laminar and turbulent flows up to Re = 10,000 are considered, and the effect of gravity is included as well. Several locations for the reference temperature are taken into consideration on the basis of the heretofore published research, and the results in terms of the overall Nusselt number are compared with those obtained by the experimental correlations. This paper provides the most suitable variant for a unique reference temperature, in terms of a constant value for all tube angles, and the Reynolds number ranges of 100–1000 and 1000–10,000 which are in good agreement with the most frequently used correlating equations.

Emission Characteristics of Pollution Gases from the Combustion of Food Waste

The emission characteristics of pollution gases produced via the combustion of food waste were studied through a laboratory-scale electrically heated tube furnace. The results showed that the pollution gases generated from the combustion of food waste were CO, H2 and NOx. Each emission curve of CO had a peak. When the combustion temperature rose from 400 °C to 1000 °C, the peak first increased (from 400 °C to 700 °C) and then decreased (from 800 °C to 1000 °C). However, the burnout time shortened with the increase in temperature. Therefore, food waste should be combusted at a higher temperature than 700 °C from the perspective of reducing CO emissions. The emissions of H2 were similar to those of CO. In other words, if CO emissions increased, H2 emissions also increased in the same temperature range. Some NOx emission curves had two peaks (the combustion of cooked rice at 1000 °C; the combustion of vegetable leaves in the temperature range of 600 °C to 1000 °C). The higher the combustion temperature, the higher the second NOx emission peak. NOx emissions from the combustion of cooked rice were greater in the temperature range of 400 °C to 500 °C, whereas for vegetable leaves, that temperature range was from 600 °C to 700 °C. Hence, from the viewpoint of reducing pollution gases, food waste should be combusted at a higher temperature than 700 °C.

Numerical Study of Heated Tube Arrays in the Laminar Free Convection Heat Transfer

Laminar free convection heat transfer from a heated cylinder and tube arrays is studied numerically to obtain the local and average Nusselt numbers. To verify the numerical simulations, the Nusselt numbers for a single cylinder were compared to other authors for the Rayleigh numbers of 103 and 104. Furthermore, the vertically arranged heated tube arrays 4 × 1 and 4 × 2 with the tube ratio spacing SV/D = 2 were considered, and obtained average Nusselt numbers were compared to the existing correlating equations. A good agreement of the average Nusselt numbers for the single cylinder and the bottom tube of the 4 × 1 tube array is proved. On the other hand, the bottom tubes of the 4 × 2 tube array affect each other, and the Nusselt numbers have a different course compared to the single cylinder. The temperature fields for the tube array 4 × 4 in basic, concave, and convex configurations are studied, and new correlating equations were determined. The simulations were done for the Rayleigh numbers in the range of 1.3 × 104 to 3.7 × 104 with a tube ratio spacing S/D of 2, 2.5, and 3. On the basis of the results, the average Nusselt numbers increase with the Rayleigh numbers and tube spacing increasing. The average Nusselt number and total heat flux density for the convex configuration increase compared to the base one; on the other hand, the average Nusselt number decreases for the concave one. The results are applicable to the tube heaters constructional design in order to heat the ambient air effectively.

EXPERIMENTAL INVESTIGATION OF HEAT TRANSFER FROM ELLIPTIC TUBE IMMERSED IN A FLUIDIZED BED

Heat transfer characteristics around an elliptic heated tube immersed in fluidized bed were studied experimentally. Experiments were carried out under uniform heat flux condition, with air as a fluidizing gas and pulverized coal as a bed material of Geldart D. An elliptic tube was heated by using a cartridge heater of 16 mm OD and 200 mm length. The heated tube had a total surface area of As ≈ 0.015 m2, and length of 200 mm. The local as well as average heat transfer coefficients were calculated at different superficial air velocities, particles sizes of 2, 4 and 6 mm, and static bed height of 250 mm. Various values of fluidization number, Usup/Umf, based on hydraulic diameter of the heated tube are utilized in the experiments which are ranged from 1 to 1.4. The results showed that, the minimum fluidization velocity increased with the increase in bed particles diameter. The local Nusselt number was quasi-uniform and having a maximum value at the sides of the heated tube and minimum at the stagnation, ϑ=0°, and top, ϑ=180°, of the tube. The local and average Nusselt numbers increase with the increase of the fluidization velocity and decrease in particles diameter.